A new program is described for fitting rotation-torsion energy levels in the molecules like toluene, in which the frame (C6H5) has C2v symmetry and the methyl top has C3v symmetry, i.e., for molecules where the internal rotation barrier is expanded in cos6n, where is the internal rotation angle and n = 1, 2, . The program is based on the theoretical framework developed by S rensen and Pedersen in their application of the Longuet-Higgins permutation-inversion group G12 to the microwave spectrum of CH3NO2. It is specifically designed for six-fold barrier molecules, and allows the user to select almost any symmetry-allowed torsion-rotation term for inclusion in the fitting Hamiltonian. This program leads to a very successful fit of transitions in the toluene microwave spectrum characterized by J 30, Ka 12, and the free rotor quantum number |m| 3. In these fits we included both published and rather extensive unpublished new measurements, for which fits using other torsion-rotation programs have not been very successful. The fit presented here used 23 parameters to give an overall standard deviation of 7.6 kHz for the 362 transitions.
Citation: Journal of Molecular Spectroscopy
Pub Type: Journals
G12 Permutation-Inversion Group, Internal Rotation, Least-Squares Fit, Microwave Spectrum, Six-Fold Barrier, Toluene