Photodissociation of alkyl and aryl iodides and effect of fluorination:: Analysis of proposed mechanisms and vertical excitations by spin-orbit ab initio study

An ab initio study of the vertical electronic excitations in CX3I, C6X5H, and C6X5I (X=H and F) is presented. All-electron basis sets are used and the relativistic effects are accounted for with the relativistic elimination of small components scheme. The structures are optimized with the complete active space self-consistent field approach and the excitation energies are computed with the spin-orbit multiconfiguration quasidegenerate perturbation theory. The n-sigma(*) transitions of CX3I, low-lying pi-pi(*) transitions of C6X5H, and low-lying n-sigma(*), pi-pi(*), and pi-sigma(*) transitions of C6X5I are elucidated. For CH3I, energy values of parallel and perpendicular transitions differ from experimental values by 455 and 1156 cm(-1), respectively. Effects of fluorination are emphasized, it is found that fluorination increases the gap between (3)Q(0) and (1)Q(1) transitions and increase is substantially more in aryl iodides than in alkyl iodides. Electronic factors influencing increased I-* quantum yield in the photodissociation on fluorination of alkyl iodides is attributed to increased gap between (3)Q(0) and (1)Q(1) transitions reducing curve crossing probability and for aryl iodides there is additional role by phenyl transitions. A correlation diagram illustrating transitions of aryl iodides is presented. (C) 2002 American Institute of Physics.