Jens Martinsen

General theory of polycrystalline ENDOR patterns. g and hyperfine tensors of arbitrary symmetry and relative orientation

Abstract A general formulation is presented for the ENDOR spectra of a randomly oriented, polycrystalline (powder) S = 1 2 paramagnet, assuming slow cross-relaxation, g anisotropy domination of the EPR spectrum, arbitrary symmetries and relative orientations of g and hyperfine tensors, and δ-function EPR and ENDOR component lineshapes. Calculations for simple, archetypical types of centers have been presented, as well as selected calculations showing the full generality of the method. In particular, at g values where the ENDOR spectra arise from multiple orientations of the paramagnet (powder-type spectra) it was found that the ENDOR shape functions, N ( A ) g , show two divergences and no steps when the g and A tensors are coaxial; noncoaxiality can introduce subsidiary maxima and can cause the occurrence of as many as four additional divergences.

General theory of polycrystalline ENDOR patterns: effects of finite EPR and ENDOR component linewidths

Recently (I) we presented a general method for calculating ENDOR spectra of a randomly oriented polycrystalline (powder) S = 4 paramagnet (2, 3) that has g and hyperfme tensors of arbitrary symmetry and relative orientation. That study (Paper I) laid a foundation for the detailed analysis of polycrystalline ENDOR spectra, as well as for the heuristic categorization of such spectra, much as EPR spectra long have been classified (isotropic, axial, rhombic (4)). To this end the calculations in I used a “double delta function” approach, employing a-function EPR and ENDOR component lineshapes. They showed that polycrystalline ENDOR spectra can exhibit a rich array of features when the static field is set to a g value where the EPR signal arises from a well-defined set of orientations of the paramagnet, rather than a single crystal-type setting. The principal values and relative orientation of a hyperline tensor can be obtained by examining such angle-selected spectra obtained as the observing g value (static field) is moved across the EPR envelope. We now have generalized the analysis to permit full simulations of polycrystalline ENDOR spectra by including finite EPR and ENDOR component linewidths. The calculations presented here show that the two types of features that arise in the double delta-function limit, divergences and maxima, both persist as absorption peaks in the full simulations. These ENDOR peaks broaden equally as the ENDOR component linewidth, W,,, increases, but in general they do not broaden equally as the EPR component linewidth, W,, increases. Instead, the rate at which a peak broadens with W, is related to (&/dg), the rate of change in its ENDOR frequency with observing g value. The essential features of the approach developed in I are first summarized, and then are extended to include EPR and ENDOR component linewidths. When the external field is set to an arbitrary value, Ho, within a a-function EPR envelope, the EPR signal intensity, and thus an ENDOR signal, arises from the selected molecular orientations associated with the curve on the unit sphere, S, comprising points for which the orientation dependent spectroscopic splitting factor, g = g(B, @), has a fixed value defined by the strength of the observing field: gmi, < g = hv/@& < g,,,. H’owever, although g is constant over the locus of points, S, in general the angledependent hyperfine coupling, A = A(& 4), is not, and the ENDOR pattern in general is more complex than a single crystal-like spectrum. The net ENDOR intensity at radiofrequency, Y, is determined by the sum of the probabilities that the

Stacked metal complexes: Structures and properties

We discuss the structural aspects of crystals that contain molecular stacks and are comprised of two components, at least one of which is a planar transition-metal chelate complex. One of the components may be viewed as an electron donor, D, or its cation, D+p, the other an acceptor, A, or its anion, A−q. In these crystals either one or both components tend to form stacks, and we employ a classification scheme whose first major division is structural: crystals either contain stacks that integrate the D and A units, or the D and A units can be segregated with D or A stacking, or both. The second major division is physical. In an integrated stack crystal the metal-ligand complex is either neutral or ionic; in a segregated stack crystal it either has an integral oxidation state or a non-integral oxidation state. The non-integral oxidation state is sometimes referred to as “partial oxidation”, “mixed valence”, or “incomplete charge transfer”.

Porphyrinic Molecular Metals

Porphyrinic molecular metals rival the organic metals in the range of their properties and surpass them in the tunability of these properties. Such Such tunability is illustrated here by the change...

Carrier Properties of Porphyrinic Molecular Metals

Abstract A series of quasi one-dimensional molecular metals based on metallomacrocycle building blocks is compared. The group of compounds Ni(L)I, where L = dian-ion of phthalocyanine (PC), tetrabenzporphyrin (TBP), or triazatetrabenzporphyrin (TATBP), are isoionic and isostructural but the nature of the charge carrier is quite different in each material. Oxidation in Ni(PC)l is exclusively from ligand π-orbitals so that conduction is associated with delocalized π-orbitals. Ni(TBP)I and Ni(TATBP)I display epr spectra that demonstrate that oxidation has occurred from both ligand ir-orbitals and Ni d-orbitals so that these compounds display a novel, doubly mixed-valence state. The compounds Ni(PC)I, Ni(TMP)I, and Ni(OMTBP)I (TMP = tetramethylporphyrin, OMTBP = octamethyltetrabenzporphyrin) form an isoionic series where Coulomb correlations are of progressively greater importance.

Magnetic transitions in a molecular metal with embedded local moments: Cu(pc)I

Abstract We discuss the low temperature magnetic properties of (phthalocyaninato)-copper(II) iodide (Cu(pc)I). This molecular metal contains conductive stacks that incorporate a one-dimensional array of local Cu +2 moments strongly coupled to conduction electrons. Below 20K, the EPR g-value of the coupled spin system increases anomalously and at 8K the EPR signal broadens abruptly and becomes unobservable. Anomalies in the proton NMR spin-lattice relaxation times are observed at the transition temperature. Preliminary EPR experiments conducted on a newly synthesized series of materials, Cu x Ni 1−x (pc)I, indicate the existence of a similar low temperature transition in the presence of various concentrations of loca moments.

Metallic Conductivity and Magnetic Interactions in Nickel and Copper Phthalocyanine Iodides

Abstract We discuss the charge transport and magnetic properties of the titled molecular metals. Replacement of the diamagnetic Ni(+2) by paramagnetic Cu(+2) introduces a novel coupling between local Cu(+2) spins the mobile charge carriers, and produces a coupled transition at 8 K.

Structural, Magnetic, and Charge-Transport Properties of Stacked Metal Chelate Complexes

Intense effort devoted to the synthesis and physical characterization of crystals containing stacked, planar, transition-metal chelate complexes has yielded many materials of widely divergent properties. Some are primarily of structural interest. Others have unusual magnetic properties; for example, the first well-documented instance of a spin Peierls transition (a solid state analogue of the Jahn-Teller effect) was observed in such a material.1 Yet others exhibit high or unusual electrical conductivities, and the greatest effort is currently being expended toward the understanding and control of charge-transport properties in molecular crystals. Here compounds prepared from metallomacrocycles are of particular interest and challenge. For example, with phthalocyaninatonickel iodide, Ni(pc)I,† a high, metallike conductivity along the stacking axis is achieved,2,3 whereas bis(diphenylglyoximato)nickel iodide, Ni(dpg)I,4 has the same structural motif but exhibits much lower conductivity. To understand and control such divergent physical properties in related solid state structures has engendered considerable research, particularly since the possibility exists of tailor-making compounds with specific transport properties through well-defined chemical modifications.

Oxidation state of binuclear iron site in azidosemimethemerythrin

Electron paramagnetic resonance spectra of azidosemimethemerythrin (from Phascolopsis gouldii) have been integrated to find the total number of spins per monomer unit. The value observed, 1.0 +/- 0.1 spins per Fe2 pair, confirms the assignment of a hybrid oxidation state, FeIIFeIII, to each site.