3916-45-8Relevant articles and documents
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Hazlet,VanOrden
, p. 2505,2506 (1942)
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Relationship between molecular association and re-entrant phenomena in polar calamitic liquid crystals
Mandle, Richard J.,Cowling, Stephen J.,Goodby, John W.,Sage, Ian,Colclough, M. Eamon
, p. 3273 - 3280 (2015/09/02)
The relationship between molecular association and re-entrant phase behavior in polar calamitic liquid crystals has been explored in two families of materials: the 4'-alkoxy-4-cyanobiphenyls (6OCB and 8OCB) and the 4'-alkoxy-4-nitrobiphenyls. Although re-entrant nematic phase behavior has previously been observed in the phase diagram of 6OCB/8OCB, this is not observed in mixtures of the analogous nitro materials. As there is no stabilization of the smectic A phase in mixture studies, it was conjectured that the degree of association for the nitro systems is greater than that for the cyano analogues. This hypothesis was tested by using measured dielectric anisotropies and computed molecular properties to obtain a value of the Kirkwood factor, g, which describes the degree of association of dipoles in a liquid. These computed values of g confirm that the degree of association for nitro materials is greater than that for cyano and offer a useful method for quantifying molecular association in systems exhibiting a re-entrant polymorphism.
4′-substituted-4-biphenylyloxenium ions: Reactivity and selectivity in aqueous solution
Novak, Michael,Poturalski, Matthew J.,Johnson, Whitney L.,Jones, Matthew P.,Wang, Yueting,Glover, Stephen A.
, p. 3778 - 3785 (2007/10/03)
Azide trapping shows that the 4′-substituted-4-biphenylyloxenium ions 1b-d are generated during hydrolysis of 4-aryl-4-acetoxy-2,5-cyclohexadienones, 2c and 2d, and O-(4-aryl)phenyl-N-methanesulfonylhydroxylamines, 3b and 3c. In addition, the 4′-bromo-substituted ester, 2d, undergoes a kinetically second-order reaction with N3 that accounts for a fraction of the azide adduct, 5d. Since both first-order and second-order azide trapping occurs simultaneously in 2d, the second-order reaction is not enforced by the short lifetime of 1d, which has similar azide/solvent selectivity to the unsubstituted ion, 1a. In contrast the 4′-CN and 4′-NO2 ions 1e and 1f cannot be detected by azide trapping during the hydrolysis of the dichloroacetic acid esters 2e′ and 2f′ even though 18O labeling experiments show that a fraction of the hydrolysis of both esters occurs through Calkyl-O bond cleavage. These esters exhibit only second-order trapping by azide. Correlations of the azide/solvent selectivities of 1a-d with the calculated relative driving force for hydration of the ions (ΔE of eq 4) determined at the pBP/DN*//HF/6-31G* and BP/6-31G*//HF/6-31G* levels of theory suggest that 1e and 1f have lifetimes in the 1-100 ps range. Ions with these short lifetimes are not in diffusional equilibrium with nonsolvent nucleophiles, and must be trapped by such nucleophiles via a preassociation mechanism. The second-order trapping that is observed in these two cases is enforced by the short lifetime of the cations, and may occur by a concerted SN2′ mechanism or by internal azide trapping of an ion sandwich produced by azide-assisted ionization. Comparison of azide/solvent selectivities of the oxenium ions 1a-c with the corresponding biphenylylnitrenium ions 8a-c shows that 4′-substituent effects on reactivity in both sets of ions are similar in magnitude, although the nitrenium ions are ca. 30-fold more stable in an aqueous environment than the corresponding oxenium ions. The magnitude of the 4′-substituent effects for electron-donating substituents suggest that both sets of ions are more accurately described as 4-aryl-1-imino-2,5- cyclohexadienyl or 4-aryl-1-oxo-2,5-cyclohexadienyl carbocations. Calculated structures of the oxenium ions are also consistent I with this interpretation.
