The microwave rotational spectrum of toluene has been investigated in the region from 8 to 23 GHz. The three types of absorption lines to be expected in the case of a molecule of this symmetry with a very low sixfold barrier hindering internal rotation of the methyl top have been found: m=0; |m|≠ 0, 3 n; | m | = 3 n. From the lines m= 0 the rotational constants and the dipole moment (0.375 ± 0.01 D) have been determined; from the lines 1 |m|=3 the internal rotation barrier could be calculated as V60 = (13.94 ± 0.1) cal/mole. This value is discussed and compared with the very much larger value derived from solid-phase heat capacity data.
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