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The direct ultraviolet irradiation of dimethyl benzyl phosphite (1) and dimethyl p-acetylbenzyl phosphite (8) was investigated in acetonitrile, cyclohexane, and benzene. Phosphite 1 gives predominantly the photo-Arbuzov product, dimethyl benzylphosphonate (2), in 67−81% accountability yields, based of phosphite consumed, along with minor amounts of bibenzyl (20) and dimethyl phosphite (10). The quantum yield for formation of 2 in cyclohexane, φP, is 0.43. By contrast, irradiation of phosphite 8 yields only 7−13% of photo-Arbuzov phosphonate (9) but relatively large amounts of radical diffusion products:  dimethyl phosphite (10) the p-acetylbenzyl radical dimer (11); and p-acetyltoluene (12). Evidently 8, closely related to acetophenone, reacts predominantly via the triplet excited estate to generate long-lived, triplet, free-radical pairs (6 and 7a). In benzene, further products (15, 16, 17a and 17b) are identified that result from addition of the phosphinoyl radical (6) to benzene to give cyclohexadienyl radical 14, followed by combination and disproportionation reactions with radical 7a. (Total product quantum yields in benzene (Σφi) = 0.47.) In benzene, accountabilities of radical 6 from photolysis of 8 as high as 56% are encountered along with up to 92% accountabilities of p-acetylbenzyl (7a) radicals. Addition of radical scavengers PhSH, PhCH2Br, and TEMPO in the three solvents establishes the cage yield of 9 as 3 − 5%. The products of radical trapping provide further proof of the radical-pair nature of the photolysis of phosphite 8, including a 95% accountability of 6 with PhCH2Br in benzene. It is proposed that the CH2−O scission of triplet 8 must occur concertedly with partial phosphoryl (PO) bond formation. The trapping of radicals 6 and 7b from irradiation of phosphite 1 as the benzene adducts 22 and 23, analogous structurally to those (16 and 17) from phosphite 8, supports the postulation that photoisomerization of 1 to 2 proceeds via short-lived, presumably singlet, free-radical pairs.

31 P chemically induced dynamic nuclear polarization (CIDNP) studies of the photochemistry of p-acetylbenzyl dimethyl phosphite (1) provide evidence for the operation of the relatively rare T --S intersystem crossing mechanism for a photochemically generated triplet free radical pair. The mechanism of intersystem crossing of the triplet radical pair p-acetylbenzyl (2) and dimethoxyphosphonyl (3), generated from 1, is influenced by the large 31 P hfcc of 3 and switches from the T --S mechanism in a relatively low magnetic field (18.8 kG) to the conventional T 0 -S pathway at a higher field (58.8 kG). This change in mechanism is evidenced by the photolysis of 1 in the presence of radical scavengers (halocarbons, thiophenol) which yields the corresponding products from atom abstraction by 3 that has escaped the initial radical cage. These escape products are absorptiVely polarized at high field (T 0 -S) but are emissiVely polarized at lower field (T --S) in a particularly well-defined display of the effect of magnetic field strength on intersystem crossing mechanism for a geminate radical pair. Photolysis of 1 in C 6 D 6 solution yields emissively polarized dimethyl p-acetylbenzylphosphonate (4) from combination of radical pair [2,3] in the initial solvent cage and following diffusive formation of [2,3] free pairs. Cage recombination and disproportionation of secondary F pairs, comprised of 2 and the phosphorus-substituted cyclohexadienyl radical (5), affords several emissively polarized products (6, 7, and 9) via the T 0 -S mechanism at both magnetic fields. The polarization of 4 remains emissive at both magnetic fields regardless of the presence of radical scavengers, a finding which is argued to be consistent with the above results.Several mechanisms can be operative in the generation of both chemically induced dynamic electron (CIDEP) and nuclear polarization (CIDNP) observed in the ESR spectra of radical intermediates and in the NMR spectra of radical-derived products, respectively. 1-3 For radical pairs (RP) in solution, the most commonly encountered mechanism responsible for both phenomena is the T 0 -S mixing of spin states. Much more rarely the T --S mechanism prevails, and usually requires the mobility of the partners in the pair to be restricted, e.g., as in highly viscous solvents 4 or in biradicals. 5 However, the T --S mechanism can be operative for a RP in solution at ambient temperature 6,7 if one of the radicals possesses an unusually large hyperfine coupling constant (hfcc). Time-resolved ESR studies 8 have implicated the possible involvement of T --S transitions in CIDEP phenomena observed on generation of the triplet primary RP [2,3] by photolysis of p-acetylbenzyl dimethyl phosphite (1) in solution in a 3.5 kG magnetic field (eq 1, Scheme 1). Indeed, the hfcc of phosphonyl radical 3 is large (ap ≈ 700 G) [8][9][10] and could facilitate intersystem crossing by the T --S pathway. However, a unique interpretation of the CIDEP data could not be made due to the simultaneous action of several elect...

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