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

摘要

An ab initio study of the vertical electronic excitations in CX_3I, C_6X_5H, and C_6X_5I (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 CX_3I, low-lying pi-pi~* transitions of C_6X_5H, and low-lysing n-sigma~*, pi-pi~*, and pi-sigma~* transitions of C_6X_5I are elucidated. For CH_3I, 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 ~3Q_0 and ~1Q_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 ~3Q_0 and ~1Q_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.