J . Phys. Chem. 1987,91, 3731-3736
3731
ARTICLES Vibronlc Spectrum of Cold, Gas-Phase Allyl Radicals by Multiphoton Ionization Andrew D. Sappey and James C. Weisshaar* Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706 (Received: December 15, 1986; In Final Form: March 19, 1987)
We have combined the techniques of multiphoton ionization (MPI), time-of-flight mass spectrometry, and creation of cold neutral free radicals by photolysis of precursors upstream in a free-jet expansion to study the-MPI optical spectrum of the allyl radical. We resolve 19 vibronic bands in the nominal two-photon resonant, 3s 2AI X ZA2,four-photon ionization spectrum from 488 to 5 1 3 nm. The shapes and widths of the vibronic bands are remarkably sensitive to the ionizing laser flux. The spectrum includes both hot and cold bands. Comparison with previous gas-phase and matrix data and with ab initio calculations of harmonic frequencies permits assignment and identification of three fundamental vibrational levels in both the ground and excited states: the CCC bend, the symmetric CH2 twist, and the antisymmetric CH2 twist. Using the same technique, we have detected the ethyl radical by MPI for the first time. The three-photon ionization yield is small and the spectrum is apparently structureless in the wavelength region 398.5-409.5 nm.
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Introduction Neutral free radicals are important intermediate species in gas-phase atmospheric chemistry, in combustion processes, and in solution-phase organic reactions. As compared to stable molecules, relatively little is known experimentally about the geometric and electronic structure of radical species. Electron spin resonance spectra and infrared and UV-visible absorption spectra of matrix-isolated free radicals' have provided considerable information. Microwave and infrared absorption2 and far-infrared laser magnetic resonance3techniques have provided high-resolution spectroscopic information for certain gas-phase radicals. Photoelectron spectra of gas-phase anions4 are another source of neutral radical vibrational frequencies and structural inferences. Hunziker and co-workerss have obtained direct optical absorption spectra of a variety of gas-phase radicals created by Hg photosensitization. The techniques of laser-induced fluorescence (LIF) and resonance-enhanced multiphoton ionization (REMPI) have permitted recent advances in the optical spectroscopy of free radicals. Hudgens and co-workers have demonstrated the utility of one-color REMPI via resonant Rydberg states for probing the vibronic structure of such diverse radicals as CH3,6 SiF: CIO and Br0,8 hydroxymethyl (CH20H): allyl (C3Hs) and methylallyl,'O and methoxy (CH3O).I1 In these experiments, atom abstraction reactions create radicals which are often internally quite warm. (1) (a) Jacox, M. E. J . Phys. Chem. Ref.Data 1984,4,945. (b) Meyer, B. Low Temperature Spectroscopy; Elsevier: New York, 1971. (2) See, for example, Endo, Y.; Saito, S.; Hirota, E. J . Chem. Phys. 1985, 83,2026. Uehara, H.; Kawaguchi, K.; Hirota, E. J . Chem. Phys. 1985,83, 5479. (3) See,for example, Brown, J. M.; Evenson, K. M.; Sears, T. J. J . Chem. Phys. 1985,83, 3275. (4) Lineberger, W. C. In Chemical and Biochemical Applications of Lasers, Moore, c. B., Ed.; Wiley: New York, 1974. (5) Wendt, H. R.; Hunziker, H. E. J . Chem. Phys. 1984, 82, 717, and references therein. (6) Hudgens, J. W.; DiGiuseppe, T. G.; Lin, M. C. J. Phys. Chem. 1982, 86,36. Hudgens, J. W.; DiGuseppe, T. G.; Lin, M. C. J . Chem. Phys. 1983, 79, 571. (7) Dulcey, C. S.; Hudgens, J. W. Chem. Phys. Lett. 1985, 118, 444. (8) Duignan, M. T.; Hudgens, J. W. J . Chem. Phys. 1985, 82, 4426. (9) Dulcey, C. S.; Hudgens, J. W. J . Phys. Chem. 1983, 87, 2296. (10) Hudgens, J. W.; Dulcey, C. S . J . Phys. Chem. 1985, 89, 1505. (11) Long, G.R.; Johnson, R. D.; Hudgens, J. W. J. Phys. Chem. 1986, 90, 4901.
0022-3654/87/2091-373 1$01.50/0
Smalley and co-workers12 obtained rotationally resolved LIF spectra of cold methoxy radicals created upstream by photolysis of an appropriate precursor and subsequently cooled in a free-jet expansion. Miller and co-workers have used a similar technique to obtain rotationally resolved L I F spectra of cold vinoxy (CH2CHO),I3CN, and SHI4radicals. Engelking and co-workers developed a novel glow discharge source of cold free radicals to obtain emission spectra of OH,I5 NH,I6 m e t h o ~ y and , ~ ~me~ thylnitrene (CH3N).17bMost recently, Chen and co-workers'* obtained a REMPI spectrum of cold CH3 radicals formed in a pyrolysis source coupled to a free-jet expansion. In this paper, we report our initial results using a new combination of techniques: photolysis of a precursor upstream in a free jet and REMPI with time-of-flight mass analysis on the resulting skimmed, cold radical beam. Our long-range goals include both rotationally resolved spectra of nonfluorescent radicals and ion vibrational state information from photoelectron spectra of cold polyatomic radicals. For allyl radical, we obtain wellresolved nominal 2f2 REMPI vibronic spectra of the previously reported 3s 2Al X 2A2transition near 500 nm. The shape and width of the vibronic bands are remarkably sensitive to the ionizing laser intensity, which is surprising for a nominal 2+2 process. Twenty-one vibronic bands are assigned by invoking activity in three vibrational modes. By combining gas-phase REMPI, matrix IR a b s ~ r p t i o n , 'and ~ * ~allylic ~ anion photoelectron2' spectroscopic data with the results of a b initio calculations22of harmonic fre-
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(12) Powers, D. E.; Hopkins, J. B.; Smalley, R. E. J . Phys. Chem. 1981, 85, 271 1.
(13) 2339. (14) 84, 1 . (15) (16) (17)
DiMauro, L. F.; Heaven, M.; Miller, T. A. J . Chem. Phys. 1984,81, Heaven, M.; Miller, T. A.; Bondybey, V. E. Chem. Phys. Lett. 1981,
Droege, A. T.; Engelking, P. C. Chem. Phys. Lett. 1983, 96, 316. Carrick, P. G.; Engelking, P. C. Chem. Phys. Lett. 1984, 108, 505. (a) Carrick, P. G.;Brossard, S. D.; Engelking, P. C. J . Chem. Phys. 1985, 83, 1995. (b) Carrick, P. G.; Engelking, P. C. J . Chem. Phys. 1984,
81, 1661. (18) Chen, P.; Colson, S. D.; Chupka, W. A,; Berson, J. A. J . Phys. Chem. 1986, 90, 2319.
(19) Maier, G.;Reisenauer, H. P.; Rohde, B.; Dehnicke, K. Chem. Ber. 1983, 116, 732.
(20) Mal'tsev, A. K.; Korolov, V. A.; Nefdov, 0. M. Bull. Acad. Sci. USSR, Chem. Ser. 1982, 31, 2131. (21) Oakes, J. M.; Ellison, G . B. J. Am. Chem. SOC. 1984, 106, 7734. (22) Takada, T.; Dupuis, M. J . Am. Chem. SOC.1983, 105, 1713.
1987 American Chemical Society
Sappey and Weisshaar
3732 The Journal of Physical Chemistry, Vol. 91, No. 14, 1987
6 ” DP 6 ” DP
Figure 1. Schematic of the apparatus. Molecular beam of cold radicals is created by excimer laser photolysis of precursor molecules seeded in a pulsed Ar expansion. The radical beam is skimmed and probed by resonance-enhanced multiphoton ionization using a pulsed dye laser in
the extraction region of a time-of-flight mass spectrometer. The timeof-flight photoelectron spectrometer is not used in the present experiment. See ref 23 for further details. quencies, we are able to assign and identify three ground-state vibrational levels of the allyl radical: the CCC bend (al symmetry, 426 cm-I); the symmetric CH2 twist (bl, 508 cm-I); and the antisymmetric C H 2 twist (az symmetry, 558 cm-I). The corresponding 3s 2Al excited-state frequencies are 393, 491, and 529 cm-l, respectively. We also report briefly on less successful studies of the three-photon ionization of cold CH3 and CZHSradicals in order to point out some limitations of the technique.
Experimental Section Figure 1 shows a schematic of the apparatus.23 An excimer photolysis laser pulse (typically 10 mJ/pulse, I O pulse/s at 193 nm) dissociates stable precursor seed molecules in a quartz extension tube24 (1 cm long, 1 mm i.d., 2 mm 0.d.) mounted on a commercial pulsed valve (Lasertechnics, LPV). The rectangular excimer laser beam is softly focused (- 3 X 1 mm spot with long axis parallel to the expansion) at the tip of the quartz tube. The resulting radicals, the remaining precursors, and the majority Ar buffer gas co-expand and cool. The free jet is skimmed (2-mm diameter, Beam Dynamics) to form a cold molecular beam which intersects the ionization laser (Quanta-ray DCR-2A Nd:YAG pumped dye laser, 8 ns fwhm, 10 pulse/s) in the ion extraction region of the time-of-flight mass spectrometer. The laser dye was Coumarin 500 for the allyl radical work. The collimated dye laser is focused by a spherical lens of either 10 or 1 5 cm f.1. The resulting photoions are extracted to 1700-eV kinetic energy and mass-analyzed by time-of-flight in a 100-cm tube. The small spatial extent of the REMPI source and the use of a WileyMcLarin extraction region2s yield fwhm resolution AM = 0.16 amu for C3H5+parent ions at mass 41 amu. A transverse electric field counteracts the kinetic energy of the beam and an electrostatic lens improves ion collection efficiency. The ion detector is a 14-stage electron multiplier (EM1 9643/2b, gain 2 X IO6). Mass spectra are digitized on each laser shot and averaged in a LeCroy (23) Sanders, L.; Sappey, A. D.; Weisshaar, J. C. J . Chem. Phys., in press. (24) We thank Dr. Alec Wodtke for suggesting the quartz extension tube. (25) Wiley, W. C.; McLaren, I . H. Rev. Sci. Instrum. 1955, 26, 1 1 50.
9400 digital oscilloscope. REMPI spectra of cold radicals are obtained as computer-controlled scans of gated, integrated parent ion current vs. dye laser wavelength. No correction is applied for variations in laser pulse energy, either shot-to-shot or across the gain curve of the dye laser. The ideal precursor would have a large absorption cross section at a convenient dissociation wavelength, large photochemical quantum yield of the desired radical, and a negligible absorption cross section at the REMPI probe laser wavelength. The latter qualification diminishes undesirable background due to ionization of precursor molecules or of hot radicals produced by the REMPI laser itself. In the most favorable case, the photolysis laser would completely dissociate the precursor in the volume probed by the REMPI laser. Photolysis of 3-chloropropene (allyl chloride) at 193 nm provided good number densities of allyl radicals. Ar carrier gas at 1-4-atm pressure was bubbled through the neat liquid; the seed ratio was varied by changing the temperature of the liquid. Possible problems with the photolysis source include the deposition of large amounts of translational and internal energy in the photofragments, recombination or abstraction reactions of the desired radical prior to cooling and expansion, and the formation of van der Waals complexes between the radical and Ar or the precursor. The energy of a 193-nm photon is 148 kcal/mol, far in excess of that required to break single bonds. Success of the source requires Ar number density large enough that the hot radical photolysis products are cooled before they can expand and find the walls of the quartz tube. For the largest stagnation pressure used, 4 atm, we estimate that the effective conductance of the pulsed valve is about half of the calculated conductance for a 1-mm orifice.
Results REMPI of CH3 and C2H5 Radicals. Preliminary studies of C H 3 and C2H5 radicals illustrate the extent of cooling possible in the source and also some important limitations of the technqiue. For sources of CH,, we tried acetone and nitromethane seeded at 2-4% in 1-2 atm of Ar and photolyzed at 193 nm. The probe was 2+1 REMPI at 333 nm with 0.6 mJ/pulse focused with a IO-cm lens. The photolysis was carried out either 1-3-mm downstream of the unmodified valve,I3 or in an aluminum nozzle extension tube similar to the Teflon extension tube of Smalley and co-workers,I2 or in the quartz tube of Figure 1. The probe laser creates a background CH3+ signal in the absence of the photolysis laser. With both lasers on, in all conditions studied a plot of CH3+signal vs. probe laser delay reveals a 15-ps fwhm forward-scattered peak above the background followed by a 20-ps fwhm hole in the background signal. The excimer laser apparently completely bleaches the precursor in the center of the photolysis region but the resulting CH, radicals either are not efficiently trapped 0,’ perhaps react with the precursor. Scans of the 3p 2A2” X 2A2/’ origin band at 333 nm indicate a rotational temperature somewhat colder than the 40 K reported in recent pyrolysis/REMPI experiments’*on CH,, as judged by the width of the Q-branch. We produced C2H5 radicals from 193-nm photolysis of C 2 H 5 N 0 2in the metal extension tube with 2.5-atm Ar backing pressure. Based on Hunziker’s5 recent discovery of the broad, one-photon absorption spectrum of hot C2H, radicals at 205 nm, we attempted two-photon excitation of C2H5 at h = 410 nm. The estimated adiabatic ionization energy of 8.5 1 eV indicates that absorption of three such photons is necessary to reach the continuum. Using 2.5 mJ/pulse focused with the 10-cm lens we observed weak C2H5+signals which vanished in the absence of either laser pulse. No structure was observed in the spectrum from 398.5 to 409.5 nm. The result is consistent with the possibility of rapid dissociation of the C2H5* resonant state on a time scale competitive with the ionization step, Le., the ionization efficiency may be inherently low. REMPI Spectrum of Cold Allyl Radicals. We created cold allyl radicals C3H5by 193-nm photolysis of C3H5C1at 2% seed ratio in 4 atm of Ar using the quartz nozzle extension tube of Figure I . Other configurations were not tested on allyl. The
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-
-
Gas-Phase Allyl Radical Spectrum
The Journal of Physical Chemistry, Vol. 91, No. 14, 1987 3733 I
REM P I -TOF MASS SPECTRUM x 499.3 nm
I
a
I
10 TIME
I
I
A,
I
I
12
14
o
MOO
+800
1
I
-400 I
I
-800 I
I
I
1,
(a) 6.8mJ/pulse, 10cm lens
40000
39600
(PSI
Figure 2. Time-of-flight mass spectrum following resonance-enhanced multiphoton ionization of allyl radicals from photolysis of allyl chloride. Conditions included 3.5 mJ/pulse at 499.3 nm focused by the 15-cm lens.
40800
40400
39200
TWO-PHOTON ENERGY (CM-')
extinction coefficient is probably large by analogy with the C3H5Br spectrumz7 (e N lo4 M-' cm-I); the quantum yield of C3H5is not known. We probed the cold allyl radicals by nominal 2+2 REMPI in the 3s 2A1 % ZA2band system discovered by Hudgens and Dulceyioat X = 490-510 nm. The laser pulse energy was varied from 1 to 7 mJ/pulse, focused by either a 10- or 15-cm lens (see below). REMPI mass spectra (Figure 2) reveal the following peaks (relative intensities) when using 3.5 mJ of ionization laser at 499.3 nm (2.48 eV) and the 15-cm lens: m / e 41 (89%); m / e 39 (4%); m / e 42 (3%); m / e 12 (2%); and smaller peaks at m / e 38, 37, and 15. The largest peak is the parent ion C3H5+;the natural abundance of a single 13Catom in C3H3+(3.3%) accounts for the intensity of the m / e 42 peak. Fragmentation and/or photodissociation channels include C3H3+,C3H2+, C3H+, CH3+, and C+. Gentle ionization of the radical to form the parent cation is the dominant process at low laser energies. At higher photon flux, we observe increasing fractions of fragment ions. From the adiabatic ionization energy of 8.13 eV,29 we calculate that absorption of four 499.3-nm photons can deposit a maximum of 1.8 eV of internal energy in the parent cation. We observed no evidence of higher mass ions that might arise from photoionization of dimers or clusters in the beam. In contrast to the CH3 behavior, plots of the C3H5+parent ion signal vs. probe laser delay exhibit a single narrow peak of 15-ps fwhm on zero background. The allyl radicals apparently are trapped and cooled efficiently by Ar collisions without undergoing extensive chemical reactions with each other or the precursor. The signal decreases rapidly with decreasing Ar pressure and vanishes when the photolysis laser is blocked. Thus 2+2 REMPI of allyl radicals from allyl chloride provides an example in which the visible wavelength probe laser does not efficiently ionize the precursor molecule. We can thus detect parent ions on an essentially zero background. Figure 3a shows a nominally 2+2 REMPI scan of integrated C3H5+ current vs. two-photon energy obtained by using 6.8 mJ/pulse focused by the IO-cm lens to ionize cold C3H5. Under these laser conditions, the widths of the most intense vibronic features (-70 cm-I fwhm a t the two photon energy) are about half the vibronic band widths observed by Hudgens and DulceyIo using 5-14 mJ/pulse and a 3.8-cm lens to ionize warm allyl radicals from the C1 + propene atom abstraction reaction. The bands at two photon energies below 40000 cm-I, which were attributed to hot bands in the previous work, are indeed less intense
-
L. Ph.D. Thesis, University of California at Berkeley, 1985. (27) UV Atlas of Organic Compounds, Vol. 111; Butterworths: London, (26) Butler,
1967. (28) Mainfray, G.; Manus, C. In Multiphoton Ionization of Atoms, Chin, S. L., Lambropoulos, P., Ed.: Academic: Toronto, 1984. (29) Houle, F. A,; Beauchamp, J. L. J. Am. Chem. Soc. 1978,100, 3290.
Figure 3. Nominal 2+2 REMPI spectra of cold allyl radicals created from 193-nm photolysis of C,H,CI seeded at 2% in 4 atm of Ar. Trace (a) was taken with 6.8 mJ/pulse of probe dye laser focused by a spherical IO-cm lens; trace (b) was taken with 2.5 mJ/pulse and a 15-cm lens. All other conditions are identical in the two experiments. Dye laser pulse energy is constant within f 10% across the spectrum.
in the molecular beam than in the atom abstraction source. In sharp contrast, Figure 3b shows the same region scanned a t laser energy of 2.5 mJ/pulse (constant to &lo%) focused by the 15-cm lens, Le., at much smaller laser flux. The photolysis and expansion conditions are identical in parts a and b of Figure 3; i.e., both are spectra of cold allyl radicals. Phenomenologically, larger photon flux broadens each vibronic band envelope asymmetrically, increasing the relative intensity on the high-energy side of the band. The relative intensities of different vibronic bands change dramatically as well. The spectrum of Figure 3b appears rotationally quite cold; vibronic bandwidths are about 15-cm-' fwhm at the two-photon energy, and we observe definite evidence of reproducible rotational structure in the bands at the two-photon laser resolution of 3-cm-' fwhm. Further reductions in laser intensity do not lead to further changes in the band shapes. The result serves as a cautionary note for high-order REMPI studies of vibronic band positions. We expect relative vibronic band intensities to vary with laser intensity in an unsaturated high-order multiphoton process. However, laser intensity dependent band shapes and peak positions in a nominal 2+2 REMPI process are surprising under such relatively modest conditions of laser energy and focus. Possibly the third photon, which falls 5600 cm-' below the ionization limit, strongly perturbs nearby Rydberg states, Le., the process may be more nearly 2+ 1 1 REMPI. We have observed the intensity of the band at 40 057 cm-' to vary with laser pulse energy E as E" with n between 0.75 and 3.5 depending on the focal length of the lens used. Atomic REMPI lines are known to broaden and shift toward higher energy with increasing photon At the present resolution, we cannot ascertain whether individual rovibronic transitions on the highenergy side of the bands are selectively enhanced by increased laser intensity or whether a wholesale shift and broadening of lines occurs. Studies of this intriguing phenomenon at higher wavelength resolution may clarify the mechanism. Multiple scans using the conditions of the cold spectrum of Figure 3b reveal 19 definitely observed, well-resolved, reproducible vibronic bands. Some of the bands listed as definitely observed in Table I were measured most accurately on spectra other than that of Figure 3. Six additional very weak bands were tentatively observed. Table I presents all of the measured peak positions at the two-photon energy. Some of the bands were previously resolved by Hudgens and Dulcey.lo The colder spectrum obtained at smaller laser intensity allows us to refine the transition energies
+
Sappey and Weisshaar
3734 The Journal of Physical Chemistry, Vol. 91, No. 14, 1987 ~~
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2A1 3 2A1 REMPI Spectrum of Allyl Radical two-photon energy re1 to origin, cm-' vibronic assignment obsd calcd" - obsd
TABLE I: Vibronic Bands' of the Two-Photon 3s
band no.
two-photon energy, cm-I
I 2 3 4 5 6 7 8
40848 (3) 40820 (3) 40782 (5) 40654 (3) 40586 (3) [40548 (7)] 40450 (3) 40418 (3) 40382 (3) 40 350 (4) [40 3 12 (6)] 40 154 (8) 40056.8 (2.0) 40024 (5) [39 986 (3)] [39 960 (8)] [39918 (lo)]
A; A:
39880 ( I O ) [39 728 (5)] 39 630 (5) 39 584 (7) 39 548 (12) 39510 (15) 39 190 (12) 39 144 (15)
L46CY1
9 10 11 12 13 14 15
16 17 18 19 20 21 22
23 24 25
[A:]
[D61
c:, [Bbl Ab A: A: [A:] AYCb
0: At A: [Ail
AY A: [BY1
C? A; A:
192 (3) 764 (3) 125 ( 5 ) 597 (3) 529 (3) [491 (711 394 (3) 362 (3) 326 (3) 294 (4) [255 (6)l 98 (8) 0 -35 (5) [-7o (3)1 [-96 (811 [-I39 ( l o ) ]
other work, cm-'
-2
+I [+91 0 0 0 -1 +2 +3 [-61
7376
390'
+5 0 0 -2 -2 1
-176 ( I O ) [-328 (5)i -427 ( 5 ) -473 (7) -508 (12) -547 (1 5 ) -866 (1 2) -913 ( 1 5 )
[+Ill
+I +4 0 -1 1 +4 -2
[-413],* 395 (165)' [-513],* -510d
Uncertainties in parentheses. Entries in square brackets are tentative. In both electronic states, modes are labeled A, B, and C in order of increasing frequency. Mode A was previously labeled B in ref IO. Calculated - observed differences are based on level energies of Table 11. Reference I O . Reference 21. dReferences 19 and 20. (I
to f3 cm-l at the two-photon energy in the best cases, which in turn permits the vibronic assignments described below. Most of the transition energies of Table I agree with those of Hudgens and Dulcey within their estimated f 3 0 cm-' accuracy. The small "satellite bands" to the low-energy side of the two strongest bands at 40450 and 40056.8 cm-I and many of the other weak bands are reproducible from scan to scan. In order to check for the possibility of red-shifted bands due to van der Waals clusters C3H5-Arnor C,HS.C3H5CI we scanned the region 40 570-40 240 cm-I under various source conditions at constant probe laser intensity. Variation of the allyl chloride seed ratio from 0.5 to 2.5% and of the Ar stagnation pressure from 1.7 to 3.8 atm had no apparent effect on the relative intensities or positions of the four largest peaks in this region. The satellite bands also appeared unchanged in relative intensity and position in lower signal-to-noise scans using He buffer gas at 3 atm, which rules out the possibility of Ar clusters. All of the experimental evidence indicates that the spectrum of Figure 3b is due to cold allyl radical monomers. The success of the vibronic assignments in explaining the weak satellites as sequence bands further argues against complications from clusters. Vibronic Band Assignments in the Allyl Spectrum. A simple model invoking three active vibrational modes plus significant vibrational excitation in the beam can explain 21 of the bands listed in Table I. Since the vibrational structure of the allyl radical is only moderately well characterized even in the ground electronic state, for the moment we label the active modes A, B, and C in order of increasing frequency. The mode labeled A matches Hudgens and Dulcey's "mode B" in the earlier work.1° In either electronic state, vibrational energy levels are given by Go(~APB,~ =c )
+
U A W ~
vXXAA
+ V& + u ~ x B B + VCW; + v E x ~ C(1)
Here the ui are vibrational quantum numbers, the W : are fundamental frequencies, and the x,, are diagonal anharmonicities. No off-diagonal anharmonicities x. are determined by the data. All energies are measured in cm-{'relative to the zero point energy
on each potential energy surface. Double primes and single primes denote ground and excited electronic state parameters as usual. The vibronic band notation A',% denotes the transition (uA' = i , uB' = j ) (uA" = m, v i ' = n), for example. To assign the spectrum, we assume the origin to be band 13 at the single photon energy 20 028 cm-I and the resonances to be at the two-photon excitation energy, as did Hudgens and Dulcey. The evidence for two photon resonances includes the sensible quantum defect of 0.92 for the 3s Rydberg statelo built from the ionization limit of 8.13 eV29and the similarity of the -400-cm-' spacing (on a two-photon energy scale) in the spectrum to the 420 f 40 cm-l spacing observed in the one-photon photoelectron spectrumz9of allyl radical. The consistency of our assignment scheme in explaining both hot and cold bands and the agreement of the resulting frequencies with matrix IR193z0and allyl anion photoelectron spectraz1strongly support both the choice of origin and the two-photon order of the resonances. Other choices of origin are less satisfactory. Given the origin, we fit the positions of bands 1, 2, 7 , 8, 9, 14, 15, 20, 21, 24, and 25 within experimental error by invoking a progression A:, hot bands A:, and associated sequence bands in vibrational mode A, as indicated in Table I. The fundamental frequencies I, = W: + xAA are 426 cm-I in the ground state and 393 cm-I in the excited state (Table 11). Both modes are quite harmonic, but the fit is improved significantly by including small positioe anharmonicities xAA" = 5.0 f 1.5 cm-I and xAA' = 2.0 f 1.0 cm-l. The frequencies and anharmonicities were adjusted by trial and error to provide the best fit to the 11 bands listed above. The fits to bands 3, 10, and 16 as A:, A:, and A: lie outside estimated error. The agreement improves substantially if we alter the level energies to A4 = 1588 cm-' and A, = 1292 cm-', suggesting that eq 1 is not accurate for those two levels. Thus far, the assignments require population of the levels Oo, A,, A*, and A3 in the beam and quantum number changes A v A = 0, f 1 , and f 2 in the spectrum. Vibrational degrees of freedom often cool less efficiently than rotational degrees of freedom in supersonic expansion. Mode A is undoubtedly the same mode responsible +
The Journal of Physical Chemistry, Vol. 91, No. 14, I987
Gas-Phase Allyl Radical Spectrum TABLE II: Recommended Gas-Phase Allyl Vibrational Levels' in *A2 and 3s 'Al Electronic States
2 2A2ground state vibronic level Ai A2
[A31 Bi
c,
Gn. cm-I 426 (3) 862 (4)
[I308 (l0)lb
508 (1 2) 558 (15)
3
3s 2A1 excited state vibronic level Go. cm-l A1 393 (3) A2 790 (3) A' 1191 (4) w41 [1596 (lo)]* [B'I [491 (7)1 C' 529 (5) [Dl1 597 (3)
'Uncertainty estimates in parentheses. Energies and assignments in square brackets are tentative. For modes vA" and vA', the diagonal anharmonicities were determined as xAA" = +5.0 1.5 cm-' and xAA' = +2.0 1.0 cm-I. bThe calculated positions of bands 3, 10, and 16 (Table I) agree much better with experiment if we adopt the level energies A' = 1292 cm-l and A4 = 1588 cm-', which suggests that eq 1 does not accurately describe these levels.
*
*
TABLE Ill: Mode Identification and Comparison with ab Initio Freouencies
neutral allvl radical frea, cm-I 2 2A2state 3s 2AIstate, allyl cation mode' svm" calcd' obsdb obsdb frea. cm-' 426 393 42040)c CCC bend a, 416 508 [491] CHI twist (sym) b, 562 558 529 CH2 twist (asym) a2 596 'Harmonic frequencies, mode identifications, and C2, symmetry species from the ab initio calculations of ref 22. bGas-phase experimental values. See Table I1 for uncertainties. Entry in square brackets is based on a tentative observation. CFromphotoelectron spectrum on ref 29. for the long progression in the photoelectron spectrum of gas-phase allylic anion. Oakes and Ellison2' thus obtained bA = 395 f 165 cm-l for the neutral radical ground state, consistent with our result. Further progress requires that we invoke activity in other modes. Band 22 at -508 cm-' matches the matrix IR absorption band observed at 510 cm-l. In addition, Hudgens and DulceyIoobserved a much stronger band at -5 13 cm-I in their hotter sample. We assign this band as the BY transition. Hot band activity in a new mode suggests that we look for the first member of a progression in the same mode of the excited electronic state. We tentatively assign band 6 at 491 cm-I as BA. Evidence of B-mode activity in our spectrum is weak; we rely on the results of Hudgens and Dulcey, the matrix IR spectra, and calculated ab initio frequencies for the plausibility of these assignments, as explained below. Similarly, we assign band 5 a t +529 cm-I as CA. In corroboration, band 12 then fits AYCA within experimental error. We estimate the C , vibrational level energy at 558 f 15 cm-l; the calculated CY and A$; transition energies then fit the positions of bands 23 and 18 at the edges of the estimated uncertainties. Further support of the C , level energy comes from the band at -562 cm-I in the hotter spectrum of Hudgens and Dulcey. The only remaining unexplained, definitely observed band is band 4 at +597 cm-I, which we tentatively assign as another excited state mode DA; it lacks a counterpart DY in the spectrum. It could also be a hot band transition involving a new upper state mode, e.g., AYDA. Three other tentatively observed bands lack assignments. Tables 1-111 include all of the results with tentative assignments and energy levels indicated by square brackets. The overall scheme is internally consistent in the following senses. Cross-checks on the energy levels A I , A,, AI, AZ,A', C1, and C' occur in the spectrum; the levels BI, B', A,, and A4 are less well established based on our spectrum alone. The assignments require population in the ground vibrational state Oo, Al, A,, BI, C,, and probably A,. Roughly speaking, the intensities of the bands diminish with increasing ground-state vibrational energy in the assigned transition, consistent with a vibrational state population distribution that peaks at the ground state. The quantum number changes AuA = 0, f l , and f 2 are observed,
3735
indicating a significant geometry change along normal coordinate A on electronic excitation. Only Aug = f l and Auc = f l bands are observed; CA is weak and BA is very weak compared with 0;. Geometry changes along the normal coordinates B and C are probably minimal. Given the populations and inferred qualitative Franck-Condon propensities, no important bands are missing from the spectrum and only one definitely observed band (band 4, Dk) lacks a plausible assignment within the framework of three active modes A, B, and C.
Discussion Both ESR spectra30 and ab initio calculations22indicate that the ground state of the allyl radical is planar with C,, symmetry. The 3s ,A, X ,Az transition studied here involves promotion of the unpaired electron from a nominally nonbonding 7r-orbital (a, symmetry) to a 3s Rydberg orbital (al). We can now combine the evidence from the gas-phase data presented here with that of Hudgens and Dulcey,lo with the matrix IR absorption spectra,19q20with the allylic anion photoelectron spectrum,*' and with the ground-state ab initio multiconfiguration Hartree-Fock calculations of Takada and Dupuis22 to infer the nature and symmetry types of the three active modes labeled A, B, and C. The three lowest ab initio ground-state harmonic frequencies and symmetry types (Table 111) are the CCC bend (al species, 476 cm-I), the symmetric C H 2 twist (bl, 562 cm-I), and the asymmetric CH, twist (a,, 596 cm-I). The next two lowest calculated frequencies are the out-of-plane CH, bends at 761 and 786 cm-I. We can identify the three lowest energy calculated modes with the experimental vibrational levels A, (426 cm-I), B1 (508 cm-I), and C I (558 cm-I) of Table I, respectively, with considerable confidence. The experimental frequencies are 89%, 90%, and 94% of the calculated harmonic frequencies. Takada and Dupuis applied a similar level of theory to the stable molecule propene, CH3-CH=CH2, with similar quantitative success: the 428-cm-' observed CCC bend frequency is 9 1 % of the calculated harmonic frequency of 469 cm-l, and the observed CH2 twist frequency of 575 cm-l is 95% of the calculated 603 cm-I. The experimental allyl CCC bending frequency is nearly identical with that of p r ~ p e n e . ~ For ' comparison with theory, we estimate an experimental harmonic frequency for mode A as wA" = up' xAA"= 416 cm-I, which is 87% of the calculated result. Table 111 summarizes the mode identification scheme. Oakes and EllisonZ1previously assigned the 395 i 165 cm-' spachg in the allylic anion photoelectron spectrum to the CCC bend of the neutral radical, in agreement with Table 111. The lowest frequency band observed in the matrix IR spectrum of allyl lies at 51 1 cm-I, in close agreement with the gas-phase level at 508 cm-I. In accord with the scheme of Table 111, IR absorption from the a, ground state to the b, CH2 twist is allowed. Maier and co-workersI9 assigned the 51 I-cm-' band as the CCC bend, which we believe is incorrect. Frequency shifts from gasphase radicals to Ar matrix isolated radicals1*are 2% at most. Thus no reported matrix IR absorption bands match either the 426- or 558-cm-I levels observed in the gas phase. In the former case, the likely reason is that the IR frequency scan was truncatedI9 at about 425 cm-', probably due to increasing absorption by the KBr windows. In the latter case, the a, symmetry of the asymmetric CH, twist forbids IR absorption. Thus the matrix IR spectral data are consistent with the gas-phase data and its interpretation in Table 111. Hudgens and Dulcey'O also observed strong hot bands at -101 1 and -1094 cm-I. These might be assigned as B: and C:, respectively. However, the Au = f 2 intensities should be negligible for these CH2 twist modes if the excited state is planar, as the small BA and CA intensities strongly suggest. These bands are more likely due to C C stretch modes. The frequencies are 92% and 91%, respectively, of the symmetric and antisymmetric CC stretch
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(30) Kochi, J. K.; Krusic, P. K. J . Am. Chem. Soc. 1968, 90, 7158, and references therein. (31) Silvi, B.; Labarbe, P.; Perchard, J. P. Spectrochim. Acta, Part A 1973, 29A, 263.
3736
J . Phys. Chem. 1987, 91, 3736-3740
harmonic frequencies of 1093 and 1204 cm-' calculated by Takada and Dupuis.22 Finally, the mode identification scheme of Table I11 permits a qualitative description of some of the changes in vibrational structure on excitation from the ground state to the 3s Rydberg state. The frequencies of all three modes A, B, and C diminish slightly from the ground to the excited state. Thus the potential softens somewhat along both the CCC bending and the C H 2 twisting coordinates on excitation of the nominally nonbonding r electron to the 3s Rydberg orbital. The progression in mode A indicates a substantial change in the equilibrium CCC bond angle from ground to excited state. We cannot determine either the sign or magnitude of the change in angle from the available data. The much weaker activity in the torsions B and C indicates that the excited state, like the ground state, is planar or nearly so. The single allyl cation vibrational spacing of 420 f 40 cm-l identified in the photoelectron spectrum of gas-phase allyl is almost surely the CCC bending frequency of the cation. Thus the C C bending frequencies of the 3s Rydberg state and of the cation are the same within current experimental error. More quantitative interpretation of the vibronic band intensity envelopes in REMPI spectra is not possible.
Conclusion We have demonstrated the utility of the new combination of a photolytic source of cold neutral free radicals in a supersonic expansion with the spectroscopic technique of resonance-enhanced multiphoton ionization. The presence of excited vibrational states in the allyl radical beam permitted the determination of both
ground- and excited-state frequencies. The availability of matrix IR and allyl anion photoelectron spectra and especially of highquality ab initio calculations of harmonic frequencies permits a firm identification of observed gas-phase levels with particular low-frequency motions. In the future, we will carry out higher resolution scans of the allyl spectrum and attempt to obtain additional cation vibrational state information from the photoelectron spectroscopy of the cold neutral radical. Use of two probe laser colors will improve the generality of the technique and the signal levels as well. Note Added in Proof. To the extent that the two-photon vibronic transition strength can be approximated by a product of electronic and vibrational (Franck-Condon) factors, bands having Av = f 1 in a nontotally symmetric mode will be symmetry forbidden in an electronically allowed two-photon transition such as 'A, *A2in C,, symmetry. (See Herzberg, G. Electronic Spectra of Polyatomic Molecules; Van Nostrand Reinhold: New York, 1966; pp 173-177 for a discussion of the analogous onephoton case.) If the symmetry labels of Table I11 are correct, then it appears that transitions such as Bh, ,C ,!, etc., are vibronically induced. This seems particularly plausible for a Rydberg state due to its close proximity to other excited states. We thank Dr. J. W. Hudgens for pointing out this possibility.
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Acknowledgment. We thank Dr. Alec Wodtke for suggesting the quartz photolysis tube, Professor Richard N. Zare for a helpful discussion of possible REMPI line broadening mechanisms, and the National Science Foundation for support of this research under Grant No. CHE-8302856.
Radtcal Catlons of Selectively Deuteriated 3-Methylpentanes and 3-Methylhexanes Produced in y-Irradiated CF,CICFC12 Matrices As Studied by ESR Nobuaki Ohta and Takahisa Ichikawa* Department of Applied Physics and Chemistry, Hiroshima University, Saijyo, Higashi- Hiroshima 724, Japan (Received: December 23, 1986)
The cations of 3-methyl~entane-h,~ (3MP-h) and 3-methylhexane-h16(3MHX-h) are explained with a model where the unpaired electrons are coupled with three trans C-H, protons with respect to the C3-C4 u bonds. The coupling constants of the protons (a") are determined in consideration of the results obtained from the deuteriated compounds: aH(3MP-h) for methylene (C2), branched methyl, and terminal methyl (C,) protons are 52.9, 39.3, and 49.9 G , respectively, and aH(3MHX-h) for methylene (C2), branched methyl, and methylene (C,) protons are 57.1, 39.1, and 61.9 G , respectively. It is suggested that the unpaired electrons in methyl-branched alkane cations higher than 3methylheptane are delocalized over the main-chain C-C c bonds. A mechanism for free radical formation is briefly discussed.
Introduction By the use of electron spin resonance (ESR) spectroscopy Iwasaki and co-workers have found that methyl-substituted butane radical cations in CF2CICFCI2 matrices undergo thermal deprotonation from CH, groups to give primary alkyl radicals.' For methyl-substituted propane radical cations, they have also found that thermal deprotonation from the cations gives alkyl radicals and they have concluded that deprotonation takes place from the bond in which the unpaired electron is highly populated.2 We have observed that y-irradiated glassy 3-methylpentane-h14 (3MP-h) yields mainly the secondary radical which is produced by rupture of the C-H bond on position 23 and that for 3methylhexane-h16(3MHX-h) the secondary radicals on positions (1) Nunome, K.; Toriyama, K.; Iwasaki, M. Chem. Phys. Lett. 1984, 105, 414. (2) Toriyama, K.; Nunome, K.; Iwasaki, M. J . Chem. Phys. 1982, 77, 5891. ( 3 ) Ichikawa, T.; Ohta, N. J . Phys. Chem. 1977, 81, 560.
0022-3654/87/2091-3736$01.50/0
2 and 5 are mainly produced in equal a m o ~ n t s . ~It has been also found that deuterium substitution on positions 2 and 4 in 3MPrh induces selective formation of the tertiary radical CH,CD2C(CH3)CD2CH3.For 3MXH-h, deuterium substitution on positions 2, 4, and 5 resulted in the formation of the tertiary radical at position 3. If these alkyl radicals produced in 3MP and 3MHX glasses are formed through deprotonation from radical cations of 3MP and 3MHX, the unpaired electron sites of the resultant alkyl radicals should reflect a distribution of the unpaired electron densities on the radical cations. At the present, there seems to be no report investigating the ESR spectra of radical cations of selectively deuteriated 3MPs and 3MHXs in halocarbon mat rice^.^ To determine the positions of the coupling protons in the 3MP (4) Ichikawa, T.; Ohta, N. Radial. Phys. Chem., in press. ( 5 ) After submitting the manuscript, we found a recently published paper about the structure of the radical cation of 3MP-h: Toriyama, K.; Nunome, K.: Iwasaki, M. Chem. Phys. L e u 1986, 132, 456.
0 1987 American Chemical Society
