[en] The reactivity and selectivity of radiolytically (⁶⁰Co γ 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 (CCl₃⁺) ion, owing to the overwhelming interference of CCl₃ radicals, by far the major reactive species produced by the γ 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 i-C₃H₇⁺ ions (para/0.5 meta = 2.7) and that displayed by the very mild t-C₄H₉⁺ cation (para/0.5 meta = 35). 4 figures; 5 tables; 29 references

[en] A previous radiolytic study on the gas-phase methylation of pyrrole and N-methylpyrrole by CH³XCH³⁺ (X = F or Cl) ions, from the ß radiolysis of CH³X, is extended to furan (3) and thiophene (4). The mechanism of the substitution and of the subsequent isomerization occurring via intramolecular 1,2 methyl-group shifts is discussed and the substrate and positional selectivity of the selected electrophilic species evaluated. As for pyrroles, gas-phase CH³FCH³⁺ methylation of furan and thiophene is characterized by a scarce substrate discrimination (k_Sk_B = 1.2 (3), 0.8 (4)), accompanied by an appreciable positional selectivity toward those substrate positions with the highest negative net charge (O:α:ß = 36%:35%:29% for 3; S:α:ß = 19%:43%:38% for 4). On the contrary, CH³ClCH³⁺ confirms its inherent affinity toward n-type nucleophilic centers by attacking preferentially the heteroatom of 3 and 4. In light of the previous results concerning CH³XCH³⁺ methylation of pyrroles, it is concluded that gas-phase attack of CH³XCH³⁺ on simple five-membered heteroaromatics is essentially regulated by the electrostatic interaction established within the encounter pair. A close correspondence does exist between this rationalization of the present gas-phase results and recent theoretical predictions

[en] A tritium nuclear magnetic resonance study is carried out on the T₂, 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 open-quotes mean-fieldclose quotes 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. copyright 1997 The American Physical Society