Comparison of STO-3G and 3-21G ab initio harmonic force fields for ethane, propane, dimethyl ether, and cyclobutane: effects of geometry and scaling on calculated frequencies, eigenvectors, and infrared absorption intensities

• Published in: Canadian journal of chemistry Vol. 66; no. 5; pp. 1318 - 1332
• Main Authors: Shaw, R. Anthony, Ursenbach, Charles, Rauk, Arvi, Wieser, Hal
• Format: Journal Article
• Language: English
• Published: Ottawa, Canada NRC Research Press 01.05.1988; National Research Council of Canada
• Subjects: Atomic and molecular physics; Exact sciences and technology; Molecular properties and interactions with photons; Molecular spectra; Physics; Basis set size; Ether; Hydrocarbon; Ab initio method; Intensity; Force field; Theoretical study; Ethane; Gaussian orbital; Saturated cyclic compound; Saturated aliphatic compound; Cyclobutane; Infrared spectrum; Monocyclic compound; Propane; Organic compounds
• ISSN: 0008-4042; 1480-3291
• DOI: 10.1139/v88-214

Ab initio harmonic force fields were calculated for ethane, propane, dimethyl ether, and cyclobutane at the STO-3G and 3-21G levels. The calculated frequencies, displacement eigenvectors, and calculated infrared absorption intensities were compared as they derive from force constants that were (i) unsealed; (ii) scaled to fit observed vibrational frequencies reported in the literature; (iii) evaluated at the optimized geometries; and (iv) evaluated at structures for which the bond lengths were corrected from the optimized geometries according to published procedures. A total of nine combinations of ab initio force field/reference geometry/ G -matrix geometry were investigated for each of the four molecules. The ability of scaling factors as the only variables to predict vibrational parameters from STO-3G and 3-21G force fields was explored. Conditions were examined for which the scaling factors are satisfactorily transferable among different molecules.