STRUCTURAL AND SUBSTITUENT EFFECTS ON THE KETENE INFRARED STRETCHING FREQUENCY

The frequencies and intensities of the ketene asymmetric stretching vibration near 2100 cm(-1) for a series of substituted ketenes and fulvenones have been calculated using ab initio molecular orbital methods. Comparisons of the calculated values with experimentally determined results show reasonably good agreement in most cases, with a typical deviation of +/-10 cm(-1) between the measured and calculated values. A test of a correlation proposed by Gano and Jacob (Spectrochim. Acta, 43A, 1023 (1987)) of the frequencies with substituent field and resonance parameters F and R shows a modest correlation coefficient of 0.86, with no significant dependence on R. These results are consistent with enhancements of ketene infrared absorption frequencies by interaction of substituents with the negative charge density at C-beta of ketenes, as indicated by the resonance structure R(C) over barH$-C=0(+). The calculated intensities I (km/mol) for non-pi acceptor substituents are correlated with the group electronegativities X by the relationship I=1841.6-305.7X (r=0.94). The pi-acceptor substituents show exaltations of the intensities, and the results are interpreted as showing an enhancement of intensities by electropositive substituents increasing the normal ketene dipole directed towards oxygen, and an opposite effect by pi-acceptor substituents delocalizing the negative charge at C-beta.