Intermolecular bonding and vibrations of phenol⋅oxirane

Abstract

The supersonically cooled hydrogen-bonded phenol⋅oxirane complex was studied using mass- and isomer-selective laser spectroscopic techniques. The S1←S0 vibronic spectrum was measured by mass-selective two-color resonant two-photon ionization. UV/UV-hole-burning experiments prove that the whole observed spectrum is due to only one isomer. High-resolution fluorescence emission spectra yielded five different intermolecular S0 state vibrational fundamentals as 15, 27, 39, 83, and 177 cm−1, which are assigned as the ρ1″, β1″, τ″, β2″, and σ″ modes, respectively, based on ab initio calculations. The analogous S1 state intermolecular vibrations were also assigned, based on frequency and Franck–Condon activity. The observation of the ρ1 and τ intermolecular vibrational transitions in both excitation and emission implies that phenol⋅oxirane is asymmetric (chiral), even though the H-donor is Cs and the acceptor C2v symmetric. Four different ab initio structure optimizations and normal-mode calculations were made, to compare the performance of the self-consistent field (SCF) and Becke–Lee–Yang–Parr (B-LYP) density functional methods, using the 6-31G(d,p) and 6-311++G(d,p) basis sets. The SCF/6-31G(d,p) method and the B-LYP method with both basis sets indeed predict chiral minimum-energy structures. The B-LYP/6-311++G(d,p) and SCF/6-31G(d,p) normal mode frequencies agree well with the experimental S0 state frequencies, with rms deviations of 4%. The MP2/6-31G(d,p) hydrogen bond well depth is De=6.9 kcal/mol and the dissociation energy is D0=5.7 kcal/mol.