• Sankar Prasad

      Articles written in Journal of Chemical Sciences

    • CNDO bonding parameters in transition metal atoms

      Sankar Prasad Bhattacharyya

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      The method of calculating CNDO bonding parameters developed recently is extended to transition metal atoms. It is shown that one of the approximations introduced earlier can also be deduced by a more complete treatment of the imbalance problem in CNDO-MO theory. The conventionally calibrated bonding parameters indirectly incorporate important contributions from two-particle interactions. The parameters developed are used to compute the coefficients of metal-to-ligand transfer of spin in many hexafluro metallate ions of transition metals. The results are compared with those obtained by conventional CNDO-MO calculation. Comparison of the computed bonding parameters with other available values is also made.

    • Change of molecular structure in the excited states: A (CNDO/2) hole-potential study on phosphine

      Manas Banerjee Sankar Prasad Bhattacharyya

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      Molecular structure of phosphine in a number of excited electronic states is studied using the method of hole-potential within the basic framework of CNDO/2 theory. Effects of including 3d-functions of phosphorus in the basis set on computed molecular geometries, transition energies and inversion barriers in the excited states have been investigated. An attempt is made to rationalise qualitatively the structural changes in the excitedstate in terms of Walsh-type correlation diagram constructed with the eigenvalues of the Fock operator in theVN-1 potential model. A simple orbital model for predicting the nature of structural changes in the excited states is proposed.

    • An average fock operator technique of approximate open-shell LCAO-MO-SCF calculation

      Sankar Prasad Bhattachararyya Mihir Chowdhury

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      The exchange part of the usual Hartree-Fock potential in the unrestricted Hartree-Fock (UHF) theory is suitablyaveraged to construct an, average one-electron model Hamiltonian which generates a set of spin-restricted one-electron orbitals in a self-consistent manner. These orbitals are then used to calculate the electronic energy of the open-shell system by using the proper functional form for the energy which handles the exchange terms correctly. The eigenvalues ofFav can be used for calculating either the spin-polarised or spin-averaged ionisation potentials of different orbitals at theKoopmans’ theorem level of approximation. Comparison ofEac with the UHF-energy shows thatEUHF<Eac in each case revealing some kind of an upper bound nature ofEac. An approximate variational argument is given. Relationship of our model with the hyper-Hartree-Fock theory of slater is explored and the general problem of eliminating ‘self-interaction’ terms in average Fock-operator based theories is discussed.

    • Applications of a novel algorithm for the calculation ofMCSCF wavefunction: a look into possible avenues of convergence acceleration

      Kalyan Das Kumar Debashis Mukherjee Sankar Prasad Bhattacharyya

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      The efficacy of a method based on the direct inversion in the iterative subspace (DIIS) in accelerating the approach to self consistency in the calculation of theMCSCF wavefunction using a novel algorithm developed earlier, is compared with that of a simple damping technique. Although the ‘damping’ turns out to be ineffective in the ‘quadratic region’, it accelerates remarkably in the rate of descent on the energy hypersurface in the early stages of the iterative process which leads to an impressive overall increase in the rate of approach to self consistency. TheDIIS based procedure turns out to be ineffective when coupled to the present method and is plagued by ill conditioning problems. Calculations are done to compute the equilibrium geometrical parameter, charge density on different atoms, and dipole moment of HNO molecule in the lowest1,3nπ* states at theINDO/2-MCSCF level.

    • The scaled one-electron Hamiltonian model for open-shelllcao-mo-scf calculations: further corrections to thesoeh energy

      Kalyan Kumar Das Sankar Prasad Bhattacharyya

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      The wavefunction generated by the scaled one-electron Hamiltonian (soeh) model has been modified further by including all single excitations from the space of thesoeh function. A perturbation-variation ansatz has been invoked for obtaining the corresponding energy correction (ΔE2). [E(soeh)+ΔE2] is shown to reproduceE(Roothaan) very closely. It has been demonstrated that by making ΔE2 stationary with respect to change in μ results. The correctedsoeh [E(soeh)+ΔE2] is shown to be quite useful for the calculation of geometrical parameters of open-shell systems even though it lacks the ‘upper-boundedness’ exhibited byE(soeh).

    • Structure and properties of a family of sugar vanadates incorporating VO3+

      Kajal Krishna Rajak Sankar Prasad Rath

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    • Dioxygen reactivity ofmeso-hydroxylated hemes: Intermediates in heme degradation process catalyzed by heme oxygenase

      Sankar Prasad Rath

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      Heme oxygenase (HO) is the only enzyme in mammals known to catalyse the physiological degradation of unwanted heme into biliverdin, Fe ion and CO. The process involves introduction of the hydroxyl group at one of itsmeso-positions as the first fundamental step of the heme cleavage process. It was also found thatmeso-amino heme undergoes similar ring-cleavage process while reacting with dioxygen in presence of pyridine as an axial ligand. The present paper briefly reviews the reactions of modelmeso-hydroxylated heme and its analogues with dioxygen, and their relevance in the heme degradation process.

    • Axial phenoxide coordination on di-iron(III) bisporphyrin: Insights from experimental and DFT studies

      Susovan Bhowmik Debangsu Sil Ranjan Patra Sankar Prasad Rath

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      Synthesis, structure and properties of new five-coordinate phenolate complexes of di-iron(III) bisporphyrin are reported here, in which phenol binds in $\eta^1$-fashion as an axial ligand. The solid and solution EPR at 120K and 1H NMR spectral pattern in solution provide unequivocal evidence for the high spin (S = 5/2) nature of the complex. Mulliken spin density calculation using DFT demonstrates the positive spin densities at the meso carbons and negative spin densities at the methylene carbons and, as a result, the meso and methylene protons are shifted in the upfield and down field regions, respectively in the 1H NMR spectra of the molecule. Also, the ortho- and para-protons of the phenolate ligands are observed to be shifted in the upfield region while meta-protons are shifted downfield. The alternating shift pattern, which is the opposite sign of the chemical shifts for meta-versus ortho- and para-protons, was also explained due to negative and positive spin densities, respectively on the carbons and indicative of 𝜋 spin delocalization on the phenolate ligand. Thus, the calculated spin density maps accounted for the essential 1H NMR spectroscopic features that are observed here for the phenolate complexes of di-iron(III) bisporphyrin. The temperature dependence of the signals follows the Curie law which is indicative of single spin state throughout the temperature range of −40 to +40°C. The single crystal X-ray structure of the corresponding chloro derivative, trans 1,2-bis(chloroiron(III) octaethyl porphyrinyl)ethene, has also been reported here which authenticates the high-spin nature of the complex.

    • Building-up novel coordination polymer with Zn(II) porphyrin dimer: Synthesis, structures, surface morphology and effect of axial ligands

      Sk Asif Ikbal Sanfaori Brahma Avinash Dhamija Sankar Prasad Rath

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      Highly flexible Zn(II)1,2-bis(meso-octaethylporphyrin)ethane (1) has been used as host in which two porphyrin rings are found to be face-to-face in non-coordinating solvents. Upon addition of one relatively smaller 4,4′-dipyridine (L1) and one extended N,N′-bispyridine-4-yl-methylene ethylenediamine (L2) guest ligands, the syn conformation of 1 is switched to the anti complexes 1·(L1)2 and 1·L2, respectively. Single crystal X-ray structures of all the complexes are reported in which a stable one-dimensional coordination polymer is produced only in 1·L2 that is, to the best of our knowledge, the first structural report of 1D-coordination polymer with porphyrin dimer. Solution structures of the complexes along with binding studies in solution between 1 and L have also been investigated. The morphology of the polymeric complex 1·L2 on silicon wafer surface was examined by Atomic Force Microscopy (AFM) in which the crystalline islands of well defined facets of size ranging from 200-550 nm perimeter and a height of 20-40 nm have been observed.

    • Iron(III) and copper(II) complexes of trans-bis(ferrocenyl)porphyrin: Effect of metal ions on long-range electronic communication

      Dipankar Sahoo Sankar Prasad Rath

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      A series of complex with a general formula of M(Fc2Ph2P) [Fc2Ph2P = 5, 10-bisferrocenyl-15,20-bisphenylporphyrin (2−); M = Fe(III)Cl Fe(III)(ClO4) and Cu(II)] have been synthesized and characterized. The single crystal X-ray structure of CuII(Fc2Ph2P) has been reported in which two ferrocene moieties are in anti form with respect to each other. The ferrocenyl groups of CuII(Fc2Ph2P) are more easily oxidized via a single two-electron quasi-reversible process compared to the free base ligand in which two 1e-oxidative response separated by 0.23 V are observed. Electrochemical study of FeIII(Fc2Ph2P)Cl revealed ferrocenebased two-electron quasi-reversible oxidation at 0.72 V indicating no observable coupling of the ferrocene moieties. The higher oxidation state of Fe(III) reduces the electron releasing tendency of the porphyrin ring and thus make the ferrocene oxidation difficult. The porphyrin, however, lack substituents at the 𝛽-pyrrolic positions, and the ferrocenyl moieties are therefore free to rotate. The observed electrochemical analyses thus demonstrate that the oxidation of the ferrocene subunit is strongly affected by porphyrin ring as well as the central metal through extended 𝜋-conjugation.

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