A fitting program for molecules with two inequivalent methyl tops and a plane of symmetry at equilibrium: Application to new microwave and millimeter-wave measurements of methyl acetate

A program, called BELGI-Cs-2Tops, for fitting the high-resolution torsion-rotation spectra of molecules with two inequivalent methyl rotors and a plane of symmetry at equilibrium is described and applied to methyl acetate [CH(3)-O-C(=O)-CH(3)]. The G(18) 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 and a two-step diagonalization procedure. Restriction to molecules belonging to the C(s) point group at equilibrium allows the use of real arithmetic throughout the program. This program was 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 root-mean-square deviations of 4 kHz for the microwave lines and of 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 CH(3)) and 422 cm(-1) (ester CH(3)). 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