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Browsing Chemistry - Publications by Author "Agranat, Israel"
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    A new class of nonlinear optical materials based on push-pull quinonoid molecules
    ( 1995-01-01) Ravi, M. ; Rao, D. Narayana ; Cohen, Shmuel ; Agranat, Israel ; Radhakrishnan, T. P.
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    Dual Influence of H-Bonding on the Solid-State Second-Harmonic Generation of a Chiral Quinonoid Compound
    ( 1998-01-01) Ravi, Mosurkal ; Gangopadhyay, P. ; Rao, D. Narayana ; Cohen, Shmuel ; Agranat, Israel ; Radhakrishnan, T. P.
    A new chiral push-pull quinonoid compound, 7,7-bis(3(R)-hydroxypyrrolidino)-8,8-dicyanoquinodimethane (BHPDQ), has been synthesized. Powder studies indicate an appreciable second harmonic generation (SHG) capability, and crystal structure analysis reveals extensive intermolecular H-bonding in BHPDQ. The influence of H-bonding and molecular chirality on the dipole alignment in the crystal is described. The H-bonds play an unusual role of modifying the molecular structure by producing an enhanced molecular twist in BHPDQ; this in turn affects the molecular hyperpolarizability. Thus we describe a novel instance wherein the powder SHG in a molecular material is determined by the dual influence of intermolecular H-bonding on the crystal structure as well as on the molecular structure and hyperpolarizability. The new dimension that this observation adds to the general theme of molecular materials design is highlighted.
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    Evaluation of Y- versus linear-delocalization and the role of charge alternation in trimethylenemethane dianion and butadiene dianions
    ( 1991-06-21) Agranat, Israel ; Radhakrishnan, T. P. ; Herndon, William C. ; Skancke, Anne
    Experimental results and theory have generally been taken to indicate that cross-conjugated trimethylenemethane dianion is more stable than isomeric linear butadiene dianions, engendering the theoretical concepts of Y-aromaticity and stabilizing topological charge alternation. The viabilities of the concepts are tested for these dianions by using RHF ab initio calculations with a series of polarization and diffuse function basis sets. At high levels of theory the relative energies of trimethylenemethane and butadiene dianions have changed to the extent that an explicit preference for the dianion Y-topology is no longer supported. Charge alternation also does not seem to be a dominating effect in stabilizing the C4H6-2 species. The present results indicate that neither factor is a significant stabilizing structural principle for the isolated dianions. © 1991.
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    Measures of pyramidalization
    ( 1991-02-01) Radhakrishnan, T. P. ; Agranat, Israel
    This article reviews common definitions of pyramidalization. An appraisal of the quantification of pyramidalization follows, and the formulation of a new measure of pyramidalization. The versatility of the new measure is tested using correlation with some molecular properties. © 1991 VCH Publishers, Inc.
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    Molecular and crystal structures of a class of push-pull quinonoid compounds with potential nonlinear optical applications
    ( 1996-01-01) Ravi, M. ; Cohen, Shmuel ; Agranat, Israel ; Radhakrishnan, T. P.
    Molecular and crystal structures of three compounds with electron donor-conjugation unit-electron acceptor framework are reported where the conjugation unit is formally a quinonoid ring. Semiempirical quantum chemical calculations indicate large hyperpolarizabilities in these push-pull molecules. Since all the crystal structures are found to belong to centrosymmetric space groups, modifications are necessary for quadratic nonlinear optical applications. © 1996 Plenum Publishing Corporation.
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    Molecular and crystal structures of a class of push-pull quinonoid compounds with potential nonlinear optical applications
    ( 2015-09-09) Ravi, M. ; Cohen, Shmuel ; Agranat, Israel ; Radhakrishnan, T. P.
    Molecular and crystal structures of three compounds with electron donor-conjugation unit- electron acceptor framework are reported where the conjugation unit is formally a quinonoid ring. Semiempirical quantum chemical calculations indicate large hyperpolarizabilities in these push-pull molecules. Since all the crystal structures are found to belong to centrosymmetric space groups, modifications are necessary for quadratic nonlinear optical applications.
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    Optical second harmonic generation in achiral bis(n-alkylamino)dicyanoquinodimethanes: alkyl chain length as the design element
    ( 1999-01-01) Gangopadhyay, P. ; Sharma, S. ; Rao, A. Jaganmohan ; Rao, D. Narayana ; Cohen, Shmuel ; Agranat, Israel ; Radhakrishnan, T. P.
    Chain length of substituted n-alkyl groups is proposed to be a convenient design element for the generation of noncentrosymmetric crystal lattices of interest in optical second harmonic generation (SHG). Powder studies on 7,7-Bis(n-alkylamino)-8,8-dicyanoquinodimethanes indicate that moderate solid-state optical SHG is obtained when the alkyl chains are of length 4, 5, and 6, and no detectable SHG occurs when the chains are of length 0, 3, 7, 8, and 12. The significant role of the intermediate alkyl chain length in generating noncentric crystal lattices is examined using the crystal structures of the propyl, butyl, and octyl derivatives presented in this paper. © 1999 American Chemical Society.
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    Push-Pull Quinonoid Compounds: Enhanced Powder SHG Utilizing the Effect of Chiral Centers on the Dipole Alignment
    ( 1997-01-01) Ravi, M. ; Rao, D. Narayana ; Cohen, Shmuel ; Agranat, Israel ; Radhakrishnan, T. P.
    The molecular and crystal structure of a chiral diamino-substituted dicyanoquinodimethane and the observation of strong solid-state optical second harmonic generation (SHG) in this material are reported. The enhanced SHG was obtained by exploiting the strong influence of the rigidity of the stereogenic center environment on the alignment of the molecular dipole vectors. The intervector angles in this class of materials are found to correlate well with the powder SHG. Since a strong molecular twist (of the diaminomethylene unit with respect to the quinonoid ring) of ∼50° is found in this class of compounds, the dependence of the molecular hyperpolarizability (β) on the twist was examined. The interesting observation was that the computed β attained the maximum value at the twist angle corresponding to the optimized molecular structure, which in turn closely resembled the structure obtained from single-crystal X-ray analysis. This unusual behavior of β is explained on the basis of the two-state model.
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    Strategic placement of stereogenic centers in molecular materials for second harmonic generation
    ( 2002-03-01) Gangopadhyay, P. ; Narayana Rao, D. N. ; Agranat, Israel ; Radhakrishnan, T. P.
    Basic aspects of the nonlinear optical phenomenon of second harmonic generation (SHG) and the assembly of molecular materials for SHG are reviewed. Extensive use of chirality as a convenient tool to generate noncentrosymmetricity in molecular lattices, an essential requirement for the development of quadratic nonlinear optical materials, is noted. An overview of our investigations of chiral diaminodicyanoquinodimethanes is presented, which provides insight into a systematic approach to the effective deployment of chirality to achieve optimal molecular orientations for enhanced solid state SHG. Extension of these ideas to the realization of strong SHG in materials based on helical superstructures is outlined.
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    Strong optical second-harmonic generation in a chiral diaminodicyanoquinodimethane system
    ( 1996-01-01) Ravi, M. ; Rao, Narayana ; Cohen, Shmuel ; Agranat, Israel ; Radhakrishnan, T. P.
    A series of amine donor-substituted dicyanoquinodimethane molecules with large calculated hyperpolarizabilities has been investigated. Molecular and crystal structure details of a prototypical system are presented. Moderate to strong phase-matched powder second-harmonic generation is observed in chiral amine-substituted compounds.
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    Two-point contact chiral distinction—a theoretical appraisal
    ( 2002-01-01) Radhakrishnan, T. P. ; Topiol, Sid ; Biedermann, P. Ulrich ; Garten, Sarit ; Agranat, Israel
    Ab initio calculations reveal chiral distinction in two-point contact CHFClBr dimers, with chiral distinction energy of 1.5 kJ mol-1between the SR and SS dimers fully optimized at the MP2/6-311++G** level. © 2002 The Royal Society of Chemistry.