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 CX[SUB3]I, C[SUB6]X[SUB5]H, and C[SUB6]X[SUB5]I (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 spinorbit multiconfiguration quasidegenerate perturbation theory. The n-σ∗ transitions of CX[SUB3]I, low-lying π-π∗ transitions of C[SUB6]X[SUB5]H, and low-lying n-σ∗, π-π∗, and π-σ&lowasr; transitions of C[SUB6]X[SUB5]I are elucidated. For CH[SUB3]I, energy values of parallel and perpendicular transitions differ from experimental values by 455 and 1156 cm[SUP-1], respectively. Effects of fluorination are emphasized, it is found that fluorination increases the gap between [SUP3]Q[SUB0] and [SUP1]Q[SUB1] 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 [SUP3]Q[SUB0] and [SUP1]Q[SUB1] 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. [ABSTRACT FROM AUTHOR]