Browsing by Author "P.C. Mishra"

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1,3(n-π*) excited states of benzaldehyde
(Springer India, 1985) R.D. Tewari; P.C. Mishra
Geometries of the first triplet and first singlet n-π* excited states of benzaldehyde have been optimized using the sindo 1 molecular orbital wave-function (with CI) and the Newton-Raphson method. The triplet excited state geometry of the molecule is found to be appreciably non-planar whereas that of the singlet excited state is planar. A crossing of molecular orbitals occurs in going from the ground state equilibrium geometry to the triplet and singlet n-π* excited state equilibrium geometries. Existence of the para-directing effect of the singlet n-π* transition for electronic charges found in an earlier work is confirmed by the present work. The triplet n-π* excitation rearranges electronic charges mainly on the CHO group. It is found that the dipole moment of the molecule would appreciably increase following the singlet n-π* excitation whereas the same would appreciably decrease following the triplet n-π* excitation. © 1985 Indian Academy of Sciences.
A directing effect of n–π* transitions on electronic charges
(1984) R.D. Tewari; P.C. Mishra
Changes of electronic charge distribution following the lowest singlet n–π* transitions of benzaldehyde, benzamide, benzoic acid, the phthalaldehydes, pyridine, and the diazines have been examined using the CNDO‐S/CI method. A Singlet n–π* transition is found as a general rule to direct electronic charges to the para position of the atom in the ring that carries the lone pair or to which the substituent carrying the lone pair is attached. Copyright © 1984 John Wiley & Sons, Inc.
A gas phase ab initio excited state geometry optimization study of thymine, cytosine and uracil
(Elsevier, 1999) M.K. Shukla; P.C. Mishra
Molecular geometries of the nucleic acid bases thymine, cytosine and uracil in the ground and the lowest two singlet excited states were optimized using the ab initio approach employing the 4-31G basis set for all the atoms except the amino group of cytosine for which the 6-311+G* basis set was used. The excited state calculations were performed employing configuration interaction involving singly excited configurations (CIS). Vibrational frequencies were computed in order to examine the nature of the stationary points on the potential energy surfaces obtained by geometry optimization. While the ground state geometries of uracil and thymine (except the methyl group hydrogens) are planar, the corresponding excited state geometries were found to be significantly nonplanar. In the case of cytosine, the amino group is pyramidal and the rest of the molecule is only slightly nonplanar in the ground state, but the excited state geometries are appreciably nonplanar. In particular, consequent to the S2(n-π*) excitation of cytosine, the amino group plane is strongly rotated. While thymine is stable in the S2(π-π*) excited state, uracil appears to be dissociative in the corresponding excited state.
A gas phase theoretical study of the hydrogen bonding properties of some androgen antagonists using molecular orbital and molecular electric field mapping methods
(Elsevier, 1996) Anil C. Nair; P.C. Mishra
A number of anilides, alkenes and an indole derivative which are known to be potent anti-androgens were studied using molecular orbital (AM1) and molecular electric field mapping (MEF) techniques. Linear correlation coefficients between the MEF values near the tertiary hydroxyl groups of these molecules and the corresponding experimentally observed hydrogen bond donating abilities (log Kα, values) were found to be satisfactory. The study illustrates the usefulness of MEF mapping in predicting the hydrogen bond donating abilities of these molecules.
A geometry optimization and molecular electrostatic potential mapping study of structure-activity relationship for some anti-Alzheimer gents
(1999) P.S. Kushwaha; M.K. Shukla; P.C. Mishra
Molecular geometries of some substituted (pyrroloamino)pyridines which possess anti-Alzheimer activity were optimized and potential-derived CHelpG point charges were computed using ab initio SCF molecular orbital approach employing the 3-21G basis set. AM1 molecular orbital calculations were performed using these optimized geometries and thus optimized Hybridization Displacement Charges (HDC) combined with Löwdin charges continuously distributed in three dimension were obtained. Molecular electrostatic potential (MEP) maps of the molecules were obtained in two ways: (i) using the HDC-based model with the help of which MEP minima near the molecules were located, and (ii) using the CHelpG point charges, MEP values on the van der Waals surfaces of the molecules were computed. The MEP maps computed using both the methods have negative MEP regions near the pyridine nitrogen atom which appears to be the main binding site of the molecules with the appropriate receptor. Both electrostatic interaction and lipophilic association between these molecules and the receptor appear to contribute to biological activity.
A molecular orbital study of electronic spectra of thymine, its photodimerization and photodeactivation of DNA
(1976) P.C. Mishra; G. Govil
[No abstract available]
A novel Raman spectroscopic approach to identify polymorphism in leflunomide: A combined experimental and theoretical study
(John Wiley and Sons Ltd, 2016) Poornima Sharma; Debraj Gangopadhyay; Sima Umrao; Shiv Kumar; A.K. Ghosh; P.C. Mishra; Ranjan K. Singh
Polymorphism is an important characteristic which affects the activity, solubility and other physical properties of a compound and can be induced by varying temperature, pressure and solvent. The presence and conversion of α to β polymorphic forms of an anti-rheumatic drug leflunomide have been studied by temperature-dependent and in situ Raman observations. Both α and β polymorphs were found to co-exist in the temperature interval 367-372 K. The α form alone exists below 367 K and the β form alone above 373 K. The C=O stretching band clearly demonstrates the α → β conversion because of breaking of N-H···O bond and formation of N-H···N bond. On cooling the Raman spectra suggest the irreversibility of this conversion. Thermodynamic stability, crystal parameters and surface morphology of both forms in the leflunomide powder used for the present study have been verified by differential scanning calorimetry, X-ray powder diffraction and scanning electron microscopy. © 2015 John Wiley & Sons, Ltd.
A quantum chemical study of reactions of DNA bases with sulphur mustard: A chemical warfare agent
(2010) P.K. Shukla; P.C. Mishra
Reactions of the sulphonium ion of sulphur mustard (SM+1) at the N7, N3 and O6 sites of guanine, N7, N3 and N1 sites of adenine, O2 and N3 sites of cytosine and O2 and O4 sites of thymine were studied theoretically in gas phase and aqueous media employing density functional theory (DFT) and second order Møller-Plesset perturbation (MP2) theory. The B3LYP, B3PW91 and B1B95 functionals of DFT and the 6-31+G* and AUG-cc-pVDZ basis sets were used in the calculations. Basis set superposition error was treated using the counterpoise method by single point energy calculations at the B3LYP/6-31+G* level in gas phase. The present study explains the mechanism of alkylation of the DNA bases and shows that SM+1 would form stable adducts at the endocyclic nitrogen sites of the DNA bases, and at the O6 site of guanine and the O2 site of cytosine. Formation of adducts at the N7 site of guanine and N3 site of adenine are found to be most favored and next most favored respectively, which agrees with experimental observations. © Springer-Verlag 2009.
A quantum theoretical study of reactions of methyldiazonium ion with DNA base pairs
(2011) P.K. Shukla; Vinay Ganapathy; P.C. Mishra
Methylation of the DNA bases in the Watson-Crick GC and AT base pairs by the methyldiazonium ion was investigated employing density functional and second order Møller-Plesset (MP2) perturbation theories. Methylation at the N3, N7 and O6 sites of guanine, N1, N3 and N7 sites of adenine, O2 and N3 sites of cytosine and the O2 and O4 sites of thymine were considered. The computed reactivities for methylation follow the order N7(guanine) > N3(adenine) > O6(guanine) which is in agreement with experiment. The base pairing in DNA is found to play a significant role with regard to reactivities of the different sites. © 2011 Elsevier B.V. All rights reserved.
A SINDO1 Study of Photoisomerization and Photofragmentation of Cyclopentanone
(1984) Peter L. Müller-Remmers; Karl Jug; P.C. Mishra
The photoisomerization of cyclopentanone to 4-pentenal and the photofragmentation to cyclobutane and carbon monoxide or to two ethylene and carbon monoxide was investigated by the SINDOl method including configuration interaction (CI). Structures and energies of all pertinent ground and excited states as well as transition states and intermediates were calculated. The mechanism for all reactions involves excitation to S1 and intersystem crossing to T1 or T2. The triplets lead to diradical intermediates from which further reaction to the products takes place. The dependence of product distribution on excitation wavelength is discussed. © 1984, American Chemical Society. All rights reserved.
A theoretical study of structures and electron affinities of radical anions of guanine-cytosine, adenine-thymine, and hypoxanthine-cytosine base pairs
(2004) Anil Kumar; Michaela Knapp-Mohammady; P.C. Mishra; Sándor Suhai
Adiabatic electron affinities (AEA) and structural perturbations due to addition of an excess electron to each of the neutral guanine-cytosine (G-C), adenine-thymine (A-T), and hypoxanthine-cytosine (HX-C) base pairs were studied using the self-consistent charge, density functional tight-binding (SCC-DFTB-D) method, augmented by the empirical London dispersion energy term. Performance of the SCC-DFTB-D method was examined by comparing the calculated results using it with those obtained from experiment as well as ab initio and other different density functional theoretical studies. An excellent agreement between the SCC-DFTB-D results and those obtained by the other calculations regarding the structural modifications, hydrogen bonding, and dissociation energies of the neutral and radical anion base pairs was found. It is shown that adiabatic electron affinity can be better predicted by considering reaction enthalpies of formation of the respective neutral and anionic base pairs from their respective molecular components instead of taking the difference between their total energies. The calculated AEAs of the base pairs were compared with those obtained by the bracketing method from Schaefer and coworkers, where a satisfactory agreement was found. It shows applicability of the SCC-DFTB-D method to study charged DNA models at a highly economical computational cost. © 2004 Wiley Periodicals, Inc.
A theoretical study of the ground‐state potential surface of guanine and its binding with oxygen and water
(1992) C. Santhosh; P.C. Mishra
A molecular orbital geometry optimization study of the potential surface of guanine, guanineO2, and guanineO2water reaction products in the ground state has been carried out. The origin of the asymmetric double‐well potential surface of guanine suggested earlier on the basis of experimental observations has been explained. The most stable binding of O2 with guanine (G) is found to occur at the C4C5 double bond of the latter molecule. In the case of GO2water reaction product (HOOGOH), the groups OOH and OH bind at C4 and C8, respectively. The possiblity of a polymeric reaction product of the type R1(GOOG)nR2 (R1, R2 = H, OH) has also been suggested. These results are broadly supported by experimental observations. The mechanism of spectral oscillations observed in UV‐irradiated guanine solutions has been discussed. Copyright © 1992 John Wiley & Sons, Inc.
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.
Addition and hydrogen abstraction reactions of an OH radical with 8-oxoguanine
(2006) N.R. Jena; P.C. Mishra
Addition reaction of an OH radical at the C2, C4, C5 or C8 position of 8-oxoguanine (8OG) and abstraction of its H9 atom by an OH radical were studied using density functional theory (B3LYP) employing 6-31G**, 6-311++G** and AUG-cc-pVDZ basis sets. Solvent effects of aqueous media were treated using the PCM model. It is found that the addition of an OH radical at the C4 position of 8OG would be most favored in both gas phase and aqueous media. These addition and abstraction reactions in aqueous media are both found to be barrierless. © 2006 Elsevier B.V. All rights reserved.
Adiabatic electron affinities of the polyhydrated adenine-thymine base pair: A density functional study
(2005) Anil Kumar; P.C. Mishra; Sándor Suhai
Adiabatic electron affinities (AEAs) of the adenine-thymine (AT) base pair surrounded by 5 and 13 water molecules have been studied by density functional theory (DFT). Geometries of neutral AT·nH2O and anionic (AT·nH2O)- complexes (n = 5 and 13) were fully optimized, and vibrational frequency analysis was performed at the B3LYP/6-31+G** level of theory. The optimized structures of the neutral (AT·nH2O) and (AT·nH2O)- complexes were found to be somewhat nonplanar. Some of the water molecules are displaced away from the AT ring plane and linked with one another by hydrogen bonds. The optimized structures of the complexes are found to be in a satisfactory agreement with the observed experimental and molecular dynamics simulation results. In the optimized anionic complexes, the thymine (T) moiety was found to be puckered, whereas the adenine (A) moiety remained almost planar. Natural population analysis (NPA) performed using the B3LYP/ 6-31+G** method shows that the thymine moiety in the anionic (AT·nH2O)- complexes (n = 5 and 13) has most of the excess electronic charge, i.e., ∼ -0.87 and ∼ -0.81 (in the unit of magnitude of the electronic charge), respectively. The zero-point energy corrected adiabatic electron affinities of the hydrated AT base pair were found to be positive both for n = 5 and 13 and have the values of 0.97 and 0.92 eV, respectively, which are almost three times the AEA of the AT base pair. The results show that the presence of water molecules appreciably enhances the EA of the base pair. © 2005 American Chemical Society.
All Valence Electron Molecular Orbital Study of Electronic Structure and Spectra of the three Isomeric Xylenes
(1972) J.S. Yadav; P.C. Mishra; D.K. Rai
The modified method of Del Bene and Jaffe1 (CNDO-CI) has been applied to study the electronic structure and spectra of the three isomeric xylenes taking a three dimensional geometry for the CH3 group. The spectral transition energies and the corresponding oscillator strengths have been computed and the results are compared with x-electron calculations and experimental results. Quite satisfactory results have been obtained. Ionization potentials have also been computed by the INDO and extended Huckel methods. © 1972 by Marcel Dekker, Inc. All Rights Reserved.
An ab initio and density functional study of microsolvation of carbon dioxide in water clusters and formation of carbonic acid
(2005) N.R. Jena; P.C. Mishra
Geometry of the CO2-H2O complex and reaction barriers leading to the formation of H2CO3were studied at the RHF/6-311++G**, MP2/6-311++G**, B3LYP/AUG-cc-pVDZ, B3LYP/AUG-cc-pVTZ, MP2/AUG-cc-pVDZ and CCD/AUG-cc-pVDZ levels of theory. The rotational barrier of the CO2-H2O complex and the reaction barrier leading to the formation of H2CO3-H2O from CO2-(H2O)2 were studied using the first three of the above-mentioned methods. Microsolvation of CO2 in water clusters having upto eight water molecules was studied using the B3LYP/AUG-cc-pVDZ method. Various methods except MP2/AUG-cc-pVDZ predict the equilibrium structure of the CO2-H2O complex to be symmetric while the MP2/AUG-cc-pVDZ method predicts it to be unsymmetric. Formation of H2CO3 from CO2-H2O is strongly catalyzed by the presence of a second water molecule. Atomic orbitals are strongly rehybridized in going from the equilibrium structures of the CO2-H2O and CO2-(H2O)2 complexes to the transition states involved in the formation of H 2CO3 and H2CO3-H2O, respectively, as shown by hybridization displacement charges. © Springer-Verlag 2005.
An ab initio study of electronic spectra and excited-state properties of 7-azaindole in vapour phase and aqueous solution
(Elsevier, 1998) M.K. Shukla; P.C. Mishra
The geometries of 7-azaindole (7AI), its tautomer (7AT), and 7AI-H2O and 7AT-H2O complexes were optimised in the ground state and some low-lying singlet excited states using the 3-21G basis set. Differences of total energies of the optimised ground and excited states and the vertical excitation energies of these systems were used to explain the observed electronic spectra. Effect of solvation of these systems in bulk water was studied using the polarized continuum model (PCM). The mode of binding of a water molecule in the S2(n-π*) excited state of 7AI was found to be quite different from those in its ground and π-π* excited states. It is shown that crossing of the lowest two singlet excited-state potential surfaces S1(π-π*) and S2(n-π*) of 7AI occurs in the vapour phase under geometry relaxation while on interaction with water, the S2(n-π*) excited state is raised up appreciably going even above the S3(π-π*) excited state. Ground- and excited-state molecular electrostatic potential mapping was carried out, which led to valuable information regarding the nature of excited states of the above-mentioned systems.
An ab initio study of electronic structure and spectra of 8-bromoguanine: A comparative study with guanine
(Elsevier, 2001) S.K. Mishra; P.C. Mishra
Ground state geometries of the four tautomeric forms keto-N9H, keto-N7H, enol-N9H and enol-N7H of 8-bromoguanine (8BG) were optimized using the ab initio RHF procedure employing a mixed basis set consisting of the 6-311 + G* basis set for the nitrogen atom of the amino group and the bromine atom, and the 4-31G basis set for all other atoms. These calculations were followed by correlation correction of the total energy at the MP2 level using the same basis set. The different tautomeric forms of 8BG in the ground state were solvated using the isodensity surface polarized continuum model (IPCM) of the SCRF theory both at the RHF and MP2 levels. Excited states were generated employing configuration interaction among singly excited configurations (CIS) obtained using a limited window of filled and empty molecular orbitals. Formation of hydrogen-bonded complexes between 8BG and three water molecules in the ground and excited states was considered in order to account for solvent effects approximately. Excited state geometry was optimized in each case for the lowest singlet excited state which was found to be of π-π* type. Vibrational frequency analysis was performed in order to ensure that the stationary points located on the potential energy surfaces by geometry optimization were minima. It is found that 8BG would occur in the ground state dominantly in the keto-N7H form both at the aqueous solution-air interface and inside the bulk liquid. The observed absorption and fluorescence spectra of 8BG can be explained satisfactorily considering only the keto-N7H form of the molecule. The enol tautomers of 8BG do not appear to be important from the point of view of ground state properties or electronic spectra. The observed differences between the behaviors of guanine and 8BG can be easily explained on the basis of the results obtained. © 2001 Elsevier Science B.V. All rights reserved.
An ab initio study of excited state molecular electrostatic potential maps and other related properties of 5-fluorouracil: A comparative study with uracil
(2001) M.K. Shukla; A. Kumar; P.C. Mishra
An ab initio study of electronic transitions of 5-fluorouracil (5FU) has been carried out using ground and excited state geometry optimization and the results thus obtained are compared with those of uracil. Excited states of the molecules were generated using configuration interaction considering single electron excitations (CIS) from the filled to the unfilled molecular orbitals. The 6-31G* basis set was used for all the calculations. While uracil is unstable in its lowest singlet π-π* excited state, 5FU, like thymine, is stable in the corresponding π-π* excited state. Further, both uracil and 5FU are planar in the ground state but uracil in the lowest singlet n-π* excited state and 5FU in the lowest singlet n-π* and π-π* excited states are appreciably nonplanar. The MEP map for the lowest singlet n-π* excited state of uracil, drawn in a plane located at a distance of about 1.8 Å from the molecular plane, shows a localized reactive region above the C5C6 bond which is involved in photodimerization. However, this feature is missing in the corresponding map of 5FU. Further, there is no such feature in the S2 (π-π*) excited state MEP map of uracil that can be correlated with the property of photodimerization which seems to suggest that the photodimerization reaction in uracil, occurring under ultraviolet irradiation, dominantly involves covalent interactions. © 2001 Elsevier Science B.V.