| Abstract
| - The photochemistry of isomeric methoxyphenyl chlorides and phosphates has been examinedin different solvents (and in the presence of benzene) and found to involve the triplet state. With the chlorides,C−Cl bond homolysis occurs in cyclohexane and is superseded by heterolysis in polar media, while thephosphate group is detached (heterolytically) only in polar solvents. Under such conditions, the isomerictriplet methoxyphenyl cations are the first formed intermediates from both precursors, but intersystemcrossing (isc) to the singlets can take place. Solvent addition (forming the acetanilide in MeCN, the ethersin alcohols, overall a SN1 solvolysis) is a diagnostic reaction for the singlet cation, as reduction and trappingby benzene are for the corresponding triplet. Solvolysis is most important with the meta isomer, for whichthe singlet is calculated (UB3LYP/6-31g(d)) to be the ground state of the cation (ΔE = 4 kcal/mol) and iscis efficient (kisc ca. 1 × 108 s-1), and occurs to some extent with the para isomer (isoenergetic spin states,kisc ca. 1.7 × 106 s-1). The triplet is the ground state with the ortho isomer, and in that case isc does notcompete, although trapping by benzene is slow because of the hindering of C1 by the substituent. Theposition of the substituent thus determines the energetic order of the cation spin states, in particular throughthe selective stabilization of the singlet by the m-methoxy group, a novel case of “meta effect”.
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