Coupled Large Amplitude Motions: A Case Study of the Dimethylbenzaldehyde Isomers

The microwave spectra of the 3,4- (syn and anti), 2,5- (syn), and 3,5-dimethylbenzaldehyde
(DMBA) molecules
have been recorded for the first time in the 2–26.5 GHz frequency
range, using the high resolution COBRA-FTMW spectrometer in Hannover.
The experimental assignments and fits are supplemented by ab initio
quantum chemical calculations of the conformational energy landscape
and dipole moment components. The analysis of the spectra of the four
observed isomers, including spectroscopic constants and large amplitude
motion parameters, are presented in this paper. The DMBA isomers belong
to a series of similar molecules obtained formally by adding one or
more methyl group(s) at the aromatic ring. These molecules serve as
prototype systems for the development of the theoretical model of
asymmetric top molecules having Cs symmetry while containing in addition two nonequivalent methyl
tops (C3v), exhibiting
different barrier heights and coupling terms. Thus, the DMBA isomers
represent good species for testing the recently written two-top internal
rotors BELGI program.