Spin Chemistry - Centre national de la recherche scientifiquemfs-cargese.grenoble.cnrs.fr/Steiner.pdf · 2007-08-23 · Spin Chemistry: How magnetic fields affect chemical reactions

U.E. Steiner Summer School Cargèse 2007 1

1

Spin Spin ChemistryChemistry::How magnetic fields affect

chemical reactions

Ulrich SteinerUniversity of Konstanz

Part I: Basic Mechanisms and Examples

Abstract Slide No. History and present status The roots of modern spin chemistry date back to the late 60s when the phe-nomena of chemically induced magnetic nuclear and electron polarization were discovered. Since 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. 2-6 However, for chemical reactions passing through intermediate stages invol- ving a set of near degenerate spin states and possessing various reaction channels to products of different spin state, the effects of an external mag-netic field and structure related variations in internal magnetic fields can be substantial. 7-8 Radical Pairs Radicals are very reactive paramagnetic (S=1/2) 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. Such radical pairs forming by diffusional encounters are called F- pairs. 9-17 Spin 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 interac-tion, hyperfine interaction and Zeeman interaction. After separation for a small distance of 1-2 molecular diameters, the former two interactions be-come negligible. Then the motion of the two radical (electron) spins is en-tirely controlled by the local magnetic fields in the two radicals. In a semi-classical picture, Zeeman interaction and hyperfine interaction can be com-bined to an effective magnetic field around which the electron spin of a radical precesses. Since the two local fields at the two radicals are not corre-lated, the individual motion of the two electron spins can lead to a change in the overall spin (multiplicity) of the radical pair. 19-23

MARY (Magnetically Affected Reaction Yields) and MIE (Magnetic Isotope Effects) Through their effects on the evolution of the overall spin in a radical pair, 22-23 Zeeman and hyperfine interaction may control the reaction yields into the different reaction channels. The dependence of a reaction yield on the exter-nal magnetic field strength is called a MARY spectrum. Typical MARY spectra of radical pair reactions (cf. Fig. 26) show a monotonic variation with a saturation limit at high fields. The B1/2 value (field where the mag-netic 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. 24-28 Spin 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 inter-actions 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, fluc-tuating components of the magnetic interactions can also affect the spin state of a radical pair even at external field strengths where the coherent mo-tion of both radicals is locked. This is because stochastic perturbations lead to spin relaxation. Spin relaxation mechanisms do show magnetic-field de-pendence and tend to saturate at high fields, too. From their magnetic field dependence, different relaxation mechanisms can be distinguished. 29-42 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 re-cord 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. 43-48

CIDNP (Chemically Induced Dynamic Nuclear Polarization) 49 Nuclear magnetic resonance (NMR) spectroscopy is a powerful method for assigning and identifying molecular structures in chemistry. 50-52 If NMR spectra are recorded during the course of a chemical reaction with radical pair intermediates, extraordinary individual line intensities (en-hanced absorption or emission) may appear. Their origin is a non-Boltz-mann 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 com-bined 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. 53-59 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 cou-pling in a molecular frame. The symmetry-adapted triplet substates are mixed by an external magnetic field. This is a time-dependent process inter-fering with the reactive behaviour of an excited triplet state in a similar way as outlined for the spin motion in a radical pair. 60-66 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. 67-70 The techniques and phenomena of Spinchemistry 71-72 i.e.

MIE magnetic isotope effects MARY magnetic field affected reaction yields RYDMR reaction yield detected magnetic resonance CIDNP 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.


1

Spin Spin ChemistryChemistry::How magnetic fields affect

chemical reactions

Ulrich SteinerUniversity of Konstanz

Part I: Basic Mechanisms and Examples

2

Magnetic field dependence of chemical equilibria?

G

Reaction coordinate

A

B

GA

GBA B

3

Magnetic field dependent chemical equilibria

Low-spin to high-spin conversion

Temperature-driven spin-crossover phenomenon in the polymorphiccompound Fe[p-IC6H4)B(3-Mepz)3]2 from high-spin Fe(II) (colorless) to low-spin Fe(II) (purple). Single crystals of two polymorphs are alternatelymounted on the fiber. (Reger, et al. Inorg. Chem. 2005, 44(6), 1852-1866).

4

G

Reaction coordinate

A

B

GA

GB

Magnetic shift of chemical equilibria

5

Magnetic field effects on chemical kinetics ?

6


7

Set of near-degenerateintermediate statesdiffering in spin quantum number

out 3

out 1

out 2out 4

in

8

9

In chemical bonds electrons are paired

stable chemical compounds are usually diamagnetic

10

Radicals can be formed by

Homolytic bond cleavage reactions

Electron transfer reactions

A + D → A-• + D+•

radical ions

OO O

O

benzoylperoxide

+heat

O•

O

•O

O

benzoyloxy radicals

11

S1

2

S2

S0

Electronic excitation = creation of an electron/hole pair opens the way for photochemical creation of radical pairs

12

S1

2

S2

S0

T1

T2

T3



13

S1

2

S2

S0

T1

T2

T3


14

S1

2

S2

S0

T1

T2

T3

phosphorescence

ISC (intersystem

crossing) ∼ 10-9 s

IC ∼10-9s

IC (internalconversion)∼ 10-12s

fluorescence∼10-8 s

IC ∼10-12s

IC ∼10-12s


15

16

17

Radical pair formation and reaction in solution

18


19

O•

O

•O

O.......

spin dependent energies

exchange energy Zeeman energy

20

from Schulten, K.; Wolynes, P. G. J. Chem. Phys. 1978, 68, 3292-3297.

a semiclassical view of spin motion

21

Evolution of singletprobability in a radicalpair created with tripletspin correlation.

The individual hyperfinecouplings correspond to effective fields of

B1 = 11 G and

B2 = 18 G

22

N

N,N-dimethylanilinepyrene

23

8 G

Bi: 2.5 G 3.7 G

theoretical B1/2 value:

)(3* 22

21 BBB +≈

17 G

Bi: 9.1 G 4.6 G

59 G

Bi: 9.1 G 34.5 G

B* = 7.7 G

B* = 17.7 G

B* = 61.8 G

24

The Magnetic Compass in BirdsDoes it involve Spin Chemistry?


25from C. Rodgers, PhD Thesis, Oxford 2007




29

The role of spin relaxation

30

SN

(n)

(m)

NN

NN

RuN

N

+

+

N

N

SN

(p)

n-DQ4p-PTZ

4p-PTZ

(p)


31

SN

(n)

(m)

NN

NN

RuN

N

+

+

N

N

SN

(p)

n-DQ4p-PTZ

4p-PTZ

(p)

photoexcitation

32

SN

(n)

(m)

NN

NN

RuN

N

+

+

N

N

SN

(p)

n-DQ4p-PTZ

4p-PTZ

(p)Intersystem crossing

33

SN

(n)

(m)

NN

NN

RuN

N

+

+

N

N

SN

(p)

n-DQ4p-PTZ

4p-PTZ

(p)1

spin conservingelectron transfer 1

34

SN

(n)

(m)

NN

NN

RuN

N

+

+

N

N

SN

(p)

n-DQ4p-PTZ

4p-PTZ

(p)

2

spin conservingelectron transfer 2

35

SN

(n)

(m)

NN

NN

RuN

N

+

+

N

N

SN

(p)

n-DQ4p-PTZ

4p-PTZ

(p)

36

SN

(n)

(m)

NN

NN

RuN

N

+

+

N

N

SN

(p)

n-DQ4p-PTZ

4p-PTZ

(p)

3spin forbiddenback transfer


37

0,0

0,2

0,4

0,6

0,8

1,0

-50 150 350 550 750ns

norm

aliz

ed tr

ansi

ent a

bsor

banc

e 0 mT

10 mT

24 mT

50 mT

100 mT

300 mT

600 mT

1900 mT

SN

(n)

(m)

NN

NN

RuN

N

+

+

N

N

SN

(p)

n-DQ4p-PTZ

4p-PTZ

(p)1

23

38

0.0

0.2

0.4

0.6

0.8

1.0

-50 150 350 550 750ns

norm

aliz

ed tr

ansi

ent a

bsor

banc

e 0 mT

10 mT

24 mT

50 mT

100 mT

300 mT

600 mT

1900 mT

0

0.2

0.4

0.6

0.8

1

-50 150 350 550 750ns

norm

aliz

ed c

once

ntra

tion

of C

SS

0 mT

10 mT

24 mT

50 mT

100 mT

300 mT

600 mT

1900 mT

0.0

0.2

0.4

0.6

0.8

1.0

-50 150 350 550 750ns

norm

aliz

ed tr

ansi

ent a

bsor

banc

e 0 mT

10 mT

24 mT

50 mT

100 mT

300 mT

600 mT

1900 mT

0

0.2

0.4

0.6

0.8

1

-50 150 350 550 750ns

norm

aliz

ed c

once

ntra

tion

of C

SS

0 mT

10 mT

24 mT

50 mT

100 mT

300 mT

600 mT

1900 mT

Ground State (S0)

1CS

kS

3CS(T+)

3CS(T0)

3CS(T-)

fast kr

kr

Nonzero Field

Ground State (S0)

1CS

kS

3CS(T+)

3CS(T0)

3CS(T-)

fast kr

kr

Nonzero Field

39

contributions to kr

1 10 100 1000 10000

Bo/mT

k r/s

-1

DCA-POZ kT=0 DCA-PSZ kT=0 k-POZ/DQ+esdi+c

k-PSZ/DQ+esdi+c' c POZ c' PSZ

104

105

106

107

40

What are the contributions to the relaxation rate constant kr?

41

Relaxation by the esdi mechanism

NN

XN

⎟⎟⎠

⎞⎜⎜⎝

⎛+

++

== −−→± 2

220

2221

20

11330

22,, 1110

30 τω

ττω

τγ aarrkk MeesdiTTesdir h

a1 = 0.6, a2 = 0.4 1-1216

1 scm1010.1 −−×= Dτ 1-12162 scm1070.7 −−×= Dτ

42

contributions to kr

1 10 100 1000 10000

Bo/mT

k r/s

-1

DCA-POZ kT=0 DCA-PSZ kT=0 k-POZ/DQk-PSZ/DQ k-POZ/DQ+esdi+c k-PSZ/DQ+esdi+c'c POZ c' PSZ k-esdi D=9E-7k-esdi D=2E-6 k-esdi D=5E-6 k-esdi D=1E-5

104

105

106

107


43

RYDMR

44

RYDMR Reaction yield detected magnetic resonance

45

detected escapeproduct

3M + H-Det → 3(MH• Det•) → MH-X• X

1(MH• Det•) → MH-Det

O

O

2-methylnaphthoquinone

= M

N O

NaO3S

X = DMNSspin probe

Okazaki, M.; Sakata, S.; Konaka, R.; Shiga, T. J. Chem. Phys. 1987, 86, 6792-6800.

46




47




48





49

CIDNP

50

Assignment of NMR signals by chemical shifts and multiplet structure

FA

ED

BC

51

O

O2.29

1.144.08

1.57

1.33

0.96

ChemNMR H-1 Estimation

Estimation Quality: blue = good, magenta = medium, red = rough

01234PPM

52

53during reaction at higher T

2.02.5

after stopping reaction at lower T

2.02.5

Ward, H. R. Acc. Chem. Res. 1972, 5, 18-24

54

α-polarisation β-polarisation

α

β

energyin magneticfield

Nuclear Spin Polarisation

thermalequilibrium


55

Mechanism of net CIDNPCIDNP formationin singlet radical pairs forming singlet products

g1 > g2

56Vector representation of radical pair spin states (after Turro and Kräutler)

57

58

Mechanism of net CIDNPCIDNP formationin singlet radical pairs forming singlet products

g1 > g2

59during reaction at higher T

2.02.5

Check whether the rule on theprevious slide is true in this case!

Ward, H. R. Acc. Chem. Res. 1972, 5, 18-24

60

The Triplet Mechanism


61

Representation of symmetry-adapted triplet spin substates

62

magnetic field || x

energy eigenstates of a triplet spin system at zero field and in high magnetic field

63

64

65

66

Stochastic Liouville equation

set of Euler anglesrelating molecularframe to laboratoryframe


67Sakaguchi, Y.; Hayashi, H. Journal of Physical Chemistry A 2004, 108, 3421-3429.

Ar3P +hν → 1(Ar3P)* → 3(Ar3P) → Ar2P• + Ar•

Ar3P P

triphenyl phosphine

Ar3P =

68Sakaguchi, Y.; Hayashi, H. Journal of Physical Chemistry A 2004, 108, 3421-3429.

Ar3P +hν → 1(Ar3P)* → 3(Ar3P) → Ar2P• + Ar•

Ar3P

P

triphenyl phosphine

Ar3P =

69

Photoelectron Transfer

70

Photoelectron Transfer

71

MIE magnetic isotope effect

MARY magnetically affected reaction yield

RYDMR reaction yield detected magnetic resonance

CIDNP chemically induced dynamic nuclear polarization

CIDEP chemically induced dynamic electron polarizatio

NMR nuclear magnetic resonance ESR electron spin resonance

72

U.E. Steiner, University of Konstanz Spin Chemistry: How magnetic fields affect chemical reactions Recommended Reading (1) Steiner, U. E.; Ulrich, T. Chem. Rev. 1989, 89, 51 - 147.

Magnetic Field Effects in Chemical Kinetics and Related Phenomena (2) Steiner, U. E.; Wolff, H. J. In Photochemistry and Photophysics; Rabek, J. J., Scott, G.

W., Eds.; CRC Press: Boca Raton, 1991; Vol. IV, p 1-130. Magnetic Field Effects in Photochemistry.

(3) Steiner, U. E.; Gilch, P. In High Magnetic Fields, Techniques and Experiments I, Vol. 2; Herlach, F., Miura, N., Eds.; World Scientific Publishing Co.: New Jersey, London, 2003, p 219 - 244. High Magnetic Fields in Chemistry.

(4) Dynamic Spin Chemistry. Magnetic Controls and Spin Dynamics of Chemical Reac-tions; Nagakura, S.; Hayashi, H.; Azumi, T., Eds.; Kodansha and Wiley: Tokyo and New York, 1998.

(5) Hayashi, H. Introduction to dynamic spin chemistry: magnetic field effects on chemi-cal and biochemical reactions; World Scientific Publishing Co. Ptc. Ltd.: Singapore, 2004.

(6) Magneto-Science. Magnetic field effects on materials: fundamentals and applications; Yamaguchi, M.; Tanimoto, Y., Eds.; Kodansha, Springer: Heidelberg, 2006; Vol. 89.

(7) McLauchlan, K. A.; Steiner, U. E. Mol.Phys. 1991, 73, 241-263. The Spin-Correlated Radical Pair as a Reaction Intermediate

(8) Hayashi, H.; Nagakura, S. Bull. Chem. Soc. Jap. 1984, 57, 322-328. Theoretical study of relaxation mechanism in magnetic field effects on chemical reac-tions

(9) Rawls, M. T.; Kollmannsberger, G.; Elliott, C. M.; Steiner, U. E. J. Phys. Chem. A 2007, 111, 3485-3496. Spin Chemical Control of Photoinduced Electron Transfer Processes in Ruthe-nium(II)-Trisbipyridine-Based Supramolecular Triads: 2. The Effect of Oxygen, Sul-fur, and Selenium as Heteroatom in the Azine Donor

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Spin Chemistry - Centre national de la recherche scientifiquemfs-cargese.grenoble.cnrs.fr/Steiner.pdf · 2007-08-23 · Spin Chemistry: How magnetic fields affect chemical reactions