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Raman and infrared spectra of isopropyl nitrate and isobutyl nitrate are reported. These spectra are used in combination with computational studies employing density functional theory at the B3-LYP/6-31G* level to assign the vibrational transitions to their corresponding normal coordinates. Similar to other alkyl nitrates, the frequency of the NO2 symmetric stretch remains relatively unchanged while the asymmetric stretch shifts to lower frequency with increasing α-carbon substitution. The mode assignments involving the photochemically relevant &z.sbnd;ONO2 chromophore agree well with those from previous infrared work. Raman depolarization ratios are also presented, and provide evidence that the condensed phase, ground-state molecular structure of isobutyl nitrate is of Cs symmetry. In contrast, the minimum energy structure of isopropyl nitrate is predicted to contain a pronounced twist around the C&z.sbnd;O bond relative to the Cs-symmetry structure that lies 2.6 kcal/mol higher in energy. Infrared intensities of isopropyl nitrate are consistent with the twisted geometry, demonstrating that this conformer is favored in solution. [Copyright &y& Elsevier]

Abstract: In order to explore the effects of incorporation of an amino-phenyl-thienyl (APT) donor moiety into state-of-the art donor-bridge-acceptor organic NLO chromophores, two new materials were synthesized and characterized. Density functional theory (DFT) quantum-mechanical modeling indicates that this structural modification, producing enhanced ground-state electron density asymmetry and increased conjugation length, should lead to appreciable enhancement in first molecular hyperpolarizability (β). Previous empirical data suggests that such an approach to enhancements in nonlinearity may overcome the tradeoffs in nonlinearity with optical absorption and thermal stability, often encountered in organic NLO chromophore design. Material properties were evaluated in terms of poling-induced electro-optic coefficients normalized against applied poling field (r 33/E p), DFT calculated β and gas-phase dipole moment, experimentally determined β HRS, absorption maxima, and thermal stability. These data were then compared against those from benchmark materials. Experimental data were determined to reflect enhancements in nonlinearity with little detriment to thermal stability and optical absorption, making them excellent candidates for use in electro-optic polymer applications. The most active material (C3) demonstrated highly reproducible values of r 33 =130pm/V. [Copyright &y& Elsevier]