Laser photofragmentation processes of the doubly charged ions SiF 2 2ϩ and SiF 3 2ϩ are studied at 1064, 532, and 355 nm. The observed photoproducts are compared with those of the reaction pathways induced by the collisions of these dications with the rare gases. Photodissociation, via absorption of a single photon occurs readily for both dications, and the photoion yields are dominated by the products of neutral-loss pathways: SiF 2ϩ from SiF 2 2ϩ and SiF 2 2ϩ from SiF 3 2ϩ . A minor contribution from photoinduced charge separation is also observed. For SiF 2 2ϩ , a neutral-loss pathway forming the atomic dication Si 2ϩ is also detected at higher photon energies. The excitation energy required for this fragmentation process is determined to be 2.2Ϯ0.2 eV. Collisions of SiF 2 2ϩ and SiF 3 2ϩ with He and Ne are also dominated by neutral-loss reactions. With the heavier rare gases, charge transfer is exothermic and dominates the product ion yield, although neutral-loss reactions still occur. Interpretation of the charge transfer results using Landau-Zener theory suggests that two electronic states of SiF 2 2ϩ , lying at 31.0Ϯ0.5 eV and 35.5Ϯ0.5 eV above the ground state of the neutral molecule, are present in the dication beam. Ab initio calculations reveal that SiF 3 2ϩ adopts an equilibrium geometry of C 2v symmetry with one elongated bond. Hence, electron capture by SiF 3 2ϩ results in the formation of SiF 3 ϩ in an energetically unfavorable conformation. By taking account of these geometry differences in the Landau-Zener algorithm, a satisfactory rationalization of the ion yields from the charge transfer reactions of SiF 3 2ϩ is achieved.

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Solid-state 13C NMR and 2H NMR techniques have been used to investigate structural and dynamic properties of the 1,4-dicyanobutane/urea and 1,5-dicyanopentane/urea 1:1 hydrogen-bonded complexes and the 1,6-dicyanohexane/urea inclusion compound. The pure crystalline phase of urea has also been investigated. The 13C NMR studies have focused on 13C chemical shift anisotropy and second-order quadrupolar effects (arising from 13C-14N interaction) for the urea molecules and the cyano groups of the alpha,omega-dicyanoalkanes. Parameters describing these interactions are derived and are discussed in relation to the known structural properties of these materials. Comparison of 13C chemical shift anisotropies of the cyano carbons and rates of 13C dipolar dephasing suggest that 1,4-dicyanobutane and 1,5-dicyanopentane are effectively static, whereas 1,6-dicyanohexane has greater mobility. 2H NMR line shape analysis for the 1,4-dicyanobutane/urea-d4 and 1,5-dicyanopentane/urea-d4 complexes indicates that the only motion of the urea molecules that is effective on the 2H NMR time scale is a rapid libration about the C=O bond over an angular range of about 26 degrees . For the 1,6-dicyanohexane/urea-d4 inclusion compound, the 2H NMR line shape is consistent with a motion comprising 180 degrees jumps about the C=O bond at rates that are intermediate on the 2H NMR time scale. In addition, rapid libration about the C=O bond also occurs over an angular range of about 20 degrees . The dynamic properties of the urea molecules in these materials are compared with those of urea molecules in other crystalline environments.

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