RESUMO
Nucleogenic cations were formed by beta-decay of phenylsilane tritiated at ortho- and para-positions of the benzene ring as well as at the silyl group and the products of their reactions with methyl tert-butyl ether were analyzed by radiochromatography. We found that the o-silatolyl cation was isomerized into the silabenzyl cation while the p-silatolyl cation was not. Furthermore, the silabenzyl cation was not converted into other isomers. The potential energy surface of the C(6)H(7)Si(+) system was constructed by B3LYP and MP2 methods using an aug-cc-pVDZ basis set. Theory predicts the low barrier for o-silatolyl-silabenzyl isomerization but high barriers for hydride shifts in the ring from para to meta- to ortho-isomers. It seems that nascent nucleogenic ions have enough internal energy to overcome the ortho-to-benzyl barrier but not enough to cross over the hydride-shift barriers. Theory also confirms that the isomerization of the silabenzyl cation to the [C(6)H(6).SiH](+) complex takes place in one step with a 62 kcal mol(-1) barrier, whereas that to either the global minimum [C(6)H(7).Si](+) or the silatropylium ion involves multisteps with 69-80 kcal/mol barriers. In addition, we find that the barrier of interconversion between the [C(6)H(6).SiH](+) complex and one of the low-lying [C(6)H(7).Si](+) complexes is only 29 kcal/mol.
RESUMO
Exposure of solid thymine and uracil at room temperature to free methyl cations, produced due to beta-decay of tritiated methane, resulted in formation of their 1-, O2-, 3-, O4-, and 6-methyl derivatives. In addition, uracil formed a 5-methyl derivative (thymine); tritium-containing thymine and uracil were also detected. Both thymine and uracil formed predominantly unidentified products which resulted presumably from their oligomerization. Incubation at -195 degrees C did not markedly change the pattern of reaction products. Aqueous-ammonia solutions of these pyrimidines formed methylated derivatives and considerable amounts of methanol and tritiated water. The possible implication of these reactions in mutagenic and carcinogenic effects of tritium-substituted hydrocarbons is discussed.
