Electron magnetic resonance and density functional theory study of room temperature X-irradiated β-D-fructose single crystals

Stable free radical formation in fructose single crystals X-irradiated at room temperature was investigated using Q-band electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR), and ENDOR induced EPR (EIE) techniques. ENDOR angular variations in the three main crystallographic planes allowed an unambiguous determination of 12 proton HFC tensors. From the EIE studies, these hyperfine interactions were assigned to six different radical species, labeled F1-F6. Two of the radicals (F1 and F2) were studied previously by Vanhaelewyn et al. [Vanhaelewyn, G. C. A. M.; Pauwels, E.; Callens, F. J.; Waroquier, M.; Sagstuen, E.; Matthys, P. J. Phys. Chem. A 2006, 110, 2147.] and Tarpan et al. [Tarpan, M. A.; Vrielinck, H.; De Cooman, H.; Callens, F. J. J. Phys. Chem. A 2009, 113, 7994.]. The other four radicals are reported here for the first time and periodic density functional theory (DFT) calculations were used to aid their structural identification. For the radical F3 a C3 carbon centered radical with a carbonyl group at the C4 position is proposed. The close similarity in HFC tensors suggests that F4 and F5 originate from the same type of radical stabilized in two slightly different conformations. For these radicals a C2 carbon centered radical model with a carbonyl group situated at the C3 position is proposed. A rather exotic C2 centered radical model is proposed for F6.     

Proton magnetic resonance studies of cyclic compounds—II: N- and 2-substituted cyclohexylamines

PMR spectra are recorded for a number of N- and 2-substituted cyclohexylamines, and are discussed in terms of stereochemical and conformational features. The cis- or trans- geometry of a 2-alkyl-cyclohexylamine is readily assessed from the PMR spectrum of the amine or the phthaloyl derivative. Analysis of the α-proton signal of cis-alkylcyclohexylamines gives only crude information about the conformational equilibrium.     

Nuclear magnetic resonance studies of intramolecular hydrogen bonds in monocations of sparteine- and α-isospartene-N-oxides

The NMR spectra of perchlorate salts of sparteine- and ga-isosparteine-N-oxides have been investigated in dimethylsulphoxide-d₆ solution. The results show a strong intramolecular hydrogen bond in the monocation of both N-oxides. This is in agreement with the anomalous high ratios of their first and second dissociation constants, and also with the conclusions from previous IR measurements. On this basis it is postulated that sparteine-N₍₁₆₎-oxide as well as α-isosparteine-N-oxide have a carbon-nitrogen skeleton in which all piperidine rings are present in the chair conformation.     

Infrared polarized spectra of hexamethylbenzene above and below the λ point at 116 K

The i.r. spectrum of hexamethylbenzene (HMB) has been investigated above and below the λ point between phase II (stability range 383-116 K) and phase III (below 116 K) in order (i) to get information on the molecular and/or crystal modifications involved in the phase transition and (ii) to assign i.r. absorptions. Spectra of randomly oriented samples and of single crystals having ab and bc as predominant faces were taken at different temperatures. In phase II the number of bands and their polarizations clearly indicate that the benzene like modes feel a potential of symmetry higher than that of the site, whilst the band broadness of the absorptions due to methyl stretchings and bendings does not allow any indication on “peripheral” potential. Changes occurring in the spectrum of the low temperature phase should indicate a more ordered situation in which the molecular symmetry is D_3d or S₆.     

Structural studies by nuclear magnetic resonance—XVII : Conformations and configurations of N-methylhydrazones

Conformations and configurations were assigned to several aliphatic N-methylhydrazones from analysis of their 60-Mc NMR spectra. Interpretation of the vicinal spin-spin coupling constants, J_H1Hα, of the syn isomers (N-methylamino cis to hydrogen) of the aldehyde N-methylhydrazones in terms of rotamers I and II gave ΔH° values that are similar to those found in hydrazones, oximes, and oxime O-methyl ethers.The anisotropic effects of the CN double bond, the association between solute and solvent, several aspects of the chemical shifts, and other problems related to configurational isomerism about the CN double bond are discussed.     

Structural studies by nuclear magnetic resonance—XII: Conformations and configurations of n-methylphenylhydrazones

Conformations and configurations were assigned to several aldehyde and ketone N-methylphenylhydrazones from analysis of their 60-Mc NMR spectra. Whereas ketone N-methyl-phenylhydrazones show detectable configurational isomerism about the CN double bond, aldehyde N-methylphenylhydrazones exist solely as the syn isomers. This observation is rationalized in terms of conformational isomers about the NN single bond.     

Nuclear magnetic resonance studies of tetraalkyl compounds of group III and group V elements—I

The N.M.R. spectra of the symmetrical ions BEt₄₋, NEt₄₊ and AsEt₄₊ have been examined and the coupling constants of the protons with the central atoms have been found. The effect of the ionic charge on the electronegativity of the metal, as measured by the differences in the chemical shifts of the protons in the methyl and methylene groups, has been observed. Evidence of ion-pair association by the collapse of spin-spin multiplets has been found.     

13C nuclear magnetic resonance spectroscopy and lifetime studies of the solution photoreactivity of 1,4-bis(β-pyridyl-2-vinyl)benzene and 2,5-distyrylpyrazine

The photoreactivity of 1,4-bis(β-pyridyl-2-vinyl)benzene and distyryl-pyrazine has been investigated using lifetime, Shpol'skii and carbon-13 nuclear magnetic resonance (¹³C NMR) techniques. Our study shows that long-lived emitting species exist in concentrated solutions indicating excited state aggregation that could lead to bimolecular processes in concentrated solutions. It is confirmed that the photoproducts do not emit in the spectral region studied by using the Shpol'skii technique.¹³C NMR has been performed only for concentrated solutions, revealing the formation of cis isomers in addition to the occurrence of the previously reported photochemical processes.     

Effect of sintering temperature and thermoelectric power studies of the system MgFe2−xCrxO4

Mixed metal oxides showing the spinel structure exhibit interesting structural and electrical properties. Substances with specific compositions in the system MgFe_2?xCr_xO₄ were synthesized by the simple co-precipitation method and have been investigated by X-ray diffraction (XRD) and scanning electron microscope (SEM) to study the effect of temperature on the size of particles and grains. The infrared spectrum shows, two strong bands around 600 and 500 cm^?1. An elemental composition of one of the samples, MgFeCrO₄ was found by energy dispersive X-ray spectroscopy (EDS). The thermoelectric power measurements carried out from room temperature to 500 °C, show both n-type and p-type behavior.     

Structural studies by nuclear magnetic resonance—XVIII : Conformational and configurational isomerism of N-alkyl imines

Conformations and configurations were assigned to several N-Me imines and N-alkyl acetaldimines from analyses of their 60-Mc NMR spectra. All aldimines exist exclusively in the syn configuration. Interpretation of the vicinal and long-range (across 4 and 5 and bonds) spin-spin coupling constants of N-Me aldimines led to the conclusion that I and II are the minimum energy conformations of these compounds. The ΔH° values for I ⇌ II fall between those of aldehydes, which are usually negative, and those of oximes and their O-methyl ethers, which are positive. Interpretation of the long-range (across 4 and 5 bonds) spin-spin coupling constants of N-alkyl acetaldimines led to the conclusion that III is far more stable than IV.The greater thermodynamic stabilities of the syn over the anti isomers of aldimines and III over IV have been explained in terms of nonbonded interactions across the CN double bond. The results of some of these calculations are given.     

Electron spin resonance studies of an iron nitroxide, [(CN)5FeN.(Ph)OH]3− and some related radicals

Methanolic solutions containing the complex ion (CN)₅FeN(Ph)O³⁻, on exposure to ⁶⁰Co γ-rays at 77 K, gave a well-defined ESR spectrum assigned to the nitroxide (CN)₅FeN^.(Ph)OH³⁻, having ESR parameters similar to those for the well-known cobalt analogue, the unpaired electron being largely localised on the NO group. In contrast, nitrosobenzene alone gave PhN^.O(OMe) in methanol. In acetonitrile, a species thought to be the negative ion, PhN^.O⁻ was formed from nitrosobenzene.On annealing above 77 K, the iron nitroxide was lost irreversibly and a new ESR spectrum, characteristic of an iron d⁷ complex was obtained. We suggest that this is formed by electron transfer from the paramagnetic ligand into the d_z² (Fe) orbital with concurrent loss of the trans-cyanide ligand.     

An octanuclear complex containing the [Fe3O]7+ metal core: structural, magnetic, Mössbauer, and electron paramagnetic resonance studies

A new asymmetrically coordinated bis-trinuclear iron(III) cluster containing a [Fe(3)O](7+) core has been synthesized and structurally, magnetically, and spectroscopically characterized. [Fe(6)Na(2)O(2)(O(2)CPh)(10)(pic)(4)(EtOH)(4)(H(2)O)(2)](ClO(4))(2).2EpsilontOH (1.2EpsilontOH) crystallizes in the P space group and consists of two symmetry-related {Fe(3)O](7+) subunits linked by two Na(+) cations. Inside each [Fe(3)O](7+) subunit, the iron(III) ions are antiferromagnetically coupled, and their magnetic exchange is best described by an isosceles triangle model with two equal (J) and one different (J ') coupling constants. On the basis of the H = -2SigmaJ(ij)S(i)S(j) spin Hamiltonian formalism, the two best fits to the data yield solutions J = -27.4 cm(-1), J ' = -20.9 cm(-1) and J = -22.7 cm(-1), J ' = -31.6 cm(-1). The ground state of the cluster is S = (1)/(2). X-band electron paramagnetic resonance (EPR) spectroscopy at liquid-helium temperature reveals a signal comprising a sharp peak at g approximately 2 and a broad tail at higher magnetic fields consistent with the S = (1)/(2) character of the ground state. Variable-temperature zero-field and magnetically perturbed M?ssbauer spectra at liquid-helium temperatures are consistent with three antiferromagnetically coupled high-spin ferric ions in agreement with the magnetic susceptibility and EPR results. The EPR and M?ssbauer spectra are interpreted by assuming the presence of an antisymmetric exchange interaction with |d| approximately 2-4 cm(-1) and a distribution of exchange constants J(ij).     

Synthesis,transport and magnetic studies of superconducting Y1−xNaxCa2Cu3O7−δ system

A series of oxides with chemical formula Y_1?xNa_xCa₂Cu₃O_7?δ (x = 0.1, 0.2, 0.3 & 0.4) have been synthesized by solid-state diffusion method. The influence of substitution of Na with different Y concentrations has been studied using XRD, electrical resistivity and ac magnetic susceptibility measurements. The formation is checked by XRD and all samples found to crystallize in a single phase with orthorhombic structure, isostructural to YCa₂Cu₃O₇. The substitution of Na at Y site has a distinct effect on the superconducting properties of the oxides. With increase in Na content the critical temperature decreases. The resistivity of the compound above T_c is in the range of semiconductors but the behavior is metallic.     

Nuclear magnetic resonance studies in the series ZrF4·xHF·yH2O—I: The constitution of the hydrates of zirconium tetrafluoride

Proton and fluorine magnetic resonance spectra prove that the hydrates of zirconium tetrafluoride are correctly formulated as ZrF₄·H₂O and ZrF₄·3H₂O. The results are related to the possible crystal structure of the trihydrate. It is found that 10–20 per cent surplus water is unavoidably incorporated into the crystal lattice of the monohydrate prepared by the usual methods, but pure monohydrate could be obtained by dehydrating ZrF₄·3H₂O at 110°C.     

Electron spin resonance spectrum and structure of the pentatungstovanadate(IV) anion,VW5O194−

The ESR spectrum of the V^IVW₅O₁₉⁴⁻ ion (I) in (CH₃NH₃)₂Na₂[V₂^VW₄O₁₉]·6H₂O host is reported. Spin Hamiltonian parameters are g_∥, 1·949 ± 0·001; g_⊥, 1·969 ± 0·001; |A|, (167·1 ± 0·5) 10⁻⁴ cm⁻¹; |B|, (60·8 ± 1·0) 10⁻⁴ cm⁻¹. Analysis of single crystal spectra shows three mutually perpendicular g directions, consistent with a structure for the anion analogous to that found for Nb₆O₁₉⁸⁻. It is not possible to reconcile the g-values for I with previously reported optical data without assuming an unusually large value (ca. 240 cm⁻¹) for the spin-orbit coupling constant of vanadium.     

Structural studies by nuclear magnetic resonance—XIX : N,N-dimethylhydrazones and general comments on configurational and conformational isomerism

Configurations and conformations were assigned to several aldehyde and ketone N,N-dimethyldrazones from analyses of their 60-Mc NMR spectra. Aldehyde, but not ketone, N,N-dimethylhydrazones exist exclusively in the syn configuration. Interpretation of the vicinal spin-spin coupling constants, J_H1Hα, led to the conclusion that II and III are the minimum energy conformations of these compounds. The ΔH⁰ values for II ⇋ III were found to be more positive than those of N-Me imines (Z = Me) but less positive than those of oximes (Z = OH) and their O-methyl ethers (Z = Me).General comments are made on several aspects of configurational and conformational isomerism about single, double and partial double bonds, and on the usefulness and limitations of NMR in studying various problems associated with such isomerism.