Rotamers and isotopomers of 3-chloro-5-fluoroanisole studied by resonant two-photon ionization spectroscopy and theoretical calculations.

The ab initio and density functional theory (DFT) calculations reveal that two rotamers, denoted by cis and trans 3-chloro-5-fluoroanisole (3C5FA), are stable for each of the S-0, S-1, and D-0 states. In the one-color resonant two-photon ionization (R2PI) spectra, the band origins of the S-1 <- S-0 electronic transition (0 degrees bands) of cis Cl-35-3C5FA and cis Cl-37-3C5FA are both located at 36,468 +/- 3 cm(-1), while the 0 degrees bands of trans Cl-35-3C5FA and trans Cl-37-3C5FA are found to be 36,351 +/- 3 and 36,354 +/- 3 cm(-1). The two rotamers display very similar vibrational frequencies in the S-1 state, and the observed active modes mainly involve the in-plane ring deformation vibrations. By the two-color R2PI spectroscopy, the adiabatic ionization energies (IEs) of both isotopomers of 3C5FA are determined to be 69,720 +/- 15 cm(-1) for the cis rotamer and 69,636 +/- 15 cm(-1) for the trans rotamer. The substitution, conformation, and isotope effects on the properties of 3C5FA, including the molecular structures, vibrational frequencies, and electronic transition and ionization energies, were also discussed in detail. (c) 2012 Elsevier B.V. All rights reserved.