Hindered internal rotation and torsion–vibrational coupling in ortho-chlorotoluene (S1) and ortho-chlorotoluene+ (D)

Abstract

The techniques of resonant two-photon ionization (R2PI) and pulsed field ionization (PFI) were used to measure absorption spectra of ortho-chlorotoluene (S1, Ã 1A1) and of ortho-chlorotoluene+ (D0, X̃ 2A1; the cation ground state) for internally cold molecules in a pulsed nozzle expansion. The adiabatic ionization potential is 71 318±5 cm−1=8.8423±0.0006 eV. Most of the observed low lying torsion–vibrational structure in both S1 and D0 can be assigned using a one-dimensional torsional model plus low frequency vibrational modes whose identity is corroborated by the ab initio normal modes of D0. The intensities of certain weak, forbidden torsion–vibration combination bands in the S1–S0 spectrum are well predicted by a nuclear coordinate dependence of the electric dipole transition moment. The threefold methyl torsional barrier is V3=144.2±2.0 cm−1 in S1 and V3=456.5±2.0 cm−1 in D0. Ab initio calculations at the HF/6-31G* level find V3=481 cm−1 in S0 with the minimum energy conformation pseudo-trans, i.e., with one CH bond lying in the plane of the ring on the opposite side of the chlorine substituent. Spectral band intensities show that the minimum is pseudo-trans in S1 and D0 as well. In both S1 and D0, excitation of either of the two lowest frequency out-of-plane bending modes, ν38 or ν37, leaves the methyl torsional potential essentially unchanged. In S0, S1, and D0, the barrier is substantially larger for ortho-chlorotoluene than for ortho-fluorotoluene, consistent with greater steric repulsion between the 3p chlorine lone pair and the CH bond pairs. The effects of π excitation and π ionization on the barrier are similar in ortho-chlorotoluene and ortho-fluorotoluene. Apparently both chlorine and fluorine are weak π donors that have similar effects on the crucial ring CC bond orders closest to methyl. Both the S1 and D0 spectra reveal several examples of torsion–vibrational coupling which perturbs torsional state energies and produces extra bands. Simple zeroth-order models of energy levels and coupling strengths fit experimental frequencies and band intensities well. Deperturbation yields coupling matrix elements between torsion and out-of-plane bending vibrations that fall in the narrow range 6–15 cm−1 in all cases. For three different molecules with widely varying methyl rotor barriers from 10 to 450 cm−1, the magnitude of such coupling matrix elements is similar, always in the range 3–15 cm−1. This provides guidance for theoretical models of intramolecular vibrational energy redistribution.