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Geometries of clusters of water molecules (Wn) and those of the LiF–Wn(n= 1–9) complexes were optimized using the B3LYP/6‐31+G** method. Geometries of the complexes up to n= 7 were also optimized using the MP2/6‐31+G** approach. Only one structure of each of Wn, n = 1–5 was considered to generate the complexes with LiF while two structures, one of a cage type and the other of a prism type, were considered for n= 6–9. The LiF–W2complex is found to be most stable among the various complexes. The LiF–W6complex, where W6is of a cage type, is predicted to be substantially less stable than that where W6is of a prism type. Certain existing ambiguities regarding the most stable structures of the LiF–Wn(n= 1–3) complexes have been resolved. The LiF molecule seems to divide the Wnclusters in the LiF–Wn(n= 3–6) complexes into different fragments where at least one W2‐like fragment is present. In LiF–W6(cage), there is one W2‐like fragment while in LiF–W6(prism), there are three W2‐like fragments. The LiF bond length is substantially increased in going from the gas phase to the different complexes, this increase being most prominent in LiF–W6,where W6is of the cage or prism type. The LiF molecule, however, does not acquire the ionic structure Li+F−in any of the complexes studied here. An appreciable amount of electronic charge is transferred from LiF to the water molecules involved in the different complexes. In this process, the Li atom gains electronic charge in some cases, while the F atom considered separately, as well as the Li and F atoms taken together, lose the same in most cases. © 2003 Wiley Periodicals, Inc. J Comput Chem 24: 1336–1347, 2003

Geometries of clusters of water molecules (Wn) and those of the LiF–Wn (n = 1–9) complexes were optimized using the B3LYP/6-31+G** method. Geometries of the complexes up to n = 7 were also optimized using the MP2/6-31+G** approach. Only one structure of each of Wn, n = 1–5 was considered to generate the complexes with LiF while two structures, one of a cage type and the other of a prism type, were considered for n = 6–9. The LiF–W2 complex is found to be most stable among the various complexes. The LiF–W6 complex, where W6 is of a cage type, is predicted to be substantially less stable than that where W6 is of a prism type. Certain existing ambiguities regarding the most stable structures of the LiF–Wn (n = 1–3) complexes have been resolved. The LiF molecule seems to divide the Wn clusters in the LiF–Wn (n = 3–6) complexes into different fragments where at least one W2-like fragment is present. In LiF–W6 (cage), there is one W2-like fragment while in LiF–W6 (prism), there are three W2-like fragments. The LiF bond length is substantially increased in going from the gas phase to the different complexes, this increase being most prominent in LiF–W6, where W6 is of the cage or prism type. The LiF molecule, however, does not acquire the ionic structure Li⁺F⁻ in any of the complexes studied here. An appreciable amount of electronic charge is transferred from LiF to the water molecules involved in the different complexes. In this process, the Li atom gains electronic charge in some cases, while the F atom considered separately, as well as the Li and F atoms taken together, lose the same in most cases. © 2003 Wiley Periodicals, Inc. J Comput Chem 24: 1336–1347, 2003

Molecular geometries of two structural forms of 2′‐deoxyguanosine (keto‐N9R and keto‐N7R, R = the sugar moiety) considering both the C2′‐endo and C3′‐endo conformations of the sugar ring and those of the complexes of these species with two water molecules each were optimized employing the ab initioRHF procedure. A mixed basis set consisting of the 6‐311+G* basis set for the nitrogen atom of the amino group and the 4‐31G basis set for all the other atoms was used. The RHF calculations were followed by correlation correction of the total energy at the MP2 level. Both the structural forms of 2′‐deoxyguanosine were solvated using the polarized continuum model (PCM) of the self‐consistent reaction field (SCRF) theory and the corresponding RHF optimized geometries at the RHF and MP2 levels. Geometry optimization was also performed in aqueous media using the Onsager model at the RHF level using the above‐mentioned mixed basis set, and subsequently, using the reoptimized geometries, single‐point MP2 calculations were performed. It is found that both the keto‐N9R and keto‐N7R forms of 2′‐deoxyguanosine as well as their complexes with two water molecules each would occur, particularly at the water–air interface. Though the normal Watson–Crick‐type base pairing would not be possible with the keto‐N7R form of 2′‐deoxyguanosine(G*), two other (G*‐C and G*‐T) base pairing schemes may occur with this form of the nucleoside, which may cause mutation. The present calculated geometry of the keto‐N9R form of the anti‐conformation of 2′‐deoxyguanosine including the dihedral angle χCNagree satisfactorily with the available crystallographic results. The present results also agree satisfactorily with those obtained by other authors earlier for the keto‐N9R form of 2′‐deoxyguanosine using B3LYP and MP2 methods employing the 6‐31G* basis set. Using transition state calculations, it is shown that tautomerism of guanine and other similar molecules where the tautomers would coexist would be facilitated by the occurrence of the H+and OH−fragments of water molecules. Further, this coexistence of the two tautomers appears to make the C8 carbon atom located between the N7 and N9 nitrogen atoms susceptible to attack by the OH−group. Thus, an explanation is obtained for the efficient formation of the reaction product 8‐hydroxy‐2′‐deoxyguanosine, which serves as a biomarker for oxidative damage to DNA in biological systems. © 2002 Wiley Periodicals, Inc. J Comput Chem 23: 530–540, 2002; DOI 10.1002/jcc.10046

Ground state geometries of the four tautomeric forms keto‐N9H, keto‐N7H, enol‐N9H, and enol‐N7H of guanine were optimized in the gas phase at the RHF level using a mixed basis set consisting of the 4‐31G basis set for all the atoms except the nitrogen atom of the amino group for which the 6‐311+G* basis set was used. These calculations were also extended to hydrogen‐bonded complexes of three water molecules with each of the keto‐N9H (G9‐3W) and keto‐N7H (G7‐3W) forms of guanine. Relative stabilities of the four above‐mentioned tautomers of guanine as well as those of G9‐3W and G7‐3W complexes in the ground state in the gas phase were studied employing the MP2 correlation correction. In aqueous solution, relative stabilities of these systems were studied using the MP2 correlation correction and polarized continuum model (PCM) or the isodensity surface polarized continuum model (IPCM) of the self‐consistent reaction field (SCRF) theory. Geometry optimization in the gas phase at the RHF level using the 6‐31+G* basis set for all atoms and the solvation calculations in water at the MP2 level using the same basis set were also carried out for the nonplanar keto‐N9H and keto‐N7H forms of guanine. Thus, it is shown that among the different tautomers of guanine, the keto‐N7H form is most stable in the gas phase, while the keto‐N9H form is most stable in aqueous solution. It appears that both the keto‐N9H and keto‐N7H forms of guanine would be present in the ground state, particularly near the aqueous solution–air interface. Vertical excitation and excited state geometry optimization calculations were performed using configuration interaction involving single electron excitation (CIS). It is found that the absorption spectrum of guanine would arise mainly due to its keto‐N9H form but the keto‐N7H form of the same would also make some contribution to it. The enol‐N9H and enol‐N7H forms of the molecule are not expected to occur in appreciable abundance in the gas phase or aqueous media. The normal fluorescence spectrum of guanine in aqueous solution with a peak near 332 nm seems to originate from the lowest singlet excited state of the keto‐N7H form of the molecule while the fluorescence of oxygen‐rich aqueous solutions of guanine with a peak near 450 nm appears to originate from the lowest singlet excited state of the keto‐N9H form of the molecule. The origin of the slow damped spectral oscillation observed in the absorption spectrum of guanine has been explained. © 2000 John Wiley & Sons, Inc. J Comput Chem 21: 826–846, 2000

Ground state geometries of the four tautomeric forms keto-N9H, keto-N7H, enol-N9H, and enol-N7H of guanine were optimized in the gas phase at the RHF level using a mixed basis set consisting of the 4-31G basis set for all the atoms except the nitrogen atom of the amino group for which the 6-311+G* basis set was used. These calculations were also extended to hydrogen-bonded complexes of three water molecules with each of the keto-N9H (G9-3W) and keto-N7H (G7-3W) forms of guanine. Relative stabilities of the four above-mentioned tautomers of guanine as well as those of G9-3W and G7-3W complexes in the ground state in the gas phase were studied employing the MP2 correlation correction. In aqueous solution, relative stabilities of these systems were studied using the MP2 correlation correction and polarized continuum model (PCM) or the isodensity surface polarized continuum model (IPCM) of the self-consistent reaction field (SCRF) theory. Geometry optimization in the gas phase at the RHF level using the 6-31+G* basis set for all atoms and the solvation calculations in water at the MP2 level using the same basis set were also carried out for the nonplanar keto-N9H and keto-N7H forms of guanine. Thus, it is shown that among the different tautomers of guanine, the keto-N7H form is most stable in the gas phase, while the keto-N9H form is most stable in aqueous solution. It appears that both the keto-N9H and keto-N7H forms of guanine would be present in the ground state, particularly near the aqueous solution–air interface. Vertical excitation and excited state geometry optimization calculations were performed using configuration interaction involving single electron excitation (CIS). It is found that the absorption spectrum of guanine would arise mainly due to its keto-N9H form but the keto-N7H form of the same would also make some contribution to it. The enol-N9H and enol-N7H forms of the molecule are not expected to occur in appreciable abundance in the gas phase or aqueous media. The normal fluorescence spectrum of guanine in aqueous solution with a peak near 332 nm seems to originate from the lowest singlet excited state of the keto-N7H form of the molecule while the fluorescence of oxygen-rich aqueous solutions of guanine with a peak near 450 nm appears to originate from the lowest singlet excited state of the keto-N9H form of the molecule. The origin of the slow damped spectral oscillation observed in the absorption spectrum of guanine has been explained. © 2000 John Wiley & Sons, Inc. J Comput Chem 21: 826–846, 2000