IR spectra simulation as auxiliary tool for gas chromatography Fourier transform IR spectroscopy mass spectrometry identification of unknown compounds: comparison between several semi-empirical methods

A set of the semi-empirical methods supplied by the HyperChem(TM) package h as been tested to find the best auxiliary tool for the gas chromatography-F ourier transform IR spectroscopy-mass spectrometry identification of cyclic amide-type compounds. The method MINDO3 has been found to be the most adva ntageous since: (1) as a rule, the IR spectra simulated by this method best match the experimental spectra; (2) within the most useful range of nu(CH) , nu(C=O), nu(C=N) and delta(NH) vibrations, MINDO3 provides the best linea rity between the calculated and experimental wavenumbers. The possibility o f IR spectral discrimination of diastereomers for the case of derivatives o f optically active amino acids has also been considered. IR spectra simulat ions using the PM3 method for various diastereomers of alanine derivative 2 ,5,7,10-tetramethylhexahydroimidazo[1,2-a]imidazo-[1,2-d]pyrazine-3,8-dione have shown the nu(C=O) and nu(C=N) frequencies to vary insignificantly, by less than 8 cm(-1), thus making spectral discrimination of the diastereome rs impossible under normal optical resolution of the commercially available FTIR detectors. (C) 1999 Elsevier Science B.V. All rights reserved.