Molecular systems with nearly-free internal rotation and nuclear quadrupole coupling: Meta-chlorotoluene

Nearly-free internal rotation produces huge (>1 GHz) tunneling splittings in the rotational spectrum of meta-chlorotoluene. Additionally, the simultaneous presence of two chlorine (Cl-35 and Cl-37) isotopologues results in dense spectra with complicated hyperfine patterns, hampering the spectral analysis by conventional spectroscopic techniques. Sub-Doppler molecular-beam Fourier transform microwave rotational spectroscopy has been used to study the internal rotation and nuclear quadrupole coupling terms in meta-chlorotoluene. Notably, with the halogen substituent in meta position of the methyl internal rotor, the periodic torsional potential contains six-fold V-6 terms that are larger than the three-fold V-3 terms. Molecular parameters for the two isotopologues of meta-chlorotoluene are reported and compared with DFT (B3LYP) and ab initio (MP2) calculations. (C) 2019 Elsevier Inc. All rights reserved.