Vibration-internal rotation coupling in trifluoronitromethane

Abstract

The barrier to internal rotation for CF3NO2 has previously been determined in the early 1960s by two independent methods (1) electron diffraction, and (2) rotational microwave spectroscopy. These two methods have indicated the barrier to rotation to be 3 kcal/mol and 74 cal/mol, respectively. This discrepancy has not been resolved in 25 years. In an attempt to explain this discrepancy, a model has been developed to incorporate the effects of coupling between the internal rotation motion and the vibration of the NO2 group. This coupling appears to be present based on symmetry considerations and based upon geometries obtained from electronic calculations of the molecule. Both electron diffraction radial distribution curves and rotational microwave spectra are simulated and compared with experimental data. A surprising result indicates that the coupling between internal rotation and the vibrational motions may be strong, but minimally visible in the rotational spectrum. Such coupling can introduce interatomic correlation which affects the appearance of diffraction experiments, yet is not apparent in microwave spectra. This model is consistent with the behavior observed by the two methods, and is offered as the explanation for the otherwise contradictory interpretations.