7772
J. Phys. Chem. 1996, 100, 7772-7777
Resonance Raman Spectroscopy of the 1-Methylallyl Radical Darcy H. Tarrant, James D. Getty, Xianming Liu, and Peter B. Kelly* Department of Chemistry, UniVersity of California, DaVis, California 95616 ReceiVed: September 5, 1995; In Final Form: January 8, 1996X
The first vibrational spectrum of the 1-methylallyl radical is detected using resonance Raman spectroscopy with excitation from 237 to 232 nm. The vibrational frequencies of five symmetric fundamentals, the H3CCCC bend, the CCC bend, the H3C-C stretch, the C-H in plane bend, and the CH2 scissors, of the 1-methylallyl radical are reported. The even overtones of the CH3 torsion, the C-H out of plane bend, the H3C-CCC bend, and the CCC bend are identified. The vibrational assignments of the resonance Raman spectra are based upon comparison with calculated vibrational frequencies generated by UHF method using a 6-31G* basis set and experimental values for the allyl radical, the β-methylallyl radical, and similar molecules. Excitation at 236.05 and 234.95 nm yields significant resonance enhancement of overtone and combination bands associated with the H3C-CCC bend and CCC bend, respectively. The variation of intensities in the Raman spectra with excitation wavelength yields excited state vibrational frequencies for the H3C-CCC bend and CCC symmetric bend of 305 and 502 cm-1, respectively. Examination of the observed intensity patterns in the resonance Raman spectra indicates that the initial excited state dynamics of the 1-methylallyl radical are dominated by bending motions of the carbon chain.
I. Introduction The allyl radical, CH2CHCH2, and its substituted forms are important key intermediates in combustion processes and many atmospheric photochemical reactions.1,2 Allyl chloride, a precursor of the allyl radical, is a key intermediate in the photochemical production of smog.3 Allyl chloride photooxidizes to produce chloroacetaldehyde in the atmosphere4 which may contribute to the photochemical depletion of ozone thorough free radical reactions.5 Chesick proposed that the photolysis of cis-2-hexene and cis-2-butene at 202.6 and 206.2 nm at low pressure yields smaller olefins via the CH3CHCHCH2, 1-methylallyl radical, intermediate.6 Collin et al. have determined that the photolysis of 3-methyl-1-butene and cis-2-pentene at 163 nm produces the 1-methylallyl radical when the C-C bond of the precursor cleaves β to the CdC double bond.7,8 Absorption9 photoelectron10 resonance-enhanced multiphoton ionization11 and electron spin resonance spectroscopic techniques12,13 have been employed to study the 1-methylallyl radical. Callear and Lee recorded the absorption spectrum of the 1-methylallyl radical through flash photolysis of 3-chloro1-butene, 3-methyl-1-butene, crotyl alcohol (CH3CHCHdCH2OH), cis- 2-pentene, 1-butene, trans-2-butene, and cis-2-butene.9 Eight broad diffuse bands consisting of a double progression were observed in the 226.0-238.0 nm region. The band origin at 237.8 nm and a weak absorption at 232.0 nm define the beginning of each band progression.9 Schultz et al. recorded the photoelectron spectrum of the 1-methylallyl radical by pyrolysis of 2-methyl-2-buten-1-yl nitrate, CH2dCHCH(CH3)CH2ONO, at 740 and 490 °C. They determined the vertical and the adiabatic ionization potentials to be 7.67 ( 0.02 and 7.49 ( 0.02 eV, respectively.10 The vibrational progressions on the structured bands with an approximate spacing of 300, 520, and 590 cm-1 could not be identified. Band broadening caused by excitation upon ionization of low-frequency torsion modes and the overlapping of the cis- and trans-photoelectron bands of the 1-methylallyl radical masked some of the structure present on the individual bands. A weak band at 9.0 eV was tentatively X
Abstract published in AdVance ACS Abstracts, April 1, 1996.
S0022-3654(95)02625-6 CCC: $12.00
assigned to the He Ib of CH2O and the formation of a small amount of 1,3-butadiene and/or 2-butene from recombination reactions.10 Hudgens has reported the REMPI spectra of cisand trans-1-methylallyl radicals [C(CH3)HCHCH2, m/z ) 55] by reactions of chlorine and fluorine with cis- and trans-2-butene at 465-485 nm.11 The REMPI spectrum of the cis radical exhibits a broad band centered at 472.5 nm and a less intense broad band at 464 nm. The spectrum of the trans radical shows six poorly resolved broad bands centered at 468, 471.5, 474.5, 477.2, 481, and 483.7 nm. Either a 1 + 2 or a 2 + 1 REMPI mechanism may account for the photon order of the resonant state since three laser photons can ionize the radicals.11 Kochi and Krusic employed ESR spectroscopy to observe a mixture of cis- and trans-1-methylallyl radicals during hydrogen abstraction of 1-butene with tert-butoxy radicals.12 Separate isomers of the cis- and trans-1-methylallyl radicals were produced with cis-2-butene and trans-2-butene precursors.12 Discroll et al. employed matrix isolation electron spin resonance spectroscopy (MIESR) to detect the production of 1-methylallyl radicals when 1-butene was passed over B2O3 at 450 °C.13 The 1-methylallyl radical has been studied by various theoretical methods. Traegar calculated ab-initio vibrational frequencies for the 1-methylallyl radical and cation at the STO3G and 3-21G levels; however, the vibrational modes were not assigned.14 Traegar correlated two of his calculated vibrational frequencies (298 and 508 cm-1) of the 1-methylallyl cation with two vibrational spacings observed in the photoelectron spectrum (300 and 520 cm-1). Both frequencies were described as in plane skeletal deformations involving changes in the CH2-CHCH bond angle.14 Pople et al. performed ab-initio molecular orbital calculations on the staggered and eclipsed conformations of the methyl group about the partial double bond of the 1-methylallyl cation, anion, and radical in the cis and trans configurations using various STO-3G and 4-31G basis sets.15 Their results predict that the trans isomer of the 1-methylallyl radical is preferred with the methyl group in an eclipsed confirmation about the partial C-C double bond.15 The trans conformer was 3-3.6 kcal/mol lower in energy than the cis conformer. Similar results were obtained with MINDO16 and © 1996 American Chemical Society
Spectroscopy of the 1-Methylallyl Radical CNDO/2 calculations.17 Jaris and Khalil have also performed MINDO calculations on the 1-methylallyl radical to determine the electron density around each atom.16 Boyd et al. used a UHF/6-31G(d) basis set to calculate the structure of the 1-methylallyl radical and determined that the species is planar at the radical center.18 The use of resonance Raman spectroscopy to examine excited state dynamics and ground state vibrational levels of molecules to very high overtones, near the dissociation limit, was pioneered by Kinsey and co-workers.19,20 We have applied the technique to examine the ground state vibrational frequencies and excited state dynamics of conjugated hydrocarbon radicals.21-23 Upon excitation at 220 nm, the initial dynamics of the allyl radical are consistent with photoisomerization to form the cyclopropyl radical as indicated by the strong intensity in the allyl CCC symmetric stretch, the allyl CCC symmetric bend, the C-H symmetric out of plane bend, and the CH2 twist.21 The resonance Raman spectrum of the 2-methylallyl radical reveals four fundamentals (the symmetric CH2 rock, the symmetric CCC stretch, the C-CH3 bend, and the CCC bend) and an overtone (the symmetric CH2 twist).23 The excited state dynamics of the 2-methylallyl radical, CH2C(CH3)CH2, appear to be similar to those of the allyl radical, CH2CHCH2. The effect of methyl substitution on the 2 position of the allyl radical is to red shift the electronic band origin without significant variation of the ground state vibrational frequencies of the allyl moiety.23 The infrared vibrational spectrum of the 1-methylallyl cation has been recorded independently from crotyl chloride and from homoallyl bromide at 180 K in a SbF5 matrix by Schleyer and his co-workers,24 but there has been no previous observation of the vibrational spectrum of the 1-methylallyl radical. The first vibrational spectrum of the 1-methylallyl radical employing resonance Raman spectroscopy is presented. The modes of vibrations and the approximate values for their corresponding frequencies are determined with ab-initio calculations using a 6-31G* basis set. The vibrational frequencies of the 1-methylallyl radical are assigned using the frequencies of the allyl radical, the 2-methylallyl radical, and similar molecules. II. Experimental Section The Nd:YAG pumped dye laser system used to study allyltype radicals has been previously described in detail.21 The 1-methylallyl radicals were generated from 213 nm photolysis of crotyl chloride (1-chloro-2-butene, Aldrich, 95% purity), 3-chloro-1-butene (Aldrich, 98% purity), and crotyl bromide (1-bromo-2-butene, Aldrich, 85% purity). The spectra were taken using 750 mm slits (bandpass 1.85 nm) with an uncertainty of 6 cm-1 in determining the band center. The relative intensity of the CdC stretch (1667 cm-1)25 of crotyl chloride with and without photolysis was examined to determine the extent of photolysis of the precursor (∼33% photolysis). The approximate radical concentration was 1.6 × 1017 molecules/cm3 after 213 nm photolysis assuming unit quantum efficiency in producing the 1-methylallyl radical. All observed radical features were due to the single photon photolysis product as indicated by the linear correlation between the photolysis power and the radical intensity. The radicals were probed using a tunable laser beam. A probe wavelength of 232-238 nm was produced as the third harmonic of a tunable dye laser. The probe radiation was generated by frequency doubling the fundamental to the dye laser in potassium dideuterium phosphate (KDP) and by sum mixing the second harmonic with the remaining fundamental in beta barium borate (BBO) to yield probe radiation at 238 nm (∼0.25 mJ/pulse). A 14 ns delay between the photolysis and probe beam allowed for sufficient
J. Phys. Chem., Vol. 100, No. 19, 1996 7773
Figure 1. Resonance Raman spectrum of the 1-methyl radical with excitation at 237.75 nm. The asterisks denote vibrational bands of the 1-chloro-2-butene precursor.
cooling of the radical through collisions with the methane buffer gas. The Raman scattering was detected with a solar blind photomultiplier tube, sampled at various time intervals with a gated integrator, and the data were stored on a PC/AT computer. Mercury lines were used to calibrate the spectra. The conformation of the radical (cis or trans) produced depends upon the experimental conditions used to generate the radical. Kochi and Krusic were able to separately produce the cis and trans forms of the radical for ESR spectroscopy from cis and trans precursors.12 Hudgens was able to obtain the REMPI spectrum of the cis and trans form of the 1-methylallyl radical during the reaction of cis- or trans-2-butene with chlorine or fluorine.11 Once produced, the conformation is locked in by the rigidity of the extended π bonding in the allylic moiety. The trans configuration is lowest in energy for both crotyl chloride and crotyl bromide. Thus photolysis of those precursors is expected to produce the trans-1-methylallyl radical. The trans conformer is lower in energy than the cis conformer by 3-3.6 kcal/mol based on ab-initio calculations.15,18 Depolarization ratio measurements were measured with a probe polarizer and Raman analyzer.26 The device incorporates a stacked quartz plate polarizer with 10 quartz slides at Brewster’s angle. A magnesium fluoride polarization scrambler was placed between the analyzer and the monochromator slit to remove any polarization effects of the spectrometer. The depolarization ratios of the C-H stretching frequencies of methane were used to check the system. The depolarization ratio of the totally C-H symmetric stretch of methane (ν1, 2917 cm-1) was 0.051 ( 0.015. The depolarization ratio for the asymmetric C-H stretch (ν3, 3020 cm-1) was 0.759 ( 0.078. The literature value for ν1 is 0.08. The depolarization ratio of 0.75 is expected for the asymmetric stretch by symmetry.27 III. Results Figure 1 shows the first vibrational spectrum of the 1-methylallyl radical. The resonance Raman spectrum of the 1-methylallyl radical was produced from crotyl chloride with excitation at 237.757 nm. The 1-methylallyl radical has 27 modes of vibration: 18 modes of A′ symmetry and 9 modes of A′′ symmetry. Five modes of A′ symmetry are clearly observed: the H3C-CCC bend (289 cm-1), the allyl CCC bend (499 cm-1), the H3C-CCC stretch (872 cm-1), the C-H in plane bend (1265 cm-1), and the CH2 scissors (1492 cm-1). The band at 1008 cm-1 may either be due to the CCC stretch, or the first overtone of the allyl CCC bend. A first overtone of
7774 J. Phys. Chem., Vol. 100, No. 19, 1996
Tarrant et al.
TABLE 1: Vibrational Assignments for the 1-Methylallyl Radical assignment
ab-initio 6-31G* (90% corr)
1-methylallyl radical, this work
2-methylallyl radicala
allyl radical b
methyl formatec
propenalc
A′ ν1 CH2 asym stretch ν2 CH asym stretch ν3 CH2 sym stretch ν4 CH sym stretch ν5 CH3 asym stretch ν6 CH3 sym stretch ν7 CH3 deform (asym) ν8 CH2 scissor (sym) ν9 CH asym bend ν10 CH3 deform (sym) ν11 CH ip sym, bend ν12 CCC asym stretch ν13 CCC sym stretch ν14 CH3-C-C asym str ν15 CH2,CH3 rock (sym) ν16 C-CH3 sym stretch ν17 C-C-C sym bend ν18 H3C-C-CC sym bend A′′ ν19 CH3 stretch (asym) ν20 CH3 deform (asym) ν21 CH3 rock ν22 CH out of pl bend ν23 CH2 out of pl bend ν24 C(H)CH3 op bend ν25 CH2, HCCH3 twist sym ν26 CH2, H-C-CH3 op bend ν27 CH3 torsion a
3092 3015 3013 3003 2949 2873 1482 1478 1449 1403 1305 1219 1132 1077 960 834 476 273 2916 1462 1007 918 695 669 499 188 84
3107 3051 2943 3045 2969 1454 1492
1492
1488
1392
3103 3028 3000 2800
1420 1445
1265
1275 1037
1184 1066 912
872 499 289
865 432
427
327 303d
1904(OT)
1936(OT) 1298(OT) 1076(OT)
205(OT)
3012 1443 1168 1032
993
1036(OT) 130
Reference 23. b References 21 and 22. c Reference 31.
the H3C-CCC bend is observed at 582 cm-1. The band at 792 cm-1 is attributed to a combination band of the H3C-CCC bend and the symmetric CCC bend of the 1-methylallyl radical. The bands labeled with an asterisk (1667 and 692 cm-1) are the CdC stretch and the CsCl stretch of the crotyl chloride precursor.25 Resonance enhancement produces radical intensities much larger than the methane or precursor signals despite the lower concentration of the radicals. Matrix isolation infrared studies have been performed on the allyl27 and 2-methylallyl radicals,29,30 but there have been no previous infrared studies of the 1-methylallyl radical. The normal modes of vibration and the symmetry of each mode were correlated with a set of calculated vibrational frequencies generated using the Spartan program (Wavefunction Corp., Irvine, CA). A 6-31G* basis set was used in calculating the vibrational frequencies for the 1-methylallyl radical in the trans-eclipsed conformation. The modes of vibration and the calculated frequencies generated with Spartan are used to make preliminary assignments to the experimentally observed features in the 1-methylallyl radical spectrum in Table 1. Ab-initio frequencies were in good agreement with the observed vibrational bands of the allyl radical20 and are expected to have similar accuracy for the 1-methylallyl radical. Errors associated with calculated vibrational frequencies using the UHF/6-31G* basis set are typically 10-12% for comparisons with experimental frequencies. The calculated vibrational frequencies of the 1-methylallyl radical agree with Traeger’s 3-21 G vibrational values (∼(53 cm-1 ( 8% difference).14 The vibrational assignments of the 1-methylallyl radical can be compared to the experimental vibrational frequencies of similar radicals and molecules. The vibrational frequencies for the 1-methylallyl radical correlate with experimental values obtained for the allyl radical,20,22 the 2-methylallyl radical,22 methyl formate,31 and propenal.31 The vibrational frequencies are compared with the eclipsed and gauche conformations of 1-butene,33 trans-1,3butadiene,34 and the two isomeric forms of butene in Table 2.33
The resonance Raman spectrum with 236.05 nm excitation (Figure 2) shows resonance enhancement of the H3C-CCC bend (ν18 ) 286 cm-1) and the overtone 2ν18, at 582 cm-1. Intensity in combination bands of the H3C-CCC bend is observed at 790 cm-1 (ν18 + ν17) and 1170 cm-1 (ν16 + ν18). A combination band of the first overtone (2ν18) is observed at 1085 cm-1 (2ν18 + ν17). The bands at 500, 876, 1013, 1259, and 1902 cm-1 are the same bands previously described in the spectrum at 237.75 nm. The C-H symmetric stretch (2917 cm-1) and the C-H asymmetric stretch (3015 cm-1) of the methane buffer gas appear in the spectrum. Assignment of the fundamental, overtone, and combination bands of the 1-methylallyl radical provides enough information to make reasonable assignments to the band progressions observed in Hudgen’s REMPI spectrum11 of the trans-1methylallyl radical and the vibrational spacing observed in the photoelectron spectrum of Beauchamp et al.10 The vibrational separations at 268, 507, and 838 cm-1 calculated for the REMPI spectrum have been assigned to the modes for the 1-methylallyl radical in Table 1 assuming a band origin located at 471.5 nm for the trans conformer of the radical. IV. Discussion Placzek’s polarizability rule (0 < F < 3/4) can be used to determine whether a particular observed frequency is classified as a totally or nontotally symmetric normal mode.26,34 The depolarization ratio (F) is defined as the ratio of the Raman scatter perpendicular to the incident radiation (I⊥) to that of the Raman scatter parallel to the incident radiation (I|). For totally symmetric modes, the depolarization ratio is between 0 and 3/4. A depolarization ratio of 3/4 is indicative of a nontotally symmetric mode. For anomalous polarization, a depolarization ratio greater than 3/4 is observed.26 The intensity of perpendicular and parallel polarized Raman scattering in the lowfrequency range of the spectra is shown in Figure 3. Depolar-
Spectroscopy of the 1-Methylallyl Radical
J. Phys. Chem., Vol. 100, No. 19, 1996 7775
TABLE 2: Vibrational Assignments for the 1-Methylallyl Radical Compared to Various Butenes ab-initio 6-31G* (90% corr)
1-methylallyl radical, this work
eclipsed 1-butenea
gauche 1-butenea
trans-1,3butadieneb
3090 3019 3008
3090 3019 3008
3087 3003 2992
1492
2982 2952 1460 1426
2982 2948 1463 1421
1265
1380 1306
1380 1296
988
1020
1128 836 540 311
1128 854 439 301
2978 1469 1264 1177 999 915 551
2978 1469 1264 1177 993 912 634
154
103
trans-2butenec
cis-2butenec
3009 2973 2869 1444
3038 2935 2870 1445
1385 1306
1406 1260
1142 863 500
986 870
A′ ν1 CH2 asym stretch ν2 CH asym stretch ν3 CH2 sym stretch ν4 CH sym stretch ν5 CH3 asym stretch ν6 CH3 sym stretch ν7 CH3 deform (asym) ν8 CH2 scissor (sym) ν9 CH asym bend ν10 CH3 deform (sym) ν11 CH in pl sym, bend ν12 CCC asym stretch ν13 CCC sym stretch ν14 CH3-C-C asym str ν15 CH2, CH3 rock (sym) ν16 C-CH3 sym stretch ν17 C-C-C sym bend ν18 H3C-C-CC sym bend A′′ ν19 CH3 stretch (asym) ν20 CH3 deform (asym) ν21 CH3 rock ν22 CH out of pl bend ν23 CH2 out of plbend ν24 C(H)CH3 op bend ν25 CH2, HCCH3 twist sym ν26 CH2, H-C-CH3 op bend ν27 CH3 torsion a
3092 3015 3013 3003 2949 2873 1482 1478 1449 1403 1305 1219 1132 1077 960 834 476 273 2916 1462 1007 918 695 669 499 188 84
872 499 289
1904(OT)
205(OT)
1438 1280 1196 894 512 301
291 2950
2975
522
Reference 32. b Reference 31. c Reference 33.
Figure 2. Resonance Raman spectrum of the 1-methyl radical with excitation at 236.05 nm. The asterisks denote vibrational bands of the 1-chloro-2-butene precursor.
ization ratios of 0.40 and 0.34 were obtained for the 298 and the 499 cm-1 bands. Depolarization ratios of 0.3-0.4 are indicative of totally symmetric fundamentals or even overtones. The band at 298 cm-1 of the 1-methylallyl radical is assigned to the H3C-CCC bend, ν18. The frequencies observed for the H3C-COC in plane bend of methyl formate (318 cm-1),28 the H3C-CCC in plane bend of eclipsed 1-butene (311 cm-1),32 the H3C-CCC in plane bend of gauche 1-butene (301 cm-1),32 and the CCC deformation of trans-1,3-butadiene-d1 (288 cm-1)31 are in close agreement to the frequency observed for the 1-methylallyl radical. The experimental frequency observed at 298 cm-1 corresponds closely to theoretical values assigned to the H3C-CCC bend of the 1-methylallyl cation (298 cm-1) and radical (283 cm-1) by Traegar.14 Analysis of the first vibrational progression in the REMPI spectrum indicates that the hot band at 268 ( 30 cm-1 may be attributed to the methyl in plane bend.11 The first overtone of 2ν18 is observed at 582 cm-1.
Figure 3. Depolarization ratios for the 1-methyl radical obtained by recording spectra with the analyzer polarizer perpendicular and parallel to the incident radiation.
The fundamental and overtone of the H3C-CCC bend are recommended for the assignment of the first and third spacings observed in the photoelectron spectrum10 at 320 and 590 cm-1 , the differences with the Raman frequencies arising from inaccuracy inherent in the photoelectron measurements. The agreement of the frequencies observed in the REMPI and resonance Raman experiments confirm that the REMPI spectrum arises from a two-photon resonance. The allyl CCC in plane symmetric bend, ν17, is assigned to the band observed at 499 cm-1. The observed frequency is consistent with the CCC deformation observed for trans-2butene (500 cm-1)33 and trans-1,3-butadiene-d1 (511 cm-1).31 The 499 cm-1 assignment agrees with the in plane deformation assignment of the 1-methylallyl cation proposed by Traegar.14 The CCC bend at 499 cm-1 agrees with the vibrational spacing
7776 J. Phys. Chem., Vol. 100, No. 19, 1996 observed in the photoelectron spectrum at 520 cm-1.10 A hot band located in the REMPI spectrum of Hudgens at 507 ( 30 cm-1 may be assigned to the allyl CCC bend.11 The ν17 feature observed at 499 cm-1 is at a higher frequency than the skeletal bending frequencies observed for the 2-methylallyl radical at 432 cm-1 or the allyl radical at 427 cm-1.21-23 A large change in frequency supports the observation that the carbon-carbon skeletal vibrations of the allyl moiety are strongly coupled to the motions of the methyl group. Coupling indicates contribution to the off-diagonal elements to the vibrational secular equation. A strong coupling effect is expected for the R-substituted species since the methyl group is attached to a terminal skeletal carbon undergoing a large displacement from equilibrium during the in plane deformation. A methyl group attached to the central β-carbon of the allyl moiety experiences little displacement with excitation of the CCC bend and will not strongly couple to that motion, thus yielding the similarity in CCC bending frequencies for the allyl and 2-methylallyl radicals. A combination band of ν17 + ν18 is observed at 792 cm-1. The feature observed at 872 cm-1 is assigned to ν16, the C-C in plane stretch between the methyl group and the first allyl carbon. The observed frequency is in good agreement with similar frequencies observed for the H3C-C stretch in the 2-methylallyl radical (865 cm-1),23 cis-2-butene (870 cm-1),33 trans-2-butene (863 cm-1),33 eclipsed 1-butene (836 cm-1),32 and gauche 1-butene (854 cm-1).32 The frequencies of the hot band observed in the REMPI spectrum of Hudgens at 838 ( 30 cm-1 for the trans isomer and 865 cm-1 ( 30 cm-1 for the cis isomer of the 1-methylallyl radical are concordant with the frequency observed for the H3C-C stretch in the Raman spectrum at 872 ( 6 cm-1.11 The band at 1008 cm-1 can either be assigned to the CCC symmetric stretch of the allyl moiety or the first overtone of the CCC symmetric bend (2ν17 ) 998 cm-1) or a superposition of the two. Analysis of the spectrum with 234.95 nm excitation reveals that the band observed at 1008 cm-1 may be attributed to 2ν17 rather than ν13. The overtone of the H3C-CCC bend (ν18), the largest peak observed in the spectrum at 237.76 nm, is clearly observed at 2ν18 ) 572 cm-1. The overtone of the of the (ν17) CCC symmetric stretch at approximately 1000 cm-1 is expected to be observed in the spectrum with 237.76 nm excitation since the fundamental of the CCC stretch (ν17 ) 499 cm-1) is almost as intense as the H3C-CCC bend (ν18 ) 289 cm-1). The resonance Raman spectrum at 234.95 nm excitation also reveals intensity in combination and overtone bands of the CCC bend (ν17 ) 505 cm-1). The spectrum at 234.95 nm excitation shows intensity in 2ν18 at 790 cm-1, 2ν17 at 1007 cm-1, and a combination of 2ν17 + ν18 at 1301 cm-1. Comparison of the relative intensities of bands associated with ν17 at 234.95 nm to the bands seen in the 237.76 nm spectrum shows increased intensity in the combination and overtone bands of ν17. An increase in combinations and overtones of the CCC bend observed in the 234.95 nm spectrum indicates a change in ν17 in the excited state. Resonance Raman spectra can be used to identify the vibrational level in the upper electronic state that gives rise to a particular absorption feature in the UV spectrum. Callear and Lee have assigned the electronic band origin to the absorption band at 237.76 nm and observed several other absorption bands.9 The spectrum taken at 236.05 nm (Figure 2) shows increased intensity of the overtone (2ν18) and combination band of the H3C-CCC bending mode (ν17 + ν18). The increased intensity in Raman features associated with ν18 indicates excitation of a quanta of ν18 in the excited state. The excited state vibrational frequency for the H3C-CCC bend was calculated to be 305
Tarrant et al. cm-1 by subtracting the corresponding energies of the observed excitation of ν18 at 236.05 nm and the electronic band origin at 237.76 nm. The resonance Raman spectra taken with 234.95 nm excitation shows similar enhancement of bands associated with ν17. An excited state vibrational frequency of the 1-methylallyl radical is calculated to be 502 cm-1 by subtracting the corresponding energies of 234.95 nm and the electronic band origin located at 237.76 nm. The calculated excited state frequencies obtained from resonance Raman experiments are consistent with the vibrational spacings observed in the absorption spectrum of Callear and Lee at 321 and 510 cm-1 and a spacing observed the REMPI spectrum of Hudgens at 317 cm-1.9,11 Resonance enhancement of combination and overtone bands of ν17 with 234.95 nm excitation and agreement of the calculated excited vibrational frequencies to vibrational spacings observed in the absorption and REMPI spectrum indicate that the observed feature at 1008 cm-1 is more likely to be 2ν17 rather than ν13. Selective enhancement of different normal modes and sequences by changing the probe laser wavelength is one feature of the resonance Raman technique for identification of ground state and excited state normal modes. Although assignment of the 1008 cm-1 feature to 2ν17 is favored, an assignment of ν13, the CCC stretch, cannot be completely excluded. A similar frequency is observed for the CCC stretch in eclipsed 1-butene at 988 cm-1 and gauche 1-butene at 1020 cm-1.32 The difference in frequency between the allyl radical (1066 cm-1),21,22 the 2-methylallyl radical (1037 cm-1),23 and the 1-methylallyl radical (1008 cm-1) could be due to coupling present between the carbon skeleton of the allyl moiety and the methyl group in the 1 methylallyl radical. Methyl substitution on a terminal carbon of the allyl moiety is expected to lower the frequency of the CCC stretch because the outer carbons are directly involved with the primary C-C stretching motion in the allyl moiety. The CCC stretch in the 1-methylallyl radical is therefore expected to have a lower frequency than that in the 2-methylallyl radical. The feature observed at 1265 cm-1 has been assigned to ν11, the C-H in plane symmetric bend associated with the β-carbon on the allyl moiety. The C-H in plane bend of propenal (1275 cm-1, Cs point group) was used to assign the mode because the molecule has similar force constants and reduced mass as the 1-methylallyl radical (1265 cm-1).31 The frequencies observed for the dCH bend in eclipsed 1-butene (1306 cm-1),32 gauche 1-butene (1296 cm-1),32 cis-2-butene (1260 cm-1),33 trans-2butene (1306 cm-1),33 and trans-1,3-butadiene (1280 cm-1)31 are consistent with the C-H bending frequency observed for the 1-methylallyl radical. A weak band observed at 1492 ( 6 cm-1 can either be attributed to the CH3 asymmetric deformation mode (asymmetric umbrella) or the CH2 scissors mode. A similar frequency has been observed in the 2-methylallyl radical (1492 cm-1) and allyl radical (1488 cm-1) which supports an assignment of the observed frequency to the CH2 scissors over the umbrella mode.21-23 Two nontotally symmetric A′′ modes of the 1-methylallyl radical are assigned as even overtones of the CH3 torsion mode (205 cm-1) and the C-H out of plane bending mode (1904 cm-1). The CH3 torsion mode is clearly observed in the 237.76 nm spectrum. The vibrational frequency for the CH3 torsion mode overtone at 205 cm-1 is in the range of torsion modes observed in acetic acid-d1 (CH3COOD, 2[93 cm-1] ) 186 cm-1),31 gauche 1-butene (2[103 cm-1] ) 206 cm-1),32 eclipsed 1-butene (2 [154 cm-1] ) 308 cm-1),32 and methyl formate (2[130 cm-1] ) 260 cm-1).31 The weak band observed at 1904 cm-1 is consistent with the assignment of the C-H out of plane
Spectroscopy of the 1-Methylallyl Radical overtone for the allyl radical (2ν10 ) 1936 cm-1).21,22 Similar frequencies observed for the overtone of the C-H fundamental in trans-1,3-butadiene-d1 (2[976 cm-1] ) 1952 cm-1)31 and trans-2-butene (2[963 cm-1] ) 1926 cm-1)33 support the assignment of the band observed at 1904 cm-1 to the C-H out of plane bend in the 1-methylallyl spectrum. The intensity of the vibrational bands in the resonance Raman spectra is indicative of the nuclear motion on the excited state surface. In general, the larger the gradient on the excited state surface in a particular coordinate, the greater the force on the nuclei in that coordinate and the greater the resonance Raman scattering in that particular normal mode. The most striking features in the 1-methylallyl radical spectra taken with excitation from 233 to 237 nm are the ν17 and ν18 carbon skeletal in plane bending modes. The bending motions may be a prelude to bond dissociation, although there is little evidence for a direct dissociation. The lack of intensity in the CH stretch region of the spectrum, and most notably the lack of intensity in the CH3 stretches ν5 and ν6, indicates that the C-H bond lengths in the excited state are similar to those in the ground state. There is no evidence for CH bond dissociation in the excited state. Thus the initial dynamics are not loss of a hydrogen atom to form 1,3-butadiene. The feeble intensity in ν16, the C-CH3 stretch, and lack of overtones in ν16 do not strongly argue for a rapid departure of the methyl group. The lack of intensity in the out of plane bending modes in the resonance Raman spectra recorded with excitation at the electronic band origin indicates that the excited state dynamics of the 1-methylallyl radical differ significantly from those of the allyl and 2-methylallyl radical. At 220 nm excitation, intensity patterns in the resonance Raman spectrum for the allyl radical have large intensity in the CCC in plane bend, the CCC stretch, the C-H out of plane bend, and the CH2 twist indicating initial dynamics of photoisomerization of the allyl radical toward the cyclopropyl radical.21 The 2-methylallyl radical spectra taken with 223-243 nm excitation yields intensity patterns similar to the allyl radical and thus has similar initial dynamics.23 Lack of intensity in the CH2 twist and little intensity of the C-H out of plane bend show that the initial dynamics of the 1-methylallyl radical should not produce a cyclopropyl species. V. Conclusion Resonance Raman spectroscopy yields information about both the ground and the excited electronic state surfaces of the 1-methylallyl radical. The resonance Raman spectrum of 1-methylallyl has been observed at probe wavelengths 233237 nm using three separate precursors. Five bands are observed for the 1-methylallyl spectrum at 237.76 nm: the H3C-CCC allyl symmetric bend, the CCC allyl symmetric bend, the H3C-C symmetric stretch, the CH in plane symmetric bend, and the CH2 scissors. A sixth band at 1008 cm-1 is attributed to the overtone of the CCC bend. The bands at 205, 582, and 792 cm-1 have been assigned to the 2ν27, the 2ν18, and the (ν17 + ν18) modes. Raman excitation at 236.05 nm yields overtones and combination overtones of ν18, indicating resonance with one quanta of ν18 in the excited electronic state. Excitation at 233.24 yields overtones and combination overtones of ν1, similarly indicating resonance with one quanta of ν17 in the excited electronic state. The excited state vibrational frequency for ν18 is 305 cm-1 using the electronic band origin at 237.76 nm and the observed excitation of ν18 at 236.05 nm. The excited state vibrational frequency for ν17 ) 503 cm-1 is calculated using excitation of the CCC bend at 234.95 nm.
J. Phys. Chem., Vol. 100, No. 19, 1996 7777 Qualitative analysis of the intensity patterns of the spectrum at various excitation wavelengths provides information of the dynamics occurring in the excited state. The 1-methylallyl radical excited state dynamics consist of significant bending of the carbon skeletal framework, but there is little evidence for any direct dissociation of either the methyl group or one of the methyl hydrogens. Acknowledgment. The authors wish to express their gratitude to the National Science Foundation (CHE8923059) for the financial support of this work. References and Notes (1) Weissman, M.; Benson, S. W. Prog. Energy Combust. Sci. 1984, 15, 273. (2) Gusel’nikov, L. E.; Volkova, V. V.; Ivanov, P. E.; Inyushkin, S. V.; Shevelkova, L. V. J. Anal. Appl. Pyrolysis 1991, 21, 79. (3) Shida, T. Annu. ReV. Phys. Chem. 1991, 42, 55. (4) Grosjean, D. J. Air Waste Manage. Assoc. 1991, 41, 182. (5) Edney, E. O.; Shepson, P. B.; Kleindienst, T. E.; Corse, E. W. Int. J. Chem. Kinet. 1986, 18, 597. (6) Chesick, J. P. J. Chem. Phys. 1966, 45, 3934. (7) Collin, G. J.; Deslauriers, H.; Auclair, S. Int. J. Chem. Kinet. 1980, 7, 17. (8) Collin, G. J.; Deslauriers, H. Can. J. Chem. 1979, 57, 863. (9) Callear, A. B.; Lee, H. K. Trans. Faraday Soc. 1968, 64, 308. (10) Schultz, J. C.; Houle, F. A.; Beauchamp, J. L. J. Am. Chem. Soc. 1984, 106, 7336. (11) Hudgens, J. W. In AdVances in Multi-Photon Processes and Spectroscopy; Lin, S. H., Ed.; World Scientific: Singapore Vol. IV, p 278. Tsai, B. P.; Johnson, R. D., III; Hudgens, J. W. In Institute of Physics Conference Series No. 94; Institute of Physics: Bristol, UK, 1988; Vol. 94, p 129. (12) Kochi, J. K.; Krusic, P. J. J. Am. Chem. Soc. 1968, 90, 7157. (13) Driscroll, D. J.; Martir, W.; Lunsford, J. H. J. Phys. Chem. 1987, 91, 3585. (14) Traeger, J. C. J. Phys. Chem. 1986, 90, 4114. (15) Schleyer, P. v. R.; Dill, J. D.; Pople, J. A.; Hehre, W. J. Tetrahedron 1977, 33, 2497. (16) Jaris, H. M.; Khalil, S. M. J. Chem. Soc., Perkin Trans. 2, 1986, 1701. (17) Grunwell, J. R.; Sebastian, J. F. Tetrahedron 1971, 27, 4387. (18) Boyd, S. L.; Boyd, R. J.; Shi, Z.; Barclay, R. C.; Porter, N. A. J. Am. Chem. Soc. 1993, 115, 687. (19) Imre, D. G.; Kinsey, J. L.; Field, R. W.; Katayama, D. H. J. Phys .Chem. 1982, 86, 2564. (20) Imre, D. G.; Kinsey, J. L.; Sinha, A.; Krenos, J. J. Phys. Chem. 1984, 88, 3956. (21) Getty,J. D.; Burmeister, M. J.; Westre, S. G.; Kelly, P. B. J. Am. Chem. Soc. 1991, 113, 801. (22) Liu, X.; Getty, J. D.; Kelly, P. B. J. Chem. Phys. 1993, 99, 1522. (23) Getty, J. D.; Liu, X.; Kelly, P. B. J. Chem. Phys. 1996, 104, 3176. (24) Vancik, H.; Gabelica, V.; Sunko, D. E.; Buzek, P.; Schleyer, P. v. R. J. Phys. Org. Chem. 1993, 6, 427. (25) Chadha, S.; Nelson, W. H.; Emrich, R.; Lindesmith, E. Appl. Spectrosc. 1993, 47, 475. (26) Getty, J. D.; Liu, X.; Kelly, P. B. Chem. Phys. Lett. 1993, 201, 236. (27) Bhagavantam, S. Nature 1932, 129, 830. (28) Maire, G.; Reisenauer, H. P.; Rohde, B.; Dehnicke, K. Chem. Ber. 1983, 116, 732. (29) Gusel’nikov, L. E.; Tsimmermann, G.; Tsigler, U.; Volkova, V. V.; Konobevskii, K. S.; Smirnov, V. I.; Ondrushka, B.; Avakyan, V. G.; Nametkin, N. S. Dokl. Akad. Nauk, SSSR 1985, 282, 591. (30) Avakyan, V. G.; Volkova, V. V.; Gusel’nikov, L. E.; Ziegler, U.; Zimmermann, G.; Ondrushka, B.; Nametkin, N. S. Dokl. Akad. Nauk, SSSR 1986, 290, 903. (31) Shimanouchi, T. Tables of Molecular Vibrational Frequencies, Consolidated Volume I; NSRDS-NBS-39; National Bureau of Standards (U.S.): Washington, DC, 1972. (32) Durig, J. R.; Compton, D. A. C. J. Phys. Chem. 1980, 84, 773. (33) Manzanares, C.; Blunt, V. M.; Peng, J. P. J. Phys. Chem. 1993, 97, 3994. (34) Hudson, B. S.; Kelly, P. B.; Ziegler, L. D.; Desiderio, R. A.; Gerrity, D. P.; Hess, W.; Bates, R. In AdVances in Laser Spectrosocopy; Garetz, B. A., Lombardi, J. R., Eds.; Heyden: Philadelphia, 1986.
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