219640-39-8Relevant articles and documents
Structural Characterization of Crystalline Ternary Inclusion Compounds at the Air-Water Interface
Plaut, David J.,Martin, Stephen M.,Kjaer, Kristian,Weygand, Markus J.,Lahav, Meir,Leiserowitz, Leslie,Weissbuch, Isabelle,Ward, Michael D.
, p. 15922 - 15934 (2003)
Crystalline ternary inclusion monolayers consisting of a two-dimensional hydrogen-bonded host network of guanidinium (G) ions and organosulfonate (S) amphiphiles, and biphenylalkane guests, can be generated at the air-water interface through synergistic structural enforcement by hydrogen bonding and host-guest packing. Surface pressure-area isotherms of the 4′ -hexadecylbiphenyl-4-sulfonate (C16BPS) amphiphile in the presence of G, with or without guest, are characterized by lift-off molecular areas expected for the GS sheet based on single-crystal X-ray structures of homologous bulk crystals. Intercalation of biphenylalkane guests (4-CnH 2n+1-C6H4-C6H5, n = 1, 4, 6, 10, 16; denoted CnBP) between organosulfonate hydrophobes, which define pocketlike cavities in the GS monolayer host, afford ternary inclusion monolayers with a 1:1 host-guest stoichiometry. These inclusion monolayers are less compressible than the guest-free host, consistent with dense packing of the biphenylalkane moieties of the host and the biphenylalkane guests. The inclusion monolayers are distinguished from the amorphous guest-free host and from selected guanidinium-free mixed monolayers by structural characterization with grazing-angle incidence X-ray diffraction (GIXD). The GIXD data for the ternary (G)C16BPS:C16BP and (G)C16BPS:C6BP inclusion monolayers obtained upon compression are consistent with a rectangular unit cell. The dimensions of these unit cells and refinement of the GIXD data suggest a "rotated shifted ribbon" GS hydrogen-bonding motif similar to that observed in some bulk GS crystals, including (G) (ethylbiphenylsulfonate). GIXD reveals that (G)C16BPS:C16BP and (G)C16BPS:C6BP are more crystalline than the corresponding guanidinium-free mixed monolayers. The (G)C16BPS:C6BP inclusion monolayer is stable upon compression, even though the alkyl-alkyl host-guest interactions are reduced due to the shorter hexyl substituents of the guest, demonstrating an important reinforcing role for the hydrogen-bonded GS sheet. The structure of a C16BPS: tetracosane (C24) mixed monolayer is independent of G; the unit cell symmetry and dimensions suggest a structure governed by alkyl-alkane interactions that prohibit formation of a GS network. These results illustrate that the existence of ternary inclusion monolayers with an intact GS network requires guest molecules that are structurally homologous with the hydrophobes of the host, in this case biphenylalkanes. The observation of these inclusion compounds suggests an approach for introducing functional nonamphiphilic molecules to an air-water interface through inclusion in a well-defined host.
New insights into the mechanism of triplet radical-pair combinations. The persistent radical effect masks the distinction between in-cage and out-of-cage processes
Chesta, Carlos A.,Mohanty, Jyotirmayee,Nau, Werner M.,Bhattacharjee, Urbashi,Weiss, Richard G.
, p. 5012 - 5022 (2008/02/07)
Steady-state and laser-pulsed irradiations of dibenzyl ketone (ACOB 0) and derivatives with a p-methyl or a p-hexadecyl chain (ACOB 1 and ACOB16, respectively) have been conducted in polyethylene films with 0, 46, and 68% crystallinities. Calculation of the fractions of in-cage combinations of the triplet benzylic radical-pair intermediates based on photoproduct yields, Fc, from ACOB 16 are shown to be incorrect as a result of the kinetic consequences of drastically different diffusion coefficients for the benzyl and p-hexadecylbenzyl radicals. Careful analyses of the transient absorption traces, based upon a new model developed here, allow the correct cage effects to be determined even from ACOB0. The model also permits the rate constants for radical-pair combinations and escape from their cage of origin to be calculated using either an iterative fitting procedure or a very simple one which requires only k-CO and the intensities of the transient absorption immediately after the flash and after the in-cage portion of reaction by the benzylic radicals is completed. Values of the rate constant for decarbonylation of the initially formed arylacetyl radicals, k-CO, have been measured from the rise portions of the laser-flash transient absorption traces. They confirm the assertion from results in liquid alkane media that decarbonylation rates are independent of microviscosity. The data separate components of a reaction from an (in-cage) "cage effect" and an (out-of-cage) "persistent radical effect" that are responsible for formation of AB-type (i.e., decarbonylated) products. The effects here are a consequence of vastly different rates of diffusion for coreacting A· and B· benzylic radicals rather than segregation of the radicals in different parts of a hetereogeneous environment (which leads to an excess of AA and BB products). Heretofore, observation of exclusive formation of AB products has been attributed to in-cage combinations of geminate radical pairs. We show that not to be the case here and provide methodologies which may be used for testing the importance of the "persistent radical effect" component of reaction.
