Abstract
Abstract Nine microwave ground-state spectra of seven isotopes of ortho-xylene have been measured between 9 and 29 GHz. From the rotational constants a partial substitution structure could be calculated. The dipole moment was determined from Stark-lobe shifts, μa = 0.640 ± 0.005 D. The high-J transitions were found split into multiplets due to the interaction of methyl top internal rotation with the overall molecular rotation; doublets through quintets with the correct nuclear spin weight dependence could be observed according to group-theoretical expectations. A weighted average, V3 = 1490 ± 50 cal/mole, was derived for the internal rotation barrier neglecting top-top coupling and presumably small, higher than threefold barrier terms. The methyl groups both stagger the bond between the two benzene carbon atoms which carry them.
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