Interpretation of carbonyl stretching band intensities in the infrared spectra: An ab initio MO study

Ab initio MO calculations at HF/6-31G** level have been used to investigate the variations in the carbonyl stretching band intensities in a series of 15 open-chain aliphatic carbonyl derivatives containing first-row elements (molecules of the type HCOX, CH3COX and X2CO, where X = -H, -CH3, -OH, -OCH3, -NH2 and -F). Geometry optimisation and frequency and intensity calculations at the optimised geometry have been performed. Two types of local intensity parameters have been evaluated: (1) dipole moment derivatives with respect to C=O bond stretching (delta p/delta r(C=O)); and (2) effective charges of the C=O bond (delta(C=O)). Atomic charges determined according to CHELPG scheme and the energy of hydrogen bond formation between hydrogen fluoride and the carbonyl oxygen have also been evaluated. Linear dependencies between effective bond charges (delta(C=O)) and carbonyl oxygen atomic charges, as well as between delta(C=O) and the energy of hydrogen bond formation have been found. The relations obtained show that the derived local infrared intensity parameters can be used to characterise electric charge properties of the carbonyl bond. (C) 1997 Elsevier Science B.V.