Infrared absorption of gaseous c -ClCOOH and t -ClCOOH recorded with a step-scan
Fourier-transform spectrometer
Li-Kang Chu and Yuan-Pern Lee
Citation: The Journal of Chemical Physics 130, 174304 (2009); doi: 10.1063/1.3122722
View online: http://dx.doi.org/10.1063/1.3122722
View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/130/17?ver=pdfcov Published by the AIP Publishing
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Infrared absorption of gaseous c-ClCOOH and t-ClCOOH recorded
with a step-scan Fourier-transform spectrometer
Li-Kang Chu1and Yuan-Pern Lee2,a兲 1
Department of Applied Chemistry, National Chiao Tung University, Hsinchu 30010, Taiwan
2
Department of Applied Chemistry and Institute of Molecular Science, National Chiao Tung University, Hsinchu 30010, Taiwan and Institute of Atomic and Molecular Sciences, Academia Sinica,
Taipei 10617, Taiwan
共Received 19 February 2009; accepted 31 March 2009; published online 4 May 2009兲
Two conformers of ClCOOH were produced upon irradiation at 355 nm of a gaseous flowing mixture of Cl2, HCOOH, and N2. A step-scan Fourier-transform infrared spectrometer coupled with
a multipass absorption cell was utilized to monitor the transient spectra of ClCOOH. Absorption bands with origins at 1808.0 and 1328.5 cm−1 are attributed to the C = O stretching and COH
bending modes of t-ClCOOH, respectively; those at 1883.0 and 1284.9 cm−1 are assigned as the
C = O stretching and COH bending modes of c-ClCOOH, respectively. These observed vibrational wavenumbers agree with corresponding values for t-ClCOOH and c-ClCOOH predicted with B3LYP/aug-cc-pVTZ density-functional theory and the observed rotational contours agree satisfactorily with simulated bands based on predicted rotational parameters. The observed relative intensities indicate that t-ClCOOH is more stable than c-ClCOOH by ⬃3 kJ mol−1. A simple
kinetic model is employed to account for the production and decay of ClCOOH. © 2009 American Institute of Physics.关DOI:10.1063/1.3122722兴
I. INTRODUCTION
The hydrocarboxyl radical共HOCO兲 is an important in-termediate in the reaction of OH with CO, which is the major reaction responsible for the oxidation of CO to CO2 in the
atmosphere and in combustion systems.1–4 HOCO is also produced in other reactions. The reaction of OH with formic acid 共HCOOH兲, the most abundant carboxylic acid in the troposphere,5is not only a source of HOCO but also a major sink for HCOOH.6 Another important source of HOCO in the atmosphere is the reaction of Cl with HCOOH. In the laboratory, this reaction also serves as a source to produce HOCO.7
West and Rollefson reported that CO2 and HCl were
produced upon irradiation of a mixture of Cl2and HCOOH;
they proposed that HOCO and ClCOOH are reaction intermediates.8 Li et al. irradiated mixtures of Cl2, O2,
HCOOH, and He with a Xe flash lamp to investigate the bimolecular rate coefficients of reactions 共2兲 and 共4兲 based on the following mechanism:
Cl2+ h→ 2Cl 共1兲
Cl + HCOOH→ HCl + HOCO 共2兲
HOCO + O2→ CO2+ HO2 共3兲
Cl + HOCO→ HCl + CO2. 共4兲
Values of k2=共1.83⫾0.12兲⫻10−13 and k4=共4.8⫾1.0兲
⫻10−11 cm3molecule−1s−1were derived on fitting the
tem-poral profile of CO2probed with the P共4兲 line in the3band
of CO2with a simplified equation of exponential rise and a
literature value of k3= 1.9⫻10−12cm3molecule−1s−1.9
By comparison of the decay rates of the reactant and a reference compound determined with infrared 共IR兲 absorption, Wall-ington et al.10 evaluated k2=共2.00⫾0.25兲⫻10−13 cm3
molecule−1s−1under 700 Torr of N2or synthetic air.
The experiment on reaction of Cl with monodeuterated formic acid HCOOD indicated that reaction共2兲proceeds pre-dominantly via abstraction of the H-atom on the carbon to form HOCO rather than abstraction of the hydroxyl hydro-gen to form HCO2.7 This finding is consistent with the
re-sults of Tyndall et al.11who reported a yield of 96⫾5% for CO2from the reaction of Cl with HCOOH in 700 Torr of air based on their measurements with Fourier-transform infrared 共FTIR兲 spectra; CO2 was presumably produced from
reac-tions共3兲 and共4兲.
Li et al. proposed that reaction 共4兲 might proceed via direct abstraction of a H-atom, formation of a ClCOOH com-plex, or a short-lived intermediate in which the C–Cl bond is rapidly formed and the H atom is rapidly abstracted.9In con-trast, West and Rollefson postulated the formation of tran-sient chloroformic acid 共ClCOOH兲 that decomposes readily to HCl and CO2 via the cis configuration共c-ClCOOH兲. For
convenience, the trans and cis notations in this paper follow the same nomenclature as for HCOOH,12 therefore c-ClCOOH has its H atom cis to the Cl atom with respect to the C–O bond.
Herr and Pimentel13 photolyzed a mixture of Cl2 and
HCOOH with a flash lamp and monitored reaction interme-diates with a rapid-scan IR spectrometer; a transient band at a兲Author to whom correspondence should be addressed. Electronic mail:
[email protected]. FAX: 886-3-5713491.
共2009兲
0021-9606/2009/130共17兲/174304/8/$25.00 130, 174304-1 © 2009 American Institute of Physics
768 cm−1was observed and assigned as the C–Cl stretching
mode of ClCOOH. Jensen and Pimentel14further studied the unimolecular decomposition
ClCOOH→ HCl + CO2 共5兲
over the temperature range 288–343 K by monitoring the decay of this 768 cm−1band and derived a rate coefficient of
k5= 5⫻1013exp共−7050/T兲 s−1. They proposed that the
rate-determining step for reaction 共5兲 is the conversion from
t-ClCOOH to c-ClCOOH with a barrier about
58⫾7 kJ mol−1. No distinction of absorption between
con-formers of ClCOOH was discussed, nor was any other IR absorption band of ClCOOH reported.
Several quantum-chemical calculations on c-ClCOOH and t-ClCOOH have been performed.15–19The conformation notation employed in this paper is consistent with that used in Refs.17and18, but is the reverse of that in Refs.14–16
and19. Conformer t-ClCOOH is predicted to be more stable than c-ClCOOH by 4 – 11 kJ mol−1 and the barrier to
con-vert from trans to cis conformers is 41– 50 kJ mol−1,
de-pending on the methods of calculation.15,17,19 Stephenson et al.17 argued that because of the disagreement between the vibrational wavenumber 共768 cm−1兲 of the only band
ob-served by Pimentel and coworkers13,14 and the value 共725 cm−1兲 predicted quantum chemically, a search for the
carbonyl absorption in the 1800– 1880 cm−1 region would
provide crucial evidence whether ClCOOH was indeed ob-served previously.
We have demonstrated that by coupling a multipass ab-sorption cell with a step-scan FTIR spectrometer, time-resolved IR absorption spectra of transient intermediates in gaseous reactions can be recorded.20–25 Here such an appli-cation is further demonstrated by our observation of transient IR absorption spectra of c-ClCOOH and t-ClCOOH upon photolysis of a gaseous mixture of Cl2/HCOOH/N2 at 363
K.
II. EXPERIMENTS
The sample compartment of a step-scan Fourier-transform spectrometer 共Thermo Nicolet, Nexus 870兲 con-tains a flow reactor inside which a set of White cell mirrors with a base path length of 20 cm and an effective path length of 6.4 m was installed.21,22The flow reactor has a volume of ⬃1.6 L and accommodates two rectangular quartz windows 共3⫻12 cm2兲 to pass the photolysis beam that propagates
perpendicularly to multiply passed IR beams. The beam from a frequency-tripled Nd:YAG共yttrium aluminum garnet兲 laser 共LOTIS TII, LS-2137/20, 11 Hz, 74 mJ pulse−1, beam
di-mension 0.5 cm2兲 emitting at 355 nm passed through these
quartz windows and was reflected eight times with a pair of external laser mirrors to photodissociate a flowing mixture of Cl2/HCOOH/N2. We obtained temporally resolved
differ-ence absorption spectra from interferograms recorded simul-taneously with ac- and dc-coupled signals of a mercury cad-mium telluride detector 共20 MHz response bandwidth兲.21,26 The ac-coupled signal was further amplified 共Stanford Re-search Systems, Model SR560, bandwidth 100 Hz–1 MHz兲 20 times before being transferred to an external 14-bit
digi-tizer 共Gage Applied Technology, CompuScope 14100, 108 sample s−1兲, whereas the dc-coupled signal was sent
di-rectly to the internal 16-bit digitizer共2⫻105 sample s−1兲 of
the spectrometer. Typically, 500 data points were acquired at 1 s integrated intervals共100 dwells at 10 ns gate width兲 to cover a period of 500 s after photolysis; these signals were typically averaged over 40 laser shots at each scan step. With appropriate optical filters to define a narrow spectral region, we performed undersampling to decrease the size of the in-terferogram, hence the duration of data acquisition. For spec-tra in the range 1055– 2100 cm−1, 1192 scan steps at a
res-olution of 2.0 cm−1 were completed within ⬃1.5 h. For spectra in the range 1000– 3150 cm−1 at a resolution of 5.0 cm−1, 1472 scan steps were completed within⬃2 h.
Experimental conditions were as follows: flow rates FHCOOH⬵0.25–1.05, FCl2⬵2.1, and FN2
⬵26.1 STP cm3s−1 共STP denotes standard temperature
273.15 K and pressure 1 atm兲; total pressure⬵58 Torr. To minimize the formation of dimeric formic acid, the flow re-actor was heated to T = 363 K with heated water circulated from a thermostat bath through the jacket of the reactor. Based on the equilibrium constant between monomeric and dimeric HCOOH, the fraction of 共HCOOH兲2 is less than 10% under our experimental conditions.27
The efficiency of photolysis of Cl2 is estimated to be ⬃4% based on the absorption cross section of ⬃1.6 ⫻10−19 cm2molecule−1 at 355 nm 共Ref. 28兲 and the laser
fluence of ⬃2.6⫻1017 photons cm−2. HCOOH 共99%,
Riedel-de Haën兲, Cl2 共99.99%, AGA Specialty Gases兲, and
N2共99.995%, AGA Specialty Gases兲 were used without
fur-ther purification.
III. QUANTUM-CHEMICAL CALCULATIONS
The equilibrium geometry, vibrational wavenumbers, and IR intensities of c-HCOOH, t-HCOOH, c-HOCO, t-HOCO, c-ClCOOH, and t-ClCOOH were calculated with the B3LYP density-functional theory using the GAUSSIAN 03
program.29 The B3LYP method uses Becke’s30 three-parameter hybrid exchange functional with a correlation functional of Lee et al.31 Dunning’s correlation-consistent polarized-valence triple-zeta basis set, augmented with s, p, d, and f functions 共aug-cc-pVTZ兲 was applied in these calculations.32,33 Analytic first derivatives were utilized in geometry optimization, and vibrational wavenumbers were calculated analytically at each stationary point. Rotational
parameters of c-HOCO, t-HOCO, c-ClCOOH, and
t-ClCOOH in their vibrational ground and excited 共vi= 1兲 states were also calculated with B3LYP/aug-cc-pVTZ for spectral simulation.
IV. RESULTS
A. Absorption spectra recorded upon photolysis of a mixture of HCOOH/ Cl2/ N2
The absorption spectrum of a static gaseous mixture of HCOOH/Cl2/N2 共1/0.6/10.7 at 54.3 Torr兲 at 363 K was
re-corded with a conventional FTIR technique. The character-istic bands of t-HCOOH at 2943, 2196, 1770, 1387, 1229, and 1105 cm−1are consistent with literature values.34Upon
174304-2 L.-K. Chu and Y.-P. Lee J. Chem. Phys. 130, 174304共2009兲
irradiation of this mixture with laser emission at 355 nm 共70 mJ pulse−1, 10 Hz for 1 min兲, an intense broad band of
CO2 near 2349 cm−1 and a rotational progression of HCl
with an origin at 2885 cm−1 were observed, consistent with previous results.9,11 No absorption bands can be assigned to HOCO or ClCOOH in these static cell experiments.
The temporally and spectrally resolved difference spec-tra recorded upon irradiation of a flowing mixture of HCOOH/Cl2/N2 共1/9/103 at 58 Torr and 363 K兲 with the step-scan FTIR spectrometer are shown in Fig. 1共a兲 as a three-dimensional plot. The consumption of HCOOH shown as downward features near 1750 cm−1is partially truncated
due to saturation of the parent absorption. Several features near 1850 and 1300 cm−1 were observed to increase with
time. Spectra integrated at 50-s intervals are shown in Fig.
1共b兲. In addition to the downward peaks near 2943, 2196, 1770, and 1229 cm−1 due to destruction of HCOOH, the
CO2 band near the 2350 cm−1 region and four bands near
1285, 1883, 1329, and 1808 cm−1, labeled as A1, A2, B1,
and B2 respectively, gradually increased in intensity and at-tained their maximal absorbance near 100– 150 s. These
four new bands diminished in intensity afterward and be-came nearly undetectable⬃500 s after laser irradiation.
B. Quantum-chemical calculations
Geometries of c-ClCOOH and t-ClCOOH predicted with B3LYP/aug-cc-pVTZ are shown in Fig. 2. For comparison, results from MP2/6-311Gⴱⴱ共Ref.17兲 and
CCSD共T兲/aug-cc-pVTZ 共Ref. 19兲 are listed in parentheses and brackets,
re-spectively; the deviations among these results are within 2% for both conformers.
The conformer t-ClCOOH is more stable than c-ClCOOH; the zero-point-energy corrected energy of t-ClCOOH is smaller than c-ClCOOH by 2 kJ mol−1 at the B3LYP/aug-cc-pVTZ level of theory, slightly smaller than values ⬃8 kJ mol−1 reported by Francisco and Ghoul15
us-ing the UMP2/6-311Gⴱⴱmethod and by Stephenson et al.17 using MP2/6-311Gⴱⴱ, but similar to a value of 4 kJ mol−1
reported by Yu et al.19using the high-level CCSD 共T兲/aug-cc-pVTZ method.
The isomerization barrier from t-ClCOOH to c-ClCOOH was calculated to be 41– 50 kJ mol−1 using various
methods,15,17,19 as compared to the experimental value of 58⫾7 kJ mol−1 determined by Jensen and Pimentel14
from the temperature dependence of the decay of the 768 cm−1
band assigned to ClCOOH; they assumed that the rate-determining step is the trans-cis isomerization of ClCOOH before its rapid decomposition to HCl and CO2.
The harmonic vibrational wavenumbers and IR intensi-ties of t-ClCOOH and c-ClCOOH predicted with the B3LYP/ aug-cc-pVTZ method are listed in Table I. Values predicted previously with MP2/6-311Gⴱⴱ 共Ref. 17兲 and HF/3-21G
共Ref. 15兲, including the fundamental 共v=0→v=1兲
vibra-tional wavenumbers reported for t-ClCOOH,17are listed also for comparison. For t-ClCOOH, the most intense absorption bands are predicted to be at 1844, 1127, and 706 cm−1,
cor-FIG. 1. 共a兲 Three-dimensional plot of time-resolved difference absorption spectra upon laser photolysis共355 nm, 11 Hz, 74 mJ cm−2兲 of a flowing
mixture of HCOOH/Cl2/N2 共1/9/103 at 58 Torr兲 at 363K; spectral reso-lution 5.0 cm−1. 共b兲 Spectra integrated over 50 s intervals: downward
features are due to consumption of HCOOH共the saturation near 1750 cm−1
is truncated兲, whereas upward features A1 and A2 correspond to formation of c-ClCOOH and features B1 and B2 correspond to formation of
t-ClCOOH. The band near 2350 cm−1is due to CO
2.
FIG. 2. Geometries predicted with the B3LYP/aug-cc-pVTZ method for
c-ClCOOH and t-ClCOOH. Bond lengths are in Å and bond angles in
de-grees. The values in parentheses are derived with MP2/ /6-311Gⴱⴱ共Ref.17兲 and the values in brackets are derived with CCSD共T兲/cc-pVTZ 共Ref.19兲.
responding approximately to C = O stretching, C–O stretch-ing, and the C–Cl stretching mixed with OCO bending modes, respectively. Intense absorption bands of c-ClCOOH are predicted to occur at 1919, 1286, and 1131 cm−1,
attrib-utable to C = O stretching, COH bending, and C–O stretching modes, respectively. The values predicted for t-ClCOOH are within 5.2% of the harmonic wavenumbers and 3.5% of the fundamental vibrational wavenumbers predicted previously with MP2/6-311Gⴱⴱ.17 Similarly, the values predicted for c-ClCOOH are within 7.0% of the harmonic wavenumbers predicted previously with MP2/6-311Gⴱⴱ.17
The predicted molecular axes, vibrational displacements 共thin arrows兲, and their corresponding dipole derivatives 共thick dashed arrows兲 for the C=O stretching and COH bending modes of c-ClCOOH and t-ClCOOH are shown in Fig. 3. The projection vectors of the dipole derivative onto the molecular axes represent the weighting of the transition types. The C = O stretching band of t-ClCOOH is a hybrid type with a ratio of a-type/b-type=1/2, whereas the COH bending band of t-ClCOOH is a hybrid type with a ratio of a-type/b-type=3/1. For c-ClCOOH, the C=O stretching band has a ratio of a-type/b-type=43/57, whereas the COH bending band has a ratio of a-type/b-type=1/1.
Rotational parameters for the equilibrium geometry, the vibrational ground state, and excited states 共vi= 1兲 of each vibrational mode of c-ClCOOH and t-ClCOOH were calcu-lated with the B3LYP/aug-cc-pVTZ method. These param-eters and the ratios of A
⬘
/A⬙
, B⬘
/B⬙
, and C⬘
/C⬙
, in which the prime and double prime indicate the excited and ground states, respectively, for the C = O stretching and COH bend-ing modes of these two conformers of ClCOOH are listed in TableII.The HOCO radical has been well characterized with quantum-chemical calculations.35–42 We performed calcula-tions on HOCO at the same level as for ClCOOH mainly for comparison purposes. Vibrational wavenumbers 共in cm−1兲 and IR intensities共in km mol−1兲 for c-HOCO are 3562 共16兲, TABLE I. Comparison of harmonic vibrational wavenumbers共cm−1兲 and IR intensities 共listed in parentheses in km mol−1兲 of c-ClCOOH and t-ClCOOH
derived from experiments and calculations.
i Mode B3LYP /aug-cc-pVTZ MP2 /6-311Gⴱⴱ MP2 fundamental HF /3-21G Gas
t-ClCOOH
1 O–H stretch 3724共92兲 3819 共100兲 3633 3829
2 C = O stretch 1844共464兲 1879 共399兲 1840 1998 1808.0共B2兲
3 COH bend 1332共63兲 1368 共65兲 1346 1438 1328.5共B1兲
4 C–O stretch 1127共366兲 1167 共406兲 1139 1174
5 OCO bend/C–Cl stretch 706共157兲 733 共162兲 725 728 768?a
6 ClCOO out-of-plane 705共48兲 714 共62兲 690 709
7 ClCOH torsion 549共82兲 576 共85兲 552 544
8 OCO bend/C–Cl stretch 480共5兲 505 共4兲 497 479
9 ClCO bend 409共2兲 423 共1兲 417 404
c-ClCOOH
1 O–H stretch 3775共88兲 3877 共108兲 3903
2 C = O stretch 1919共429兲 1944 共369兲 2108 1883.0共A2兲
3 COH bend 1286共407兲 1304 共430兲 1321 1284.9共A1兲
4 C–O stretch 1131共167兲 1157 共171兲 1205
5 OCO bend/C–Cl stretch 691共114兲 706 共131兲 698
6 ClCOO out-of-plane 679共3兲 686 共4兲 695
7 ClCOH torsion 519共108兲 485 共139兲 454
8 OCO bend/C–Cl stretch 452共22兲 473 共18兲 437
9 ClCO bend 402共8兲 415 共10兲 392
References This work 17 17 15 This work
aReferences13and14.
FIG. 3. Displacement vectors共thin solid arrows兲 and vectors of dipole de-rivatives 共thick dashed arrows兲 predicted with the B3LYP/aug-cc-pVTZ method for the COH bending and C = O stretching modes of共a兲 t-ClCOOH and共b兲 c-ClCOOH. Molecular axes a and b are shown in the plane; the axis
c points out of the plane perpendicularly.
174304-4 L.-K. Chu and Y.-P. Lee J. Chem. Phys. 130, 174304共2009兲
1860共339兲, 1293 共1兲, 1067 共165兲, 598 共31兲, and 588 共105兲; corresponding values for t-HOCO are 3786 共125兲, 1897 共246兲, 1239 共238兲, 1079 共79兲, 620 共4兲, and 550 共85兲. For c-HOCO, the most intense absorption bands are at 1860 and 1067 cm−1, corresponding approximately to the C = O stretching and C–O stretching modes, respectively, whereas the most intense bands of t-HOCO are at 1897 and 1239 cm−1, corresponding to the C = O stretching and COH bending modes, respectively. The COH bending mode of c-HOCO at 1293 cm−1 has a very small intensity, as indi-cated above.
V. DISCUSSION
A. Assignments of c-ClCOOH and t-ClCOOH
At 363 K, negligible dimeric formic acid is present; therefore only the reactions of Cl atom with HCOOH need to be considered. Based on theoretical calculations, the reaction might involve two reaction intermediates, HOCO and ClCOOH.19 The 1 共⬃3636 cm−1兲 and 2 bands
共⬃1853 cm−1兲 of gaseous t-HOCO have been characterized
previously.43–45 Most vibrational bands of t-HOCO isolated in CO,46Ar,47Ne matrices,48and c-HOCO isolated in a CO matrix have been reported.46 Only one band, the C–Cl stretching 共5兲 mode near 768 cm−1, of gaseous ClCOOH
共presumably due to trans configuration兲 was reported.13,14
Upon excitation of a flowing mixture of
HCOOH/Cl2/N2, four transient bands near 1285, 1883,
1329, and 1808 cm−1, showed as A1, A2, B1, and B2 in Fig. 1共b兲, were observed. Bands A1 and A2 correlate well in in-tensity under various experimental conditions; bands B1 and B2 also correlate with each other, even though we cannot positively rule out the possibility that other combinations of these features are correlated.
The possibility that these bands are due to t-HOCO is eliminated because the C = O stretching band of t-HOCO re-ported by Sears et al.44 lies at 1852.567 cm−1, at least
32 cm−1 separated from the observed bands. Furthermore,
the3mode of t-HOCO isolated in Ar or Ne was observed to absorb at ⬃1210 cm−1,47,48
74 cm−1 from the observed A1
band.
According to the only report on IR absorption of c-HOCO, an intense band at 1797 cm−1and a weaker one at
1261 cm−1were assigned to absorption of the C = O
stretch-ing and COH bendstretch-ing modes of c-HOCO isolated in solid CO.46 Observed wavenumbers of the B2 共1808 cm−1兲 and
A1 共1285 cm−1兲 bands in this work fit satisfactorily with
these reported values. However, according to quantum-chemical calculations, two most intense absorption bands of c-HOCO are at 1860 and 1067 cm−1, the latter corresponds
to approximately the C–O stretching mode. The intensity of the COH bending mode 共predicted to be 1293 cm−1 at
B3LYP/aug-cc-pVTZ兲 has an IR intensity 1/340 that of the C = O stretching band predicted at 1860 cm−1. Observed
in-tensity ratio for these two bands disagrees with theoretical predictions.
In Fig. 4, the observed transient spectrum in the 1000– 2000 cm−1 region is compared to unscaled harmonic vibrational wavenumbers of c-HOCO, t-HOCO, c-ClCOOH, and t-ClCOOH predicted with B3LYP/aug-cc-pVTZ; bands
TABLE II. Comparison of rotational parameters of c-ClCOOH and
t-ClCOOH in ground and vibrationally excited states predicted with B3LYP/
aug-cc-pVTZ. Equilibrium v = 0 v = 1 v = 1/v=0 t-ClCOOH C = O stretch A/cm−1 0.401 508 0.398 943 0.397 499 A⬘/A⬙= 0.9964 B/cm−1 0.171 810 0.171 132 0.170 964 B⬘/B⬙= 0.9990 C/cm−1 0.120 322 0.119 605 0.119 412 C⬘/C⬙= 0.9984 COH bend A/cm−1 0.401 508 0.398 943 0.398 600 A⬘/A⬙= 0.9991 B/cm−1 0.171 810 0.171 132 0.171 151 B⬘/B⬙= 1.0001 C/cm−1 0.120 322 0.119 605 0.119 469 C⬘/C⬙= 0.9989 c-ClCOOH C = O stretch A/cm−1 0.390 067 0.387 953 0.386 525 A⬘/A⬙= 0.9963 B/cm−1 0.170 474 0.169 698 0.169 697 B⬘/B⬙= 1.0000 C/cm−1 0.118 628 0.117 908 0.117 785 C⬘/C⬙= 0.9990 COH bend A/cm−1 0.390 067 0.387 953 0.387 793 A⬘/A⬙= 0.9996 B/cm−1 0.170 474 0.169 698 0.169 617 B⬘/B⬙= 0.9995 C/cm−1 0.118 628 0.117 908 0.117 747 C⬘/C⬙= 0.9986 -1 0 1 0 2 4 0 2 4 0 2 4 1000 1200 1400 1600 1800 20000 2 4 0 1 2 B1A1 B2 Ab s . A b s ./1 0 -2 A2 m m (c) c-HOCO m g m m (d) t-HOCO In te ns it y / a .u. (f) c-ClCOOH (e) t-ClCOOH Wavenumber / cm-1 (a) Expt. (b) HCOOH
FIG. 4.共a兲 Transient difference absorption spectrum recorded upon photoly-sis at 355 nm of a flowing mixture of HCOOH/Cl2/N2共1/9/103 at 58 Torr兲 at 363 K; resolution is 5.0 cm−1and averaging period is 100– 150 s.共b兲
Absorption of HCOOH with the saturated spectral regions indicated with grey color.关共c兲–共f兲兴 Stick spectra of c-HOCO, t-HOCO, t-ClCOOH, and
c-ClCOOH, respectively, based on harmonic vibrational wave numbers and
IR intensities predicted with the B3LYP/aug-cc-pVTZ method. Experimen-tal values of HOCO are also shown as arrows关m for matrix isolation 共Refs. 46–48兲 and g for gas phase 共Ref.44兲兴.
previously observed for c-HOCO and t-HOCO are also indi-cated with dashed lines and marked with m for matrix and g for gaseous experiments. The spectrum of HCOOH is also shown in Fig. 4共b兲 to indicate the regions共1730⫺1810 and 1000– 1140 cm−1, with gray color兲 that are unusable because
of saturated absorption of the parent. If we assume small corrections to derive expected vibrational wavenumbers from calculated harmonic vibrational wavenumbers of ClCOOH, similar to those indicated in frames共c兲 and 共d兲 of Fig.4for HOCO, the expected vibrational wavenumbers and relative IR intensities of t-ClCOOH and c-ClCOOH共TableI兲
fit well with observed 共B1, B2兲 and 共A1, A2兲 groups of bands, respectively. If bands A1 and A2 are assigned to the COH bending and C = O stretching modes of c-ClCOOH, the experimental values deviate only about 2% and 3%, respec-tively, from our calculated harmonic wavenumbers. Simi-larly, if bands B1 and B2 are attributed to the COH bending and C = O stretching modes of t-ClCOOH, the deviations are 2% and 1%, respectively.
As a derivation of rotational parameters from observed spectra is unlikely to be practicable with the present spectral resolution, we simulated the band contour using the molecu-lar parameters predicted with B3LYP/aug-cc-pVTZ for com-parison with observed spectra. With theSPECVIEWprogram49 we simulated the spectrum of each band using predicted ro-tational parameters A
⬘
, A⬙
, B⬘
, B⬙
, C⬘
, and C⬙
共Table II兲,Jmax= 120, T = 363 K, and a Doppler width共full width at half
maximum兲 of 2.0 cm−1. The resultant rotational contours
simulated for these four bands with a-type/b-type ratios cal-culated quantum chemically共Sec. IV B兲 are shown in frames 共a兲 and 共b兲 of Fig.5for t-ClCOOH and c-ClCOOH, respec-tively. A comparison of the observed transient spectrum av-eraged over 100– 150 s 共with open circle marks兲 with the combined simulated contour共solid lines兲 of these four bands is shown in Fig.5共c兲. The missing data for the P branch of the B2 band in the 1750– 1810 cm−1region are due to
inter-ference from saturated absorption of HCOOH. For this simu-lation, A1 band has 0= 1284.9 cm−1 with a-type/b-type
= 1/1, A2 band has 0= 1883.0 cm−1 with a-type/b-type
= 43/57, B1 band has 0= 1328.5 cm−1 with a-type/b-type
= 3/1, and B2 band has 0= 1808.0 cm−1 with
a-type/b-type=1/2; values of 0 are listed in Table I for
comparison with calculations. Observed integrated intensity ratios of C = O stretching to COH bending modes are ⬃6.5 and ⬃1.1, respectively, for t-ClCOOH and c-ClCOOH, which are consistent with the quantum-chemically predicted values of 7.2 and 1.1, respectively.
When the quantum-chemically predicted IR intensities are used, the population ratios of t-ClCOOH to c-ClCOOH can be estimated to be ⬃2.5⫾0.3 based on the observed integrated intensities of B1 and A1 bands; the error limits reflect errors only in simulation, not the errors in the pre-dicted IR intensities. If we assume that the simulation of the B2 band is reliable even though only part of the band was observed, the observed population ratio is⬃2.6⫾0.3 based on the integrated intensities of simulated B2 and A2 bands, consistent with the result derived from B1 and A1 bands. A concentration of t-ClCOOH greater than that of c-ClCOOH is consistent with the quantum-chemically predicted result that t-ClCOOH is slightly more stable than c-ClCOOH. As-suming a Boltzmann population at 363 K, we derive from the observed intensity ratio an energy difference of 2.8⫾0.3 kJ mol−1 between these two configurations. If an
error of a factor of 2 is assumed for the predicted IR inten-sity, then ⌬E=2.8⫾1.9 kJ mol−1. This value is consistent
with values of 2 kJ mol−1 calculated with
B3LYP/aug-cc-pVTZ in this work and 4 kJ mol−1 reported by Yu et al.19
using the CCSD共T兲/aug-cc-pVTZ method, but slightly smaller than values⬃8 kJ mol−1 reported by Francisco and Ghoul15using UMP2/6-311Gⴱⴱ, and by Stephenson et al.17 using MP2/6-311Gⴱⴱ; the latter two methods used a smaller basis set and are expected to be less accurate.
Considering possible chemical reactions, vibrational wavenumbers, rotational contours, relative IR intensities, and relative concentrations, we are confident of the assignments of the four transient features A1, A2, B1, and B2 observed near 1285, 1883, 1329, and 1808 cm−1to the COH bending
and C = O stretching modes of c-ClCOOH, COH bending, and C = O stretching modes of t-ClCOOH, respectively. Ac-cording to quantum-chemical calculations, the C–Cl stretch-ing 共691 cm−1兲 and C–O stretching 共1131 cm−1兲 modes of
c-ClCOOH are expected to have intensities ⬃0.25 those of the A1 and A2 bands, and the C–Cl stretching 共706 cm−1兲
and C–O stretching 共1127 cm−1兲 modes of t-ClCOOH are
expected to have intensities slightly smaller than that of the B2 bands. The C–Cl stretching mode is beyond our detection range 共950–7000 cm−1兲, whereas the C–O stretching mode
of ClCOOH is severely overlapped by the intense absorption of HCOOH 共Fig. 4兲. The 768 cm−1 band observed by
Pi-mentel and coworkers13,14 does not match the predicted wavenumber for the C–Cl stretching mode of c-ClCOOH or t-ClCOOH.
According to literature values, t-HOCO absorbs at 1852.6 共Ref. 44兲 and ⬃1210 cm−1.47,48
The 1852.6 cm−1
band might be buried between the B2 and A2 bands, but we 1250 1300 1350 1750 1800 1850 1900 1950 0 4 8 0 1 0 1 A1 B1 B2 Abs. / 1 0 -3 Wavenumber / cm-1 A2 (b) c-ClCOOH (a) t-ClCOOH In te ns it y / a .u. (c) Experiment
FIG. 5. Comparison of the observed and simulated spectra of ClCOOH in the region 1250– 1950 cm−1. 共a兲 Simulated bands for the COH bending
mode共0= 1328.5 cm−1and a-type/b-type=3/1兲 and the C=O stretching
mode共0= 1808.0 cm−1and a-type/b-type=1/2兲 of t-ClCOOH. 共b兲
Simu-lated bands for the COH bending mode 共0= 1284.9 cm−1 and
a-type/b-type=1/1兲 and the C=O stretching mode 共0= 1883.0 cm−1and
a-type/b-type=43/57兲 of c-ClCOOH. 共c兲 Simulated 共in solid line兲 and
ob-served spectra 共open circles兲 100–300 s upon irradiation of a flowing mixture of HCOOH/Cl2/N2 共1/2.1/24.6 at 58.1 Torr兲 at 363 K; spectral resolution 2.0 cm−1. See text.
174304-6 L.-K. Chu and Y.-P. Lee J. Chem. Phys. 130, 174304共2009兲
observed no discernible feature in the 1200– 1250 cm−1
re-gion near the A1 band. It is likely that t-HOCO was not produced directly from reaction共2兲because t-HCOOH is the dominant conformer and the Cl atom only abstracts the hy-drogen on the C atom. Conformer c-HOCO was observed only in a CO matrix,46 with two intense lines at 1797 and 1088 cm−1; the 1261 cm−1 line is expected to have a small
intensity according to calculations with the B3LYP/aug-cc-pVTZ method. Both absorption regions 共⬃1800 and 1090 cm−1兲 for c-HOCO were inaccessible in this work be-cause of the intense absorption of HCOOH.
B. Possible mechanism of the Cl+ HCOOH reaction
The reaction of Cl with HCOOH is believed to proceed via reaction 共2兲,
Cl + HCOOH→ HCl + HOCO. 共2兲
Tyndall et al.11determined a yield of 96⫾5% for CO2 and
suggested that the other channel
Cl + HCOOH→ HCl + HCO2. 共6兲
is unimportant. Miyoshi et al.7also showed that reaction共6兲 is unimportant by deuteration experiments. Yu et al.50 employed UQCISD共T, full兲/6-311+ +G共3df ,2p兲//UMP2 共full兲/6-311+G共d,p兲 and reported that reactions共2兲 and共6兲 have barriers of 3.7 and 68 kJ mol−1, respectively. They
concluded that the C-site hydrogen abstraction of t-HCOOH
to form c-HOCO dominates, consistent with
experimental observations. Rate coefficient k2
=共1.83⫾0.12兲⫻10−13 cm3molecule−1s−1 near 298 K was
reported by Li et al.,9 similar to k2=共2.00⫾0.25兲
⫻10−13 cm3molecule−1s−1reported by Wallington et al.10
Further reaction of HOCO with Cl2 is unimportant
be-cause this reaction is expected to have a large barrier. In contrast, reaction of Cl with HOCO to form HCl and CO2is
expected to be rapid,
Cl + HOCO→ HCl + CO2 共4兲
and the rate coefficient was determined to be k4 =共4.8⫾1.0兲⫻10−11 cm3molecule−1s−1 near 298 K, ⬃250
times greater than k2.9Yu et al.19employed CCSD共T兲 theory to investigate reaction共4兲and found that the reaction occurs via a ClCOOH intermediate that is formed through the bar-rierless addition reaction of Cl to the carbon atom in HOCO. The ClCOOH intermediate dissociate into HCl and CO2
through a four-center 共Cl–C–O–H兲 transition state that lies ⬃234 kJ mol−1below the asymptotic reactant channel. They
derived a thermal rate coefficient of k4= 3.0
⫻10−11 cm3molecule−1s−1, consistent with experimental
results. They also employed a direct MP2/6-31G共d兲 molecu-lar dynamics method to find reactive trajectories of two kinds, both of which proceed through ClCOOH with life-times⬃0.31 and 1.9 ps, respectively. They suggested that the long-lived ClCOOH might be stabilized with a third collision partner, consistent with our experimental observation of both conformers of ClCOOH upon photolysis of a mixture of Cl2
and HCOOH under 58 Torr of N2.
We employ a simple mechanism to describe the reac-tions in our system,
Cl + HCOOH→ HCl + HOCO 共2兲
Cl + HOCO→ ClCOOH 共7兲
ClCOOH→ HCl + CO2 共5兲
in which k7 is expected to be much larger than k2, as
sup-ported by resup-ported values of k4Ⰷk2; hence the
rate-determining step for formation of ClCOOH is reaction 共2兲. Under such conditions, 关HOCO兴 is expected to be in the steady state and
关HOCO兴 = k2关HCOOH兴/k7. 共8兲
By solving the differential equations of d关ClCOOH兴
dt = k2关Cl兴关HCOOH兴 − k5关ClCOOH兴, 共9兲 d关Cl兴
dt = − 2k2关Cl兴关HCOOH兴, 共10兲
we derived the concentration of ClCOOH as 关ClCOOH兴 = 关Cl兴0
k2关HCOOH兴 2k2关HCOOH兴 − k5
⫻关exp共− k5t兲 − exp共− 2k2关HCOOH兴t兲兴.
共11兲 The temporal profile of the absorbance integrated over the 1350– 1250 cm−1 region, including absorption of both c-ClCOOH and t-ClCOOH, upon photodissociation at 355 nm of a flowing mixture of HCOOH/Cl2/N2 共1/9/103 at 58
Torr兲 is shown in Fig. 6. When we fitted this profile to Eq.
共11兲, we derived first-order rate coefficients as k2关HCOOH兴
=共2.7⫾0.7兲⫻103 s−1and k5=共9.2⫾2.7兲⫻103 s−1,
respec-tively. Using the initial concentrations of 关HCOOH兴=1.5 ⫻1016 molecule cm−3and assuming that reaction共2兲is
rate-determining for the formation of ClCOOH, we estimated a bimolecular reaction rate coefficient of k2=共1.8⫾0.5兲
⫻10−13 at 363 K, consistent with the literature value of
⬃2⫻10−13 cm3molecule−1s−1at 298 K.9,10
The k5value at
363 K derived in this work is about one twentieth of the
0 100 200 300 400 500 0 3 6 9 12 Integrate di ntens ity Time /s
FIG. 6. Temporal profile of ClCOOH integrated over 1350– 1250 cm−1
upon 355 nm photolysis of a flowing mixture of HCOOH/Cl2/N2共1/9/103
at 58 Torr兲. Fitted results are represented with solid lines; see text.
value, 1.8⫻105 s−1, derived from the rate equation reported
by Jensen and Pimentel.14 This also indicates that the 768 cm−1 band observed by them might not be due to
ClCOOH.
VI. CONCLUSION
Four IR absorption bands of two conformers of ClCOOH were observed with a step-scan Fourier-transform spectrometer upon irradiation at 355 nm of a flowing gaseous mixture of Cl2and HCOOH in N2. By considering possible chemical reactions, vibrational wavenumbers, rotational con-tours, relative IR intensities, and relative concentrations pre-dicted with quantum-chemical calculations, we attributed ab-sorption bands with origins at 1808.0 and 1328.5 cm−1to the
C = O stretching and COH bending modes of t-ClCOOH, re-spectively. Similarly, band origins at 1883.0 and 1284.9 cm−1 are assigned as the C = O stretching and COH
bending modes of c-ClCOOH, respectively. From the ob-served and predicted relative intensities, we estimate that
t-ClCOOH is more stable than c-ClCOOH by
⬃3⫾2 kJ mol−1, consistent with quantum-chemical
calcula-tions.
ACKNOWLEDGEMENTS
We thank J. S. Francisco for bringing to our attention the importance of ClCOOH. National Science Council of Taiwan 共Grant No. NSC97-2113-M009-009-MY3兲 and the MOE-ATU project of National Chiao Tung University supported this work. We thank V. Stakhursky and T. A. Miller for pro-viding the SpecView software for spectral simulation.
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