Electronic structure of some adenosine receptor antagonists: I. Equilibrium geometries, charge density distributions, and substituent effects

Abstract

ABSTRACT:Ab initio molecular orbital calculations were implemented to investigate the variation of the electronic structure of a series of different xanthine derivatives. MP2(full)/6-31G calculations have been carried out and used to investigate the geometric features of the studied compounds. Alkyl xanthines studied in the present work are highly polarized with the and frameworks polarized in opposite directions. This polarization is sensitive to substitution. The trend observed for the variation of dipole moment with substitution is interesting. Thus, electron-withdrawing substituents lower the magnitude of the dipole moment, which reaches its smallest value in the case of OCF 3 substituent. The ONO 2 group not only reduces the dipole moment but it also inverts its direction as well. The effect of the basis set on these geometric features has been investigated through implementation of polarization function in MP2(full)/6-31G/6-311G** calculations, where it was found that inclusion of the polarization function led to a noticeable redistribution of the charge density. Thus, charge density that was originally accumulated on the atomic centers is redistributed into the bonding regions and this is the reason for the smaller net charges and the larger bond orders predicted at the 6-311G** level.