1980,13, 369 Magnetic Field and Magnetic Isotope Effects ... Magnetic Field and Magnetic Isotope

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  • Acc. Chem. Res. 1980,13, 369-377 369

    Magnetic Field and Magnetic Isotope Effects in Organic Photochemical Reactions. A Novel Probe of Reaction

    Mechanisms and a Method for Enrichment of Magnetic Isotopes

    NICHOLAS J. TURRO* and BERNHARD KRAEUTLER Chemistry Department, Columbia University, New York, New York 10027

    Received August 8, 1979 (Revised Manuscript Received M a y 2, 1980)

    The possibility that the reactivity and efficiency of reactions involving radical pairs will depend on nuclear spin states (or laboratory magnetic fields) is a rather fascinating idea in view of the tiny interactions that exist between electron spins and nuclear spins (or electron spins and laboratory magnetic fields).

    Many chemists are accustomed to thinking of nuclei as passive mass points, whose inertia only allows them to follow but not to influence electronic processes in a significant way.l However, besides mass, nuclei possess a second important property (or nonproperty): mag- netism or magnetic spin (or diamagnetism or lack of magnetic spin). The latter property is not usually considered in a discussion of reaction mechanisms. Similarly, the possible influence of laboratory magnetic fields on chemical reactions represents an intriguing notion but most chemists would be hard pressed to cite any famous documented cases (or even any obscure, undocumented examples).2

    A simple thermodynamic argument has been em- ployed to summarily dismiss any serious possibility of significant magnetic field or magnetic isotope effects on the rates and/or efficiencies of chemical processes. The logic goes as follows: The strongest laboratory magnetic fields ( w 1OOOOO G) can induce at most energy changes of -0.03 kcal/mol even in paramagnetic molecule^.^ Such energy changes are negligible relative to the commonly encountered activation energies of chemical reactions (210 kcal/mol). The magnetic strengths and corresponding energy changes that can be induced by even the strongest nuclear magnets (5100 G) are, of course, even tinier (S0.00003 kcal/mol). Neither laboratory nor nuclear magnets can be expected to produce measurable effects on the energetics of chemical processes; therefore, there can be no serious possibility of magnetic field effects or magnetic isotope effects on chemical processes.

    This is a pretty good argument, as far as it goes, and it would be difficult to refute if the rates and efficiencies of chemical reactions depended only on energetic con- siderations. However, reaction rates and efficiencies

    Nicholas J. Turro was born in MMdletown, CN. He dM undergraduate work at Wesleyan University in his home town and took his Ph.D. in 1963 from Caltech. After a years postdoctoral research at Harvard, he joined the faculty at Columbia where ha is a Professor of Chemisby. HIS research interests have Included synthetic applications of cyclopropanones, mechanisms of photo- reactions, chemiluminescent organic reactions, and chemistry in micellar ag- gregates. He received the ACS Award In Pure Chemistry in 1974.

    Bernhard Kraeutler Is a native of Austria and received his Ph.D. from the E.T.H. in Zurich, Swltzerland. He spent a year as a postdoctoral associate as a Swiss NRF Fellow at the University of Texas before joining Professor Turros group for a second postdoctoral year in 1978. He is a member of the chemistry department at E.T.H. Zurich.

    0001-4842/80/0113-0369$01.00/0

    depend not only on energetic factors but also on en- tropic factors. In terms of the Arrhenius formulation of rates, a rate constant is given by k = Ae-hE/RT. Substantial magnetic effects on the A factor are pos- sible if states that possess different magnetic proper- ties (i.e., triplet states and singlet states) are involved in a rate-determining step whose rate constant is given by k. Intersystem Crossing in Radical Pairs

    The Wigner spin conservation rule states that during an elementary chemical step, electron and nuclear spins are c~nserved .~ For most organic systems this rule forbids singlet to triplet or triplet to singlet intersystem crossing (S-T ISC or T-S ISC, respectively) in an ele- mentary chemical reaction. The rule breaks down when an elementary step involves states whose lifetimes are long enough to allow the relatively slow ISC mecha- nisms (spin-orbit coupling and electron-nuclear hy- perfine interactions) to operate. Reactions involving carbon radical pairs represent a large class for which the spin conservation rule fails, Le., for which electron and nuclear spins are not conserved. The collapse of the spin conservation rule for organic radical pairs leads to several striking and unusual phenomena: (1) the observation (via NMR) of non-Boltzmann distribu- tions of nuclear spins in the products of chemical re- actions, i.e., chemically induced nuclear polarization (CIDNP);5 ( 2 ) the observation of different reaction probabilities and efficiencies for radical pairs possessing nuclear spins relative to those possessing no nuclear spin, i.e., a magnetic spin isotope effect on chemical reaction^;^ (3) the observation of a dependence of the reaction probability and efficiency of a radical pair on the strength of a relatively weak (

  • 370 Turro and Kraeutler Accounts of Chemical Research

    THE STATES OF A SPIN CORRELATED RADICAL PAIR T+ - n T+

    T- - TO S TO T+ To T- S

    T- -

    I HYPERFINE II HYPERFINE m EXTERNAL FIELD DOMINATES DOMINATES DOMINATES

    I T * I T o } d s {TO) + S {To)--

    { T t } it. S {T* P- Figure 2. At zero magnetic field, all three triplet levels may undergo electron-nuclear hyperfine-induced intersystem crossing to S. When the magnetic field strength is much larger than the magnitude of the hyperfine interaction, T+ and T- are split away from S and are incapable of undergoing hyperfine-induced crossing to S. To, however, remains degenerate with S and hyperfine-in- duced To - S crossing is still possible. When the external field interaction is much larger than the hyperfine interaction, ISC is determined by the external magnetic field strength.

    denote an electron spin that is pointing in the direction of an arbitrary magnetic field, H,, and let P denote an electron spin that is pointing in a direction opposed to H,. The three triplet levels of a radical pair are labeled T+, T-, and To and have spin configurations aa, Po, and a& The singlet state has a spin configuration -a@ (These functions are, of course, abbreviated symbols of the correct spin function^.)^ The To state has net spin-angular momentum but the projection of this momentum along H, is zero (the magnetic quantum number Ms = 0). The singlet state S of the radical pair is similar to the To triplet sublevel in that it possesses an a spin and a P spin. The only vectorial difference between To and S is due to the phasing of the spins; in To the vectors are 0 out of phase, whereas in S the vectors are 180 out of phase. The conversion of To to S requires the rewinding of one spin vector relative to the other by 180. In the absence of an external magnetic field, the spin vectors of a triplet pair (Rl and Ri) are at some indefinite overall orientation but at a well-defined (triplet) relative orientation or phase. This means that although the three magnetic sublevels T+, T-, and To are strongly mixed (rapidly intercon- verting), the net spin of the triplet is always equal to 1.

    An interesting consequence of the application of a strong magnetic field to a radical pair is the splitting of T, and T- from To (Figure 2). However, To and S remain degenerate. This means that T, + S and T-

    S transitions in the radical pair may be quenched by the application of a strong magnetic field because interconversions of T, P S are slowed down and only To F? S transitions remain probable. A General Qualitative Theory of CIDNP, Magnetic Isotope Effects, and Magnetic Field Effects for Reactions Involving Radical Pairs

    The modern theory of CIDNP provides all of the essential concepts required for an understanding of magnetic isotope and magnetic field effects on reactions involving radical pairs.5 Two important aspects of the theory involve a sorting principle and a correlation principle. These principles state that for a radical pair nuclear spin states (+l/z or -l/z spins), electronic spin states (singlet or triplet), and triplet sublevels (T+, T-,

  • Vol. 13, 1980 Magnetic Effec

    I n W W

    p u r e t r i p l e t s t a t e m i x e d singlet and p u r e s i n g l e t S t a t e t r i p l e t s t a t e

    = a , P 2 = R 2 , = a , . d 2 = 5 .d, = a , J2 = R Ooout of p h a s e -90 o u t of p h a s e l S O o o u t of p h a s e

    Figure 3. Schematic vector description of the rephasing mech- anism for To S intersystem crossing in a radical pair. Bottom: At t = 0 the system is a pure triplet, at a later time (t < 7r/2Au) the system is a mixed state and at t = n/Aw the system is a pure singlet state. Top: A view of the spin vectors in terms of a spin clock.

    and To) may be sorted from one another if the pair can undergo geminate cage reactions of a correlated radical pair in competition with an escape process that either destroys the correlation or involves a reaction of the radical pair. The Mechanism of Intersystem Crossing in Radical Pairs

    In the presence of magnetic fie!ds,. the correlated electronic spins of a triplet pair R1 R2 will tend to precess about the direction of the strongest field to which they can couple. If these fields are exactly the same for R1 and R2, then the precession rates w1 and wz of the two spins are identical, the difference in pre- cessional rates Aw is zero, and the relative triplet phase is preserved forever. If, on the other e d , the magnetic fields experienced by the spins of Rl and Rz are dif- ferent, then Aw # 0, and the i