Rotamers of <em>m</em>-Methylanisole Studied by Mass-Analyzed Threshold Ionization Spectroscopy

Abstract

The S-1 <- S-0 electronic transition and threshold ionization of the cis and trans rotamers of m-methylanisole were investigated by using one-color resonant two-photon ionization and mass-analyzed threshold ionization techniques. The first electronic excitation energies (E-1) of the cis and trans rotamers were determined to be (36049 +/- 2) and (36117 +/- 2) cm(-1), while the adiabatic ionization energies (I-p) were (64859 +/- 5) and (65110 +/- 5) cm(-1), respectively. The results of ab initio and density functional theory calculations provide a satisfactory interpretation for our experimental findings concerning the difference in the transitional energies of the cis and trans rotamers and assist in assigning the vibronic and cation spectra obtained in the present study. The observed active vibrations of both rotamers in the S-1 and D-0 states primarily consist of methyl torsion, in-plane ring deformation, and substituent-sensitive bending modes. Both experimental and theoretical results show that, for both cis and trans isomers, the geometry of the cation in the D-0 state is somewhat different from that of the neutral species in the S-1 state. In addition, the strengths of both the through-space substituent-substituent and substituent-ring interactions were found to follow the order: S-0<S-1<D-0.