(CH3CO) and CH3-CO complex
Prasanta Das and Yuan-Pern Lee
Citation: The Journal of Chemical Physics 140, 244303 (2014); doi: 10.1063/1.4883519 View online: http://dx.doi.org/10.1063/1.4883519
View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/140/24?ver=pdfcov Published by the AIP Publishing
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Bimolecular reaction of CH
3+ CO in solid
p
-H
2: Infrared absorption
of acetyl radical (CH
3CO) and CH
3-CO complex
Prasanta Das1and Yuan-Pern Lee1,2,a)
1Department of Applied Chemistry and Institute of Molecular Science, National Chiao Tung University,
Hsinchu 30010, Taiwan
2Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan
(Received 17 April 2014; accepted 3 June 2014; published online 24 June 2014)
We have recorded infrared spectra of acetyl radical (CH3CO) and CH3-CO complex in solid para-hydrogen (p-H2). Upon irradiation at 248 nm of CH3C(O)Cl/p-H2 matrices, CH3CO was identified as the major product; characteristic intense IR absorption features at 2990.3 (ν9), 2989.1 (ν1), 2915.6 (ν2), 1880.5 (ν3), 1419.9 (ν10), 1323.2 (ν5), 836.6 (ν7), and 468.1 (ν8) cm−1were observed. When CD3C(O)Cl was used, lines of CD3CO at 2246.2 (ν9), 2244.0 (ν1), 1866.1 (ν3), 1046.7 (ν5), 1029.7 (ν4), 1027.5 (ν10), 889.1 (ν6), and 723.8 (ν7) cm−1 appeared. Previous studies characterized only three vibrational modes of CH3CO and one mode of CD3CO in solid Ar. In contrast, upon pho-tolysis of a CH3I/CO/p-H2 matrix with light at 248 nm and subsequent annealing at 5.1 K before re-cooling to 3.2 K, the CH3-CO complex was observed with characteristic IR features at 3165.7, 3164.5, 2150.1, 1397.6, 1396.4, and 613.0 cm−1. The assignments are based on photolytic behavior, observed deuterium isotopic shifts, and a comparison of observed vibrational wavenumbers and rel-ative IR intensities with those predicted with quantum-chemical calculations. This work clearly indi-cates that CH3CO can be readily produced from photolysis of CH3C(O)Cl because of the diminished cage effect in solid p-H2but not from the reaction of CH3+ CO because of the reaction barrier. Even though CH3has nascent kinetic energy greater than 87 kJ mol−1 and internal energy∼42 kJ mol−1 upon photodissociation of CH3I at 248 nm, its energy was rapidly quenched so that it was unable to overcome the barrier height of ∼27 kJ mol−1 for the formation of CH3CO from the CH3 + CO reaction; a barrierless channel for formation of a CH3-CO complex was observed instead. This rapid quenching poses a limitation in production of free radicals via bimolecular reactions in p-H2. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4883519]
I. INTRODUCTION
The acetyl radical (also known as methyl carbonyl and ethanoyl, designated CH3CO) is an important intermediate in the oxidation of hydrocarbons in the troposphere1 and in
combustion of biofuels.2It is also involved in the metabolism
of acetaldehyde, CH3C(O)H.3 The CH3CO radical might be formed in the atmosphere via several channels: the reac-tions of OH and Cl with CH3C(O)H,4–6reaction of CH3with CO,7,8 and UV photolysis of acetyl halides [CH3C(O)X; X = F, Cl, Br, I] and methyl ketones such as CH3C(O)CH3, CH3C(O)CH2CH3, and CH3C(O)C(O)CH3.6,9,10
Despite its importance, spectral investigations of CH3CO are limited. Bennett and Mile prepared CH3CO on depositing alternative layers of CH3C(O)Cl and Na atoms with various matrix hosts at 77 K and characterized the products with elec-tron paramagnetic resonance (EPR).11Their analysis showed
that CH3CO is a σ−type radical with the unpaired electron located primarily on the carbon atom of the carbonyl moi-ety, but appreciable spin density was located on the adja-cent carbon and oxygen atoms. The infrared (IR) spectrum of gaseous CH3CO is unreported, and only three IR lines
a)Author to whom correspondence should be addressed. Electronic mail:
yplee@mail.nctu.edu.tw.
were reported for matrix-isolated CH3CO despite numerous attempts. Shirk and Pimentel identified the IR absorption of CH3CO, produced on reaction of CH3C(O)Cl with Li atoms in an Ar matrix;12 two lines at 1844 and 1328 cm−1 were
observed. Jacox deposited Ar and CH3CO, produced from the gaseous reaction F+ CH3C(O)H in which F atoms were generated in a microwave discharge, and reported three lines at 1875, 1420, and 1329 cm−1 for CH3CO isolated in solid Ar.13 Thompson et al. reported a weak feature of CH3CO at 1875.3 cm−1 after a CH3C(O)H/Ar matrix was bombarded with electrons.14Because of the matrix cage effect, it is diffi-cult to prepare CH3CO from photolysis in situ of precursors such as CH3C(O)Cl isolated in a matrix. Several workers re-ported that predominantly complexes of HCl and ketene were observed upon UV photolysis of CH3C(O)Cl in its amorphous or crystalline form,15,16 or isolated in solid Ar or Xe.16–18
Only in experiments when CH3C(O)Cl isolated in Xe was ir-radiated with laser light at 193 nm were two weak features at 1874 and 1320 cm−1 observed and assigned to CH3CO; the authors proposed that under such conditions some Cl might escape from the original Xe cage.18
Acetyl chloride served as a convenient precursor of CH3CO in the gaseous phase. Arunan reported that the C−C bond is stronger than the C−Cl bond in CH3C(O)Cl by 12 kJ mol−1 based on experimental enthalpies for the
formation of CH3C(O)Cl, CH3, ClCO, CH3CO, and Cl.19 Person et al. investigated the photodissociation dynamics of gaseous CH3C(O)Cl in a molecular-beam and observed a preferential fission of the C−Cl bond over the C−C bond at photolysis wavelength 248 nm.20,21The observed anisotropic
angular distribution of Cl is characteristic of a prompt, im-pulsive dissociation of the C−C1 bond. The anisotropic an-gular distribution was observed also by Deshmukh et al. us-ing photofragment ion-imagus-ing.22 These authors found also
that a fraction of the primary photofragment CH3CO subse-quently decomposes to form CH3and CO. Shibata et al. em-ployed the photofragment ion-imaging with a femtosecond laser to demonstrate that the dissociation of CH3C(O)Cl oc-curs within a period comparable to the laser pulse duration of 200 fs.23 Using photofragment translation spectroscopy, North et al. investigated the photolysis dynamics of CH3C(O)Cl at 248 nm and reported that ∼35% of CH3CO underwent secondary decomposition to form CH3 + CO,24 consistent with a quantum yield of 0.28 for CH3from CH3CO reported by Desmukh and Hess.25 North et al. also
de-termined the barrier for decomposition of CH3CO to be 71 ± 4 kJ mol−1;24 this value was supported by Shibata
et al.26 In contrast, Tang et al. excited CH
3C(O)Cl at 235 nm and employed velocity map imaging to determine the barrier height for dissociation of CH3CO to CH3 + CO as 59 kJ mol−1.27 The enthalpy of formation (H0
f) for CH3CO at 298 K was determined to be−(9.8 ± 1.8) kJ mol−1 with the threshold photoelectron-photoion coincidence technique.28
Liu et al. photolyzed at 248 nm CH3C(O)Cl in the presence of Ar or O2 and reported an additional dissociation channel leading to formation of HCl, CO, and CH2; the latter two were proposed to result from the secondary decomposition of CH2CO, the co-product of HCl.29
Several theoretical investigations of the decomposition of CH3C(O)Cl have been reported.30–32 Chen and Fang em-ployed the B3LYP and CAS(10,8) methods with cc-pVDZ and cc-pVTZ basis sets to optimize the geometries and the MR-CI method with CAS(10,8) wave functions to calculate the single-point energy to investigate the photodissociation of CH3C(O)Cl. They reported a nearly barrierless path involving fission of C−Cl bond on the first excited-singlet (S1) surface, resulting in fragmentation on a time scale of picosecond, fol-lowed by further decomposition of CH3CO to CH3and CO.
In addition to photolysis of CH3C(O)Cl, CH3CO radi-cal might be produced from the reaction CH3+ CO. Watkins and Word performed the CH3 + CO reaction in the gaseous phase to generate chemically activated CH3CO and analyzed the products with gas-liquid chromatograph.7They estimated
an activation energy of 25.0 kJ mol−1 for this reaction and enthalpy of −17 kJ mol−1 for formation of CH3CO from CH3 + CO. Anastasi and Maw photolyzed azomethane in the presence of CO in the gaseous phase and monitored CH3 and CH3CO using molecular modulation;33 they determined the activation energy of 27.6 kJ mol−1 for the reaction CH3 + CO → CH3CO, in agreement with 27 kJ mol−1 predicted with the CCSD(T)/aug-cc-pVTZ method.34According to the
calculations, the complex of CH3and CO, denoted CH3-CO, is stabilized by∼3 kJ mol−1relative to CH3+ CO, but no ex-perimental observation of CH3-CO has been reported. Hence,
it would be interesting to record the IR spectrum of CH3-CO and to compare it with that of CH3CO.
Solid para-hydrogen (p-H2) is known to have a di-minished cage effect that allows the production of free radicals via photofragmentation in situ or photo-induced bimolecular reactions.35–39 In this laboratory we took
advantage of this unique property of p-H2 to prepare free radicals from C−Cl bond fission of their Cl-containing pre-cursors because the Cl fragment can escape from the orig-inal matrix cage upon photolysis. Examples include the photolysis of acryloyl chloride, CH2CHC(O)Cl, to generate 3-propenonyl (•CH2CHCO) radical40 and the photolysis of methoxysulfinyl chloride, CH3OS(O)Cl, to yield CH3OSO.41 We, thus, expect to produce CH3CO from photolysis of CH3C(O)Cl isolated in p-H2 to yield an IR spectrum of CH3CO much improved from those in previous reports. We irradiated with UV light also a p-H2matrix containing CH3I and SO2 to produce five prominent IR features of CH3SO upon annealing of the matrix to induce the CH3 + SO2 reaction.42 The reaction of CH
3 + CO is, hence, expected to be feasible in solid p-H2 for the purpose of learning whether CH3CO or CH3-CO is formed.
We report here the observation of eight IR lines of CH3CO upon photolysis of CH3C(O)Cl. The correspond-ing features of CD3CO were observed from photolysis of CD3C(O)Cl to confirm the assignments. In performing the re-action CH3 + CO in solid p-H2, we observed IR features of CH3-CO instead of CH3CO, indicating that the barrier height to form CH3CO prevents this reaction in solid p-H2.
II. EXPERIMENTS
The matrix isolation system for IR absorption employed in this work is the same as previously described.40,42,43
In brief, the gold-plated copper block, cooled to 3.2 K with a closed-cycle refrigerator system, serves as both a matrix sample substrate and a mirror to reflect the inci-dent IR beam to the detector. For photodissociation experi-ments, a gaseous mixture of CH3C(O)Cl/p-H2, CD3 C(O)Cl/p-H2, or CH3C(O)C(O)CH3/p-H2 (1/2000−1/2500, flow rate ∼15 mmol h−1) was deposited over a period of ∼9 h. For photo-induced bimolecular reactions, gaseous mixtures of CH3I/p-H2 (1/1000) and CO/p-H2 (1/1000) were co-deposited over a period of 9 h with a flow rate of 7 mmol h−1 for each mixture. IR absorption spectra were recorded with a Fourier-transform infrared (FTIR) spectrometer equipped with a KBr beam splitter and a HgCdTe detector at 77 K to cover a spectral range 400−4100 cm−1. The spectrum was typically recorded at a resolution of 0.25 cm−1and averaged with 600 scans at each stage of the experiment. The IR beam was passed through a filter (2.4 ILP-50, Andover) to block light of wavenumber greater than 4100 cm−1to avoid the re-action of Cl with vibrationally excited H2.44
The primary photolysis light at 248 nm was gener-ated from a KrF excimer laser. For photodissociation of CH3C(O)Cl/p-H2 and CD3C(O)Cl/p-H2, the matrices were irradiated for up to 120 min and 30 min, respectively, us-ing an average pulse energy of 5−6 mJ at a repetition rate of 3 Hz. For the photo-induced reaction, CH3I/CO/p-H2
matrix was irradiated for 15 min using pulses of average energy 3−4 mJ at repetition rate of 3 Hz. For photolysis of CH3C(O)C(O)CH3/p-H2 and secondary photolysis we em-ployed light at 355 nm and 532 nm generated from the third and second harmonic of a pulsed Nd:YAG laser, respectively. CH3C(O)Cl (98%, Fluka), CD3C(O)Cl (99%, Aldrich), CH3C(O)C(O)CH3 (99%, Acros), CH3I (99%, Riedel-de Haën), and CO (99.999%, Specialty Gases of America) were used without further purification. We used catalytic conver-sion to prepare gaseous p-H2. Normal H2 (99.9999%, Scott Specialty Gases) was passed through a trap at 77 K and a cop-per cell filled with hydrous iron (III) oxide catalyst (Aldrich) that was cooled to 10−15 K with a closed-cycle refrigera-tor. The temperature for p-H2conversion was typically set at 13 K at which the concentration of o-H2 was less than 100 ppm.
We derived mixing ratios of reactants and products ac-cording to the method described in Ref. 40. For CH3CO, CH3, and CH2CO, the integrated absorbance of 2−3 lines and predicted IR intensities predicted with the B3PW91/aug-cc-pVTZ method were used, whereas for CO (Ref.45) and HCl (Ref.46), integration of a single line and the reported experi-mental absorption cross sections were used.
III. THEORETICAL CALCULATIONS
We employed the GAUSSIAN09 program47 to calculate
the harmonic and anharmonic vibrational wavenumbers and IR intensities of CH3C(O)Cl, CH3CO, and CH3-CO using the B3PW91 and B3LYP density functional theories48,49with the
aug-cc-pVTZ and 6-311G(2d,2p) basis sets, respectively.50,51 Analytic first derivatives were utilized to optimize geometry, and anharmonic vibrational wavenumbers were calculated an-alytically at each stationary point.
A. CH3C(O)Cl
The geometry and vibrational wavenumbers of CH3C(O)Cl have been well characterized.52 The structural parameters of CH3C(O)Cl optimized with the B3PW91/aug-cc-pVTZ method are compared with those measured previously with electron diffraction (listed parenthetically)53 in Fig. 1(a). The harmonic and anharmonic vibrational wavenumbers and relative intensities predicted with the B3PW91/aug-cc-pVTZ method for the fundamental modes of all vibrations of CH3C(O)Cl and CD3C(O)Cl are listed in Table SI of the supplementary material.54 The anharmonic
(harmonic) vibrational wavenumbers of CH3C(O)Cl with IR intensities greater than 20 km mol−1 are 1836 (1893), 1351 (1381), 1092 (1114), 935 (962), 600 (606), and 435 (439) cm−1. Those of CD3C(O)Cl are 1855 (1891), 1103 (1145), 948 (967), and 554 (560) cm−1.
B. CH3CO radical and other possible products
The geometry and vibrational wavenumbers of CH3CO have been reported.55–57The structural parameters of CH
3CO optimized with the B3PW91/aug-cc-pVTZ method are shown
FIG. 1. Geometries of CH3C(O)Cl (a), CH3CO (b), and•CH2C(O)Cl (c)
predicted with the B3PW91/aug-cc-pVTZ method, and geometry of the CH3-CO complex (d) predicted with the B3LYP/6-311G(2d,2p) method.
Re-ported parameters obtained by gas-phase electron diffraction for (a)53and those predicted for (b) with the MP(full)/6-31G* method57and (d) with the B3PW91/6-311G(2d,2p) method are listed in parentheses. Bond distances are in Å and angles are in degrees.
in Fig. 1(b); those predicted with the MP(full)/6-31G* method57 for CH
3CO are listed in parentheses for compar-ison. The harmonic and anharmonic vibrational wavenum-bers and relative intensities predicted with the B3PW91/aug-cc-pVTZ method for the fundamental modes of all vibra-tions of CH3CO and CD3CO are listed in TableI. The anhar-monic (haranhar-monic) vibrational wavenumbers of CH3CO with IR intensities greater than 10 km mol−1 are 1918 (1938), 1405 (1447), 1402 (1443), 1315 (1346), and 1039 (1049) cm−1; corresponding values of CD3CO are 1910 (1936), 1014 (1033), 1018 (1042), 1112 (1074), and 882 (904) cm−1, respectively.
Possible products in our photofragmentation experi-ments of CH3C(O)Cl include ketene (CH2CO), 2-chloro-2-oxo ethyl [also known as chloroformylmethyl,•CH2C(O)Cl], and chloromethane (CH3Cl). Infrared spectra of CH2CO and CH3Cl and their deuterated species have been reported.17,58 We have calculated vibrational wavenumbers and IR inten-sities of CH2CO, •CH2C(O)Cl, and CH3Cl and compared them with available experimental vibrational wavenumbers in
TABLE I. Comparison of observed wavenumbers (cm−1) and relative IR intensities of CH3CO and CD3CO in solid p-H2with their harmonic and anharmonic
vibrational wavenumbers and IR intensities calculated with the B3PW91/aug-cc-pVTZ method.
CH3CO CD3CO
νi Sym. Modea Harmonic Anharmonic p-H2 Ar Harmonic Anharmonic p-H2c Isotopic ratiob
ν1 A a-νCH3 3122 (4.9)d 2974 2989.1 (1.9)d 2313 (3.1)d 2230 2244.0 (3.4)d 0.751 (0.750) ν2 A s-νCH3 3026 (4.7) 2901 2915.6 (2.9) 2170 (1.5) 2152 ν3 A νC=O 1938 (100) 1918 1880.5 (100) 1875 1936 (100) 1910 1866.1 (100) 0.992 (0.996) ν4 A δs-CH2 1447 (14.2) 1405 1419.9 (8.0)e 1420 1033 (11.1) 1014 1029.7 (10.9) 0.725 (0.722) ν5 A uCH3 1346 (9.6) 1315 1323.2 (10.9) 1329 1074 (4.7) 1112 1046.7 (3.5) 0.791 (0.845)f ν6 A δCCH 1049 (9.7) 1039 ? 904 (7.9) 882 889.1 (9.3) ν7 A νC−C 871 (2.8) 838 836.6 (4.5) 769 (0.1) 729 723.8 (<0.1) 0.865 (0.870) ν8 A δCCO 468 (3.3) 466 468.1?(2.8) 417 (2.2) 416 ν9 A a-νCH2 3128 (0.2) 2979 2990.3 (4.2) 2314 (0.2) 2237 2246.2 (sh) 0.751 (0.751) ν10 A a-δCH3 1443 (7.9) 1402 1419.9 (8.0)e 1042 (3.4) 1018 1027.5 (2.7) 0.724 (0.726) ν11 A oop-δ 948 (0.0) 896 747 (0.3) 718 ν12 A τ 112 (0.5) 100 93 (0.2) 82
Reference This work This work 13 This work This work This work
aν: stretch, δ: bend or deformation, δs: scissor, u: umbrella, τ : torsion, oop: out-of-plane, a:antisymmetric, and s: symmetric. bDefined as the ratio of wavenumber of the isotopic species to that of natural species; theoretical values are listed in parentheses.
cAdditional weak infrared feature observed at 2029.0, 1929.1, and 1914.2 cm−1is tentatively assigned to 2ν4or (ν4+ ν10), (ν5+ ν6), and (ν4+ ν6), respectively.
dPercentage IR intensities relative to the most intense lines at 1918 and 1910 cm−1for CH3CO and CD3CO with corresponding IR intensities 163.9 and 171.3 km mol−1, respectively. ePreferred assignment of this line is ν
10, but the possibility of ν4cannot be positively ruled out. See text for discussion. fIsotopic ratio of calculated harmonic vibrational wavenumbers is 0.798. See text for discussion.
Table SII of supplementary material.54 The corresponding
values of the fully deuterated species are listed in Table SIII of supplementary material.54
The geometry of •CH2C(O)Cl optimized with the B3PW91/aug-cc-pVTZ method is presented in Fig.1(c). The predicted anharmonic (harmonic) vibrational wavenumbers of•CH2C(O)Cl with IR intensities greater than 20 km mol−1 are 1682 (1718), 1416 (1444), 1136 (1157), 769 (774), and 602 (609) cm−1.
C. CH3-CO complex
The geometry of the CH3-CO complex predicted with the B3LYP/6-311G (2d,2p) method is presented in Fig.1(d); we were unable to obtain a stable optimized structure of CH3 -CO with the B3PW91/aug-cc-pVTZ method. The most
sta-ble structure of CH3-CO is the anti-conformer in which O is anti to one C−H bond of CH3. The distance between two C atoms is predicted with the B3LYP/6-311G (2d,2p) method to be 3.634 Å; this distance became 4.103 Å when the B3PW91/6-311G(2d,2p) method was used. At the B3LYP/6-311G (2d,2p) level of theory, the energy of the complex ex-ceeded that of CH3 + CO by 0.2 kJ mol−1. A single-point energy calculation was performed on the B3LYP/6-311G (2d,2p) optimized structures with the CCSD/6-311G (2d,2p) method; the energy of CH3-CO became−1.9 kJ mol−1 rela-tive to CH3+ CO.
The predicted anharmonic (harmonic) vibrational wavenumbers and IR intensities of CH3, CO, and CH3-CO are listed in Table II. Those of CH3-CO with IR intensities greater than 5 km mol−1 are 3142 (3288), 2190 (2214), and 669 (517) cm−1. The most intense line near 669 cm−1
TABLE II. Comparison of observed wavenumbers (cm−1) and relative IR intensities of CH3and the CH3-CO complex in solid p-H2with their harmonic and
anharmonic vibrational wavenumbers and IR intensities predicted with the B3LYP/6-311G(2d,2p) method.
CH3 CH3-COb
νia Harmonic Anharmonic p-H2 Harmonic Anharmonic p-H2
ν1 3290 (8.9)c 3155 3171.6/3170.6 (21.0)c 3291 (5.4)c 3144 3165.7 (sh)c ν2 3290 (8.9) 3140 3288 (6.5) 3142 3164.5 (15) ν3 3114 (0.0) 3002 3113 (0.0) 2995 ν4 2214 (86) 2190 2150.1 (67) ν5 1411 (3.7) 1385 1402.7/1402.4 (1.9) 1412 (2.7) 1380 1397.6 (1.0) ν6 1411 (3.7) 1386 1401.7 (2.6) 1410 (3.4) 1379 1396.4 (3.8) ν7 504 (100) 703 624.3/623.1 (100) 517 (100) 669 613.0 (100)
Ref. This work 42 This work This work
aThe order of mode follows the predicted harmonic vibrational wavenumbers of the CH
3-CO complex.
bAdditional modes are predicted to have harmonic (relative intensities)/anharmonic vibrational wavenumbers: 70 (0.1)/289, 63 (0.0)/0.1, 49 (0.0)/195, 25 (0.2)/168, and 20 (0.0)/−
95 cm−1for the CH3-CO complex.
cPercentage IR intensities relative to the most intense line near 600 cm−1are listed in parentheses. IR intensities of these lines of CH3and the CH3-CO complex are predicted to be
69 and 81 km mol−1, respectively.
corresponding to the C−H out-of-plane bending mode shows significant, likely overestimated, negative anharmonicity.
IV. EXPERIMENTAL RESULTS
A. Photolysis of CH3C(O)Cl/p-H2matrices
The IR spectrum of a sample of CH3C(O)Cl/p-H2 (1/2500) deposited at 3.2 K for 9 h is shown in Fig. 2(a); observed lines are compared with values from gaseous experiments59 and calculations in Table SI of
supplemen-tary material.54Our experimental observations are consistent
with those reported for gaseous CH3C(O)Cl. Lines at 1814.0, 1428.6, 1425.1, 1362.6, 1105.4, 952.0, and 603.9 cm−1 are more intense.
Gaseous CH3C(O)Cl at 298 K has UV absorption cross sections of ∼5.5 × 10−20 cm2 molecule−1 at 248 nm and ∼2.5 × 10−20 cm2 molecule−1 at 266 nm, respectively.60
Light at these two wavelengths was initially tested for pho-tolysis of CH3C(O)Cl, but irradiation of the matrix sample with light at 248 nm produced lines of CH3CO with intensi-ties greater than after irradiation at 266 nm. Upon irradiation of the CH3C(O)Cl/p-H2 (1/2500) matrix at 248 nm for 2 h, lines of CH3C(O)Cl decreased in intensity and new features in several groups appeared. A difference spectrum obtained on subtracting the spectrum recorded upon deposition from that recorded after irradiation at 248 nm is presented in Fig.2(b); lines pointing upwards indicate production and those pointing downward indicate destruction. Lines in group X at 2990.3, 2915.6, 2989.1, 1880.5, 1419.9, 1323.2, 836.6, and 468.1(?) cm−1 show similar behavior under varied conditions; the ? mark indicates an uncertain assignment because of a small in-tensity. The intensities of these lines decrease upon secondary photolysis at 532 nm, as shown in the difference spectrum of the matrix upon further irradiation at 532 nm, Fig.2(c). These lines in group X are assigned to acetyl (CH3CO) radical, to be discussed in Sec.V A.
As shown in Figs.2(b)and2(c), lines of ketene (CH2CO) at 3064.5/3065.4, 2146.4/2147.6, 1383.0, 600.0, and 522.6 cm−1 also appeared upon photolysis at 248 nm but their intensities remained unchanged upon secondary photol-ysis at 532 nm; for the observed doublets, the more intense ones are listed first. The observed vibrational wavenumbers are similar to those reported in an Ar matrix17 at 3053, 2138,
1378, 618, and 528 cm−1 and in the gaseous phase at 3069, 2151, 1388, 588, and 528 cm−1,61as compared in Table SII.54
Weak lines at 2141.8 and 2140.7 cm−1, marked CH2CO in Fig.2(b), are tentatively assigned to a CH2CO-HCl complex because a line at 2145 cm−1 was reported for this complex produced after photolysis at 266 nm of CH3C(O)Cl in solid Ar.18 The line of HCl at 2894.3 cm−1(Refs.62and63) was
observed upon photolysis at 248 nm; its intensity increased after further irradiation at 532 nm.
Weak lines of CH3 at 3170.7, 1401.7, and 624.1 cm−1 appeared upon irradiation of the matrix at 248 nm;42 their intensities increased significantly upon secondary photolysis at 532 nm, as shown in Fig. 2(c). Observed lines at 2146.0, 2143.0, 2140.6, and 2137.6 cm−1 are assigned to the rota-tional lines of CO according to the reports on CO isolated in p-H2.40,64,65
The temporal evolution of the concentrations of these species as a function of photolysis period at 248 nm is shown in Fig. 3. We estimated the decrease in the mixing ratio of CH3C(O)Cl to be (13.6± 0.9) ppm and the increases in the mixing ratios of CH3CO, CH2CO, HCl, CH3, and CO to be approximately (9.5 ± 1.7), (2.5 ± 0.2), (3.6 ± 0.7), (1.4 ± 0.3), and (0.5 ± 0.1) ppm upon photolysis of the matrix at 248 nm for 2 h. After secondary photolysis with light at 532 nm, the changes in the mixing ratios of CH3CO, CH3, and CO were estimated to be−(3.1 ± 0.8), (3.7 ± 0.2), and (2.6± 0.5) ppm, respectively.
An alternative precursor 2,3-butanedione, CH3C(O)C(O) CH3, was also employed to produce CH3CO. The ab-sorption spectrum of CH3C(O)C(O)CH3/p-H2 (1/2500)
FIG. 2. (a) IR absorption spectrum of a CH3C(O)Cl/p-H2(1/2500) matrix after deposition at 3.2 K for 9 h. (b) Difference spectrum of the matrix in (a) upon
irradiation at 3.2 K with light at 248 nm for 2 h. (c) Difference spectrum obtained on further irradiation at 532 nm for 1 h. Lines in groups X, CH2CO, CH3,
FIG. 3. Variations of mixing ratios as a function of period of photolysis at 248 nm for the precursor CH3C(O)Cl (a) and products CH3CO, CH2CO,
HCl, CH3, and CO (b). The initial concentration of CH3C(O)Cl was 214
± 41 ppm.
matrix at 3.2 K deposited for 10 h is shown in Fig. S1(a) of the supplementary material.54 Similar lines in
group “X” were observed when this matrix was irradiated at 355 nm for 3 h, as shown in the difference spectrum in Fig. S1(b), but their intensities remained small even after ir-radiation prolonged to 6 h. This effect is likely due to sec-ondary photolysis of CH3CO at 355 nm. The difference spec-trum obtained after photolysis of CH3C(O)Cl/p-H2(1/2500) at 248 nm is shown in Fig. S1(c) for comparison.
B. Photolysis of CD3C(O)Cl/p-H2matrices
Similar experiments were performed with CD3C(O)Cl as a precursor. The IR spectrum of a sample of CD3 C(O)Cl/p-H2 (1/2500) deposited at 3.2 K is shown in Fig. 4(a). Lines observed at 2273.2, 1817.1, 1129.0, 1039.8, 1034.7, 956.4, 873.4, 814.9, 558.8, 453.1, and 440.0 cm−1 are assigned to CD3C(O)Cl, consistent with those reported for the gaseous phases,59 as compared in Table SI of the supplementary
material.54Figure4(b)shows a difference spectrum of the
ma-trix after irradiation at 248 nm for 30 min, indicating that the intensities of lines of CD3C(O)Cl decreased and new features in several groups appeared. Lines corresponding to group X (CH3CO) that were observed in CH3C(O)Cl experiments shifted to 2244.0, 2246.2 (shoulder), 2029.0, 1929.1, 1914.2, 1866.1, 1046.7, 1029.7, 1027.5, 889.1, and 723.8 (not shown) cm−1, marked as XDin Fig.4(b)and listed in TableI. The dif-ference spectrum obtained after secondary photolysis at 532 nm is shown in Fig.4(c); lines in group XDdecreased in in-tensity. We assigned these lines in group XDto the CD3CO radical, to be discussed in Sec.V B.
Lines of CH2CO observed in CH3C(O)Cl/p-H2 experi-ments shifted to 2376.5, 2262.3/2262.2, 2116.2, 1226.2, and 849.9 cm−1, as listed in Table SII;54 they were readily
as-signed to CD2CO according to values for CD2CO in an Ar matrix.17 The line of HCl at 2894.3 cm−1 shifted to 2092.6
cm−1, identical to the value reported for DCl in p-H2.62 Lines of CH3observed at 3170.7 and 624.1 cm−1shifted to 2383.9/2379.2 and 475.8/467.6 cm−1, consistent with the re-ported values 2379 cm−1 for CD3 in a p-H2 matrix,66 2381, 1026, and 463 cm−1in a Ne matrix,67and 453 and 465 cm−1 in an Ar matrix.68 The lines of CH3 at 1401.7 cm−1 are ex-pected to shift to∼1034 cm−1, but this region was severely interfered by absorption of CD3C(O)Cl near 1034 cm−1. As expected, lines of CO did not shift. A weak line of ClCO observed at 1880.4 cm−1 is identical to that observed in p-H2 upon photolysis of a CH2C=CHC(O)Cl/p-H2 matrix at
FIG. 4. (a) IR absorption spectrum of a CD3C(O)Cl/p-H2(1/2500) matrix after deposition at 3.2 K for 9 h. (b) Difference spectrum of the matrix in (a) upon
irradiation at 3.2 K with light at 248 nm for 30 min. (c) Difference spectrum obtained on further irradiation at 532 nm for 45 min. Lines in groups XD, CD2CO,
CD3, and CO are indicated; CD2COindicates a complex of CD2CO with DCl or HCl.
FIG. 5. (a) Partial IR spectra in regions of 500−660, 860−920, 1220−1265, 1380−1470, 2126−2162, 2955−3080, and 3140−3200 cm−1of a CH3I/CO/p-H2
(1/1/2000) matrix after deposition at 3.2 K for 8 h. (b) Difference spectrum of the matrix in (a) upon irradiation at 3.2 K with light at 248 nm for 15 min. (c) Difference spectrum of the irradiated matrix after annealing at 5.1 K for 2 min and cooling to 3.2 K. Lines in groups Y, CH3I, CH3, C2H6, and CO are marked.
193 nm40 and is consistent with the reported value of
1885 cm−1in the gaseous phase.69
C. Photolysis of CH3I/CO/p-H2matrices
We undertook blank tests with CO and CH3I isolated in p-H2 separately. The IR spectrum of a sample of CO/p-H2 (1/4000) at 3.2 K exhibits intense vibration-rotational lines at 2146.0, 2143.0, 2140.6, and 2137.6 cm−1.40The IR spectrum
of a sample of CH3I/p-H2(1/2000) at 3.2 K exhibits intense lines at 2965.1(ν1), 1248.4 (ν2), and 884.5 (ν6) and weaker lines at 3057.5 (ν4), 1432.5 (ν5), and 531.3 (ν3) cm−1. An-nealing of these matrices at 4.8 K for 5 min produced no ob-servable change in the spectrum.
Gaseous CH3I at 300 K has an absorption cross section of ∼1.0 × 10−18 cm2 molecule−1 near 254 nm.70 We em-ployed light at 248 nm from a KrF excimer laser to irradi-ate the matrix with the expectation that only CH3I, but not CO, would be photolyzed so that reactions among only CH3, CO, and I might occur. Irradiation of a CH3I/p-H2 (1/2000) matrix at 248 nm for 15 min yielded four sets of new lines at 3171.6/3170.6, 2780.1/2779.3, 1402.7/1402.4/1401.7, and 624.3/623.1 cm−1. These lines are assigned to ν3, 2ν4, ν4, and ν2 modes of CH3 in p-H2.42 Weak lines of C2H6 at 2981.9, 1467.5, and 821.3 (?) cm−1 were also observed, indicating that some CH3diffused and reacted with CH3to form C2H6.
The IR spectrum of a sample of CH3I/CO/p-H2 (1/1/2000) deposited at 3.2 K for 8 h is shown in Fig. 5(a). We observed lines of CH3I and CO, but no additional fea-ture assignable to the CH3I-CO complex even after retain-ing the matrix overnight or annealretain-ing at 4.5 K for 10 min. Figure 5(b) represents the difference spectrum recorded af-ter irradiation of the CH3I/CO/p-H2 (1/1/2000) matrix at 3.2 K with laser light at 248 nm for 15 min; the intensities of lines due to CH3I decreased, the features due to CO show a first-derivative shape, and lines of CH3appeared. The first-derivative shape for lines of CO in the difference spectrum indicates that lines of CO might have shifted slightly upon
ir-radiation. Extremely weak features at 2150.1 and 613.0 cm−1 were also observed; their intensities increased after annealing. Upon subsequent annealing at 5.1 K for 2 min, the intensi-ties of lines due to CH3and CO decreased, those of CH3I in-creased, and a group of new lines appeared at 3165.7, 3164.5, 2150.1, 1397.6, 1396.4, and 613.0 cm−1, indicated as “Y” in Fig.5(c). These features in group Y are assigned to the CH3 -CO complex, to be discussed in Sec.V C.
V. DISCUSSION
A. Assignment of lines in group X to the acetyl (CH3CO) radical
Butler and co-workers observed that C−Cl bond fis-sion was the major channel upon photolysis of gaseous CH3C(O)Cl at 248 nm.20,21 Some CH3CO fragments, thus, formed contained sufficient internal energy to enable fur-ther dissociation to CH3+ CO; 28−35% of CH3CO radicals decomposed.24,25 Hence, CH
3CO, Cl, CH3, and CO are the major products in this system.
Lines of group X were observed in UV photolysis of both CH3C(O)Cl and CH3C(O)C(O)CH3. The more intense lines in group X at 1880.5, 1419.9, and 1323.2 cm−1 have wavenumbers similar to values 1875, 1420, and 1329 cm−1 reported for CH3CO isolated in solid Ar.13 They are also near the anharmonic vibrational wavenumbers of 1918, 1405, and 1315 cm−1 for the C=O stretching (ν3), CH2 scissor (ν4), and CH3umbrella (ν5) modes of CH3CO predicted with the B3PW91/aug-cc-pVTZ method. These observed lines in group X are, hence, likely due to CH3CO. Because of the su-perior signal-to-noise (S/N) ratio, we were able to identify additional lines at 2990.3, 2989.1, 2915.6, 836.6, and, ten-tatively, 468.1 cm−1 that were previously unreported. These values are near quantum-chemically predicted values of 2979 (ν9, CH2 antisymmetric stretch), 2974 (ν1, CH3 antisym-metric stretch), 2901 (ν2, CH3 symmetric stretch), 838 (ν7, C−C stretch), and 466 (ν8, CCO bend) cm−1for CH3CO, as compared in Table I. The IR intensities of these lines were
FIG. 6. Comparison of experimental spectrum with those simulated for possible candidates. (a) Inverted spectrum of Fig.2(c), obtained upon secondary photolysis of the UV-irradiated matrix of CH3C(O)Cl/p-H2 with light at 532 nm for 1 h. IR spectra of CH3CO (b),•CH2C(O)Cl (c), and CH3Cl (d) were
simulated according to anharmonic vibrational wavenumbers and IR intensities predicted with the B3PW91/aug-cc-pVTZ method. Regions of interference due to absorption of CH3C(O)Cl and CO are marked grey.
predicted to be less than 15% of the most intense line at 1880.5 cm−1.
We inverted the difference spectrum in Fig. 2(c) and present it in Fig. 6(a) to compare with spectra of CH3CO, •CH2C(O)Cl, and CH3Cl in Figs.6(b)–6(d)simulated accord-ing to anharmonic vibrational wavenumbers and IR intensi-ties predicted with quantum-chemical calculations. The ob-served lines in group X agree satisfactorily with the simu-lated spectrum of CH3CO in terms of vibrational wavenum-bers and IR intensities, but do not match with those predicted for•CH2C(O)Cl or CH3Cl; the reported experimental values for CH3Cl (Ref. 58) are near their predicted anharmonic vi-brational wavenumbers, as listed in Table SII.54
After considering the photolytic behavior observed upon primary and secondary photolysis of the CH3C(O)Cl/p-H2 and CH3C(O)C(O)CH3/p-H2matrices and the agreement be-tween predicted and observed vibrational wavenumbers and relative IR intensities, we assigned these features in group X to the IR absorption of acetyl (CH3CO) radical. The most in-tense line at 1880.5 cm−1 corresponds to the C=O stretch-ing mode. This value is near a value of 1875 cm−1 reported by Jacox,13 but much greater than the value of 1844 cm−1 reported by Shirk and Pimentel;12 the latter line might be
perturbed by nearby Li atoms. The ν1 (CH3 antisymmet-ric stretching) and ν9(CH2antisymmetric stretching) modes were predicted at 2974 and 2979 cm−1 with IR intensities 4.9 and 0.2 km mol−1, respectively. We observed a line at 2990.3 cm−1and a shoulder at 2989.1 cm−1, which we ten-tatively assigned to ν1and ν9,respectively, according to ob-served relative IR intensities. The ν4 (CH2 scissor) and ν10 (CH3deformation) modes were predicted to be 1405 and 1402 cm−1with IR intensities 14.2 and 7.9 km mol−1, respectively, but only one line at 1419.9 cm−1 was observed in this re-gion. The intense absorption of CH3C(O)Cl at 1425.1 and 1428.6 cm−1 and that of CH3C(O)C(O)CH3 at 1422.7 cm−1 might have interfered with the observation of the second line. We are, hence, unsure whether the assignment of the line
at 1419.9 cm−1 should be ν4 or ν10. However, according to the observed relative IR intensity and relative position of ν4 or ν10, we prefer to assign this line at 1419.9 cm−1 to ν10 and the line for ν4 might be interfered by absorption of CH3C(O)Cl and CH3C(O)C(O)CH3. The CCH bending (ν6) mode was predicted to be 1039 cm−1 with IR inten-sity of 9.7 km mol−1, but we observed no line in this region assignable to this mode. The reason for this absent line is un-clear, but less likely to be due to interference of parent absorp-tion, because we observed this mode in experiments on nei-ther CH3C(O)Cl nor CH3C(O)C(O)CH3, which has distinct parent absorption lines in this region. The average deviation between observed wavenumbers and predicted anharmonic vibrational wavenumbers for CH3CO is (14 ± 10) cm−1 with the largest deviation of ∼37 cm−1 for the ν3 mode at 1880.5 cm−1.
B. Assignment of lines in group XD
to the perdeuterated acetyl (CD3CO) radical
The deuterium-substitution experiments provide addi-tional support for the assignment of lines in group X to the acetyl radical. We show in Fig. S2(a) the inverted difference spectrum of the CD3C(O)Cl/p-H2matrix after secondary pho-tolysis with light at 532 nm [Fig.4(c)]; the matrix was initially irradiated with light at 248 nm for 30 min before this step. The spectrum is compared with the IR spectra of CD3CO, •CD2C(O)Cl, and CD3Cl, presented in traces (b)−(d) of Fig. S2, simulated according to predicted anharmonic vibra-tional wavenumbers and IR intensities. The agreement be-tween the observed spectrum and the simulated spectrum of CD3CO further supports our assignments of lines in group X (XD) to CH3CO (CD3CO).
In Table I we compare the vibrational wavenumbers of lines in group XD with the harmonic and anharmonic vibrational wavenumbers of CD3CO calculated quantum-chemically. The assignments were made according to This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
predicted wavenumbers and IR intensities without ambiguity. A line of ν2 predicted near 2152 cm−1 was unobserved be-cause of interference from absorption of CO. The missing line of ν6in CH3CO appeared at 889.1 cm−1. The calculated iso-topic ratios, defined as the ratio of vibrational wavenumbers of deuterated species to that of the natural species, are within 0.5% of observed values except for ν5for which observed ra-tio of 0.791 is much smaller than the predicted value of 0.845. The reason is that the predicted anharmonicity for this mode of CD3CO is negative but that for CH3CO is positive. If we use the harmonic vibrational wavenumbers, the predicted iso-topic ratio of 0.798 becomes much nearer the experimental value.
Jacox observed a line at 1855 cm−1 in the D-substituted experiments and tentatively assigned it to CD3CO.13 We observed a characteristically intense line in group XD at 1866.2 cm−1. Considering the observed redshift of 6 cm−1 for CH3CO from p-H2to Ar matrices, the redshift of 11 cm−1 for CD3CO is slightly large, but not unacceptable. This line near 1855 cm−1 was the only one reported for CD3CO; we observed seven additional lines in this experiment.
Three weak lines observed at 2029.0, 1929.1, and 1914.2 cm−1 in group XD correspond to no predicted fun-damental vibrational wavenumber of CD3CO. The line ob-served at 2029.0 cm−1 might be tentatively assigned to the first overtone of ν4(fundamental mode at 1029.7 cm−1) or a combination band of (ν4+ ν10), whereas lines at 1929.1 and 1914.2 cm−1 might be assigned to the combination bands of (ν5+ ν6) and (ν4+ ν6), respectively.
C. Assignment of lines in group Y to the CH3-CO complex
As discussed previously, reactions among CH3, CO, and I might occur upon UV photolysis of a CH3I/CO/p-H2 ma-trix at 248 nm; possible new products are CH3CO, CH3 -CO complex, I-CO, I2, and C2H6. Intense lines of CH3 and weak lines of C2H6 were readily identified upon photoly-sis of the CH3I/CO/p-H2matrix sample. In experiments with CH3C(O)Cl/p-H2 matrices, CH3CO was identified with a characteristic intense line at 1880.5 cm−1and four moderately intense lines at 2989.1, 1419.9, 1323.2, and 836.6 cm−1, as discussed in Sec.V A. The intensities of observed new lines in group Y increased significantly upon annealing of the UV-irradiated matrix of CH3I/CO/p-H2; these lines do not corre-spond to those of CH3, C2H6, or CH3CO.
Among the observed lines in group Y, two weak features at 3164.5/3165.7 and 1396.4/1397.6 cm−1and an intense line at 613.0 cm−1 have vibrational wavenumbers only slightly smaller than those observed for CH3 at 3170.6/3171.6, 1401.7/1402.7/1402.4, and 624.3/623.1 cm−1,42 indicating the presence of a perturbed CH3moiety. The intense line ob-served at 2150.1 cm−1in group Y has a vibrational wavenum-ber slightly greater than those of CO observed in the range of 2138−2146 cm−1in solid p-H2, indicating a perturbed moi-ety of CO. These lines in group Y, enhanced upon annealing of the UV-irradiated CH3I/CO/p-H2 matrix, are hence likely due to a CH3-CO complex produced from the reaction CH3 + CO. Lines of the CH3 moiety were observed to be
red-shifted from those of isolated CH3 by∼6, 5, and 11 cm−1 for lines at 3164.5/3165.7, 1396.4/1397.6, and 613.0 cm−1, whereas the line of the CO moiety at 2150.1 cm−1 was blue-shifted by 9.5 cm−1from the Q(0) line of CO at 2140.6 cm−1. The observed and quantum-chemically predicted vibra-tional wavenumbers of CH3 and CH3-CO are compared in TableII. Quantum-chemical calculations using the B3LYP/6-311G (2d,2p) method predicted that the two most intense IR lines of the CH3-CO complex have anharmonic vibrational wavenumbers at 2190 and 669 cm−1and four weaker ones at 3144, 3142, 1380, and 1379 cm−1; our observed value devi-ates within 8.0% of these predicted values. Predicted redshifts of 4−5 cm−1 for lines near 3165 and 1397 cm−1 are in sat-isfactory agreements with experimental observations, but the line at 613 cm−1, observed to have a redshift of 11.1 cm−1 from that of CH3, was predicted to have a redshift of ∼30 cm−1. This effect might be due to the large negative an-harmonic corrections from 504 to 703 cm−1for CH3and from 517 to 669 cm−1for CH3-CO; a more sophisticated method is needed to describe properly the potential-energy surface cor-responding to this vibrational mode. Observation of a single line at 2150.1 cm−1slightly blue-shifted from the rotational lines of CO is consistent with the fact that upon complex for-mation CO does not rotate and the C=O bond is strengthened. A similar blueshift of the CO line was observed in the com-plex Cl2-CO with the carbon atom as the interacting site; in contrast, a redshift was observed for the complex CO-Cl2with the oxygen atom as the interacting site.71
D. Mechanism for photolysis of CH3C(O)Cl
in solidp-H2
Five primary channels are energetically accessible at 248 nm (482 kJ mol−1): CH3C(O)Cl→ CH3CO+ Cl, (1) → CH2CO+ HCl, (2) → CH3Cl+ CO, (3) → CH3+ ClCO, (4) →•CH2C(O)Cl+ H. (5)
The experimental dissociation energy of the C−Cl bond, D0(C−Cl), is 350 kJ mol−1,72 which is near the theo-retical values of 349 and 345 kJ mol−1 calculated with the UCCSD(T)/CBS//UCCSD(T)/aug-cc-pVTZ27 and the G4//B3LYP/6-311++G(3df,2p) methods.73Experimental
en-thalpies of reaction at 298 K for channels (2)−(4) were re-ported to be 103, 50, and 367 kJ mol−1.60 According to CCSD(T)/cc-pVTZ//B3LYP/6-311G (d,p) calculations, chan-nels (2) and (3) involve reaction barriers of heights 190−309 and 364 kJ mol−1, respectively.29 Ho
f of •CH2C(O)Cl at 0 K was reported to be −26 kJ mol−1 and the C−H bond energy in CH3C(O)Cl is ∼422 kJ mol−1,74 consis-tent with the value of 402 kJ mol−1 predicted with the
CCSD(T)/cc-pVTZ//B3LYP/6-311G(d,p) method.29 Accord-ing to molecular-beam experiments, upon irradiation of CH3C(O)Cl at 248 or 235 nm, the primary dissociation chan-nel is the C−Cl bond fission;20,27 about 30% of the CH
3CO product dissociates further to CH3 + CO.24,25 Because en-ergy quenching is expected to be facile in solid p-H2, likely a greater fraction of CH3CO radicals is stabilized in our experiment.
The mixing ratios of CH3C(O)Cl and photolysis prod-ucts CH3CO, CH2CO, HCl, CH3, and CO as a function of photolysis period, presented in Fig. 3, indicates that the fis-sion of the C−Cl bond leading to the formation of stabilized CH3CO and Cl (reaction (1)) is the primary channel. The spin-orbit transition of Cl at 943.7 cm−1was unobservable be-cause of its small absorption cross section of 9.5× 10−26km molecule−1.75 The growth patterns of HCl and CH2CO are similar to that of CH3CO, indicating that they might be pro-duced directly upon photo-irradiation, but the possibility of formation due to secondary photolysis or reaction could not be definitively excluded because of the nature of our exper-iments using continuous radiation for photolysis. We iden-tified no CH3Cl;58 because CH3Cl cannot be photolyzed at 248 nm, this result indicates the absence of reaction (3). The product ClCO in reaction(4)was reported to absorb at 1880.4 cm−1 in solid p-H2 (Ref. 40) and at 1884.6 cm−1 in the gaseous phase,69 but the absorption of CH
3CO at 1880.5 cm−1 interfered with this observation. No absorption line of •CH2C(O)Cl, predicted to have characteristic lines near 1682, 1136, and 602 cm−1, was identified.
The formation of CH3CO increased more rapidly ini-tially, but the increase became slower subsequently. Consid-ering that the initial concentration of CH3C(O)Cl was 210 ± 40 ppm, and that the decay was only ∼14 ppm after ir-radiation for 2 h, CH3CO likely underwent secondary pho-todissociation or recombination with Cl to yield CH3C(O)Cl and other products. This condition is rationalized by its UV absorption cross section of ∼1 × 10−18 cm2 molecule−1 at 248 nm.5The intensities of lines of products HCl and CH
2CO also increased with behavior similar to those lines of CH3CO, but the small intensities defy a definitive confirmation of such behavior.
To have an acceptable S/N ratio and to minimize sec-ondary processes as much as possible, we chose an irradiation period of 30 min to estimate the mixing ratios of CH3CO, CH2CO, CO, and CH3 as approximately (4.5 ± 0.6), (1.15 ± 0.1), (0.20 ± 0.03), and (0.5 ± 0.1) ppm, respectively; the corresponding branching ratios of (0.71 ± 0.09), (0.18 ± 0.01), (0.03 ± 0.01), and (0.08 ± 0.02) were estimated. The error limits reflect only one standard deviation among results obtained from several lines of each species. The sys-tematic error might be as large as twice the estimated values for CH3CO and CH3 because theoretically predicted IR in-tensities were used and we assumed that all products were primary photofragments. Because of the limitations posed by the nature of our experiments, the branching ratio for channel (2), 0.7± 0.1, should be taken as a lower limit because some CH3CO might dissociate upon irradiation or reacts further with Cl or other species. The estimated yield for formation of CH2CO and HCl, 18%, should be taken as an upper limit for
reaction (2) because of possible secondary reaction to form CH2CO. The most significant result is that CH3CO is the ma-jor product upon irradiation of the CH3C(O)Cl/p-H2 matrix, in contrast to the observation of only the HCl-CH2CO com-plex when CH3C(O)Cl isolated in an Ar-matrix was irradiated with light at 248 nm.16Our observation of CH
3CO rather than the HCl-CH2CO complex as a major product demonstrates again the advantage of a diminished cage effect of p-H2 to produce free radicals via photolysis in situ.
E. Mechanism for formation of the CH3-CO complex
The purpose of the CH3I/CO/p-H2 photolysis experi-ments was to investigate the products of the reaction of CH3 + CO. In the gaseous phase, photolysis of CH3I at 254 nm produces CH3with translational energy of∼134 kJ mol−1.76 This translational energy of CH3 corresponds to ∼87 kJ mol−1 in the center-of-mass coordinates of the CH3 + CO system, greater than the barrier of 25−27 kJ mol−1for the for-mation of CH3CO.7,33,34The internal energy of CH3was also estimated to be 42 kJ mol−1.76 The observation of only CH
3 -CO, not CH3CO, upon UV irradiation of the CH3I/CO/p-H2 matrix indicates that the excess energy of CH3 upon photol-ysis of CH3I was readily quenched in solid p-H2 so that the barrier for formation of CH3CO was not overcome. The ki-netic energy of the CH3+ CO system was near zero upon an-nealing so that the barrier could not be overcome either. The formation of the CH3-CO complex is barrierless so that the reaction of CH3+ CO → CH3-CO is feasible upon annealing of the UV-irradiated matrix. Bennett et al. irradiated CH4/CO in ice at 10 K with electrons at 5 keV to generate CH3 radi-cal for the reaction CH3+ CO and also failed to observe the formation of CH3CO.34
We have performed several bimolecular reactions in solid p-H2 to produce free radicals.35 Most reactions are barrierless, for example: Cl + CS2 → ClSCS,77 CH3 + SO2 → CH3SO2,42 and reactions of Cl with CH3CH=CH2, t-butadiene, C6H6, and C5H5N to form •CH2CHClCH3,78 •(CH2CHCH)CH2Cl,79 σ-complex ClC6H6,80 and C5H5 N-Cl,80respectively. A small barrier∼2 kJ mol−1was predicted
for the reaction of Cl with C2H4and C2H2, and reaction prod-ucts CH2CH2Cl (Ref. 81) and C2H3Cl + CHClCH3 were observed.82 Apparently, the barrier height of ∼27 kJ mol−1 was too large for reactions to occur in solid p-H2. This condi-tion indicates a limitacondi-tion to the applicacondi-tion of photo-induced bimolecular reactions to produce free radicals in solid p-H2 such that only reactions with negligible barriers can occur within these low-temperature solid p-H2environments.
VI. CONCLUSION
Irradiation at 248 nm of a CH3C(O)Cl/p-H2 matrix at 3.2 K produced new features at 2990.3, 2989.1, 2915.6, 1880.5, 1419.9, 1323.2, 836.6, and possibly 468.1 cm−1 that are assigned to the CH3CO radical; we extended pre-vious observation of only three most intense lines to eight lines. When a matrix of CD3C(O)Cl/p-H2was used, lines at 2246.2, 2244.0, 1866.1, 1046.7, 1029.6, 1027.5, 889.1, and This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
723.8 cm−1were observed and assigned to CD3CO; all lines except that at 1866.1 cm−1 are newly observed. When a ma-trix of CH3I/CO/p-H2 at 3.2 K was irradiated at 248 nm and, subsequently, annealed at 5.1 K, new lines at 3165.7, 3164.5, 2150.1, 1397.6, 1396.4, and 613.0 cm−1 were ob-served and assigned to the CH3-CO complex. All spectral as-signments are based on their photochemical behavior, avail-able D-isotopic shifts, and comparison of observed wavenum-bers and intensities with calculations.
The observation of CH3CO radical as the major product from photolysis of CH3C(O)Cl serves as an additional exam-ple to illustrate that solid p-H2 has a diminished cage effect such that isolated CH3CO radicals and Cl atoms were pro-duced upon UV photolysis of CH3C(O)Cl, in contrast to ex-periments in the Ar matrix in which only CH2CO and HCl were observed. Similarly, the formation of CH3radical from CH3I and its subsequent reaction with CO to form a CH3-CO complex also demonstrate the diminished cage effect of solid p-H2. Observation of a CH3-CO complex but not the CH3CO radical upon photolysis of a CH3I/CO/p-H2 matrix indicates that the kinetic energy of CH3upon photolysis of CH3I was readily quenched so that the barrier height of∼27 kJ mol−1 for the reaction of CH3+ CO could not be overcome. This re-sult indicates that the application of photo-induced bimolec-ular reactions to prepare free radicals in solid p-H2is limited to reactions with negligible barrier.
ACKNOWLEDGMENTS
Ministry of Science and Technology, Taiwan (Grant No. NSC102-2745-M009-001-ASP) and Ministry of Education, Taiwan (“Aim for the Top University Plan” of National Chiao Tung University) supported this work. The National Center for High-Performance Computation provided computer time.
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