A tritium nuclear magnetic resonance study is carried out on the T 2 , HT, and DT isotopomers of dihydrogen dissolved in various nematic phases, including a zero-electric-field-gradient mixture. Ab initio calculations are performed to reproduce the observed dipolar couplings. Within the framework of the ''mean-field'' approximation, the results provide support for a picture in which two independent contributions to the solute orientation can be distinguished. One contribution involves a liquid-crystal-dependent interaction between the mean solvent electric-field gradient and the solute molecular quadrupole moment. The other contribution is of unknown origin; however, it is essentially identical in all liquid crystals and it can be modeled adequately with a phenomenological mean-field interaction. A remarkable feature of this second interaction is that it causes the average orientation of the asymmetrical isotopomers, and especially of HT, to behave differently from the symmetrical species.
Labeled arylium ions XC6H4+ (X = NOz, CN, C1, Br, OH, OCH3) (ortho, 40%; meta, 40%; para,\ud
20%) from the decay of uniformly multitritiated arenes XC6H5 have been allowed to react with\ud
methanol in the liquid and gaseous phase (P = 5-65 Torr) and with methyl halides in the gas\ud
phase (P = 10-760 Torr) at room temperature. The isomeric composition of the products from\ud
02NCsH4+, characterized by a pronounced depletion of the ortho isomers, is regarded as due to\ud
effective singlet-carbenic to biradicalic state crossing in o-O2NC&+, favored by proximity between\ud
a lone pair of the NO2 substituent and the formally vacant orbital. The same process is prevented\ud
in o-NCC6H4+ by unfavorable arrangement of the corresponding orbitals. Nuclear decay Of XCsH5\ud
(X = C1, Br, OCH3) in liquid CH30H generates the corresponding methoxy derivatives in proportions\ud
reflecting those of their corresponding XC6H4+ precursors (ortho:meta:para = 2:2:1). The specific\ud
solvation mode in o-HOC&+ allows instead its extremely fast conversion to phenoxenium ion\ud
C6H50+, prior to addition to CH30H. The nature and the isomeric composition of the labeled\ud
products formed in gaseous CH30H (5-65 Torr) and methyl halides (10-760 Torr) point to Xc&+\ud
ions (X = C1, Br) with no tendency to undergo ring-hydrogen migration. In HOC6H4’ ions, instead,\ud
intramolecular 1,2-hydrogen transfers readily take place whose phenomenological rate constants\ud
have been evaluated. In CH30C6H4’ ions, the same process is obscured by a rapid l,4-hydride ion\ud
transfer in o-CH30C6H4+ from the methyl moiety to the vacant ring orbital. The effects of the X\ud
substituent upon the behavior of arylium ions in both gaseous and liquid media have been compared\ud
with the data of previous related studies and are discussed in light of theoretical predictions
The reactivity and selectivity of radiolytically (60Co y rays) generated unsolvated trifluoromethylium (CF3+) ions toward selected aromatic substrates have been measured under conditions where nonionic processes can be safely ruled out. The same situation could not be attained for the trichloromethylium (CC13+) ion, owing to the overwhelming interference of CC13 radicals, by far the major reactive species produced by the y radiolysis of gaseous carbon tetrachloride. The unsolvated CF3+ ion is a moderate electrophile, whose attack on differently substituted aromatic substrates is rather indiscriminate; its positional selectivity toward a toluene molecule (para/0.5 meta = 4.3) appears intermediate between the one of t-C3H7+ ions (para/0.5 meta = 2.7) and that displayed by the very mild t-C4H9+ cation (para/0.5 meta = 35).Publication costs assisted by Consigno Nazionale deile Ricerche, RomaThe triplet state lifetime for 3-benzoylpyridine has been determined in aqueous solution at different pH values. The lifetime was found to decrease with decreasing pH and with increasing concentration of 3-benzoylpyridine. These findings are interpreted in terms of a complex formation with hydroxonium ion and of self-quenching of the triplet. Kinetic parameters for these interactions have been determined. The results obtained are compared with those of benzophenone.
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