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 (³⁵Cl and ³⁷Cl)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 V₆ terms that are larger than the three-fold V₃ terms. Molecular parameters for the two isotopologues of meta-chlorotoluene are reported and compared with DFT (B3LYP)and ab initio (MP2)calculations.