4138
J. Phys. Chem. 1992, 96, 4138-4140
Photoelectron Spectrum of the Vinyl Radical. Downward Revision of the C,H, Ionization Potential Joel A. Blush and Peter Chen*J Mallinckrodt Chemical Laboratory, Haruard University, Cambridge, Massachusetts 02138 (Received: January 2, 1992; In Final Form: April 10, 1992)
We report the vacuum-UV photoelectron spectrum of the vinyl radical, C2H3, in a supersonic molecular beam. We find IP[C,H,] = 8.25;od:i eV. Our ionization potential, which is lower than other reported values, combined with AWBD60[o[C,H3-H] = 105-1 10 kcallmol, supports the conclusion from low-temperatureion-molecule chemistry that AHor,,[C2H3+]IS several kcal/mol lower than the value derived from the dissociative photoionization of ethylene.
Introduction We report the vacuum-UV photoelectron spectrum of the vinyl radical, C2H3, in a supersonic molecular beam. The ionization potential, measured in this experiment, and appropriate auxiliary thermochemical data,2relates the heats of formation of C2H3 and C2H3+ to the homolytic C-H bond dissociation energy of ethylene, all of which have been the subject of much controversy. We measure the onset of the photoelectron spectrum of C2H3 at 8.25 f 0.05 eV, with the actual adiabatic ionization potential no more than 0.2 eV higher. Our ionization potential, which is considerably lower than other reported values, in the equations IP[C2H3I = m0f,0[C2H3+l - MBDE,O[CZH3-H] + ~ O r , o [ H ' l- uof,o[C2H41 IP[CZH,] = AHor,o[C2H3+]- AHoBDE,O[CZH3-H] + (37.05 f 0.07) kcal/mol determines one of the two quantities, AHor,o[C2Hs+]or MOBDE,,[C~H~-H], given a value for the other. The value for IP[C2H3]reported in this work, combined with the recent determinations of WBDEo[C2H3-H],supports the conclusion from low-temperature ion-molecule chemistry that M f , o [ C 2 H 3 +=] 265:; kcal / mol.
Experimental Section The spectrometers used in this work, as well as pyrolytic techniques used to prepare C2H3, have been previously Vinyl radicals were produced from two separate precursors in a resistively heated alumina tube nozzle (heated length = 0.8 cm; inner diameter = 1 mm) at temperatures up to 1000 K. Photoionization mass spectra of the pyrolysate were obtained in a molecular beam time-of-flight ~pectrometer.~Photoelectron spectra were obtained with a magnetic-focusing time-of-flight ~pectrometer.~ The radical beam was skimmed and ionized by either 118.2-nm (10.49-eV) photons, generated by frequencytripling the 355-nm third harmonic of a Nd3+:YAG laser (Spectra-Physics DCR-3G) in xenon, or 129-nm (9.61-eV) photons, generated by frequency-tripling, also in xenon, the 387-nm sum frequency of a YAG pumped dye laser (Spectra-Physics PDL-2, Sulforhodamine 640) and the Nd3+:YAG fundamental at 1064 nm. Two separate precursors, fert-butyl peracrylate or nitroethylene (-1 mbar), were seeded into 1700 mbar of He and expanded, via a pulsed valve (General valve) operating at 10 Hz, through the hot tube nozzle. rert-Butyl peracrylate was prepared by our published p r ~ c e d u r e . ~Nitroethylene-h3 and nitroethylene-d, were prepared by the procedure of Buckley and Scaife.6 (1) NSF Presidential Young Investigator, David and Lucile Packard Fellow, Camille and Henry Dreyfus Teacher-Scholar, Alfred P. Sloan Research Fellow. (2) Auxiliary thermochemical data on C2H, and H' were taken from: Chase, M. W.; Davies, C. A.; Downey, J. R.; Frurip, D. J.; McDonald, R. A,; Syverud, A. N . J . Phys. Chem. Ref. Dora 1985, 1 4 (Suppl. No. 1). (3) Blush, J. A.; Park, J.; Chen, P. J . Am. Chem. SOC.1989, 111, 8951. (4) Minsek, D. W.; Chen, P. J . Phys. Chem. 1990, 94, 8399. (5) Kohn, D. W.; Clauberg, H.; Chen, P. Rev. Sci. Instrum., in press.
These two precursors were chosen from a large number of structures which we prepared and screened for ease and specificity in their thermal decomposition to C2H3. Results Photoionization mass spectra of pyrolyzed tert-butyl peracrylate and nitroethylene, taken using 129-nm (9.61-eV) photons, are shown in Figure 1. Vinyl radical, C2H3, is the only detectable product from tert-butyl peracrylate. The other product^,^ COz, CH3, and acetone, all have ionization potentials above 9.61 eV. The peak for vinyl dominates the 9.61-eV mass spectrum of pyrolyzed nitroethyleneunder conditions of mild pyrolysis. At higher temperatures, peaks corresponding to nitric oxide (plus ketene and CH3 in the 10.49-eV spectrum) were also observed in the mass spectrum, presumably arising from fragmentation of a vinyl nitrite rearrangement product' of nitroethylene. A photoelectron spectrum of pyrolyzed tert-butyl peracrylate, taken with 118.2-nm (10.49-eV) photons, is shown in Figure 2. We interpret the absence of all other peaks in the 9.61-eV photoionization mass spectrum of pyrolyzed tert-butyl peracrylate to indicate that all of the photoelectrons faster than 0.88-eV electron kinetic energy in the 10.49-eV photoelectron spectrum in Figure 2 derive from ionization of the vinyl radical. The 10.49-eV photoelectron spectrum of pyrolyzed nitroethylene-h3 (not shown) is identical in the threshold region with the corresponding part of Figure 2. Having obtained the same spectrum from two different precursors, we therefore assign that part of the spectrum to the vinyl radical, C2H3*
Unfavorable Franck-Condon factors for photoionization of C2H3, in conjunction with our -7O-meV energy resolution (fwhm) at 2.0-eV electron energy, produce the poorly resolved, gradually rising threshold in the photoelectron spectra. If there were vibrational excitation in the vinyl radical, there would be strong hot bands in the photoelectron spectrum, which would lower the apparent ionization potential measured under these circumstances. However, we argue, along two independent lines of reasoning, that our measured threshold of 8.25 f 0.05 eV, while rigorously setting a lower limit on IP[C2H3],cannot be much less than the actual adiabatic ionization potential. We have seen efficient vibrational cooling, by our jet expansion, of other small hydrocarbon radicals, C3H2 and C3H5,in separate studies where the initial pyrolysis was done at substantially higher temperatures. In the other studies, vibrational cooling by supersonic jet expansion was confirmed by detailed Franck-condon modeling of the photoelectron ~pectrum*9~ or by spectroscopic analysis of resonant MPI spectral0 for the radicals. In both of those cases, there were geometric changes that would have given hot band activity if vibrational excitation were present. Secondly, the 10.49-eV photoelectron spectrum of C2D3, produced by pyrolysis of nitroethylene-d3,is nearly indis( 6 ) Buckley, G. D.; Scaife, C. W. J . Chem. SOC.1947, 1471. (7) Dewar, M. J. S.;Ritchie, J. P.; Alster, J. J . Org. Chem. 1985,50, 1031. (8) Clauberg, H.; Minsek, D. W.; Chen, P. J . Am. Chem. Soc. 1992, 114,
99. (9) Clauberg, H.; Chen, P. J . Phys. Chem., submitted for publication. (10) Minsek, D. W.; Blush, J. A.; Chen, P. J . Phys. Chem. 1992,96,2025.
0022-365419212096-4138%03.00/0 0 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, No. 11, 1992 4139
Letters 9.61 e V PHOTOIONIZATION
=." 27
f r o m pyrolysis of nitroethylene
NO
30
I
=."27 j
f r o m pyrolysis of t-butylperacrylate
mass (amu)
Figure 1. Time-of-flight photoionization mass spectra of pyrolyzed tert-butyl peracrylate (bottom) and pyrolyzed nitroethylene (top), using 129-nm (9.61-eV) photons. Other primary products of the pyrolysis (except a small amount of nitric oxide in the top trace) have ionization potentials above 9.61 eV and do not appear in the spectrum.
10.49 eV PHOTOIONIZATION I
3.0 2 . 0 1 . 5
1.0
'
0.88
0.75
p h o t o e l e c t r o n k i n e t i c e n e r g y (eV) Figure 2. Time-of-flight photoelectron spectra of pyrolyzed tert-butyl peracrylate, using 118.2-nm (10.49-eV) photons. Electron kinetic energies are indicated on the scale at bottom. Ionization potentials, obtained by subtraction of the electron kinetic energy from the 10.49-eV photon energy, are indicated by labels on the peaks. The absence of ions in the 9.61-eV mass spectrum allows assignment of all other than C2H3+ The threshold region is photoelectrons faster than 0.88 eV to C2H3. expanded for clarity. Photoelectron spectra of nitric oxide and naphthalene were used to calibrate the spectra.
tinguishable from that of CzH3in the threshold region. Hot bands would show an isotopic shift, which would be greater for higher vibrational excitation in the radical. The absence of an observable shift, at our resolution, places the adiabatic ionization potential no more than 0.2 eV higher than our reported threshold. We therefore believe that, while our photoelectron spectrum gives an unambiguous lower bound for the ionization potential, our spectrum is that of vibrationally cold vinyl radicals, and IP[C2H3] = 8.253;; eV. Discussion Recent work has sharpened interest in CzH, thermochemistry and bonding. Vinyl radicals are putative intermediates in hydrocarbon combustion" reactions; both the radical and cation are believed to be intermediates in interstellar chemistry of interest to the astrophysical communityI2as well. CzH3has an ethylenic (1 1) Gardiner Jr., W. C. Combustion Chemistry; Springer-Verlag: New York. 1984.
(minus one H) structure, but CzH3+resembles a side-protonated acetylene. Ab initio calculations by Curtiss and Pople13 predict the bridged cation to be more stable than the classical (nonbridged) structure by 3.1 kcal/mol (including zero-point energies). In an infrared study, Crofton et al.I4 assign transitions to the u6 fundamental band of the bridged cation. Kantor et al.I5 find a bridged structure for C2H3+via a Coulomb explosion experiment. Vibrational frequencies for the classical and bridged cation states have been ~ a l c u l a t e d . ~ ~The J ' large change in equilibrium geometry from the ethylene-like radical to the bridged cation complicates spectroscopic and thermochemical measurements. Measurements of the homolytic C-H bond dissociation energy of ethylene, PHoBDE,0[C2H3-H], or, equivalently, the heat of formation of vinyl radical, AHof,o[C2H,],have produced values ranging over a few kcal/mol. Kinetic studies of forward and backward rates for hydrogen abstraction reactions from ethylene have yielded bond energies in the low end of the range, with = 104.4 f 0.4 kcal/mol by Parmar and PHoBDEO[C2H3-H] Bensonld from the C1' + CzH4 + HCl + C2H3 reaction. Russell et all9 redetermined the rate for the backward reaction by a more direct method and, using Parmar and Benson's forward rate, found m o ~ ~ ~ , 0 [ C z H 3 -=H ]105.1 f 0.3 kcal/mol. Photofragment translational energy studies by Cao et aL20 and Wodtke et aLZ1 find 106.5 f 2 and 108 f 3 kcal/mol, respectively, at 0 K. Measuring the gas-phase acidity of ethylene, AGadB8[C2H4],and the electron affinity of vinyl radical, EA[C2H3],Ervin et a1.22find ~ B D E , o [ C ~ H ~=-109.7 H ] f 0.8 kcal/mol. A range Of 105-1 10 kcal/mol would be consistent with the measurements by many methods from different laboratories. The mass spectrometric determinati~n~~ of MBDE,o[C,H,-Hl relies upon an appearance potential measurement of [C2H3+]and the ionization potential of vinyl radical, IP[CzH3]. The most reliable24appearance potential determinations gave M f , o [ C 2 H 3 +=] 268.4 f 1.P and 267.8 f 0.5 kcal/mo1.26 An early electron impact ionization efficiency curve measurement by Lasing2' found IP[C2H,] = 8.95 eV. The recent photoionization efficiency curve measurement by Berkowitz et al.28found IP[C2H3]= 8.59 f 0.03 eV, which, strictly speaking, is an upper bound with a possibility for a lower threshold. The latter ionization potential, with M f , o [ C z H 3 +=] 267.8 f 0.5 kcal/mol, was used to derive ~ B D E , o [ C ~ H ~= -106.8 H ] f 0.8 kcal/mol, which iS in general agreement with the values above. Our IP[C,H,] = (12) Smith, D. Philos. Trans. R. SOC.London, A 1988, 324, 257. 1131 Curtiss. L. A.: Pode. J. A. J. Chem. Phvs. 1988. 88. 7405. (14j Crofton, M. W.; jagod, M.-F.; Rehfus; B. D.; Oka, T. J . Chem. Phys. 1989, 91, 5 139. (15) Kantor, E. P.; Vaner, 2.;Both, G.; Zaifman, D. J . Chem. Phys. 1986, 85,'7487. (16) Raine, G. P.; Schaefer 111, H. F. J . Chem. Phys. 1984, 81, 4034. (17) DeFrees, D. J.; McLean, A. D. J . Chem. Phys. 1985,82, 333. (18) Parmar, S. S.; Benson, S. W. J . Phys. Chem. 1988, 92, 2652. (19) Russell, J. J.; Senkan, S. M.; Seetula, J. A.; Gutman, D. J . Phys. Chem. 1989. 93. 5184. (20) Cao,'J. R.;Zhang, J. M.; Zhong, X.;Huang, Y. H.; Fang, W. Q.;Wu, X.J.; Zhu, Q.H. Chem. Phys. 1989,138, 377. (21) Wcdtke, A. M.; Hintsa, E. J.; Somorjai, J.; Lee, Y. T. Zsr. J . Chem. 1989, 29, 383. (22) Ervin, K. M.; Gronert, S.; Barlow, S.E.; Gilles, M. K.; Harrison, A. G.; Bierbaum, V. M.; DePuy, C. H.; Lineberger, W. C.; Ellison, G. B. J . Am. Chem. Soc. 1990, I 1 2, 5750. (23) For a review, see: Rosenstock, H. M. Znr. J . Mass Specrrosc. Zon Phys. 1976, 20, 139. (24) An appearance potential for C2H3+from the dissociative photoionization of vinyl chloride has been reported to give Lwo&2H3+] = 265 f 2 kcal/mol: Reinke, D.; Kraeasig, R.; Baumgartel, H. Z . Naturforsch. A 1973, 28, 1021. However, vinyl chloride thermochemistry is itself poorly known, and the AHf[C2H,C1]used in that derivation was nor corrected to 0 K. Use of the 0 K value gives the same, higher Wf,,,[C2H3+] as the ethylene studies. Moreover, at threshold, there was a competing dissociation to C2H2'++ HCI, making the appearance potential from vinyl chloride subject to the same problems as that from ethylene. (25) Chupka, W. A.; Berkowitz, J.; Refaey, K. M. A. J . Chem. Phys. 1969, 50, 1938. (26) Stockbauer, R.; Inghram, M. G. J . Chem. Phys. 1975, 62, 4862. (27) Lossing, F. P. Can. J . Chem. 1971, 49, 357. (28) Berkowitz, J.; Mayhew, C. A.; Ruscic, B. J . Chem. Phys. 1988,88, 7396.
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J. Phys. Chem. 1992, 96, 4140-4143
8.25Zdp250 eV is considerably lower than the Berkowitz number. r f we take the highest of the acceptable ethylene bond energies, AZP31SkO[C2H3-H] = 109.7 f 0.8 kcal/mol, and IP[C2H3] = 8.25+0,20 eV, we find that AWf,o[C2H3+]= 261.8-267.5 kcal/mol. This range can barely be made consistent with AHof,o[C2H3+] = 267.8 f 0.5 kcal/mol (and only if one includes the error bars) and extends well below the range of values derived from a p pearance potentials. Using any of the lower values of would reduce AH'f,o[C2H3+]even further. AHoBDE,0[C2H3-H] We propose that a downward adjustment of AH'f,o[C2H3+]by several kcal/mol is more consistent with the data. The same adjustment has been suggested by Hawley and SmithB on the basii of a thorough investigation of the C2H2'++ H2 [C2H4'+]* C2H3' H' ion-molecule reaction at temperatures below 3 K. They report rate measurements and a large deuterium kinetic isotope effect for a slightly exothermic tunneling reaction through an activation barrier on the C2H4" potential surface, which gives LU!I"f,o[C2H3+] = 265;: kcal/mol. The presence of a small activation barrier for the dissociation of energized C2H4'+to C2H3+ is precisely the condition that would cause appearance potential measurements to overestimate AZPf,o[C2H3+].The adjustment, strictly speaking, is not inconsistent with the original measurement in that the appearance potential of C2H3+ from C2H4 rigorously places only an upper bound23on AHof,o[C2H3+].That upper bound can be safely interpreted as the actual value only if the C2H4'+ C2H3+ + H' reaction is a simple bond rupture, and
+
-
-
-
(29) Hawley, M.; Smith, M. A. J . Am. Chem. Soc. 1989, 1 1 1 , 8293. Hawley, M.; Smith, M. A. J . Chem. Phys., 1992, 96, 1121.
competing reactions are absent at threshold. Given the extensive bonding changes in going from ethylene radical cation to the bridged vinyl cation, and the reported25*26 C2H4'+ C2H2'++ H2 dissociation at very nearly the same appearance potential, neither condition is fulfilled, and one may question the derived value for AH'f,o[C2H3+]. We suggest that the different reported ionization potentials for C2H3 arise from improvements in instrumental sensitivity and/or the different sensitivity to an ionization threshold in photoelectron spectra (PES), photoionization efficiency (PIE) curves, and electron impact ionization efficiency (EIE) curves, for an ionization with a poor Franck-Condon factor at threshold. In the simplest model,30the PES would be the derivative spectrum of the PIE curve, which would be the derivative spectrum of the EIE curve. For an ionization accompanied by gross changes in equilibrium geometry, as is the case for C2H3, both the PIE and EIE curves would miss the threshold and overestimate an adiabatic ionization potential seen by PES.
-
Acknowledgment. We acknowledge the preliminary work of
Dr.Jeunghee Park on this project and helpful discussions with Dr. J. Berkowitz and Prof. W. A. Chupka. Funding from the National Science Foundation, for the purchase of laser equipment, the Department of Energy, and the Exxon Educational Foundation is acknowledged. (30) For a discussion of threshold laws, and a comparison of photoelectron spectra to the derivative of photoionization efficiency curves, see Figure 84 on p 306 of Berkowitz, J. Photoabsorption, Photoionization, and Phoroelectron Spectroscopy; Academic Press: New York, 1979.
Infrared Absorptlon Spectroscopy of the Weakly Bonded CO-CI, Complex S. W. Bunte, J. B. Miller,+ Z. S. Huang,*J. E. Verdasco,s C. Wittig,* and R. A. Beaudet Department of Chemistry, University of Southern California, Lm Angeles, California 90089-0482 (Received: January 13, 1992; In Final Form: April 6, 1992)
High-resolution rovibrational absorption spectra of the weakly bonded COC12 complex have been recorded in the 2143-cm-' region by exciting the CO chromophore with a tunable diode laser. The spectra indicate that C 0 - Q is linear and semirigid. By fitting the data to a linear-molecule Hamiltonian, the following constants (in cm-I) were obtained: vo = 2149.5424 (4), B"= 0.031 5823 (39), B'= 0.031 4867 (52), D," = 4.37 (25) X lo-*, and DJ' = 4.58 (35) X lo-*. The distance between the CO and C12centers of mass is approximately 4.78 A. The orientation of CO is not determined experimentally. However, CI2appears to act as a classical a-electron acceptor, while CO behaves like a weak Lewis base, donating charge from the carbon side via the weakly antibonding 5u orbital, thereby raising the CO vibrational frequency.
Introduction Spectroscopic studies of weakly bonded binary complexes containing molecular chlorine have provided much information on their properties, structure, and dynamics. For example, H&12, Ne-C12, Ar-C12, Xe-C12, and Kr-C12 have been shown to be T-shaped in both the ground state and the electronically excited state that correlates to C12(B3&+),with bonding dominated by van der Waals forces in both electronic states.lV2 Going one step further with these systems, Boivineau et al. carried out experiments in which Xe-CI2 was excited to a chemically reactive surface by a two-photon proces~.~The first photon excites the C12moiety to dissociative Illu, but before the complex can dissociate a second photon is absorbed and excites either the X e C l or the Cl-Cl pair. 'Present address: Department of Chemistry, Denison University, Granville, OH 43023. *Present address: Department of Chemical Engineering, University of Florida, Gainesville, FL 3261 1. 'Present address: Departmento de Quimica-Fisica, Facultad de Quimica, Universidad Complutense de Madrid, 28040 Madrid, Spain.
The excited complex then dissociates and forms a X&l* excimer. In addition, such weakly bonded complexes can provide a basis for detailed studies of photoinitiated reactive and/or inelastic proces~es.~ Baiocchi et al. determined the structure of the HF-C12 complex from molecular beam electric resonance spectroscopy by fitting to the H F hyperfine s t r ~ c t u r e . Their ~ results indicate that the three heavy atoms are linear and that the hydrogen atom is off-axis with an average H-F-CI angle of 125'. They argue against a ~~
~~~
(1) Janda, K. C.; Bieler, C. R. Atomic and Molecular Clusters; Bemstein, E. R., Ed.; Elsevier: Amsterdam, 1990; pp 455-506. (2) Bieler, C. R.; Spence, K. E.; Janda, K.C. J . Phys. Chem. 1991, 95, 5058.
(3) (a) Boivineau, M.; LeCalve, J.; Castex, M. C.; Jouvet, C. Chem. Phys. Lett. 1986, 128. 528. (b) Boivineau, M.; LeCalve, J.; Castex, M. C.; Jouvet, C. Chem. Phys. Lett. 1986, 130, 208. (4) Shin, S.K.;Chen, Y.; Nickolaisen, S.;Sharpe, S.W.; Beaudet, R.A.; Wittig, C. Advances in Photochemistry; Volman, D., Hammond, G., Neckers, D., Eds.; Wiley: New York, 1991; pp 249-363. (5) Baiocchi, F. A.; Dixon, T. A.; Klemperer, W. J. Chem. Phys. 1982,77, 1632.
0022-3654/92/2096-4 140$03.00/0 0 1992 American Chemical Society
