The microwave spectrum of m-tolunitrile: Methyl internal rotation and N-14nuclear quadrupole coupling

The microwave spectrum of m-tolunitrile (3-methylbenaonitrile, m-C6H4CH3CN) has been investigated in the frequency range from 1 to 4 and 8 to 26.5 GHz . The spectra in the two lowest states of internal methyl rotation (m = 0, +/-1) were recorded by means of pulsed molecular beam Fourier transform mic rowave (MB-FTMW) spectrometers. The interpretation of the spectra was based on an asymmetric frame-symmetric top Hamiltonian with inclusion of centrif ugal distortion terms and first-order contributions from N-14 nuclear quadr upole coupling. A least-squares analysis yielded the rotational constants A = 3295.9103(10) MHz, B = 1199.1188(2) MHz, C = 883.9223(1) MHz, all elemen ts of the nuclear quadrupole coupling tensor chi(aa) = -3.626(1) MHz, chi(b b) = 1.684(1) MHz, chi(cc) = 1.943(1) MHz, and chi(ab) = -1.870(3) MHz, as well as the threefold barrier to internal rotation, V-3 = 14.2 cm(-1), and the angle between the internal rotor axis and the principal moment of inert ia a axis, theta = 42.66 degrees, using fixed values for the sixfold barrie r term V-6 (-11 cm(-1)) and the moment of inertia of the methyl top I-alpha (3.16 u Angstrom(2)). (C) 2000 Academic Press.