Methyl group internal rotation and the choice of Hamiltonian for the rotational spectrum of 1,1-difluoroacetone

Abstract

The effects of partial fluorination on acetone have been examined using broadband rotational spectroscopy within the 7.5–16.5GHz region. The broadband spectrum, which contains information on the methyl group internal rotation, was analyzed using a principal axis method, and subsequently analyzed using (i) a combined axis method and (ii) a method in which the Hamiltonian possesses matrix elements expressed as Fourier series. Spectroscopic constants and effective barriers to methyl group internal rotation are presented for the first time. Results are compared to related molecules where it is shown that the carbonyl group effectively limits the effects of fluorination on the adjacent C(O)CH3 structure. The spectroscopic analyses performed demonstrate a satisfactory convergence of the Hamiltonian in all combinations of Watson reduction and representation with the exception of A and Ir. Within this combination the ratio (2A−B−C)/(B−C) is large resulting in unreasonably large values of the quartic centrifugal distortion constant δK which in turn prohibits a satisfactory eigenvalue solution.