J . Phys. Chem. 1987, 91, 913-919 belonging to pentagons or hexagons. Primitive heptagonal rings are rare at all temperatures, reflecting the fact that most heptagons are bridged by smaller polygons. The chance that a heptagon survives bridging to remain primitive is only about 2%. In terms of primitive polygons then, the network topology - --_in water is dominated increasingly by pentagons and hexagons as the temperature is reduced. These are the primitive polygons which feature in the low density ices and the common clathrate crystals. They are also the smallest polygons in which the 0-0-0 angles can approach their energetically optimal geometry and, as Figure 5 shows, the molecules belonging to five- and sixmembered primitive polygons make stronger H bonds than those belonging to smaller or larger rings. As noted earlier, one advantage of the primitive polygon concept over the non-short-circuited polygon concept is that the primitive polygon distribution in water can be characterized without going beyond heptagons. Another important difference may be noted by referring to Figure 3a. The two pentagons shown there be counted as being non-short-circuited, but not as being primitive. Because the structure shown includes a quadrilateral, several of
913
the 0-0-0angles involved are necessarily distorted away from the ideal tetrahedral geometry. Thus, the primitive polygon concept may focus more precisely on energetically stable structures in the network.
Conclusions In the previous work it was established that the isobaric-isothermal TIP4P Monte Carlo simulations correctly reproduce the anomalous expansion of water when it is cooled below 4 "C at 1 atm. The present analysis shows that the hydrogen-bonded network topology becomes increasingly dominated by the primitive hexagons which feature in the low density ices and the primitive pentagons which feature in the low density clathrate lattices, as the water is cooled. The water molecules belonging to the pentagons and hexagons are more strongly hydrogen bonded than those belonging to smaller or larger rings. Acknowledgment. Gratitude is expressed to the National Science Foundation for support of this research at Purdue, Registry No. H20, 7732-18-5.
Alkylperoxy and Alkyl Radicals. 3. Infrared Spectra and Ultraviolet Photolysis of I-C3H,02 and i-C,H, Radicals in Argon Oxygen Matrices
+
G. Chettur and A. Snelson* IIT Research Institute, Chicago, Illinois 60616 (Received: April 30, 1986)
Isopropyl radicals, formed by the pyrolysis of bis(1-methylethyl)diazene, were isolated in matrices of Ar + 10% O2and pure Ar. IR spectra of the trapped species were obtained. By use of oxygen isotopic labeling, the i-C3H702radical was identified and a partial vibrational frequency assignment was made. Under Hg arc irradiation, the i-C3H702radical was destroyed. Some of the photolysis products were identified. IR spectral data were obtained for the i-C3H7radical. Under Hg arc irradiation, i-C3H7was converted to n-C3H7,CH4, C2H4, possibly CH3, and some unknown species.
Introduction In the first paper in the series' we reported on the IR spectra of the methylperoxy radical and its dimer, dimethyl tetroxide. As noted in the previous paper, alkylperoxy radicals are important intermediates in the low-temperature oxidation of organic materials. Little is known spectrally about these species. The isopropylperoxy 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 r e g i ~ n ) . ~More , ~ recently, a structured electronic spectrum of the isopropylperoxy radical has been identified in the near-infrared showing a well-defined sequence of bands characteristic of an 0-0stretching mode.4 No infrared spectrum has been reported for the species. Alkyl radicals are also important intermediates in combustion processes.s At low temperatures and in the presence of oxygen they react readily to form alkylperoxy radicah6 Recent work has greatly improved the spectral characterization of low molecular weight alkyl radicals. UV spectra of the isopropyl and other alkyl radicals have been reported7 in the 195-370-nm wavelength range. (1) Ase, P.; Bock, W.; Snelson, A. J . Phys. Chem. 1986, 90, 2099. (2) Thomas, J. K. J . Phys. Chem. 1967, 71, 1919. (3) Adachi, H.; Basco, N. Int. J . Chem. Kine?. 1982, 14, 1125. (4) Hunziker, H. E.; Wendt, H. R. J . Chem. Phys. 1976, 64, 3446. (5) Benson, S . W.; Nangia, P.S . Acc. Chem. Res. 1977, 12, 223. (6) Ruiz, R.P.; Bayes, K. D. J . Phys. Chem. 1984, 88, 2592.
0022-3654/87/2091-0913$01 S O / O
Infrared spectra, of varying degrees of completeness, have been reported for the C2 through C, alkyl radicals by using the matrix isolation technique.* As a result of these studies, the general form of primary, secondary, and tertiary Siicyl radical IR spectra has been characterized. In this paper the iriirared spectrum of the isopropylperoxy radical is presented, together with data on its photolysis in the UV. During the study, the IR spectrum of the isopropyl radical was also obtained. Some new data are presented on this radical's vibration frequencies and on its photolysis in the UV.9
Experimental Section The matrix isolation cryostat and the molecular beam pyrolysis tube furnace assembly used in the study have been described previously.10 Isopropyl radicals were produced by the pyrolysis of bis( 1-methy1ethyl)diazene at -300 O C . To form isopropylproxy radicals, isopropyl radicals were allowed to react with argon matrices containing 10% oxygen during the trapping process. Isopropyl radicals were trapped in argon matrices when IR spectra of this species were studied. Matrix deposition times varied from 20 to 70 h. Absolute values of the radical matrix gas dilution ratios (7) Wendt, H. R.; Hunziker, H . E. J . Chem. Phys. 1984, 81, 717. (8) Pacansky, J.; Brown, D. W.; Chang, J. S. J . Phys. Chem. 1981, 85,
2562 and references cited. (9) Pacansky, J.; Coufal, H . J . Chem. Phys. 1980, 72, 3298. (10) Butler, R.; Snelson, A. J . Phys. Chem. 1979, 83, 3243.
0 1987 American Chemical Society
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The Journal of Physical Chemistry, Vol. 91, No. 4, 1987
Chettur and Snelson
Figure 1. IR spectra (280-920 cm-’) of reaction products formed in the interaction of i-C3H1radicals and a matrix of Ar + 10% 02:(a) spectrum of reaction products in an “as-deposited”matrix of Ar + 10% l602; (b) spectrum of the “as-deposited”matrix in (a) after photolysis for 14 h with an Hg arc; (c) spectrum of reaction products in an “as-deposited”matrix of Ar + 10% I8O2. PA s propane, PE propene, AE = bis(1-methylethyl)diazene, and i-Pr = i-C3H,
were not known. Most experiments were run at matrix dilution ratios sufficient to minimize dimerization of reactive species. In a few experiments, matrix dilution ratios were deliberately lowered to encourage dimerization of reactive intermediates to help in their identification. Bis( 1-methylethy1)diazene was synthesized according to a literature procedure.” Oxygen isotopes, 1802 (99%), and a 50% scrambled mixture of 160180 were obtained from ProChem. (Figures in parentheses are the manufacturer’s stated isotopic and the Ar matrix gas were Matheson Research purity.) 1602 Grade. Reference spectra of the following materials isolated in Ar + 10% O2and pure Ar matrices were generated: CH4, C2H4, C2H6,C3H6,C3H8,CH3COCH3,i-C3H70H,and (CH3)$HCH(CH3),. These materials were used as supplied since their purities were stated as 198%. Matrices were subject on occasion to irradiation with light. Two sources were used: an Osram 500-W Hg arc and a 1000-W GE tungsten filament lamp. Matrices were irradiated for periods up to 20 h through the cesium iodide windows of the cryostat. IR spectra (4000-200 cm-I) were recorded on a Perkin-Elmer 273 spectrophotometer. Reported frequencies are accurate to f 2 cm-’.
Results and Discussion Isopropylperoxy Radical Spectra. In preliminary experiments, conditions were established for the effective pyrolysis of bis( 1 methy1ethyl)diazene using pure argon matrices for trapping the decomposition products. At temperatures of -300 OC almost all the bis( 1methylethyl)diazene (estimated 195%) was pyrolyzed; the major stable products identified were propane and propene together with small amounts of methane and trace amounts of ethylene. At the matrix dilution ratios used, only trace amounts of tetramethylethane, formed by dimerization of isopropyl radicals, were detected. The formation of substantial amounts of the isopropyl radical was indicated by the appearance of the characteristic9 radical center, out-of-plane bending mode, at -374 (11) Renaud, R.; Leitch, L. C. Can. J. Chem. 1954, 32, 545.
cm-’. Annealing the matrix to -40 K resulted in tetramethylethane absorption bands growing in intensity at the expsense of isopropyl radical bands. By raising the pyrolysis temperature somewhat, we achieved complete destruction of the bis( 1methylethyl)diazene, albeit at the expense of a reduction in the amount of isopropyl radical stabilized. For this latter reason the lower pyrolysis temperature was used in most experiments. Based on the known chemistry of the system at ambient temperatures,’ reactions of potential importance in the matrix experiments were assumed to be as follows:
--
+
i-C3H7 O2
2i-C3H,02
-+
(adduct)
-
i-C3H,O2
(1)
+ O2
2i-C3H70
CH3COCH, + i-C3H70H+ O2 i-C3H702i-C3H7+ O2 (2)
Under conditions of good isolation, formation and trapping of the i-C3H702radical should be possible. Under conditions of poor isolation, or on annealing the matrix, self-reaction of the radicals (eq 2) may be expected to occur. A series of experiments was made in which the isopropyl radical was isolated in argon matrices containing either 10% I6O2,10% 1802, 5% I6O2+ 5% I8O2,or 2.5% I6O2+ 5% ’60180 + 2.5% 1 8 0 2 . Characteristic spectra for the 10% I6O2and 10% containing matrices are shown in Figures 1 and 2 for the spectral region 270-1520 cm-’. Spectra above 1520 cm-l are not shown since absorption band overlapping problems prevented the identification of new features being made. Absorption bands assignments in these spectra were made as follows: (1) A process of elimination, i.e. identification of absorption bands attributable to known stable species: CH4, C2H4,C3H6, C3H8(CH3)2CHCH(CH3)2,C3H60,i-C3H70H, and unreacted (GH,)2N!. (2) Identification of those absorption bands that decreased in intensity on annealing matrices to 35-40 K and recooling to 12 K. Presumably, because of the relatively large size of the isopropylperoxy radical, diffusion of the species in the matrix was
The Journal of Physical Chemistry, Vol. 91, No. 4, I987
Alkylperoxy and Alkyl Radicals
915
W
I
1
I
I loo
I
d]
1200
1300
1
I
1400
I500
C M-’
+
Figure 2. IR spectra (1000-1520 cm-I) of reaction products formed in the interaction of i-C3H7radicals and a matrix of Ar 10% 02:(a) spectrum of reaction products in an “as-deposited”matrix of Ar 10% I6O2;(b) spectrum of “as-deposited”matrix in (a) after photolysis with an Hg arc for 14 h; (c) spectrum of reaction products in an “as-deposited”matrix of Ar + 10% 1802. Absorption bands labeled (PA+) and (PE+) are overlapped to some degree by absorption bands of other species, probably i-C3H702.TEM = tetramethylethylene.
+
very slow. Only small changes in absorption band intensities, both reductions in existing bands and growth of new features, were recorded. (3) Identification of those absorption bands (generally the same as in ( 2 ) above) that decreased in intensity under irradiation from a Hg arc. (4) Identification of those absorption bands that showed oxygen with ISO2. isotopic frequency shifts on substitution of 1602 By use of the above criteria, the absorption bands shown in and i-C3H7l8O2.The Figures 1 and 2 were assigned to i-C3H71602 effects of Hg arc irradiation on the matrices are clearly shown by comparing spectral curves a and b in Figures 1 and 2. In spectra of the “as-deposited” matrices, Figures 1 and 2, essentially all the stronger absorption bands in the spectra could be assigned to the stable species noted above and the isopropylperoxy radical. A few weaker bands were also present in the spectra for which assignments were not made. Analogy with the results of a previous matrix study on the methylperoxy radical suggests that these bands
may be due to the dimer ( i - C 3 H 7 0 J 2and peroxide (i-C3H70)2. Indeed, a few weak absorption bands attributable to the dimer (based on limited data from matrix annealing experiments) appeared to be present; definitive assignments for these bands were not pursued further. In spectra of reaction products from the interaction of i-C3H7 radicals with argon matrices containing 5% I6O2 5% I8O2,no new major absorption bands were found to be present that were and Ar 10% l 8 0 , spectra. not identified in the Ar 10% 1602 This finding was consistent with the assignment of the “new” absorption bands to a species containing no more than two oxygen atoms. Finally, spectra were recorded of the reaction products formed by the interaction of isopropyl radicals with argon matrices containing 10%of an isotopic mixture of 1602:’60’80:’802 in the mole ratio 1:2:1, Spectral regions of interest are shown in Figure 3. Vibrational Assignment for the Isopropylperoxy Radical. The geometry of the isopropylperoxy radical has not been determined.
+
+
+
916
The Journal of Physical Chemistry, Vol. 91, No. 4, 1987 I
Chettur and Snelson
I
2;t
40
100
I
I
II
I
I ,
800
500
12 3
I100
I'
CM-'
Figure 3. IR spectrum in regions of interest of reaction products formed in the interaction of i-C3H7radicals and a matrix of Ar '60180 and 2.5% I8O2.
+ 2.5% 1602+ 5%
TABLE I: Vibrational Assignment (em-') for Various Oxygen Isotopically Labeled Isopropylperoxy Radicals (Frequencies below 1500 cm-') experimentally obsd freq
approx mode description for i-C3H71602"
'*
"9
'lo
'I I 'I2/ "I3 'I4
"I5 'I6 'I7 "I8 "19
vz0 '21
'22
u2, uZ4
1 in-plane def 1 out-of-plane def sym def
/ out-of-planerock 1 in-plane rock 1 CH bends 0-0 str c-c 1 c-c Str
C-0 str CCOO out-of-phase bend
u Z 5 CCOO '2,
in-phase bend
ccc ccc 1 bends
est freq assignb 1458 1458 1461 1461 1412 1412 1277 986 1226 1042 1368 1349 1105 1016 1016 825 532
i-C,H,I6O2
1178 1130 1153 1372 1310 1101 1101 884 789 515
300 465 367
305 450 348
i-C3H7160'80, i-C3H7l8Oi60
1114 1150
1088
511 516
i-C,H71802
U(~~O~)/U(~~O~)
1178 1107 1147
1.000 1.020 1.005
1372 1308 1059 I IO7 884 770 502
1.021 1.002 1.040 0.995 1.000 1.025 1.026
298 442 347
1.024 1.018 1.003
through u I 7 , CH, bending modes. bReference 12.
As with the methylperoxy radical,' a nonlinear C-0-0 group is assumed, resulting in a radical with low symmetry belonging to either the C, or C, point groups. For either structure, a total of 30 fundamental vibrational frequencies is expected, all of which are IR active. Three of these frequencies will correspond to internal rotations in the radical and will probably lie below the long-wavelength limit of the present study. Recently,I2 a vibrational assignment has been proposed for the isopropylperoxy radical, based on existing assignments for CH302,' i-C3Ha,13and i-C3H7.9 This proposed vibrational assignment for i-C3H,0z, together with the observed experimental data on oxygen isotope effects, was used in making the vibrational assignment shown in Table I for the isopropylperoxy radical. Of the vibrational modes shown in Table I, v2,, (0-0 stretch), "23 (C-0 stretch), and v24, u25 (C-C-0-0 bends) are expected to show pronounced oxygen isotope effects. Assuming complete decoupling of these modes from those of the rest of the radical, the value of the ratio 16v/1av may be estimated at 1.06 (0-0 stretch), 1.025 (C-0 stretch), and 1.025 (C-C-0-0 bend). In
-
(12) Wagner, A. F.; Melius, C., private communication. (13) Shimanouchi, T. J . Phys. Chem. Ref: Data 1978, 7, 1323.
the spectra of the radicals formed containing the mixed 160i80 species, each of these oxygen-dependent vibration modes will potentially have four discrete frequency components.1 The assignments of the experimental frequencies to these modes will be discussed. uZ4 and v25, CCOO Bending Modes. Two sets of bands at 5 15 and 305 cm-l and at 502 and 298 cm-' in the I6O2and I8O2spectra, respectively, have appropriate isotope shifts for assignments to the bending modes. The present assignment for v24 is compatible with that found for the analogous mode in CH3I6O2at 492 cm-'. The quartet structure of this band in the spectrum of the scrambled oxygen isotopic species was also observed for this mode in the methylperoxy radical spectrum.' In the spectrum containing the scrambled oxygen isotopes, the v25 absorption band appeared as a rather broad feature with a single maximum at -301 cm-'; apparently, the quartet structure of this band was not resolved. u22 CO Stretching Mode. Bands at 789 and 770 cm-' in the I6O2and I8O2spectra showed appropriate isotope shifts for assignment to the C-I6O and C-'*O stretching modes. This mode in C H 3 0 2occurred at 902 cm-I. The lower value found in iC3H702 is consistent with the larger effective mass of the alkyl group in this radicals. In spectra containing the scrambled oxygen
Alkylperoxy and Alkyl Radicals isotopes, these bands did not have a quartet structure. This finding implies that the C - 0 stretching modes in the pairs i-C3H71602 i-C3H71s0160 are essentially i-C,H7160-'80 and i-C3H71802, identical. A similar situation was also found for these frequencies in the methylperoxy radicals. vzo, @O Stretching Mode. The 0-0 stretching frequencies in CH3O2,' CF3O2," and HO2I4have been reported at 1112, 1092, and 1101 cm-I, respectively. Frequencies appearing at 1101 and 1059 cm-I in the 1602 and IsO2spectra (Figure 2), with an appropriate isotopic frequency ratio v ( ' ~ O ~ ) / V (of' ~1.04, O ) are and iassigned to the 0-0 stretching modes of i-C3H71602 C3H7I802, respectively. In radicals containing the scrambled 0 C3H7180160 oxygen isotopes, the mixed species C 3 H 7 1 6 0 1 8and are expected to have similar 0-0stretching frequencies; the single absorption band at 1088 cm-' (Figure 3) is assigned to these species. In the methylperoxy spectra' these two isotopically labeled radicals also had coincident 0-0 stretching frequencies. An apparent inconsistency in the above assignment for v20 resides in the observed relative absorption band intensities of the triad at 1059, 1088, and 1101 cm-' (Figure 3), which was not in the ratio 1:2:1 required by the random distribution of the respective isotopic species; the absorption band at 1101 cm-' was approximately twice as intense as expected. This difficulty is apparently due to the absorption band at 1101 cm-' (0-0 stretch) in the i-C3H7I6O2 spectra being coincident with another vibrational mode of the radical not directly associated with the 0-0 stretching ( ~ ~de~), mode. Absorption band intensity ratios, I ( V ~ ~ ) / Iwere termined for the-i-C,H7l6O2and i-C3H7I8O2 radicals at l .54f 7% and 0.92 f 8%, respectively, with the error quoted as the standard deviations of the means from measurements on four sets of spectra for each isotope. Although transference of intensity ratios for similar vibrational modes from one species to another is at best an approximation, the above findings are consistent with has two different vibrational modes the suggestion that i-C3H71602 with a frequency of 1101 cm-I. There are some other difficulties in this region of the spectrum concerning absorption bands which exhibit small oxygen isotope spectrum (Figure 2a), the absorption shifts. In the i-C3H7I6O2 band at 1101 cm-I appeared with high intensity. In the iC3H702180, spectrum (Figure 2c) the 1101-cm-' band is absent, and a strong absorption feature is found at 1107 cm-I. It would thus appear that the i-C3H7I6O2absorption band, assumed to be coincident with the 0-0 stretching mode at 1101 cm-I discussed above, has been shifted in frequency to 1107 cm-I in i-C3H71802. Although, an "up"-shifted frequency is not generally associated with replacement of a light by a heavy nuclei, there are welldocumented examples of such b e h a ~ i 0 r . I ~An absorption band this band at 1130 cm-' (Figure 2a) is assigned to i-C3H7I6O2; overlaps a much weaker band of undecomposed bis( l-methylethy1)diazene. An analogous band does not appear at this frequency in the corresponding i-C3H7"02spectrum (Figure 2c). However, in the spectrum of the scrambled oxygen isotopes (Figure 3) a new absorption band appeared at ~ 1 1 1 cm-I, 4 with approximately twice the absorption intensity of the 1130-cm-' band; the 1107-cm-' band was also present with a somewhat greater intensity than the 1130-cm-' absorption band. Because of the close proximity of the 1107- and 11 14-cm-' bands, only qualitative estimates for these band intensities were possible. The above situation would be consistent if it assumed that the 1107-cm-' absorption band consists of two coincident vibrational modes of i - C 3 H 7 1 S 0one 2 , of which is downshifted to 1101 cm-I and one in which is up-shifted to 1130 cm-I, on replacing I8O2with 1602 the radical. Assuming the validity of the above interpretation, these and the remaining absorption bands in the spectra of the isopropylperoxy radicals were assigned as indicated in Table I. The assignments to specific .C-H bending modes must be regarded as somewhat arbitrary; interchanging the various frequencies among the same modes would in many cases be equally acceptable in the (14) Jacox, M. E.; Milligan, D. E. J . Mol. Specfrosc. 1972, 42, 495. (15) Falk, M.; Whalley, E. J . Chem. Phys. 1961, 34, 1554.
The Journal of Physical Chemistry, Vol. 91, No. 4, 1987 917 absence of more data. Methyl rocking modes in C H 3 0 2 and C2H5I6 have been assigned at 1175 and 1112 cm-I and at 1138 and 11 16 cm-', respectively. The values assigned in the present study are of the same magnitude. The two C-H bending frequencies of the central carbon atom were assigned based on their similarity to the estimated frequencies for the radical.I2 The C-C stretching modes were assigned by analogy with the values observed for these modes in propane, at 1054 and 867 cm-', and in the ethyl radical,I6 at 1175 cm-I. The two skeletal C-C-C bends were assigned based on their similarity to the estimated values.12 It was not possible to make assignments for the remaining C H , bending modes of the radical, v8 through v i 3 since these frequencies were overlapped by those of C3Hsand C3H6. However, it was clear that these modes were present in the spectra, both from the distortion of the observed absorption band envelopes of C3H8and C3H, and from the displacements of the absorption band maxima by a few wavenumbers from those found in the reference spectra. Photolysis of the Isopropylperoxy Radical. Absorption bands assigned to the isopropylperoxy radical in this study could be removed completely by exposure of the matrices to radiation from an Hg arc lamp for 20 h. Exposure to radiation from 1000-W tungsten lamp over a period of 20 h did not affect the intensities of the i-C3H702absorption bands. During the course of the UV photolysis, new absorption bands appeared in the spectrum. Easily identified were absorption bands of C 0 2 , C O , and H 2 0 which showed multiplet structure characteristic of either several trapping sites and/or formation of hydrogen-bonded c o m p l e ~ e s . ~ ~ ~ ~ In similar experiments in which matrices containing methylperoxy radicals were irradiated, absorption band were identified which indicated the formation and stabilization of HO, radicals and formaldehyde. In the present study no positive identification for H 0 2 was made. Broad absorption bands -25 cm-I wide at half peak height were observed at 1716 cm-' (I6O2)and at 1688 in spectra containing the respective isotopic species. cm-I ( "02) In matrices containing a scrambled isotopic mixture 2.5% I6O2 5% 160180 2.5% IsO2,again only the two absorption bands were observed in this region at 1716 and 1688 cm-', strongly suggesting that the responsible species contained only one 0 atom. By analogy with the C H 3 0 2system, it is tempting to assign these bands to two oxygen isotopic species of acetone in which the C=O stretching mode is perturbed by hydrogen bonding.18 In Ar 10% O2matrices, the C=l6O stretching frequency of acetone in the reference spectrum was recorded at 1725 cm-I. In the photolysis spectrum the band was red-shifted by -9 cm-I, a shift which is similar in magnitude to values reported for a matrixisolated hydrogen-bonded carbonyl groups in formaldehyde.18 Other bands were present in the photolysis spectra which could plausibly be assigned to acetone; some of these bands were found at frequencies essentially identical with those of the reference spectrum of ( C H 3 ) 2 C 0in an Ar + 10% 0, matrix, while others showed frequency shifts. The mechanism involved in the photolysis of the isopropylperoxy radicals appeared to be similar to that reported for the methylperoxy radica1.l Photolysis occurs by absorption of energy in the radicals UV absorption band in the 200-300-nm region. Analogy with the UV photolysis of HO2I9suggests the primary process may be
+
+
+
i-C3H702
+ hv (200-300
nm)
-
+
i-C3H70 O('D)
followed by further reaction of the products to produce C 0 2 ,CO, H 2 0 , and probably (CH3),C0. Insufficient data are available to indicate the details of the overall decomposition process. I R Spectrum and UV Photolysis of the Isopropyl Radical. In the initial phase of the study bis( 1-methy1ethyl)diazene was pyrolyzed and the products were isolated in argon matrices. The isopropyl radical was identified by its characteristic out-of-plane (16) Pacansky, J.; Dupuis, M. J . Am. Chem. SOC.1982, 104, 415. (17) Cyvin, B. N.; Cyvin, S. J.; Snelson, A,, accepted for publication in Z . Anorg. Allg. Chem. (18) Diem, M.; Lee, E. K. C . J . Phys. Chem. 1982, 86, 4507. (19) Lee, L. C. J . Chem. Phys. 1982, 76, 4909.
918
Chettur and Snelson
The Journal of Physical Chemistry, Vol. 91, No. 4, 1987 I
I
I
‘I
Figure 4. IR spectra (300-800 cm-I) of bis( 1-methylethy1)diazene pyrolysis products isolated in an Ar matrix: (a) spectrum of the “as-deposited” matrix; (b) spectrum of “as-deposited”matrix in (a) after 14-h photolysis with an Hg arc.
0 ) i-Pr IN
Ar MATRIX
I
N
C
W
C
I c
-
ICI ,
i
1
‘“A00
mlo
900
t
I cM-I
1100
I ,,
I
1200” 3ooo
l
3100
Figure 5. IR spectra (800-1200 and 3000-3150 cm-I) of bis(1-methy1ethyl)diazene pyrolysis products isolated in an Ar matrix: (a) spectrum of the “as-deposited”matrix; (b) spectrum of “as-deposited”matrix in (a) after 14-h photolysis with an Hg arc.
-
bending frequency mode.9 In the present investigation this appeared as a rather broad absorption band ( 15 cm-’ wide at half peak height) centered at 364 cm-l. In the only literature report on the IR spectrum of the isopropyl radical isolated in argon mat rice^,^ this band was stated to be broad, having a doublet structure, with two well-defined maxima at 382 and 369 cm-I. In the previous study, it was noted that a number of the isopropyl radicals absorption bands, in particular the radical center C-H stretching mode at 3069-3058 cm-l in addition to the radical center bending mode, exhibited splitting; this was taken as evidence for different radical conformers present in the matrix. However, in our spectra, neither of these bands showed any evidence of splitting; for this reason the spectrum of the isopropyl radical was investigated in a little more detail. Some spectral regions of interest containing absorption bands of the isopropyl radical are shown in Figures 4a and 5a. The spectral regions 2800-3000 and 1300-1 500 cm-’, containing the bulk of the C-H stretching and C-H bending modes, are not presented since there was extensive overlapping of the radical bands with those of propane and propene; within the limits of these difficulties, the earlier assignmentsg for absorption bands in these regions were confirmed. From spectra shown in Figures 4 and 5 it is clear that the radical center C-H stretching and the outof-plane bending modes at 3052 and 364 cm-’, respectively, appear as well-defined singlets at somewhat lower frequencies than those reported in the previous study. These findings suggest that the
previous splitting observed in these absorption bands might, in part, be a consequence of the more unsymmetrical matrix environment around the trapped isopropyl radicals (COz and Ar) in the earlier study as opposed to Ar only in this study, rather than definitive evidence for the existence of several radical conformer^.^ In the earlier study,g it was noted that the intensity of the out-of-plane bending mode of i-C3H7 appeared to be relatively low with respect to other absorption bands in the spectrum. This observation was made based on comparison with spectra for methyl and ethyl radicals in which the out-of-plane bending modes were clearly the most intense absorption bands of the radicals. The possibility that the band’s low relative intensity in i-C3H7might be connected with orientation effects was investigated. To this end, spectra were recorded with the matrix window surface positioned at angles of either 90’ or 60’ with respect to the analyzing beam in the spectrometer. Assuming random orientation of the trapped species, the geometry of the system would dictate an increase in absorption band intensities 160/190 of -1.16. In Table I1 the experimental data are present for selected absorption bands of several different trapped species. The experimental error associated with the measured intensity ratios is estimated at -24%. The magnitudes of these ratios strongly suggest that, in the present study, the slightly nonplanar i-C3H7radical center is tending to orient itself parallel to the matrix surface. Similar orientation effects for planar molecules trapped in matrices have been reported.*O In the earlier study? the relatively low intensity
The Journal of Physical Chemistry, Vol. 91, No. 4 , 1987
Alkylperoxy and Alkyl Radicals TABLE 11: Intensity Measurements on Some Absorption Bands Appearing in the Matrix Spectrum from the Pyrolysis Products of Bis( 1-methylethy1)diazene as a Function of Matrix Surface-IR Analyzing Beam Orientation absorp band freq, cm-' 364 879 1119 1050 1043 99 1
window orientation species
190
16,
160/190
i-C3H7 i-C3H7 (CH3),C,H,
0.207 0.051 0.116 0.050 0.297 0.517
0.446 0.063 0.127 0.069 0.364 0.604
2.15 1.24 1.09 1.38 1.22 1.16
C3H8 C3H6
C3H6
of this band was taken as evidence in favor of the existence of several conformers of the isopropyl radical; the present findings suggest that orientation effects may also be significant in lowering the apparent intensity of this absorption band with respect to other bands in the radical's spectrum. The results of irradiating an argon matrix containing isopropyl radicals with an Hg arc for 14 h are shown in Figures 4b and 5b. The isopropyl radical bands at 364 and 3052 cm-' have almost disappeared. (Further irradiation resulted in complete bleaching.) In addition, two absorption bands in the spectrum were identified at 1165 and 879 cm-I, which consistently (a total of six experiments) disappeared on irradiation. On the basis of this behavior, these two bands were assigned to the isopropyl radical. The 879-cm-' frequency could plausibly be assigned to a C-C stretching mode and that at 1165 cm-I to a C H 3 rocking mode. From the spectra in Figures 4 and 5 it is clear that several new absorption bands appeared on irradiation of the matrix containing i-C3H7. Four of these new absorption bands, those at 523, 1035, 3028, and 31 18 cm-', whose intensity variations tracked each other during the photolysis, were assigned to the same species. Comparison of three of the frequencies with those reported for the n-propyl radical at 530, 3017.5, and 3100 cm-I strongly suggests that these new features be assigned to the n-propyl radical. Annealing the matrix to 40 K resulted in absorption bands assignable to n-hexane appearing and a concomitant reduction in intensity of those bands attributed to the n-propyl radical. Although the frequency correspondence is not perfect, the differences are plausibly the result of the different matrix environments: Ar + C02*'in the earlier study and Ar in the present study. After photolysis, there was evidence for a very small increase in the amount of propane and propene trapped in the matrix, whereas methane and ethylene concentrations approximately doubled. In addition to these identifiable changes in the matrix composition, some other species were obviously present as indicated (20) Snelson, A. J. Phys. Chem. 1967, 71, 3202. (21) Pacansky, J.; Home, D. E.; Gardini, G. P.; Bargon, J. J. Phys. Chem. 1977, 81, 2149.
919
by three new absorption bands appearing at 604, 746, and 801 cm-I. Relative intensity measurements made on these bands in several experiments indicated they did not arise from the same species. The absorption band at 604 cm-l could possibly be dssigned to the out-of-plane bending mode of the CH, radical. This mode has been reported at 61 1 cm-I (photolysis of CH4 in Ar),22 617 cm-' (pyrolysis of CH31, Ne matrix),23 612 cm-I (photolysis of acetyl benzoyl peroxide in Ar),24 and 608 cm-I (pyrolysis of CH31, Ar m a t r i ~ ) . ' ~ None of the above matrix environments were exactly the same as used in this investigation. The differences between the observed frequency at 604 cm-', tentatively assigned to the CH, radical in this study, and those reported for CH3 in the literature could be due to matrix effects. At the present time we are unable to make assignments for the two bands at 746 and 801 cm-I. Recently, the UV photolysis (
