Vibrational investigations of guanine, thioguanine and their singly charged cations and anions

Abstract

The complete vibrational studies have been done with help of quantum mechanics for the neutral Guanine (Gua) and Thioguanine (TGua) molecules and their singly charged cations and anions employing the B3LYP/6-311++G** method. Neutral Thioguanine and cations of Guanine and Thioguanine show planar structures and belong to Cs point group symmetry while the neutral Guanine and anions of Guanine and Thioguanine possess non-planar structure with C1 point group symmetry. Vibrational studies of ionic radicals of Gua and its thio- derivative are not available in literatures. Such extensive studies have been attempted for the first time. The normal modes of all the species have been assigned on the basis using potential energy distributions (PEDs) using GAR2PED software. The PEDs have also been calculated to make a conspicuous assignment as animation available in GaussView is not a guarantee for correct normal mode assignment. Charge transfer occurs in the molecule have been shown by the calculated highest occupied molecular orbital—lowest unoccupied molecular orbital (HOMO–LUMO) energies. The mapping of electron density iso-surface with electrostatic potential, has been carried out to get the information about the size, shape, charge density distribution and site of chemical reactivity of the molecule. The electronic properties HOMO and LUMO energies have been measured. The energy gap from HOMO to LUMO of the Gua is 5.0547 eV and TGua 4.0743 eV.