Structure-efficiency relationships in hydrogen photoabstraction reactions by ketones: thermal activation versus nuclear tunnelling

Radiationless transitions and photochemical reactivity

“…Then dCO= 0.308 A° and the value used in the calculation is the average of the two values, d = (dC_Q + d_H)/2, i. e., d = 0.377 A0. The calculated rate constant for tunnelling for the H-abstraction of (n,ir ) benzophenone from isopropanol is 5.Ox 1O5M1s1, in excellent agreement with the experimental value L4.5 x iO M1 51; other relevant data D(CH) = 367.8 kJ mor1, D(OH) = L&35 ki mor1, Ee 287.8 kJ mor1 (18). At the transition state we have neglected any resonance interaction which effectively decreases slightly the harrier height and slightly thickens the barrier at the top.…”

“…Many other alkoxy-type intermolecular and intramolecular H-photoabstraction reactions by l,3(n,ir*) and 3(ir, *) carbonyl compounds with several substrates have been studied, and calculations support the conclusion that at room temperature tunnelling is always a few orders of magnitude higher than thermal activation (15,18). Reactions which have a strong charge-transfer character have n* values higher than the radical-type reactions and consequently have smaller d values.…”

“…However large temperature changes affect the physical properties of the solvent medium and can also affect the kinetics of chemical reactions (20). Arnaut et al (18) have shown that the study of quantum efficiencies under current experimental conditions provides a criterium to distinguish between the mechanisms of tunnelling and thermal activation.…”

“…For example E was estimated as 22000 cm1 for the reaction between benzophenone 3(n,r*) and isopropanol (18). The effective reduced mass is i112 4I2 ÷ The reactions from n,ir* triplet and singlet states which will be discussed here are adiabatic processes (15).…”

“…However for a tunnelling mechanism the same situation only arises when w is below the reactive excited state, i. e., E<E(Ti or si); when E>E(Ti or i) the crossing point will not be found along the reaction coordinate and r= 1.0. Table 1 estimated from the change in C 0 bond lengths betwen S and T1 and the ratio of frequencies; the values (S0) -d(T1) = 0.12 A0 and f50 = 60x104 cmA02 (18) were employed in all calculations.…”