Field-induced level crossings in spin clusters: Thermodynamics and magneto-elastic instability

Quantum spin clusters with dominant antiferromagnetic Heisenberg exchange interactions typically exhibit a sequence of field-induced level crossings in the ground state as function of magnetic field. For fields near a level crossing, the cluster can be approximated by a two-level Hamiltonian at low temperatures. Perturbations, such as magnetic anisotropy or spin-phonon coupling, sensitively affect the behavior at the level-crossing points. The general two-level Hamiltonian of the spin system is derived in first-order perturbation theory, and the thermodynamic functions magnetization, magnetic torque, and magnetic specific heat are calculated. Then a magneto-elastic coupling is introduced and the effective two-level Hamilitonian for the spin-lattice system derived in the adiabatic approximation of the phonons. At the level crossings the system becomes unconditionally unstable against lattice distortions due to the effects of magnetic anisotropy. The resultant magneto-elastic instabilities at the level crossings are discussed, as well as the magnetic behavior.Comment: 13 pages, 8 figures, REVTEX

[S-Allyl-4-(4-hy­droxy-2-oxidobenzyl­idene-κO)-1-(2-oxidobenzyl­idene-κO)isothio­semicarbazidato-κ2 N 1,N 4](ethanol-κO)dioxido­uranium(VI) ethanol monosolvate

In the title compound, [U(C18H15N3O3S)O2(C2H5OH)]·C2H5OH, the UVI ion is in a distorted penta­gonal–bipyramidal coordination geometry, with two oxide O atoms in axial sites. Two N and two O atoms of the tetra­dentate ligand and an O atom of an ethanol ligand form the equatorial plane. The dihedral angle between the mean planes of the two benzene rings is 34.8 (3)°. In the crystal, relatively strong O—H⋯O hydrogen bonds connect the complex and ethanol solvent mol­ecules into alternating centrosymmetric R 2 2(8) and R 4 4(16) ring motifs, forming chains along [100]. Weak inter­molecular C—H⋯O hydrogen bonds are also present

(4Z,6Z,12Z,14Z)-2,10-Dimethyl-2,8,10,16-tetra­hydro­dipyrazolo[3,4-e:3′,4′-l][1,2,4,8,9,11]hexa­azacyclo­tetra­decine-4,12-diamine

The title compound, C12H16N12, is a centrosymmetric mol­ecule which comprises of a hexa­aza[14]annulene macrocyclic ring fused with two pyrazole rings. The macrocyclic ring is essentially planar, with an r.m.s. deviation of 0.0381 Å. The electron pairs of the amino groups are delocalized with the conjugated system of the macrocycle. Strong intra­molecular N—H⋯N hydrogen bonds arranged in an S 2 2(10) graph-set motif are present in the macrocyclic ring. In the crystal, the amino groups act as donors for inter­molecular N—H⋯N inter­actions with the N atoms of the heterocyclic system, forming a network of two types of extended chains oriented parallel to the [101] and [011] directions. The crystal packing is also stabilized by weak inter­molecular C—H⋯N hydrogen bonds formed between pyrazole C—H groups and N atoms of the macrocyclic ring, running in the [10] direction

Magnetothermal properties of molecule-based materials

We critically review recent results obtained by studying the low-temperature specific heat of some of the most popular molecular magnets. Perspectives of this field are discussed as well.Comment: 12 pages text + 14 pages figures, Submitted as "feature article" to Journal of Materials Chemistr

Spin dynamics of molecular nanomagnets fully unraveled by four-dimensional inelastic neutron scattering

Molecular nanomagnets are among the first examples of spin systems of finite size and have been test-beds for addressing a range of elusive but important phenomena in quantum dynamics. In fact, for short-enough timescales the spin wavefunctions evolve coherently according to the an appropriate cluster spin-Hamiltonian, whose structure can be tailored at the synthetic level to meet specific requirements. Unfortunately, to this point it has been impossible to determine the spin dynamics directly. If the molecule is sufficiently simple, the spin motion can be indirectly assessed by an approximate model Hamiltonian fitted to experimental measurements of various types. Here we show that recently-developed instrumentation yields the four-dimensional inelastic-neutron scattering function S(Q,E) in vast portions of reciprocal space and enables the spin dynamics to be determined with no need of any model Hamiltonian. We exploit the Cr8 antiferromagnetic ring as a benchmark to demonstrate the potential of this new approach. For the first time we extract a model-free picture of the quantum dynamics of a molecular nanomagnet. This allows us, for example, to examine how a quantum fluctuation propagates along the ring and to directly test the degree of validity of the N\'{e}el-vector-tunneling description of the spin dynamics

Binding CO2 by a Cr8 Metallacrown.

The {Cr8 } metallacrown [CrF(O2 C(t) Bu)2 ]8 , containing a F-lined internal cavity, shows high selectivity for CO2 over N2 . DFT calculations and absorption studies support the multiple binding of F-groups to the C-center of CO2 (C⋅⋅⋅F 3.190(9)-3.389(9) Å), as confirmed by single-crystal X-ray diffraction

A Facile Synthetic Route to a Family of Mn(III) Monomers and their Structural, Magnetic and Spectroscopic Studies

This is an accepted manuscript of an article published by Wiley in European Journal of Inorganic Chemistry on 07/10/2016, available online: https://doi.org/10.1002/ejic.201601124 The accepted version of the publication may differ from the final published version.© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim We report a rapid and facile route to the synthesis of a family of MnIIImonomers of general formula [MnIIIF3(H2O)(L1–6)]·xH2O·yMeOH [L1= 2,2′-bipyridyl, x = 2, y = 0 (1′); L2= 1,10′-phenanthroline, x = y = 0 (2′); L3= 6-methyl-2,2′-dipyridyl, x = y = 0 (3), L4= 4,4-dimethyl-2,2′-dipyridyl, x = 2, y = 0 (4), L5= 5,5′-dimethyl-2,2′-dipyridyl, x = 0, y = 0.5 (5); L6= 5-chloro-1,10-phenanthroline, x = y = 0 (6)]. Magnetic susceptibility and magnetisation experiments have been employed to elucidate the D tensor for each family member (ranging from –3.01 cm–1in 2′ to –4.02 cm–1in 5), while multi-frequency/high-field EPR spectroscopic measurements and subsequent simulations gave similar values for complexes 1′ (–4.25 cm–1), 2′ (–4.03 cm–1), 4 (–3.90 cm–1) and 5 (–4.04 cm–1). The terminal Mn–F vibrational stretches in 1′–6 have been probed using Raman spectroscopy.The authors would like to thank the Irish Research Council for Science, Engineering and Technology (IRSCET) Embark Fellowship (E. H.) and the Engineering and Physical Sciences Research Council (EPRSC) (S. S. and E. K. B.) for funding.Published versio