Molecular structures and vibrations of cis and trans m-cresol in the electronically excited S1 and cationic D0 states

The optimized molecular structures of cis and trans m-cresol in the ground S-0, electronically excited S-1, and cationic D-0 states are predicted by ab initio and density functional theory (DFT) calculations. Their vibrational spectra in the S-1 and D-0 states are recorded by two color resonant (1 + 1') two photon ionization (2C-R2PI) and mass analyzed threshold ionization (MATI) methods. In consideration of the optimized geometries, the trans rotamer is more stable than the cis one in the S-0 state. Upon the S-1 <- S-0 excitation, the aromatic ring expansion is expected, and the interaction of the OH group with the ring is enhanced. On the D-0 <- S-1 transition, the bond length of the Cl-O7 bond is further shortened, exhibiting a partial double bond character in the D-0 state. The band origins of cis and trans m-cresol are measured to be 35,982 +/- 2 and 36,098 +/- 2 cm(-1) by the 2C-R2PI method, and their adiabatic ionization energies (IE) are determined to be 66,933 +/- 5 and 67,084 +/- 5 cm(-1) by the MATI technique. Comparison of the IE of o-, m-, p-cresol, and phenol gives the order as: p < o < m < phenol. Analysis of the spectroscopic features of cis and trans m-cresol in the S-1 and D-0 states shows that different orientations of the OH group with respect to the CH3 group slightly influence the vibrational frequency of the in-plane ring deformation. (C) 2007 Elsevier B.V. All rights reserved.