Diffusion pyridine

Photoexcited fluorescence from spread monolayers may be studied [158,159] if the substance has both a strong absorption band and a high emission yield as in the case for chlorophyll [159]. Gaines and co-workers [160] have reported on the emission from monolayers of Ru(bipyridine)3, one of the pyridine ligands having attached C g aliphatic chains. Ruorescence depolarization provides information about the restriction of rotational diffusion of molecules in a monolayer [161], Combining pressure-area... 

Amines can also swell the polymer, lea ding to very rapid reactions. Pyridine, for example, would be a fairly good solvent for a VDC copolymer if it did not attack the polymer chemically. However, when pyridine is part of a solvent mixture that does not dissolve the polymer, pyridine does not penetrate into the polymer phase (108). Studies of single crystals indicate that pyridine removes hydrogen chloride only from the surface. Kinetic studies and product characterizations suggest that the reaction of two units in each chain-fold can easily take place further reaction is greatiy retarded either by the inabiUty of pyridine to diffuse into the crystal or by steric factors. 

In a previous work [1,2], we were interested in the calculation of second order hyperpolarizabilities of eonjugated systems including substituted benzenes, pyridine N-oxydes and vinyl oligomers, in relation with non linear optical activity [3]. We showed that MNDO ealeulations were in good agreement with SCF ab initio results obtained using a double zeta basis set plus polarization and diffuse orbitals. 

The mobilities of alkylpyridines were modeled and predicted in capillary zone electrophoresis.35 The model predicted that compounds adopt a preferred orientation, and additionally predicted mobilities of structural isomers to within 4%, a higher degree of accuracy than can be obtained from simple considerations of van der Waal s radius. Quantitative prediction of the mobilities of some pyridines, such as alkenylpyridines, was not possible. Mobilities of small solutes in capillaries filled with oligomers of ethylene glycol were related to solution viscosity and the diffusion coefficient.36... 

At negative potentials in alkaline solutions, adsorbed NA retains K+ ions, as demonstrated by Auger spectroscopy, Figure 5-B. This retention of K+ ions is due to interaction of K+ with the pendant carboxylate moiety and greatly exceeds the amounts expected simply from diffuse double-layer interactions. Potential-dependence of K+ retention is essentially absent for compounds incapable of potential-dependent carboxylate pendancy (pyridine, picolinic acid, isonicotinic acid and 2,6-pyridine dicarboxylic acid). 

Triad formation in Scheme 10 is a two-step process (88) involving the metastable A-nitropyridinyl radical, whereas the adiabatic electron transfer in Scheme 19 is likely to occur irreversibly with the simultaneous cleavage of the N-N02 bond, as in (89). As a result, the nascent pair (Py and N02) in (88) can suffer greater diffusive separation from ArH+- compared with that in (89). If so, the complexation of the aromatic cation radical by pyridine (90), as recently delineated by Reitstoen and Parker (1991) is (kinetically)... 

The relatively high mobilities of bromide and iodide in pyridine may be the result of poor anion solvation by the diffuse positive end of the pyridine dipole. The other aryl solvent shown in Fig. 9, nitrobenzene, appears to solvate somewhat more readily, though the nature of anion interaction is not clear. 

In another study (40) we found that protonation of pyridine is diffusion-controlled with a one-to-one solute-methanol complex as the reactive species. Thus, while methanol plays essentially no role in the proton transfer to dipicrylamine in the first study, it is indeed intimately involved in the proton transfer to pyridine. 

Not surprisingly, formalisms with very diffuse density functions tend to yield large electrostatic moments. This appears, in particular, to be true for the Hirshfeld formalism, in which each cos 1 term in the expansion (3.48) includes diffuse spherical harmonic functions with / = n, n — 2, n — 4,... (0, 1) with the radial factor rn. For instance when the refinement includes cos4 terms, monopoles and quadrupoles with radial functions containing a factor r4 are present. For pyridin-ium dicyanomethylide (Fig. 7.3), the dipole moment obtained with the coefficients from the Hirshfeld-type refinement is 62.7-10" 30 Cm (18.8 D), whereas the dipole moments from the spherical harmonic refinement, from integration in direct space, and the solution value (in dioxane), all cluster around 31 10 30 Cm (9.4 D) (Baert et al. 1982). 

Fig. 1. Counter-diffusion of (a) Benzene (B) vs. Pyridine (Py) and (b) ethylbenzene (EB) in Hydrogen Mordenite...

The sequence of IR spectra demonstrates that the molecules of the preloaded component A (pyridine, benzene) are displaced by the ingoing component B (benzene, ethylbenzene) when the preloaded sample is contacted with the vapour phase of the second compound. The process is slow because in both cases component A is more strongly held by the sorbent than component B (vide infra). But these experiments showed, that in principle, it should be possible to monitor counter-diffusion in zeolites via the IR method. 

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