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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

par Tudorie, Marcela ;Kleiner, Isabelle;Hougen, Jon J.T.;Melandri, Sonia;Sutikdja, Lilian L.W.;Stahl, Wolfgang
Référence Journal of Molecular Spectroscopy, 269, 2, page (211-225)
Publication Publié, 2011-10

Article révisé par les pairs

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Résumé :

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 [CH3OC(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 and a two-step diagonalization procedure. Restriction to molecules belonging to the Cs 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 CH3) and 422 cm-1 (ester CH3). 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. © 2011 Elsevier Inc. All rights reserved.