摘要:

The variation of the number and position of OH substituents at the phenyl ring of α-phenyl nitronyl nitroxide radicals yields different H-bonded molecular self-assemblies in the solid state but do not change essentially their molecular spin density distributions. In accordance with the distinct structural dimensionalities, the obtained radicals show a large variety of magnetic behaviors, being the monoorthosubstituted isomer the most remarkable one since it shows a 3-D network of weak H-bonds exhibiting abulk ferromagnetictransition below 0.45K.

ISSN:1058-725X    

DOI:10.1080/10587259508034033

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Recent advances of our study of magnetic properties of a series of molecular conductors, Cu salts of DCNQIs, are presented. First, we have found weak ferromagnetism in the antiferromagnetically ordered state of Cu spins in the insulating phase. Second, the metal-insulator (M-I) transitions of these systems have been characterized by the abrupt change of Cu 3d spins from the itinerant state to the completely localized state, even in the metal-insulator-metal (M-I-M) reentrant system, (DMe-DCNQI-α,α'-d2)2Cu. Finally, (DI-DCNQI)2Cu is found to exhibit enhanced and temperature dependent susceptibility, suggesting the effect of spin fluctuations of itinerant electrons in a Cu 3d narrow band.

ISSN:1058-725X    

DOI:10.1080/10587259508034034

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Spin densities for nitronylnitroxide, phenyl nitronyl nitroxide(PNNO) and p-NPNN in the β-phase crystal were calculated by UHF, APUHF and UKS B(S)-LYP methods. Ab initio UHF overestimated a negative spin densitiy induced on the α-carbon atom of nitronylnitroxide group by spin polarization effect, whereas the approximately spin-projected UHF(APUHF)/6-31G* method provided a reasonable negative spin density on the α-carbon atom and almost equal distributions of the spin densities on nitrogen and oxygen atoms in PNNO and p-NPNN, in accord with the recent experiments. The spin populations on the benzene ring in these species by APUHF/6-31G* and APUHF/INDO, and UKS B-LYP/6-31G* methods were similar to the experimental values, indicating the important role of the spin polarization effect.

ISSN:1058-725X    

DOI:10.1080/10587259508034035

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The Electron Spin Resonance in ferromagnetic β-p-NPNN (Tc= 0.6 K) has been studied for the spherically shaped samples down to 0.4 K. It is shown that the demagnetization effect is particularly important to understand the results both in paramagnetic and ferromagnetic states. The ESR measurements for the shaped samples down to 1 K reveal the completely different g-value temperature dependence from those for the unshaped samples due to the shape demagnetization effect. The resonance line shapes alonga- andc-axes become rapidly asymmetric from 2K and below, indicating the existence of anisotropic relaxation process in the ferromagnetic state. It is shown that the sharp resonance g-shift exists at the ferromagnetic transition temperature in the magneic field alongb. Such a shift had been unidentified in the early studies due to the aspheric sample shape. These results can be ascribed to the anisotropic susceptibility due to the low dimensionality of the system. The magnetic resonance below 1 K can be understood as the ferromagnetic resonance with theb-axis as the easy axis.

ISSN:1058-725X    

DOI:10.1080/10587259508034036

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

We have determined, by polarized neutron diffraction, the spin density of two nitrophenyl nitroxides: the para-nitrophenyl nitronyl nitroxide (p-NPNN), which is a ferromagnet at low temperature, and the meta-nitrophenyl imino nitroxide (m-NPIN), which exhibits antiferromagnetic coupling. Several methods were used to reconstruct the spin density distribution, including the maximum entropy method. Our data provide the basis for a better knowledge of the electronic configuration. The nodes of the SOMO are directly observed and the spin polarization effects which give rise to a negative spin density on several atomic sites is discussed.

ISSN:1058-725X    

DOI:10.1080/10587259508034037

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The molecular building block of the first well-characterized purely organic crystalline ferromagnet is 4,4,5,5-tetramethyl-3-oxide-2(p-nitrophenyl)-2-imidazolin-1-yloxyl doublet radical. Among the diverse subjects of organic molecular crystalline ferromagnetism, the spin density distribution of the molecular building unit gives fundamental bases for understanding the underlying mechanism. In this work, an attempt has been made to determine experimentally the magnitudes and relative signs of the π-spin densities on the aromatic ring carbon sites (except the site 1) of 4,4,5,5-tetramethyl-3-oxide-2-phenyl-2-imidazolin-1-yloxyl doublet radical (abbreviated to 2- phenylnitronyl nitroxide) by liquid-phase ENDOR/TRIPLE spectroscopy. A series of partially deuterated 2-phenylnitronyl nitroxide compounds have given an unequivocal experimental identification of the ring protons: the spin densities of the ortho (2′,6′), meta (3′,5′), and para (4′) proton are ρortho= − 0.023, ρmeta= +0.013, and ρpara= -0.0207, respectively, and σiρi= -0.0407. The results show that 2-phenylnitronyl nitroxide undergoes negative spin polarization in the phenyl ring because the negative spins exceed the positive ones. The results have been compared with those obtained from polarized neutron diffraction measurements on the single crystal of 2-(p-nitrophenyl)- nitronyl nitroxide by Schweizer et al. The INDO calculation has been tested, indicating that the calculation gives an overestimation a great deal for the hyperfine coupling constants of the aromatic protons.

ISSN:1058-725X    

DOI:10.1080/10587259508034038

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The thermally excited triplet state ofp-phenylenebis(phenylmethylene) (p-PBPM) which is a topological isomer of the first organic high-spin molecule,m-phenylenebis(phenylmethylene) (m-PBPM, S=2), was examined using singlecrystal1H-ENDOR (Electron-NuclearDOubleResonance) spectroscopy. From the1H-ENDOR spectrum of the triplet state we found that the absolute sign of the fine-structure parameterDwas negative and thatp-PBPM in the triplet state has eight negative π-carbon sites and six positive ones. The π-spin density distribution determined experimentally indicates that the carbon atoms of the centralp-phenylene ring have negative π-spin density. The spin-spin interaction between the induced π-spins causes the fine-structure parameter D to be negative. It is concluded that the π-π spin interaction predominates in the triplet state ofp-PBPM.

ISSN:1058-725X    

DOI:10.1080/10587259508034039

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The ability of various ab initio methods (UHF, ROHF, MCSCF, MP2, CISD, CCD) to reproduce the experimental information on the spin distribution over various substituted α-nitronyl nitroxide radicals was tested using various basis sets which range from single zeta to near Hartree-Fock limit quality.

ISSN:1058-725X    

DOI:10.1080/10587259508034040

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

A new type of α-asymmetric bicyclic nitroxides were synthesized by three electron reduction of certain homoallylic nitroenones followed by the reaction with nitro-substituted benzoyl chlorides. Some nitroxides thus obtained turned out to exist as a conglomerate (racemic mixture), an equimolecular mixture of two crystalline enantiomers, and hence induced spontaneous optical resolution upon recrystallization. The relationship between the formation of conglomerates and their molecular and crystral structures is described, together with the preliminary results of their paramagnetic susceptibility mesurements.

ISSN:1058-725X    

DOI:10.1080/10587259508034041

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The magneto-structural correlation in somep-N-alkylpyridinium nitronyl nitroxide salts has been studied. In the iodide salts ofp-N-R-pyridinium nitronyl nitroxides with R=methyl, ethyl,n-propyl, andn-butyl, the magnetic property varies from antiferromagnetic to ferromagnetic with the extension of theN-alkyl chain. The nearest-neighbor molecular arrangements of the nitroxides observed in the four crystals, can be classified into two groups: the radicals are connected by a intermolecular contact between the NO groups (Type I) or between the NO group and the pyridinium ring (Type II). The observed magnetic behaviors can be interpreted in terms of an antiferromagnetic intermolecular interaction for the Type I contact and a ferromagnetic for Type II. Effects of replacement of the I−ion by Br−or Cl−, have been also studied in thep-N-methyl derivative.

ISSN:1058-725X    

DOI:10.1080/10587259508034042

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor