Rotational Spectrum of 1,1,1-Trifluoro-2-butanone Using Chirped-Pulse Fourier Transform Microwave Spectroscopy

The pure rotational spectra of 1,1,1-trifluoro-2-butanone and its four C-13 isotopologues have been studied using the new chirped-pulsed Fourier transform microwave spectrometer at the University of Manitoba in combination with a conventional Balle-Flygare-type instrument. Quantum chemical calculations, at the MP2/6-311++G(d,p) level, were carried out to obtain information about the structure, relative stability, and difference in populations of the three lowest energy conformers corresponding to dihedral angles of 0 degrees, 82.8 degrees, and 119.2 degrees along the carbon back-bone. The observed spectra are that of conformer I (dihedral angle 0), and, based on analysis of the observed splitting, the V-3 barrier to internal rotation of the methyl group has been determined to be 9.380(5) kJ mol(-1). The spectroscopic constants of the five isotopologues were used to precisely derive the r(s) and partial r(0) geometries of this conformer based on an assumed planar carbon backbone (as supported by the spectra and ab initio calculations).