Digital Computer System

Barddhamān, India

Digital Computer System

Barddhamān, India
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The mechanism of substitution reaction of chloroaqua and diaqua complexes, trans-[Pt(2-pic)(NH3)Cl(H2O)]+ and trans-[Pt(2-pic)(NH3)(H2O)2]2+, which are formed after intracellular aquation of the anticancer drug AMD443, with the DNA purine bases guanine (G) and adenine (A) were studied computationally by TST-DFT method. The aqua ligand of such complexes undergoes ligand exchange reactions with N7 atom of G or A as the preferential nucleophilic centers. The predominance of attack toward G over A has been observed. We have estimated the degree of trigonality (τ) and synchronicity (σy) for all transition states. The computed free energy of activation (ΔG‡(aq)) for chloroaqua complexes are 15.1 kcal/mol and 18.5 kcal/mol for guanine and adenine, respectively. The respective values for the diaqua complexes are higher. For the bifunctional adduct we observed that GG adduct with head-to-head (HH) conformation prefers over GA adduct by ≈8.5 kcal/mol and over the GG head-to-tail (HT) conformation by ≈0.9 kcal/mol. © 2013 Elsevier B.V. All rights reserved.


Pandey P.,Indian Association for The Cultivation of Science | Pandey P.,Indian Central Mechanical Engineering Research Institute | Chakraborty T.,Indian Central Mechanical Engineering Research Institute | Mukherjee A.K.,Digital Computer System
Molecular Physics | Year: 2013

Ab initio theory at the HF/6-311G(d,p) level has been used to compute the hydrogen bonding thermodynamics in bulk liquid ethanol. Inter-cluster hydrogen bonding is assumed to mimic the H-bonding in bulk ethanol. Rotation of the clusters has been neglected, but translational and vibrational motions are taken into account for calculating bulk thermodynamic parameters. Results are well in agreement with an earlier report [J. Chem. Phys. 116, 4212 (2002)]. For a more accurate dipole moment of monomer, MP2/6-311++G(d,p) calculation was done. Use of the computed thermodynamic data in a statistical model yields the Kirkwood-Frohlich correlation factor and the dielectric constant of ethanol (21.0) close to the experimental value, 24.3 at 298 K. © 2013 Taylor & Francis.


Tiwary A.S.,Netaji Mahavidyalaya | Mukherjee A.K.,Digital Computer System
Chemical Physics Letters | Year: 2014

The B3LYP and the M06 family of functionals were found to be of nearly equal efficiency for predicting the 13C and 15N NMR chemical shifts (which are ground state properties) of molecular complexes of TCNE with naphthalene and pyrene. But the B3LYP functional could not find any charge transfer (CT) absorption band of the complexes. The M06, M06-2X, M06-HF and M06-L functionals could find two CT absorption bands in CCl4 medium by a TDDFT calculation under the PCM formalism for solvation; the calculated CT transition energies are comparable with the reported experimental values, M06-2X and M06-L agreeing best with experiment. © 2014 Elsevier B.V.


Tiwary A.S.,Netaji Mahavidyalaya | Datta K.,Muc Womens College | Mukherjee A.K.,Digital Computer System
Computational and Theoretical Chemistry | Year: 2015

Ground state intermolecular interactions in the molecular complexes of TCNE with a series of methylated indoles have been predicted with almost equal efficiency by the B3LYP and the M06 family of functionals. However, TDDFT calculation with the B3LYP functional failed to find any charge transfer (CT) absorption band of the complexes while each of the functionals M06, M06-2X, M06-HF and M06-L succeeded; the calculated CT transition energies vary with the calculated vertical ionization potentials of the indoles, complying with Mulliken's theory of charge transfer complexes and are fairly close to reported experimental values. © 2015 Elsevier B.V.


Ghosh B.C.,Durgapur Government College | Mukherjee A.K.,Digital Computer System
Molecular Physics | Year: 2013

The pKα values of different dissociable groups of L-Aspartic and L-Glutamic acids in vacuo and in aqueous medium over a wide pH range have been estimated by DFT/B3LYP/6-31G(d) and 6-31G++(d,p) methods. For both the amino acids discrete water molecules (n = 0, 3 and 6) have been used to get the first hydration sphere. Starting from a low pH, all possible tautomers resulting from each dissociation step are assumed to exist in a cyclic equilibrium. The structures of the species involved in the dissociation and tautomerization processes have been optimized in vacuo and also in aqueous medium considering H-bonded water molecules under the PCM formalism. For obtaining pKa in aqueous medium the difference in Gibbs energy of the clusters H3O+.mH2O and (H2O) m+1 (m=an integer) is necessary and has been evaluated from computed literature data. Calculations reveal that in vacuo the neutral or less charged species predominate but in aqueous medium the zwitterionic or more chargeable forms contribute appreciably. The Gibbs energy changes for the microscopic cyclic equilibria have been estimated theoretically. These lead to overall (macroscopic) pKα values for the ionization steps which are in good agreement with available experimental data for both the amino acids. © 2013 Taylor & Francis.


The inductive effect of methyl group has been quantified by expressing highest occupied molecular orbital (HOMO) and HOMO-1 energies of indole and a series of methylated indoles using a combination of graph theory (GT) and the Coulson-Longuett-Higgins perturbation method. By correlating these expressions with the corresponding Kohn-Sham orbital energies of the indoles obtained by density functional theory (DFT) calculation at the B3LYP/6-31+ +G(d,p) and M06-2X/6-31+ +G(d,p) levels of theory, the inductive effect parameter h Me has been estimated; the Coulomb integral α of π-conjugated carbon atom also comes out from the analysis. A correlation of the GT results with the HOMO and HOMO-1 energies obtained by the HF/STO-3G method yield almost the same values of h Me and α. Finally, when these estimated h Me and α are used to calculate the vertical ionization potentials of the methylated indoles in the series, an excellent correlation with experimental charge transfer transition energies of their molecular complexes with tetracyanoethylene is obtained which complies with Mulliken's theory of charge transfer. [Figure not available: see fulltext.] © 2013 Indian Academy of Sciences.

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