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Pan S.,Indian Institute of Technology Kharagpur | Gupta A.,Udai Pratap Autonomous College | Mandal S.,Indian Institute of Technology Kharagpur | Chattaraj P.K.,Indian Institute of Technology Kharagpur
Physical Chemistry Chemical Physics | Year: 2015

Ab initio computations are carried out to explore the structure and stability of FNgEF3 and FNgEF (E = Sn, Pb; Ng = Kr-Rn) compounds. They are the first reported systems to possess Ng-Sn and Ng-Pb bonds. Except for FKrEF3, the dissociations of FNgSnF3 and FNgEF, producing Ng and SnF4 or EF2, are only exergonic in nature at room temperature, whereas FNgPbF3 has a thermochemical instability with respect to two two-body dissociation channels. However, they are kinetically stable, having positive activation barriers (ranging from 2.2 to 49.9 kcal mol-1) with respect to those dissociations. The kinetic stability gradually improves in moving from the Kr to Rn analogues. The remaining possible dissociation channels for these compounds are found to be endergonic in nature. The nature of the bonding is analyzed by natural bond order, electron density, and energy decomposition analyses. Particularly, the natural population analysis reveals that they are best represented as F-(NgEF3)+ and F-(NgEF)+. All the Xe/Rn-E bonds in FNgEF3 and FNgEF are covalent in nature. © the Owner Societies 2015. Source


Gupta A.,Udai Pratap Autonomous College | Jaeger H.M.,University of Rochester | Compaan K.R.,University of Georgia | Schaefer H.F.,University of Georgia
Journal of Physical Chemistry B | Year: 2012

The guanine-cytosine (GC) radical anion and its interaction with a single water molecule is studied using ab initio and density functional methods. Z-averaged second-order perturbation theory (ZAPT2) was applied to GC radical anion for the first time. Predicted spin densities show that the radical character is localized on cytosine. The Watson-Crick monohydrated GC anion is compared to neutral GC•H2O, as well as to the proton-transferred analogue on the basis of structural and energetic properties. In all three systems, local minima are identified that correspond to water positioned in the major and minor grooves of macromolecular DNA. On the anionic surface, two novel structures have water positioned above or below the GC plane. On the neutral and anionic surfaces, the global minimum can be described as water interacting with the minor groove. These structures are predicted to have hydration energies of 9.7 and 11.8 kcal mol-1, respectively. Upon interbase proton-transfer (PT), the anionic global minimum has water positioned in the major groove, and the hydration energy increases to 13.4 kcal mol-1. PT GC•H2O•- has distonic character; the radical character resides on cytosine, while the negative charge is localized on guanine. The effects of proton transfer are further investigated through the computed adiabatic electron affinities (AEA) of GC and monohydrated GC, and the vertical detachment energies (VDE) of the corresponding anions. Monohydration increases the AEAs and VDEs by only 0.1 eV, while proton-transfer increases the VDEs substantially (0.8 eV). The molecular charge distribution of monohydrated guanine-cytosine radical anion depends heavily on interbase proton transfer. © 2012 American Chemical Society. Source


Mukherjee V.,Sambalpur University | Singh N.P.,Udai Pratap Autonomous College
Spectrochimica Acta - Part A: Molecular and Biomolecular Spectroscopy | Year: 2014

Molecular structure in optimum geometry and vibrational frequencies of pentafulvalene [bicyclopentyliden-2,4,2′,4′-tetraene], tetrathiafulvalene [2,2′-bis(1,3-dithiolylidene)] and its cation are calculated. All the calculations are carried out by employing density functional theory incorporated with a suitable basis set. Normal coordinate analysis is also employed to scale the DFT calculated frequencies and to calculate potential energy distributions. The molecular structures and vibrational frequencies are compared for both the pentafulvalene and tetrathiafulvalene molecules. The effect upon geometry and vibrational frequencies of TTF due to charge transfer has also been studied. The vibrational partition function and hence, the thermodynamical properties, such as Helmholtz free energy, entropy, specific heat at constant volume and enthalpy are also calculated and compared for the title molecules. The reason of conductivity of tetrathiafulvalene has been tried to explain on the basis of molecular geometry and normal modes. Study of vibrational partition function exhibits that below 109 K, PFV starts to condense. © 2013 Elsevier B.V. All rights reserved. Source


Singh V.B.,Udai Pratap Autonomous College
RSC Advances | Year: 2014

The conformational landscapes of neutral caffeine and its hydrated complex have been investigated by MP2 and DFT methods. The ground state geometry optimization yields six lowest energy structures for bare caffeine and five lowest energy conformers of the caff1-(H2O)1 complex at the MP2/6-311++G(d,p) level of theory for the first time. We investigated the low-lying excited states of bare caffeine by means of coupled cluster singles and approximate doubles (CC2) and TDDFT methods and a satisfactory interpretation of the electronic absorption spectra (Phys. Chem. Chem. Phys., 2012, 14, 10677-10682) is obtained. The difference between the S0-S1 transition energy due to the most stable and the least stable conformation of caffeine was found to be ∼859 cm-1. One striking feature is the coexistence of the blue and red shift of the vertical excitation energy of the optically bright state S1 (1ππ∗) of caffeine upon forming a complex with a water at isolated and conjugated carbonyl sites, respectively. The lowest singlet ππ∗ excited-state of the caff1-(H2O)1 complex involving isolated carbonyl is strongly blue shifted which is in agreement with the result of R2PI spectra of singly hydrated caffeine (J. Chem. Phys., 2008, 128, 134310). While for the most stable and the second most stable caff1-(H2O)1 complexes involving conjugated carbonyl, the lowest singlet ππ∗ excited-state is red shifted. The effect of hydration on the S1 (1ππ∗) excited state due to the bulk water environment was mimicked by a combination of a polarizable continuum solvent model (PCM) and a conductor like screening model (COSMO), which also shows a blue shift in accordance with the result of electronic absorption spectra in an aqueous solution (Phys. Chem. Chem. Phys., 2012, 14, 10677-10682). This hypsochromic shift is expected to be the result of the changes in the π-electron delocalization extent of the molecule because of hydrogen bond formation. This journal is © 2014 The Royal Society of Chemistry. Source


The spectra and structures of theophylline monomer and dimer and their hydrated complex have been investigated by MP2 and DFT methods. The ground state geometry optimization yields five lowest energy conformers of the Tph1-(H2O)1 complex at the M06-2X/6-311++G(d,p) level of theory for the first time. We investigated the low-lying excited states of bare theophylline by means of coupled cluster singles and approximate doubles (CC2) and TDDFT methods and a satisfactory interpretation of the electronic absorption spectra (Phys. Chem. Chem. Phys., 2012, 14, 10677-10682) is obtained. One striking feature is the coexistence of the blue and red shift of the vertical excitation energy of the optically bright state S1 (1ππ∗) of theophylline upon forming a complex with water at C2=O and C6=O carbonyl sites, respectively. The optimized structure of newly characterized theophylline dimer Form IV is computed for the first time by MP2 and DFT methods. The binding energy of this dimer linked by double N-H⋯O=C hydrogen bonds was found to be 88 kJ mol-1 at the MP2/6-311++G(d,p) level of theory. The geometry optimization of dimer Form M has also been performed and it is found that further stability is being conferred to the dimer IV after hydration. Computed IR spectra are found to be in remarkable agreement with the experiment (Cryst. Growth Des. 2010, 10, 3879-3886) and the out of phase (C=O)2 stretching mode shows a tripling of intensity upon dimerisation. The vertical excitation energy of the optically bright state S1 (1ππ∗) of the theophylline monomer upon forming dimer IV is shifted towards red as well as blue. © The Royal Society of Chemistry 2015. Source

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