KETO-ENOL TAUTOMERIZATION OF 2-(2'-HYDROXYPHENYL)BENZOXAZOLE AND 2-(2'-HYDROXY-4'-METHYLPHENYL)BENZOXAZOLE IN THE TRIPLET-STATE - HYDROGEN TUNNELING AND ISOTOPE EFFECTS .1. TRANSIENT ABSORPTION KINETICS

Hydrogen tunnel effects in the metastable triplet states of 2-(2'-hydroxyphenyl)benzoxazole (HBO) and 2-(2'-hydroxy-4'-methylphenyl) benzoxazole (m-MeHBO) and their deuteriooxy analogues DBO and m-MeDBO have been investigated between 35 and 250 K in three alkane solvents that have drastically different viscosities at low temperatures. In the triplet states of HBO the hydrogen transfer between the enol and keto tautomers is reversible because the two triplet states are isoenergetic. In m-MeHBO the hydrogen transfer is virtually unidirectional because the initially populated keto triplet state has a higher energy than the enol triplet state. In spite of these differences the observed hydrogen-transfer rate constants of the two compounds have nearly the same values. The temperature dependence of the rate constants is insensitive to large changes in solvent viscosity, i.e., hydrogen tunneling in HBO and m-MeHBO depends on solvent friction neither at intermediate nor at very low temperatures. The couples HBO, DBO and m-MeHBO, m-MeDBO are tbe first examples where the determination of tunneling rate constants has been extended for both isotopomers into a temperature region where the rates and the isotope effect on these rates become temperature independent.