George V. D. Tiers

Materials science of organic compounds: Part 2. Thermal studies of acentric (second-harmonic-generative) and centrosymmetric crystals of achiral straight-chain para-nitrophenylcarbanyl compounds

Abstract Straight-chain alkyl and analogous polymethylene p -nitrophenylurethanes and ureas yield acentric crystals having nonlinear-optical (NLO) properties. DSC onset points are shown to agree with metal-calibrated capillary melting ranges within experimental error. Heats of fusion and melting/onset points for the alkylurethanes (C 1 -C 22 ) show a sharp minimum at C 4 -C 5 . The high second-harmonic-generation (SHG) maximum at C 6 appears unrelated. Several of the alkylurethanes (C 2 , C 5 , C 7 , and C 16 ), when crystallized using different solvents or temperatures, yielded SHG-inactive forms differing in heats of fusion, though not in onset points; in other cases lower heats of fusion upon remelting suggested analogous changes. Similar behavior was noted for some ureas and for some omega-substituted polymethylene urethanes.

Computational Modelling And Design Considerations Of Nonlinear Optical Materials

Compounds having large second-order nonlinear optical (NLO) response are those composed of suitable donor and acceptor groups in a delocalized π-system. In addition to the relative strengths of the donor/acceptor groups, the performance of a NLO compound depends also on the nature of the conjugated π-system, the molecular conformation and other geometric factors. To design better NLO materials, it is essential to have a good understanding of these factors. These molecular properties are addressed from the theoretical point of view. Their relation to NLO response will also be discussed.

NMR DETECTION OF ORIENTED ASSOCIATION VIA DILUTION SHIFTS IN TETRAMETHYLSILANE SOLVENT. 1. N-ALKYL HALIDES

Infinite‐dilution deshielding values, in one or more specified pure solvents, serve to characterize a known or unidentified molecule, even in mixtures. The usual solvents tend to interact strongly with solutes, thus concealing weak intermolecular associations. By definition, all solvents interact somewhat with solutes. The most favorable solvent in which to detect weak self‐association of a solute must be a non‐polar one having low and spherically symmetrical polarizability. Tetramethylsilane (TMS) is the most practical choice. The slopes, HSV and CSV, of linear plots (by volume in TMS) of 1H and 13C deshielding values for n‐alkyl halides are reported. These slopes (termed ‘sticking indices’) differ substantially within the molecules, being largest at the halogen position and thus demonstrating oriented self‐association. The slopes increase with substituent polarizability, not with electronegativity. Substantial negative CSV but near‐zero HSV for all chain methylene groups is tentatively ascribed to alteration of conformational equilibria upon dilution. The reference TMS line position was located by means of its 29Si satellites (for 13C) and its 13C and 29Si satellites (for 1H). The latter show an unusual previously unreported positive isotope shift of +0.7 ppb. Although perdeuterocyclohexane was used for the NMR ‘lock,’ perdeutero‐TMS, if available, would permit much greater latitude in dilutions. Copyright