HETEROATOM DERIVATIVES OF INDENE .1. VIBRATIONAL FREQUENCIES AND A REFINED SCALED OVERLAY OF THE AMI FORCE-FIELD OF INDOLE, BENZOFURAN BENZOTHIOPHENE, BENZOXAZOLE AND BENZOTHIAZOLE

The vibrational freqeuncies of indole, benzofuran, benzothiophene, benzoxazole and benzothiazole are analyzed by scaling of the AM1 generated force field. Predicted, or zero-order, frequencies are calculated by transferring scale factors determined from a similar analysis of the vibrations of the constituent single-ring molecules. A method is presented for symmetry factoring the frequencies calculated from the AM1 cartesian coordinate basis. Application of symmetry to the refinement of the single-ring species results in improved monocyclic scale factors. The planar and non-planar average standard deviations for the zero-order vibrations of the bicyclics are approximate to 13 and 23 cm(-1). Much of the deviation in the non-planar vibrations is systematic. Refined scale factors are determined for the two-ring molecules using a five-molecule overlay regression. Here, the root-mean standard deviations of the planar and non-planar vibrations improve to 9.0 (133 freqeuncies, 25 scale factors) and 7.5 cm(-1) (59 frequencies, 13 scale factors), respectively. The scale factors and the assignments are discussed.