PRIMARY ISOTOPE EFFECTS ON EXCITED-STATE HYDROGEN-ATOM TRANSFER-REACTIONS - ACTIVATED AND TUNNELING MECHANISMS IN AN ORTHO-METHYLANTHRONE

Phosphorescence intensities and lifetimes of 1,4-dimethylanthrone (1,4 -MAT) and 1,4-dimethylanthrone-d(8) (1,4-DMAT) were measured to determ ine the involvement of activated and quantum mechanical tunneling mech anisms (QMT) in their hydrogen and deuterium atom transfer reactions. The thermal-dependence of the radiative and thermal decay of the anthr one chromophore and the effect of methyl substitution were evaluated b y using anthrone (AT), 2,3-dimethylanthrone, (2,3-MAT), and 10,10-dime thylanthrone (10,10-MAT). Measurements were carried out in methylcyclo hexane (MCH) glasses between 18 and 80 K. The unreactive molecules AT, 2,3-MAT, and 10,10-MAT present phosphorescence parameters typical of diarylketones with (3)n,pi configurations and show a relatively small temperature dependence changing from monoexponential at 77 K to nonex ponential at the lowest temperature values. The phosphorescence intens ity from 1,4-DMAT was extremely weak and highly temperature-dependent. In contrast, no phosphorescence was detected in 1,4-MAT at all temper atures studied. Differences between deuterio and protio compounds were analyzed in terms of a large isotope effect on the hydrogen atom tran sfer reaction. A quantum mechanical tunneling mechanism was confirmed from nonlinear Arrhenius plots constructed with the average deuterium transfer rates of 1,4-DMAT. A temperature-independent quantum mechanic al tunneling reaction with a rate of 2 x 10(3) s(-1) was calculated be tween 30 and 18 K. The involvement of reaction was confirmed by accumu lation and detection of the photoenol product in ethanol glasses at al l the temperatures studied. Changes in phosphorescence intensity obser ved even under conditions where the triplet lifetimes remain constant (18-30 K) were analyzed in terms of an avoided crossing mechanism pred icted by orbital and state symmetry considerations.