Devashis Majumdar

Excited-state intramolecular proton transfer and rotamerism of 2-(2'-hydroxyphenyl) benzimidazole

Abstract Excited-state intramolecular proton transfer (ESIPT) and rotamerism of 2-(2′-hydroxyphenyl benzimidazole (HPBI) have been studied using steady-state and time-resolved emission spectroscopy and by semi-empirical quantum-chemical methods. Two ground-state rotamers ( I and II ) with distinct excitation and emission spectra and lifetimes have been identified. Excitation of one of them ( I ) produces the normal emission while the other after excitation undergoes ultrafast ESIPT to form the tautomer ( III ) with Stokes-shifted emission. At 77 K the tautomer emission is markedly supressed as the rotamer, II , responsible for it, is less stable than I . CNDO/S-CI calculations were performed optimising the ground-state geometry by the AM1 method. These calculations give good estimates of the ground- and excited-state energies of the rotamers I and II and the tautomer, III .

Global optimization of molecular geometry: A new avenue involving the use of Metropolis simulated annealing

The viability of what has been known as the Metropolis simulated-annealing method (MSAM) in the optimization of molecular geometry is tested. The ability of MSAM to seek out the global energy minimum on a potential energy hypersurface with multiple minima is demonstrated in model calculations.

Molecular electrostatic potential : a tool for the prediction of the pharmacophoric pattern of drug molecules

Abstract Molecular electrostatic potential (MEP) calculations were used to explain the diverse pharmacological properties of γ-aminobutyric acid (GABA), the celebrated inhibitory neurotransmitter, and several GABA mediators. MEP calculations were also performed on several bipyridine cardiotonics, to account for their observed cardiotonic properties.

Ground and excited states of benzil: A theoretical study

Abstract The ground and low-lying excited state potential energy curves of benzil have been studied. The ground state geometry is fully optimized by the AM1 method. The excited states are calculated by using the CNDO/S-CI method. The calculations confirm that benzil in the ground state has a skewed conformation whereas the first excited singlet and triplet states of this molecule are trans-planar. The geometry relaxation in the excited states of benzil agrees well with the experimental findings. The absorption and emission bands of benzil have been compared with the observed bands. The dipole moments of the ground and excited states at some conformations are also reported. The effect of solvents on the electronic states of benzil has been studied using the continuum dielectric model.

Effect of Configuration Interaction on Crystal Field Splitting of Fe2+ Ion

The effect of excited configurations 3d54p, 3d54d, and 3d54f on the crystal field levels Γ3 and Γ5 of tetrahedral and octahedral Fe2+ ion via crystal field interaction has been discussed. The shift of the Γ3 and Γ5 levels due to configuration interaction has been estimated in the two cases within the framework of crystal field model carrying out the perturbation calculation up to second order and using Slater functions for the radial integrals. Such calculation shows that in the case of tetrahedral Fe2+ ion in ZnS, the net effect of the excited configurations 3d54p, 3d54d, and 3d54f on the crystal field spectra is to close up the levels Γ5 and Γ3 by an appreciable amount of about 1000 cm−1 and the major part of the contribution comes from 3d54p. In the octahedral case also (Fe2+ in MgO), the effect arising from 3d44d only is not negligible although comparatively smaller, the separation (Γ3−Γ5) is narrowed by about 380 cm−1.

Modelling of reaction paths from bond-order profiles : a local versus global description of A-B-C → B-C-A photoisomerization

Abstract Quantum-chemical bond-order profiles are shown to be useful as ‘local’ reaction path indicators. A few photochemical isomerization reactions of the type A-B-C → B-C-A were studied in this context. The bond-energy bond-order principle was used to interpret some of the observed features. Some consequences of a ‘local’ version of the Hammond postulate proposed earlier is also tested in the present context.

Hyperpolarizabilities of twisted-intramolecular-charge-transfer (TICT) molecules: A case study on p-dimethylaminobenzonitrile (DMABN) and p-dimethylaminonitrosobenzene (DMANB)

Abstract The hyperpolarizabilities of DMABN and DMANB in the ground and a number of excited states (vertical) have been calculated by combining the MNDO geometry optimization with finite-field SCF-CI at the CNDO/S level of approximation. The procedure has been calibrated with respect to p -nitroaniline (PNA). The role of twisting around the ue5f8Cue5f8Nue608bond — so important in the TICT process — and of pyramidality at the dimethylamino nitrogen centre in shaping hyperpolarizabilities in different electronic states is examined.

On the ground and excited state molecular electronic structures and properties of benzanilide and its tautomer and their solvent dielectric modulations

Abstract The electronic and structural properties of benzanilide have been computed in the gas phase as well as in various solvents with the help of the CNDO/S-CI reaction-field (RF) method in the image-charge representation. The ground state geometries have been optimized with the AM1 method. The predicted normal S1 → S0 fluorescence of the amide form of the molecule, as observed experimentally also, has a mixed nπ ∗ + ππ ∗ character. The imidol tautomer is also predicted to have similar S1′ → S0′ fluorescence. The predicted magnitudes of ΔES1′ − S0′ of the imidol form and ΔES1 − S0 of the amide form compare well with the experimental results. The theoretically predicted ΔES0′ − S0 value also concurs with the experimental observation. The singlet-triplet splittings of both the tautomers of benzanilide are also well reproduced. The computed dipole moments, the Cue5f8N and the Cue5f8O bond orders and the energies for different conformations have also been reported for both the tautomers in the ground as well as excited states, shedding some light on their important structural features and their solvent modulations.

Solvent modulation of the first hyperpolarizability of DMABN: an approximate theoretical analysis

Abstract An approximate molecular orbital theoretical calculation of the solvent modulation of the frequency-dependent first hyperpolarizability β(ω) of p-dimethylaminobenzonitrile (DMABN) shows a significant solvent polarity effect, especially in the low dielectric constant (∈) region. The zero-frequency β turns out to be a quadratic function of the solvent polarity function f(∈).

QUANTUM CHEMICAL BOND ORDER AS A LOCAL DESCRIPTOR FOR MODELLING REACTION PATHS

The feasibility of studying chemical reactions by focusing attention only on the bonds actively involved in the reaction process is explored. Active bond order (BO) profiles are shown frequently to be characterized by the occurrence of sharp inflexions at a point close to the saddle point on the reaction path. The sum of the active BOs is shown to remain approximately conserved. Active BOs may therefore be used as a local descriptor of the reaction path. This leads to extended bond-energy bond-order (EBEBO) postulates which are proposed to transcribe an essentially local picture of a reaction as a superposition of elementary chemical events into a semi-global description. A local Hammond postulate and its ramifications are explored along with a local measure of the asynchronicity of the bond-breaking and bond-making processes in a chemical reaction. The origin of BO inflexions is theoretically sought to be connected with Fermi correlation.