A program for fitting the high-resolution torsion-rotation spectra of molecules having two inequivalent methyl rotors and a plane of symmetry at equilibrium is described and then applied to methyl acetate [CH3-O-C(=O)-CH3]. The G18 permutation-inversion group-theoretical considerations (allowed coordinate transformations, symmetry species for the basis set functions and various operators, etc.) used in the design of the program are presented, followed by a description of the structure of the program, which uses an extreme principal axis method, a two-step diagonalization procedure, and banded matrix techniques. Restriction to molecules belonging to the Cs point group at equilibrium allows the use of real arithmetic throughout the program. This program is then used to carry out a weighted least-squares fit of more than 800 new microwave and millimeter-wave measurements on ground-state transitions in methyl acetate, leading to a root-mean-square residual near 4 kHz for the microwave lines and near 40 kHz for the millimeter-wave lines, i.e., to residuals essentially equal to the experimental measurement errors, and to heights for two internal rotation barriers of 102 cm-1 (acetyl CH3, attached to C=O) and 422 cm-1 (ester CH3, attached to O). Some of the difficulties in determining molecular parameters for a two-top molecule from a data set containing only torsional ground state information are discussed.
Citation: Journal of Molecular Spectroscopy
Pub Type: Journals
Fitting Program, Internal Rotation, Methyl Acetate, Methyl Tops, Microwave Spectrum, Two-Top Hamiltonian