Recent advances in spin chemistry*

The great success in controlling chemical reactivity by spin manipulation was achieved in the last decade, and many remarkable spin and magnetic phenomena have been discovered. Among those discoveries, the most chemically important highlights are magnetic isotope effect, spin catalysis, and single-spin tunneling spectroscopy. This paper summarizes recent advances in these new and hot areas of modern chemistry. GENERAL OUTLOOK OF SPIN CHEMISTRY Spin chemistry as a new land and hot area in modern chemistry is based on the universal and fundamental principle—all chemical reactions are spin selective. They allowed only for those spin states of products whose total electron spin is identical to that of reagents. The reactions are forbidden if they require a change of spin. Some typical examples of spin-selective reactions are given below. The recombination of radicals into the diamagnetic molecule is allowed from singlet state of the reaction precursor, radical pair: Triplet state is strictly forbidden for the reaction, so that triplet pair either dissociates to free radicals, or experiences spin conversion (shown by dashed arrow), which transforms nonreactive triplet state into the chemically reactive singlet state. Chemical addition of radical to triplet molecule (oxygen, for instance), as well as physical quenching of excited triplet molecule by radical, takes place only in doublet state: Quartet spin state is forbidden to react and quartet-doublet spin conversion is required to switch over the reaction channel. Again, spin selectivity is controlled by competition between the dissociation of the quartet pair on the starting reagents and spin transformation of the pair into the reactive doublet state. In the reaction of two triplets (recombination of carbenes, delayed fluorescence from the fusion of two excited triplet molecules, etc.) only one of the nine spin states is open to react: *Plenary lecture presented at the 15 International Conference on Physical Organic Chemistry (ICPOC 15), Göteborg, Sweden, 8–13 July 2000. Other presentations are published in this issue, pp. 2219–2358. The other eight triplet and quintet spin states are forbidden for the reaction, and only spin conversion is able to stimulate their chemical reactivity and to switch over the reaction channel. And again, spin conversion competes with dissociation of T and Qi pairs. Spin chemistry is unique because it introduces in chemistry magnetic interactions. Contributing almost nothing in chemical energy, being negligibly small and traditionally ignored, magnetic interactions produce spin conversion and switch over the reaction from spin forbidden to spin-allowed channels (or vice versa). Ultimately, they implement a spin control of the reactions, they modify chemical reactivity of the reagents, and they write a new, magnetic scenario of chemical reactions. All remarkable phenomena in spin chemistry, the acts of the magnetic scenario, may be exemplified by the radical pair which plays in spin chemistry testifying role similar to that of H2 in quantum chemistry. Radical pair functions as an electron and nuclear spin selective microreactor in which spin conversion of nonreactive states into the chemically reactive ones is controlled by Zeeman and Fermi interactions, and microwaves (Fig. 1). The rate of spin conversion and, therefore, the probability of the reaction (e.g., recombination) of the pair P is a function of magnetic field H, hyperfine coupling constant a, nuclear spin I and nuclear magnetic moment μn, nuclear spin projection mI, resonance frequency ω and amplitude H1 of microwaves, the energy J of exchange interradical interaction in pair. The dependence of P on these parameters generates numerous magnetic phenomena in chemistry: A. L. BUCHACHENKO © 2000 IUPAC, Pure and Applied Chemistry 72, 2243–2258 2244 Fig. 1 General scheme of spin conversion induced by Zeeman and Fermi interactions and microwaves. Radical pair functions as a spin-selective microreactor in which the reaction probability P is a function of magnetic parameters of the radicals (a, μn, In, mI), magnetic fields (H, H1, ω), and exchange interaction J. • magnetic field effect • magnetic isotope effect • chemically induced nuclear polarization • chemically induced electron polarization • radiowave emission of chemical reactions • chemically detected magnetic resonance • optically detected magnetic resonance • microwave-induced magnetic isotope effect • microwave-stimulated nuclear polarization • spin coherency in chemical reactivity • spin catalysis • single-spin tunneling spectroscopy In this paper, I will focus the reader’s attention on the three of the most outstanding, significant (for chemistry), and famous phenomena—magnetic isotope effect, spin catalysis, and single-spin tunneling spectroscopy. MAGNETIC ISOTOPE EFFECT Discovery of magnetic isotope effect (MIE) is one of the greatest events in modern chemistry [1], comparable in importance only with that of classical, mass-dependent isotope effect (CIE). In contrast to CIE, MIE demonstrates dependence of the reaction rates on the nuclear spin, magnetic moments, and hyperfine, electron-nuclear interaction in reagents. It sorts isotope nuclei and directs them into the different reaction products according to their spin and magnetic moment. MIE results in fractionation of magnetic and nonmagnetic isotopes in chemical processes, geochemistry, and space chemistry. Now it is evident that the monitoring of contents and isotope distribution in ores, minerals, oils, coals, and space matter should take into account both mechanisms of isotope fractionation, CIE and MIE on a par, in order to accurately reconstruct genesis and pathways of chemical evolution of substances in nature. MIE may also operate in biochemical processes, so it can be considered a mechanistic tool in biochemistry. Being a phenomenon of fundamental importance, MIE offers a new tool for probing and testifying the reaction mechanisms, kinetics, and chemical physics of reactions. It can arise in spin-selective reactions of carbenes, triplet molecules, ions, and high-spin species, so that radical pair is not the only source of MIE [2]. MIE-induced isotope sorting can be illustrated by the photolysis of dibenzyl ketone, which is known to occur via fragmentation of triplet molecule and generation of triplet radical pair (Fig. 2). Triplet–singlet conversion of magnetic pairs (with C nuclei) is much faster than that of nonmagnetic pairs (with C nuclei), so that magnetic pairs predominantly recombine and regenerate the © 2000 IUPAC, Pure and Applied Chemistry 72, 2243–2258 Recent advances in spin chemistry 2245 Fig. 2 Scheme of chemically induced isotope fractionation in the photolysis of dibenzylketone. starting ketone molecules, while the delay of spin conversion of nonmagnetic pairs favors their dissociation and transformation into the reaction products. As a result, the regenerated ketone molecules accumulate C nuclei. Thus, due to the difference in the rates of spin conversion radical pair sorts the nuclei according to their magnetic moments and dispatches magnetic and nonmagnetic nuclei into the different reaction products. Discovery of MIE in the photolysis of dibenzyl ketone made this reaction famous and popular; physics and chemistry of MIE have been extensively studied; physical and kinetic theories of MIE have been experimentally tested by this key reaction [3]. Oxygen MIE was discovered in the reactions of chain oxidation of polymers and hydrocarbons by molecular oxygen [4]. Two reactions in the chain process are spin selective—the chain termination reaction (e.g., recombination of peroxyradicals),and chain propagation reaction (e.g., the addition of molecular oxygen to alkyl radicals). The former dominates in solid-state polymer oxidation, the latter prevails in the liquid-phase oxidation. The scheme of isotope sorting in the termination step is shown in Fig. 3. The encounter pair of freely diffusing peroxy radicals either recombines resulting unstable tetraoxide RO4R, which decomposes, regenerating an oxygen molecule from central oxygen atoms, or dissociates regenerating peroxy A. L. BUCHACHENKO © 2000 IUPAC, Pure and Applied Chemistry 72, 2243–2258 2246 Fig. 3 Oxygen MIE and O enrichment of molecular oxygen in the radical chain termination reaction of polymer oxidation as a function of chemical conversion of oxygen. Fig. 4 O isotope enrichment as a function of chain length in γ-initiated polyethylene oxidation. radicals. Due to MIE, which differentiates the rates of spin conversion, the recombination probability of peroxy radicals with terminal O atoms is higher than that of radicals with O and O atoms. As a result, the tetraoxide and, consequently, the recovered oxygen is enriched with O, while the hydroperoxide molecules are enriched with O and O. Figure 3 demonstrates isotope enrichment of molecular oxygen as a function of oxygen conversion; the higher chemical conversion the larger number of recovered oxygen molecules and the higher isotope enrichment. It is impressive that MIE-induced O isotope enrichment strongly exceeds O enrichment induced by CIE. Figure 4 shows the chain length effect on the isotope separation which evidences in favor of the mechanism of isotope selection: the shorter the chain length, the more spin-selective termination reactions and, therefore, the higher isotope sorting. Figure 5 demonstrates the impressive distinction of the temperature behavior of isotope fractionation induced by MIE and CIE. In contrast to CIE, which exhibits only small temperature dependence (in agreement with its physical nature), MIE reveals a severe dependence; the maximum MIE reflects the most favorable conditions for isotope selection when the timescale of molecular dynamics is compatible with that of spin dynamics, providing the best nuclear spin selection in radical pair [5]. In liquid-phase oxidation, the dominating reaction responsible for oxygen isotope fractionation is the propagation reaction The reaction product, doublet spin state peroxy radical, originates only from the doublet pair [R ̇...O2]; the quartet spin states of the pair are forbidden to react. MIE in quartet-doublet spin conversion selects magnetic O nuclei into peroxy radicals, resulting in impoverishment of the remaining molecular oxygen with O. In contrast, oxygen molecules with O react slower than those with O, resulting in CIE-induced enrichment of molecular oxygen with O. Experiment has perfectly confirmed these predictions (Fig. 6); the ratio of the radical addition rate constants is found to be k(OO)/k(OO) = 1.011; k(OO)/k(OO) = 0.990 that is, OO molecules react 1.1% faster than O2 molecules, but OO molecules react 1.0% slower than O2 molecules. The former is due to MIE, the latter is induced by CIE [5]. © 2000 IUPAC, Pure and Applied Chemistry 72, 2243–2258 Recent advances in spin chemistry 2247 Fig. 5 Temperature dependence of O and O isotope enrichment in oxidation of polyethylene (1), polypropylene (2) and polymethylpentene (3). Open circles, squares, and triangles correspond to O; the filled ones, to O. Si MIE has been demonstrated in the photolysis of silylketone according to the scheme [6]: which was verified by CIDNP and by inspection of the photolysis products. In triplet-sensitized photolysis, MIE in triplet radical pair selects magnetic Si nuclei and provides an enrichment of recovered ketone with Si isotope (Fig. 7). The other impressive result is that the inversion of the spin multiplicity of radical pair (in direct photolysis, which occurs in singlet excited state of ketone) is accompanied by inversion of the sign of MIE: Si enrichment is replaced by small, however, reliably measured impoverishment (Fig. 7). A. L. BUCHACHENKO © 2000 IUPAC, Pure and Applied Chemistry 72, 2243–2258 2248 Fig. 6 Oxygen isotope fractionation in the chain propagation reaction of ethylbenzene oxidation by molecular oxygen; S is the isotope enrichment, 1-F is oxygen chemical conversion. Fig. 7 Silicon isotope fractionation induced by photolysis of silyl ketone as a function of chemical conversion. CIE-induced fractionation of Si is identical for both singlet and triplet channels of photolysis; MIE-induced Si isotope separation is sensitive to the spin multiplicity of channels. Now one can summarize qualitative symptoms of MIE in comparison with CIE (Table 1); they are specific and clearly contrasting for both effects. Figure 8 illustrates the origin of S MIE in direct photolysis of sulfur-containing ketone, which has been proved by CIDNP to occur via triplet radical pair according to scheme: Hyperfine coupling with magnetic S nuclei induces triplet–singlet conversion of the radical pair and stimulates regeneration of the starting ketone, which accumulates magnetic isotope as far as ketone itself is exhausted (Fig. 8) [8,9]. Uranium MIE has been observed in the photoreduction of uranyl salts (perchlorate, in particular) in presence of substituted phenols (Fig. 9). The intermediate triplet radical pair, composed of uranoyl and phenoxyl radicals, regenerates the starting uranyl ion via triplet–singlet conversion if the pair contains magnetic U nuclei. The pairs with U nuclei have a little chance to experience spin conversion, so they react further into the products of U ions (unsoluble UF4 in the presence of NH4F, for instance) [8,9]. © 2000 IUPAC, Pure and Applied Chemistry 72, 2243–2258 Recent advances in spin chemistry 2249 Table 1 MIE versus CIE: quality.