A new method of mapping molecular electrostatic potentials using a dipole of arbitrary length and strength has been developed. The dipole is allowed to move on a geometrical surface enclosing the molecule under study such that the closest distance of approach (CDA) between the two species is fixed. Potential maps of water, formaldehyde and ethylene epoxide molecules have been studied by this approach for CDA values of 2 and 5 A. Experimental geometries for the molecules and net charge distributions obtained by the MNDO molecular orbital method were used in the calculations. Dipole potential maps obtained by the present method are compared with those obtained by the prevalent monopole mapping method. The two approaches broadly predict the same sites for potential minima in the molecules studied. The present dipole maps are however more informative, especially with regard to electric field directions, than the monopole maps, and it has been shown that this additional information is very useful.

Electrostatic potential maps of guanine, cytosine and the G-C base pair in the Watson-Crick and Hoogsteen configurations have been studied using a mapping dipole of length 1Åand strength 1 Debye. Net charges on the atoms of the molecules were obtained by the MNDO molecular orbital method. Several closest distances of approach (CDA) between the mapping dipole and atomic sites of the molecules were considered and potentials calculated using interactions of net charges keeping the dipole in the molecular plane in each case. While on one hand this work supports the existence of potential minima obtained by monopole isopotential mapping, on the other it yields additional useful information, e.g. electric field directions which correspond to the directions of probable hydrogen bonds with other species. A deep localized potential region is observed around N4 of cytosine in the Hoogsteen configuration while it is not present in the Watson-Crick base pairing.

Electronic absorption and fluorescence spectra of 8-azaguanine in aqueous solution have been studied. Relative intensities of the two peaks observed in absorption are found to change with time. This result can be interpreted in terms of the double-well potential surfaces of the molecule as has been done earlier for guanine. Like guanine, 8-azaguanine also forms a complex with oxygen dissolved in the solution on irradiation in the absorption region and this makes the¹∑_g⁺-³∑_g^- transition of oxygen in the complex allowed by dipole selection rule. The peak of 8-azaguanine fluorescence when excited by radiations of different wavelengths upto 305 nm lies near 400 nm. This fluorescence differs appreciably from that of guanine and can be interpreted as mainly arising from the second triplet excited state of the molecule which may be populated by intersystem crossing from the first singlet excited state.

Molecular electrostatic potential (MEP) maps of acetaldehyde, fluoroacetaldehyde, trifluoroacetaldehyde, hydroxyacetaldehyde and aminoacetaldehyde were studied usingab initio SCF wavefunctions with 6–31G** basis set as well as a recently developed and optimised new method in which a combination of Löwdin and hybridization displacement charges (HDC) at the MNDO level is used. An important point of this method is the distribution of electronic charges continuously and spherically in three dimensions, which helps reproduce most of the important features of the correspondingab initio MEP maps that cannot be achieved using point charges. Electrophilic reactivity of the molecules has been discussed using the MEP results.

Mechanisms of anti-oxidant action of 6-gingerol as a hydroxyl radical scavenger have been investigated using the transition state theory within the framework of density functional theory. Hydrogen abstraction by a hydroxyl radical from the different sites of 6-gingerol and addition of the former to the different sites ofthe latter were studied. Electron transfer from 6-gingerol to a hydroxyl radical was also studied. Solvent effect in aqueous media was treated using the integral equation formalism of the polarizable continuum model (IEFPCM). Reaction rate constants in aqueous media were generally found to be larger than those in gas phase. The tunneling contributions to rate constants were found to be appreciable. Our results show that 6-gingerol is an excellent anti-oxidant and would scavenge hydroxyl radicals efficiently.