Molecular Orbital Studies on Optimized Geometries of 1 -Methyl-2(1H)-pyridinimine HCI Salt and 2-Amino-1-methylpyridinium Cation and the Simulation of Their Infrared Spectra

Abstract

Before the IR spectral measurement to ascertain the formation of 2-amino-1-methylpyridinium cation (abbrev. pyridinium cation) in the aquous solution of 1-methyl-2 (1H) -pyridinimine (abbrev. pyridinimine) is done, the normal coordinate calculation within the ab initio 6-31G level was carried out for the optimized models of pyridinium cation and pyridinimine-HC1 salt using the Gaussian 90 source program on a FACOM VP-100E computer to make sure the IR spectral features of pyridinium cation. The optimized model of the latter compound showed that the HC1 salt is consisted of pyridinium cation and chloride anion moieties. Therefore, the IR spectra of the two compounds were expected to be close to each other. From the present calculation the following conclusion was drawn. (1) The strong two bands appear in the region 3500 to 3000 cm^-1. These bands were assigned to the N-H stretching vibration. Their band position, band separations and relative intensities are different from each other due to the existence of chloride anion or no existence. Pyridinimine has no such strong two bands in this region. (2) Pyridinium cation and cation moiety have four prominent bands in the region 1700 to 1500cm^-1, while pyridinimine has the three bands in the same region. The band at the highest wavenum ber side among the four bands is strong in intensity and it was assigned to the scissoring vibration of the amino group of the cation. Pyridinimine has no such band in the same region. Therefore, the corresponding band of cation is a key band for the recognition of pyridinium cation from the IR spectrum of pyridinimine. (3) The band assignment of the IR spectrum of pyridinimine hydroiodide was done by making reference to the results of the normal coordinate treatment of pyridinimine hydrochloride model. The correspondence between the IR spectrum of the HI salt and the results of the normal coordinate treatment of the HC1 salt model is relatively good in the band position but poor in the relative intensity of the band.