In this project, permutation-inversion group-theoretical techniques are used to treat the rovibronic energy levels in a variety of small molecules exhibiting one or more large-amplitude vibrational motions.
Programs for fitting to experimental accuracy the microwave and infrared spectra of molecules with one, two, or three equivalent or inequivalent methyl tops have been developed, are under development, or are under consideration, principally in collaboration with I. Kleiner (LISA, Creteil, France) and V. Ilyushin (RIAN, Kharkov, Ukraine). The principal difficulty involves achieving the computational speed necessary to treat multi-top spectra to high J values (e.g., J = 60).
One of these programs has been applied to fitting a very large data set (20 000 MW and IR transitions) for CH3OH, principally in collaboration with Li-Hong Xu (UNB, Saint John, NB, Canada). The principal difficult involves correctly assessing measurement uncertainties for data from many different laboratories and many different instrumentation eras. This work includes application of Gaussian calculations to aid in adding small-amplitude vibrational spectra to the above methanol data mix.
Different versions of a generalized multi-dimensional tunneling formalism have been or are being applied to various high-barrier large-amplitude motion problems, e.g.,: H transfer and internal rotation in 2-methylmalonaldehyde and 5-methyltropolone (see publication titles), H migration in C2H3+, and cis-trans isomerization in excited electronic states of HC≡CH.