Trigonal Centers Research Articles

The identity of the AB1 and NE4 electron paramagnetic resonance spectra inhigh-pressure–high-temperature diamond

Similar electron paramagnetic resonance (EPR) spectra caused by the trigonalcentres NE4 and AB1 have been detected in nitrogen-rich diamond synthesizedwith Ni catalyst by two different groups. The NE4 centre is commonlyconsidered to be the basic structure of several nickel–nitrogen centres that areformed upon the annealing of this type of diamond. However, its EPRspectrum is observed so far only in diamond crystals grown in Novosibirsk,whereas in similar crystals grown in the NIRIM the AB1 centre could bedetected. This discrepancy in the EPR detection could be solved by are-analysis of the published EPR data of the NE4 centre and through EPRinvestigations of synthetic diamonds from Novosibirsk and the NIRIM. Thestudy revealed that the EPR spectrum of the NE4 centre corresponds infact to the spectrum of the trigonal AB1 centre, which is described withS = 1/2 and theg-values g∥ = 2.0027 ± 0.0002 andg⊥ = 2.0923 ± 0.0002.Furthermore, we study whether the experimentalg-valuescan be successfully explained, within the framework of crystal-field theory, by thepreviously proposed models for the AB1 (NE4) centre.

Synthesis of heteronuclear bifunctional Lewis acids by transmetalation of 1,8-bis(trimethylstannyl)naphthalene with BCl3

Reaction of 1,8-bis(trimethylstannyl)naphthalene (1) with an excess of BCl3 at –78°, followed by warming to 0°C, results in the exclusive formation of the novel bifunctional Lewis acid 1-(chlorodimethylstannyl)-8-(dichloroboryl)naphthalene (2), a compound in which a boryl and a stannyl moiety coexist at the peri-positions of a naphthalene core. At elevated temperature compound 2 undergoes a chloride–methyl exchange, which affords 1-(dichloromethylstannyl)-8-(chloromethylboryl)naphthalene (3). Compounds 2 and 3 have been characterized by multi-nuclear NMR spectroscopy. The single crystal X-ray analysis for compound 3 reveals a sterically crowded structure with an essentially trigonal planar boron center, and a tin center pentacoordinated in a [4+1]-fashion. Upon exposure to traces of water, compound 3 is converted into the borinic acid derivative 1-(dichloromethylstannyl)-8-(hydroxymethylboryl)naphthalene (4), which has been characterized by multi-nuclear NMR spectroscopy and single crystal X-ray analysis.Key words: multidentate Lewis acids, organoboranes, organostannanes.

Selective averaging of EPR transitions of a high-spin center in the vicinity of their accidental coincidence

The averaging of a part of spin packets for two transitions of the Gd3+ trigonal center in ferroelectric lead germanite in the vicinity of coincidence of their positions leading to the emergence of an additional EPR signal is investigated. A computer simulation of the experimental spectrum gives the temperature dependence of the spin-lattice relaxation time connecting the doublets in which resonance transitions occur and leading to selective averaging of the packets. An increase in the relaxation rate for intra-and interdoublet transitions is observed in the vicinity of the structural ferroelectric transition.

Selective Synthesis, Characterization, and Configurational Flexibility of the Coordinatively Unsaturated Metal Center of Half-Sandwich Type Complexes with the Less-Hindered Hydrotris(3, 5-dimethyl-4-X-1-pyrazolyl)borate Ligands [TpMe2,XMII(κ2-O,O′-L)] (M = Ni, Co; L = NO3, OAc; X = Me, H, Br)

Abstract The selective synthesis of half-sandwich type complexes of Ni(II) and Co(II) with the less-hindered hydrotris(3,5-dimethyl-4-X-1-pyrazolyl)borate [X = Me (TpMe3; a series), H (TpMe2; b series), and Br (TpMe2,Br; c series)] ligands, [TpMe2,XMII(κ2-O,O′-L)] [L = NO3 (2), OAc (3)], has been achieved by dropwise addition of a THF solution of NaTpMe2,X to an excess amount of ML2 dissolved in MeOH. The solid state structures of the nitrato complexes 2 have been characterized by X-ray crystallography as their solvated form, 2·solv. (solv. = MeCN, MeOH), in which the metal centers have the six-coordinated octahedral geometry. Solvates are readily dissociated from the metal center upon dissolution in non-coordinationg solvents, such as CH2Cl2 and toluene, as well as drying of a solid sample to give the non-solvated five-coordinated species 2. The crystal structure of the non-solvated TpMe2Ni derivative 3bNi consists of a highly distorted trigonal bipyramidal nickel center.

Theoretical Calculations of g Factors and Hyperfine Structure Constants for Co2+ in Cd2+(I) and Cs2+(II) Sites of CsCdCl3 Crystal**The project supported by Key Laboratory of Radiation Physics and Technology of Ministry of Education of China

The factors and hyperfine structure constants for two trigonal centers (i.e., in (I) and (II) sites) in crystals are calculated from the high-order perturbation formulas based on the cluster approach. In the calculation, the contributions from covalency effect and configuration interaction effect are considered and the parameters related to both effects are obtained from the optical spectrum and the structure data of the studied system. The results are in good agreement with the observed values.

Hydroxy group interactions in stannylated carbohydrates. Structures and thermal stabilities of 5-deoxy-5-C-(Ph3Sn)-1,2-O-isopropylidene-α-D-xylofuranose, 5-deoxy-5-C-(IPh2Sn)-1,2-O-isopropylidene-α-D- and -L-xylofuranose, and 5-deoxy-5-C-(I2PhSn)-1,2-O-isopropylidene-α-D- and -L-xylofuranose

Structures and thermal stabilities of 5-deoxy-5-C-(Ph3Sn)-1,2-O-isopropylidene-α-D-xylofuranose, [(D)-3], 5-deoxy-5-C-(IPh2Sn)-1,2-O-isopropylidene-α-D- and -L-xylofuranose, [(D)-4 and (L)-4], and 5-deoxy-5-C-(I2PhSn)-1,2-O-isopropylidene-α-D- and -L-xylofuranose, [(D)-5 and (L)-5], are reported. The hydroxy groups in the stannylated xylofuranose derivatives, 3–5, exhibit roles as Lewis bases, Bronsted acids and hydrogen bonding centres. The 5-deoxy-1,2-O-isopropylidene-α-D- and –L-xylofuran-5-yl ligands operate as C5,O4 chelating ligands (4-membered chelate rings) in 3, C5,O3 chelating ligands in 4 (molecule 2) and 5 (5-membered chelate rings), and a C5,O4,O3 tridentate ligand in 4 (molecule 1) [both 4- and 5-membered chelate rings]. Compounds 4, in contrast to 3 and 5, undergo slow proton-dephenylation reactions in chloroform solutions at ambient temperature. All decompose on heating at 145–155 °C with evolution of PhH, Me2CO and H2O. The trigonal bipyramidal tin centres in (D)-5 and (L)-5 are chiral, at least in the solid state. Different modes of O–H–O bonding are found in 3, 4 (molecule 2) and 5 compared to that in 4 (molecule 1).

Scandium chloride, alkyl and phenyl complexes of diamido-donor ligands

Reactions of the lithiated diamido-pyridine and -amine ligands Li2N2TMSNpy or Li2N2NC2,TMS with ScCl3 in tetrahydrofuran (THF) afforded the five-coordinate scandium chloride derivatives [Sc(N2TMSNpy)Cl(THF)] 1 and [Sc(N2NC2,TMS)Cl(THF)] 2 where N2TMSNpy = MeC(2-C5H4N)(CH2NSiMe3)2 and N2NC2,TMS = Me3SiN(CH2CH2NSiMe3)2. The corresponding reactions of ScCl3 with the dilithium salts of the amino N-methylated two-carbon analogue N2NC2,Me (N2NC2,Me = MeN(CH2CH2NSiMe3)2) or of the amino N-silylated three-carbon chain analogue N2NC3,TMS (N2NC3,TMS = Me3SiN(CH2CH2CH2NSiMe3)2) afforded no tractable products. In contrast, reaction of ScCl3with the homologous N-methylated three-carbon chain species Li2N2NC3,Me (N2NC3,Me = MeN(CH2CH2CH2NSiMe3)2) cleanly gave the THF-free dinuclear, chloride-bridged compound [Sc2(N2NC3,Me)2(μ-Cl)2] 3. The compounds 1–3 have been crystallographically characterised. Organometallic analogues of 1 and 2 have been prepared by protonolysis reactions of H2N2R′Npy (N2R′Npy = MeC(2-C5H4N)(CH2NR′)2 where R′ = SiMe3, Tol (4-C6H4Me) or Mes (2,4,6-C6H2Me3)) and H2N2NC2,R′ (R′ = Me or SiMe3) with [ScR3(THF)2] (R = CH2SiMe3 or Ph) in benzene which gave the five-coordinate alkyl or phenyl compounds [Sc(N2R′Npy)R(THF)] (R = CH2SiMe3, R′ = SiMe3 4, Tol 5 or Mes 6; R = Ph, R′ = SiMe3 7) and [Sc(N2NC2,R′)(CH2SiMe3)(THF)] (R′ = Me 8 or SiMe3 9). The compound 4 can also be prepared by the reaction of 1 with LiCH2SiMe3. Reaction of [Sc(CH2SiMe3)3(THF)2] with H2N2NC3,Me afforded no tractable product, and with H2N2NC3,TMS in deuterobenzene the labile compound [Sc(N2NC3,TMS)(CH2SiMe3)(THF)] 10 was observed by 1H NMR spectroscopy but could not be isolated. On one occasion the THF-free dimeric alkyl species [Sc2(N2TolNpy)2(CH2SiMe3)2] 11 was obtained. This compound possesses one bridging and one terminal amido nitrogen per N2TolNpy ligand. The X-ray crystal structures of 6 and 11 have been determined. The monomeric compounds 1, 2 and 6 all have trigonal bipyramidal Sc centres in the solid state; the neutral donors take up the axial sites, and the amido nitrogens and either Cl or CH2SiMe3 occupy the equatorial ones.

Electronic structure of the paramagnetic boron oxygen hole center in B-dopedSiO2

We have studied the ground-state properties of boron-related dia- and paramagnetic point defects in B-doped silica. Hartree-Fock and density functional theory calculations have been performed to determine the structure, charge, and spin distribution of the boron oxygen hole center (BOHC). The currently accepted model of the BOHC is that of a hole localized on a nonbonding $2p$ orbital of an O atom in a bridge position between a B and a Si atom, $\ensuremath{\equiv}{\mathrm{B}\mathrm{---}\mathrm{O}}^{\mathbf{\ensuremath{\cdot}}}---\mathrm{Si}\ensuremath{\equiv}.$ Our calculations do not support this model and show that the structure is not stable and spontaneously evolves into a planar trigonal diamagnetic boron center, $g\mathrm{B}---,$ and a nonbridging oxygen, $\ensuremath{\equiv}{\mathrm{Si}\mathrm{---}\mathrm{O}}^{\mathbf{\ensuremath{\cdot}}}.$ The results of this study suggest that the BOHC consists of a three-coordinated B atom bound to a non bridging oxygen, $g\mathrm{B}---{\mathrm{O}}^{\mathbf{\ensuremath{\cdot}}}.$ The computed hyperfine coupling constants for this model are in quantitative agreement with those measured experimentally for B-doped silica. This assignment is consistent with recent magnetic resonance studies on borosilicates and alkali borate glasses.

On the Structural and Electronic Factors Governing the Magnetic Properties of the Hexagonal Perovskite-Type Oxides AxBO3 (A=Ca, Sr, Ba; B=Co, Ni)

The hexagonal perovskite-type oxides AxBO3 (A=Ca, Sr, Ba; B=Co, Ni) consist of (BO3)∞ chains made up of face-sharing BO6 octahedra and BO6 trigonal prisms. On the basis of molecular orbital calculations for their structural building blocks we examined why the transition metal atoms in the trigonal prisms of Sr9/7NiO3 and Sr14/11CoO3 occupy the positions away from their trigonal prism centers and why Ba6/5NiO3 and Ca3/2CoO3 exhibit apparently puzzling magnetic properties. Our analysis indicates that the structural building units of the (BO3)∞ chains in these oxides adopt high-spin states to reduce the metal–metal σ-antibonding interactions between adjacent metal atoms as well as the on-site repulsion. This finding led us to predict the number of unpaired spins that the oxides Sr9/7NiO3, Sr6/5CoO3, and Sr14/11CoO3 are expected to have.

The synthesis and structural characterisation of the first phosphavinyl–Group 13 complexes

The bisphosphavinyl indium complex, [CyIn{C( t Bu)PCy} 2], has been prepared from the reaction of two equivalents of [CyPC( t Bu)MgCl(OEt 2)] with CyInBr 2. Its crystal structure shows it to be monomeric with a trigonal planar indium centre. The reactions of [CyPC( t Bu)MgCl(OEt 2)] with MX 3, M=Al, Ga or In, X=Cl or Br, leads to facile phosphavinyl coupling reactions and the formation of the diphosphametallobicyclo[1.1.1]pentane complexes, [M{C 2( t Bu) 2P 2Cy 2}{C( t Bu)PCy}], which contain terminal phosphavinyl ligands.

Bonding analysis of inorganic transition-metal cubic clusters. 6. Copper(I)-dithiolato species and related compounds.

Extended Hückel and density functional calculations carried out on 128-MVE Cu(8)(dithiolato)(6) edge-bridged cubic clusters indicate that their stability is mainly driven by the chelating effect of the ligands, which provide a stable 16-electron configuration to the approximately trigonal planar metal centers. Nevertheless, a weak but significant d(10)-d(10) bonding interaction is present which is rather independent from the dithiolato bite effect. The metal centers have a nonbonding 4p(z)() vacant AO pointing to the center of the cube available for bonding to an encapsulated atom. The electronic closed-shell requirement is satisfied for the 136-MVE and 140-MVE counts, respectively, when a main-group atom or a transition-metal atom is incorporated in the middle of the cube. The bonding within these dithiolato compounds is compared to other edge-bridged M(8) cubic clusters. In particular, it is shown that clusters of higher nuclearity but containing an M(8) cubic core are related to the dithiolato species. Indeed, their outer metal atoms can be considered as belonging to the ligand shell, interacting with the M(8) cube in a way similar to the dithiolato ligands in the Cu(8) species.

Superhyperfine interaction in the trigonal center BaF2: Gd3+ and the lattice distortion analysis in the vicinity of the impurity ion

The constants of the superhyperfine interaction of Gd3+ with the 19F nuclear spins in the first four coordination shells were determined from the ENDOR spectra of a trigonal BaF2: Gd3+ center. These data were used for analysis of the crystalline lattice distortions in the vicinity of the impurity ion. It was found that the largest displacements of fluorine nuclei occurred in the vicinity of the ion compensator and the impurity ion. To calculate the anion positions within the first coordinate shell, whose electron-nuclear interaction with Gd3+ depends considerably on chemical bonds in the Gd3+F8− complex, an empirical model is used for the isotropic constants of the superhyperfine interaction of Gd3+ with fluorine nuclei in cubic centers, with allowance made for the impurity ion polarization.

C s–H 2* defect in crystalline silicon

An infrared absorption study has been carried out on hydrogen-doped Si crystals with high carbon content. Hydrogen (deuterium) was introduced into the crystals by high-temperature (1250–1350°C) in-diffusion from H 2 (D 2) gas ambient. After hydrogenation several local vibrational mode lines were observed in the ranges of 660–690, 790–820, 1920–1960, 2120–2230, and 2680–2950 cm −1. Two Si–H (Si–D) lines with frequencies at 792.0 (571.6) and 1921.8 (1401.2) cm −1 were found to have the same dependence on hydrogenation temperature and the same annealing behaviour, therefore they were assigned to a single defect. Measurements on samples co-doped with hydrogen and deuterium revealed the appearance of new lines at 793.2, 1400.3, and 1922.7 cm −1 indicating that the defect of interest incorporates two weakly interacting hydrogen atoms. Uniaxial stress measurements showed that the 792.0 cm −1 line represents an E mode (two-fold degenerate) of a trigonal centre. A correlation was found in annealing behaviour of the lines at 792.0 and 1921.8 cm −1 with that of a line at 2752.3 cm −1. The lines at 792.0 and 1921.8 cm −1 were assigned to Si–H bend and stretch modes, respectively, and the line at 2752.3 cm −1 to a C–H stretch mode of a CH 2* complex with C 3v symmetry. One hydrogen atom of the complex is bound to a C atom and locates on C 3v axis close to bond-centre site, and the other is bound to a Si atom and locates at an antibonding site.

Identification of the Gold-Lithium Defect L1 in Silicon with the Trigonal Centre Au2--(Li+)3 by Electrical, Optical and Magnetic Resonance Spectroscopy

N-type silicon doped with gold and lithium was investigated by Electron Paramagnetic Resonance (EPR) and Deep Level Transient Spectroscopy (DLTS), using two sets of samples with different defect concentrations of both gold and lithium. Photoluminescence (PL) on both sets of samples allowed to correlate the results of EPR and DLTS measurements. It is shown that the gold–lithium donor state, Au2––(Li+)3, with trigonal symmetry is most likely the origin of the well known DLTS level L1 at EC – 0.41 eV.

Identification of the hexavacancy in silicon with theB804optical center

The ring hexavacancy (V{sub 6}) has been found by previous theoretical modeling to be a particularly stable defect, but it has not been identified with any observed center to date. Here, we use ab initio calculations to derive the structure and properties of two forms of V{sub 6}H{sub 2} and identify these defects with the trigonal optical centers B{sub 41} and B{sub 71}{sup 1}, which are known to contain two hydrogen atoms in equivalent and inequivalent sites, respectively. It follows from the calculations that V{sub 6} should also be optically active and we identify it with the B{sub 80}{sup 4} (J line) center. This allows us to place the acceptor level of V{sub 6} at E{sub c}-0.04 eV. (c) 2000 The American Physical Society.

Asymmetric photodeconjugation of ammonium ene-carboxylates: temperature effects and evidence for the α-carbon of the dienolic species as a latent trigonal centre

The diastereoselective photodeconjugation of CH 2Cl 2 solutions of enantiopure ammonium ene-carboxylates was studied and the results compared with those of the enantioselective photodeconjugation of ene-esters in the presence of catalytic amounts of chiral aminoalcohols. We have observed a significant entropic effect and shown that a C-β control of the selectivity is operative in the asymmetric protonation of the photodienolic species.

Enantioselective Preparation of γ ‐Amino Acids and γ ‐Lactams from Nitro Olefins and Carboxylic Acids, with the Valine‐Derived 4‐Isopropyl‐5,5‐diphenyl‐1,3‐oxazolidin‐2‐one as an Auxiliary

Titanium enolates of acyl-oxazolidinones 1, derived from acetic, propanoic, 3-methylbutanoic, and 4-methylpentanoic acids and 4-isopropyl-5,5-diphenyl-1,3-oxazolidin-2-one, are added to aliphatic and aromatic nitro olefins in the presence of TiCl4 (Schemes 2 – 4). The products, 4-nitro carboxylic-acid derivatives 2, are formed in high diastereoselectivities (ds 80 to >99%) and in good yields (50 – 75% of purified samples of ds >98%). Hydrogenation over Raney-Ni of the NO2 group in the adducts leads directly to the corresponding γ-lactams (3 and 8; 80 – 92%), with recovery of the insoluble auxiliary (ca. 95%). Ring opening is achieved through the N-Boc-lactams (4), which are converted to N-Boc-protected γ-amino acids 5 or to their benzyl and methyl esters (6 and 7; Scheme 5). The configuration of the products (containing up to three new stereogenic centers; Scheme 1) is assigned by comparison with literature data, by X-ray crystal-structure analysis (for 2c, g, f, 8, Fig.), and by analogy. Thus, the (S)-auxiliary gives rise to combination of the trigonal centers of enolate and nitro olefin with Si/Si topicity (relative topicity all-lk; cf. A).

Pt and Li complexes in silicon. 99-07-22 16.52

Two different Pt- and Li-related centers in silicon have been studied by excitation spectroscopy combined with uniaxial stress and isotope doping experiments. The dominating spectrum consists of a series of sharp lines between about 5000 and 8000cm−1 and is due to excitation from a deep initial state to various shallow donor states. The energy positions of the electronic lines in Li6-doped samples shift by about 1cm−1 to lower energies compared to those in Li7 samples. Phonon replicas of the no-phonon lines are detected with ℏω(Li7)=528.6cm−1 and ℏω(Li6)=547.8cm−1. Other replicas of the no-phonon lines are also observed involving an energy of about Δ=2229cm−1 as well as combinations of Δ and hω. The uniaxial stress data indicate that the center has either tetrahedral or orthorhombic I symmetry. The second center gives rise to sharp lines at 1965.64, 1974.12, and 2001.00cm−1 at T<30K. The 2001cm−1 line has a full width at half maximum of 1.4 and 0.12cm−1 at room temperature and T=10K, respectively. Uniaxial stress experiments show that all three lines originate from excitations of a trigonal center. The isotope shift is small and for Li6 samples the 2001cm−1 line shifts only by 0.06cm−1 to higher energy compared to Li7 samples.

Piezospectroscopy study of the trigonal FeIn pair center in silicon

The electronic structure of an iron- and indium-related center in silicon has been studied by Fourier transmission infrared spectroscopy, employing uniaxial stress and polarization spectroscopy. The center gives rise to an excitation spectrum centered at about 6550 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, and is closely related to the orthorhombic FeIn pair center with an excitation spectrum at about 6100 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. The two spectra are always observed in our Fe and In codoped samples, and show metastability properties similar to those that have been reported previously for the trigonal and orthorhombic FeIn pair centers. In this paper, uniaxial stress results are reported which show unambiguously that the ${6550\ensuremath{-}\mathrm{c}\mathrm{m}}^{\mathrm{\ensuremath{-}}1}$ spectrum involves a center with trigonal symmetry. The polarization properties of the stress split lines show that the initial state of the transitions transforms as the ${\ensuremath{\Gamma}}_{4}$ irreducible representation of the ${C}_{3v}$ point group which is identical to the one previously deduced for the trigonal FeIn center by electron paramagnetic resonance. One of the excitation lines shows interesting polarization rules, which are discussed in detail and explained as a superposition of a \ensuremath{\sigma} and a \ensuremath{\pi} dipole at a ${C}_{3v}$ center. The origin of the spectrum is tentatively assigned to transitions from the deep ground state to shallowlike hole states. The symmetry assignment of the center and its ground state gives strong evidence that the center studied is indeed the trigonal version of the FeIn pair center.

MMFF VI. MMFF94s option for energy minimization studies.

This article describes the derivation of MMFF94s, which is the "s" (static) variant of MMFF94. MMFF94s incorporates altered out of plane bending parameters that yield planar (or nearly planar) energy-minimized geometries at unstrained delocalized trigonal nitrogen centers. Some experimental and most theoretical structures show appreciable puckering at nitrogen in isolated structures. However, condensed-phase effects or even strong intermolecular hydrogen bonding tends to reduce, but need not eliminate, such puckering; in contravention to results reported on the lower level Hartree-Fock surface, we show in the correlated LMP2/6-31G** calculations for the Watson-Crick guanine-cytosine base pair that one of the hydrogen-bonded NH2 groups remains appreciably puckered. The resultant MMFF94s geometries emulate the "time-averaged" structures typically observed in crystallographic and most other experimental structure determinations. MMFF94s also employs modified torsion parameters for interactions that involve such centers, but is identical to MMFF94 for other systems. Isolated instances are found in which MMFF94s fails to locate a (probably shallow) local minimum found on the MMFF94 and reference MP2/6-31G* surfaces. In general, however, MMFF94s describes conformation energies for delocalized trigonal nitrogen systems nearly as well as MMFF94 does. ©1999 John Wiley & Sons, Inc. J Comput Chem 20: 720-729, 1999.