• S Ramasesha

      Articles written in Journal of Chemical Sciences

    • Is tetramethyleneethane a ground state triplet?

      Aparna Chakrabarti I D L Albert S Ramasesha S Lalitha Jayaraman Chandrasekhar

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      We have examined a number of possible ways by which tetramethyleneethane (TME) can be a ground state triplet, as claimed by experimental studies, in violation of Ovchinnikov’s theorem for alternant hydrocarbons of equal bond lengths. Model exact π calculations of the low-lying states of TME, 3,4-dimethylenefuran and 3,4-dimethylenepyrrole were carried out using a diagrammatic valence bond approach. The calculations failed to yield a triplet ground state even after (a) tuning of electron correlation, (b) breaking alternancy symmetry, and (c) allowing for geometric distortions. In contrast to earlier studies of fine structure constants in other conjugated systems, the computedD andE values of all the low-lying triplet states of TME for various geometries are at least an order of magnitude different from the experimentally reported values. Incorporation of σ mixing by means of UHF MNDO calculations is found to favour a singlet ground state even further. A reinterpretation of the experimental results of TME is therefore suggested to resolve the conflict.

    • Lattice embedding and equilibrium geometry of metal-halogen chains in the two-band extended Hubbard model

      Y Anusooya S Ramasesha

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      Two-band extended Hubbard model studies show that the shift in optical gap of the metal-halogen (MX) chain upon embedding in a crystalline environment depends upon alternation in the site-diagonal electron-lattice interaction parameter (εM) and the strength of electron-electron interactions at the metal site (UM). The equilibrium geometry studies on isolated chains show that the MX chains tend to distort for alternating εM and smallUM values.

    • Frontiers in chemistry - Foreword

      S Ramasesha

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    • Quantum phenomena in magnetic nano clusters

      C Raghu Indranil Rudra Diptiman Sen S Ramasesha

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      One of the fascinating fields of study in magnetism in recent years has been the study of quantum phenomena in nanosystems. While semiconductor structures have provided paradigms of nanosystems from the stand point of electronic phenomena, the synthesis of high nuclearity transition metal complexes have provided examples of nano magnets. The range and diversity of the properties exhibited by these systems rivals its electronic counterparts. Qualitative understanding of these phenomena requires only a knowledge of basic physics, but quantitative study throws up many challenges that are similar to those encountered in the study of correlated electronic systems. In this article, a brief overview of the current trends in this area are highlighted and some of the efforts of our group in developing a quantitative understanding of this field are outlined.

    • A density matrix renormalization group study of low-lying excitations of polythiophene within a Pariser-Parr-Pople model

      Mosumi Das S Ramasesha

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      Symmetrized density-matrix-renormalization-group calculations have been carried out, within Pariser-Parr-Pople Hamiltonian, to explore the nature of the ground and low-lying excited states of long polythiophene oligomers. We have exploitedC2 symmetry and spin parity of the system to obtain excited states of experimental interest, and studied the lowest dipole allowed excited state and lowest dipole forbidden two photon state, for different oligomer sizes. In the long system limit, the dipole allowed excited state always lies below the lowest dipole forbidden two-photon state which implies, by Kasha rule, that polythiophene fluoresces strongly. The lowest triplet state lies below two-photon state as usual in conjugated polymers. We have doped the system with a hole and an electron and obtained the charge excitation gap and the binding energy of the 11Bu exciton. We have calculated the charge density of the ground, one-photon and two-photon states for the longer system size of 10 thiophene rings to characterize these states. We have studied bond order in these states to get an idea about the equilibrium excited state geometry of the system. We have also studied the charge density distribution of the singly and doubly doped polarons for longer system size, and observe that polythiophenes do not support bipolarons.

    • Computing magnetic anisotropy constants of single molecule magnets

      S Ramasesha Shaon Sahoo Rajamani Raghunathan Diptiman Sen

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      We present here a theoretical approach to compute the molecular magnetic anisotropy parameters, $D_M$ and $E_M$ for single molecule magnets in any given spin eigenstate of exchange spin Hamiltonian. We first describe a hybrid constant $M_S$-valence bond (VB) technique of solving spin Hamiltonians employing full spatial and spin symmetry adaptation and we illustrate this technique by solving the exchange Hamiltonian of the Cu6Fe8 system. Treating the anisotropy Hamiltonian as perturbation, we compute the D$_M$ and E$_M$ values for various eigenstates of the exchange Hamiltonian. Since, the dipolar contribution to the magnetic anisotropy is negligibly small, we calculate the molecular anisotropy from the single-ion anisotropies of the metal centers. We have studied the variation of D$_M$ and E$_M$ by rotating the single-ion anisotropies in the case of Mn12Ac and Fe8 SMMs in ground and few low-lying excited states of the exchange Hamiltonian. In both the systems, we find that the molecular anisotropy changes drastically when the single-ion anisotropies are rotated. While in Mn12Ac SMM $D_M$ values depend strongly on the spin of the eigenstate, it is almost independent of the spin of the eigenstate in Fe8 SMM. We also find that the $D_M$ value is almost insensitive to the orientation of the anisotropy of the core Mn(IV) ions. The dependence of $D_M$ on the energy gap between the ground and the excited states in both the systems has also been studied by using different sets of exchange constants.

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