Spin Chemistry: How magnetic fields affect chemical reactions

Transcription

1 pin Chemistry: How magnetic fields affect chemical reactions Part I: Basic Mechanisms and Examples Ulrich teiner University of Konstanz 1 U.E. teiner ummer chool Cargèse 007 1

2 Abstract lide o. History and present status The roots of modern spin chemistry date back to the late 60s when the phenomena of chemically induced magnetic nuclear and electron polarization were discovered. ince then, a thorough theoretical framework and many specific methods and experiments were developed that nowadays make up the field of spin chemistry. Its applications range from very low fields of the order of the earth s magnetic field of some tens of Mikrotesla up to the highest convenient laboratory fields of several tens of Tesla, from chemical systems of all types between the solid and gaseous state, from biological systems to material science. Chemical Reactions Essential aspects of chemical reactions are usually dealt with in terms of thermodynamics and chemical kinetics. Insofar as magnetic energies have to be counted against thermal energies, thermodynamic and kinetic effects of magnetic fields are very small. -6 However, for chemical reactions passing through intermediate stages involving a set of near degenerate spin states and possessing various reaction channels to products of different spin state, the effects of an external magnetic field and structure related variations in internal magnetic fields can be substantial. 7-8 Radical Pairs Radicals are very reactive paramagnetic (=1/) chemical species because they have an odd number of electrons. To create radicals, it usually requires the input of external energy. Electronic excitation by light is a main route to create radicals. When they are formed from diamagnetic precursors, radicals are created in pairs. A pair of radicals generated from the same diamagnetic precursor is called a geminate radical pair (G-pair). In liquid solutions, geminate radical pairs can separate and eventually meet radicals from other pairs. uch radical pairs forming by diffusional encounters are called F- pairs pin Motion in Radical Pairs. The Radical Pair Mechanism (RPM). Chemical reactions are usually spin-conserving. Therefore, initially, the overall spin and multiplicity (singlet or triplet) of a geminate radical pair is the same as that of its precursor. The initial spin of an F-pair is random. 18 Radical pairs may adopt four different spin states. The energies of these are mainly determined by exchange interaction, electron spin dipolar interaction, hyperfine interaction and Zeeman interaction. After separation for a small distance of 1- molecular diameters, the former two interactions become negligible. Then the motion of the two radical (electron) spins is entirely controlled by the local magnetic fields in the two radicals. In a semiclassical picture, Zeeman interaction and hyperfine interaction can be combined to an effective magnetic field around which the electron spin of a radical precesses. ince the two local fields at the two radicals are not correlated, the individual motion of the two electron spins can lead to a change in the overall spin (multiplicity) of the radical pair. 19-3

3 MARY (Magnetically Affected Reaction Yields) and MIE (Magnetic Isotope Effects) Through their effects on the evolution of the overall spin in a radical pair, -3 Zeeman and hyperfine interaction may control the reaction yields into the different reaction channels. The dependence of a reaction yield on the external magnetic field strength is called a MARY spectrum. Typical MARY spectra of radical pair reactions (cf. Fig. 6) show a monotonic variation with a saturation limit at high fields. The B 1/ value (field where the magnetic change of the yield reaches half its saturation value) is determined by the hyperfine coupling constants of the two radicals. As a consequence, changes in the magnetic isotope composition of a radical will also change the reaction yields (magnetic isotope effect). Effects of low magnetic fields in biological systems The radical pair mechanism has been suggested as a potential mechanism to explain the magnetic compass of migrating birds. Increasing evidence for this mechanism has been accumulated during the last 30 years. 4-8 pin Relaxation. The Relaxation Mechanism. If the diffusional life time of a geminate radical pair is less than typically 10 ns, only the rotationally averaged values of the hyperfine and Zeeman interactions are effective. Under such conditions, the spin motion in a radical pair is a coherent process. The individual differences in spin motion of the two radical spins disappear as the external magnetic field gets much larger than the hyperfine fields in the two radicals. Then the motions of the two spins become locked. However, if the diffusional life time of a geminate radical pair, i.e. the time window during which geminate re-encounters take place, gets much longer (e.g. in chemically linked radical pairs, or in radical pairs enclosed in micelles as nanoscopic supercages) the non-isotropic, fluctuating components of the magnetic interactions can also affect the spin state of a radical pair even at external field strengths where the coherent motion of both radicals is locked. This is because stochastic perturbations lead to spin relaxation. pin relaxation mechanisms do show magnetic-field dependence and tend to saturate at high fields, too. From their magnetic field dependence, different relaxation mechanisms can be distinguished. 9-4 RYDMR (Reaction Yield Detected Magnetic Resonance) At magnetic fields, where MARY spectra are saturated, microwave induced resonance transitions between the Zeeman levels can still be employed to achieve level repopulation and thereby affect the yields into different chemical reaction channels of radical pairs. In this way it is possible to record the magnetic resonance spectra of the radical pairs by monitoring the chemical reaction yield as a function of microwave frequency at constant field or (usually) as a function of the magnetic field at constant microwave frequency

4 CIDP (Chemically Induced Dynamic uclear Polarization) 49 uclear magnetic resonance (MR) spectroscopy is a powerful method for assigning and identifying molecular structures in chemistry If MR spectra are recorded during the course of a chemical reaction with radical pair intermediates, extraordinary individual line intensities (enhanced absorption or emission) may appear. Their origin is a non-boltzmann population of nuclear spin levels in the stable diamagnetic products. From the sign of the polarization it can be determined whether the products were formed in G-pairs of in F-pairs. These phenomena are due to the combined effects of hyperfine controlled spin motion and the spin selectivity of the chemical reaction channels in the geminate radical pairs. Mechanisms of this kind can also lead to electron spin polarization (CIDEP) of the free radicals originating from such radical pairs The Triplet Mechanism (TM) In photochemistry, excited triplet states are important intermediates. The triplet spin substates may differ in the rates at which they are populated and depopulated. This is due to symmetry constraints imposed on spin-orbit coupling in a molecular frame. The symmetry-adapted triplet substates are mixed by an external magnetic field. This is a time-dependent process interfering with the reactive behaviour of an excited triplet state in a similar way as outlined for the spin motion in a radical pair Prominent examples for manifestations of the triplet mechanism are given and the theoretical basis for extracting kinetic parameters of interest from the MARY curves is indicated The techniques and phenomena of pinchemistry 71-7 i.e. MIE magnetic isotope effects MARY magnetic field affected reaction yields RYDMR reaction yield detected magnetic resonance CIDP chemically induced dynamic nuclear spin polarization CIDEP chemically induced dynamic electron spin polarization span a wide range and form a continuous link between classical chemical kinetics and magnetic resonance spectroscopy.

5 Magnetic shift of chemical equilibria G pin Chemistry: How magnetic fields affect chemical reactions G A G B A B Reaction coordinate Part I: Basic Mechanisms and Examples Ulrich teiner University of Konstanz 1 4 Magnetic field dependence of chemical equilibria? Magnetic field effects on chemical kinetics? G A B G A G B A B Reaction coordinate 5 Magnetic field dependent chemical equilibria Low-spin to high-spin conversion Temperature-driven spin-crossover phenomenon in the polymorphic compound Fe[p-IC6H4)B(3-Mepz)3] from high-spin Fe(II) (colorless) to low-spin Fe(II) (purple). ingle crystals of two polymorphs are alternately mounted on the fiber. (Reger, et al. Inorg. Chem. 005, 44(6), ). 3 6 U.E. teiner ummer chool Cargèse 007 1

6 Radicals canbeformedby in out 4 out 1 out 3 out et of near-degenerate intermediate states differing in spin quantum number Homolytic bond cleavage reactions benzoylperoxide heat Electron transfer reactions A D A - D radical ions benzoyloxy radicals 7 10 Electronic excitation = creation of an electron/hole pair opens the way for photochemical creation of radical pairs In chemical bonds electrons are paired stable chemical compounds are usually diamagnetic Electronic excitation = creation of an electron/hole pair opens the way for photochemical creation of radical pairs T 3 T 1 T U.E. teiner ummer chool Cargèse 007

7 Electronic excitation = creation of an electron/hole pair opens the way for photochemical creation of radical pairs T 3 T 1 T Electronic excitation = creation of an electron/hole pair opens the way for photochemical creation of radical pairs Radical pair formation and reaction in solution T 3 IC 10-1 s IC (internal conversion) 10-1 s T 1 fluorescence 10-8 s IC 10-9 s IC (intersystem crossing) 10-9 s phosphorescence IC 10-1 s T U.E. teiner ummer chool Cargèse 007 3

8 spin dependent energies exchange energy Zeeman energy pyrene,-dimethylaniline a semiclassical view of spin motion theoretical B 1/ value: B * 3( B 1 B ) B i :.5 G 3.7 G B i : 9.1 G 4.6 G 8 G B i : 9.1 G 34.5 G B* = 7.7 G 17 G B* = 17.7 G 59 G B* = 61.8 G from chulten, K.; Wolynes, P. G. J. Chem. Phys. 1978, 68, Evolution of singlet probability in a radical pair created with triplet spin correlation. The individual hyperfine couplings correspond to effective fields of B 1 = 11 G and B = 18 G The Magnetic Compass in Birds Does it involve pin Chemistry? 1 4 U.E. teiner ummer chool Cargèse 007 4

9 from C. Rodgers, PhD Thesis, xford from C. Rodgers, PhD Thesis, xford The role of spin relaxation from C. Rodgers, PhD Thesis, xford (n) (m) Ru n-dq from C. Rodgers, PhD Thesis, xford U.E. teiner ummer chool Cargèse 007 5

10 U.E. teiner ummer chool Cargèse (n) (m) Ru n-dq photoexcitation 3 (n) (m) Ru n-dq Intersystem crossing 33 (n) (m) Ru n-dq 1 spin conserving electron transfer 1 34 (n) (m) Ru n-dq spin conserving electron transfer 35 (n) (m) Ru n-dq 36 (n) (m) Ru n-dq 3 spin forbidden back transfer

11 normalized transient absorbance 1,0 0,8 0,6 0,4 0, 0,0 (n) 1 n-dq (m) 3 Ru ns 0 mt 10 mt 4 mt 50 mt 100 mt 300 mt 600 mt 1900 mt What are the contributions to the relaxation rate constant k r? normalized concentration of C normalized transient absorbance ns mt 10 mt 4 mt 50 mt 100 mt 300 mt 600 mt 1900 mt 0 mt 10 mt 4 mt 50 mt 100 mt 300 mt 600 mt 1900 mt 1 C k 3 C(T ) k fast r 3 C(T 0 ) k r 3 C(T - ) onzero Field Ground tate ( 0 ) k r, esdi Relaxation by the esdi mechanism = k a 1 = 0.6, a = = h γ e r0 r 10 X a1τ 1 aτ 1 ω0τ1 1 ω0 3 T± T0, esdi M τ τ = cm s D τ = cm s D ns contributions to kr contributions to kr DCA-PZ kt=0 DCA-PZ kt=0 k-pz/dq DCA-PZ kt=0 DCA-PZ kt=0 k-pz/dqesdic k-pz/dq k-pz/dqesdic k-pz/dqesdic' c PZ c' PZ k-esdi D=9E k-pz/dqesdic' c PZ c' PZ 10 7 k-esdi D=E-6 k-esdi D=5E-6 k-esdi D=1E kr/s -1 kr/s Bo/mT Bo/mT U.E. teiner ummer chool Cargèse 007 7

12 X 3 M H-Det 3 (MH Det ) MH-X 1 (MH Det ) MH-Det RYDMR 43 kazaki, M.; akata,.; Konaka, R.; higa, T. J. Chem. Phys. 1987, 86, RYDMR Reaction yield detected magnetic resonance X 3 M H-Det 3 (MH Det ) MH-X 1 (MH Det ) MH-Det 44 kazaki, M.; akata,.; Konaka, R.; higa, T. J. Chem. Phys. 1987, 86, X 3 M H-Det 3 (MH Det ) MH-X detected escape product X 3 M H-Det 3 (MH Det ) MH-X 1 (MH Det ) MH-Det 1 (MH Det ) MH-Det -methylnaphthoquinone = M a 3 X = DM spin probe kazaki, M.; akata,.; Konaka, R.; higa, T. J. Chem. Phys. 1987, 86, kazaki, M.; akata,.; Konaka, R.; higa, T. J. Chem. Phys. 1987, 86, U.E. teiner ummer chool Cargèse 007 8

13 CIDP 49 5 Ward, H. R. Acc. Chem. Res. 197, 5, 18-4 Assignment of MR signals by chemical shifts and multiplet structure A F C B D E during reaction at higher T after stopping reaction at lower T 53 ChemMR H-1 Estimation Estimation Quality: blue = good, magenta = medium, red = rough energy in magnetic field β thermal equilibrium uclear pin Polarisation α-polarisation β-polarisation α 4 3 PPM U.E. teiner ummer chool Cargèse 007 9

14 Mechanism of net CIDP formation in singlet radical pairs forming singlet products Mechanism of net CIDP formation in singlet radical pairs forming singlet products g 1 > g g 1 > g Ward, H. R. Acc. Chem. Res. 197, 5, 18-4 Check whether the rule on the previous slide is true in this case!.5.0 Vector representation of radical pair spin states (after Turro and Kräutler) 56 during reaction at higher T 59 The Triplet Mechanism U.E. teiner ummer chool Cargèse

15 Representation of symmetry-adapted triplet spin substates energy eigenstates of a triplet spin system at zero field and in high magnetic field magnetic field x 6 65 tochastic Liouville equation set of Euler angles relating molecular frame to laboratory frame U.E. teiner ummer chool Cargèse

16 Ar 3 P hν 1 (Ar 3 P)* 3 (Ar 3 P) Ar P Ar Photoelectron Transfer Ar 3 P = P triphenyl phosphine Ar 3 P akaguchi, Y.; Hayashi, H. Journal of Physical Chemistry A 004, 108, Ar3P hν 1 (Ar3P)* 3 (Ar3P) ArP Ar Ar3P Ar 3 P = P MIE magnetic isotope effect triphenyl phosphine MARY magnetically affected reaction yield RYDMR reaction yield detected magnetic resonance CIDP chemically induced dynamic nuclear polarization CIDEP chemically induced dynamic electron polarizatio akaguchi, Y.; Hayashi, H. Journal of Physical Chemistry A 004, 108, MR nuclear magnetic resonance ER electron spin resonance 71 Photoelectron Transfer 69 7 U.E. teiner ummer chool Cargèse 007 1

17 U.E. teiner, University of Konstanz pin Chemistry: How magnetic fields affect chemical reactions Recommended Reading (1) teiner, U. E.; Ulrich, T. Chem. Rev. 1989, 89, Magnetic Field Effects in Chemical Kinetics and Related Phenomena () teiner, U. E.; Wolff, H. J. In Photochemistry and Photophysics; Rabek, J. J., cott, G. W., Eds.; CRC Press: Boca Raton, 1991; Vol. IV, p Magnetic Field Effects in Photochemistry. (3) teiner, U. E.; Gilch, P. In High Magnetic Fields, Techniques and Experiments I, Vol. ; Herlach, F., Miura,., Eds.; World cientific Publishing Co.: ew Jersey, London, 003, p High Magnetic Fields in Chemistry. (4) Dynamic pin Chemistry. Magnetic Controls and pin Dynamics of Chemical Reactions; agakura,.; Hayashi, H.; Azumi, T., Eds.; Kodansha and Wiley: Tokyo and ew York, (5) Hayashi, H. Introduction to dynamic spin chemistry: magnetic field effects on chemical and biochemical reactions; World cientific Publishing Co. Ptc. Ltd.: ingapore, 004. (6) Magneto-cience. Magnetic field effects on materials: fundamentals and applications; Yamaguchi, M.; Tanimoto, Y., Eds.; Kodansha, pringer: Heidelberg, 006; Vol. 89. (7) McLauchlan, K. A.; teiner, U. E. Mol.Phys. 1991, 73, The pin-correlated Radical Pair as a Reaction Intermediate (8) Hayashi, H.; agakura,. Bull. Chem. oc. Jap. 1984, 57, Theoretical study of relaxation mechanism in magnetic field effects on chemical reactions (9) Rawls, M. T.; Kollmannsberger, G.; Elliott, C. M.; teiner, U. E. J. Phys. Chem. A 007, 111, pin Chemical Control of Photoinduced Electron Transfer Processes in Ruthenium(II)-Trisbipyridine-Based upramolecular Triads:. The Effect of xygen, ulfur, and elenium as Heteroatom in the Azine Donor