Intermolecular Exchange Interactions Research Articles

Density-functional theory calculation of the intermolecular exchange interaction in the magneticMn4dimer

The dimeric form of the single-molecule magnet [Mn$_4$O$_3$Cl$_4$(O$_2$CEt)$_3$(py)$_3$]$_2$ recently revealed interesting phenomena: no quantum tunneling at zero field and tunneling before magnetic field reversal. This is attributed to substantial antiferromagnetic exchange interaction between different monomers. The intermolecular exchange interaction, electronic structure and magnetic properties of this molecular magnet are calculated using density-functional theory within generalized-gradient approximation. Calculations are in good agreement with experiment.

On the richness of supra-molecular chemistry and its openings in physics

Some branches of molecular and supra-molecular chemistry deal with magnetic ions (mainly of the 3d transition series). This leads to the subject of molecular magnetism. When magnetic molecules are sufficiently separated from each other to avoid inter-molecular exchange interactions, they form networks of spin-clusters, sometimes called ‘single molecule magnets’. However, this label can only apply to high-spin molecules, with high classical energy barriers. This paper gives a short presentation of the results obtained on the low-spin molecule V15, in collaboration with the Bielefeld group of Achim Müller.

A comparative high frequency EPR study of monomeric and dimeric Mn4 single-molecule magnets

We compare high frequency single crystal electron paramagnetic resonance (EPR) data for monomeric and dimeric Mn4 complexes where, in the case of the latter, intra-dimer exchange interactions are known to significantly alter the low temperature quantum behavior. The monomeric system exhibits typical single-molecule S=9/2 EPR spectra; analysis of these data have enabled us to characterize the effective spin Hamiltonian parameters for this material. In the dimeric system, the ground state transition shows a splitting over a narrow range of fields and frequencies. The transfer of spectral weight between these split peaks is consistent with the exchange bias picture which has been developed to explain the absence of a zero-field tunneling resonance in low temperature hysteresis experiments. Indeed, these measurements provide the first independent confirmation for this exchange bias model. The absence of a significant intensity in excited state EPR transitions for the dimer contrasts the results for the monomer. This likely suggests important differences concerning the coherence of excited states of the dimer versus the monomer. No clear indications for intermolecular exchange interactions were observed in the EPR for the monomer.

Synthesis, Characterization, Crystal Structure and Magnetic Properties of [(en)CuII(ImH)2](ClO4)2 (en = Ethylenediamine, ImH = Imidazole): A Metalloligand as a Building Block for Heterometal Chains

The reaction of Cu(ClO4)2·6H2O, imidazole (ImH), and ethylenediamine (en) in a 1 : 2 : 1 ratio afforded in high yield dark-blue single-crystals of [(en)CuII(ImH)2](ClO4)2, 1, which can be used after deprotonation as a building block for heterometallic ferrimagnetic chains. The structural and electronic properties of 1 were examined by X-ray crystallography, FTIR, ESI-MS, electrochemistry, UV/vis/NIR, EPR, and magnetic susceptibility measurements. The structure of 1 exhibits intermolecular π-π interactions and hydrogen bonds but the magnetic data exclude the presence of intermolecular exchange interactions.

High-frequency electron paramagnetic resonance investigations of tetranuclear nickel-based single-molecule magnets

We report preliminary high-frequency electron paramagnetic resonance (EPR) investigations for several tetranuclear nickel complexes which exhibit single-molecule magnetism, including low-temperature (below ∼1 K) hysteresis loops and resonant magnetic quantum tunneling. The combination of a cavity perturbation technique and a split-coil magnet facilitates high-sensitivity, multifrequency (40 to 200+ GHz), angle dependent single-crystal EPR measurements. The data confirm the expected S=4 ground states, and a negative magnetocrystalline anisotropy for each member in the series. An unusual splitting of the easy-axis EPR peaks is observed, which may be interpreted in terms of distinct Ni4 species within the crystals. Overall, however, the trends associated with the splitting, as well as the EPR linewidths and shapes, suggest that intermolecular exchange interactions are important. Indeed, differences between the EPR spectra obtained for different complexes correlate nicely with the expected strength of exchange interactions, as determined both from intermolecular contact distances and from independent hysteresis measurements.

Mn4 single-molecule magnets with a planar diamond core and S=9

The syntheses and magnetic properties are reported for three Mn4 single-molecule magnets (SMMs): [Mn4(hmp)6(NO3)2(MeCN)2](ClO4)2·2MeCN (3), [Mn4(hmp)6(NO3)4]·(MeCN) (4), and [Mn4(hmp)4(acac)2(MeO)2](ClO4)2·2MeOH (5). In each complex there is a planar diamond core of MnIII2MnII2 ions. An analysis of the variable-temperature and variable-field magnetization data indicate that all three molecules have intramolecular ferromagnetic coupling and a S=9 ground state. The presence of a frequency-dependent alternating current susceptibility signal indicates a significant energy barrier between the spin-up and spin-down states for each of these three MnIII2MnII2 complexes. The fact that these complexes are SMMs has been confirmed by the observation of hysteresis in the plot of magnetization versus magnetic field measured for single crystals of complexes 3 and 4. The hysteresis loops for both of these complexes exhibit steps characteristic of quantum tunneling of magnetization. Complex 4 shows its first step at zero field, whereas the first step for complex 3 is shifted to −0.10 T. This shift is attributable to weak intermolecular antiferromagnetic exchange interactions present for complex 3.

Exchange Interactions at the Supramolecular Level − Synthesis, Crystal Structure, Magnetic Properties, and EPR Spectra of [Mn(MAC)(TCNQ)2] (MAC = Pentaaza Macrocyclic Ligand; TCNQ·− = Radical Anion of 7,7,8,8‐Tetracyano‐p‐quinodimethane)

AbstractThe reaction between [Mn(MAC)(H2O)2]Cl2·4H2O and LiTCNQ (MAC = 2,13‐dimethyl‐3,6,9,12,18‐pentaazabicyclo[12.3.1]octadeca‐1(18),2,12,14,16‐pentaene) affords a complex with the formula [Mn(MAC)(TCNQ)2] (1), whose crystal structure has been determined. Its structure consists of neutral mononuclear entities. The manganese(II) ion is heptacoordinated, with a pentagonal bipyramidal geometry. The apical positions are occupied by the TCNQ·− radicals, while the macrocyclic ligand is coordinated at the equatorial positions. The seven Mn−N distances range from 2.273(3) to 2.301(6) Å. The strong intermolecular π−π stacking interactions between the TCNQ radicals (3.2 Å) leads to weave‐like infinite chains, which propagate along the crystallographic c axis. The cryomagnetic investigation of 1 revealed a weak intermolecular antiferromagnetic coupling of the Mn2+ ions (J = −0.18 cm−1), which is mediated by the diamagnetic (TCNQ)22− pairs resulting from the stacking interactions in the crystal. The intermolecular exchange interaction between the Mn2+ ions was further confirmed by variable temperature EPR spectroscopic measurements [|J| = 0.15(5) cm−1], which have been carried out in both the X and Q bands. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)

Magnetic Properties of Langmuir−Blodgett Films: A Theoretical Study. 1. Photoswitching of the Magnetic Properties of Langmuir Films

The change of intermolecular spin exchange interaction within a half-filled band of two-dimensional Langmuir films formed by photoresponsive radicals has been investigated theoretically. The photoswitching of magnetic properties arises as a result of a reversible photochemical reaction within the building units of the Langmuir films. These units consist of a photochromic moiety connected with a polymethine radical. The various contributions to the Heisenberg effective exchange integral, the direct (Coulomb), kinetic, and indirect spin exchange, have been calculated using the band theory of organic open-shell π-electron systems.

Defects, Tunneling, and EPR Spectra of Single-Molecule Magnets

ABSTRACTWe examine theoretically electron paramagnetic resonance (EPR) lineshapes as functions of resonance frequency, energy level, and temperature for single crystals of three different kinds of single-molecule nanomagnets (SMMs): Mn12 acetate, Fe8Br, and the S = 9/2 Mn4 compound. We use a density-matrix equation and consider distributions in the uniaxial (second-order) anisotropy parameter D and the g factor, caused by possible defects in the samples. Additionally, weak intermolecular exchange and electronic dipole interactions are included in a mean-field approximation. Our calculated linewidths are in good agreement with experiments. We find that the distribution in D is common to the three examined single-molecule magnets. This could provide a basis for a proposed tunneling mechanism due to lattice defects or imperfections. We also find that weak intermolecular exchange and dipolar interactions are mainly responsible for the temperature dependence of the lineshapes for all three SMMs, and that the intermolecular exchange interaction is more significant for Mn4 than for the other two SMMs. This finding is consistent with earlier experiments and suggests the role of spin-spin relaxation processes in the mechanism of magnetization tunneling.

Half-Integer Spin Molecular Nanomagnets

ABSTRACTSingle-molecule magnets (SMM) are molecules that function as single-domain nanomagnets. SMMs have been characterized with a ground-state spin ranging from S = 4 to S = 13. A few SMMs have been identified that have half-integer spin ground states. [Cation][Mn12O12(O2CR)16(H2O)4] complexes, where R is some substituent, are SMMs that have either a S = 19/2 or 21/2 ground state. Quantum tunneling of magnetization (QTM) is observed for these half-integer-spin Kramers [Mn12]- degenerate SMMs in zero external magnetic field, as well as for a class of S = 9/2 Mn4 SMMs. The presence of QTM in zero external field is attributed to a transverse component of a nuclear spin field, dipolar interactions and intermolecular exchange interactions. The Landau-Zener method is used to measure the tunnel splitting as a function of transverse magnetic field for a single crystal of the S = 9/2 SMM [Mn4O3(OSiMe3)(OAc)3(dbm)3]. Spin parity dependent QTM is established. The effect of a magnetic exchange interaction between two S = 9/2 Mn4 SMMs upon QTM was studied for another compound. The hydrogen bonding and Cl…Cl contacts within a supramolecularly linked [Mn4]2 dimer lead to a weak antiferromagnetic exchange interaction between the two S = 9/2 SMMs. This interaction causes a shift (exchange bias) from zero field for the magnetic field at which QTM occurs.

Synthesis, Crystal Structure and Magnetic Properties of a Terephthalato-bridged Binuclear Nickel(II) Complex

A new terephthalato-bridged binuclear nickel(II) complex with a tetraazamacrocyclic compound as the terminal ligand, [Ni 2 (cth) 2 (µ-TPHA)](ClO 4 ) 2 (1) [cth= rac -5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane] has been synthesized and characterized. According to X-ray crystallographic studies on the solvated species 1·2CH 3 OH, each Ni(II) ion lies in a distorted octahedral environment, and the terephthalato ligand bridges two Ni(II) ions in a bis bidentate fashion. Cryomagnetic measurements revealed Curie-Weiss behaviour with è = m 1.4 K. Such behaviour may be due to a very weak intramolecular superexchange interaction through the extended bridge, a weak intermolecular exchange interaction or the local zero-field splitting of Ni(II) ions.

Metal Complexes with Nitronyl Nitroxide Substituted Phenolate Ligands Providing New Magnetic Exchange Interaction Pathways − Synthesis, Structures, Magnetic Dilution Studies, and Ab Initio Calculations

The synthesis and magnetic properties of seven new nitronyl nitroxide substituted phenolates with chelating amine groups are reported. Copper(II) and nickel(II) complexes prepared with these ligands are structurally and magnetically characterized, showing exchange interactions between the metal and the radical ligands through the phenolate oxygen. Magnetically diluted samples are prepared by embedding the compounds in a poly(vinyl chloride) film to suppress intermolecular exchange interactions. This method is used for the first time in a quantitative way to separate intra- and intermolecular exchange interactions in radical-metal systems. The experimentally determined ground states are further confirmed by DFT calculations.

Synthesis, Crystal Structure and Magnetic Properties of Zinc (II) Chloride Complexes with Pyridyl‐Substituted Nitronyl Nitroxides

AbstractZnCl2(NIT‐pPy) 1, (NIT‐pPy = 2‐(4‐pyridyl)‐4,4,5,5‐tetramethyl‐4,5‐dihydro‐1H‐imidazolyl‐1‐oxyl‐3‐oxide) crystallizes in the orthorhombic space group Pnn2 with the cell parameters a = 9.4083(9), b = 39.686(3), c = 7.4417(14) Å. ZnCl2(NIT‐6M‐oPy) 2, (NIT‐6M‐oPy = 2‐(6‐methyl‐2‐pyridyl)‐4,4,5,5‐tetramethyl‐4,5‐dihydro‐1H‐imidazolyl‐1‐oxyl‐3‐oxide) crystallizes in the monoclinic space group Pn with a = 7.377(2), b = 19.096(4), c = 11.853(2) Å, β = 95.65(3)°. The temperature dependence magnetic susceptibility of complex 1 revealed an intermolecular ferromagnetic exchange interaction. A simple spin‐polarization model has been used to justify the observed ferromagnetic exchange interaction between the spins of the radical NO group in complex 2.

Crystal Structure and Magnetic Properties of Pyridyl‐Substituted Nitronyl Nitroxides

AbstractX‐ray crystal structure and cryomagnetic properties of three organic radicals, 2‐(2‐pyridyl)‐4, 4,5,5‐tetramethyl‐4,5‐dihydro‐1‐H‐imidazol‐1‐oxyl‐3‐N‐oxide(NIT‐oPy) 1, 2‐(3‐pyridyl)‐4,4,5,5‐tetramethyl‐4,5‐dihydro‐l‐H‐imidazol‐1‐oxyl‐3‐N‐oxide(NIT‐mPy) 2 and 2‐(6‐methyl‐2‐pyridyl)‐4,4,5,5‐tetramethyl‐4, 5‐dihydro‐1‐H‐imidazol‐1‐oxyl‐3‐N‐oxide(NIT‐6M‐oPy) 3 were investigated. The crystal structures of compounds 1‐3 show the same feature of a P21/c space group of a monoclinic system. Two crystallographically independent molecules were found in the crystals of compounds 1‐3. The temperature dependence of magnetic susceptibility of compounds 1‐3 revealed an intermolecular antiferromagnetic exchange interaction.

Magnetic Properties of Purely Organic Radical Magnets, N-(Arylthio)-2,4,6-Triarylphenylaminyls: Susceptibility and Heat Capacity Measurements

The magnetic properties of N-[(dichlorophenyl)thio]-2,4,6-tris(chlorophenyl)phenylaminyl radical crystals are reported. The stable radical crystals of N-[(2,4-dichlorophenyl)thio]-2,4,6-tris(4-chlorophenyl)phenylaminyl (1) and N-[(2,4-dichlorophenyl)thio]-2,4,6-tris(3-chlorophenyl)phenylaminyl (2) have shown one-dimensional ferromagnetic behavior which have described using the Bonner-Fisher model with the large ferromagnetic exchange interaction of J/kB = 14.0 K and J/kB = 5.7 K, respectively. In addition to these ferromagnetic compounds, N-[(3,5-dichlorophenyl)thio]-2,4,6-tris(4-chlorophenyl)phenylaminyl (3) has also shown the ferromagnetic intermolecular exchange interaction of J/kB = 1.8 K. For these crystals, we have carried out low-temperature susceptibility and heat capacity measurements below 2 K. Radical 1 has shown a magnetic phase transition at 0.4 K. The susceptibility in the ordered state, however, shows spinflip transition, indicating antiferromagnetic long range ordering. For the ordered state, the interchain interactions are also estimated to be J'/kB = 0.02 K and J″/kB = −0.01 K. In order to clarify the electronic structures, ab-initio molecular orbital calculations of their substituted radicals are carried out. The correlation between the magnetic interactions and the electronic structures is discussed.

A Prerequisite for Purely Organic Molecule-Based Ferrimagnetics: Breakdown of Simple Classical Pictures

Spin state energies were calculated for an antiferromagnetic linear chain composed of a doublet (S = 1/2) radical and a biradical with a triplet (S = 1) ground state by numerical diagonalization of a finite-size Heisenberg−Dirac spin Hamiltonian. A ferrimagnetic-like ground state with a spin value of S = N/2 (N stands for the number of repeating units) appears if the intermolecular antiferromagnetic exchange interactions are uniform between all the S = 1/2 sites. This finding is consistent with a simple picture of differing and neighboring spin angular moments S's ordered in sublattices in an antiparallel manner. On the other hand, quasi-degenerate low-lying states appear as the spatial symmetry of the intermolecular antiferromagnetic interactions is lowered. The quasi-degeneracy destabilizes the ferrimagnetic-like ground state. This instability is inherent in multicentered antiferromagnetic interactions between open-shell organic molecular systems, suggesting a prerequisite for long-range magnetic order in purely organic molecule-based ferrimagnetics. Such prerequisite has never been reported until now. The results of the calculations are invoked to explain the susceptibility and the spin−spin relaxation time measured for the first model system for purely organic ferrimagnetics, a nitronyl nitroxide-based molecular crystal composed of an S = 1/2 and S = 1 molecules (Izuoka, A.; Fukuda, M.; Kumai, R.; Itakura, M.; Hikami, S.; Sugawara, T. J. Am. Chem. Soc. 1994, 116, 2609−2610.)

Paramagnetic spin catalysis of a radical recombination reaction

Recombination of the triplet state radical pair consisting of two hydrogen atoms catalysed by molecular oxygen is considered as a simulating example of a paramagnetic-exchange catalytic process. Intermolecular exchange interaction in the collision complex between the H2 and O2 molecules is calculatedab initio in STO-6G and 6–31 G* basis sets with complete active space configuration interaction. Calculations are done at a fixed O−H distance (≃3 A), scanning the H−H bond length from 0.6 till 12 A at the linear geometry of collision. The mixture of the triplet (‘T’)3∑ u + and singlet (‘S’)1∑ g + states of the hydrogen moiety is possible because both states have the same triplet symmetry in the ‘collision’ complex with O2 (3∑ g − ). A strong mixture of the ‘S’ (1∑ g + , H2 +3∑ g − , O2) and ‘T’) and ‘T’ (3∑ u + , H2 +3∑ g − , O2) states is actually obtained even at large H−H distances. The quintet and singlet states5,1(3∑ u + , H2 +3∑ g − , O2) are also considered for comparison of the exchange potentials. Atr(H−H)≥4.4 A the ‘S-T’ splitting is approximately constant (12 cm-1 in the STO-6G basis set; 55.5 cm-1 in the 6–31 G* basis set) and is determined by the exchange interaction between O2 and the nearest hydrogen atom in the O−O...H fragment. The paramagnetic catalyst can accelerate radical recombination through the ‘triplet-singlet’ nonadiabatic transition to the lowest ‘S’ reactive state when the radical encounter takes place in the vicinity of the catalyst. Though we do not consider the radical dynamics in a real solvent, which modulates the exchange potentials and the ‘T-S’ transitions, the nature of this mechanism of spin catalysis is obvious. The electric polarization and charge transfer are important in the analysis of the exchange interaction and radical recombination potentials for all multiplets. In accordance with the concept of spin catalysis, the electronic spin-uncoupling mechanism, induced by O2 perturbation, has the same nature as other known catalytic processes of paramagnetic-exchange type.

Studies on the magnetic property of μ3‐oxotriiron(III) complex [Fe3O(OBz)6(CH3OH)3](NO3)(CH3OH)2 (HOBzBenzoic acid)

AbstractThe magnetic property of μ3‐oxotriiron(III) complex [Fe3O(OBz)6(CH3OH)3](NP3)‐(CH3OH)2 (HOBzbenzoic acid) has been studied. We use isosceles triangle model and molecular field correction to fit the experimental magnetic susceptibility data. It shows that an intramolecular antiferromagnetic exchange interaction occurs with J = ‐31.27 cm−1, J'= ‐27.26 cm−1, and a weaker intermolecular antiferromagnetic exchange interaction occurs with zJ' = ‐ 3.76 cm−1. We give the d5‐d5‐d5 energy level diagram of triiron(III) complex as a function of J'/J. From the diagram we can get the total spin ST of the complex as 1/2 in the ground state.

Macrocyclic dimeric vanadium(IV) and heterodinuclear vanadium(IV)–nickel(II) complexes. Structure, magnetic, electronic and redox properties

The oxovanadium(IV) complexes [{VO(H2L1)}2(µ-SO4)][NO3]2, [(VO)L1Ni(H2O)2(SO4)]·H2O and [(VO)L1Ni(µ-SO4)(H2O)]·2H2O have been synthesized from the dinucleating tetraaminodiphenol macrocycle H2L1. The crystal structure of the first, in which two unco-ordinated amino nitrogens are protonated, has been determined. The two distorted-octahedral vanadium centres in the complex, separated by 6.741(4)A, are bridged by sulfate and have an anti-oxo configuration. In the solid state the magnetic moment of this complex per vanadium decreases from 1.71 to 1.64 µB on lowering the temperature from 299 to 5 K, indicating very weak intra-/or inter-molecular exchange interactions. Its ESR spectra in fluid solution and glass, however, showed the absence of VO ⋯ VO electron-spin interaction. This complex undergoes stepwise oxidation to produce VOIVVOV and VOVVOV species with E1/2(1)= 0.445 V and E½(2)= 0.60 V vs. Ag–AgCl electrode; the comproportionation constant Kc= 4.2 × 102. The other two complexes have identical composition but differ in their IR and UV/VIS spectra and electrochemistry. In the first the oxovanadium(IV) and nickel(II) centres are revesibly oxidized at 0.436 and 0.756 V, respectively, while in the second irreversible oxidation of both metal centres takes place at 1.05 and 1.25 V. Both behave ferromagnetically and their exchange coupling constants J are 10(1) and 6(1) cm–1, respectively.

Calculation of third-sound velocities for superfluid4He films with3He impurities

The velocities of third sound in superfluid4He films containing submonolayer densities of3He are calculated from the variation of film height with the concentration of3He. The present model assumes that the population of the first-excited, transverse, surface state of the3He impurities directly increases the height of the film because these atoms are preferentially found within the film rather than on the surface. The relative populations of the ground-state and the first excited-state are calculated from a many-body, mean-field theory which takes account of the first two transverse branches of the spectra of the states. Intermolecular and one-ripplon exchange interactions are included. Graphs of the variation of third-sound velocity with3He density are given.