Fourier-transform IR spectroscopy
Chong-Kai Huang, Zhen-Feng Xu, Masakazu Nakajima, Hue M. T. Nguyen, M. C. Lin, Soji Tsuchiya, and Yuan-Pern Lee
Citation: The Journal of Chemical Physics 137, 164307 (2012); doi: 10.1063/1.4759619 View online: http://dx.doi.org/10.1063/1.4759619
View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/137/16?ver=pdfcov Published by the AIP Publishing
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Dynamics of the reactions of O(
1D) with CD
3OH and CH
3OD studied
with time-resolved Fourier-transform IR spectroscopy
Chong-Kai Huang,1Zhen-Feng Xu,2Masakazu Nakajima,1,a)Hue M. T. Nguyen,3
M. C. Lin,1,2,b)Soji Tsuchiya,1,b),c)and Yuan-Pern Lee1,4,b)
1Department of Applied Chemistry and Institute of Molecular Science, National Chiao Tung University, Hsinchu 30010, Taiwan
2Department of Chemistry, Emory University, Atlanta, Georgia 30322, USA
3Department of Physical Chemistry, Hanoi University of Technology, Hanoi, Vietnam 4Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan
(Received 16 August 2012; accepted 4 October 2012; published online 24 October 2012)
We investigated the reactivity of O(1D) towards two types of hydrogen atoms in CH3OH. The
re-action was initiated on irradiation of a flowing mixture of O3 and CD3OH or CH3OD at 248 nm.
Relative vibration-rotational populations of OH and OD (1≤ v ≤ 4) states were determined from their infrared emission recorded with a step-scan time-resolved Fourier-transform spectrometer. In O(1D)+ CD
3OH, the rotational distribution of OD is nearly Boltzmann, whereas that of OH is
bi-modal; the product ratio [OH]/[OD] is 1.56± 0.36. In O(1D)+ CH
3OD, the rotational distribution
of OH is nearly Boltzmann, whereas that of OD is bimodal; the product ratio [OH]/[OD] is 0.59 ± 0.14. Quantum-chemical calculations of the potential energy and microcanonical rate coefficients of various channels indicate that the abstraction channels are unimportant and O(1D) inserts into the
C−H and O−H bonds of CH3OH to form HOCH2OH and CH3OOH, respectively. The observed
three channels of OH are consistent with those produced via decomposition of the newly formed OH or the original OH moiety in HOCH2OH or decomposition of CH3OOH. The former decomposition
channel of HOCH2OH produces vibrationally more excited OH because of incomplete
intramolec-ular vibrational relaxation, and decomposition of CH3COOH produces OH with greater rotational
excitation, likely due to a large torque angle during dissociation. The predicted [OH]/[OD] ratios are 1.31 and 0.61 for O(1D)+ CD3OH and CH3OD, respectively, at collision energy of 26 kJ mol−1,
in satisfactory agreement with the experimental results. These predicted product ratios vary weakly with collision energy. © 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4759619] I. INTRODUCTION
The reactions of electronically excited oxygen atom O(1D) with hydrocarbons have been the focus of many
in-vestigations concerned with atmospheric chemistry. For the simplest system, the reaction of O(1D) + CH
4, several
re-action channels that yield products, such as CH3 + OH,
H2COH/CH3O + H, and H2CO/HCOH + H2, have been
reported.1 According to experiments with crossed molecular beams, the branching ratios of these three product channels are∼77%, 18%, and 5%, respectively.1A significant propor-tion of the product OH scattered in the forward and backward directions from the O(1D) beam, indicating that the
abstrac-tion channel is important in the channel of OH formaabstrac-tion. The observations of non-statistical vibration-rotational distri-butions of OH (Refs.2–8) and CH3(Refs.9and10) indicate
that they are produced via a path with the lifetime of inter-mediate CH3OH† too small to complete the intramolecular
vibrational relaxation (IVR).11,12The H products are notice-a)Present address: Department of Basic Science, University of Tokyo,
Komaba 3-8-1, Meguro-ku, Tokyo, 153-8902, Japan.
b)Authors to whom correspondence should be addressed. Electronic addresses: chemmcl@emory.edu, tsuchis@sepia.plala.or.jp, and yplee@mail.nctu.edu.tw.
c)Present address: Research Institute of Science and Engineering, Waseda University, Ookubo, Shinjuku-ku, Tokyo 169-8555, Japan.
ably scattered backward, whereas the H2products exhibit an
isotropic angular distribution; the latter likely arises through a long-lived insertion intermediate. Direct information on the lifetime of the intermediate was given by half-collision ex-periments through the UV photodissociation of the O3–CH4
van der Waals complex;13the largest OH formation interval,
5.4 ps, corresponds to the reaction of the intermediate of which the internal energies are randomized statistically, whereas the smallest formation interval, 0.2 ps, corresponds to direct abstraction.
The potential-energy surface (PES) of the O(1D)+CH4
reaction calculated at a higher level indicates that the path of minimum energy follows a barrierless insertion of O(1D)
into the C−H bond to form intermediate CH3OH and a
di-rect abstraction path through a nearly collinear O−H−C geometry.14–16 An additional abstraction path with a small
barrier might occur on the electronically excited 2 1A
sur-face. Based on this PES, quasiclassical trajectory calculations assuming the CH3group to act as a pseudoatom depict an
in-ternal distribution of OH in agreement with the experimental observations.8,17
In the reaction of O(1D) with species that possess two types of H atoms, such as methanol (CH3OH), the reaction
paths are expected to be more complicated than those of O(1D)+ CH4. O(1D) might insert into either the C−H or the 0021-9606/2012/137(16)/164307/14/$30.00 137, 164307-1 © 2012 American Institute of Physics
O−H bond, or abstract a H atom directly from the CH3or OH
moiety of CH3OH. Goldstein and Wiesenfeld reacted O(1D)
with partially deuterated methanol CH3OD and CD3OH
and determined product ratios of [OH]/[OD] from the laser-induced fluorescence (LIF) of OH.18 These authors reported
that∼70% OH product arises from the hydroxyl site, whereas the reactions of O(1D) with the methyl group account for
the remaining OH product. This result is in accord with the rate coefficient of the reaction O(1D)+ CH
3OH reported as
5.1 × 10−10 cm3molecule−1s−1,19 which is much greater
than the value (1.5 ± 0.4) × 10−10 cm3molecule−1s−1 for
the reaction of O(1D)+ CH
4.20 The greater rate coefficient
for the reaction of O(1D) with CH
3OH than with CH4 was
attributed to an increased reactivity of O(1D) towards the
OH moiety of CH3OH. Goldstein and Wiesenfeld also
determined the vibrational populations of OH(v = 0, 1) and OD(v = 0−2) and concluded that an attack on the methyl moiety of methanol generates greater vibrational excitation of hydroxyl radicals than that from attack on the hydroxyl moiety. In these experiments, vibrational states of the product OH or OD beyond v= 1 or 2, respectively, were unreported due to the limitation of the LIF method associated with the predissociation of the electronically excited state.18
Moreover, their reaction system included a buffer gas (He at 10 Torr), precluding information on the rotational distribution of OH; the vibrational distribution of OH might also be affected by collisional relaxation under such conditions. Matsumi et al. detected H atoms with the LIF method and determined a quantum yield of 0.18 for production of H atom in the reaction of O(1D) + CH3OH.19 These authors
reported that [H]/[D] = 0.26 ± 0.03 and 7.1 ± 0.8 for the reaction of O(1D) with CD3OH and CH3OD, respectively.
To explain the selectivity in the formation of product H, two possible mechanisms were considered: (1) insertion of an O atom into the C−H bond is followed by preferential breaking of the new O−H bond because of incomplete IVR in the reaction intermediate, and (2) cleavage of the C−H bond is more likely to occur than of the O−H bond of the reaction intermediate HOCH2OH because the dissociation energy of
the former is smaller than of the latter. Despite all these pre-ceding works, information on the rotational distribution and a more complete vibrational distribution of OH is lacking.
Here, we report our observations of rotationally resolved infrared (IR) emission spectra of OH and OD that were pro-duced from the reaction of O(1D) with CH3OD and CD3OH.
One advantage of IR emission spectroscopy is the ability to monitor OH and OD in vibration-rotational levels greater than v= 2, which are difficult to detect with the LIF method because of predissociation. In addition to the rotational distributions of OH and OD, we determined also the ratio of [OH]/[OD]. We calculated quantum-chemically the PES of various reaction channels and predicted accordingly the branching ratios to assist the interpretation of experimental data.
II. EXPERIMENTS
The apparatus employed to obtain time-resolved emis-sion spectra is based on a step-scan Fourier-transform
spec-trometer (FTS) that was described previously;21–23 only a brief summary appears here. For experiments on O(1D) + methanol, ozone (O3), and methanol were injected
sepa-rately into the reaction chamber. A telescope mildly focused the photolysis beam from a KrF laser (248 nm, 19 Hz) to an area∼12 × 10 mm2at the reaction center to yield a fluence of
∼53 mJ cm−2; this laser beam decomposed O
3to form O(1D).
IR emission was collected with two Welsh mirrors (focal length 10 cm), directed into the step-scan FTS, and de-tected with an InSb detector (Kolmar Technologies) equipped with a preamplifier (rise time 0.34 μs and responsivity 3.2 × 106 V W−1). The transient signal from the InSb detector
was further amplified 20 times with a voltage amplifier (Stan-ford Research Systems, model 560, bandwidth 1 MHz) before being digitized with an external data-acquisition board (12-bit) at 25-ns resolution. Data were typically averaged over 60 laser pulses at each scan step; 4569 scan steps were performed to yield an interferogram resulting in spectra of resolution 0.8 cm−1 in a spectral range of 3800–2170 cm−1 to cover the emission of OH and OD. For the reaction of O(1D)+ CH
3OH,
3473 scan steps were performed to yield an interferogram re-sulting in spectra of resolution 0.5 cm−1in a spectral range of 3650–2950 cm−1to cover emission of OH. To increase further the ratio of signal to noise, five spectra recorded under similar conditions were averaged. The temporal response function of the instrument was determined with a pulsed IR laser beam, as described in Ref.24. The temporal response period of the de-tection system is approximately 1 μs, determined with an IR laser emission. The spectral response function was calibrated with a black-body radiation source.
To decrease the collision quenching of OH and OD, a minimal pressure yielding satisfactory signals was used: PO3 = 0.018−0.022 Torr, PCD3OH = PCH3OD ∼= 0.105 Torr.
Flow rates were FO3 = 0.7−1.0 sccm, FCD3OH = FCH3OD
= 3.9−4.5 sccm, in which sccm denotes cm3min−1 under
standard conditions (1 atm and 298.15 K). Approximately 60% of O3 was dissociated upon irradiation at 248 nm
according to the reported absorption cross section of ∼1.1 × 10−17cm2molecule−1and O(1D) quantum yield of∼0.90
± 0.05 for O3at 248 nm25,26and the laser fluence used. The
depletion of O3 in the flowing system after each laser pulse
was modest, as was confirmed by the negligible variation of the signal intensity when we varied the repetition rate of the photolysis laser from 19 Hz to 30 Hz.
CH3OH (99.9%, Mallinckrodt), CD3OH (isotopic purity
99.5%, Cambridge Isotope Laboratories), and CH3OD
(isotopic purity 99%, Cambridge Isotope Laboratories) were employed without further purification. For experiments with CH3OD, the reaction chamber was heated to 373 K under
vacuum followed by passivation with D2O (10 Torr) at
298 K for 1 h. After evacuation, the system was filled with D2O (10 Torr) overnight and was treated with passivation
and evacuation three times before each experiment. O3 was
produced from O2 (Scott Specialty Gases, 99.995%) with
an ozone generator (Polymetrics, Model T-408) and stored over silica gel at 196 K. The partial pressure of O3 was
determined from the absorption of Hg emission at 254 nm in a cell of length 7.0 cm; the absorption cross section of∼1.1 × 10−17cm2for O
III. COMPUTATIONS
A. Potential-energy surfaces and branching ratios The geometries of reactants, products, intermediates, and transition states on the PES of the O(1D) + CH
3OH
reac-tion were optimized with the B3LYP/aug-cc-pVTZ density-functional theory (DFT).27–30 The vibrational wavenumbers
were calculated at this level to characterize local minima and the transition state and to correct for the zero-point energy. To obtain reliable energies and predictions of rate coefficients, we calculated single-point energies at the CCSD(T)/aug-cc-pVTZ31,32 level based on the structures predicted with
B3LYP/aug-cc-pVTZ, expressed as CCSD(T)//B3LYP/aug-cc-pVTZ. For the abstraction channels, whose transi-tion states could not be found by the DFT method, we calculated energies with the MRCI(8,8)//CAS(10,10)/6-311+G(3df,2p) methods.33,34Calculations of the intrinsic
re-action coordinate35 were performed to connect each transi-tion state with the designated reactants and products. All calculations of electronic structure were performed with the
GAUSSIAN03 (Ref.36) andMOLPROprograms.37
The minimum energy path (MEP) representing a bar-rierless insertion or association process was obtained by calculating the potential energy curve at the CASPT2(8,8)/6-311+G(3df,2p)//CAS(8,8)/6-311+G(3df,2p) level of theory along its reaction coordinate: O· · ·H (for C−H or O−H abstraction), O· · ·C (for C−H insertion), or O· · ·O (for O−H insertion) from its equilibrium separation to ∼5 Å at step size of 0.1 Å and with other geometric parameters fully optimized. The MEP was fitted to the Morse potential function E(R) = De [1 – exp(−β(R – Re))]2 and employed
to approximate the variational transition state in calculations of rate coefficients. In the above equation, R is the reaction coordinate (i.e., the distance for O· · ·H, O· · ·C, or O· · ·O), De
is the bond dissociation energy excluding zero-point energy, β is an exponential factor, and Re is the equilibrium value
of R of an insertion product or an association intermediate. The numbers of states for the tight transition states are evaluated according to the rigid-rotor harmonic-oscillator approximation. The lower vibrational modes in the transition states were treated as classical one-dimensional free rotors. For those paths involving hydrogen atom transfer, Eckart tunneling effects38 were taken into account in the rate
coefficient calculations. The employment of the statistical theory in these rate coefficient calculations implies rapid energy redistributions among all internal degrees of freedom. The rate coefficients for production of H, D, OH, OD, H2O, HOD, and D2O were predicted according to the
fol-lowing method. We employed the variational transition-state theory for the direct abstraction reactions and the variational Rice-Ramsperger-Kassel-Marcus (RRKM) theory39–42 with
Eckart tunneling correction for the insertion/decomposition processes using theVARIFLEX code43 by solving the master
equation for the formation and removal of the excited intermediates (HOCH2OH* or CH3OOH*). The removal
processes include collisional deactivation and various decom-position reactions. Thus the effects of pressure, temperature, isotopic substitution as well as the kinetic energy carried by the O(1D) atom on the formation of individual products can
be reliably examined. To obtain rate coefficients at excess en-ergy E, the energies of all transition states and intermediates involved in the insertion/decomposition paths were increased by the amount because the excess translational energy can be quantitatively injected into the excited adducts (HOCH2OH*
or CH3OOH*) before their isomerization and fragmentation.
The energy increment was fixed at 10 cm−1 in all cal-culations of sums and densities of states using the modified Beyer-Swinehart algorithm.44 For the decomposition with
loose transition states, the potential energy paths are approx-imated with three parts.45 The first part is represented with
a Morse function. The second part corresponds to the inter-nal degrees of freedom of the separate fragments and is as-sumed to be the same as the normal-mode vibrations of the fragments. The third part is the potential energy for the transi-tional modes, which is described in terms of a set of inter-nal angles with a sum of products of sinusoidal functions. In the RRKM calculations, the L-J values for HOCH2OH
and CH3OOH, were approximated with σ = 4.317 Å and
ε/κ= 450.2 K.46
B. EinsteinA coefficients for vibration-rotational transitions of OH/OD
The rotational states of the OH radical, an open-shell molecule with ground state X 2, consist of two spin sub-levels F1 and F2 corresponding to J= N + 1/2 and J = N
− 1/2, respectively; each J level splits into e and f states through the -doubling. Because the spin uncoupling de-pends on J, OH is intermediate between Hund’s cases (a) and (b). For the assignment of our emission spectra of OH, we re-ferred to the high-resolution data and molecular parameters of OH47,48and OD.49–51
A diatomic hydride molecule OH has a large rotational constant and the large rotational constant causes a large extent of centrifugal distortion effect on the radial part of the wavefunction that changes significantly the magnitude of the transition dipole moment (the Herman-Wallis effect);52
the Einstein A coefficients must consequently be evaluated for respective vibration-rotational transitions. Nelson et al. derived the electric dipole-moment function of OH from observations of the relative intensities of vibration-rotational transitions of OH.53 Using this function, they calculated
the Einstein A coefficients of vibration-rotational transitions according to A(vJF→ vJF)=16π 3ν3 3ε0hc3 |vJF|μ (r)|vJF|2, (1) in which μ(r) is the dipole-moment function and |vJF is the eigenfunction of the vibration-rotational state of OH in which the -type splitting is disregarded. To estimate the population distribution of OH in this work, we employed a ta-ble of the Einstein A coefficients reported by Holzclaw et al.54
who calculated the transition dipole moment according to the dipole-moment functions reported by Nelson et al.53 from
the numerical solution of the Schrödinger equation in which the RKR potential was introduced.
Since there is no available data for the Einstein A co-efficients of the OD Meinel bands, we calculated them fol-lowing the method described by Nelson et al.55 The details of our calculation and the resultant Einstein A coefficients of OD are available in the supplementary material.56The
transi-tion strength for various Jvalues of the R-branches of OD, as also seen in the case of OH, is significantly smaller than the corresponding values of the P-branches because of a negative contribution of the vibration-rotational coupling to the transi-tion dipole moment. We therefore analyzed only the P-branch transitions with spin-sublevel transitions of F1 → F1 and
F2→ F2, which are denoted P1and P2, respectively.
IV. RESULTS AND DISCUSSION A. IR emission from the O(1D)+ CD
3OH system
Figure1shows the time-resolved emission spectra in the spectral regions 2800–3700 and 2160−2650 cm−1recorded at
1-μs intervals for the first 3 μs upon irradiation at 248 nm of a flowing mixture of O3/CD3OH (1/5.3, 0.125 Torr); O(1D) was
generated on photolysis of O3 to react with CD3OH. Sharp
lines in two groups located in the regions 2800–3700 cm−1 and 2160–2600 cm−1 are identified, together with two broad feature covering the regions 2160−2350 cm−1 with a maxi-mum near 2200 cm−1. The first and second groups of sharp lines are readily assigned to the v= −1 transitions of OH and OD radicals, respectively, and the observed broad feature is likely co-products of OH or OD. Because it is difficult to identify positively the carrier of this feature from the available information, we concentrate on the analysis of spectral lines of OH and OD.
The intensities of emission lines of OH and OD rise towards their maxima∼2 μs after irradiation of the flowing sample at 248 nm. Because the initial concentration of CD3OH, ∼3.4 × 1015 molecule cm−3, is much greater than
that of O(1D), the condition of pseudo-first-order reaction is valid. The pseudo-first-order rate coefficient kI= 2 × 106s−1, in which the rate coefficient of O(1D)+ CH
3OH was reported
by Matsumi et al.,19is in accord with the observed rise time
for emission of OH or OD. For an improved ratio of signal to noise, we analyzed the distribution of internal states of OH or OD according to the emission spectra averaged over 0−5 μs, and corrected for the small rotational quenching effect, as described later.
The assignments of the observed lines to vibration-rotational transitions of OH and OD, indicated at the top part of Fig.1, were made according to the high-resolution spectral data for OH (Refs. 47 and48) and OD;49–51 most lines are
associated with transitions of P1(F1→ F1) and P2(F2→ F2)
for v = −1 (v = 1−3). Lines associated with R1, R2, Q1,
and Q2transitions have much smaller intensity as J increases,
consistent with the report by Nelson et al.53,55 Most doublet e and f lines are unresolved because the resolution was set at 0.8 cm−1 in these experiments. For J≥ 10.5, the e/f splitting exceeds 1 cm−1, resulting in broadened lines or partially resolved lines. In this experiment, we made no attempt to separate the populations of the e and f transitions, but used the total population for each vibration-rotational
FIG. 1. Observed IR emission spectra of the reaction system O(1D)
+ CD3OH recorded at 1-μs intervals. The spectral resolution is 0.8 cm−1.
Partial pressures of O3 and CD3OH are 0.018−0.022 and
0.104−0.106 Torr, respectively. The assignments of vibration-rotational transitions are shown as stick diagrams for OH and OD, respectively; the numbers correspond to J.
transition. To analyze these observed lines, we assumed a Gaussian shape for each line and performed curve fitting on each line, including overlapped lines.
To improve the ratio of signal to noise in these exper-iments, the micropopulation was determined from spectral lines integrated for 0−5 μs after photoirradiation. Each vibration-rotational line in the P branch was normalized with the instrument spectral-response function, and divided by its respective Einstein coefficient to yield a relative population Pv(v, J, F). In Fig.2, the logarithm of the micropopulation
in a rotational state (v, J, F), defined as Pv(v, J, F)/(2J
+ 1), is plotted as a function of the rotational energy that is defined as the average of the e and f term values from which the vibrational term value is subtracted. The micropopula-tions of the F1and F2components are similar, as indicated in
FIG. 2. Semilogarithmic plots of the rotational populations of OH and OD as a function of the rotational energy in respective vibrational levels formed from the O(1D)+CD
3OH reaction. Average period is 0−5 μs.
Fig.2for values derived from P1(shown as blue dots) and P2
(red triangles) branches.
As shown clearly in Fig.2, the rotational distribution of OH is not singly exponential, whereas that of OD is nearly ex-ponential (Boltzmann). Average rotational energies for each vibrational level of OH and OD, Er(v), were obtained on
sum-ming the product of rotational level energy and normalized population over all observed rotational levels. The average ro-tational energy Er was derived on multiplying the observed
Er(v) with the respective vibrational population, to be
dis-cussed later. The average rotational energies Er of OH and
OD are thus calculated to be 11± 3 and 5 ± 1 kJ mol−1, re-spectively; Erof OD is much smaller than that of OH.
For OD, we derived rotational temperatures (TR) of
470 ± 60, 450 ± 50, 440 ± 50, and 430 ± 40 K for v = 1−4, respectively. Because the rotational distributions of OH(v = 1) and OH(v = 2) are non-Boltzmann, we fitted them with biexponential functions and derived the rotational temperatures 450± 90 K (v = 1) and 400 ± 40 K (v = 2) of the low-J component and 1100± 110 K (v = 1) and 680 ± 110 K (v = 2) of the high-J component. For OH(v = 3), we could fit only a rotational temperature of 420 ± 10 K because of the limited levels observed. The population ratio of the high-J component to the low-J component is∼35/65. Because the spectra were integrated for 0−5 μs, the rotational temperatures reflect the average value in this period.
To derive the nascent rotational energy, we estimated the quenching effects by measuring the rotational temperature as a function of time. By integrating the observed lines at 1-μs intervals, we determined the rotational temperature of OD and the high-J and low-J components of OH as a function of time. The rotational temperatures of OD(v) determined at 1-μs intervals are shown in Fig.3. The nascent rotational tempera-tures of 570± 190 K, 560 ± 100 K, and 560 ± 60 K were de-rived for OD(v= 1−3), respectively, after a short
extrapola-tion to t= 0. According to this measurement of the rotational quenching, we estimated that the nascent rotational tempera-ture should be multiplied by 1.2± 0.1 for OD, compared with those determined for 0−5 μs. Using a similar method, we
FIG. 3. Plot of rotational temperature of OD (v= 1−3) as a function of time. The data extrapolated to t= 0 are indicated with o.
TABLE I. Summary of experimental results for reactions O(1D) +
CH3OD/CD3OH.
O(1D)+ CD3OHa O(1D)+ CH3ODa
Products OH OD OH OD
rotation bimodal Boltzmann Boltzmann bimodal TR(v= 1) (K) 450± 90 470± 60 410± 50 250± 20 1100± 110 990± 30 TR(v= 2) (K) 400± 40 450± 50 440± 70 280± 30 680± 110 920± 40 TR(v= 3) (K) 420± 10 440± 50 410± 50 350± 20 TR(v= 4) (K) 430± 50 Er(kJ mol−1) 11± 3 5± 1 5± 1 9± 2 (12± 4)b (6± 2)b (6± 2)b (10± 3)b Ev(kJ mol−1) 28± 6 43± 8 65± 15 27± 5 [OH]/[OD] 1.56± 0.36 0.59± 0.14 aUnless otherwise noted, the data are derived from spectra integrated over 0−5 μs after photolysis of O3.
bEstimated nascent average rotational energy. For bimodal distributions, average ener-gies for the high-J and low-J components are 18± 5 and 6 ± 2 for the reaction O(1D)+ CD3OH and 14± 3 and 5 ± 2 for the reaction O(1D)+ CH3OD. The population ratio of these two components are∼50/50.
estimated that the nascent rotational temperature should be multiplied by 1.2± 0.1 for the low-J component of OH, and 1.8 ± 0.3 and 2.1 ± 0.4 for the high-J component of OH (v= 1 and 2), respectively, compared with those determined for 0−5 μs. We hence estimate the average nascent rotational energies to be 6 ± 2 kJ mol−1 for OD and 18 ± 5 and 6± 2 kJ mol−1, respectively, for the high-J and low-J compo-nents of OH; the energy of the high-J component is less accu-rate because of the large factor of correction. After correction for quenching, the population ratio of the nascent high-J component to the low-J component is ∼50/50; the nascent average rotational energy of OH is thus 12 ± 4 kJ mol−1, as listed in TableI.
The relative vibrational populations were derived on counting levels up to the observed maximal Jvalues in each vibrational level, J,F P(v, J, F). We normalized the
val-ues of J,F P(v, J, F) associated with each vibrational
state to yield a relative vibrational population (v= 1) : (v = 2) : (v= 3) = 54.8 : 33.7 : 11.5 for OH and (v= 1) : (v
= 2) : (v= 3) : (v= 4) = 34.9 : 28.6 : 22.7 : 13.8 for OD.
In our IR chemiluminescence experiments, it is impossi-ble to obtain directly information about the population of the vibrational ground state; the population of v= 0 was hence estimated on extrapolation from the populations of levels v≥ 1 assuming a Boltzmann distribution. The populations of v= 0 relative to v= 1 were thus derived to be 2.4 ± 0.3 and 1.5± 0.2 for OH and OD, respectively. After renormalizing, we derived vibrational distributions of OH as (v = 0) : (v = 1) : (v= 2) : (v= 3) = 56.8 : 23.7 : 14.6 : 4.9 and of OD
as (v= 0) : (v= 1) : (v= 2) : (v= 3) : (v= 4) = 34.1 : 23.0 : 18.9 : 14.9 : 9.1. These vibrational distributions of OH and OD are shown in Fig.4(a).
The observed ratios of total populations of OH (v = 1−3) and OD (v = 1−4) are derived to be ∼50/50. When we include the estimated populations for the v= 0 levels of OH and OD, the total ratios of populations of OH (v= 0−3) and OD (v = 0−4) are derived to be (61 ± 9)/(39 ± 9)
FIG. 4. Relative vibrational populations of OH and OD produced from the O(1D)+ CD
3OH reaction (a) and O(1D)+ CH3OD reaction (b) as a
func-tion of vibrafunc-tional quantum number v. Populafunc-tions in v= 0 are derived by extrapolation.
= 1.56 ± 0.36. This ratio is slightly smaller than, but within the error limit of, the ratio (71 ± 12)/(29± 12) = 2.45 ± 1.01 determined from OH (v = 0, 1) and OD (v = 0−2) by Goldstein and Wiesenfeld.18 The discrepancy is partly due to the fact that their observations were limited to pop-ulations in small vibrational levels of OH and OD. As seen from Fig. 4(a), the populations of OD (v > 2) contribute much more significantly than those of OH (v≥ 2).
Using this distribution of vibrational populations, we cal-culated the average vibrational energies of OH and OD to be 28 ± 6 and 43 ± 8 kJ mol−1, respectively, as listed in TableI. The vibrational energy of OH is slightly smaller than that of OD; in contrast, the rotational energy of OH is greater than that of OD.
B. Infrared emission from the O(1D)+ CH
3OD system
To discriminate the reaction sites of O(1D) towards the
methyl and hydroxyl groups in methanol, we investigated also the reaction O(1D)+ CH
3OD. In the reaction of O(1D)
FIG. 5. Observed IR emission spectra of the reaction system O(1D)+CH
3OD recorded at 1-μs intervals. The spectral resolution is
0.8 cm−1. Partial pressures of O3 and CH3OD are 0.018−0.022 and
0.104−0.106 Torr, respectively. The assignments of vibration-rotational transitions in P branches are given as stick diagrams for OH and OD, respectively; the numbers correspond to J.
to channels that produce OD and OH, respectively, in the reaction of O(1D)+ CD
3OH.
The time-resolved IR emission spectra observed at 1-μs intervals from the O(1D) + CH
3OD system are shown in
Fig. 5. Similar to the reaction of O(1D) + CD
3OH, two
groups of sharp lines located in regions of 3700−2800 and 2650−2160 cm−1 are assigned as the v = −1
vibration-rotational transitions of OH and OD, respectively. In ad-dition, broad emission bands are evident in the regions of 3600−3200, 3100−2350, and 2350−2160 cm−1 that might
have originated from the co-products of OH or OD, likely CH2DO or CH3O. Compared with the spectra of OH and OD
in the reaction system of O(1D)+CD
3OH, the line intensities
are smaller for both OH and OD and the distributions of in-ternal states differ. The relative rotational micropopulations of the respective vibrational states of OH and OD determined
from the line intensities integrated for 0−5 μs are shown as a function of rotational energy in Fig.6. The rotational distribu-tions of OH are nearly singly exponential as in the case of OD produced from the reaction of O(1D)+ CD3OH. In contrast,
the rotational distributions of OD are biexponential, similar to those of OH from the reaction O(1D)+ CD
3OH. The result is
in accord with the expectation for such isotopic substitutions. Following a method similar to that described in Sec. IV A, the average rotational energies of OH and OD for the period 0−5 μs are thus calculated to be 5 ± 1 and 9 ± 2 kJ mol−1,
respectively, as listed in TableI.
For OH, we derived rotational temperatures of 410± 50, 440± 70, and 410 ± 50 K for v = 1−3, respectively. We fit-ted the rotational distributions of OD(v= 1) and OD(v = 2) with biexponential functions and derived the rotational tem-peratures 250± 20 K (v = 1) and 280 ± 20 K (v = 2) of the low-J component and 990± 320 K (v = 1) and 920 ± 400 K (v = 2) of the high-J component. For OD(v = 3), we could fit only a rotational temperature of 350± 20 K for the low-J component because of the limited levels observed. The ratio of the high-J component to the low-J component of OD is ∼48/52. Because the spectra were integrated for 0−5 μs, the rotational temperatures reflect the average value during this period.
According to measurements of rotational quenching we estimated that the nascent rotational temperature should be multiplied by 1.3± 0.2 for OH, 1.2 ± 0.2 for the low-J com-ponent of OD, and 1.3 ± 0.3 for the high-J component of OD, respectively. We hence estimate the average nascent ro-tational energy to be 6± 2 kJ mol−1for OH, and 14± 3 and 5± 2 kJ mol−1, respectively, for the high-J and low-J compo-nents of OD. After correction for quenching, the population ratio of the nascent high-J component to the low-J component is∼50/50; the average rotational energy of OD is thus 10 ± 3 kJ mol−1, as listed in TableI.
We normalized the values of J,F P(v, J, F)
associ-ated with each vibrational state to yield a relative vibrational population (v= 1) : (v= 2) : (v = 3) : (v = 4) = 32.8 : 28.0 : 21.5 : 17.7 for OH and (v= 1) : (v= 2) : (v= 3) : (v = 4) = 45.9 : 32.1 : 15.8 : 6.2 for OD. The population of v= 0 is estimated on extrapolation from the populations of levels v≥ 1 according to the Boltzmann distribution; the populations of v= 0 relative to v= 1 were thus derived to be 1.3± 0.2 and 2.3 ± 0.5 for OH and OD, respectively. After renormalizing, we derived the vibrational distributions of OH as (v= 0) : (v= 1) : (v= 2) : (v= 3) = 29.3 : 23.2 : 19.8 : 15.2 : 12.5 and of OD as (v= 0) : (v= 1) : (v= 2) : (v = 3) : (v = 4) = 51.4 : 22.3 : 15.6 : 7.7 : 3.0. These
vibra-tional distributions of OH and OD are shown in Fig.4(b). The observed ratios of total populations of OH (v= 1−3) and OD (v = 1−4) are derived to be 46/54. When we include the estimated populations for the v= 0 levels of OH and OD, the ratio of total populations of OH (v = 0−3) and OD (v= 0−4) become (37 ± 9)/(63 ± 9) = 0.59 ± 0.14. This ratio is slightly larger than the ratio (30± 6)/(70 ± 6) = 0.43 ± 0.09 reported by Goldstein and Wiesenfeld.18 Part
of the discrepancy arises because their observations are lim-ited to populations in levels v = 0 and 1 of OH and OD. As seen from Fig.4(b), the populations of OH (v≥ 2) contribute
FIG. 6. Semilogarithmic plots of the rotational populations of OH and OD as a function of the rotational energy in respective vibrational levels formed from the O(1D)+ CH
3OD reaction. Average period is 0−5 μs.
much more significantly than that of OD (v≥ 2). The deter-mined ratio [OH]/[OD]= 0.59 ± 0.14 in O(1D)+ CH
3OD
is, notably, practically the same as the inverse of the value [OH]/[OD]= 1.56 ± 0.36 observed for O(1D)+ CD
3OH.
Using this distribution of vibrational populations, we cal-culated the average vibrational energies of OH and OD to be 65 ± 15 and 27 ± 5 kJ mol−1, respectively, listed in TableI. The vibrational energy of OH is larger than that of OD; in contrast, the rotational energy of OD is greater than that of OH.
C. Potential-energy surface and branching of the O(1D)+ CH
3OH reaction
A simplified figure showing only the important reaction paths that produce OH and H is shown in Fig.7. A more com-plete scheme of PES including other product channels is avail-able in Fig. S-2 of the supplementary material.56 The O(1D)
+ CH3OH reaction is initiated according to two mechanisms
– insertion and abstraction. The insertion process can proceed via two paths: into a C−H bond to form trans-HOCH2OH
with exothermicity of 634 kJ mol−1and into the O−H bond via the CH3O(O)H association complex lying 190 kJ mol−1
below the reactants, to give rise to CH3OOH with the total
exothermicity of 373 kJ mol−1 via a small insertion barrier of 20 kJ mol−1 (TS1/2s in Fig.7). One of the MEP curves for the C−H insertion reaction is shown in Fig. S-3 of the supplementary material.56Trans-HOCH2OH can isomerize to
cis-HOCH2OH with a barrier∼13 kJ mol−1. Both energized
trans-HOCH2OH and CH3OOH can further decompose to
form H, OH, and H2O via various channels. The direct paths
for H abstraction can proceed via two channels: attacking the CH3 moiety to form CH2OH+ OH (HC-abstraction) or
at-tacking the OH moiety to form CH3O+ OH (HO-abstraction);
both abstraction reactions proceed via shallow pre-reaction complexes (LMa and LMb) and transition-states (TSa and TSb) that have energy within 1 kJ mol−1of the reactants (see Fig.7). The structures of CH3O(O)H, CH3OOH, HOCH2OH,
and related transition states, predicted with the B3LYP//aug-cc-pVTZ method, are shown in Fig.8.
The production of OH and OD in the reaction O(1D) + CH3OD was investigated for the following dominant
channels:
O(1D)+ CH3OD→ HOCH2OD∗(C−H insertion),
(R1) HOCH2OD∗→ CH2OD+ OH
(fission of new C−O bond), (R2) → CH2OH+ OD
(fission of old C−O bond), (R3) O(1D)+ CH3OD→ CH3OOD∗ (O−D insertion), (R4)
CH3OOD∗→ CH3O+ OD
( fission of O−O bond), (R5) O(1D)+ CH3OD→ CH2OD+ OH (HC-abstraction),
(R6) O(1D)+ CH3OD→ CH3O+ OD (DO-abstraction).
(R7) A more complete mechanism including channels for pro-duction of HOD, DOD, and HOH from decomposition of HOCH2OD* is available in the supplementary material.56
For the reaction of O(1D) with CH
3OH at room
FIG. 7. Simplified potential-energy scheme for production of OH and H in the reaction O(1D)+ CH
3OH computed with the
CCSD(T)//B3LYP/aug-cc-pVTZ method. TS indicate transition states. Energy is in kJ mol−1.
× 10−10cm3molecule−1s−1, is in agreement with the
exper-imental value of (5.1 ± 0.1) × 10−10 cm3molecule−1s−1.19
About 88% of the reaction occurs by C−H insertion, 11% by O−H insertion, and the rest by direct abstraction from a C−H bond; the negligible contribution from abstraction from the O−H bond is expected according to the predicted potential energy profiles presented in Fig. 7. For the direct abstraction processes, the effect of the multiple reflections57
above the pre-reaction complexes LMa and LMb have been included; the effect of the reflections might reduce predicted rate coefficients by as much as a factor of 2 at T < 200 K (depending on the temperature) and have a negligible effect above 500 K. The abstraction channels are unimportant because they have tight transition states, TSa and TSb, with the energies of −0.8 and 0.8 kJ mol−1, respectively, relative to reactants, whereas the two insertion channels dominate because they are barrierless with less restriction on the con-figuration of the collisions. At temperatures below 1000 K, the total rate coefficients and the OH product branching ratio were found to be weakly dependent on temperature and independent of pressure below 10 atm because of the large
FIG. 8. Geometry of CH3O(O)H, trans-HOCH2OH, cis-HOCH2OH,
CH3OOH and transition states TS1/2S, TS3/4S, TSa and TSb predicted with
the B3LYP/aug-cc-pVTZ method. Bond lengths are in Å and bond angles are in deg.
excess energy carried by the insertion products and the small molecular size involved.
For the reaction of O(1D)+ CD
3OH, the following paths
for production of OH and OD are possible:
O(1D)+ CD3OH→ DOCD2OH∗(C−D insertion),
(R8) DOCD2OH∗→ CD2OH+ OD
(fission of new C−O bond), (R9) → CD2OD+ OH
(fission of old C−O bond), (R10) O(1D)+ CD3OH→ CD3OOH∗(O−H insertion),
(R11) CD3OOH∗→ CD3O+ OH
(fission of O−O bond), (R12) O(1D)+ CD3OH→ CD2OH+ OD (DC-abstraction),
(R13) O(1D)+ CD3OH→ CD3O+ OH (HO-abstraction).
FIG. 9. Predicted microcanonical rate coefficients for reactions O(1D) + CH3OD (a) and O(1D)+ CD3OH (b) as a function of excess energy
rela-tive to reactants.
The effect of isotopic substitution on the formation of various products can be readily evaluated with separate cal-culations on the system’s vibrational frequencies and mo-ments of inertia based on the predicted PES. The predicted individual rate coefficients for OH and OD production at room temperature from the O(1D) reactions with CH
3OD and
CD3OH as a function of the kinetic energy are presented in
Figs. 9(a) and9(b), respectively. The initial average kinetic energy of O(1D) that is produced upon photolysis of O
3 at
248 nm was determined to be 36 kJ mol−1 in the laboratory coordinates.58 In the center-of-mass coordinates of the colli-sion of O(1D) with CH3OD or CD3OH, the kinetic energy is
25.5 or 25.9 kJ mol−1, respectively.
The predicted results reveal one major OH (k2) and two
OD (k3and k5) production channels in the O(1D)+ CH3OD
reaction and one OD (k9) and two OH (k10 and k12)
produc-tion channels in the O(1D) + CD
3OH reaction. For O(1D)
+ CH3OD, the rate coefficient for OH/OD production is
dom-inated by k2 and k3 (insertion of O into the C−H bond
fol-lowed by rupture of the new and old C−O bonds); k3 is
slightly greater than k2 at low collision energies but they
be-come comparable (∼8.0 × 10−11cm3molecule−1s−1) at
col-lision energies greater than 26 kJ. The rate coefficients k3and
k2 remain nearly constant for higher energies because of the
large exothermicity of these reaction channels. The bimolec-ular rate coefficient k5 (insertion of O into the O−D bond,
followed by rupture of the O−O bond) remains nearly con-stant (5.2 × 10−11 cm3molecule−1s−1) throughout the
en-ergy range 0−60 kJ mol−1 and is∼0.65 times k
2 and k3 for
energies at 26 kJ mol−1; reaction (R5) is the only dominant channel from the O−H insertion. For O(1D)+ CD
3OH, the
rate coefficient for production of OH/OD is also dominated by k9 and k10 (insertion of O into the C−D bond before
fis-sion of the OH and OD groups); k9is about 1.3 times k10 at
low collision energy but become comparable at 60 kJ mol−1. k12 (insertion of O into the O−H bond, followed by rupture
of the OH group), similar to k5, remains nearly constant (4.8
× 10−11cm3molecule−1s−1) in the 0−60 kJ energy range; it
is about 63% of k10and 50% that of k9at 26 kJ mol−1.
The ratios of kOH/kOD from the reactions O(1D)
+ CH3OD/CD3OH, shown in Fig. 10, were calculated
according to these rate coefficients. For the reaction O(1D)
+ CH3OD, kOH/kOD = k2/(k3 + k5) ∼= 0.51, whereas for
the reaction O(1D) + CD
3OH, kOH/kOD = (k10 + k12)/k9
∼
= 1.23 at E = 0 kJ mol−1. These ratios increase gradually
and become 0.61 and 1.31, respectively, at 26 kJ mol−1, in satisfactory agreement with the experimental results of 0.59 ± 0.14 and 1.56 ± 0.36.
Similarly, we calculated also the ratios of kH/kDfor the
reaction O(1D)+ CH3OD according to the reactions
HOCH2OD∗→ OCH2OD+ H (fission of new O−H bond),
(R15) → OCH2OH+ D (fission of old O−D bond), (R16)
→ HOCHOD + H (fission of C−H bond), (R17)
FIG. 10. Predicted kOH/kOD for reactions O(1D)+ CH3OD and O(1D)
FIG. 11. Predicted ratios kH/kD for reactions O(1D) + CH3OD and
O(1D)+ CD
3OH as a function of excess energy relative to reactants.
and for the reaction O(1D)+ CD
3OH according to the
reac-tions
DOCD2OH∗→ OCD2OH+ D (fission of new O−D bond),
(R18) → OCD2OD+ H (fission of old O−H bond), (R19)
→ DOCDOH + D (fission of C−D bond). (R20) The O−H insertion product, CH3OOH*, which accounts
for only 11% of the insertion/decomposition processes as aforementioned, decomposes mainly by breaking its weak O−O bond to form OH radical. As shown in Fig.11, the ratio of kH/kD = (k15 + 2k17)/k16 predicted with RRKM
calcula-tions for the reaction O(1D)+ CH
3OD decreases from 4.1
to 3.3 as the excess energy increases from 0 to 60 kJ mol−1, whereas the ratio of kH/kD= k19/(k18+ 2k20) for the reaction
O(1D)+ CD
3OH increases from 0.23 to 0.27. These values
are insensitive to the temperature or the kinetic energy carried by the O atom, again attributable to the large amount of en-ergy carried by the insertion products. At the collision enen-ergy of 26 kJ mol−1, the ratios are 3.9 for O(1D)+ CH3OD and
0.23 for O(1D)+ CD3OH.
D. Mechanism for formation of OH and OD
According to our theoretical calculations, the abstraction channels are unimportant. For the insertion channels, two in-termediates are possible: HOCH2OH from insertion into the
C−H bond and CH3OOH from insertion into the O−H bond.
The formation of HOCH2OH is supported by the observation
of this species in an UV-irradiated Ar matrix containing O3
and CH3OH.59The CH3OOH intermediate has not been
iden-tified directly.
The observation of bimodal rotational distributions for OD and Boltzmann distribution for OH produced from the O(1D) + CH
3OD reaction indicates that the reaction
might proceed via three major channels. This experimental
observation is consistent with the predicted mechanism that has a single channel for formation of OH, reaction (R2), and two channels for formation of OD, reactions (R3) and (R5). Similarly, the observation of a Boltzmann rotational distribution for OD and a bimodal rotational distribution for OH in the reaction O(1D)+ CD
3OH is consistent with the
predicted mechanism that has a single channel for formation of OD, reaction(R9), and two channels for formation of OH, reactions(R10)and(R12).
For the reaction O(1D)+ CH
3OD, the observed OH has
greater vibrational energy than OD, whereas the additional rotational component of OD has greater rotational excitation than OH and the low-J component of OD. Similarly, for the reaction O(1D)+ CD
3OH, the observed OD has greater
vi-brational energy than OH, whereas the additional rotational component of OH has greater rotational excitation than OD and the low-J component of OH. These results indicate that the fission of the O−O bond of CH3OOD and CD3OOH,
re-actions (R5) and(R12), produces the hydroxyl radical with smaller vibrational energy but greater rotational energy. The bond angle HOO in CH3OOH is 100.6◦, whereas the bond
angle COH in trans-HOCH2OH is 108.5◦ (Fig.8).
Consid-ering the torque angle during bond fission, one would expect that the OH produced from CH3OOH receives greater
rota-tional excitation than that from HOCH2OH, consistent with
experimental observations.
Regarding the vibrational excitation of the hydroxyl rad-ical product, as described above, we observed that the hy-droxyl radical produced from the methyl moiety has vibra-tional excitation greater than that from the hydroxyl moiety of methanol. In the case of O(1D)+ CD3OH, the vibrational
en-ergy of OD is about 20% of the reaction enen-ergy, which is more than the statistical partitioning if one considers the complexity of the co-product CD2OH. This fact indicates that the reaction
does not proceed through a long-lived intermediate. There are two possibilities for the observed extensive vibrational ex-citation for the hydroxyl radical produced from the methyl moiety. One mechanism is that O(1D) abstracts directly one
H atom from the methyl group, as was observed in the reac-tion O(1D)+ CH
4.1In the reaction O(1D)+ CH4in crossed
molecular beams, a large proportion of the OH product scat-ters in the forward direction because of direct abstraction, and OH had much more vibrational than rotational excitation. The second mechanism is that when the reacting O atom, indi-cated as O*, inserts into the H−C bond of the CH3 moiety,
the newly formed HO*−C bond breaks more rapidly than the original C−OH bond; that is, k2> k3and k9> k10. This
condi-tion implies that the lifetime of a porcondi-tion of the HO*CH2OH
intermediate is too small for complete IVR, consistent with the conclusion derived from comparison of observed and cal-culated [OH]/[OD], to be discussed later. The reaction energy thus becomes localized near the newly formed bond upon in-sertion of O(1D) into the H−C bond, and highly vibrationally
excited O*H is produced quickly. Some HO*CH2OH survives
to allow energy redistribution before the fission of the C−OH bond to form less vibrationally excited OH. Both elimination channels from HOCH2OH likely produce OH with little
ro-tational excitation because of a smaller torque angle as indi-cated above.
A similar non-equilibrium reaction of O(1D) was
re-ported for the reaction 16O(1D)+ H
218O→16OH+18OH
in which 16OH is from the newly formed moiety, whereas 18OH is a part of the original reactant.60–66The product16OH
was found to be highly vibrationally excited, whereas 18OH was vibrationally much cooler. This condition indicates that
16O(1D) reacts quickly with H in H
218O to form16OH, with
the remaining product18OH being a spectator. Our observa-tion of vibraobserva-tionally excited OH (OD) from the channel asso-ciated with the rupture of the new C−O bond in HOCH2OD
(DOCD2OH) is consistent with the results of O(1D)+ H2O.
In the reaction of O(1D)+ CD
3OH, the observed ratio is
[OH]/[OD] ∼= 1.56 ± 0.36, whereas in the reaction of O(1D)
+ CH3OD, the product ratio is [OH]/[OD] ∼= 0.59 ± 0.14.
This indicates that the hydroxyl radical products derive their H or D atoms preferably from the hydroxyl site of methanol. However, because insertion of the O atom into the C−H bond might be followed by fragmentation of the original hydroxyl moiety, listed as reactions(R3)and(R10), these product ratios do not necessarily indicate the preference of the reaction of O(1D) with the hydroxyl moiety over the methyl moiety of methanol. In fact, the calculations predicted that about 88% of the reaction occurs by C−H insertion and 11% by O−H insertion.
In the reaction of O(1D) + CH
3OD, the ratio of
[OH]/[OD] depends mainly on kOH/kOD = k2/(k3 + k5)
ac-cording to reactions (R1)−(R5) listed in Sec. IV C. In the reaction of O(1D)+ CD
3OH, the ratio of [OH]/[OD] depends
on kOH/kOD = (k10 + k12)/k9. If one ignores the kinetic
iso-topic effects, k9 ∼= k2 for fission of the newly formed C−O
bond, k10∼= k3for fission of the old C−O bond, and k12∼= k5
for fission of the O−O bond. Hence, the ratio of [OH]/[OD] for the reaction O(1D)+ CH
3OD is expected to be
approx-imately the inverse of the [OH]/[OD] ratio for the reaction O(1D)+ CD3OH. Our observed values of [OH]/[OD]= 0.59
for the reaction O(1D)+ CH3OD and [OH]/[OD]= 1.56 for
the reaction O(1D)+ CD3OH are consistent with this scheme;
these observed ratios indicate that (k3 + k5) > k2 and (k10
+ k12) > k9. Without considering the kinetic isotope effect,
one would expect that k2∼= k3and k9∼= k10, if the IVR is much
more rapid than bond fission. If the bond rupture is more rapid than energy randomization, one would expect that k2/k3 and
k9/k10becomes greater than 1.
At the center-of-mass collision energy of 26 kJ mol−1for O(1D) with methanol, the ratio of [OH]/[OD]= (k
10+ k12)/k9
= 1.31, predicted for O(1D)+ CD
3OH such that energy is
randomized before bond rupture, is slightly smaller than our experimental value of 1.56± 0.36; the value of [OH]/[OD] = 1/(0.4 ± 0.2) reported by Goldstein and Wiesenfeld has a large error.18 These results are consistent with the condi-tion in which the breaking of the newly formed C−OD bond is more rapid than the breaking of the old C−OH bond in DOCD2OH such that observed k9/k10 is greater than
predic-tion. Similarly, the ratio of [OH]/[OD]= k2/(k3+ k5) is
pre-dicted to be 0.61 for O(1D)+ CH
3OD. This predicted value
is in close agreement with our experimental value of 0.59 ± 0.14; both are greater than the value of 0.43 reported by Goldstein and Wiesenfeld.18Considering possible errors, this
result is not inconsistent with the condition in which IVR is
slower than the bond rupture such that the breaking of the newly formed C−OH bond is more rapid than the breaking of the old C−OD bond in HOCH2OD, i.e., observed k2/k3 is
greater than prediction; the slower IVR was indicated by the observation of vibrationally excited OH, as discussed above.
The reported experimental value of [H]/[D]= 7.1 ± 0.8 for O(1D)+ CH
3OD at room temperature19 is much larger
than the value of kH/kD = (k15 + 2k17)/k16 = 3.90 predicted
according to the microcanonical rate coefficients at E = 26 kJ mol−1. On the other hand, for O(1D)+ CD
3OH, the
ob-served ratio of [H]/[D]= 0.26 ± 0.03 (Ref.19) is in agree-ment with the value of kH/kD = k19/(k18 + 2k20) = 0.23
predicted according to the microcanonical rate coefficients in-sensitive to the collision energies from 0−60 kJ mol−1. The
much larger discrepancy might indicate that the rate of bond rupture for formation of H or D atoms is much more rapid than IVR. The IVR rates of the skeletal modes involving the O−H bond are much smaller than those of the C−O bonds; the former are associated with formation of H, whereas the latter are with formation of OH. Furthermore, part of the dis-crepancy might be accounted for by secondary decomposition of OCH2OD, HOCH2O, and HOCHOD products.
According to our calculated rate coefficients, the quan-tum yields for production of H (and D) and OH (and OD) are 0.158 and 0.548 for the reaction O(1D)+ CH
3OD and 0.186
and 0.540 for the reaction O(1D)+ CD
3OH. Matsumi et al.
reported a quantum yield of 0.18 for production of H atom in the reaction of O(1D)+ CH
3OH,19consistent with our
calcu-lation.
V. CONCLUSION
By monitoring the IR emission of products OH and OD with a step-scan Fourier-transform infrared spectrometer, we investigated the reactions O(1D)+ CD3OH/CH3OD. For the
reaction of O(1D)+ CD3OH, the nascent average rotational
energies are estimated to be 6 ± 2 and 12 ± 4 kJ mol−1 and the average vibrational energies are 43 ± 8 and 28 ± 6 kJ mol−1 for OD and OH, respectively. Product OD is
vibrationally more excited than OH, whereas OH shows a bi-modal rotational distribution with the additional component having greater rotational excitation than OD. The product ra-tio [OH]/[OD] is estimated to be 61/39= 1.56.
For the reaction O(1D)+ CH
3OD, the nascent rotational
energies are estimated to be 6 ± 2 and 10 ± 3 kJ mol−1 and the average vibrational energies are 65 ± 15 and 27 ± 5 kJ mol−1 for OH and OD, respectively. Product OH is
vibrationally more excited than OD, whereas OD shows a bi-modal rotational distribution with the additional component having greater rotational excitation than OH. The product ra-tio [OH]/[OD] was estimated to be 37/63= 0.59, nearly the inverse of that of the reaction O(1D)+ CD3OH.
The experimental observations are explicable according to a mechanism of O(1D) + CH
3OD involving two
inser-tion intermediates, HOCH2OD and CH3OOD, and three
ma-jor decomposition channels, reactions (R2),(R3), and(R5). Insertion of O(1D) into the C−H bond to form HOCH
2OD
followed by fission of the newly formed HO−C bond, reac-tion (R2), produces vibrationally more excited OH because
of incomplete IVR, whereas fission of the old C−OD bond in HOCH2OD, reaction(R3), produces OD with less
vibra-tional excitation. Insertion of O(1D) into the O−D bond to form CH3COOD followed by fission of the O−O bond,
re-action(R5), produced OD with greater rotational excitation, likely due to a large torque angle during dissociation. A sim-ilar mechanism applies for O(1D)+ CD
3OH: two insertion
intermediates, DOCD2OH and CD3OOH, and three
decom-position channels, reactions (R9),(R10), and(R12), are in-volved.
For O(1D)+ CH
3OD, the observed ratio of [OH]/[OD]
= 0.59 ± 0.14 is close to the value of 0.61 predicted accord-ing to microcanonical rate coefficients. For O(1D)+ CD
3OH,
the observed ratio of [OH]/[OD]= 1.56 ± 0.36 is also close to the predicted value of 1.31. The small deviations are likely due to incomplete IVR.
For O(1D) + CD3OH, the observed ratio of [H]/[D]
= 0.26 ± 0.03 is close to the predicted value of 0.23. For O(1D) + CH3OD, the reported observed ratio of [H]/[D]
= 7.1 ± 0.8 is much larger than the value of 3.90 predicted ac-cording to microcanonical rate coefficients. The much larger discrepancy might indicate that the rate of bond rupture for formation of H or D atoms is much more rapid than IVR. Part of the discrepancy might be accounted for by secondary decomposition reactions of OCH2OD, HOCH2O, and
HO-CHOD which were assumed to be negligible in our prediction. It should be emphasized that even though the observed [OH]/[OD] indicates a preference of formation of OH from the hydroxyl moiety over the methyl moiety of CH3OH, it
is not in conflict with the theoretical prediction that O(1D) prefers to attack the methyl moiety of CH3OH. This is partly
because, upon insertion of O(1D) to a C−H bond to form HOCH2OH*, subsequent dissociation occurs for both the
newly formed OH and the original OH group, and partly because this inserted intermediate HOCH2OH* can also
de-compose to form H and H2O, whereas the decomposition of
CH3OOH, produced from insertion of O(1D) into the O−H
bond, produces mainly CH3O+ OH.
ACKNOWLEDGMENTS
National Science Council of Taiwan (Grant No. NSC100-2745-M009-001-ASP) and the Ministry of Education, Tai-wan (“Aim for the Top University Plan” of National Chiao Tung University) supported this work. The National Center for High-Performance Computing provided computer time. M.C.L. acknowledges the support from National Science Council of Taiwan for the distinguished visiting professor-ship and Taiwan Semiconductor Manufacturing Co. for the TSMC distinguished professorship (2005–2011) at the Na-tional Chiao Tung University.
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