Magnetic Field Dependence of the ³¹P CIDNP in the Photolysis of a Benzyl Phosphite. Evidence for a T_-−S Mechanism

Abstract

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...

Spin nature of genetic code

“…DNA or RNA in living cells has no external magnetic field (Earth's natural magnetic field is highly low, 5 G, to initiate biochemical processes [16]), but has the internal field, B [16]. The latter originates mostly from the hyperfine coupling (B = B hfc = I A S, where I and S are nuclear and electron spin operators, and A is the hfc tensor [17]), which in phosphorus-containing systems can reach the level of weak Zeeman fields [18,19]. In organic radicals ike ours it is common to neglect the spin-orbit coupling l SOC [17].…”

“…The found structures (Figures 1(a) and (c)) were considered as initial ones in the [T 1 ,T 2 ] region. The hyperfine coupling (hfc) between a free electron of the RP (see below) and magnetic nuclei-31 P nuclei showing 100% natural abundance [9] and large hfc [10,11]-was included in the vicinity of the T 1 and T 2 (T/S) crossings (Figure 2). Including the hyperfine coupling slows computations significantly but, as we shall see, its presence is absolutely vital for our purposesspecifically, when the electron is able to change its spin polarization upon the influence of hfc.…”

Spin effects govern DNA/RNA nucleotide polymerization

“…Note that varying R(P  O 3 (ATP)-HO-C 3' (AMP)), see above, automatically affects the other distances. The values for J 0 and β, typical for radical pairs in solution, are taken from the previous works [25,26] and correspond to J 0 = 7 × 10 9 G and β = 2.15 Å (AMP)) = 5 G. Despite the fact that these values are much smaller than those for J(R) their contribution however is still remarkable at small distances.…”

“…When in 'cage', such a radical pair with the identically oriented electron spins prefers being separated rather than being recombined [38][39][40]. The reason for keeping the spins parallel on the both radicals is in the highly large HF constant , produced by the AMP -radical [25]. According to our computations (CPMD, section II), the value of  reaches 0.078 T. The strong magnetic coupling between the 31 P atom and the unpaired electron on the AMP -aligns the spin of OH (the  in the OH-(5H 2 O) complex at 310 K is 0.036 T-a result of the OH magnetic field enhancement by surrounding water molecules [41]) in the identical direction to that on the AMP -, and the environment cannot destroy the effect in the distance range 4.75 -1.50 Å between the AMP -and OH.…”

The photo-Arbuzov rearrangement of dimethyl 9-anthrylmethyl phosphite and the photodimerization of the corresponding phosphonate

“…The naphthyl phosphite analogs of 1 have been shown to efficiently undergo the photoArbuzov rearrangement as well [5,6]. Recently, the dynamics of the interactions of triplet radical pairs, generated from the triplet excited states of benzyl and 1-naphthylmethyl phosphites, have been investigated by CIDEP [7] and 31 P CIDNP [8] techniques. We now report the efficient photorearrangement of 9-anthrylmethyl dimethyl phosphite (2) to yield the corresponding phosphonate (3) that upon further irradiation yields head-to-tail 9,10-anthracene photodimer 4.…”