J. Phys. Chem. 1988, 92, 1495-1502
-
on the above values of lu*I and P,and the Eckart function parameters anl = 2rVl/hclw*l = 2.7, a2= 2rVz/hclw*l = 13.2, where VI and V2 refer to the potential barrier of forward and reverse reaction and are evaluated from VI = P - E,(C-H), V, = P lMool - E,(C-H), and Moo = Doo(C-H) - DoO(H-C1) and Kw(300) is from Table VI. The agreement between quite surprising for the fairly large lu*l value and was not to be anticipated based on the prescriptions in Johnston's booki0 (p 200). Qualitatively similar results were obtained by Jeong and Kaufman,12 though in their case the computed imaginary frequencies for hydrogen abstraction by OH were substantially lower. We note, however, that the criterion for separability of the reaction c o ~ r d i n a t e ' was ~ , ~ ~not met in either case. If the Wigner transmission coefficient is indeed applicable, its inclusion into the Arrhenius expression leads to an apparent lower activation energy
but the effect is relatively small. This function goes through a minimum at about lu*l 5 when AW/A= 0.736. Again, formally, for Iu*l > 10, AW/A> 1. For the selected sample calculation with w*/i = 1132 an-'eq 13 and 14 predict a lowering of the activation energy and preexponential factor by AE = E'" - E = -0.64* kcal mol-', AW/A= 0.75 at 273 K and AE = -0.656 kcal mol-', AW/A = 0.74 at 373 K which corresponds to an increase in the rate constant by factors of 2.5 and 1.8, respectively. It is clear that, in Arrhenius plots, the local effect of tunneling a t a given temperature is dominated by the decrease in the activation energy and produces an appreciable increase in k. However, the net changes in AE and AW/Abetween 273 and 373 K are too small to be experimentally ascertainable. W e therefore conclude that given the still questionable validity of the expression for Kw( T ) for Iu*l values in the neighborhood of 4-6, and the sensitivity of BEBO calculations on the input data, attempts at tunneling corrections to Arrhenius parameters over a relatively narrow temperature range are probably not warranted for this class of reactions.
+
E'" = E - R T ( l ~ * 1 ~ / 1 2 ) / ( + 1 lu*I2/24)
(13)
-
since the second term is negative for a nonzero value of lu*l and 0 (formally it approaches approaches zero in the limit as lu*l but then the expression is certainly not valid). -2RT as lu*l At a given T Wigner's tunneling correction predicts a lowering of the A factor for lu*l < 10 as given by the relation
-
Aw/A = P(T)exp[-lu*12/12Kw(T)]
1495
Acknowledgment. The financial support of the Natural Sciences and Engineering Research Council of Canada is gratefully acknowledged. Registry No. CH3Br,74-83-9; CH2Br2,74-95-3; CH2BrC1,74-97-5; CH2C12, 75-09-2;CHZCIF, 593-70-4.
(14)
Photochemlcal Processes on Chloromethyl Formate and Vlnyl Formate in Low-Temperature Matrices: Infrared Spectra and ab Initio Calculations on Chloromethanol and Vinyl Alcohol Henrik Kunttu,* Martti Dahlqvist? Juhani Murto, and Markku Rasanen Department of Physical Chemistry, University of Helsinki, Meritullinkatu 1- C, SF-001 70 Helsinki, Finland, and Department of Chemistry and Biochemistry, University of Turku, SF-20500 Turku. Finland (Received: April 21, 1987; In Final Form: September 8, 1987)
--
The UV photodecomposition of chloromethyl formate and vinyl formate has been studied in low-temperaturenoble gas matrices at wavelengths between 200 and 260 nm. Two distinct channels in cage photolysis were observed: (i) CIHzCOCHO ClH2COH + CO; HzCCHOCHO HzCCHOH CO. (ii) CIHzCOCHO HzCO HC1+ C O HzCCHOCHO CH3CH0 + CO. There is a well-established wavelength dependence influencing the product ratios in these photochemical processes. Decomposition due to irradiation at wavelengths near 250 nm prefers channel i, where the formic acid esters decompose to the corresponding alcoholic species, chloromethanol (a new compound), and vinyl alcohol. On the other hand, at shorter wavelengths, channel ii dominates. Neither chloromethanol nor vinyl alcohol were observed to decompose at wavelengths above 200 nm. The photoprocesses of vinyl formate were also studied in NO-doped Ar matrices as well as in solid Xe in order to get information concerning the mechanisms of photochemical decomposition of formic acid esters. The assignment of the vibrational spectra is based on ab initio calculations performed at the Hartree-Fock 6-31G** level for chloromethanol and at the MP2/6-31G** level for vinyl alcohol. A detailed vibrational analysis is given for chloromethanol, chloromethanol-0-d, and vinyl alcohol.
-
Introduction We have previously reported the UV-induced conformer interconversion process of chloromethyl formate (ClH2COCHO; CMF) in solid Ar and have characterized the vibrational spectrum of its lowest energy conformer Z,sc as well as that of the other stable conformer E,sc.' The conformer interconversion process between these species was i n d u d in a narrow wavelength region between 255 and 275 nm, and a photochemical 1:l conformer steady state was observed. Irradiation of C M F at shorter wavelengths was found to decompose this molecule. The purpose of this paper is to analyze the photoproducts of C M F as well as those of vinyl formate (H2CCHOCHO; VF). University of Turku.
0022-3654/88/2092-1495$01.50/0
-
+
+
Only a few formic acid esters have been studied in low-temperature matrices. Methyl formate conformers have been found to interconvert (cis to trans) upon So Sl(ar*)excitation.2 It was suggested that the resulting trans species decomposes in a consecutive photolysis step into stable products such as methanol and CO, acetic acid, methane, and COz.z As far as we know hydroxymethyl formate is the only other ester of formic acid that has been studied in low-temperature mat rice^.^ This unstable
-
(1) Rashen, M.; Kunttu, H.; Murto, J.; Dahlqvist, M. J . Mol. Struct. 1987, 159, 65.
(2) Muller, R. P.; Hollenstein, H.; Huber, J. R. J . Mol. Spectrosc. 1983, 100, 95. ( 3 ) Hawkins, M.; Kohlmiller, C. K.; Andrews, L. J . Phys. Chem. 1982, 86, 3154.
0 1988 American Chemical Society
1496 The Journal of Physical Chemistry, Vol. 92, No. 6, 1988
compound was synthesized in either pyrolysis or photolysis of the secondary ozonide of ethylene? Two conformers of hydroxymethyl formate were found, the trans conformer decomposing photolytically into formic anhydride and the cis species into a specific formaldehyde-formic acid dimer.3 Chloromethanol (ClH,COH; CM) is stoichiometrically equal to formaldehyde plus HCl. It has been suggested that C M is an intermediate in syntheses with formaldehyde + HC1.4 Chloromethanol has been proposed to be an intermediate in the reaction between O(lD) and CH3C15as well as in the reaction of CHzC12 OH-.6 There is only one report in the literature of a direct observation of CM: a mixture of formaldehyde and HCI was heated in the gas phase to ca. IO OC, and C M was concluded to be present in the mixture. This conclusion was based on the observation of absorptions a t 3650, 1080, and 700 cm-l.’ Since C M is one of the smallest substituted methanols, there is considerable interest in its properties, vibrational spectra, and conformers. In order to facilitate an interpretation of the observed spectra, we calculated its spectrum at the HF/6-31G** level. In this study it was found that, besides chloromethanol, formaldehyde is one of the main products in the photolysis of CMF. Formaldehyde complexes seem to be important intermediates in several reactions. For example, nitrosomethanol can be prepared in solid Ar from methyl nitrite, a complex between formaldehyde and H N O being the intermediate in the photolysis.* Also, when formaldehyde dimers are photolyzed in matrices, they give gly~olaldehyde.~ Vinyl alcohol (H2CCHOH; VA) is another unstable alcohol that we obtained by in situ photolysis. Its precursor was vinyl formate. There are two reports concerning the IR spectra of VA in matrices.I0J1 Hawkins and Andrews were able to trap VA in Ar by reacting ozone with ethene (Hg-arc irradiation).’O In another study Rodler et al.” produced VA in a low-pressure pyrolysis of cyclobutanol or 3-thietanol and trapped the products in Ar. There exist several ab initio calculations on VA. The ’~ agreement calculated geometry at the HF l e ~ e l ~is ~in, reasonable with the experimental 011e.l~ Also, the HF/4-31G calculated force constants of VA have been used to refine the general valence force field of the syn conformer of VA.” In the present study we have optimized the geometry and calculated the vibrational spectrum of VA at an improved level of theory in order to obtain a complete assignment of the observed spectra.
Kunttu et al.
.50/
C
+
Experimental Section C M F was synthesized as described earlier.’ According to the proton N M R spectrum the sample was more than 99% pure (a small amount of methyl chloroformate was detected, however, but this impurity was of no significance since we could not induce any processes by photolyzing a sample of methyl chloroformate). Deuteriated C M F (ClH,COCDO; CMF-CD) was prepared in a manner analogous to C M F by using deuteriated formic acid. The samples used were more than 99%deuteriated. Monomeric formaldehyde was prepared from paraformaldehyde by mild heating in vacuum and purified by trap to trap distillation.I6 Hydrogen chloride was obtained from E. Merck AG. A sample of “vinyl formate”, which was found to contain almost equal amounts of vinyl formate, vinyl acetate, and acetaldehyde, was (4) Vartanayan, S.A.; Tosunyan, A. 0.;Kostochka, L. Arm. Khim. Zh. 1967, 20, 110.
( 5 ) Lin, M. C. J . Phys. Chem. 1972, 76, 811. (6) Roberts, J. D.; Caserio, M. C. Basic Principles ofOrgunic Chemistry; Benjamin: Reading, MA, 1965; p 330. (7) Rossi, I.; Levy, A.; Haeusler, C. C.R. Acad. Sci., Ser. C 1967, 133. (8) Miiller, R. P.; Huber, J. R. J. Phys. Chem. 1983, 87, 2460. (9) Sodeau, J. R.; Lee, E. K. C. Chem. Phys. Lett. 1978, 57, 71. (10) Hawkins, M.; Andrews, L. J. Am. Chem. SOC.1983, 105, 2523. (11) Rodler, M.; Blom, C. E.; Bauder, A. J. Am. Chem. SOC.1984,106, 4029. (12) Plant, C.; Spencer, K.;Macdonald, J. N. J . Mol.Struct. 1986, 140, 317. (13) Bouma, W. J.; Radom, L. J. Mol. Struct. 1978, 43, 267. (14) Bouma, W. J.; Radom, L.; Rodwell, W. R. Theor. Chim. Acta 1980, 56, 149. (15) Saito, S. Chem. Phys. Lett. 1976, 42, 399. (16) Spence, R.; Wild, W. J. Chem. SOC.1935, 338.
Figure 1. Transmission of the interference filters (a and b) and the long-pass filter (c) used in this study. Trace d displays the emission structure of our Hg arc.
SCHEME I: Matrix Cage Photochemical Proceases for Chloromethyl Formate I
0 200 < A < 280nm t----------)
1 : l equlibrium H
/
z . sc
255 nm
J
E, sc
no reaction
hvL HCI + 2 C O + H2 (D)
purchased from ICN and K&K Co. Vinyl formate was separated from this mixture by distillation in a Todd apparatus fitted with a 2-m column. The rare gas matrices were prepared as previously described.’ M I A ratios were between 500 and 1000. NO was used as a dopant to quench possible radical reactions and was purified by trap to trap distillation. The samples were photolyzed by using a 100-W mediumpressure Hg lamp (Illumination Industries Inc., USA) or a 50-W hydrogen discharge lamp (Beckman). A 5-cm water filter was used with the H g lamp to prevent I R irradiation from heating the sample. The irradiation wavelength regions used are shown in Figure 1. In filter experiments a long-pass filter WG 290 (Schott Glaswerke, Mainz, West Germany) as well as interference filters with their transmission centered at 270 and 240 nm of 15 nm half-bandwidth (from Omega Export Inc., USA) were used.
Results and Discussion Computational Methods. The a b initio calculations were performed with the GAUSSIAN 82 program” on the Finnish State Computer Centre V A X 8600 and IBM 3090 computers. The basis set used was split-valence 6-31G** containing d-type polarization functions on C and 0 atoms and p-type polarization functions on (,l7). Binkley, J. S.; Frisch, M. J.; DeFrees, D. J.; Raghavachari, K.; Whiteside, A.; Schlegel, H.B.; Fluder, E. M.;Pople, J. A. Department of Chemistry, Carnegie-Mellon University, Pittsburgh, PA.
The Journal of Physical Chemistry, Vol. 92, No. 6, 1988 1497
Photochemical Processes on Formates SCHEME II: Matrix Cage Photochemical Processes for Vinyl Formate
2500 I
I
I
203 I
I
I
I
I
1500 I
I
l
l
1
km-')
1000 I
I
I
I
I
l
l
1
.H H,
/H
H
y
+co
(
H
' T\1 slow hU
CH4
+ 2CO
TABLE I: Ab Initio HF/d31G** Results for the Gauche Conformer of Chlorometbanol'
6-31G*** rl QH 94.4 r, HIC 107.6 r3 H2C 108.0 r, CO 136.3 r5 CC1 180.9 HICO 108.3 a2 HZCO 113.4 aj HICHI 110.9 aq HICCl 106.4 a5 H2CCI 105.3 a6 OCCl 112.3 COH 110.6
local sym coord SI= Ar4 S2 = Ar, S3 = Ars S4 = (Ar, + Ar3)/21/, Ss= (Ar2- Ar3)/2lIZ s 6 S7
Aa7
perimental results of CM, they were multiplied by constant factors
= Pa6 (one for stretchings and another for bendings and torsions) as SB= (4Aa3 - A a , - ACY,- A a 4 - A ( U ~ ) / ~ O ~ / ~ indicated in Table IV (V), giving the optimal fit. No reduction Sp= (Pa, + Pa, - Am4 - Aas)/2 was used for the MPZcalculated wavenumbers of VA. Slo= ( h a l - Aa, - Aa4 + Aa5)/2 The HF/6-31G** calculation on C M did not give any minima Sll= (Aal - Pa, + Act4 - Aa5)/2 for the trans conformer, and the results given refer to the gauche SI2
(Aal
red. T
Figure 2. UV (Hg arc) induced changes in the spectrum of chloromethyl formate-C-d in Ar. A: conformer Z,sc. B: mainly a mixture of conformers Z,sc and E p c . C : spectrum of primary photoproducts chloromethanol-0-d + CO and formaldehyde + DC1 + CO. D: results of prolonged UV irradiation showing decomposition of formaldehyde.
+ Aa2 + Aa3 + Aa4 + Aa5 + Aa6)
HOCH, 177.6 S13= AT
"The bond lengths are in picometers, the angles are in degrees, and the dipole moment ( p ) is in debyes. Included are also the local symmetry coordinates of methanol, which were used in the calculations. See also Scheme I. bE,ot = -573.953 185 5 au; p = 2.111.
H atoms. All geometries were optimized according to gradient methods. Force constants were calculated at the optimized geometry as analytical second derivatives of the SCF energy. Electron correlation effects were included by second-order Moller-Plesset perturbation theory (MP2(Fu11)).I8 In the post-HF calculations force constants were evaluated numerically. A separate program was written to transform the initial Cartesian force field (output of GAUSSIAN 82) to local symmetry coordinates and to calculate the potential energy districution (PED) (cf. also ref 19). The atom numbering for C M and VA is given in Schemes I and 11, respectively. The optimized geometry, internal coordinates, and local symmetry coordinates used for C M and VA are given in Tables I and VII, respectively. The nonreduced ab initio force fields are available from the authors upon request. The wavenumbers calculated at the Hartree-Fock S C F level are ca. 10% too large.*O To allow for comparison with the ex(18) Pople, J. A.; Binkley, J. S.; Seeger, R. Int. J. Quantum Chem. Symp. 1976, 10, 1.
(19) Hamada, Y.; Hashiguchi, K.; Hirakawa, A. Y.; Tsuboi, M.; Nakata, M.; Tasumi, M.; Kato, 9.;Morokuma, K. J . Mol. Spectrosc. 1983, 102, 123 (20) Fogarasi, G.; Pulay, P. In Vibrational Spectra and Structure; Durig, J. R., Ed.; Elsevier: Amsterdam, 1985; Vol. 14.
species. Also we could not experimentally detect any features that could be attributed to another conformer of CM. No search for conformers was attempted for vinyl alcohol, and our calculations refer to the planar syn conformer. This is the only experimentally observed one in the gas phase.15 There has not been any indication to date of the existence of a second conformer of VA in the matrix spectra.,' The geometry at the MP2/6-31G** level does not differ much from the geometry calculated p r e v i ~ u s l y . ' ~However, ~'~ it is to be noted that the calculated value for the angle CCH, obtained in this study is 122.7', which is close to the experimental value reported by 5aitol5 but differs more than 6' from the unexpectedly large value reported in another microwave The internal rotation of vinyl alcohol has been studied extensively (cf. ref 12 and 22 and references cited therein). Calculations made in these studies predict that the planar syn conformer is of lowest energy, the energy difference between the syn and anti conformers being ca. 7-8 kJ mo1-'.'2,22 In connection with trapping higher energy conformers in low-temperature matrices, the energy barrier between the higher energy and lower energy species is relevant. For the anti conformer the barrier height has been estimated as 5-10 kJ mol-'.12*22 If the potential energy barriers from higher energy conformers to lower energy ones are low (e.g., a t 13-15 K lower than ca. 4 kJ mol-] (ref 23)), they can be surmounted thermally in matrices. The process anti
-
(21) Rodler, M.; Bauder, A. J. Am. Chem SOC.1984, 106, 4025. (22) Nobes, R. H ; Radom, L.; Allinger, N L J . Mol. Struct. 1981, 85, 185. (23) Barnes, A. J. J . Mol. Struct. 1984, 113, 161
Kunttu et al.
1498 The Journal of Physical Chemistry, Vol. 92, No. 6,1988 TABLE II: Observed Photoproduct Bands for Chloromethyl Formate in A P vObsd/cm-l assignt lit. value uobad/cm-’assignt lit. value 1246 B 1245.2f 3591 A 3498 2981 2971 2913 2894 2830 2801 2593 2343 2154 2144 2140 sh 1726 1720 1498 1443 1429 1393 1368 1361 1355 1323 1319
A, dim. A A A
B B
C B CO2
C B
CO B Be B A A A
D D D A A
29OOb 2835.3; 2830.7* 28 12.lC 2591b 2344d 2158d 2143d 1727.3; 1725.8b 1496.4‘
A
1231 1218 1 I89 1177 1154 1114 1096 1036
v,,/cm-l
assignt
A
B
1178.1b
D A
A Dg
I
I 469 I 463 456 664 669 530
1
A
D A?
C02
666.5d
A
D A
B
431’
A
B?
C
“The symbols refer to species: A, C1H2COH + CO; B, HzCO + HCl + CO; C, HCI + CO + CO; D,unspecified product. bReference 26. CReference30. dReference 3. ‘Due to dimeric formaldehyde. /Reference 42. g Possibly due to methanol, produced from dimeric CMF.
syn comprises of the motion of the light OH group of VA; it is then possible that due to tunneling the effective potential barrier is somewhat lower than the calculated one. These factors may contribute to the failure to observe higher energy conformers of VA in matrices at temperatures of ca. 15 K.
Spectral Results Photolysis of Chloromethyl Formate. Figure 2 shows the evolution of the spectrum of the acyl-deuteriated compound CMF-CD upon UV irradiation. The collected wavenumbers of the observed photoproduct bands are shown in Tables I1 and I11 for C M F and CMF-CD, respectively. There is a dependence in the product yield ratio on photolysis wavelength. As will be shown shortly, the primary products are chloromethanol (complexed with CO) and formaldehyde (complexed with HCl and CO) (see also Scheme I). The yield of formaldehyde increases on irradiation with shorter wavelengths. In the UV absorption spectrum of C M F there are two bands in the region between 200 and 300 nm in solution: a very weak band at ca. 280 nm and a very strong one at ca. 220 nm. In practice, then, while using either a H g arc of H2 lamp as the photolysis source, one can excite the molecule into two different electronic states which then lead to different products. C M F is trapped into the matrix initially as conformer Z,sc, but a photochemical steady state between species 2,sc and E,sc results upon irradiation with long-wavelength UV (without decomposition).’ For this reason we examined the possibility of finding conformer selectivity in the photodecomposition of CMF. No such dependence was observed for the products. Two sets of bands grew at approximately equal rates, the most indicative of these sets being at 3591 and 2593 cm-I. The first of these occurs in the region typical for the OH stretching absorption for alcohols. Other significant bands were found at 1096 and 372 cm-I, which are typical for the C O stretching and OH torsional absorption of an alcohol. In the photolysis of CMF-CD the corresponding bands were found at 265 1, 1085, and 3 17 cm-I and were considered to be due to an ad-deuteriated alcohol. The photolysis of both C M F and CMF-CD were accompanied with the production of CO. Since C M F is stoichiometrically equal to chloromethanol + CO, these bands prompted us to perform ab initio calculations on the vibrational spectrum of chloromethanol.
2974 2980 2912 2895 2883 2830 2810 2802 265 1 2642 2590 2343 2155 2145 2030
lit. value
vobd/cm-’ assignt lit. value
A A?
3591 2996
D
959 965 887 793 689.5
368 372 341 247
TABLE III: Ohewed Photoprodwet Bands for Chloromethyl Formate-C-d in A P
1
1282? 1249 1177
B B C
29OOb
C
2835.3; 2830.7b
1
E
28 12.l C
1
B
11031 1085 1098 sh
D CO2 F F
I 1
C C
2591b 2344d 2158d 2034c
C C D C
12459 11748
A
1055 10571 1049 1029’
1031 1035}
D
1877 1894 1724 1704 1498 1469? 1395 1326 1322
B
948 872
B? B
660 664}
B
464 469}
A
433 br
C
367 317 248
A
431b
1719’ 1496.4b
B
D
1396
B
B E
247e
“The symbols refer to species: A, ClHDCOH + CO, B, CICHzOD + CO; C, HzCO + DCI + CO; D, HDCO + HCI + CO; E, HCI + CO + CO; F, DCl + CO + CO. bReference 26. ‘Reference 30. dReference 3. CReference43. /Reference 44. #Reference28.
TABLE I V Obsened Wavenumbers (cm-I) for Chloromeulurol in Ar and Their Camparison with the 6316**Calculated, Reduced
Wavenumbers“
wavenumbers exptl
calcd
3591
3702
PED^ 100 (2)
297 1 2981}
2981
87 (5), 12 (4)
2913 1393
2893 1410
87 (4), 13 (5) 87 (8), 8 (9)
1323} 1319 1231 1114 1096 959
1300
41 (6), 30 (9), 19 (10)
1268 1112 1102 903
68 60 85 82
(9), 10 (6), 13 (10) (lo), 31 (6) (l), 8 (6) ( l l ) , 12 (6)
approx description uOH
GH2 v*CH2 dCH2 dCOH CH2 wag CH2 twist
vco
669) 664
659
73 (3), 22 (7)
CH2 rock YCCl
469} 463
396
33 (7), 48 (13), 15 (3)
6ClCO
321
48 (13), 37 (7), 10 (3)
rOH
izi}
“The reduction factors are 0,887 for stretchings and 0.853 for bendings and torsion. bPotential energy distribution. The numbers within the parentheses refer to the local symmetry coordinates (Table I). PED contributions smaller than 5% have been omitted. The experimental and calculated wavenumbers, together with the potential energy distribution based on the ab initio calcplated force field, are listed in Tables IV and V. Discrepancies between the experimental and calculated values occur in the OH stretching and torsional wavenumbers. It is well-known that complexation of alcohols with CO in rare gas matrices shifts the OH stretching band to lower and the OH torsion band to higher wavenumbers from the values for the uncomplexed molecules.24 In addition, the torsion is quite anharmonic and the calculated harmonic value may not be very reliable. For methanol, several of the alcoholic bands were found to be split into doublets due to complexation with CO: presumably this indicates the presence of two different (24) Murto, J.; Ovaska, M. Spectrochim. Acta, Part A 1983, 39A, 149.
The Journal of Physical Chemistry, Vol. 92, No. 6, 1988 1499
Photochemical Processes on Formates
TABLE VI: Vinyl Formate: Matrix Infrared Wavenumben (cm-')
TABLE V Experimental and Calculated Wavenumbers for Chloromethanol-0-8 wavenumbers exptl 2974 2980
1
2912
2981
1049 872
}
410} 403 317
gasb 3122.3 2941.5
1734 FR 1757}
1759.9
1735 1745}
1798
1651 1388 1376 1296 1197 1168 1113
1667.1 1409 1371.2 1296
1649
1653
1102
93 (1)
vco
991
48 ( l l ) , 25 (6), 23 (10)
CHI rock
804
58 (a), 35 (11)
967 959
73 (3), 19 (7)
1 g79 1 873
964.3
653
6COD UCCl
369
59 (7), 21 (3), 14 (13)
6ClCO
801 712
821.5 705.0
249
81 (13), 13 (7)
TOD
1276
1410
86 (4). 13 ( 5 ) 100 (2) 87 (8), 8 (9)
1175.3 1106.2
liquid' 3124 2949
SR,SVIAI
sp,aplAr 31 10 2963 2917
82 (lo), 10 ( l l ) , 7 (6)
1
;A;}
literature
1213
I
2697
1326 1322 1282?
approx description
v.CH2 &H2 uOD 6CH2 CHI wag CH2 twist
2892
2651 2642 1469?
664 658
PED
calcd
for the Fundamentals of the Most Stable Conformer sp,ap Compared with Literature Values"
1373 1295 1161
1136
864 807 714 62 1
" Included are also the observed wavenumbers for species sp,sp. *Reference 34. "Reference 36.
"See Table IV, footnotes a and b.
orientations of the CO molecule with respect to the O H group of methanol.24 As can be seen in Tables IV and V, several of the bands attributed to C M (or CM-OD) are doublets, and we have interpreted them as having arisen from different C O complexes of CM. The main alcoholic photoproduct for CMF-CD was the O-ddeuteriated chloromethanol. In addition to this, a small amount of the OH species was also formed. This product cannot have originated from CMF, since the degree of deuteriation was very high. The structure of this species is most probably CIHDCOH, and its OH stretching absorption is at exactly the same wavenumber as that for CM. For deuteriated methanols, deuterium substitution of the methyl protons did not affect the position of the OH stretching band.25 As there was only a small yield of CIHDCOH, we could identify only four bands for it. However, it is interesting to note that in the photolysis the hydrogen atom shift is not only limited to the preferred one between the acyl and ester groups. No decomposition of C M was observed over prolonged UV irradiations at wavelengths greater than 200 nm. The 2593-cm-' band is in a region that is unusual for simple undeuteriated molecules. However, for the system H2C0 + HCl the HCl stretching absorption has been found at 2591 cm-' in N2 matrices.26 On this basis the 2593-cm-' band belongs to HCl, shifted 277 cm-I lower from the value for monomeric HC12' as a result of its complexation with formaldehyde and CO. The influence of C O on the position of the HCl absorption in this complex is small. The C=O stretching absorptions for the formaldehyde monomer and dimer in Ar are at 1742 and 1738.3 cm-l, respectively.28 The value for HC1-complexed formaldehyde is 1725.8 cm-'.% For the ternary formaldehyde system obtained in the present study the C=O stretching absorption is at 1726 cm-I. We found evidence of the presence of dimeric formaldehyde during photolysis of more concentrated samples of CMF; the C = O stretching band at 1720 cm-' has been assigned to this photoproduct. Of the remaining absorptions, the 1498-cm-' band is very characteristic of the presence of formaldehyde-we found that complexation influenced its position very little. It has been found that the photodecomposition of H 2 C 0 is mainly effected via the cage dimer? producing methanol CO
+
as well as glycolaldehyde. In experiments with formaldehyde we indeed found that the monomeric species is not affected in photolysis while the dimeric species was reacting. Also the ternary formaldehyde-CO-HC1 system decomposes in a slow process giving HC1 (CO),. This supports the idea that some other molecule is needed to catalyze photochemical processes of formaldehyde.29 The bands attributed to the products of the slow process were found at wavenumbers of 2801,2154, and 247 cm-I in Ar. In Ar matrices, by varying the concentrations of HCl and CO, the HCl stretching absorption band for the system HCl (CO),has been found at 2812.1 cm-I with additional bands at 2805 and 2798 c ~ - ' . ~ O In N2a librational band has been found at 431 cm-' for HCl complexed with formaldehyde;26in our case the 247-cm-' band may correspond to librational motion in HC1 + (CO),. In addition to the assigned bands of the photolysis products of C M F we found, especially in concentrated matrices, one set of bands (marked by D in Table 11) produced in a relatively rapid process. It is probable that this set of bands originates from the photolysis of dimeric CMF, and the observed production of CO, may be connected with it. If this product came from monomeric CMF, the presence on COz would require chloromethane as its counterpart, but no chloromethane could be identified in the matrices. However, the amount of CO, produced in the photolysis is less than 10% in the concentrated matrices as compared with the products connected with CO. This estimate is based on the ca. 10-fold larger absorption coefficient of vl of CO, than that of C O found in the gas p h a ~ e . ~ ' In . ~ this ~ connection it is interesting to note that Hawkins et aL3also observed the production of C 0 2 in addition to production of CO, while photolyzing hydroxymethyl formate in Ar. According to these authors, C O was complexed with an unknown m ~ l e c u l e .The ~ results of the present study suggest that the unknown species might have been methanediol. The photolyzing of CMF-CD proceeded mainly in the channel leading to the alcohol chloromethanol-0-d, as described earlier. On the other hand, the photolysis leading to formaldehyde was
+
+
dominated by the production of DCl complexed with formaldehyde
and CO. In addition, small amounts of partially deuteriated (29) Kemper, M. J. H.; Hoeks, C. H.; Buck, H. M. J. Chem. Phys. 1981,
(25) Serrallach, A.; Meyer, R.; Gilnthard, Hs. H. J . Mol. Specrrosc. 1974, 52, 94. (26) Strandman-Long, L.; Nelander, B.; Nord, L. J . Mol. Struct. 1984, 117, 217. (27) Barnes, A. J. J . Mol. Struct. 1983, 100, 259. (28) Khoskhoo, H.; Nixon, E. R. Specrrochim. Acta, Part A 1973, 29A, 603.
74. 5744.
(30) Barnes, A. J.; Hallam, H. E.; Scrimshaw, G. F. Trans. Faraday Soc. 1969, 65, 3172. (31) Pugh, L. A.; Rao, K. N. In Molecular Spectroscopy; Modern Research; Rao, K. N., Ed.; Academic: New York, 1976; Vol. 11. (32) Herzberg, G. In Molecular Spectra and Molecular Srructure; Van Nostrand: New York, 1950; Vol. I.
1500 The Journal of Physical Chemistry, Vol. 92, No. 6, 1988
formaldehyde molecules were identified. The bands assigned to these species are listed in Table I11 and are compared with literature values where applicable. As chloromethanol is stoichiometrically equal to formaldehyde HCl, we attempted to photolyze formaldehyde-HC1 complexes in Ar matrices. The only product was HC1 + CO, one molecule of H, possibly escaping from the matrix cage. Conformers of Vinyl Formate. After deposition the spectrum of VF consists of bands due to mainly one conformer. A very small amount of another species (less than 1%) could be detected in the C=O and C-0 stretching regions with the aid of bands at 1798 and 1136 cm-I. The wavenumbers for the most characteristic absorptions of VF in Ar are collected in Table VI and are compared with values given in previous studies. According to a b initio calculations the sp,sp ( 2 , a p in the notation of ref 33) conformer of VF is the lowest in energy.34 This conformer is defined as having the values Oo and 180' for the dihedral angles O=C-0-C and C=C-0-C, respectively. The calculated energy difference between the species sp,ap and the next conformer sp,sp (which can also be denoted as Z,sp, both dihedral angles having the value Oo) varies from 2.2 to 14.4 kJ mol-',34 depending on the basis set used, being 11.9 kJ mol-' at the HF/4-3 1G Experimentally the sp,ap conformer has been found to be at least 9.6 kJ mol-I more stable than any other species.34 Upon deposition of a matrix at low temperatures, the gas-phase equilibrium is usually trapped. Thus we conclude that the initial spectrum of VF is due to conformer sp,ap. We did not observe any infrared induced conformer interconversions between the conformers of VF. Although the purpose of this study was not to analyze the spectrum of VF in detail, a few comments concerning this are, however, presented. The C=O stretching band of species sp,ap is a doublet with components at 1757 and 1734 cm-l. A similar structure has been observed for VF in this region in the spectrum of the liquid?6 We suggest that there is Fermi resonance between the first overtone level of the 879-cm-' fundamental and the C=O stretching fundamental level. During UV photolysis new bands due to another conformer of VF appeared. It was impossible to produce a large amount of this species since decomposition of VF (into vinyl alcohol or acetaldehyde) was too rapid. The VF conformer concentration ratio seemed to be close to 1:l during photolysis and was independent of photolysis wavelength. It is to be noted that the growing conformer absorptions were at the same wavenumbers where traces of the other species were detected after trapping the gas-phase equilibrium. On this basis we conclude that the initially observed less stable conformer of VF is the same as that produced in the UV-induced conformer interconversion process. The most significant difference between the spectra of the two VF conformers is to be found in the C=O stretching region: for the second conformer this band is ca. 50 cm-I blue shifted from the wavenumber for the sp,ap conformer. Similar shifts of the carbonyl stretching fundamental band have been observed for various formate esters, the higher frequency band belonging to the conformer with anti skeletal O=C-0-C structure.'S2 The reasons for the stabilities of different conformers of carboxylic acid esters are not known.37 When explaining the stabilities, diple4ipole interactions must at least be taken into account. The second most stable VF conformer has been suggested to be sp,sp,34*35 the O=C-0-C structure of which is syn, and hence the structural dependence of the YC=O wavenumber found for formate esters is not applicable for VF. Photolysis of Vinyl Formate. Entirely different products were obtained in cage photolysis of VF using either a Hg arc or
Kunttu et al.
+
T A
-
pT----
'I Figure 3. Photolysis of V F in Ar. A: after deposition. B: after Hg-arc irradiation, 10 h (by using filter b, Figure 1). C: after prolonged irradiation under similar conditions as in B. D: final situation obtained by irradiating A for 30 h using a H2-discharge lamp.
TABLE M: Ab Initio MP2/6-31C** Results for the Syn Conformer of Vinyl Alcohol",* internal coord r l OH r 2 CH3
r3 CH, r4CHI r s CC 16
co
CCH2 a3 CCH, a4 CCHl a6 CCO
coord value MP2/ 6-31G** exptle 96.6 108.2 107.6 108.0 133.5 136.5 119.9 122.1 22.7 26.8 08.1
95.6 109.0 107.8 107.9 133.2 137.3 121.2 119.5 123.8 126.0 108.9
0.0
0.0
1.093 (33) Dahlqvist, M.; Euranto, E. K. Specrrochim. Acta, Parr A 1978, 3 4 4 863. (34) Pyckhout, W.; van Alsenoy, C.; Geise, H. J.; van der Veken, B.; Coppens, P.; Traetteberg, M.J . Mol. Srrucr. 1986, 147, 85. (35) Aroney, M. J.; Bruce, E. A. W.; John, I. G.; Radom, L.; Ritchie, G. L. D. Ausrr. J. Chem. 1976, 29, 581. (36) Gardenina, A. P.,Kotorlenko, L. A. Opt. Specrrosc. 1968, 24, 495. (37) Jones, G. I. L.f Owen, N. L. J . Mol. Srrucr. 1973, 18, 1.
local sym coord
SI= Arl S2 = (Ar, - Ar3)/21/2 S 3 = (Ar2 + Ar3)/Z1'2 S4 = Ar4 S5 = Ar5 s6 = AT6 5'7 = (2Aal - A012 - Aa3)/2 Ss = (Aa2 - ALY,)/ZI/~ S9= (Ay2 + AY,)/Z'/~ SI,, = (Aa4 SI1 = (2Aa6 - Aa4 Aas)/z S I 2 = AYI S I 3 = ACY, SI4= AT] SIs= (AOCCH, + AOCCH, +AHlCCH3 + AHICCH2)/2
approx assignt uOH USH2 V W 2
uCH
ucc uco 6CH2 CH2 rock CHz wag CH rock 6CCO CH wag 6COH rOH CH2 twist
1.016
'The bond lengths are in picometers, the angles are in degrees, and the dipole moments ( p ) are in debyes (see Scheme I1 for atom numbering). Included are also the local symmetry coordinates used. *Eto, (au) = -153.3683697. 'Reference 15.
H2-discharge lamp as the photolysis source, as is shown in Figure 3. The UV spectrum of VF in ethanol resembles that of CMF;
The Journal of Physical Chemistry, Vol. 92, No. 6,1988 1501
Photochemical Processes on Formates
TABLE VIII: Observed (Ar Matrix) and Calculated (MP2/631G**) Wavenumbers (cm-I) for Vinyl Alcohol"
wavenumbers exptl
calcd
3584 3576 3130 3096
3869
1649 1608 1421 1404 1310 1282 1112 1109 975 970 952 819 713 708 516 475
PED^ 100 (1) (21, 5 (3) (4) (31, 5 (2) (5), 12 (7), 12 (lo), 10 (6)
exptl
approx description
ref 11
ref 10
uOH
3619.9
3625.0
3373 3302 3258 1748
93 96 93 61
VaCH2 uCH hCH2
1495
71 (7), 17 (lo), 5 (6). 5 (13)
bCH2
1388 1356 1142
34 (lo), 27 (13), 14 (6), 13 (5), 9 (7) 29 ( 1 3 , 34 (lo), 14 (8), 9 (111, 7 ((9, 5 (5) 38 (61, 36 (131, 9 (8), 6 (5)
C H rock 6COH
1012
52 (12), 48 (15)
C H wag
1300.2 1079.0 1121.3 97 1.4
982 805 723
58 (81, 27 (6), 6 (lo), 5 (5) 93 (9), 6 (12) 40 (15), 52 (12), 7 (14)
CH2 rock CHI wag CH2 twist
942.6 813.7 698.2
495 467
76 ( l l ) , 19 (8) 92 (14), 5 (15)
bCC0 rOH
486.1 413.3
vcc
uco
3241 1622.4 1661.9
1624.5 1631.0
1079.0
818.5
407.0
'The approximate description is based on the potential energy distribution in terms of the ab initio force field. bPotential energy distributions. PED contributions smaller than 5% have been omitted. there are two absorption bands between 200 and 300 nm for VF, a weak one at ca. 275 nm, and one that is at least 100 times stronger a t ca. 210 nm. The products obtained in the Hg-arc photolysis were easily identified as vinyl alcohol and CO by comparing the spectra with the reported matrix IR wavenumbers of VA.'O*" Best agreement was found in modes not involving motion of the OH group (for example, CH wag, CH2 rock, C H 2 wag). In these bands the average deviation from the reported wavenumbers was on the order of 10 wavenumbers. The considerable perturbation of the OH stretching and torsional wavenumbers can be explained by the fact that CO is mainly complexed via the O H group of the alcohol. In order to obtain as complete an assignment as possible for the fundamental bands of VA, we optimized the geometry of the syn conformer of VA and calculated its vibrational spectrum, carrying out both calculations a t the MP2/6-31G** level. The results of the a b initio calculations (including the symmetry coordinates used) are given in Table VI1 and the observed and calculated wavenumbers for VA (with potential energy distribution) in Table VIII. In order to obtain an IR spectrum of pure VA, we attempted to trap VA at liquid nitrogen temperature, pump the C O away, and deposit the purified VA back onto the CsI window with excess Ar. This procedure was of limited success, since most of VA was lost during the handling of the sample, but the strongest bands due to uncomplexed VA could be identified (as compared with the values reported by Rodler and Bauder, ref 21). It has been suggested that the lifetime in the gas phase of VA is 15-30 s . ~ ' Improvement in the trapping system would certainly allow one to prepare VA in larger quantities, by employing the quantitative Hg-arc photolysis of matrix-isolated VF into VA. N o decomposition of VA was observed on irradiating it a t wavelengths greater than 200 nm. During H2-lamp photolysis of VF, acetaldehyde was obtained, its wavenumbers in Ar being 2841,2730, 1742,1431, 1354, 1123, 777, and 515 cm-'. These wavenumbers are similar to those reported for acetaldehyde in the gas phasea3* The estimated energy difference between acetaldehyde and VA is only 57 kJ (the barrier between these species being ca.400 kJ mol-'@), and since VA is stable at the matrix temperatures, we attempted
to produce VA from acetaldehyde. Matrix cage photolysis of acetaldehyde did not proceed to its tautomer VA; only slow decomposition to methane and C O was observed. Mechanistic Considerations. The wavelength dependence of the product yields for C M F and especially for VF are interesting. The question arises whether the products are dependent on initial conformation of the precursor or whether the mechanisms leading to different products are different. It has been suggested that conformation of the precursor determines the products in the ~ the photolysis of methyl formate2 or hydroxymethyl f ~ r m a t e .In present study, no unequivocal proof for the role of the conformation of the precursor could be given: experimentally we found the same products when starting photolysis with either one conformer or with two conformers of C M F or VF present. Schemes I and I1 display the cage-photochemical processes in Ar for chloromethyl and vinyl formate, respectively. The photoprocess of a formate ester leading to an alcohol is obviously initialized by the production of the radicals HCO' and RO'. The short-lived HCO' fragment can then easily reorient in the cage into a position favorable to H-atom donation to RO'; eventually, the corresponding alcohol and CO are formed. In order to get additional information on the possibility of a radical mechanism in the production of alcohol, we tried to quench the radical reaction with NO. Hg-arc photolysis of a NO-doped matrix of VF (VF:NO:Ar = 1:10:500) did not yield any vinyl alcohol. This supports the idea of a radical mechanism in the photolysis of formate esters. The electronic states from which the processes occur are not known. Another experiment in order to throw some light to the possible involvement of a triplet state in the processes was undertaken for VF. The initial excitation by Hg-arc irradiation most probably leads to the SIstate. Enhancement of the SI TI intersystem crossing rate may be obtained by external perturbation, for example, by using heavier matrix atoms such as Xe atoms.41 In Xe matrices the photolysis yielded at least 5 times more acetaldehyde than similar irradiation conditions in Ar. This result supports the proposition of participation of a triplet state in the process that leads from photoexcited VF to acetaldehyde. On the other hand, short-wavelength excitation of the formate esters (using a H2-discharge lamp) most probably ends up with the
-
~~
(38) Shimanouchi,T. Tables of Molecular Vibrational Frequencies;Vol. I, NSRDS-NBS6; Superintendent of Documents, US.Government Printing Office: Washington D.C., 1967. (39) Rcdler, M. Chem. Phys. 1986, 105, 345. (40) bouma, W. J.; Poppinger, D.; Radom, L. J. Am. Chem. SOC.1977, 99, 6443.
(41) Horrocks, A. R.; Kearvill, A.; Tickle, K.; Wilkinson, F. Trans. Faraday Soc. 1966, 62, 3393. (42) Nelander, B. J . Chem. Phys. 1980, 72, 77. (43) Andrews, L.; Arlinghaus, R.T.; Johnson, G.L. J . Chem. Phys. 1983, 78, 6341. (44) Nelander, B. J . Chem. Phys. 1980, 73, 1026.
J . Phys. Chem. 1988, 92, 1502-1506
1502
-
methanol, and vinyl alcohol, respectively. In addition to these products, the corresponding aldehydes are obtained. The production of aldehydes is enhanced in short-wavelength-initiated processes as compared with long-wavelength-initiated ones. Formate esters as precursors allow the production of novel substituted methanols like chloromethanol. Extensive ab initio calculations rendered complete interpretation of the infrared spectra of the alcohol possible. The photoprocesses studied in this paper suggest that methanediol has been produced previously in a similar phot~lysis.~ The preparation of fluoromethyl formate, a possible precursor of fluoromethanol, is now being attempted. Registry NO.CIH2COCH0, 30566-3 1-5; HZCCHOCHO, 692-45-5; ClHzCOH, 15454-33-8; H,CCHOH, 557-75-5; CO, 630-08-0; H2C0, 50-00-0; CH3CH0, 75-07-0; Ar, 7440-37-1; NO, 10102-43-9; Xe,
precursor in the S2 state. Combined together, these observations (the same reaction route followed while enhancing the S1 TI intersystem crossing or while exciting into the S2state) suggest that intersystem crossing from the S2state to the triplet manifold is much mor. probable than from the SI state. Further investigations are needed to explain the differences in photolysis of formate esters at different wavelengths. The quantum-chemical simulations of the excited-state potential energy surfaces of the precursors are of particular interest. Spectroscopic studies might also reveal triplet-state populations after excitation at certain wavelengths.
Summary The main channels in cage photolysis of chloromethyl and vinyl formates lead to formation of the unstable alcohols, chloro-
7440-63-3.
Kinetic Study of the Reactions of Diacetylene with Atomic Oxygen and Atomic Chlorine M. B. Mitchell: J. Brunning,* W. A. Payne, and L. J. Stief* Astrochemistry Branch, Laboratory for Extraterrestrial Physics, NASAIGoddard Space Flight Center, Greenbelt, Maryland 20771 (Received: May 15, 1987; In Final Form: October 6, 1987)
The absolute rate constants for the reactions of O(3P) and C1(2P) with diacetylene (C4H2)have been determined at 298 K in a discharge flow system near 1-Torr pressure. The decay of C4H2in the presence of excess O(3P) or C1(2P) was followed by collision-free sampling mass spectrometry. For the O('P) ClH2 reaction, we measure k l = (1.5 0.2) X cm3 s-I, while the corresponding result for the C1(2P) C4H2 reaction is k, = (4.8 f 0.9) X lo-'' cm3 s-I. The result for the O('P) C4H2 reaction is compared with previous determinations using both discharge flow-mass spectrometry and flash photolysis-resonance fluorescence techniques. This study represents the first determination of the rate constant for the C1 + C4H2 reaction, and the result is compared to the analogous C1 + C2H2reaction.
+
Introduction The reactions of diacetylene (C4Hz) with atomic species are of interest for several reasons. The reactions of H and O(3P) with C4H2 are important for models of the atmosphere of Titan' and for their roles in the combustion of hydrocarbons in general and of acetylene in particulars2 Comparison of rate data for this molecule with the more extensive data for acetylene may contribute to our understanding of the factors which control the kinetics of the addition of atoms to the carbon-carbon triple bond. For example, in a recent study3 of the reaction H C4H2, it is shown that the high-pressure limiting rate constant k, at 298 K increases by more than an order of magnitude in going from C2H2to C4H2. Further, it was found that the increasing rate is due entirely to a larger preexponential factor since the activation energies for both reactions are essentially the same. Finally, the magnitude of the observed pressure effect was shown to decrease with a fairly strong effect for C2HZ4and no detectable pressure dependence for C4H2 over the pressure range 5-700 Torr.3 The comparison for the systems O(3P) + C2H2and O(3P) + C4H2is quite different from that of the corresponding H atom reactions. In both O(3P) systems, no effect of pressure on the rate constants has been observed at 298 K. The absence of a pressure effect indicates that the backward decomposition to reactants is negligible compared to stabilization, isomerization, or decomposition of the adduct." Similar to the H system, there is a b u t an order of magnitude increase in the rate constant at 298 K in going from O(3P) + C 2 H 2 to O('P) + C4H2.799-'1 In our study of the O(3P) + C4H2 reaction using the flash photolysis-resonance fluorescence (FP-RF) t e c h n i q ~ ek(298 ,~ K)
+
NAS/NRC Resident Research Associate. Present address: Union Carbide Corp., P.O. Box 8361, Blda. 770-224, South Charleston, WV 25303. *NAS/NRC Resident Researcfi Associate. Present address: Department of Chemistry, The University of Birmingham, P.O. Box 363, Birmingham 815 2TT, United Kingdom.
*
+
+
was determined to be (1.37 f 0.19) X cm3 s-l. This is in good agreement with the result of Niki and Weinstock9 (1.5 X cm3 s-l) but lower than the values determined by Homann et a1.l0 (2.1 X cm3 s-l) and Homann and Wellmannll (2.5 X cm3 s-l). Two of the s t ~ d i e s ~had . ' ~ atomic oxygen in excess (- lOI4 ~ m - and ~ ) monitored the decay of C4H2 (initial ~ ) the third" had C4H2 concentration (0.4-1.4) X l O I 3 ~ m - while in excess (- 1014~ m - and ~ ) monitored the decay of O(3P) (initial ) . three studiesg-" employed the concentration 3 X lOI3 ~ m - ~ All discharge flow-mass spectrometric (DF-MS) technique. We have now measured k(298 K) for the reaction O(3P) + C4H2
- OC4H2
products
(1)
using the DF-MS technique with atomic oxygen in excess but (1) Strobel, D. F. Planet. Space Sci. 1982,30,839. Strobel, D. F. Int. Rev. Phys. Chem. 1983, 3, 145. Yung, Y. L.; Allen, M.; Pinto, J. P. Astrophys. J., Suppl. Ser 1984, 55, 465. (2) Homann, K. H.; Schweinfurth,H. Ber. Bunsen-Ges. Phys. Chem. 1981, 85, 569. Glass, G. P.; Kistiakowsky, G. B.; Michael, J. V.; Niki, H. J. Chem. Phys. 1965,42,608. (3) Nava, D. F.; Mitchell, M. B.; Stief, L. J. J. Geophys. Res. 1986, 91, 4585. (4) Payne, W.A.; Stief, L. J. J. Chem. Phys. 1976, 64, 1150. (5) Arrington, C. A.; Brennen, W.; Glass, G. P.; Michael, J. V.; Niki, H. J . Chem. Phys. 1965,43, 525. (6) Harding, L.; Wagner, A. J. Phys. Chem. 1986, 90,2974. (7) Mitchell, M. B.; Nava, D. F.; Stief, L. J. J. Chem. Phys. 1986, 85, 3300.
(8) Westenberg, A. A.; deHaas, N. J. Chem. Phys. 1977, 66,4900. (9) Niki, H.; Weinstock, B.J . Chem. Phys. 1966.45, 3468. Errata: Niki, H. J. Chem. Phys. 1967, 47, 3102. (10) Homann, K. H.; Schwanebeck, W.; Warnatz, J. Ber. Bunsen-Ges. Phys. Chem. 1975, 79, 536. ( 1 1) Homann, K. H.; Wellmann, Ch. Ber. Bunsen-Ges. Phys. Chem. 1983, 87. 527.
0022-3654/88/2092-1502$01.50/0 0 1988 American Chemical Society
