Application of an IAM theory of internal rotation to the analysis and prediction of torsional-rotational spectra for selected molecules with partially deuterated methyl groups



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Texas Tech University


A theory for internal rotation-overall rotation interactions has been developed for the asymmetric-asymmetric molecules in the case where both top and frame separately have planes of symmetry using the internal axis approach. The essential features of the theory are expansion of the rotational coefficients in series in the internal rotation coordinate, and three coordinate tranformations which remove the zeroth order Coriolis coupling terms and define the internal axis system. The method of solution of the reduced torsion-rotation Hamiltonian involves solving for the torsional energy in the free rotor basis, obtaining the interaction matrix in the new basis, and diagonalizing for the torsional-rotational energy levels. Numerical calculations were made for the partially deuterated methyl alcohols and methyl mercaptans for analysis of the spectral data. The structural parameters were varied from those determined for the normal species with a slight breaking of the symmetry of the methyl group. However, the variation in the potential energy coefficients provides for very large variation in the calculated term values while the molecular structure variation provides for small changes. The structural parameters and potential energy coefficients giving the "best fit" were determined.



Methyl groups -- Molecular rotation, Molecular rotation, Deuterons -- Molecular rotation