The microwave spectra of m-xylene and m-Xylene-d(10). Determination of thelow methyl internal rotation barrier

The rotational spectra of m-xylene, (CH3)(2)C6H4, and of m-xylene-d(10), (C D3)(2)C6D4, have been recorded between 6 and 26.5 GHz using pulsed beam Fou rier transform microwave spectroscopy. The clue for the assignment of the i nternal rotation multiplets was the inertial defect derived from the A(1)A( 1) species transitions. The rotational constants for m-xylene and m-xylene-d(10) are A = 3572.1117( 1) MHz / 2896.1195(17) MHz, B = 1761.8621(1) MHz I 1446.0236(15) MHz, C =11 97.3943(2) MHz / 988.2357(7) MHz, the barrier to internal rotation of the t wo methyl groups are V-3 = 53.7(16) J/mol / 39.8(5) J/mol, their moments of inertia were assumed to be I-alpha = 3.14 u Angstrom (2) /6.28 u Angstrom (2).