Atsushi Kajiwara

No-overlap and orientation principle for ferromagnetic interactions between nitroxide groups

Abstract Approximately spin-projected UMP4(2) and UCC SD(T) computations show that ferromagnetic intermolecular interactions are feasible for T-shape and rhombus conformations in parallel interplane orientations for nitroxide pairs because of no-overlap between π ∗ -type SOMOs (no kinetic exchange) and non-zero potential exchange via SOMO-SOMO through-space contact. On the other hand, antiferromagnetic kinetic exchange interactions via SOMO-SOMO overlap are predominant for rectangular conformations. The no-overlap and orientation principle proposed previously is a useful guide for the theoretical understanding of ferromagnetic exchange interactions between nitroxide groups. Implications of the calculated results are discussed in relation to the discovery of ferromagnetic interactions in the crystals of adamantane dinitroxide, TEMPO derivatives and MOTMP.

Low temperature magnetic properties of organic radical magnet, MOTMP

The magnetic properties of MOTMP 4-methacryloyloxy-2,2,6,6-tetramethyl-piperidin-1-oxyl are studied at tempratures below 1 K. The dominant magnetic coupling between the radicals is ferromagnetic. The one-dimensional behavior observed in the temperature dependence of susceptibility is consistent with previous result of heat capacity measurement in which the intrachain ferromagnetic interaction is estimated to be J / k =0.9 K. The susceptibility in the ordered state, however, shows spin-flip transition, which reveals antiferromagnetic long range order. From the analysis of ordering temperature T N and critical field H C , the interchain interactions are also estimated by the mean field theory.

E.s.r. measurements of the propagation rate coefficient for styrene free radical polymerisation

Electron spin resonance (e.s.r.) spectroscopy should be useful for the measurement of propagation rate coefficients (k(p)) in free radical polymerisations, since the propagating radical concentration can be directly observed. However, in previous studies, k(p) values have generally been inconsistent, and disagreed with data from other accepted experimental techniques, such as pulsed laser polymerisation. This study shows that for styrene, consistency can be obtained, but only when experimental conditions are selected such that polymer of sufficiently high molecular weight is formed; otherwise incorrect rate constants may be measured. The measured Arrhenius parameters were in good agreement with IUPAC accepted data. The values at 343 K for both thermally and photochemically initiated experiments were found to be consistent, suggesting that photoinitiated systems may polymerise in the ground state. Some previously inconsistent data could be explained in terms of selection of experimental conditions

Very low-temperature heat capacities and magnetic thermal anomalies of the organic free radical magnet, 4-methacryloylamino-2,2,6,6-tetramethylpiperidine-1-oxyl (MATMP)

Abstract The magnetic properties of 4-methacryloylamino-2,2,6,6-tetramethylpiperidine-1-oxyl (MATMP) have been investigated by means of adiabatic heat capacity calorimetry in the 0.08–24 K range. The heat capacities of MATMP showed a magnetic phase transition at 0.15 K and a hump centered around 1 K, arising from the short-range ordering characteristic of low-dimensional magnets. The entropy gain due to these two magnetic thermal anomalies was ΔS = 5.49 J K−1 mol−1. The hump can be well accounted for in terms of the spin S = 1 2 one-dimensional ferromagnetic Heisenberg model with an intrachain exchange interaction of J ∥ k B = 0.70 K , where kB is the Boltzmann constant. The temperature dependence of the magnetic heat capacities below the transition temperature is approximated by T 3 2 . This fact, on the basis of the spin-wave theory, implies that MATMP would be a ferromagnet below the transition temperature. However, the magnetic susceptibility measurement reported recently suggests that MATMP would be antiferromagnetic below the transition temperature. A possible origin responsible for this contradictory feature is discussed.

Mn11-thiophenolate and -selenophenolate complexes as catalyst in air oxidation of benzoin, benzaldehyde and hydrazobenzene

Catalytic air oxidation of benzoin to benzil in N,N-dimethylformamide (DMF) occurred in the presence of [Mn(SePh)4]2- or [Mn(SPh)4]2- (with conversion of 70% and 45%, respectively ([complex]/[benzoin] =1/20, 50 h, 20 °C)). Similar oxidation of other substrates, e.g. hydrazobenzene, benzaldehyde, and benzhydrol, was also found.

Magnetic phase transition of organic radical magnets

Abstract Antiferromagnetism of quasi-one-dimensional organic / molecular ferromagnets MOTMP and MATMP and an antiferromagnet AOTMP are studied by susceptibility measurements. The ordered states are well described as an typical antiferromagnet with uniaxial anisotropy. Weak ferromagnetic behavior is also observed in MATMP and AOTMP along the direction of the spin easy axis.

Magnetic Properties of Organic Radical Magnets, MOTMP, MATMP and AOTMP

Magnetic properties of quasi-one-dimensional organic/molecular ferromagnets MOTMP and MATMP and an antiferromagnet AOTMP are studied. All samples shows the long-range order at low temperatures below 1K due to the interchain interaction. The ordered states are well described as an typical antiferromagnet. In MATMP and AOTMP, the weak ferromagnetic behavior is also observed in the magnetic field along the spin easy axis just below T N .

Structure and photolysis of the dimer of 2,2′-(m-phenylene)bis(4,5-diphenylimidazole-2-yl) diradical

Abstract The oxidation of 2,2′-( m -phenylene)bis(4,5-diphenylimidazole) 1 with potassium ferricyanide in benzene-aqueous KOH heterogeneous solution gave two dimers 2 and 3; 2 : piezochromic and photochromic dimer whose structure is deduced by NMR as an anti isomer of 3,3 : photochromic dimer with syn -[3.3]metacyclophane structure elucidated by X-ray structural analysis. Photolysis (>310 nm) of these dimers in frozen CH 2 Cl 2 ~MTHF (8:1) produced the same triplet species which can be assignable as 2,2′-( m -phenylene)bis(4,5-diphenylimidazole-2-yl) diradical.

Preparation and characterization of novel high-spin organoborane dianions

Abstract New borane-chelates, 2,4,6-tribenzoyl-1,3,5-tris(diethyl- or diphenylboryloxy)benzenes were synthesized. These borane-chelates showed multi-step reduction behavior. The metal-reduction with a Naue5f8K alloy in THF produced the dianionic species whose ground state was shown to be a triplet for the diphenylboryl derivative.

Invalidity of simple application of spin-prediction topology rule to heteroatomic pi-conjugated systems: 10,10'-(m-phenylene)diphenothiazine dication and 10,10',10''-(1,3,5-benzenetriyl)triphenothiazine trication

Abstract Invalidity of simple application of hydrocarbon spin-prediction topology rule to heteroatomic π-conjugated systems were exemplified for 10, &10′-(m-phenylene)diphenothiazine dication and 10,&10′,10″(l,3,5-benzenetriyl)triphenothiazine trication.