Nuclear Magnetic Resonance Relaxivities: Investigations of Ultrahigh‐Spin Lanthanide Clusters from 10 MHz to 1.4 GHz

Paramagnetic relaxation enhancement is often explored in magnetic resonance imaging in terms of contrast agents and in biomolecular nuclear magnetic resonance (NMR) spectroscopy for structure determination. New ultrahigh‐spin clusters are investigated with respect to their NMR relaxation properties. As their molecular size and therefore motional correlation times as well as their electronic properties differ significantly from those of conventional contrast agents, questions about a comprehensive characterization arise. The relaxivity was studied by field‐dependent longitudinal and transverse NMR relaxometry of aqueous solutions containing Fe^III₁₀Dy^III₁₀ ultrahigh‐spin clusters (spin ground state 100/2). The high‐field limit was extended to 32.9 T by using a 24 MW resistive magnet and an ultrahigh‐frequency NMR setup. Interesting relaxation dispersions were observed; the relaxivities increase up to the highest available fields, which indicates a complex interplay of electronic and molecular correlation times.     

Rotational coupling effects on the spin relaxation of free ferromagnetic clusters

A model based on resonant spin-rotation coupling is used to reproduce the magnetization of free iron clusters experimentally observed by Stern-Gerlach deflections. Due to the interaction with the rotating magnetocrystalline anisotropy field, the dynamics of the magnetic moment of the clusters cannot be explained only in terms of thermal relaxation. As in magnetic resonance experiments, modified Bloch equations can describe the interaction of the particle spin with the deflecting field and the rotating anisotropy field. The dependence of the magnetization on the rotational temperature of the clusters and the relaxation time is discussed.     

Gradient echo single scan inversion recovery: application to proton and fluorine relaxation studies

Single scan longitudinal relaxation measurement experiments enable rapid estimation of the spin‐lattice relaxation time (T₁) as the time series of spin relaxation is encoded spatially in the sample at different slices resulting in an order of magnitude saving in time. We consider here a single scan inversion recovery pulse sequence that incorporates a gradient echo sequence. The proposed pulse sequence provides spectra with significantly enhanced signal to noise ratio leading to an accurate estimation of T₁ values. The method is applicable for measuring a range of T₁ values, thus indicating the possibility of routine use of the method for several systems. A comparative study of different single scan methods currently available is presented, and the advantage of the proposed sequence is highlighted. The possibility of the use of the method for the study of cross‐correlation effects for the case of fluorine in a single shot is also demonstrated. Copyright © 2015 John Wiley & Sons, Ltd.     

Development of the spin crossover model for compressible and nonuniform chains of exchange clusters

A model of a compressible chain of exchange clusters, which retains the cooperative behavior during thermal crossover, was developed. The model was generalized for periodical chains with two and more exchange clusters per unit cell. A general expression was obtained for the effective magnetic moment of the unit cell with several exchange clusters. A two-position approximation for the partition function of the Jahn-Teller paramagnetic center Cu^II (spin 1/2) was proposed for chains with the “head-head” motif, which makes it possible to derive the equation for the effective magnetic moment in quadratures. The model developed describes the cases of both smooth and sharp (cooperative) spin crossover for chains of three-spin exchange clusters.     

Rational Design of [13C,D14]Tert‐butylbenzene as a Scaffold Structure for Designing Long‐lived Hyperpolarized 13C Probes

Dynamic nuclear polarization (DNP) is a technique to polarize the nuclear spin population. As a result of the hyperpolarization, the NMR sensitivity of the nuclei in molecules can be dramatically enhanced. Recent application of the hyperpolarization technique has led to advances in biochemical and molecular studies. A major problem is the short lifetime of the polarized nuclear spin state. Generally, in solution, the polarized nuclear spin state decays to a thermal spin equilibrium, resulting in loss of the enhanced NMR signal. This decay is correlated directly with the spin‐lattice relaxation time T₁. Here we report [¹³C,D₁₄]tert‐butylbenzene as a new scaffold structure for designing hyperpolarized ¹³C probes. Thanks to the minimized spin‐lattice relaxation (T₁) pathways, its water‐soluble derivative showed a remarkably long ¹³C T₁ value and long retention of the hyperpolarized spin state.     

Structural and dynamical properties of magnesium microclusters

We present systematic Density Functional Theory-Local Density Approximation computations for neutral Magnesium clusters Mg _n withn≤13. For the smaller sizes the ground state structure is optimized starting from selected symmetries and allowing for relaxation, Jahn-Teller distorsion and spin polarization. For the larger sizes we perform a simulated annealing based on the ab-initio Molecular Dynamics. By the same method, we study the thermal and dynamical properties of Mg₁₀ and Mg₁₆. The general picture emerging from these computations shows that already atn ≈10 these clusters have acquired many characteristic features of metallic Magnesium.     

Structural Investigation of the High Spin→Low Spin Relaxation Dynamics of the Porous Coordination Network [Fe(pz)Pt(CN)4]⋅2.6 H2O

The Hoffman‐type coordination compound [Fe(pz)Pt(CN)₄] ? 2.6 H₂O (pz=pyrazine) shows a cooperative thermal spin transition at around 270 K. Synchrotron powder X‐Ray diffraction studies reveal that a quantitative photoinduced conversion from the low‐spin (LS) state into the high‐spin (HS) state, based on the light‐induced excited spin‐state trapping effect, can be achieved at 10 K in a microcrystalline powder. Time‐resolved measurements evidence that the HS→LS relaxation proceeds by a two‐step mechanism: a random HS→LS conversion at the beginning of the relaxation is followed by a nucleation and growth process, which proceeds until a quantitative HS→LS transformation has been reached.     

Effect of rapidly relaxed electron spin anisotropically coupled to nearby nuclear partners

In practice, many situations arise when a perturbed nuclear spin relies upon the aid of a rapidly relaxed spin neighbor in order to realize thermal equilibrium. Conventional treatments view the efficiently relaxed spin as part of the 'lattice', invoke the secular approximation, and consider the associated time correlation of the lattice variables, phenomenologically. Recently, an ab initio perturbative approach has been proposed for investigation of these spin systems. In this work, a similar formalism is applied to the scenario, in which nuclear spin relaxation is effected via an anisotropically coupled, efficiently relaxed, spin. Interesting conflicts with standard theory are revealed. Furthermore, although left mostly unexplored, this approach lends insight into numerous related aspects of magnetic relaxation including separation of timescales, distinction between spin and spatial averaging, paramagnetic relaxation, Curie spin relaxation, and the dynamic frequency shift.     

Two‐Step Thermal Spin Transition and LIESST Relaxation of the Polymeric Spin‐Crossover Compounds Fe(X‐py)2[Ag(CN)2]2 (X=H, 3‐methyl, 4‐methyl, 3,4‐dimethyl, 3‐Cl)

In the series of polymeric spin‐crossover compounds Fe(X‐py)₂[Ag(CN)₂)]₂ (py=pyridine, X=H, 3‐Cl, 3‐methyl, 4‐methyl, 3,4‐dimethyl), magnetic and calorimetric measurements have revealed that the conversion from the high‐spin (HS) to the low‐spin (LS) state occurs by two‐step transitions for three out of five members of the family (X=H, 4‐methyl, and X=3,4‐dimethyl). The two other compounds (X=3‐Cl and 3‐methyl) show respectively an incomplete spin transition and no transition at all, the latter remaining in the HS state in the whole temperature range. The spin‐crossover behaviour of the compound undergoing two‐step transitions is well described by a thermodynamic model that considers both steps. Calculations with this model show low cooperativity in this type of systems. Reflectivity and photomagnetic experiments reveal that all of the compounds except that with X=3‐methyl undergo light‐induced excited spin state trapping (LIESST) at low temperatures. Isothermal HS‐to‐LS relaxation curves at different temperatures support the low‐cooperativity character by following an exponential decay law, although in the thermally activated regime and for aX=H and X=3,4‐dimethyl the behaviour is well described by a double exponential function in accordance with the two‐step thermal spin transition. The thermodynamic parameters determined from this isothermal analysis were used for simulation of thermal relaxation curves, which nicely reproduce the experimental data.     

Magnetic properties of free alkali and transition metal clusters

The Stern-Gerlach deflections of small alkali clusters (N<6) and iron clusters (10<N<500) show that the paramagnetic alkali clusters always have a non-deflecting component, while the iron clusters always deflect in the high field direction. Both of these effects appear to be related to spin relaxation however in the case of alkali clusters it is shown that they are in fact caused by avoided level crossing in the Zeeman diagram. For alkali clusters the relatively weak couplings cause reduced magnetic moments where levels cross. For iron clusters however the total spin is strongly coupled to the molecular framework. Consequently this coupling is responsible for avoided level crossings which ultimately cause the total energy of the cluster to decrease with increasing magnetic field so that the iron clusters will deflect in one direction when introduced in an inhomogeneous magnetic field. Experiment and theory are discussed for both cases.     

Revisited crystal symmetry of the high‐spin form of the iron(II) spin‐crossover complex di­cyano­[2,13‐di­methyl‐6,9‐dioxa‐3,12,18‐tri­aza­bi­cyclo­[12.3.1]­octadeca‐1(18),2,12,14,16‐pentaene]­iron(II) monohydrate

The title iron(II) complex, [Fe(CN)₂(C₁₅H₂₃N₃O₂)]·H₂O, is of interest to the spin‐crossover community because of its unusual temperature‐dependent magnetic behaviour as well as its relatively high relaxation temperature for the light‐induced spin‐crossover phenomenon. Structural modifications are strongly suspected to cause the unusual thermal spin‐crossover features. Recently, the high‐spin crystal structure has been reported but with an inadequate space group. In the present paper, the crystal structure is corrected by a new investigation, and some consequences for the structure–property relationships of this complex are discussed. The Fe^II ion is seven‐coordinate and lies on a twofold axis.     

Description of spin-crossover in a heterogeneous chain of exchange clusters

Spin transition theory is developed for periodic chains with two and more exchange clusters in a unit cell. A general expression is obtained for the effective magnetic moment of a unit cell μ_eff with several exchange clusters. For heterospin complexes Cu(hfac)₂L^R characterized by chains with the head-to-head motif a twosite approximation for the statistical sum of the Cu²⁺ Jahn-Teller paramagnetic center Cu²⁺ with spin 1/2 is proposed, within which a comparison with the available experimental data is performed. It is shown that the model developed describes both smooth and abrupt (cooperative) spin-crossover cases for the chains of three-spin exchange clusters.     

Polynuclear manganese grids and clusters—A magnetic perspective

High nuclearity paramagnetic, spin-coupled transition metal clusters and grids are fascinating chemists and physicists partly because of their structural beauty, and the challenge of creating them, but also because of their novel physical properties. Magnetic interactions between the spin centers are a primary focus. This review will examine a selection of Mn(II) polynuclear grids and clusters, with nuclearities in the range Mn₄ to Mn₉. Theoretical treatments of the magnetic properties are discussed, and approaches to solving the exchange problem for ‘large’ spin systems related to computational difficulties. A freely available software package (MAGMUN4.1) is presented as a means of dealing simply with spin-coupled clusters in general, and symmetry reduction methods are discussed briefly as a means of dealing with ‘large’ spin systems.     

Stabilities of Al<Subscript>n</Subscript>Cu (n = 1–19) Clusters and Magnetic Properties of Three Cu-Doped Al Clusters

By using the Amsterdam Density Functional program, we have studied the geometric features, stabilities and magnetic properties of Al_nCu (n = 1–19) clusters. The magnetic structures of Al₁₇Cu₂ and Al₁₉Cu clusters are found. Although the high spin ground state of Al₁₂Cu cluster is in accordance with the Hund’s rule under spherical Jellium model (SJM), it is difficult to explain why the Al₁₇Cu₂ and Al₁₉Cu clusters exhibit larger magnetic moments by the model. A superatom model under equivalent charge distribution is proposed. The magnetic properties of the Cu-doped Al clusters can be explained well by combination of the superatom model with SJM.     

Density functional based calculations for Fen (n⩽32)

We investigate magnetic and structural properties of iron clusters up to Fe₃₂, well extending into the size range accessible by experiment. A density-functional based tight-binding scheme fully incorporating the effects of spin polarisation and charge transfer in a self-consistent manner has been used. The potential hypersurfaces have been scanned by an unconstrained search using a genetic algorithm. Results for smaller clusters up to Fe₁₇ are validated against more sophisticated density functional theory calculations. Our magnetic moment data show a strong change around Fe₁₃ being unique in this size range. For the larger cluster sizes a smooth decrease of the clusters average spin magnetic moments is found in good agreement with experimental data.     

Effects of motional narrowing on the plasmon resonance in small metal clusters

A model accounting for the time dependence of shape fluctuations in metal microclusters and their effect on the damping width of the plasmon resonance is discussed. An estimate of the relaxation time of the quadrupole shape is given, and calculations demonstrating the effects of time-dependent thermal fluctuations on the plasmon in K ₉ ⁺ are presented.Lee A. DuBridge research fellow in Physics     

Solid‐state NMR analyses of the crystalline–noncrystalline structure and its thermal changes for ethylene ionomers

The crystalline–noncrystalline structure and its structural changes from thermal treatments for ethylene ionomers have been investigated with solid‐state ¹³C and ²³Na NMR spectroscopy. ¹³C spin–lattice relaxation time (T_1C) measurements reveal that as‐received ethylene ionomers have much enhanced molecular mobility in the crystalline region in comparison with conventional polyethylene samples. By appropriate annealing, however, polyethylene‐like morphological features reflecting T_1C behavior can also be observed. ¹³C spin–spin relaxation time (T_2C) measurements for the noncrystalline region reveal the existence of two components with different T_2C values, and these two components have been assigned to the crystalline–amorphous interfacial and rubbery–amorphous components. These results indicate that the structure of the major part of the noncrystalline region in the ethylene ionomers is similar to that of bulk‐crystallized polyethylene samples, regardless of possible ionic aggregates. The origin of the lower temperature endothermic peak in the heating process of the differential scanning calorimetry curve observed for the as‐received sample has also been examined somewhat in detail. As a result, it is proposed that the melting of smaller crystallites produced during storage at room temperature is the origin of the lower temperature peak. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1142–1153, 2002     

Spin‐State Ordering on One Sub‐lattice of a Mononuclear Iron(III) Spin Crossover Complex Exhibiting LIESST and TIESST

The two‐step spin crossover in mononuclear iron(III) complex [Fe(salpm)₂]ClO₄ ? 0.5 EtOH ( 1 ) is shown to be accompanied by a structural phase transition as concluded from ⁵⁷Fe Mössbauer spectroscopy and single crystal X‐ray diffraction, with spin‐state ordering on just one of two sub‐lattices in the intermediate magnetic and structural phase. The complex also exhibits thermal‐ and light‐induced spin‐state trapping (TIESST and LIESST), and relaxation from the LIESST and TIESST excited states occurs via the broken symmetry intermediate phase. Two relaxation events are evident in both experiments, that is, two T(LIESST) and two T(TIESST) values are recorded. The change in symmetry which accompanies the TIESST effect was followed in real time using single crystal diffraction. After flash freezing at 15 K the crystal was warmed to 40 K at which temperature superstructure reflections were observed to appear and disappear within a 10 000 s time range. In the frame of the international year of crystallography, these results illustrate how X‐ray diffraction makes it possible to understand complex ordering phenomena.     

Ab initio study of magnetic interactions of manganese-oxide clusters

The manganese clusters have attracted much attention in relation with the oxygen evolving center (OEC) in photosystem II (PS II) system, which catalyzes the water oxidation reaction. Previously, we examined various spin-structures of Mn(II)₄O₄ model clusters, of which all of magnetic interactions are antiferromagnetic. In this study, we investigated electronic and magnetic structures of simple model clusters, Mn₄O₄(OAc)₆ and Mn₃CaO₄(OAc)₆ using spin unrestricted B3LYP (UB3LYP) method. The UB3LYP method is a standard tool for this study and has been in fact employed by many researchers. However, several peculiar features are observed for these model clusters: for instance the most stable spin state becomes the highest spin state for Mn(IV)₄O₄(OAc)₆ although this model cluster consists of superexchange type of units, Mn(IV)₂O₂ that usually favors antiferromagnetic spin alignments. Implications of the comparative results are discussed in relation to the electrophilic (or radical) mechanism for the O-O bond formation in the OEC.