Unusual paramagnetic centers in PbTe undoped crystals

We present the results of the first experimental observation of unusual paramagnetism in solid when magnetic susceptibility of paramagnetic centers doesn't depend on temperature but drastically decreases when the applied magnetic field increases. This unusual combination of the field and temperature dependences of magnetic susceptibility was observed in the studies of magnetization and magnetic susceptibility performed in the wide range of temperatures (1.7–300 K) and magnetic fields (0–5.0 T) on the bulk and surface PbTe powder samples manufactured from crystal ingots grown by Bridgman method out of high-purity Pb and Te. We believe that presence of these features indicate that we are dealing with unknown untypical paramagnetism of paramagnetic centers in solid. We observed that the concentration of such unusual paramagnetic centers in PbTe crystal ingots increases towards their surface. Increase of the concentration of the centers can be so strong that it causes a transition of PbTe from the diamagnetic state to the paramagnetic one in quite wide range of low magnetic fields. Possible nature of the observed unusual paramagnetic centers is discussed.     

Optically detected acoustic paramagnetic resonance in molecular crystals

The optically detected acoustic paramagnetic resonance of the metastable triplet state of 1,2,4,5-tetrachlorobenzene (TCB) in zero field is reported. The spin-phonon interaction in molecular crystals is discussed in terms of the direct process single phonon absorption by the paramagnetic spin system. t is estimated that in many molecular solids, sufficient acoustic power densities may be produced to observe acoustic paramagnetic resonance by optical detection methods.     

Radiation induced paramagnetic defect centers in sodium fluoroborate single crystals

Two radiation induced defect centers, F⁰ and BF^?₃, have been identified in NaBF₄ single crystals. EPR data are presented for their characterization.     

The structure of the paramagnetic impurities in triglycine sulfate single crystals

ENDOR investigations of the bis(glycinato)-Cu(II) (NH₂CH₂COO)₂Cu and bis(glycinato)-oxo-V(IV) (NH₂CH₂COO)₂VO complexes in the ferroelectric triglycine sulfate (TGS) single crystals are reported. Magnetic hyperfine constants of the ligand nuclei are given. The ENDOR investigati confirm the conclusion derived from EPR studies that the copper and vanadyl ions occupy interstitial lattice sites forming bis(glycinato) complexes wit triclinic symmetry C₁. Molecular theoretical calculations of the EPR and ENDOR parameters of different paramagnetic impurities in TGS were performe using the self-consistent charge and configuration (SCCC) version of the extended Hückel theory (EHT). By using perturbation theory the parameters of the spin hamiltonian were determined.     

Electron paramagnetic resonance of gamma-irradiated single crystals of 3-nitroacetanilide

The electron paramagnetic resonance of single crystals of 3-nitroacetanilide has been observed and analyzed for different orientations of the crystal in the magnetic field, after being damaged at 300 K by γ-irradiation. The crystals have been investigated between 123 and 300 K. The spectra were found to be temperature independent. The irradiation of 3-nitroacetanilide by γ-rays produces radicals at the nitrogen atoms in the molecule. The principal values of the hyperfine coupling tensor of the unpaired electron and the principal values of the g-tensor were determined.     

Synthesis,characterization and magnetostructural correlation studies on three binuclear copper complexes of pyrimidine derived Schiff base ligands

Three binuclear Cu(II) complexes of two pyrimidine derived Schiff base ligands, 2-S-methyl-6-methyl-4-formyl pyrimidine-N(4)-ethyl thiosemicarbazone (HL₁) and salicyl hydrazone of 2-hydrazino-4,6-dimethylpyrimidine (HL₂), have been prepared. HL₁ produces a bis(μ-thiolato) Cu(II) complex co-crystallizing with its mononuclear analog, [Cu₂(L₁)₂(NO₃)₂][Cu(L₁)(NO₃)] (1). On the other hand HL₂ shows versatility by producing two different classes of binuclear Cu(II) complexes, a bis(μ-phenoxo) complex [Cu₂(L₂)₂(NO₃)₂] (2) and another a (μ-4,4′-bipyridyl) complex, [Cu₂(L₂)₂(μ-4,4′-bipyridyl)(NO₃)₂] (3) under suitable conditions. All the three complexes show distorted square pyramidal geometry around each Cu atom but to a varied extent. Magnetic behavior of complex 1 shows that it is strongly ferromagnetic in nature whereas compounds 2 and 3 are weakly antiferromagnetic in nature. A magnetostructural correlation study combined with molecular modelling on complexes 1 and 2 has thrown light on the difference on magnetic interaction between the Cu atoms in these two complexes. Various factors that may be responsible for such differences are also explored. A novel and potentially useful pH dependant conversion of 3 to 2 has also been noticed.     

Crucial role of fluorescence in the solid-state thermochromism of salicylideneanilines

The thermochromism of salicylideneanilines in the solid state has been so far explained only in terms of the temperature-induced change of light absorption, which is caused by the shift of the tautomeric equilibrium between the enol and cis-keto forms. In this study, we measured variable temperature diffuse reflectance and fluorescence spectra of the crystalline powders of salicylideneanilines. We also determined their fluorescence quantum yields. The chromaticity coordinates at each temperature were calculated from these data and compared to the perceived colors of the powders. We found that the temperature-induced shift of the tautomeric equilibrium is not the main cause of the thermochromic color change of the thermochromic crystals and that the thermochromism can be explained only by taking account of the temperature-induced change of fluorescence.     

Crucial role of linker portion in acridine-bearing oligonucleotides for highly efficient site-selective RNA scission

Through a series of linkers, 9-amino-2-methoxy-6-nitroacridine and 9-amino-6-chloro-2-methoxyacridine were tethered to the middle of oligonucleotide, and the abilities of these conjugates for site-selective activation of RNA (inducing site-selective scission by Lu(III)) were compared. The RNA-activating ability was strongly dependent on the structures of both acridine and linker. By tethering 9-amino-2-methoxy-6-nitroacridine with a rigid and chiral linker, derived from l-threoninol, quite fast site-selective RNA scission was accomplished.     

Crucial role of elusive isomeric eta-complexes in gas-phase electrophilic aromatic alkylations

[isomers: see text] The kinetics and stereochemistry of the protonation-induced unimolecular isomerization of (R)-1-D1-3-(p-fluorophenyl)butane have been investigated in the gas phase at 40-100 degrees C and 70-760 Torr. This process leads to the formation of the relevant meta and ortho isomers with partial racemization of the migrating sec-butyl moiety. Complete racemization is observed, instead, when the isomerization reaction involves a 1,2-H shift in the moving alkyl group. These results, together with the relevant activation parameters, fully confirm the previous evidence of the occurrence in the alkyl cation/arene PES of noncovalent eta-type intermediates of defined structure and stability, lying well below the classical pi-complexes, as confirmed by ab initio calculations. Their crucial role in determining the positional selectivity of gas-phase electrophilic aromatic substitutions clearly emerges from the comparison of the present results with the site selectivity measured in the corresponding bimolecular arene alkylation carried out at the same temperatures and pressures.     

Electron paramagnetic resonance study of vanadyl doped in zinc maleate tetrahydrate single crystals

Spectroscopy of vanadyl doped zinc meleate tetrahydrate single crystals has been studied by EPR in the X-band frequencies (∼9.5 GHz) and optical spectra. It is found that vanadium enters the zinc host lattice substitutionally.     

Toward a magnetostructural correlation for a family of Mn6 SMMs

We have structurally and magnetically characterized a total of 12 complexes based on the Single-Molecule Magnet (SMM) [MnIII6O2(sao)6(O2CH)2(MeOH) 4] (1) (where sao2- is the dianion of salicylaldoxime or 2-hydroxybenzaldeyhyde oxime) that display analogous structural cores but remarkably different magnetic behaviors. Via the use of derivatized oxime ligands and bulky carboxylates we show that it is possible to deliberately increase the value of the spin ground state of the complexes [Mn6O2(Me-sao)6(O2CCPh3)2(EtOH)4] (2), [Mn6O2(Et-sao)6(O2CCMe3)2(EtOH)5] (3), [Mn6O2(Et-sao)6(O2CPh2OPh)2(EtOH)4] (4), [Mn6O2(Et-sao)6(O2CPh4OPh)2(EtOH)4(H2O)2] (5), [Mn6O2(Me-sao)6(O2CPhBr)2(EtOH)6] (6), [Mn6O2(Et-sao)6(O2CPh)2(EtOH)4(H2O)2] (7), [Mn6O2(Et-sao)6{O2CPh(Me)2}2(EtOH)6] (8), [Mn6O2(Et-sao)6(O2C11H15)2(EtOH)6] (9), [Mn6O2(Me-sao)6(O2C-th)2(EtOH)4(H2O)2] (10), [Mn6O2(Et-sao)6(O2CPhMe)2(EtOH)4(H2O)2] (11), and [Mn6O2(Et-sao)6(O2C12H17)2(EtOH)4(H2O)2] (12) (Et-saoH2 = 2-hydroxypropiophenone oxime, Me-saoH2 = 2-hydroxyethanone oxime, HO2CCPh3 = triphenylacetic acid, HO2CCMe3 = pivalic acid, HO2CPh2OPh = 2-phenoxybenzoic acid, HO2CPh4OPh = 4-phenoxybenzoic acid, HO2CPhBr = 4-bromobenzoic acid, HO2CPh(Me)2 = 3,5-dimethylbenzoic acid, HO2C11H15 = adamantane carboxylic acid, HO2C-th = 3-thiophene carboxylic acid, HO2CPhMe = 4-methylbenzoic acid, and HO2C12H17 = adamantane acetic acid) in a stepwise fashion from S = 4 to S = 12 and, in-so-doing, enhance the energy barrier for magnetization reorientation to record levels. The change from antiferromagnetic to ferromagnetic exchange stems from the "twisting" or "puckering" of the (-Mn-N-O-)3 ring, as evidenced by the changes in the Mn-N-O-Mn torsion angles.