Internal rotation potential and relaxation of structure in nitrobenzene studied by microwave spectroscopy supported by quantum chemistry

Rotational constants for the vibrational ground state and three torsionally excited states of nine nitrobenzene isotopologues were used to investigate the internal rotation potential and the structure of nitrobenzene. Relaxation parameters were calculated by the B3LYP and MP2(full) methods with the aug-cc-pVDZ and aug-cc-pVTZ basis sets. The four sets of relaxation parameters all led to a clear improvement in the fit to rotational constants as compared with fits without relaxation of structure. The improvement was different from set to set, with B3LYP/aug-cc-PVDZ as a surprising winner. However, letting three of the relaxation parameters vary freely, gave virtually identical results for the molecular structure and potential function in all cases. Assuming higher order potential coefficient to vanish, the resultant potential parameters were, with 1σ uncertainties, V2 = 1678 ± 19 cm−1 and V4 = −207 ± 6 cm−1. The barrier to internal rotation was estimated to 1575 ± 150 cm−1. A small change in structure upon ortho deuteration was found.