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 S0, electronically excited S1, and cationic D0 states are predicted by ab initio and density functional theory (DFT) calculations. Their vibrational spectra in the S1 and D0 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 S0 state. Upon the S1 ← S0 excitation, the aromatic ring expansion is expected, and the interaction of the OH group with the ring is enhanced. On the D0 ← S1 transition, the bond length of the C1–O7 bond is further shortened, exhibiting a partial double bond character in the D0 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 S1 and D0 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.