Intertorsional Interactions Revealing Absolute Configurations: The V6 Internal Rotation Heavy-Top Case of Benzotrifluoride



The microwave spectroscopic signatures of multiple torsional states of the CF3 internal rotation in benzotrifluoride (α,α,α-trifluorotoluene) are reported. Individual rotational transitions are observed in a total of eight different torsional states, a quite challenging task for heavy tops even with Fourier transform microwave techniques. Accidental mixings of m=0 and m=3 torsional states as well as m=1 and m=2 torsional states, which can complicate the assignment of the spectra severely, are observed. These accidental mixings are probably systematic for molecules with heavy tops exhibiting an almost free internal rotation, and give an opportunity to determine the sign in the (1/2) V6 (1±cos6τ) potential function hindering internal rotation and in consequence the orientation of the CF3 top versus C6H5 frame. A recently developed torsion–rotation program reproduces all line positions within an experimental accuracy of about 2.0 kHz. The V6 barrier is determined to be 3.229949(32) cm−1. The corresponding torsional spacings are determined with the seven-digit accuracy underlying the supersonic-jet Fourier transform microwave technique.