Critical comparison of the ab initio and spectroscopic methyl-CH bond length difference in acetyl compounds, CH3C(O) X

✍ Scribed by V. J. Klimkowski; P. Pulay; J. D. Ewbank; D. C. McKean; Lothar Schäfer

• Publisher: John Wiley and Sons
• Year: 1984
• Tongue: English
• Weight: 558 KB
• Volume: 5
• Category: Article
• ISSN: 0192-8651
• DOI: 10.1002/jcc.540050603

✦ Synopsis

In order to provide additional data for the relative lengths of methyl-C-H bond distances in acetyl derivatives, which are difficult to determine accurately by the conventional tools of structural chemistry, the geometries of CH3COH, CH,COF, CH,COCH,, CH,COOH, and CH,CONH, were determined by ab initio SCF gradient optimization at the 5-31G** level and compared with previous 4-21G results. For acetaldehyde 6-311G4 * calculations were also performed and the correlated methyl-C-H stretching potential energy functions were determined. It is found that the calculated differences between the in-plane and out-of-plane methyl-C-H bonds are practically independent of the computational scheme. The calculated results are in contrast to relative bond lengths obtained by some vibrational overtone spectroscopic studies, but are in perfect agreement with C-H bond length differences determined from isolated C-H stretching frequencies of partially deuterated compounds. The reliability of the latter, and other spectroscopic data concerning the assignment of the methyl-C-H vibrations are critically analyzed. On the basis of the available evidence we conclude: (1) the methyl groups of the CH,C(=O)X systems here discussed contain one strong (in-plane) and two weak (out-of-plane) C-H bonds; (2) intensities of C-H local mode spectra do not provide a reliable basis for assignment to individual bonds.