J . Phys. Chem. 1990, 94, 8399-8401
photoproduct ethylenic line from 1590 to 1615 cm-I may be one indication of single-bond conformational relaxation. This frequency upshift is very similar to the upshift observed for the cis-hexatriene photoproduct of 1,3-CHDe7QCFF-pi calculations performed in this laboratory and ab initio calculations by Tasumi and co-workersi3 predict that, in cis-hexatriene, an upshift in ethylenic frequency will accompany single-bond conformational relaxation to the more extended s-trans,cis,s-trans form and a similar effect is expected for cis,cis-octatetraene. QCFF-pi calculations performed in this laboratory show that s-trans,cis,s-cis,cis,s-trans-octatetraene still has significant steric interaction at the ring-opened end of the molecule, resulting in distortion of one of the central double bonds away from planarity. This torsional strain would be consistent with the presence of an intense double-bond torsional mode a t 351 cm-I. It is also possible that the observed frequency upshifts are due to anharmonicity of the ground-state potential surface. At earliest times, higher vibrational levels may contribute significantly to the observed Stokes spectrum resulting in vibrational frequencies which are lower than those observed from a thermally relaxed photoproduct. An upshift in frequency would accompany vibrational relaxation on the ground-state surface. The ethylenic frequency of the OT photoproduct also upshifts as a function of time. The ring opening of a-PHE proceeds along the direction of conrotatory rotation that minimizes steric interactions of the isopropyl group with the remainder of the molecule.9b The ring-opened all-cis photoproduct would still be the high-energy conformer due to the steric interaction of the terminal hydrogens similar to all-cis-he~atriene.~~ Therefore, (15) Tai, J. C.; Allinger, N. L. J . Am. Chem. SOC.1976, 98, 7928.
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it is reasonable to suggest that the observed upshift in ethylenic frequency of the 3,7-dimethyl-l,3,5-octatrienephotoproduct represents ground-state conformational relaxation to s-cis-cis,strans-OT and/or s-trans,cis,s-trans-OT, although vibrational cooling may also contribute to this evolution. To further elucidate the conformational composition of the OT photoproduct, more detailed information about the vibrational spectra of the various conformers is needed. In conclusion, the similar photoproduct formation times for COT, a-PHE, and I,3-CHD indica.te that the IO-ps time scale for the appearance of photoproduct on the ground-state surface is a general feature of pericyclic photochemical ring-opening reactions and this time does not depend on the stereochemistry of the reaction or chemical substitution. The fact that the appearance time does not depend on the stereochemistry or substitution of the reactant argues that the structural products of electrocyclic ring-opening reactions are determined predominantly by the excited states which participate in the photochemistry. These reactions are characterized by a rapid excited-state evolution which dictates the product composition, followed by a 10-ps relaxation to the ground-state surface. Resonance Raman intensity analysis should prove useful in comparing the femtosecond excited-state dynamics of COT and a-PHE with 1,3-CHD.I6 It will also be important to examine the long-lived excited states of these systems to understand the cause of the unusually strong coupling to the ground-state surface.
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Acknowledgment. This work was supported by a grant from the NSF (CHE 86-15093). (16) Lawless, M. K.; Mathies, R. A. Manuscript in preparation.
Photoelectron Spectrum of the Ptopargyl Radical in a Supersonic Beam David W. Minsek and Peter Chen*.' Mallinckrodt Chemical Laboratories, Harvard University, Cambridge, Massachusetts 021 38 (Received: August 16, 1990)
The laser photoionization mass and photoelectron spectra of propargyl radical ( K 3 H 3 )and two partially deuterated isotopomers are measured in a supersonic molecular beam. Radicals were produced by flash pyrolysis in a heated supersonic tube. nozzle. The PES exhibits an intense origin band at 8.67 f 0.02 eV, which is assigned to the adiabatic ionization potential previously seen by electron impact mass spectroscopy. A second photoelectron band at higher energy is assigned to the previously unobserved first excited state of the propargyl cation. One highly anharmonic vibrational progression in the cation is also observed.
Introduction We report' preliminary results of the vibrationally resolved photoelectron spectrum of the hydrocarbon free radical propargyl (C3H3)in a supersonic beam. Propargyl has attracted considerable attention as one of the simplest conjugated organic free radicals. It has been discussed as a possible precursor to benzene and other aromatic hydrocarbons in flames2 The propargyl cation appears commonly in mass spectrometry and also appears in abundance in flames.3 Despite its importance, gas-phase spectroscopic and thermochemical data on propargyl remain scarce. Lossing measured an ionization potential (IP) of 8.68 eV for propargyl by electron i m p a ~ t . ~Ramsay and Thistlethwaite reported an ultraviolet absorption spectrum of propargyl in 1968, but the ( I ) David and Lucile Packard Fellow, NSF Presidential Young Investigator, Camille and Henry Dreyfus Distinguished New Faculty Fellow. ( 2 ) Westmoreland, P. R.; Dean, A. M.; Howard, J. B.; Longwell, J. P. J . Phys. Chem. 1989, 93, 8171, and references therein. (3) Goodings, J. M.; Bohme, D. K.; Ng,C.-W. Combust. Flame 1979, 36, 21. (4) Lossing, F. P. Can. J . Chem. 1972, 50, 3973.
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observed lines were diffuse, and definite assignments could not be madeus In matrix isolation studies of propargyl the infrared and ESR spectra have been reported. Flash pyrolytic production of reactive free radicals followed by supersonic expansion has been shown to be a useful technique for the isolation of radicals under stabilizing conditions, Le., jet cooled and collision free.*-" We have applied this technique for the production and isolation of the propargyl radical and obtained the first photoelectron spectra of this important species. From these spectra we have (i) confirmed the previous IP measurement; (ii) found spectroscopic evidence for a low-lying excited state of the corresponding cation, predicted by theory12but never observed; ( 5 ) Ramsay, D. A.; Thistlethwaite, P. Can. J . Phys. 1966, 44, 1381. (6) Jacox, M.E.; Milligan, D. E. Chem. Phys. 1974, 4, 45. (7) Fessenden, R. W.; Schuler, R. H. J . Chem. Phys. 1963, 39, 2147. (8) Blush, J. A.; Park, J.; Chen, P. J . Am. Chem. SOC.1989, I l l , 8951. (9) Chen, P.; Colson, S. D.; Chupka, W. A.; Berson, J. A. J. Phys. Chem. 1986, 90, 2319. (IO) Chen, P.; Colson, S.D.; Chupka, W. A. Chem. Phys. Lett. 1988,147, 466. ( 1 I ) Dunlop, J. R.; Karolczak, J.; Clouthier, D. J. Chem. Phys. Len. 1988, 151, 362. Clouthier, D. J.; Karolczak, J. J. Phys. Chem. 1989, 93, 7542.
0 1990 American Chemical Society
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Letters
The Journal of Physical Chemistry, Vol. 94, No. 22, 1990 1.61 ov
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Figure 2. Time-of-flightphotoelectron spectrum of the HCCCDl radical
following 10.49-eV photoionization.
1
78
(b) heat
on
mass Figure 1. The 10.49-eV photoionization mass spectrum of HCCCD,Br (a) without and (b) with pyrolysis. Benzene-h6( m / e = 78) was added to the sample as a mass calibrant.
and (iii) observed and tentatively assigned a highly anharmonic vibrational progression in the cation.
Experimental Section Photoionization mass spectra were obtained with a previously described time-of-flight mass spectrometer.* Briefly, ca. 1 Torr of radical precursor was seeded into 2 atm of helium and expanded via a pulsed valve (General Valve) at 20 Hz through a resistively heated zirconium oxide tube nozzle ( 1 .O mm id.) with a 2.0 cm heated zone. Twenty watts heated the nozzle to about 1500 K. A contact time of 10-50 ps with the nozzle resulted in flash pyrolysis of the radical precursor. The pyrolysis products were expanded into the source region of the mass spectrometer. After skimming, the radical beam was ionized by a pulse of 118.2-nm (10.49-eV) photons obtained by frequency tripling the 355-nm output of a Q-switched Nd:YAG laser in xenon.'O The cations were then accelerated by an electric field and mass analyzed by time of flight. Deuterated radicals were prepared from the corresponding propargyl or allenyl bromides. Figure 1 shows mass spectra obtained for HCCCD2Br (a) without and (b) with pyrolysis (benzene-h6 was added as a mass calibrant). Trace a shows only HCCCD2Br at m / e = 120 and 122. Trace b shows that flash pyrolysis cleanly produced a peak at m / e = 41, which we assign to the HCCCD2 radical from homolysis of the carbon-bromine bond. Bromine atom does not appear since IPB, > 10.5 eV. Of particular note is the nearly quantitative conversion of propargyl bromide to propargyl, and the absence of side products, particularly dimerization products a t m / e = 82. At higher concentrations of precursor a peak at m / e = 82 appeared. Pyrolysis of HCCCH2Br and DCCCH2Br produced peaks at m / e = 39 (HCCCH,) and 40 (DCCCH,), respectively. Pyrolysis of allenyl bromide also produced a peak at m / e = 39, ascribable to HCCCH,. Photoelectron spectra (PES) were obtained by using a recently constructed time-of-flight photoelectron spectrometer. The propargyl radical was produced by flash pyrolysis of propargyl bromide, immediately followed by supersonic expansion, as described above. The vacuum chamber outside the nozzle was maintained at Torr by a 235 cfm Roots blower. The jet was skimmed twice with differential pumping; it then entered the ionization region of a magnetic-focusing time-of-flight photoelectron spectrometer. The electron energy analyzeri3 was obtained from Applied Laser Technology. We measured an absolute (12) Cameron, A,; Leszczynski, J.; Zerner, M. C.; Weiner, B. J . Phys. Chem. 1989, 93, 139. ( 1 3 ) Kruit. P.; Read. F. H. J . Phys. E.. Sci. Instrum. 1983, 16, 313.
electron collection efficiency of ~ 3 0 % A . 118.2-nm photon pulse ionized the radicals, and the kinetic energies of the electrons were measured by their flight times down a flight tube. The instrumental resolution (fwhm) is a function of the electron flight time, ranging from about 0.030 eV for low-energy electrons to about >0.10 eV at higher energies. Thus, the spectral resolution can be improved by applying a small dc retarding voltage (0.1-1.0 V) to the flight tube. PES were obtained of the pyrolysis products from HCCCH2Br, DCCCH2Br, HCCCD,BR, as well as allenyl bromide. Pyrolysis of allenyl bromide gave a PES identical with that from HCCCH2Br. Both are assigned to the propargyl radical, HCCCH,. The time axes of the spectra were calibrated by taking PES'4J5 of nitric oxide under the same conditions. Propargyl bromide (80% in toluene) was purchased from Aldrich and purified by distillation prior to use. DCCCH2Br and HCCCD2Br were obtained by treatment of DCCCH20HI6and HCCCD20H17with phosphorus tribromide. Isotopic purities were determined to be >95% by ' H NMR. Allenyl bromide was prepared from propargyl bromide by a literature procedure.I* Results and Discussion The time-of-flight photoelectron spectrum of HCCCD, is shown in Figure 2. The spectrum displays a sharp, intense band at 1.81 f 0.02 eV which we assign as the origin band. The PES of HCCCH, and DCCCH, also display sharp origin bands. Subtraction of the origin band energies for the three isotopomers from the laser energy (10.49 eV) yields an average adiabatic IP of 8.67 f 0.02 eV. The IP's of the three isotopomers (more than three measurements each) are within experimental error of each other and are in excellent agreement with the IP of 8.68 eV for HCCCH, measured by electron impact by L o ~ s i n g . ~ The prominence of the origin band in the PES indicates that the geometry of the cation is similar to that of the neutral. Both theoryf9 and experiments" support a planar, CZuracetylenic structure for the neutral propargyl radical, and theory has also predicted a C , structure for the cation.I2 Although PES in general cannot determine absolute geometries, the strong origin band suggests that the geometry change between the neutral and cation is small. Figure 2 also displays a slower photoelectron peak at 0.09 eV, which we ascribe to the formation of an electronically excited state of the propargyl cation upon ionization. This feature also appears in the PES of HCCCH, and DCCCH, at similar energies. Subtraction of this peak energy from the origin band energy at 1.8 1 eV yields an excited state energy of 1.72 eV. Theory has predicted a vertical transition from the ground to the first excited state of the cation at 2.2 eV.12 However, 1.72 eV should be considered a lower limit, since the photon energy of 10.49 eV was insufficient to resolve the entire band. Investigations with higher (14) Amiot, C.; Bacis, R.;Guelachvili, G.Can. J . Phys. 1978, 56, 251. (15) Edqvist, 0.; Asbrink, L.; Lindholm, E. Z . Naturforsch. 1971, 26a, 1407. (16) McNab, H . J . Chem. SOC.Perkin Trans. 2 1981, 1283. (17) Trennery, V. C.; Shelton, J. C.; Bowie, J. H. N o w . J . Chem. 1981, 5, 1081. (18) Jacobs, T.L.; Brill, W. F. J . Am. Chem. SOC.1952, 75, 1314. (19) Honjou, H.; Yoshimine, M.; Pacansky, J. J . Phys. Chem. 1987,91, 4455. (20) Oakes, J. M.; Ellison, G. B. J. Am. Chem. SOC.1983, 105.2969 and references therein.
J . Phys. Chem. 1990, 94, 8401-8403
1 1 1 1
2.0 1.8
I I 1.4 1.2
I
1
1
1.0
0.8
0.6
Electron Energy (eV) Figure 3. Time-of-flight photoelectron spectrum of the HCCCDl radical following 10.49-eV photoionization,with the application of a 0.4-V dc retarding voltage (see Experimental Section). The electron energies for peaks a-h are listed in Table 1. TABLE I: Photoelectron Peak Energies (KE) and Spacings for HCCCDZ. Deak KE." eV re1 intensity AE," cm-l a 1.81 1 .o b 1.56 0.9 2020 C 1.33 0.7 1850 d 1.15 0.5 1450 1 .oo
0.90
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1210
f g
0.8 1 0.73
0.3
730 650
e h *0.02 ev.
0.2
810
& 150 cm-I.
energy photoionization are under way to further characterize this excited state. A higher resolution PES of HCCCD2 is shown in Figure 3. This spectrum was obtained with the use of a dc retarding potential to increase resolution, as described above. Peak positions, differences, and relative intensities are listed in Table I. The peaks appear to be due to a single vibrational progression in the cation, showing a smooth Franck-Condon envelope with peaks decreasing in intensity and frequency. The frequencies and relative integrated
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peak intensities were obtained by least-squares deconvolution of the observed photoelectron spectrum using an instrument function fitted to a quadratic. Inclusion of additional peaks to simulate unresolved, lower frequency vibrations improved the fit, but did not significantly alter the positions or relative intensities of the main peaks. The most striking feature in the observed vibrational progression is the large anharmonicity. The spacing between the first and second bands is 2020 cm-I, which decreases to 650 cm-l for the seventh and eighth bands. A fit of the vibrational progression to a Morse potential gives we = 2270 cm-l and x,w, = 130 cm-l. We tentatively assign the active vibration to a symmetric CC acetylenic stretch in propargyl cation. The harmonic frequency agrees well with the ab initio valueI2 of 2194.6 cm-I. The alternate assignment to a C-D stretch can be discounted because the dissociation limit for such an anharmonic C-D bond, estimated from xcwe, would be = I eV. This would give a AHr(HCCCD'+) 3 eV below the accepted valuez1and would suggest that we should observe the HCCCD'+ daughter ion in the 10.49-eV photoionization mass spectrum. Inspection of Figure 1 shows no such ion dissociation. The large anharmonicity of a C C acetylenic stretch may be due to strong configuration mixing induced by changes in bond length. The harmonic frequency in the cation suggests a predominant acetylenic, rather than allenic, character for propargyl cation, and, by the Franck-Condon factors, propargyl radical as well, at their equilibrium geometries. Increased contribution by an allenic structure as the C=C bond is stretched could result in a reduced force constant and the large anharmonicity. Vibrational structure was not clearly discernible in the PES of HCCCHz and DCCCH2, presumably because of spectral congestion by unresolved CHz modes. The improved resolution in HCCCDz is likely due to the reduction of these frequencies by deuteration. Acknowledgment. We acknowledge assistance in the photoelectron spectrometer by J. A. Blush and financial support from the donors of the Petroleum Research Fund, administered by the American Chemical Society, and the Department of Energy. (21) Lias, S.G.; Bartmess, J. E.; Liebman, J. F.; Holmes, J. L.; Levin, R. D.; Mallard, W. G. J . Phys. Chem. Ref Dafa 1988, 17, Supplement I .
Fleid Strength Dependence of Dielectric Saturation in Liquid Water Howard E. Alper and Ronald M. Levy* Department of Chemistry, Rutgers University, New Brunswick, New Jersey 08904 (Received: September 5, 1990)
Molecular dynamics simulations have been used to study the field strength dependence of dielectric saturation in liquid water. The equilibrium polarization response in a box containing 216 water molecules is followed as a function of the magnitude of an applied electric field in a series of simulations. The polarization is approximately linearly proportional to the applied field strength up to fields -4 X IO* V/m. Large nonlinear effects associated with dielectric saturation are observed between 1 X IO9 and 2 X 1Olo V/m, at which point the dielectric response is fully saturated. Additional simulations of solvated ions and dipoles show that the hydration layer waters are orientationally saturated even though the hydration free energies of these solutes are closely approximated by continuum solvent formulas. While hydration free energies are relatively insensitive to dielectric saturation, many other solute-solvent properties will be sensitive to such effects.
The study of the dielectric properties of aqueous solutions has a long and venerable history.' Yet there remains considerable uncertainty regarding the relative importance of dielectric satu( I ) (a) Born, M. 2.~ h p 1920, . I , 45. (b) Debye, P.Polar Molecules; Dover: New York, 1929. (c) Onsager, L. J . Am. Chem. Soc. 1936.58, 1486. (d) Kirkwood, J . G.J . Chem. Phys. 1939, 7, 911. (e) Bottcher, C. J. F. Theory of Elecrric Polarization; Elsevier: Amsterdam, 1973; Vol. I .
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ration effects on either solvation energies or dielectric screening of electrostatic interactions. On the one hand, the electric field strengths at the surface of ions are very large, exceeding 1Olo V/m. This leads to the expectation that dielectric saturation will occur in solvent regions close to the ion.'-' It has been argued, however, (2) Booth, F. J . Chem. Phys. 1951, 19, 391.
0 1990 American Chemical Society
