SILYL GROUP INTERNAL-ROTATION IN S1 PHENYLSILANE AND PHENYLSILANE CATION - EXPERIMENTS AND AB-INITIO CALCULATIONS

Resonant two-photon ionization (R2PI) and pulsed field ionization (PFI) were used to measure S1-S0 and cation-S, spectra of internally cold phenylsilane. We measure the adiabatic ionization potentials IP (phenylsilane) = 73 680 +/- 5 cm-1, IP (phenylsilane . Ar) = 73 517 +/- 5 cm-1 and IP (phenylsilane . Ar2) = 73 359 +/- 5 cm-1. We assign many low lying torsion-vibration levels of the S1 (A 1A1) state and of X B-2(1) of phenylsilane+. In both states, the pure torsional transitions are well fit by a simple sixfold hindered rotor Hamiltonian. The results for the rotor inertial constant B and internal rotation potential barrier V6 are, in S1, B = 2.7 +/- 0.2 cm-1 and V6 = -44 +/- 4 cm-1; in the cation, B = 2.7 +/- 0.2 cm-1 and V6 = +19 +/- 3 cm-1. The sign of V6 and the conformation of minimum energy are inferred from spectral intensities of bands terminating on the 3a1'' and 3a2'' torsional levels. In S1 the staggered conformation is most stable, while in the cation ground state the eclipsed conformation is most stable. For all sixfold potentials whose absolute phase is known experimentally, the most stable conformer is staggered in the neutral states (S0 and S1 p-fluorotoluene, S1 toluene, S1 p-fluorotoluene) and eclipsed in the cationic states (ground state toluene+ and phenylsilane+). In phenylsilane+ we estimate several potential energy coupling matrix elements between torsional and vibrational states. For small V6, the term P(alpha)P(a) in the rigid-frame model Hamiltonian strongly mixes the 6a1' and 6a2' torsional states, which mediates further torsion-vibrational coupling. In addition, the cation X B-2(1) vibrational structure is badly perturbed, apparently by strong vibronic coupling with the low-lying A 2A2 state. Accordingly, ab initio calculations find a substantial in-plane distortion of the equilibrium geometry of the X B-2(1) state, while the A 2A2 state is planar and symmetric. The calculations also correctly predict the lowest energy conformer for S0 states and for cation ground states. Finally, we adapt the natural resonance theory (NRT) of Glendening and Weinhold to suggest why sixfold barriers for methyl and silyl rotors are uniformly small, while some threefold barriers are quite large. The phase of the sixfold potential is apparently determined by a subtle competition between two types of rotor-ring potential terms: attractive donor-acceptor interactions and repulsive van der Waals interactions (steric effects).