J. Phys. Chem. 1994, 98, 6919-6923
6919
ARTICLES An ab Initio and Photoelectron Spectroscopic Study of the Trichloromethyl Radical and Cation Eric S. J. Robles and Peter Chen' Mallinckrodt Chemical Laboratory, Harvard University, Cambridge, Massachusetts 02138 Received: March 2, 1994; In Final Form: May 4, 1994"
W e report the 10.49 eV laser photoelectron spectrum of pyrolytically produced CCl3 and extract an adiabatic ionization potential of IPad = 8.06 f 0.02 eV by a deconvolution of the partially resolved spectrum using a b initio potential functions. The heat of formation for the trichloromethyl cation derived from the ionization potential is AZPf,o[CC13+]= 202.6 f 0.8 kcal/mol, which is compared to a Gaussian-2 (G2) theory prediction of W ~ , o [ C C 1 3 + ]= 200.73 kcal/mol. W e find the G 2 prediction of AZPf,o[CCl3] = 17.30 kcal/mol to be in excellent agreement with the most recent experimental determination for the radical; the G2 prediction for the cation is slightly low but within the claimed accuracy of the method.
Introduction The CC13 radical has been the focus of several recent studied-3 because of its importance in high-energy environments such as halocarbon plasmas? PCB in~ineration,~ and the upper atmosphere.6 The most detailed spectroscopic work to date on CC13 was performed by Hudgens and co-workers,l who, for the first time, observed its mass-resolved resonant multiphoton ionization (MPI) spectrum between 336 and 440 nm via two-photon resonances. The MPI spectrum was attributed to electronic transitions from the pyramidal electronic ground state of CC13 toa total of seven planar Rydberg states. Hot bands were observed from which the barrier to inversion, Bin",in the electronic ground state was derived to be 460 f 40 cm-l by a fit to a quarticperturbed harmonic oscillator. An adiabatic ionization potential of 8.109 f 0.005 eV was also derived by extrapolation of a Rydberg series. Hudgens et ai. also performed ab initiocalculations which confirmed previous predictions of a pyramidal (C3") radical and planar (D3h)cation. MP4SDQ/6-3 lG* single-point calculations at the HF/6-3 1*-optimized geometries for CC13 (constrained to C3"and D3h) and CC13+were used to predict an inversion barrier of 620 cm-1 and adiabatic ionization potential of 7.80 eV, both of which are relatively far from the experimentally fitted values. The objective of the present work is twofold: (i) We report the 10.49 eV laser photoelectron spectrum of the CC13 radical from which an adiabatic ionization potential is extracted by a fit of the partially resolved band to a computed Franck-Condon envelope, and (ii) we compare the experimentally determined ionization potential and derived heats of formation to Gaussian-2 calculations' (G2), which purportedly predict absolute heats of formation to within 2 kcal/mol. G2 theory uses a set of additivity assumptions to combine the results of a series of smaller ab initio calculations and cancel out systematic errors that skew absolute thermochemical predictions. Benchmark studies for a range of small molecules and ions composed of first- and second-row elements havevalidated the claimed 2 kcal/mol absolute accuracy. The first objective continues work that has been ongoing in this laboratory on the experimental determination of thermochemical data for reactive intermediates by photoionization-based techniques and highlights the use of computed Franck-Condon factors (using an ab initio potential function) in the deconvolution of photoelectron spectra of radicals for which a significant geometry change accompanies ionization. The results of the thermo*Abstract published in Advance ACS Abstracts, June 15, 1994.
0022-3654/94/2098-6919$04.50/0
chemical determinations are then compared to G2 predictions for CC13 and CC13+. These species are among the largest molecular systems for which G2 calculations have been done and, hence, serve as further benchmarks for calibration of the theory as a predictive tool.
Experimental Section Trichloromethyl radicals were produced in a skimmed molecular beam by the supersonic jet flash pyrolysis of
