Quantum chemistry calculation studies of nonlinear electronic and vibrational properties
Graduate School Korea University
Department of Chemistry
Quantum chemistry nonlinear electronic vibrational properties;
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Two photon absorption (TPA) cross sections of four representative series of planar octupolar molecules and nonlinear optical (NLO) properties of tetrahedral octupolar molecules are theoretically investigated and studied by using ab initio calculation methods. In the case of planar octupolar molecules, as the charge-transfer character of the ground electronic state increases by introducing strong donors or acceptors, (ⅰ) the transition dipole matrix elements between the ground and two-fold degenerate excited states increase, (ⅱ) the energy gap decreases, and consequently (ⅲ) the TPA transition amplitude monotonically increases. In the case of tetrahedral octupolar molecules, as the charge-transfer character of the ground electronic state increases, the first hyperpolarizability monotonically increases. The linear relationship between the structural distortion of C=O bond and the amide I mode frequency of N-methylacetamide(NMA) solvated by a few water molecules is studied by carrying out a series of ab initio calculations for NMA-H₂O clusters. By using a multivariate least square fitting method, the effective transition charges of six NMA sites were determined to obtain general equation. Dielectric effects on the molecular structure and amide I mode frequency of NMA were theoretically investigated. The low-frequency shift of the amide I mode is found to be linearly proportional to the C=O bond length elongation. The solvatochromic amide I mode frequency shift and fluctuation were taken into account by carrying out molecular dynamics simulations of acetylproline dissolved in liquids water and chloroform and by using the extrapolation method developed recently. For isolated di- and tri-peptides, intramolecular ionic field effect on the amide I mode frequency shift is found to be large. The vibrational coupling strength between the two nearest neighboring peptides does not strongly depend on the side chain as long as the amide I vibrations are not strongly coupled to any of the side chain vibrational degrees of freedom. The linear relationship between the structural distortion of CN bond in modified amino-acids and the CN stretching mode frequency shift is shown to be strongly dependent on the three-dimensional structure.