Browsing by Author "D.M. Upadhyay"

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Ab initio study of water clusters in gas phase and bulk aqueous media: (H2O)n, n = 1-12
(John Wiley & Sons Inc, 2001) D.M. Upadhyay; M.K. Shukla; P.C. Mishra
Geometries of several clusters of water molecules including single minimum energy structures of n-mers (n = 1-5), several hexamers and two structures of each of heptamer to decamer derived from hexamer cage and hexamer prism were optimized. One structural form of each of 11-mer and 12-mer were also studied. The geometry optimization calculations were performed at the RHF/6-311G* level for all the cases and at the MP2/6-311++G** level for some selected cases. The optimized cluster geometries were used to calculate total energies of the clusters in gas phase employing the B3LYP density functional method and the 6-311G* basis set. Frequency analysis was carried out in all the cases to ensure that the optimized geometries corresponded to total energy minima. Zero-point and thermal free energy corrections were applied for comparison of energies of certain hexamers. The optimized cluster geometries were used to solvate the clusters in bulk water using the polarized continuum model (PCM) of the self-consistent reaction field (SCRF) theory, the 6-311G* basis set, and the B3LYP density functional method. For the cases for which MP2/6-311++G** geometry optimization was performed, solvation calculations in water were also carried out using the B3LYP density functional method, the 6-311++G** basis set, and the PCM model of SCRF theory, besides the corresponding gas-phase calculations. It is found that the cage form of water hexamer cluster is most stable in gas phase among the different hexamers, which is in agreement with the earlier theoretical and experimental results. Further, use of a newly defined relative population index (RPI) in terms of successive total energy differences per water molecule for different cluster sizes suggests that stabilities of trimers, hexamers, and nonamers in gas phase and those of hexamers and nonamers in bulk water would be favored while those of pentamer and decamer in both the phases would be relatively disfavored.
Ab-initio and density functional study of l- and d-forms of alanine and serine in gas phase and bulk aqueous medium
(Elsevier, 2007) D.M. Upadhyay; Amareshwar Kumar Rai; D.K. Rai; A.N. Singh; Anup Kumar
The molecular geometry of l and d-forms of alanine and serine in gas phase have been studied by using ab-initio quantum chemical calculations at the restricted Hartree-Fock (RHF) level by employing 6-31G and 6-311++G** basis sets. Subsequently, for considering the electron correlations, Density functional Calculations at the Becke3LYP (B3LYP) and Moller-Plesset second order (MP2) level of calculations have been carried out with the same basis sets for these optimized geometries. Effect of solvation in water on the optimized geometries was studied using the polarized continuum model of the self-consistent reaction field (SCRF) theory. The dipole moment, energy, polarizabilities and vibrational frequencies have been calculated in all cases. Frequency analysis was carried out to ensure that optimized geometry corresponds to a total energy minimum. © 2006 Elsevier B.V. All rights reserved.
An ab initio study of microsolvation of LiF in water: Structures and properties of LiF-Wn, n = 1-9 complexes
(2003) D.M. Upadhyay; P.C. Mishra
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.
An ab initio study of water-oxygen complexes (O2-Wn, n = 1-6) in the ground and lowest singlet excited states
(Elsevier, 2003) D.M. Upadhyay; P.C. Mishra
Geometries of the O2-Wn (n = 1-4) complexes with triplet multiplicity were optimized at the UMP2/6-311++G** level of theory. The triplet ground and lowest singlet excited state geometries of O2 and the O2-Wn (n = 1-6) complexes were optimized at the B3LYP/6-31 + G* level of theory. Various methods were tested to study the lowest singlet excited states of O2 and the O2-Wn (n = 1-6) complexes, and it was found that the B3LYP/6-31 + G* method worked well and was economical enough to permit a detailed study of O2-Wn (n = 1-6) complexes. The CCD/AUG-cc-pVDZ and B3LYP/6-31 + G* methods were found to yield similar results for the triplet ground and lowest singlet excited states of O2 and the O2-W1 complex. Microsolvation of O2 in water clusters (n = 1-5) in the ground state does not affect the structures of the latter. However, microsolvation of O2 in the ground state modifies the structure of the cage form of water hexamer appreciably such that the latter acquires a non-planar cyclic structure. The structure of the O2-W1 complex is strongly modified in going from the triplet ground state to the singlet excited state. Thus W1 in the singlet excited state gets dissociated into the H and OH fragments both of which are bound to the same oxygen atom of the O2 molecule. This structure of the O2-W1 complex in the singlet excited state is retained in the O2-Wn (n = 2,3) complexes also but not in the O2-Wn (n = 4-6) complexes where none of the molecules is dissociated and are bound to one another as in the triplet ground state. The singlet O2-W2 complex is an isomer of a hydrogen bonded complex of 2H2O2 and the former represents an activated state of the latter by about 1.06 eV. An appreciable amount of electronic charge is transferred to O2 in the O2-Wn (n = 1-3) complexes from the water molecules, particularly from the hydrogen atoms. Thus O2 in the singlet excited state of each of these complexes would behave somewhat like O2-. © 2003 Elsevier Science B.V. All rights reserved.
Binding of benzene with water clusters (H2O)n, n = 1-6, in the ground and lowest singlet excited states
(Elsevier, 2002) D.M. Upadhyay; P.C. Mishra
Ground state geometries of the complexes of benzene (Bz) with different water clusters (Wn, n = 1-6) were fully optimized using the B3LYP/6-31 + G** approach. MP2/6-31 + G** single point calculations were carried out for all the complexes in the ground state. Excited states were generated using configuration interaction involving all the singly excited configurations (CIS) using the ground state optimized geometries and employing the 6-31 + G** basis set, and subsequently excited state geometries of the complexes were also optimized fully. The complex of water dimer with benzene (Bz-W2) appears to be most stable among the various complexes. The lowest singlet electronic transition of benzene gets weakly allowed in the complexes. The calculated excitation energies show that small blue shifts would take place in the absorption spectra, particularly in going from benzene to the three higher complexes (Bz-Wn, n = 4-6). The geometry of Bz-W4 is modified much more strongly due to excitation than those of the other complexes. The benzene ring gets expanded and the CH bond lengths are shortened in going from the ground to the excited states of all the complexes, as in a free benzene molecule. The HOH bond angles of the water molecules are increased, OH bond lengths are decreased and interatomic distances between the water molecules of Wn and those between the atoms of Bz and Wn are mostly increased due to excitation of the complexes. Thus, in each case, the water cluster Wn as well as the whole complex undergoes an expansion in size due to excitation. Thus excitation of the systems is not localized only on the benzene ring but is also extended to the water clusters. © 2002 Elsevier Science B.V. All rights reserved.
Electronic structure and vibrational spectra of common pain killer (Aspirin and Ibuprofen) in gas phase and different medium: An ab initio and DFT approach
(2012) Anup Kumar; Amareshwar Kumar Rai; D.M. Upadhyay; D.K. Rai; A.N. Singh
The molecular geometries of common pain killer drugs Aspirin and Ibuprofen have been investigated using ab initio quantum chemical calculations at the Restricted Hartree-Fock (RHF) and the Density functional method at the Becke3LYP (B3LYP) level by employing 6-31+G*basis set in gas phase, water, methanol and ethanol. The structure, dipóle moments, vibrational frequencies with IR intensities and other thermodynamic properties have also been studied. It is found that both Aspirin and Ibuprofen molecules are relatively more stable in water. A tentative assignment of the intense infrared calculated frequencies (IR) for both Aspirin and Ibuprofen corresponding to the different modes has been carried out. The observed IR spectra of these molecules were also recorded in the range 4000-400 cm.
Lowest singlet excited state geometries, rotational constants and molecular electrostatic potentials of some substituted benzenes: An ab initio study
(Elsevier, 2000) D.M. Upadhyay; M.K. Shukla; P.C. Mishra
Ground and lowest singlet excited state geometries of certain substituted benzenes (fluorobenzene, chlorobenzene, p-difluorobenzene, p-dichlorobenzene, p-fluorochlorobenzene and aniline) were optimized using the ab initio RHF procedure employing the 6-31 +G* basis set. The calculations were also carried out using the 6-311 + +G** basis set for two molecules. Excited states were generated using configuration interaction involving singly excited configuration (CIS). The calculated lowest singlet transition energies of the molecules agree with experimental ones satisfactorily. It is found that our calculations reproduce the ground and excited state rotational constants A, B and C of the molecules obtained from high-resolution spectroscopy quite well. Excited state geometries of the molecules have not been determined experimentally precisely but certain approximate estimates of the changes in bond lengths and bond angles consequent to excitation are available in the literature. There is a satisfactory agreement between these and our calculated changes in ring bond lengths and bond angles of the molecules consequent to excitation. Studies of excited state molecular electrostatic potential (MEP) maps are scarce in the literature. Our study of ground and excited state MEP maps of the molecules has revealed several interesting features some of which, e.g. the ortho, para directing property of the substituents in the ground state, are in agreement with experiment. It is indicated that reactions at the ortho, para positions in the ground state would take place in planes located much above the ring planes and there is an appreciable modification in this respect following excitation of the molecules. (C) 2000 Elsevier Science B.V.