Photodissociation of C–H and C–O bonds of p-methoxytoluene and p-methoxybenzyl alcohol in solution

Abstract

Picosecond real time study of the bimolecular reaction O ( 3 P)+ C 2 H 4 and the unimolecular photodissociation of CH 3 CHO and H 2 COThe photodissociation of p-methoxytoluene and p-methoxybenzyl alcohol at 266 nm in n-heptane solution is studied by nanosecond fluorescence and absorption spectroscopy. The formation of a p-methoxybenzyl radical is identified by its fluorescence which is induced by excitation at 308 nm. The yields of the radical are of the order of ϳ10 Ϫ3 for dissociation of p-methoxytoluene and p-methoxybenzyl alcohol. The growth rate of 1.5ϫ10 8 s Ϫ1 for the radical is equal to the decay rate of (1.5Ϯ0.3)ϫ10 8 s Ϫ1 for the precursor fluorescence in dissociation of p-methoxytoluene, whereas the growth rate of Ͼ1.0ϫ10 9 s Ϫ1 for the radical is much faster than the decay rate of (1.8Ϯ0.3) ϫ10 8 s Ϫ1 for the precursor fluorescence in dissociation of p-methoxybenzyl alcohol. The formation of the radical depends linearly on the photolysis pulse fluence for dissociation of p-methoxytoluene and p-methoxybenzyl alcohol. The data show existence of two distinct dissociation channels. p-Methoxytoluene dissociates from thermally equilibrated levels of the S 1 state after vibrational relaxation, whereas p-methoxybenzyl alcohol dissociates from vibrationally excited levels of the S 1 state in competition with vibrational relaxation. The difference of these channels is explained on a model of electronic coupling between the precursor and product states in the geometry where the C-H and C-O bonds are stretched in a plane perpendicular to the benzene rings. For p-methoxytoluene, the S 1 state does not correlate adiabatically to the ground state of the C-H bond fission products, so intersystem crossing or internal conversion precedes dissociation. For p-methoxybenzyl alcohol, avoided crossing between the * ͑benzene͒ configuration and the np(O)*(C-O) repulsive configuration results in the adiabatic potential-energy surface which evolves to the ground state of the C-O bond fission products allowing rapid dissociation.

Monitoring of the Heterogroup Twisting Dynamics in Phenol Type Molecules via Different Characteristic Free-Electron-Transfer Products

“…(ii) We further assume that competing with the fast deprotonation for S 1* type conformers, and more or less affected by an interconversion between S 2* and S 1* due to an −OH, −SH, or −SeH group twisting (internal rotation), the excited conformers will rapidly relax with a typical rate constant of k rel ∼ 10 11 s -1 …”