Graham Carver

Studies of Finite Molecular Chains: Synthesis, Structural, Magnetic and Inelastic Neutron Scattering Studies of Hexa- and Heptanuclear Chromium Horseshoes

We report the synthesis and structural characterisation of a family of finite molecular chains, specifically [{[R(2)NH(2)](3)[Cr(6)F(11)(O(2)CCMe(3))(10)]}(2)] (in which R=nPr 1, Et 2, nBu 3), [{Et(2)NH}(2){[Et(2)NH(2)](3)[Cr(7)F(12)(O(2)CCMe(3))(12)][HO(2)CCMe(3)](2)}(2)] (4), [{[Me(2)NH(2)](3)[Cr(6)F(11)(O(2)CCMe(3))(10)]2.5 H(2)O}(4)] (5) and [{[iPr(2)NH(2)](3)[Cr(7)F(12)(O(2)CCMe(3))(12)]}(2)] (6). The structures all contain horseshoes of chromium centres, with each Cr...Cr contact within the horseshoe bridged by a fluoride and two pivalates. The horseshoes are linked through hydrogen bonds to the secondary ammonium cations in the structure, leading to di- and tetra-horseshoe structures. Through magnetic measurements and inelastic neutron scattering studies we have determined the exchange coupling constants in 1 and 6. In 1 it is possible to distinguish two exchange interactions, J(A)=-1.1 meV and J(B)=-1.4 meV; J(A) is the exchange interactions at the tips of the horseshoe and J(B) is the exchange within the body of the horseshoe (1 meV=8.066 cm(-1)). For 6 only one interaction was needed to model the data: J=-1.18 meV. The single-ion anisotropy parameters for Cr(III) were also derived for the two compounds as: for 1, D(Cr)=-0.028 meV and |E(Cr)|=0.005 meV; for 6, D(Cr)=-0.031 meV. Magnetic-field-dependent inelastic neutron scattering experiments on 1 allowed the Zeeman splitting of the first two excited states and level crossings to be observed. For the tetramer of horseshoes (5), quantum Monte Carlo calculations were used to fit the magnetic susceptibility behaviour, giving two exchange interactions within the horseshoe (-1.32 and -1.65 meV) and a weak inter-horseshoe coupling of +0.12 meV. Multi-frequency variable-temperature EPR studies on 1, 2 and 6 have also been performed, allowing further characterisation of the spin Hamiltonian parameters of these chains.

Quantized antiferromagnetic spin waves in the molecular Heisenberg ring CsFe8

We report on inelastic neutron-scattering (INS) measurements on the molecular spin ring

Electronic Raman spectroscopy of the vanadium(III) hexaaqua cation in guanidinium vanadium sulphate: Quintessential manifestation of the dynamical Jahn–Teller effect

{\text{CsFe}}_{8}

Electronic Raman transitions from the vanadium(III) hexa-aqua cation, in guanidinium vanadium sulphate

, in which eight spin-5/2 Fe(III) ions are coupled by nearest-neighbor antiferromagnetic Heisenberg interaction. We have recorded INS data on a nondeuterated powder sample up to high energies at the time-of-flight spectrometers FOCUS at PSI and MARI at ISIS, which clearly show the excitation of spin waves in the ring. Due to the small number of spin sites, the spin-wave dispersion relation is not continuous but quantized. Furthermore, the system exhibits a gap between the ground state and the first excited state. We have modeled our data using exact diagonalization of a Heisenberg-exchange Hamiltonian together with a small single-ion anisotropy term. Due to the molecules symmetry, only two parameters

Magnetic relaxation studies on a single-molecule magnet by time-resolved inelastic neutron scattering

J

Exchange-coupling constants, spin density map, and Q dependence of the inelastic neutron scattering intensity in single-molecule magnets

and

High-Frequency Electron-Spin-Resonance Study of the Octanuclear Ferric Wheel CsFe8

D

Standing spin waves in an antiferromagnetic molecular Cr6 horseshoe

are needed to obtain excellent agreement with the data. The results can be well described within the framework of the rotational-band model as well as antiferromagnetic spin-wave theories.

Pressure dependence of the exchange interaction in the dimeric single-molecule magnet [Mn4O3Cl4(O2CEt)3(py)3]2 from inelastic neutron scattering

Single-crystal Raman spectra are presented for the salt [C(NH2)3][V(OH2)6](SO4)2, displaying electronic transitions between the trigonal components of the vanadium(III) 3T1g(Oh) ground term. The 3A-->3E(C3) electronic Raman band is centered at approximately 2720 cm-1, and exhibits extensive structure, revealing the energies of the spinor components of the 3E(C3) term for the two crystallographically distinct [V(OH2)6]3+ cations. The data are interpreted in conjunction with parameters previously reported from an electron paramagnetic resonance study of the salt. A satisfactory reproduction of the electronic Raman profile and ground-state spin-Hamiltonian parameters is achieved by employing a (3A plus sign in circle3E)multiply sign in circle e vibronic coupling model, in which the spin-orbit splitting of the 3E(C3) is quenched significantly by the Ham effect, and the intensity of harmonics of the Jahn-Teller active vibration enhanced by their proximity to the electronic Raman bands. The model gives an excellent account of the intensities of the electronic Raman bands, which are shown to depend profoundly on both temperature and the selected component of the polarizability tensor. The electronic Raman profile changes notably upon deuteriation, a result that exposes deficiencies in the single-mode coupling model.

The dependence of the spin-Hamiltonian parameters of the [Ti(OH2)6]3+ cation on the mode of water co-ordination

Abstract Electronic Raman transitions ( 3 A → 3 E ( C 3 ) ) have been observed between the trigonally split components of the 3 T 1 g ( O h ) ground term of the vanadium(III) hexa-aqua cation in guanidinium vanadium sulphate hexa-hydrate. The magnitude of the trigonal field splitting is considerable, ∼2720 cm −1 , which is consistent with expectations based on the stereochemistry of the [V(OH 2 ) 6 ] 3+ complex. It is shown that a satisfactory reproduction of the electronic Raman band profile can be obtained only by assuming a ( 3 A ⊕ 3 E )⊗e vibronic coupling model.