5. The Scientific Case 67 5.

5.1 Introduction whereas, including cooperative Panel: magnetic interactions, the U.E. Steiner (Konstanz) In chemistry, the application of modern field of molecular (Chairman) magnetic fields has a long magnetism offers new possibilities R. Bittl (Berlin) tradition and has led to well for structural control by magnetic M. Ernst (Nijmegen) established disciplines and fields. In the following sections O. Kahn (Bordeaux) techniques. Two main purposes in these several areas are discussed using high magnetic fields may be from the point of view of the distinguished; the probing of likely impact of intense magnetic structural and dynamical fields. properties and the controlling of chemical processes and structures. 5.2 Molecular magnetism The ability of a magnetic field to function as a probe is usually based Molecular magnetism is a rather on the exploitation of the field new field of research which has dependence of some property of emerged during the last decade or the or system, so. It deals with the and including its interaction with chemistry of open-shell radiation; i.e. various kinds of and of molecular , of which NMR is assemblies involving open-shell the most vital and powerful one. units. What characterizes this field Thereby detailed qualitative and of research is its deeply quantitative information can be multidisciplinary character. It gained as to chemical composition brings together organic, inorganic, (chemical analysis) as well as and organometallic synthetic structural and dynamical along with theoreticians, properties. Structural aspects solid state physicists, materials pertain to the characterization of and life scientists. The heart of the details of molecular geometry the discipline nowadays concerns as well as of the energetics and the design and the investigation spatial characteristics of the of the physical properties of valence , while dynamical molecular assemblies exhibiting aspects pertain to internal motions bulk properties such as long-range of the probed molecules as well as magnetic ordering or molecular their molecular environment and bistability. to the rates at which elementary chemical processes take place. The first factor explaining the development of molecular At the molecular level, magnetic magnetism, at the expense of the control of chemical processes is traditional field of possible by the mechanisms of , is the shift of chemistry, applying in interest in molecular chemistry particular to chemical reactions from isolated to collectively with pair intermediates, organized molecules 68 5. The Scientific Case Chemistry

(). The exciting perspectives in molecular initial successes in the synthesis chemistry is the use of isolated of molecular conductors and molecules or assemblies of superconductors prompted some molecules in electronic circuits or scientists to try to synthesize devices. It is probably a rather molecular magnets, with a shift long-term prospect, and the of interest from simple challenge is not to replace paramagnetic behavior to traditional (silicon) electronics, collective behavior. The but to use -based systems development of supramolecular to perform functions that are not chemistry has some of its roots in possible with silicon. Some ideas the challenge of imitating have already been proposed, with biological molecules, in which some advances made in switching, properly organized building blocks amplification, information form macromolecules capable of storage, and signal processing. highly selective functions. Similarly, in the area of 5.2.1 Molecule-based magnets magnetism, nature sometimes For more than half a century it uses molecular techniques to has been a dream for chemists to develop complex magnetic design molecule-based magnets. behavior patterns. Thus, the Today such exotic materials exist. second important factor behind The first ones were reported in the emergence of molecular 1986, both in America and in magnetism was the new Europe. Ten years later, a few tens understanding of the properties of molecule-based compounds of complex biological systems exhibiting a spontaneous ranging from heme proteins to magnetization below a certain iron-sulfur proteins, from temperature are known. In superoxide dismutase to ferritin, contrast with the classical and thus the mechanism of magnets which are opaque, the biomineralization. A third factor molecule-based magnets are promoting renewed interest in usually weakly colored, so that molecular magnetism was the one of the main issues concerns interest of solid state physicists in the synergy between magnetic low-dimensional physics. During and optical or photophysical the last few years molecular properties. A very appealing magnetism has offered to the perspective would be to design physicists quite a few exotic low- molecule-based magnets whose dimensional magnetic systems magnetic properties could be fine- with unprecedented spin tuned through light irradiation at topologies. The final factor was given wavelengths. the interest in new classes of advanced materials and the The spin topology of most of the appearance of molecular molecule-based magnets is rather electronics. One of the most complex, so that the effect of a 69

high magnetic field is not as this minimum should disappear in straightforward as for classical high fields somewhere between 30 magnets. In many cases, even at and 80 T. There is a strong interest 20 or 30 T, saturation is not in the community about this reached. This situation is due to prediction, and only a large field the fact that the compounds may facility will allow confirmation or present both ferromagnetic and rejection of this conjecture. antiferromagnetic interactions between uncompensated spins. At high enough fields, some decoupling of the antiferromagnetically coupled spins may occur, leading to unusual magnetization versus magnetic field curves (Fig. 1). In Fig. 1: Magnetization (in Bohr magnetons per molecular unit) versus other respects, in purely molecule- magnetic field curves for two molecular-based magnetic compounds showing long range magnetic ordering below 15 K based ferrimagnets, when the field (A) and 22.5 K (A). The magnetization curves were measured at 5 is not too high, the temperature K (A) and 4.2 K (B), respectively. In compound A the dependence of the product of the ferrimagnetically coupled magnetic centers MnII and CuII are magnetic susceptibility exhibits a connected in a two-dimensional network whereas in compound B, where a third spin carrier rad+ replaces the diamagnetic cation minimum at a certain + NBu4 in A, the network becomes three-dimensionaly ordered. In A, temperature. This minimum is full saturation of magnetization is attained within a few tesla, whilst considered as the fingerprint of in B, after a fast rise of magnetization at low fields, a slower rise the ferrimagnetic regime. It has follows which is not even saturated at 20T. This field dependence, the full analysis of which still needs measurements up to several been suggested very recently from times higher fields, bears important clues to the role of the magnetic density matrix renormalization coupling between the spin subsystems, the understanding of which group (DMRG) calculations that is essential for developing new magnetic materials. 70 5. The Scientific Case Chemistry

5.2.2 Molecular bistability and 5.2.3 Nanoscale magnetic spin transition materials materials The most spectacular example of A third exciting area in molecular molecular bistability is certainly magnetism, which was developed provided by the phenomenon of more recently, concerns molecular spin crossover, or spin transition. clusters of nanoscale dimensions. Some transition metal molecular The interest here derives from the compounds can present a same ideas which result in the crossover between a low-spin (LS) miniaturization of electronic and a high-spin (HS) state. This devices. Matter on the nanoscopic crossover may be induced by a scale has different properties, variation of temperature, which could open up many new pressure, or by irradiation with perspectives. One of these light. In some cases, the perspectives is to detect a thermally induced transition transition from the quantum between LS and HS states is regime of microscopic particles to cooperative, occurring with well that of the thermodynamical pronounced thermal hysteresis. In regime, which applies to bulk the family of Fe(II)-1,2,4-triazole magnets. Another appealing issue compounds, for instance, the here is to demonstrate the effects transitions occur around room of quantum tunnelling. Such

temperature, with thermal effects have been found in a Mn12 hysteresis widths which may molecular cluster. This cluster has reach 50 K. Furthermore, the a ground state with spin S=10 and transitions are accompanied by a a large magnetic anisotropy. When spectacular change of colour, the cluster is in its ground state, from violet in the LS state to the magnetization is preferentially white in the HS state. Several of oriented parallel to a tetragonal these compounds have already symmetry axis. It can be oriented been used as active elements of either up or down, the two display devices. Of course, the orientations having the same spin transition regime may be energy. For the system to reorient, influenced by a magnetic field it must overcome an energy which shifts in energy the barrier between the

Zeeman components of the spin MS = +10 and MS = -10 states. states. It would be very promising Above about 60 K, the system to investigate the response of easily passes over this energy such spin transition materials to barrier. On the other hand, at low an intense magnetic field. It has temperature, reorientation been suggested, but not verified to becomes increasingly difficult, date, that this type of bistability and at 2 K the time required for could be induced by a very high inverting the magnetization field in materials which show no becomes of the order of one change of spin state in zero or month. The cluster may be weak field. considered as a genuine molecular 71

magnetic bistable device. Here metallopolymers the molecular again, the access to very high ordering induced by the magnetic magnetic fields would be of the field due to the diamagnetic utmost importance to get new anisotropy of the mesogen would insights into the physics of this be accompanied by a very high new class of compounds. magnetic anisotropy and it would be most interesting to study 5.2.4 Chemical manipulation cooperative magnetic phenomena under very high magnetic fields of such media in intense The last aspect of molecular magnetic fields. magnetism covered here is the problem of knowing to what Finally we note that recent extent chemical manipulations experiments in magnetic fields up under very high fields could to 8 T have demonstrated that the provide new phases, or could growth of crystals of diamagnetic preferentially yield a phase which compounds, even of low is difficult to obtain under normal molecular weight, can be conditions. For example, the controlled by a magnetic field. compound para-nitrophenyl- The crystallization of proteins is nitronyl-nitroxide, the first one of the most difficult purely organic magnet, discovered problems that must be solved to by Kinoshita and coworkers [1], prepare samples suitable for may exist in at least four phases, structure determination by X-ray and only one of these phases, the or neutron diffraction. The β-phase , shows a long-range application of very high magnetic ferromagnetic ordering. This fields to promote the phase is difficult to obtain, and it crystallization of large anisotropic has been suggested that a very macromolecules could make way high field would favor this for new break-throughs in the magnetic phase during the crystal structure determination of growing process. More generally, biologically important the use of very high magnetic macromolecules. fields to modify the output of chemical manipulations 5.3 Magnetic resonance represents a virgin field, in which much can be expected (see also In this section we will stress only section 4). the general chemical interest in developing magnetic resonance The orientational ordering of at higher magnetic large molecules is discussed in fields. For more specific and more detail by section 6 in relation to technological aspects we refer to Soft Condensed Matter. Here we the section 8 report below. want to add the interesting possibility that when using mesogenic magnetic 72 5. The Scientific Case Chemistry

5.3.1 Nuclear magnetic will make the chemical shift of resonance quadrupolar nuclei also more One can hardly overemphasize the interesting as probes of their value of NMR as the ’s chemical environment. most important analytical tool for molecular structure Apart from the structural determination. The information regarding the average measurements of chemical shifts positions of nuclei within a of various probe nuclei, their molecular frame, which are isotropic and anisotropic couplings encoded in NOE cross-peak and relaxation measurements have intensities and the spectral made detailed topological features positions and splittings, NMR of molecular structure accessible relaxation times also hold a large almost routinely. The main gain information potential, during the last two decades was illuminating dynamical aspects of achieved by increasing the intra- and inter-molecular motion resolution due to the use of higher (cf. section 5.3.4) and also of fields, and hence frequencies, and chemical reactions. The latter case the exploitation of more shows up in spectral exchange sophisticated pulse sequences in processes between lines. The NMR FT-NMR. The ultimate time-resolution for such kinetics achievements in this respect is determined by the chemical represent the recent structure shift difference between the lines determinations of soluble proteins involved which will increase in comprising as many as almost 300 proportion to the applied amino acids. Indeed, expanding magnetic field. the limits in resolution and sensitivity requires the Yet another kind of dynamical development of NMR technology information, i.e. regarding beyond 18.8 T (800 MHz for 1H- kinetics and mechanisms of NMR). Pushing these limits elementary chemical processes, forward would not only allow the may be encoded in the non- analysis of larger molecules, with Boltzmann population of nuclear the use of the standard nuclei 1H, spin levels belonging to products 13C, and 15N but, due to the gain in formed during a sensitivity resulting from higher with radical pair intermediates in magnetic fields, would also expand a magnetic field (chemically the range of nuclei to less induced nuclear polarization, sensitive ones, so extending the CIDNP). For the exploitation of range of chemical compounds such effects, too, higher fields amenable to this powerful would open up new possibilities. analytical tool. Since, in higher fields, quadrupolar splittings 5.3.2 paramagnetic become less significant relative to resonance chemical shift differences, this Being restricted to molecular 73

species with unpaired electron decreasing separation. Higher spins, EPR does not have as wide a magnetic fields will move forward chemical scope as NMR, but it is, the frontiers of the study of such nevertheless, an invaluable interactions by EPR. It is of spectroscopic technique in the interest that the detection of study and characterization of magnetic resonance in reactive paramagnetic chemical species. radical pairs does not necessarily depend on a detection method The group of S = 1/2 systems which uses radiation in the comprises the important class of frequency range of the resonance, organic and inorganic radicals but can utilize reaction yields to which usually occur as reactive probe the field dependence of intermediates in radical reactions resonance absorption (that is, the and very often are involved in reaction yield detected magnetic photochemical and radiation resonance, RYDMR, cf. section chemical reactions. On the other 5.4 on spin chemistry). hand, a number of stable S = 1/2 systems are also known, e.g. stable Molecular systems of interest, organic and inorganic radicals of having more than one unpaired second row elements which are spin and S > 1/2, comprise excited often utilized as spin probes, and molecular triplet states, organic numerous transition metal polyradicals and mono- to poly- complexes, including nuclear transition metal metalloproteins. Structural complexes, including many information on S = 1/2 systems metalloproteins and building results from g-tensor and blocks of new magnetic materials. hyperfine-coupling (hfc) tensor Also noteworthy are complexes of parameters which allow chemical lanthanoid most of which identification and electronic and belong to this class of S>1/2 conformation-related structural compounds. For all these, EPR is characterization, also with respect an important tool when seeking to to the molecular environment of understand the relationship the spin probes. Here, as in NMR, between electronic and geometric an increase of field strength is structure and for the tantamount to better spectral identification of structure resolution and sensitivity. including, in particular, the molecular environment of active It is sometimes possible in radical enzymatic centres. For S>1/2 reactions for radical pairs to be systems, structure dependent trapped as they are generated. In interaction between the unpaired closely coupled radical pairs the spins causes zero-field splittings spin-spin interaction, in particular which represent an important the exchange interaction, can be characteristic of the spin quite high due to an exponential Hamiltonian of such systems and increase of exchange energy with hence an important key to the 74 5. The Scientific Case Chemistry

structure. In fact, zero-field here and in the context of splittings can be much larger than molecular or metallic the Zeeman splitting in available magnetism). From the magnetic fields, in which case polarization patterns and their only incomplete information is dependence on various available from magnetic resonance experimental parameters, transitions. For this reason, as well including the magnetic field as for the general argument strength, detailed information can concerning the improvement in be obtained about the mechanism sensitivity, a considerable increase and dynamics of the underlying in the scope of interesting reactions. The application of high molecular magnetic systems can fields can be particularly useful in be expected for EPR by increasing the case of radical pairs with the range of accessible EPR fields. strong exchange interaction, such as occur if the radical centres are Finally we mention that, as with closely connected by chemical nuclear spins, electron spin links (small biradicals) or when relaxation and chemically induced the pairs are trapped in spin polarization effects are also environments representing fairly highly informative phenomena rigid cages, i.e. in the solid state or which will benefit from the in microheterogenous media. availability of higher magnetic fields. In principle, chemically induced nuclear polarization (CIDNP) is 5.3.3 Chemically induced easier to detect than electron magnetic polarization polarization. Due to the long The combination of magnetic nuclear, T1, spin relaxation times interactions in radical pairs with in diamagnetic compounds, which spin conserving processes in the may be as long as several seconds, creation and recombination of separate magnetic fields may be such intermediates of radical used to create the polarization, reactions, can be a source of where the reaction is carried out, considerable deviation of spin and then to measure the level populations from thermal polarization by sample transfer in equilibrium. It gives rise to a flow line to a conventional NMR strongly enhanced absorption or spectrometer. This technique has emission lines detectable in the been successfully applied to EPR spectra of paramagnetic investigate the field dependence intermediates or in the NMR of CIDNP in low fields and spectra of diamagnetic products should be possible to employ with from radical reactions. These the very highest fields without effects are known as chemically the need to change the NMR induced magnetic polarization technique. (One should note the different usage of the term spin polarization 75

In the case of chemically induced motions to the spin system and electron spin polarization also that this coupling be (CIDEP), the polarization decays modulated by stochastic lattice within microseconds or less and motions. The spectral power has to be measured in the field density of the stochastic wherein it is created with time- perturbational force at the resolved techniques. This resonance frequency of the spin represents a very direct transition determines the spin- -1 observation method for lattice energy exchange rate, T1 . paramagnetic reaction Thus, with this parameter intermediates. It appears that as measured as a function of the applied fields and frequencies magnetic resonance frequency get higher, less microwave pulse (magnetic relaxation dispersion) it energy is necessary to generate is possible to scan the power detectable signals, so that the spectrum and thereby determine pulses can be made shorter with a the dynamical characteristics of concomitant increase in time- the lattice environment of the resolution. Very high fields spin probe. Such modes of interest together with the corresponding comprise molecular rotations and high-field EPR technology are translational diffusion as well as necessary to separate overlapping conformational motions and oppositely polarized (so other reversible chemical compensating each other) spectral transformations. Different modes regions of two radical may be identified by their τ components in the case of radical characteristic correlation times c, pairs with very small difference in leading to a characteristic drop of ω τ -1 electronic g-factors. the power spectrum at 0= c . Thus it is clear that lattice modes 5.3.4 Spin relaxation of increasingly shorter correlation While the spectral positions of times may be probed through magnetic resonance transitions magnetic relaxation dispersion by yield information about time increasingly higher magnetic independent or time-averaged fields. interactions of probe spins with their environment, other The highest EPR frequencies in important information is encoded presently available commercial in dynamic observables, in spectrometers allow the probing particular in the spin lattice of lattice modes up to 90 GHz (3 cm-1, 3 T). In commercial NMR relaxation time, T1. Energy exchange between a spin spectrometers much higher transition at some resonance magnetic fields (currently up to ω 18.8 T) are employed, but of frequency, 0, and the other intra- and inter-molecular degrees of course the corresponding NMR (“lattice”) motional freedom, frequencies are lower (up to requires a coupling of these lattice 800 MHz). There is however a way 76 5. The Scientific Case Chemistry

to utilize NMR fields and reactivity in paramagnetic spectroscopy to probe EPR reaction intermediates, mostly transitions indirectly through radical pairs, with unpaired NMR in paramagnetic electron spins. These effects, compounds, as in many transition which result in magnetic field metal complexes or dependent reaction yields and metalloproteins. Here the rates, or in non-equilibrium relaxation time of the electron electron or nuclear spin spin can be detected by its effect polarization of products emerging on the nuclear spin. Thus the from such reactions (cf. section highest NMR fields presently 5.3.3), are essentially dynamical in available can give access to nature and not a matter of electron spin lattice relaxation thermodynamics, i.e. of chemical with energy quanta of 25 cm-1. In equilibrium. In spin chemistry an NMR field of 40 T, this range one faces the remarkable situation could be extended to 40 cm-1. A that magnetic interactions that further possibility to probe are small compared to thermal electron spin relaxation arises energies at usual reaction through spin chemical effects, as temperatures ( > 200 K), and even discussed in the next section, much smaller than the reaction which, since they usually rely on energies of the chemical optical probe techniques, could be transformations which they measured even in pulsed fields of control, can be used to switch 100 T. Thereby, the effects of reaction probabilities and to lattice motions could be assessed control the flow into various with ‘formal’ correlation times reaction channels. These effects shorter than 10-13 s. have a high potential for probing and acquiring an understanding 5.4 Spin chemistry of the dynamics of reaction intermediates. Spin chemistry denotes the area of chemical research devoted to The majority of spin chemical the elucidation and exploitation reaction effects are accounted for of spin and magnetic field by the so-called radical pair induced effects on chemical mechanism. Important classes of reactions. Although attempts to reactions, among them many control chemical reactions by photochemical ones, including magnetic fields have a long photosynthesis, proceed through history, the beginning of a radical pairs as reaction systematic and successful intermediates. These radical pairs evolution of the field dates back can equilibrate between a set of only about 30 years and depended nearly degenerate electron- largely on the discovery of some nuclear spin states which, in spite basic molecular mechanisms, of their near degeneracy, fall in involving spin control of two manifolds of states 77

(electronic singlet and triplet) g-factors and hence different differing extremely in their coupling strength of the two chemical reactivity. Transitions radical spins to the external between these manifolds can be magnetic field. As a consequence, induced by relatively weak the external magnetic field interactions, e.g. hyperfine disrupts the mutual alignment of coupling, and are effective in the two radical spins which is modifying chemical reaction equivalent to a field-induced rates. External magnetic fields mixing of triplet and singlet spin may have three different effects multiplicity. The transition rate, ω ∆ γ of spin chemical importance. = g eB/2, is proportional to the magnetic field and to the (i) The first is due to the Zeeman difference in g-factors. Large effect, causing level separations effects will ensue if the spin and crossings. Particular interest transition rate can be tuned into in using high external magnetic the specific kinetic range of the fields arises where radical pairs chemical rate processes taking fixed at close distances are place within the radical pair. The involved. Here the energy highest magnetic fields applied so splitting between singlet and far to drive such transitions are triplet (exchange interaction) may still below 20 T. Recently it has be too large to allow hyperfine been possible to probe fast induced singlet-triplet transitions electron transfer reactions with in zero field, while such time constants of a few transitions may be feasible due to picoseconds in some reaction a singlet/triplet level crossing in a systems with high ∆g involving suitable external magnetic field. transition metal complexes The present progress in the (Fig. 2). Other high field of specially applications of this type of effect tailored supramolecular systems have been reported with the allows to make biradicals with primary radical pair in well defined separation of radical photosynthetic reactions centers. centers for which high magnetic The availability of higher field spin chemistry as well as magnetic fields, in the range 50- magnetic resonance spectroscopy 100 T, will allow the modification can provide suitable methods of of the kinetics of many more probing the electronic interaction systems in the nano- to between the two radical centers. femtosecond time domain and will be of particular interest in (ii) The second type of magnetic combination with detailed time- field effect is found in systems resolved studies of fast reactions where the two radicals of a pair in supramolecular entities. exhibit different spin-orbit coupling. This difference is reflected in different electronic 78 5. The Scientific Case Chemistry

Fig. 2: Magnetic field dependence of the yield of separated electron transfer products in the photoinduced oxidation of various RuII-trisdiimine complexes by methylviologen. The yield decreases with increasing field due to a field-induced, ∆g- dependent spin process, accelerating fast backward electron transfer prior to product separation in the primary pairs of electron transfer products. For these systems with their ∆g~1 being exceptionally high, saturation of the effect could be reached at about 20 T, however for the great majority of systems of interest ∆g is smaller and higher magnetic fields are necessary for attaining the full kinetic information inherent in this type of magnetic field effect.

(iii) As was already pointed out in methods usually applied for the section 5.3.4, a third type of spin detection of spin chemical effects chemical magnetic field effect do not require special results from the magnetic field technological adaptations when dependence of spin lattice applied in high magnetic fields. relaxation. In radical pairs chemical reaction rates are sensitive to spin transitions 5.5 Optical spectroscopy between the split Zeeman levels. Such transitions are brought about Whereas, in the optical by the stochastic modulation of spectroscopy of , the study interactions with the lattice. Thus of magnetic level splittings, i.e. of the magnetic relaxation dispersion the , is of can be probed through the paramount importance for magnetic field dependence of characterizing and assigning chemical reaction rates or yields. electronic states, such effects are The advantage of this spin less prominent in molecular chemical method, as compared to spectroscopy. The reasons are the magnetic resonance one, is twofold. First, the orbital that, in contrast to the case of contributions to the magnetic magnetic resonance moment of electronic states is spectroscopies, the optical much reduced in molecules 79

because usually they lack relative energies of these states rotational symmetry. Thus orbital change. Dynamic effects have contributions to the Zeeman been widely studied as magnetic effect are small. Second, as a result field effects in gas phase of the greatly increased line luminescence spectroscopy. widths in molecules, as compared Extending the range of available to atoms, it is difficult to magnetic fields will open up new determine shifts and splittings. ranges of level interactions. Such Because the Zeeman effect effects will very likely help to increases linearly with magnetic increase our understanding of the field, the availability of higher dynamics of intramolecular magnetic fields will extend the energy redistribution. A second interest in, and the practical aspect arises because the magnetic potential of, Zeeman effect field itself can provide a coupling studies in molecular spectroscopy. between different electronic Of course, direct observations of states, thereby enhancing existing, molecular Zeeman effects will be or introducing new, channels of most promising under conditions electronic relaxation, or allowing of high spectral resolution, i.e. in radiative transitions between gas phase spectroscopy and low states that are hidden in zero or temperature spectroscopy of low magnetic fields. The magnetic crystalline solids. But using field induced predissociation of special techniques, like spectral iodine is a famous historical hole burning, less ordered systems example of such phenomena. can be investigated with narrow Chemical effects of magnetic band resolution at low state mixing are of particular temperatures. interest in the context of spin chemistry. Besides energetic shifts displayed in the position of spectroscopic Finally, an important effect of the transitions, a second important Zeeman separation of state effect of magnetic fields lies in energies in a magnetic field is to the induction of dynamic effects, introduce chirality into optical eventually observable in transitions. This can be observed linewidths or excited state life as magnetic circular dichroism times. This is a consequence first (MCD), a phenomenon not of changing energy separations, restricted to high resolution including level crossings, for close spectroscopy. What circular lying states with different dichroism spectroscopy achieves magnetic moments. for chiral molecules or Intramolecular electronic supermolecular systems with relaxation is largely determined by chiral properties (e.g. proteins of a the coupling between different regular secondary structure), state manifolds like singlet and namely the discrimination of triplet and is modified if the small yet specific contributions of 80 5. The Scientific Case Chemistry

their optical absorption against a 5.6 Conclusion large absorption background of non-chiral contributions, is Intense magnetic fields, beyond possible to exploit also for many those currently available, are of molecular systems which do not wide application in many areas of have chiral chromophores or Chemistry; molecular magnetism, chirally ordered chromophores if magnetic resonance, spin one uses MCD spectroscopy. chemistry and optical spectroscopy. In some cases, e.g. MCD is an established optical spectroscopy, the spectroscopic technique with requirements on field applications to molecules of high homogeneity in space and symmetry or at least derived from constancy in time, are a parentage of high symmetry considerably less stringent than in (aromatic molecules) and, magnetic resonance applications. furthermore, to many transition Nevertheless, notwithstanding the metal coordination compounds interest in 100 T fields for with degenerate or near exploring new forefront effects, degenerate ground states. for obtaining quantitative and Important representatives of the precise data, steady fields of 50 T latter systems are found in will be of wider application in this metalloprotein enzymes. Here area than pulsed 100 T fields with MCD is used as a structural a low repetition rate. investigation method offering an alternative and complementary [1] Y. Nakazawa, K. Nozawa, D. approach to EPR. Significantly Shiomi, K. Awagawa, T. Inabe, Y. enhanced magnetic fields will Maruyama and M. Kinoshita, Phys. inevitably widen the scope and Rev. B 1992, 46, 8906, potential of MCD for applications in structural investigation without requiring significant changes of the spectroscopic technique as in the case of EPR. For S>1/2 systems with large zero field splitting, high magnetic fields are particularly promising for extending the scope of possible applications.