Skeletal Torsion Tunneling and Methyl Internal Rotation: The Coupled Large Amplitude Motions in Phenyl Acetate.

The rotational spectrum of phenyl acetate, CH 3 COOC 6 H 5 , is measured using a free jet absorption millimeter-wave spectrometer in the range from 60 to 78 GHz and two pulsed jet Fourier transform microwave spectrometers covering a total frequency range from 2 to 26.5 GHz. The features of two large amplitude motions, the methyl group internal rotation and the skeletal torsion of the CH 3 COO group with respect to the phenyl ring C 6 H 5 (tilted at about 70°), characterize the spectrum. The vibrational ground state is split into four widely spaced sublevels, labeled as A0, E0, A1, and E1, each of them with its set of rotational transitions and with additional interstate transitions. A global fit of the line frequencies of the four sublevels leads to the determination of 51 spectroscopic parameters, including the Δ E A0/A1 and Δ E E0/E1 vibrational splittings of ~36.4 and ~33.5 GHz, respectively. The V 3 barrier to methyl internal rotation (~136 cm -1 ) and the skeletal torsion B 2 barrier to the orthogonality of the two planes (~68 cm -1 ) are deduced.