J. Phys. Chem. 1987, 91, 3483-3488
3483
Alkylperoxy and Alkyl Radicals. 4. Matrix I R Spectra and UV Photolysis of C2HsO2 and C2H5 Radicals G . Chettur and A. Snelson* IIT Research Institute, Chicago, Illinois 60616 (Received: November 13. 1986)
+
Ethyl radicals, formed by the pyrolysis of azoethane, were isolated in matrices of Ar 10% O2 and pure Ar. IR spectra of the trapped species were obtained. Oxygen isotopic labeling was used to identify the C2H5Oz radical and its dimer. Vibrational frequency assignments were made for both species. Under Hg arc irradiation the C2HS0 radical was destroyed, and some photolysis products were identified. New IR spectral data were obtained for the C2H5 radical. C2Hjin argon matrices was found to be stable toward Hg arc irradiation.
Introduction In the first three papers1” in this series, we reported on the IR matrix isolation spectra of CH302, CH304CH3,i-C3H702,and i-C3H7,and on the behavior of these species under the influence of UV irradiation. As noted in the earlier papers, alkylperoxy radicals are important intermediates in the low-temperature oxidation of organic materials. Spectral information on R 0 2 radicals is limited. The ethylperoxy radical, one of the topics of the present paper, is known to have a broad and unspecific absorption band in the UV (220-280 nm).4 A structured electronic spectrum of the CZH5O2 radical has been identified in the near-infrared showing a well-defined sequence of bands characteristic of an 0-0 stretching mode.5 No infrared spectra appear to have been reported for the species. Alkyl radicals are also important intermediates in combustion processes.6 Recent work has greatly increased data on the spectral characterization of the low-molecular-weight alkyl radicals. The UV spectrum of the ethyl radical has recently been reported7 in the 195-370-nm wavelength range. The matrix isolation IR spectra of several isotopically labeled ethyl radicals, produced by the photolysis of dipropionyl peroxide, have been reported in the literature.* In the same study the UV photolysis of the C2H5 radical was reported. In this paper, the infrared spectrum of the ethylperoxy radical is presented together with data on its photolysis in the UV. Some new data are presented on the IR spectrum of the ethyl radical and on its behavior under UV photolysis. Experimental Section The matrix isolation cryostat and molecular beam pyrolysis tube furnace assembly used in the study have been described previously? Ethyl radicals were produced by the pyrolysis of azoethane. To form ethylperoxy radicals, ethyl radicals were allowed to react during the trapping process with argon matrices containing 10% 02.Ethyl radicals were trapped in argon matrices when the IR spectrum of this species was studied. Matrix deposition times varied from 20 to 70 h. Absolute values of the radical matrix gas dilution ratios were not known. Most experiments were run at matrix dilution ratios sufficient to minimize dimerization of reactive species. Azoethane was synthesized according to a literature procedure.1° Oxygen isotopes, ’802 (99%) and a 50% scrambled mixture of 160180, were obtained from Prochem. (The values in parentheses (1) Ase, P.: Bock, W.; Snelson, A. J. Phys. Chem. 1986, 90, 2099. (2) Cyvin, B. N.; Cyvin, S.J.; Snelson, A. Z. Anorg. Allg. Chem. 1986, 542, 193. (3) Chettur, G.; Snelson, A., J . Phys. Chem. 1987, 91, 913. (4) Adachi, H.; Basco, N.; James, D.G. L. Int. J . Chem. Kinet. 1979, 11,
1211. ( 5 ) Hunziker, H. E.; Wendt, H. R. J . Chem. Phys. 1976, 64, 3446. (6) Benson, S.W.; Nangia, P. S . Ace. Chem. Res. 1977, 12, 223. (7) Wendt, H. R.; Hunziker, H. E. J. Chem. Phys. 1984, 81, 717. (8) Pacansky, J.; Dupuis, M. J. Am. Chem. SOC.1982, 104, 415. (9) Snelson, A. J . Phys. Chem. 1970, 74, 537. (IO) Renaud, R.; Leitch, L. C. Can. J . Chem. 1954, 32, 545.
0022-3654/87/2091-3483$01.50/0
are the manufacturer’s stated purity.) The Ar matrix gas and 1602 were Matheson Research Grade. Reference IR matrix spectra of the following materials were generated: CH4, CzH4, C2H6, n-C4Hlo,C2H50H, C H 3 C H 0 , and CH,COOH. These materials, stated purities 398%, were used as supplied. Matrices were subject on occasion to irradiation by either a 100-W low-pressure Hg arc lamp or by a 1000-W tungsten filament lamp for periods up to 20 h. IR spectra in the 4000200-cm-’ range were recorded on a Perkin-Elmer 273 spectrophotometer. Reported frequencies are accurate to f 2 cm-I.
Results and Discussion Ethyl Radical Spectra. Initially experimental conditions were established for the effective pyrolysis of azoethane using pure argon matrices for trapping the decomposition products. At temperatures of -400 OC, almost all the azoethane was pyrolyzed (estimated 395%); the major stable products identified were ethylene and ethane, together with smaller amounts of methane. Butane, formed by dimerization of ethyl radicals, was detected at trace levels. The formation of substantial amounts of the ethyl radical was indicated by the appearance of the characteristic8 radical center bending mode at 532 cm-I. By raising the pyrolysis temperature somewhat, complete destruction of the azoethane was achieved albeit at the expense of a reduction in the amount of the ethyl radical stabilized. For this reason the lower pyrolysis temperature was used in most experiments. A series of experiments was made in which the ethyl radical was isolated in argon matrices containing either 10% I6O2,10% I8O2,5% 1602 5% 1802,or 2.5% 1602 + 5% 1601802.5% I8O2. Reactions of potential importance in the matrix experiments were assumed to be as follows, based on the known chemistry of the system at ambient temperatures:]
+
+
Under conditions of good isolation, formation and trapping of the C2H5O2 radical should be the major process occurring on matrix deposition. Under conditions of poor isolation, or on annealing the matrix, self-reaction of the radicals (eq 2) may be expected to occur. Characteristic spectra of regions of interest are shown in Figures 1 and 2. Absorption bands in these spectra were identified as follows: (1) Identification of absorption bands attributable to expected stable species, CHI, C2H4, C2H6,n-C4Hlo,C2HjOH, CH,CHO, and unreacted azoethane. (2) Identification of those absorption bands that decreased in intensity on annealing matrices at 35 K and recooling to 12 K. (3) Identification of those absorption bands, generally the same as in (2), that decreased in intensity under irradiation from a low-pressure Hg arc. 0 1987 American Chemical Society
The Journal of Physical Chemistry, Vol. 91, No. 13, 1987 I
Chettur and Snelson
I
Figure 1. 1R spectra (in regions of interest between 1280 and 3150 cm-I of reaction products formed from the interaction of C2HSradicals and a matrix of Ar + 2.5% I6O2+ 5% 160180 and 2.5% (a) Spectrum of reaction products in an "as-deposited" matrix, and (b) spectrum after 20 h of Hg
arc irradiation.
(4) Identification of those absorption bands that showed oxygen isotopic frequency shifts on substitution of I6O2with I8O2or '60'80.
With the above criteria, and by analogy with data for CH302,' the absorption band assignments shown in Figures 1 and 2 were made for the various isotopically labeled C2HS02radicals and other species. In Figure 1, spectral regions of interest in the 12504000-cm-' range are presented. These spectra are of the products isolated from the interaction of CIHS radicals with an Ar matrix containing a scrambled mixture of oxygen isotopes and show the effect of UV irradiation on the "as-deposited" matrix. With the exception of the two absorption bands shown at 1697 and 1728 cm-I in (Figure Ib), to be discussed later, spectra recorded from the reaction of C2H5 with matrices containing either pure I6O2, or an equimolar mixture of I6O2 I8O2were identical with those shown in Figure 1. This finding implies that frequencies assigned to C2H5O2 in these spectral regions were oxygen mass independent. In Figure 2, the spectral region below 1250 cm-', a number of the absorption bands assigned to the C2H502radical showed a marked oxygen mass dependence and for this reason spectra are presented to demonstrate these effects. The isotopic substitution observed in these spectra (to be discussed) were consistent with assignment of these bands to the ethylperoxy radical. Vibrational Assignment for the Ethylperoxy Radical. The geometry of the ethylperoxy radical has not been determined. As with the methylperoxy radical' a nonlinear C-0-0 group is assumed, resulting in a radical of low symmetry belonging to either the C , or C, point groups. For either structure a total of 21 fundamental vibration frequencies is expected; all are IR active. Two of the vibrational modes will correspond to hindered internal rotations in the radical and will probably lie below the long wavelength limit of present study. Recently," a vibrational as-
+
( I 1) Wagner, A. F.; Melius, C., private communication.
signment has been proposed for the ethylperoxy radical, based on spectral data for CH3O2Iand C2H5,' and on theoretical U H F calculations of the radical's vibration frequencies." This proposed vibrational assignment, together with experimental data on oxygen isotope effects, was used in making the vibrational assignments shown in Table I. C-H Stretching Modes. In the 2800-3 100-cm-' frequency range shown in Figure l a , C-H stretching modes obviously attributable to C2H6, C2H4,and CH4 were identified. In addition, a well-defined absorption band at 3016 cm-' was present that disappeared completely on exposure to 14-h irradiation from a Hg arc lamp, and also diminished in intensity on annealing the matrix. This band was assigned to an ethylperoxy radical C-H stretching mode. Comparison of reference spectra for C2H6, C2H4, CH4, and AE (azoethane), with the spectrum in Figure 1, strongly suggested that a number of absorption bands, in addition to those of the reference species, were present in the 2800-3000-cm-' region. Because of absorption band overlapping problems in this region, specific vibrational assignments to the C2H5O2 radical could not be made. The one resolved C-H stretching mode at 3016 cm-' was assigned to v4, the asymmetric stretching mode of the C2H502 radical. C-H Bending Modes > 2300 cm-I. In the spectral region 1300-1 500 cm-I, five absorption bands were assigned to the C2H502radical. Four of these bands, those at 1474, 145 I , 1389, and 1380 cm-I, all showed reductions in absorption intensity on irradiation and annealing the matrices. On annealing the matrix, the 1351 -cm-l band decreased in intensity but, as shown in Figure Ib, irradiation resulted in a new band "growing in" at 1353 cm-I. Evidence will be presented later that this new band is probably due to a perturbed vibrational mode of acetaldehyde. These five frequencies were assigned to C-H bending modes of the C2H502 radical, distributed in accordance with the proposed vibrational mode assignment for C2H,021' referred to earlier. Oxygen Mass Dependent Vibrational Modes. Four of the C2H5O2 radical's vibration modes, v l 5 (0-0 stretch), v I 7 (C-0
The Journal of Physical Chemistry, Vol. 91, No. 13, 1987 3485
Alkylperoxy and Alkyl Radicals
I
I
500
800
I 1000
I 1100
I 1200
CM-'
Figure 2. IR spectra (400-1250 cm-I) of "as-deposited" reaction products formed from the interaction of C2H5radicals and a matrix of Ar + 10% I6O2 + 5% 160's0 + 2.5% (b) matrix (a) after Hg arc irradiation for 20 h; (c) matrix of Ar = 5% I6O2+ 5% '*02; (d) matrix of Ar + 10% (e) matrix of Ar + 10% I6O2. matrix dilution approximately a factor of five lower than in previous spectra; and ( f ) matrix (e) annealed to 35 K and cooled to 12 K.
stretch), and u I 8 and ~ 1 (9C - C - G O bends) are expected to show pronounced oxygen isotopic frequency shifts. If complete decoupling of these modes from those of the rest of the radical is assumed, an upper limit for the ratio 1 6 u / 1 8 umay be estimated at -1.06 (0-0 str), 1.025 (C-0 str), and 1.025 (C-C-0-0 bends). In addition, in the spectra of C 2 H 5 0 2radicals containing I6O2,'60180, and '*02 isotopes, each of these oxygen-dependent vibration modes potentially will have four discrete frequency components.' Inspection of the spectral curves shown in Figure 2a-e clearly indicates the presence of an absorption band at -500 cm-' which had a quartet structure in the spectra of the scrambled
oxygen isotopes and appeared as a singlet in the monoisotopically labeled radicals. In CH3OZ1and i-C3H,02 radicals, the C-0-0 bending modes have been identified at 492 and 515 cm-I, re~ p e c t i v e l y . ~The . ~ absorption band appearing at 499 cm-' in the C2H502radical is similarly assigned. A second skeletal C X 4 - 0 bending mode is expected for the C 2 H 5 0 2radical. The frequency has been estimated at ~ 3 0 cm-I;'I 0 the analogous mode in iC 3 H 7 0 2was observed experimentally at 305 cm-I. In the present investigations no frequency was observed which could be assigned to this mode. Possibly, in C 2 H 5 0 ,this mode has a low extinction coefficient making its identification difficult.
3486 The Journal of Physical Chemistry, Vol. 91, No. 13, 1987
Chettur and Snelson
TABLE I: Vibrational Assignment (em-') for Various Oxveen Isotooieallv Labeled Ethvlwroxv Radicals _ _ ~ expt obsd freq
approx mode description CH, stretches u1 in-plane v2 out-plane v3 symmetric CH, stretches u4 asymmetric u5 symmetric CH, bends v6 in-plane def u7 out-plane def us symmetric def ug out-plane rock uI0 in-plane rock CH, bends u I 1 scissors wag u I 3 twist VI2
ui4 vis ui6
rock
Y ,
0-0 str C-C str C-0 7 str C-C-0-0 ~
ui9
C-C-0-0 in-phase bend
~ 1
out-of-phasebend
proposed freq assignment" C2HSI602
C,H1I6O2
-
C2~5160180 C,H1180160 C,H1180,
U('~O,)/~(~~O,)~
2936 2874 2955 2901
3016
3016
3016
1.000
1467 1454 1410 1152 1129
1451 1389 1380
1451 1389 1380
1451 1389 1380
I.000 1.000 I.000
1120
1.014
1493 1371 1259 786 1145 1006 859 49 1
1474 1351 1242 800 1112 1009 838 499
1474 1351 1242 800 1095
1474 1351 1242 800 1065 1005
1.016 1.000 1.000 1.000 1.044 1.004
494 488
484
1 I36
1.01
300
"Reference 11. Isotopic frequency ratio. Peroxy radical 0-0 stretching frequencies in HO2,I2CH302,1 CF3O2,I3and i-C3H70? have been reported at 1101,1112, 1092, and 1101 cm-I, respectively. Absorption bands at 11 12 cm-' (C2H5I6O2), 1065 cm-I (C2H5is02),and 1095 cm-I (C2H5160180 and C2Hs180160) were assigned to the respective 0-0stretching modes of the radicals. In both C H 3 0 2and i-C3H702,the radicals containing the R-i60-180 and R-180-i60 scrambled isotopes had identical 0-0 stretching frequencies. A similar situation also appears to exist for the ethylperoxy radicals. The intensity ratios of the three bands in the scrambled oxygen isotopic spectrum (Figure 2a) at = 1:2:1 is consistent with the assignment. The C - 0 stretching frequencies in CH3O2,I CF302,13 and iC3H7023have been reported at 902, 870, and 789 cm-', respectively. In the C2H5I6O2spectrum (Figure 2e) an absorption band at 838 cm-' has an appropriate frequency for the C - O stretching mode. In the C2H51802 radical spectrum, the corresponding frequency was not identifiable. In the spectra of CH3O2, CF3O2, and i-C3H702,the C - O stretching frequency in R i 6 0is essentially identical with that in R160180with a similar situation for the radical pairs RI8O2and R1s0160.If this is the case for C2H5O2, the C-isO stretching mode would be expected in the 823-818-cm-I range, assuming an isotope shift similar to than found for this mode in CH3O2, CF302,and i-C3H702.A strong absorption band of ethane, centered at 821 cm-I, would thus overlap that of the C-0 stretching mode in C2HSi802and preclude it obvious identification. The C - 0 stretching mode of C2H5I6O2is accordingly assigned at 838 cm-I. C-C Stretching Mode. Two absorption bands appeared in the and ethylperoxy radical spectrum (Figure 2c) at 1009 cm-' (1602) 1005 cm-' (I8O2);they were also present in the spectrum of the scrambled oxygen isotopes, without any additional absorption bands appearing. These bands were assigned to the C-C stretching modes in CzH5l6O2 and C,H5"02. This assignment is in good agreement with the proposed value" and falls within the range usually associated with this vibrational mode. C-H Bending Modes < 1300 cm-'. The lowest C-H bending frequency proposed" for the C2H502radical, vI4, was estimated at 786 cm-I (Table I). In the present study, one absorption band, showing no oxygen isotopic frequency shifts, was observed in all spectra at 800 cm-I. This band was assigned to q 4 based on the (12) Jacox, M. E.; Milligan, E. E. J. Mol. Spectrosc. 1972, 42, 495. ( 1 3 ) Butler, R.; Snelson, A. J . Phys. Ckem. 1979, 83, 3243.
similarity of the observed and estimated frequencies. For similar reasons, a very weak absorption band at 1242 cm-I, whose intensity variations tracked, qualitatively, those of other bands of C2H50, was assigned to the radical's CH, twisting mode, uI3. Two C-H bending modes of the CH! group, ug and vlo, with frequencies of 1150 cm-I, remain to be identified. From spectral curves c, d, and e of Figure 2, an absorption band was easily identified at 1120 cm-I in the I8O2system which was clearly absent system. The corresponding band in the later system in the 1602 appeared at 1136 cm-I. In the spectrum shown in Figure 2e, this band was partially overlapped by a strong (C2H5I6O2)2band at 1 128 cm-'. Most experiments were made at matrix dilutions that suppressed dimer formation completely. (Spectral curves e and f of Figure 2 were included to show that absorption bands attributed to the dimer could be enhanced both by decreasing the matrix dilution and annealing the matrix.) At high matrix dilutions, the 1136-cm-I band in the C2H5I6O2spectrum was well defined. Returning to the I8O2system, a weak absorption band consistently appeared at 1136 cm-' that behaved on matrix photolysis or annealing in the same way as other bands in the spectrum assigned to C2H5I8O2. A similar weak band in the corresponding spectrum of C2H51602was not identified, possibly because in this system it was overlapped by the stronger oxygen mass sensitive mode of C2H51602and 1136 cm-I. If this were the case, it would imply that the vibrational mode, responsible for the weak band at 1136 cm-I in the CzHS1802 spectrum, was oxygen mass independent. The spectrum shown in Figure 2a for the system containing the scrambled oxygen isotopes is not obviously consistent with the above interpretation for the 1 1 20- and 1 136-cm-I features. Although three absorption bands were found at 1136, 1127, and 1120 cm-I, which would not be inconsistent with one of the vibrational modes being oxygen isotope mass sensitive, the expected relative intensity ratio for the triplet of = 1:2:1 was not realized. Because of these difficulties, only one C-H rocking mode was assigned, v I o , at 1136 cm-I (I6O2)and at 1120 cm-I (I8O2). Photolysis of the Ethylperoxy Radical. Absorption bands assigned to the ethylperoxy radical could be bleached completely by exposure of the matrices to Hg arc radiation for 20 h. Irradiation with a 1000-W tungsten lamp over a similar period had no effect on the spectrum. On UV photolysis, new absorption were found in the spectrum. Easily identified were absorption bands of C 0 2 ,CO, and HzO. They showed multiplet structure char-
The Journal of Physical Chemistry, Vol. 91 No. 13, I987
Alkylperoxy and Alkyl Radicals
I
TABLE 11: Frequencies (cm-') of Absorption Bands Tentatively Assigned to CH3CH0 on the UV Photolysis of Matrices Containing C2Hq02Radicals
strong absorption bands" in spectrum of C H ~ C H O isolated in Ar + 10% O2 1736 1436 1431
ii;
} } }
doublet doublet
i :: doublet 506
photolysis product bands of C2HS02tentatively assigned to acetaldehyde
TABLE 111: Vibrational Frequencies (cm-') Assigned to the Dimer of the C2HS02 Radical
approx mode description C-0-0 bend
CH3CH160
CH3CHI80
1728 1431
1697 1431
1353
1353
C-0-0 bend 0-0 str C-O str C-O str C-H bend C-H bend
1123
1123
"Frequency ratio.
517
508
" Reference spectrum. acteristic of either several different trapping sites and/or formation of hydrogen bonded c ~ m p l e x e s . ' ~ In similar experiments in which matrices containing CH302 radicals were irradiated, absorption bands were identified which indicated the formation and stabilization of H 0 2 radicals and formaldehyde in the matrix. In the present study no positive identification for HOz was made. Two absorption bands appeared at 1728 cm-' (I6O) and 1697 cm-' (I8O) on photolysis of ethylperoxy radicals containing scrambled oxygen isotopes (Figure 1b). On photolysis of matrices containing only C2H5I6O2radicals, the higher frequency band was present; the lower frequency band appeared on photolysis of matrices containing only CzH5I8O2. These observations suggested that the species responsible for these bands probably contains only one oxygen atom. The observed '~O 1.019, ~ ) is frequency ratio for these two bands, V ( ' ~ O ) ~ / V ( at close to that expected for a carbon-oxygen (carbonyl) vibrational mode, at 1.025. Formation of CH20on photolysis in the CH3O2 system suggests that the carbonyl bands in the photolyzed C2H502 system are those of acetaldehyde. In Table 11, absorption bands whose intensities changes during photolysis tracked those of the 1728 or 1697 cm-l bands are tabulated, together with the frequencies recorded for the stronger absorption bands of acetaldehyde isolated in an Ar 10% O2matrix (reference spectrum). The matrix cavity containing CH3CH0 formed in the photolysis of C2HSO2 is likely to contain species capable of hydrogen bonding to the aldehyde. Absorption band frequency shifts, both positive and negative, with respect to those of the reference spectrum may be anticipated by analogy with similar frequency shifts found in matrix spectra of formaldehyde formed by the photolysis of CH302.l The C-C-0 bending frequencyI5 of acetaldehyde was
+
(14) Diem, M.; Lee, E. K. C . J . Phys. Chem. 1980, 72, 1769.
3487
species (C2H5'602)2 (C2H51802)2 v ( ~ ~ O ~ ) / V " 351 464 785 868 878 1020 1128
340 450 750 851 859 1010 1119
1.032 1.031 1.047 1.020 1.022 1.010 1.008
observed at 506 cm-l in the reference spectrum. In the C2H5O2 photolysis spectra, two absorption bands were found in this region at 517 cm-l (I6O) and 508 cm-l ( I 8 0 ) . The isotopic frequency O ) , for these bands at 1.018 is consistent ratio, v ( ' ~ O ) / V ( ' ~found with that expected for acetaldehyde containing these isotopes. The primary photolytic process in C2H5Oz on UV irradiation is not known. In gaseous H02, the primary process has been identifiedL6as
H 0 2 + hu (248 nm)
-
HO(X211)+ O(lD)
If a similar process takes place on photolysis of CzHSO2, acetaldehyde could be formed on H-atom abstraction by O('D) to form OH. Hydrogen bonding between the trapped pair could be the source of the acetaldehyde frequency shifts noted earlier. No spectral evidence for OH formation was found, possibly for the same reasons as suggested in an earlier study.' Tentative Vibrational Assignments for Dimer Bands of Ethylperoxy Radicals. From matrix annealing experiments several absorption bands were identified in the CzH5 I6O2and C2H5 1802 systems that could reasonably be assigned to dimers of the ethylperoxy radical. Some of these bands are shown in the spectra presented in Figure 2f. The frequencies of all the experimentally observed absorption bands attributed to (C2H5I6O2)2 and (C2HS'*O2)2 dimers are collected in Table 111 and tentative vibrational assignments made. As with the methylperoxy dimer,] only one absorption band was identified which could be assigned to an 0-0 stretching mode, based on the observed isotopic frequency shift ratio. The values obtained for the 0-0 stretching frequencies in the two dimers are very similar at 775 cm-' Both these fre(CH3l6O4CH3)and 785 cm-' (CZH5O4C2HS).
+
+
(15) Colthup, N. H.; Daly, L. H.; Wiberly, S. E. Introduction to IR and Raman Spectroscopy: Academic: New York, 1964. (16) Lee, L. C. J . Chem. Phys. 1982, 76, 1409.
3488 The Journal of Physical Chemistry, Vol. 91, No. 13, 1987 TABLE IV: Vibrational Assignments (cm-') for the C,H, Radical approx mode description CH, stretches asymmetric symmetric symmetric C H 2 stretches asymmetric symmetric C H 3 bends internal internal internal rocking rocking CH, bends scissors asymmetric C-C stretch C-H radical center bend
previous assignmenta
this study
2987 2920 2842
2844
3112 3033
31 14 3036
1440 1439 1366 1138
1442 1442 1369 1133 1025 1383
1175 540
1185 532
Reference 8.
quencies are in the range found in many organic peroxides] and contrast with the values found for 0-0 stretching modes in organoperoxy radicals of E 1 100 cm-*. IR Spectrum and UV Photolysis of the Ethyl Radical. Reference was made earlier in this paper to some preliminary studies on azoethane pyrolysis product isolation in argon matrices. The C2H5radical was identified by the appearance of a strong broad absorption band centered at 532 cm-I. In the only previous IR matrix isolation study on the radical,s this frequency was reported at 540 cm-I. Additionally, it was noted that other absorption bands of the C2H5radical recorded in the present study appeared at slightly different frequencies to those reported earlier. For this reason the radical's spectrum was subjected to a more detailed study. In Table IV, the vibration frequencies assigned to the C2H5 radical in this study are presented, together with those from the previous study.* With the exception of two of the CH3 stretching frequencies that could not be positively identified in this investigation because of absorption band overlapping problems, the present study confirms the results of the earlier vibrational assignments for the C2H5radical. In general, the differences between the two studies are small, probably reflecting the perturbing effect
Chettur and Snelson of the different matrix environments on the radical's vibration frequencies. Two absorption bands in the present investigation, not reported in the earlier study, at 1025 and 1384 cm-I were assigned to the C2H5 radical. Spectral regions of interest are shown in Figure 3. The radical center bending mode at 532 cm-' is shown to indicate the intensity of this band relative to some of the other bands in the radical's spectrum. The intensity of the absorption band at 1025 cm-' tracked that of other confirmed absorption bands in the radical's spectra, both during matrix deposition and on matrix annealing. Its nonassignment in the earlier study was probably a combination of the band's fairly low extinction coefficient and the presence of some residual absorption bands of the radical precursor, dipropionyl peroxide used in that investigation. The presence of a radical absorption band at 1384 cm-' was inferred from broadening of the ethane absorption band, a doublet at 1383 and 1379 cm-I, and changes in the relative intensity of the absorption band's two maxima. In C2H6reference spectra, the intensity ratio v( 1379 cm-')/v(1383 cm-') was measured at 2.2 whereas in the radical spectra this ratio varied between 1.3 and 1.5, implying the presence of an overlapping band at E 1383 cm-I. From the reporteds results of ab initio calculations on the C2H5 radical vibration frequencies, the two new frequencies were assigned to a CH, rocking mode (1025 cm-') and a CH, scissors bend (1383 cm-I). In an earlier study,8 photolysis of the C2H5radical by radiation S280 nm was reported to produce ethyl propionate, propionic acid, and acetylene. In the present study, matrices containing the C2H5 radicals were irradiated for periods of up to 20 h with a lowpressure Hg arc. No indication of photolysis of the ethyl radical was observed under these conditions. This finding strongly suggests that acetylene, found in the earlier photolytic investigation, was not formed directly by the suggested reaction pathway, C2H5 hv (5280 nm) C2H2+ H, H. It seems likely that some other species present in the radical's immediate environment in the matrix must have been involved in the acetylene formation.
-
+
+
Acknowledgment. The authors are grateful to the Atmospheric Sciences Branch of the National Science Foundation for supporting this study under Grant No. ATM-81-01294. Registry No. C2H502,3170-61-4; CIH, 2025-56-1; C2H5'80160, 107960-12-3; C 2 H 5 ' 8 0 1 8 0 , 107960-13-4; C H 3 C H l 6 0 , 75-07-0; CH3CH1'0, 3752-37-2; (C2H5I6O2)2, 107960-14-5; (C2H5'80,)2, 107960-15-6; azoethane, 821-14-7.
