Large Amplitude Motions in 2,3-Dimethylfluorobenzene: Steric Effects Failing to Interpret Hindered Methyl Torsion

The microwave spectrum of 2,3-dimethylfluorobezene,
one of the
six isomers of dimethylfluorobenzene, was recorded using a pulsed
molecular jet Fourier transform microwave spectrometer operating in
the frequency range from 2 to 26.5 GHz. The internal rotations of
two inequivalent methyl groups, causing splittings of up to several
hundred MHz of all rotational energy levels into quintets, were analyzed
and modeled. The torsional barriers of the methyl groups at the ortho and the meta positions were determined
to be 215.5740(56) cm–1 and 488.53(11) cm–1. A comparison with the barrier heights observed for the two isomers
2,6-dimethylfluorobenzene and 3,4-dimethylfluorobenzene has shown
that the methyl group at the meta position seems
to be invisible to its neighboring ortho-methyl group,
while the meta-methyl group clearly senses the ortho one. Steric effects are not able to explain this observation,
and electrostatic effects are most probably the reason. Highly accurate
molecular parameters determined experimentally were compared with
those obtained from quantum chemical calculations at different levels
of theory.