Single Crystal ESR Study Research Articles

Electronic instabilities and irradiation effects in the (TMTTF)2X series

The paper presents a single-crystal ESR study of a series of pristine and X-ray irradiated (TMTTF)2X salts, where TMTTF is tetramethyltetrathiafulvalene and X is either a centro-symmetrical (SbF6 and Br) or a non-centro-symmetrical (ReO4, ClO4, BF4, SCN and NO3) monovalent anion. Besides standard line-width and spin-susceptibility measurements, the analysis of the asymmetry of the ESR line shape allows to obtain simultaneous information on the electrical conductivity. This whole set of data is used to determine the charge and spin gaps related to the charge ordering (CO) and anion ordering (AO) ground states and their evolution as a function of X-ray irradiation damages. We show in particular that the sensibility of (TMTTF)2X salts to irradiation depends upon the nature of the anion X and that the CO ground state is drastically affected by irradiation damages at the difference of the AO ground state. We also present evidence of a CO transition and of a decoupling between the AO transition and the opening of a spin gap in (TMTTF)2NO3.

Magnetic superexchange interactions: trinuclear bis(oxamidato) versus bis(oxamato) type complexes.

The diethyl ester of o-phenylenebis(oxamic acid) (opbaH2Et2) was treated with an excess of RNH2 in MeOH to cause the exclusive formation of the respective o-phenylenebis(N(R)-oxamides) (opboH4R2, R = Me , Et , (n)Pr ) in good yields. Treatment of with half an equivalent of [Cu2(AcO)4(H2O)2] or one equivalent of [Ni(AcO)2(H2O)4] followed by the addition of four equivalents of [(n)Bu4N]OH resulted in the formation of mononuclear bis(oxamidato) type complexes [(n)Bu4N]2[M(opboR2)] (M = Ni, R = Me , Et , (n)Pr ; M = Cu, R = Me , Et , (n)Pr ). By addition of two equivalents of [Cu(pmdta)(NO3)2] to MeCN solutions of , novel trinuclear complexes [Cu3(opboR2)(L)2](NO3)2 (L = pmdta, R = Me , Et , (n)Pr ) could be obtained. Compounds have been characterized by elemental analysis and NMR/IR spectroscopy. Furthermore, the solid state structures of and have been determined by single-crystal X-ray diffraction studies. By controlled cocrystallization, diamagnetically diluted and (1%) in the host lattice of and (99%) (@ and @), respectively, in the form of single crystals have been made available, allowing single crystal ESR studies to extract all components of the g-factor and the tensors of onsite (Cu)A and transferred (N)A hyperfine (HF) interaction. From these studies, the spin density distribution of the [Cu(opboEt2)](2-) and [Cu(opbo(n)Pr2)](2-) complex fragments of and , respectively, could be determined. Additionally, as a single crystal ENDOR measurement of @ revealed the individual HF tensors of the N donor atoms to be unequal, individual estimates of the spin densities on each N donor atom were made. The magnetic properties of were studied by susceptibility measurements versus temperature to give J values varying from -96 cm(-1) () over -104 cm(-1) () to -132 cm(-1) (). These three trinuclear Cu(II)-containing bis(oxamidato) type complexes exhibit J values which are comparable to and slightly larger in magnitude than those of related bis(oxamato) type complexes. In a summarizing discussion involving experimentally obtained ESR results (spin density distribution) of and , the geometries of the terminal [Cu(pmdta)](2+) fragments of determined by crystallographic studies, together with accompanying quantum chemical calculations, an approach is derived to explain these phenomena and to conclude if the spin density distribution of mononuclear bis(oxamato)/bis(oxamidato) type complexes could be a measure of the J couplings of corresponding trinuclear complexes.

A Single-Crystal Esr Study of Magnetism and Spin Dynamics in N-[(2,4-Dichlorophenyl)Thio]-2,4,6-Tripheyl-Phenylaminyl Radical Crystal

A single-crystal ESR study of the magnetic properties of N-[(2,4-dichlorophenyl)thio]-2,4,6-triphenylphenylaminyl radical crystals is reported. The stable radical crystal shows a one-dimensional antiferromagnetic behavior which has described in terms of an alternating linear chain model with J/kB = -7.5 K and an alternating parameter α=0.55. A detailed analysis of the angular and temperature dependence of the ESR linewidth and g-value has been carried out. The angular dependence of the line-width and line shape observed at 5 K has shown characteristic features expected for a one-dimensional magnetic system: (3cos2θ-1)4/3 angular dependence of line width and Lorentzian line shape at θ= 55°. Nagata's theory has applied for the analysis of the temperature dependence of the g-shift. However, the temperature dependence has been only qualitatively reproduced by the theory which is proposed for the one-dimensional regular Heisenberg model. We discuss on the origin of this unsatisfactory agreement. The magneto-structural correlation is also discussed based on the X-ray crystallographic and single-crystal ESR data.

Syntheses, properties and crystal structures of a series of mixtures of dmit and dmise metal complexes (DMIT1,3-DITHIOLE-2-THIONE-4,5-DITHIOLATE, DMISE1,3-DITHIOLE-2-SELONE-4,5-DITHIOLATE). Single-crystal esr study of [Bu 4N] 2[Cu/Ni(C 3S 4.4Se 0.6) 2

A series of mixtures of symmetric metal complexes of C 3S 5 - and C 3S 4Se 2- ligands (C 3S 5 -1,3-dithiole-2-thione-4,5-dithiolate, DMIT; C 3S 4Se 2- = 1,3-dithiole-2-selone-4, 5-dithiolate, DMISE), [Bu 4N] 2[M(C 3S 4.4Se 0.6) 2] (MNi, 1 ; M = Cu, 2), [Bu 4N] [Ni(C 3S 4.4Se 0.6) 2], 3, [Bu 4N] 2[Ni(C 3S 4.7Se 0.3) 2], 4, and [Bu 4N][Ni(C 3S 4.7Se 0.3) 2], 5, have been prepared. Oxidation reactions of 1 with excess iodine afforded the neutral complex [Ni(C 3S 4.4Se 0.6) 2], 6, which exhibited electrical conductivity of 0.29 S cm -1 at 25°C for a compacted pellet. Single-crystal X-ray analysis of 1 revealed that the anion structure as a whole is planar. Both 3 and 5 form a one-dimensional arrangement through the intermolecular sulfursulfur contacts. The S and Se at the terminal positions of the ligand are disordered. A single-crystal ESR study on [BuN 4] 2[Cu/Ni(C 3S 4.4Se 0.6) 2] demonstrated that the M-S coordination bond in the complex has a strong covalent σ-bond character.

Electronic and Molecular Structures of 1,3,5-Tris(Phenylmethylene)Benzene (S = 3) as Models for Discotic High-Spin Liquid Crystals: Single-Crystal ESR and X-Ray Diffraction Studies

Abstract The electronic and molecular structures of the two-dimensional septet-state molecules(S=3), l,3,5-tris(p-mcthoxyphenylmcthylcne)benzcnc A and 1,3,5-tris(phenylmethylene)benzene B, were investigated by single-crystal ESR spectroscopy. A was designed for a prototypical core model for discotic liquid-crystalline organic high-spin molecules. We determined the fine-structure constants of A by the ESR spectroscopy and analyzed the crystal and molecular structures of B by X-ray diffraction. The effect of the mcthoxy functional group on the fine-structure constants was found: the effect showed up in the significant reduction of the fine-structure constant D value for A, indicating the modification of the topologically controlled robust π spin polarization. The observed non-vanishing E value is in accord with the molecular symmetry and crystal structure of the host molecule, 1,3,5-tris(α-benzoyl)benzene.

ESR studies of electron loss and gain centers in orthoperiodic acid (H5IO6) at 77 K

Abstract ESR studies of single crystals of H5IO6 γ-irradiated at 77 K reveal two paramagnetic centers, one exhibiting 11 hyperfine components in the free-spin region and the other revealing two broad features at higher field. The radical which is in the free-spin region has a low hyperfine coupling and is identified as (HO)5IO⨪OI(OH)+5 (dimer hole center). The spin density of the unpaired electron on the iodine 5s and 5p orbitals is found to be 0.014. The center at the higher field, however, has a larger coupling and is identified as H5IO−6 (electron gain center). The spin density for this center has been found to be 0.34. The temperature variation studies reveal that both the centers are unstable above 120 K.

ESR and ENDOR Study of Single Crystals of Deoxyadenosine Monohydrate X-Irradiated at 10 K

Five free radicals have been detected by detailed ESR/ENDOR experiments on single crystals of deoxyadenosine monohydrate (AdRm), X-irradiated and observed at 10 K. In a previous study of adenosine (Radiat. Res. 117, 367-378, 1989), only the anion (protonated at N3) and the cation (deprotonated at the exocyclic NH2) were detected at 10 K. In AdRm, Radical R1 is the N3-protonated anion, similar to that observed previously in adenosine. Radical R3 is a C5' hydroxyalkyl radical formed by net H-abstraction from C5'. A second sugar radical is formed by net C1' H-abstraction. Two other base radicals observed in AdRm at 10 K are the C2 and C8 H-addition radicals. The C2 H-addition radical (Radical R5) exhibits inequivalent methylene hydrogen couplings of 5.43 and 3.29 mT, while in the C8 H-addition radical (Radical R6) the couplings are somewhat more equivalent (3.63 and 4.17 mT). No link between RAdical R1 and the H-addition radicals has been observed. The reduced base appears to protonate rapidly even at 10 K, while at the same time both H-addition radicals are clearly present. On warming, Radical R1 appears to decay at about 80 K with no apparent successor. Although no base cation was stabilized in AdRm at 10 K, it is interesting to note that Radicals R3 and R4 could both be formed as the result of deprotonation of primary oxidation products.

Formation of Pt(III) complexes in irradiated potassium tetranitroplatinate(II): a single crystal ESR study

X-irradiation of single crystals of K 2[Pt(NO 2)] and of K 2[Pt(NO 2) 4]·2H 2O leads to the formation of Pt(III) complexes which have been studied by EPR at 77 K. The crystal structure of the hydrated compound has been determined in order to interpret the orientation of the g and 195Pt hyperfine tensors and to reassign the powder spectrum previously attributed to this compound. The nature of the trapped species is shown to be dependent upon the presence of water molecules: one of the two Pt(III) complexes formed in the hydrated crystal probably results from oxidation of the original molecule by the radiogenic OH · radical whereas, for the second species, the water molecule seems only to participate in the stabilization of the Pt(III) complex. For the anhydrous compound this stabilizing effect is probably assured by a distant NO 2 group.

Remarks on the ground state of low-spin Cobalt(II) triazene-1-oxide complexes

Single-crystal ESR studies on low-spin bis(3-phenyl-1-ethyltriazene-1-olato) cobalt(II) doped into the diamagnetic host lattice of the isomorphous nickel(II) complex have been performed. McGarvey's theory of low-spin d 7 ions and modified INDO calculations have been applied in order to account for the spin-Hamiltonian parameters obtained. On the basis of the data obtained, g x = 2.67, g y = 2.07, g z = 1.98, A x = 16 × 10 −4 cm −1, A y = 28 × 10 −4 cm −1, A z = 36 × 10 −4 cm −1, and the observed superhyperfine structure due to equatorially disposed 14N nuclei, the ¦ z 2 > 1 ground state has been proposed. This same result was obtained on the basis of modified INDO (GRINDOL) calculations.

Radical pair formation in X-irradiated croconate ions: A single crystal ESR study

The crystal structure of CsNa(C 5O 5) has been determined and shows that the croconate rings stack along the crystallographic a direction. Single crystals of this compound have been X-irradiated at 77 K. The angular dependence of the resulting ESR signals shows that a radical pair is trapped; the corresponding electron-electron dipolar interaction indicates that the vector which links the two components of the pair is aligned along the stacking direction and is consistent with a pair formed by two paramagnetic croconate anions separated by a (C 5O 5) 2- ring.

Intermolecular-directed reactivity in solid media. Radiogenic formation of phosphorus-centered radicals in chiral diphosphine disulfides studied by ESR

AWct Singlecrystal, powder, and frozen-matrix ESR experiments have been performed to study the radiogenic electron-capture properties of several diastereoisomeric and asymmetric diphosphine disulfides (R,RzP(S)P(S)R3R4). The principal values of the hyperfine couplings of several phosphorus-centered radical configurations are determined and related to the spin density distribution. Attention is focused on the strong differences in radiogenic properties, observed between the meso and racemic forms of phenyl- and tolyl-substituted diphosphine disulfides. The most striking result is that X irradiation of the crystalline meso compounds MePhP(S)P(S)MePh, Me@-Tol)P(S)P(S)Me@-Tol), and Ph(PhCHz)P(S)P(S)Ph(CH2Ph) does not lead to the formation of a three-electron bond PAP u* radical but invariably results in configurations in which the unpaired electron is primarily localized on one half of the molecule. X irradiation of the corresponding racemic forms, on the other hand, gives rise to PLP u* configurations. The observed discrimination between symmetric and asymmetric configurations is explained in terms of intermolecular steric interactions affecting the geometry relaxation of the precursor molecule after initial electron addition. For a quantitative assessment, the change in van der Waals energy resulting from elongation of the P-P bond of the molecules in their respective crystal lattices was calculated with X-ray crystallographic data. The calculations reveal significantly stronger steric interactions for the aromatic meso compounds than for their racemic forms, in agreement with the absence of PAP u* configurations in the first. X irradiation of diphosphine disulfides in a frozen THF matrix almost invariably results in a single radical product, being the PAP u* configuration, and differences between meso and racemic isomers disappear. This is a consequence of the fact that in a randomly oriented solid matrix the molecular packing is less tight than in a molecular crystal, making more space available to the precursor molecule. It is concluded that in case stabilization of the initial electron adduct via P-P bond length elongation is unfavorable because of steric interactions, other relaxation pathways become accessible, resulting in alternative phosphoranyl radical configurations. In recent years the radiochemistry of organophosphorus compounds has received considerable attention. The phosphoruscentered radicals, formed in the radiation process, are of special interest because of their diverse structural and dynamical prop erties and their possible relevance to radiation damage in biochemical systems.’” It has been shown from a number of single-crystal ESR studies that the precise nature of the configuration of a phosphoranyl radical depends on the type of ligands surrounding the central phosphorus nucleus. Only recently have effects other than the nature of the substituents been envisaged as being essential to the formation of radiogenic phosphoruscentered radicals. In this respect, aspects of conformation and configuration of the precursor molecules and the involvement of the precursor environment in the electron-capture process have been studied.4d Substituted diphosphine disulfides (R,R,P(S)P(S)R,R,) can serve as good probes to study phosphoranyl radical formation because of their intrinsic ability to adopt various radical configurations from a single molecule (Figure l).’ X irradiation of a diphosphine disulfide almost invariably results in the formation of an electron-capture radical in which the unpaired electron is accommodated in the antibonding u* orbital of the inter-phosphorus bond. As a consequence, the electron is symmetrically distributed over the two phosphorus nuclei. Complementary to this symmetrical PAP u* configuration, several radical structures with an asymmetrical spin density distribution have been established. One of the asymmetric structures frequently observed is a radical in which the odd electron is located in the vacant equatorial position of a trigonal-bipyramidal configuration (TBP-e). Another asymmetric structure has been identified as a phosphoranyl radical in which the unpaired electron resides in an antibonding u* orbital of the phosphorussulfur bond (PS U’).

Electron capture by trivalent phosphorus compounds: A single crystal ESR study of the chlorodithiaphospholane anion

The 2-chloro-1,3,2-dithiaphospholane anion has been trapped in an X-irradiated single crystal of 1. The anion has been identified by comparing its 31P and 35Cl hyperfine tensors with the corresponding couplings obtained from ab initio calculations on H 2PCl −. and PCl 3 −.. The structure of R 2PCl − is discussed. Increasing the temperature causes the dissociation reaction to occur, and the phosphinyl radical has been observed after annealing at 220 K.

On the electronic structure of metastable nitroprusside ion in Na 2[Fe(CN) 5NO].2H 2O. A comparative single crystal esr study

Laser irradiation of Na 2[Fe(CN) 5NO].2H 2O at low temperature produces two long-living nitroprusside metastable states which, upon heating, decay at two different temperatures. In addition, laser illumination of previously X-ray irradiated crystals generates traces of the closely related [Fe(CN) 5NO] 3- species with linear (axial) NCFeNO geometry where the unpaired electron is localized on a d z 2 orbital as detected by ESR spectroscopy [g |=2.0055(6), g ∥=2.0348(4); A |=16.9(1)G, A |=14.8(1)G]. There also exists another stable form of [Fe(CN) 5NO] 3- with bent FeNO group and the odd electron located in a π∗NO orbital. The comparison of metastable [Fe(CN) 5NO] 2- and the [Fe(CN) 5NO] 3- (d z 2) species suggest that the electronic structure of the high-temperature metastable state should be described as 1B 2: 6e(d xz,d yz) 4 2b 2(d xy) 1 5a 1(d z 2) 1. This assignment is consistent with reported optical absorption, Mössbauer, Raman, and IR data.

Copper(I) and copper(II) complexes of entwined or interlocked phenanthroline type ligands: ESR and crystallographic investigations

The crystal structure of [Cu(dap) 2]BF 4 has been determined and single crystals of 63Cu(dap) 2BF 4 were doped with [ 63Cu(dap) 2] 2+ ions and studied by ESR. The resulting spectra are compared with those obtained with frozen solutions containing [ 63Cu(dap) 2] 2+ or [Cu(cat-30)] 2+. The complex [Cu(dap) 2] + is pseudotetrahedral (crystal structure) as is [Cu(dap) 2] 2+ in frozen solution. The 63Cu hyperfine coupling shows however that this coordination is not maintained for the dication in a crystalline environment, and the single crystal ESR study suggests the presence of a fifth ligand. Such a versatility of the structure is not observed for the Cu(II) catenate in agreement with the more pronounced rigidity of the interlocked system as compared to its open chain analogue.

Intermolecular effects on the radiogenic formation of electron-capture phosphorus-centered radicals. A single-crystal ESR study of diastereoisomeric precursors

ESR experiments on X-irradiated single crystals of the 2R,4S,5R and 2S,4S,5R diastereoisomers of 2-chloro- 3,4-dimethyI-5-phenyl- 1,3,2-oxazaphospholidine 2-sulfide reveal that the yield of radiogenic electron-capture reactions in the solid state strongly depends on intermolecular interactions in the crystal. In the present case a high yield of P-CI three-electron-bond phosphoranyl radical anions is found in crystals of the 2R,4S,5R isomer, whereas no radical formation can be detected for the 2S,4S,5R isomer. An analysis of nonbonded interactions with neighboring molecules reveals that the geometry relaxation necessary for the radical stabilization is easily accommodated in crystals of the 2R,4S.5R isomer but not in the 2S,4S,5R isomer, explaining the observed difference in electron-capture efficiency. Experiments on radical formation in a MeTHF host matrix give further insight into the importance of the environment on radiogenic radical formation. The possible concurrent effect of the matrix on the electronic configuration and spin density distribution of the resulting phosphoranyl radical is discussed. The radiogenic formation of phosphorus-centered radicals has been extensively studied over the last decades.' The continuing interest in phosphorus-centered radicals originates partly from their possible role in the radiation chemistry of nucleic acids and is partly a consequence of their interesting electronic, structural, and dynamical properties. It has been established that the radical configuration, formed as a result of the electron-addition process, strongly depends on the nature of the substituents linked to the central phosphorus atom.2 Recently, however, it has become clear that other effects, which are not generally taken into account, can also be decisive for the nature of the observed radiation products. In particular we reported on the pronounced differences observed in electron capture of rac- and meso- 1,2-dimethyl- 1,2-di- phenyldiphosphine disulfide, demonstrating the importance of stereochemical effect^.^ Simultaneously, Cattani-Lorente et al. showed, by studying the electron capture of 1,2-phenylene phosphorochloridate in the pure compound and in frozen solutions, that the environment of the precursor molecule influences the resulting radical configuration after X-irradiati~n.~ In this paper we examine the influence of the surrounding matrix by studying radical formation in single crystals of two diastereoisomeric compounds, (2R,4S,5R)- and (2S,4S,5R)-2- chloro-3,4-dimethyl-5-phenyl-l,3,2-oxazaphospholidine 2-sulfide

ChemInform Abstract: Dithiosquarato Cuprate and the Dithiosquarato Palladate Ions: Single Crystal ESR Study and Crystallographic Investigations.

AbstractThe crystal structure of the Pd complex (Ia) (space group P21/n, Z=4) is determined and the single crystal ESR study of the bis(dithiosquarato)‐Cu(II) ion is performed in (Ib) and in doped (Ia).

Esr and Endor Studies of X-Irradiated Single Crystals of Guanine Derivatives

Esr and Endor Studies of X-Irradiated Single Crystals of Guanine Derivatives

A single crystal ESR study of Gd(bipy) 2(NO 3) 3 in La(bipy) 2(NO 3) 3

The single crystal ESR spectrum of Gd 3+ doped into La(bipy) 2(NO 3) 3 has been recorded at 290 K for many orientations of the crystal axes with respect to the magnetic field. A good fit of the spin Hamiltonian parameters to the experimental data has been achieved with a more efficient algorithm than previously used in this context, a significant saving being the computation of the eigenvalues only. It has been shown that previously reported spin Hamiltonian parameters for the title compound are not sufficient to describe the spectra recorded for arbitrary orientations of the single crystal and that low symmetry Gd 3+ complexes require all possible parameters to be included.

Electron spin resonance studies of low symmetry oxomolybdenum(V) ions diluted in the pseudo-isostructural chlorotin(IV) moiety

The results of a single crystal ESR study of LMoOCl2 diluted in the isostructural host LSnCl3 are reported. These results confirm the monoclinic ESR symmetry deduced previously from frozen solution measurements giving the rotation angle between the tensor axes as 35° compared with 33° in previous study. The single crystal data show that none of the principal g- or A-tensor axes coincide with metal-ligand directions.

Lactose and “Tris” Lyoluminescence dosimetry systems and ESR correlation studies

Lyoluminescence (LL) dosimeters have been developed using lactose monohydrate (disaccharide) and tris(hydroxymethyl)aminomethane (“Tris”) systems and attempts have been made to understand the LL mechanism through ESR correlation studies. Tris LL dosimeter has a γ-ray sensitivity with a linear response in the absorbed-dose range 0.05–200 Gy (5 −2 × 10 4 rad), while the lactose response extends to a higher range from 1 to 10 4 Gy (10 2–10 6 rad). The LL output of lactose and Tris did not show any appreciable decay for a period of 6 months after irradiation. ESR measurements show that free-radical concentration in both the systems increases with γ-ray dose in the range 10 2–10 5 Gy. The minimum dose required to measure the radiation-induced ESR signal for Tris is ∼ 500 Gy, the dose at which the LL output saturates, while lactose shows a radiation-induced ESR signal right at the minimum dose where LL could be detected. The estimated value of free-radical concentration for lactose was 10 14–10 17 spins/g in the dose range 10 2–10 5 Gy, while for Tris it is 10 16–10 17 spins/g in the dose range 10 3–10 5 Gy. ESR spectral features of the irradiated Tris show the presence of two distinct radical species and one of these species is found to decay with time. This species has been assigned to a R-cHOH radical on the basis of a detailed single-crystal ESR study. The principal g factors of the radicals are g x = 2.0021, y = 2.0041, g z = 2.0024 and the principal hyperfine couplings are found to be A x = 15.8 G, A y = 23.4 G and A z = 13.8 G. The estimated spin density on the radical carbon atom is 0.7. ESR signal stabilities of lactose and Tris were also studied. Lactose did not show any appreciable ESR decay for a period of 3 months after irradiation, while, for Tris, one of the radicals showed a decay of 45% for the same period.