Direct Coupling Constants Research Articles

Synthesis and structure of fluorinated tetraphenylantimony β-diketonates

The reactions of pentaphenylantimony with fluorinated β-diketones have been studied. The structure of the obtained compounds has been determined by the X-ray diffraction method and confirmed by IR, 1H, 13C and 19F NMR spectroscopy. According to X-ray diffraction analysis data, the reaction products are tetraphenylantimony 1,1,1-trifluoropentane-2,4-dionate (3а), 1,1,1-trifluoro-5,5-dimethylhexane-2,4-dionate (3b) and 6,6,7,7,8,8,8-heptafluoro-2,2-dimethyloctane-3,5-dionate (3c): Ph4Sb[OC(R)CHC(R')O] (R = CH3 (3a), Bu-t (3b, 3c); R' = CF3 (3a, 3b), C3F7 (3c)), the antimony atoms in the molecules have a distorted octahedral coordination. The CSbC angles equal 158.26(14)°, 157.90(9)°, 157.75(15)° (3 a‒c), the CSbO angles are 164.70(12)°, 168.05(12)° (3a); 165.47(9)°, 167.37(7)° (3b); 164.96(14)°, 169.66(13)° (3c). The Sb–C bond lengths vary in the ranges of 2.142(4)–2.164(4) Å, 2.144(2)–2.160(2) Å, 2.142(4)–2.160(4) Å (3 a‒c), the Sb–O distances are 2.358(3), 2.333(3) Å (3a); 2.306(2), 2.339(2) Å (3b); 2.376(3), 2.289(3) Å (3c). The 3a and 3b13C NMR spectra contain short spin-spin interactions between carbon and fluorine atoms with a large direct spin-spin coupling constant, 1JCF ≈ 286.0 Hz, which are characteristic for fluorinated compounds.

Transmission of the spin-spin coupling constants through hydrogen bonds in ammonia clusters

Spin-spin coupling constants are reported using six ab initio and fifteen DFT methods for dimers and larger clusters of ammonia. An analysis of components (Fermi contact, spin dipole, paramagnetic spin-orbit, and diamagnetic spin-orbit) of more relevant coupling constants 1JNH′, 1hJNH′ and 2hJNN has been carried out. Fermi contact is the dominant term in the total value for all constants. For dimers, a relationship between the addition of direct and intermolecular coupling constants gives the direct constants of monomer. From the comparison of all ab initio and DFT methods for dimers, SOPPA(CCSD) and S55VWN5 methods are, respectively, more reliable taking into account their accuracy and the computing time. Both methods are employed for the analysis of the transmission of coupling constants through the hydrogen bond for ammonia clusters. A linear relation between the intermolecular constants 1hJNH′ and the length of the hydrogen bond is found.

Improved Isotopic Profiling by Pure Shift Heteronuclear 2D J-Resolved NMR Spectroscopy.

Quantitative information on the carbon isotope content of metabolites is essential for flux analysis. Whereas this information is in principle present in proton NMR spectra through both direct and long-range heteronuclear coupling constants, spectral overlap and homonuclear coupling constants both hinder its extraction. We demonstrate here how pure shift 2D J-resolved NMR spectroscopy can simultaneously remove the homonuclear couplings and separate the chemical shift information from the heteronuclear coupling patterns. We demonstrate the power of this method on cell lysates from different bacterial cultures and investigate in detail the branched chain amino acid biosynthesis.

Direct 13C-31P coupling constant of coordinated triphenylphosphine as a characteristic of electron-withdrawing power of the metal center

Direct 13C-31P coupling constant of coordinated triphenylphosphine as a characteristic of electron-withdrawing power of the metal center

Nuclear spin–spin constants, rotational g factor and susceptibility of sulphur hexafluoride

Following our previous study on spin–rotation and shielding constants of the SF6 molecule, the rotational g factor and the magnetic susceptibility are calculated here, using ab initio methods to evaluate the electronic contribution to the nuclear hyperfine constants, and compared with experimental results. It is shown, for the first time, that the electronic component of the rotational g factor is proportional to a constant, which is given by a sum over electronic states. We also evaluate for the SF6 molecule the indirect, or electron-coupled spin–spin interaction, theoretically described by Ramsey, and show that it gives non-negligible corrections to direct coupling constants d 1 and d 2. The contributions of the terms included in this interaction (DSO, PSO, SD and FC) are also analysed.

Stereospecificity of 13C shielding constants in acetone azine as a tool for configurational assignment of ketone azines

According to the 13C NMR data, the chemical shift of the methyl carbon atom in acetone azine in the trans position with respect to the lone electron pair on the neighboring nitrogen atom is lower by 7 ppm than that of the methyl carbon atom in the cis position. The corresponding direct 13C-13C coupling constant for the trans-methyl groups is lower by 10 Hz as compared to the cis-methyl groups. The experimental spectral data are reproduced well by nonempirical quantum-chemical calculations. The observed stereospecificity of 13C chemical shifts and 13C-13C coupling constants may be used as an effective tool in configurational analysis of various ketone azines.

Is the 1 J PSe Coupling Constant a Reliable Probe for the Basicity of Phosphines? A 31P NMR Study

The influence of different heteroaryl and functionalized aryl substituents on the electron-donating ability and basicity of the phosphorus atoms in heteroaryl phosphines and diphosphines has been determined by the use of the direct 1JPSe coupling constants of the corresponding selenides. The generality of the use of 31P–77Se spin–spin coupling constants as probe for the basicity of phosphines is discussed as well as the scope and limits of this concept. GRAPHICAL ABSTRACT

Relativistic hybrid density functional calculations of indirect nuclear spin–spin coupling tensors — Comparison with experiment for diatomic alkali metal halides,

The accurate calculation of the isotropic (Jiso) and anisotropic (ΔJ) parts of indirect nuclear spin–spin coupling tensors is a stringent test for quantum chemistry, particularly for couplings involving heavy isotopes where relativistic effects and relativity – electron correlation cross terms are expected to play an important role. Experimental measurements on diatomic molecules in the gas phase offer ideal data for testing the success of computational approaches, since the data are essentially free from intermolecular effects, and precise coupling anisotropies may be reliably extracted in favourable cases. On the basis of available experimental molecular-beam coupling-tensor parameters for diatomic alkali metal halides, we tabulate known values of Jiso and, taking rotational–vibrational corrections to the direct dipolar coupling constant into account, precise values of ΔJ are determined for the ground rovibrational state. First-principles calculations of the coupling tensors were performed using a recently developed program based on hybrid density functional theory using the two-component relativistic zeroth-order regular approximation (ZORA). Experimental trends in Jiso and ΔJ are reproduced with correlation coefficients of 0.993 and 0.977, respectively. Periodic trends in the coupling constants and their dependence on the product of the atomic numbers of the coupled nuclei are discussed. Finally, the hybrid functional method is also successfully tested against experimental data for a series of polyatomic xenon fluorides and group-17 fluorides.

A13C and15N Solid-State NMR Study of Structural Disorder and Aurophilic Bonding in AuIand AuIIICyanide Complexes

Solid-state nuclear magnetic resonance has been used to study several cyanoaurates. Carbon-13 and nitrogen-15 NMR spectra of samples enriched with isotopically labeled 13C,15N cyanide ligands were recorded for stationary samples and samples spinning at the magic angle. Several salts of the dicyanoaurate(I) anion, M[Au(CN)2], where M = n-butylammonium, potassium, and thallium, were studied via solid-state NMR. A gold(III) cyanide, K[Au(CN)4], was also investigated. Carbon-13 and nitrogen-15 chemical shift tensors are reported for each salt, as are the measured 13C,15N direct dipolar coupling constants together with the related derived cyanide bond lengths, r(C,N). The value for r(C,N) in [(n-C4H9)4N][Au(CN)2], 1.17(5) A, was determined to be more realistic than a previously reported X-ray diffraction value of 1.03(4) A. Large 13C NMR line widths from Tl[Au(CN)2], 250-315 Hz, are attributed to coupling with 197Au (I = 3/2) and/or 203/205Tl (I = 1/2), as confirmed by measurements of the transverse relaxation constant, T2. Investigation of the carbon-13 chemical shifts for cyanide ligands bound to gold involved in a variety of metallophilic bonding environments demonstrates that the chemical shift is sensitive to metallophilic bonding. Differences in Au-Tl metallophilic bonding are shown to cause a difference in the isotropic carbon-13 chemical shift of up to 15.7 ppm, while differences in Au-Au aurophilic bonding are found to be responsible for a change of up to 5.9 ppm. The disordered polymeric material gold(I) monocyanide, AuCN, was also investigated using 13C and 15N SSNMR. Two-dimensional 13C,13C double-quantum dipolar-recoupling spectroscopy was used to probe connectivity in this material. The 13C NMR site multiplicity in AuCN is explained on the basis of sensitivity of the carbon-13 chemical shift to aurophilic bonding of the directly bonded gold atom. This assignment allows estimation of the position of the linear [-M-CN-]infinity chain's position with respect to the neighboring polymer chain. For the samples studied, a range of 7 +/- 2% to 25 +/- 5% of the AuCN chains are found to be "slipped" instead of aligned with the neighboring chains at the metal position.

Lightest scalar resonances and the dynamics of the γγ → ππ reactions

The high-statistics Belle data on the γγ → π+π− and γγ → π0π0 reactions have been jointly analyzed. The main dynamical mechanisms of these reactions for energies below 1.5 GeV have been revealed. It has been shown that the direct coupling constants of the σ(600) and f 0(980) resonances with a γγ pair are small and that the σ(600) → γγ and f 0(980) → γγ decays are four-quark transitions due primarily to π+π− and K + K −-loop mechanisms, respectively. The role of the chiral shielding of the σ(600) resonance is emphasized. The widths of the f 0(980) → γγ and σ(600) → γγ decays averaged over the resonance mass distributions, as well as the width of the f 2(1270) → γγ decay, are estimated as \( \langle \Gamma _{f_0 \to \gamma \gamma } \rangle _{\pi \pi } \approx 0.19 keV, \langle \Gamma _{\sigma \to \gamma \gamma } \rangle _{\pi \pi } \approx 0.45 keV \), and \( \Gamma _{f_2 \to \gamma \gamma } (m_{f_2 }^2 ) \approx 3.8 keV \).

Lightest scalar and tensor resonances inγγ→ππafter the Belle experiment

New high statistics Belle data on the {gamma}{gamma}{yields}{pi}{sup +}{pi}{sup -} reaction cross section measured in the range of pion-pair invariant masses {radical}(s) between 0.8 GeV and 1.5 GeV are analyzed to clarify the current situation around the {sigma}(600), f{sub 0}(980), and f{sub 2}(1270) resonances in {gamma}{gamma} collisions. The present analysis shows that the direct coupling constants of the {sigma}(600) and f{sub 0}(980) resonances to {gamma}{gamma} are small, and the {sigma}(600){yields}{gamma}{gamma} and f{sub 0}(980){yields}{gamma}{gamma} decays are four-quark transitions caused by the {pi}{sup +}{pi}{sup -} and K{sup +}K{sup -} loop mechanisms, respectively. The chiral shielding of the {sigma}(600) resonance takes place in the reactions {gamma}{gamma}{yields}{pi}{pi} as well as in {pi}{pi} scattering. Some results of a simultaneous description of the {gamma}{gamma}{yields}{pi}{sup +}{pi}{sup -} and {gamma}{gamma}{yields}{pi}{sup 0}{pi}{sup 0} Belle data are also presented. In particular, the following tentative estimate of the f{sub 2}(1270){yields}{gamma}{gamma} decay width is obtained: {gamma}{sub f{sub 2}}{sub {r_reversible}}{sub {gamma}}{sub {gamma}}(m{sub f{sub 2}}{sup 2}){approx_equal}3.68 keV.

Localising gravity in composite monopole brane worlds without bulk cosmological constant

Abstract We study a 7-dimensional brane world scenario with a Ricci-flat 3-brane residing in the core of a composite monopole defect, i.e., a defect composed of a 't Hooft–Polyakov and a global monopole. Admitting a direct interaction between the two bosonic sectors of the theory, we analyse the structure of the space–time in the limits of small, respectively large direct interaction coupling constant. For large direct interaction, the global monopole disappears from the system and leaves behind a negative cosmological constant in the bulk such that gravity-localising solutions are possible without a priori introduction of a bulk cosmological constant.

X-ray diffraction, Raman spectroscopic, and solid-state NMR studies of the group 14 metal-(tetracarbonyl)cobalt complexes Ph3MCo(CO)4(M = Si, Sn, Pb)

Single-crystal X-ray diffraction studies illustrate that the three title compounds are isomorphous, belonging to the triclinic space group P[Formula: see text], with slightly distorted trigonal bipyramidal geometry about cobalt. The solid-state29Si,119Sn, and207Pb cross-polarization magic angle spinning (CP MAS) NMR spectra are presented. The indirect spin–spin coupling constant (J), quadrupolar–dipolar shift (d), direct dipolar coupling constant (D' ), anisotropy in spin–spin coupling (ΔJ), and the chemical shift tensor were extracted. A plot of the reduced coupling constant vs. s-electron densities at the nucleus indicates that the Fermi contact term may be dominant for the tin and lead complexes; however, the large ΔJ for all complexes indicate that there are also significant anisotropic terms. Trends in the Raman scattering spectra are also discussed.Key words:29Si,119Sn, and207Pb CP MAS NMR, tetracarbonyl cobalt, spin–spin coupling, chemical shift tensor, quadrupole coupling, Fermi contact, cobalt–group 14.

Conformational studies by [formula omitted] in a nematic solvent: methyl isonicotinate. methyl nicotinate and methyl picolinate

The structures of methyl isonicotinate (MI), methyl nicotinate (MN) and methyl picolinate (MP), i.e., 4-, 3-, and 2-pyridinecarboxylic acid methyl esters, were studied by 1 H−NMR at 296 K using nematic liquid–crystal solvent ZLI 1167. Conformational analysis was performed according to the model of Emsley, Luckhurst and Stockley to take account of the correlation between rotation and internal rotation. Only the conformer with a planar skeleton was detected for MI but s- trans and s- cis conformers were found for MN and MP. The relative abundance of the s- trans form was determined to be 64(2) and 68(2)% for MN and MP, respectively, which are in agreement with those determined by gas-phase electron diffraction. The positions of the ring protons of MI in ZLI 1167 agree with those in the gas phase within experimental errors. In the case of MN and MP, however, observed direct coupling constants between the ring protons are in poor agreement with those calculated from the gas-phase structures due to the deformation of the pyridine rings in ZLI 1167.

13C and19F NMR study of α,β-difluorostyryl derivatives of transition metal carbonylates. A method of signal assignment based on the carbon—fluorine spin-spin coupling constants

The13C and19F NMR spectra ofZ- andE-isomers of β-X-substituted α,β-difluorostyrenes (X=F, Cl, CpFe(CO)2, Re(CO)5, Re2(CO)9Na) were studied. Direct and long-range (across 1–5 bonds) spin-spin coupling constants and the (13C−12C) isotope shifts in the19F NMR spectra were determined. The study of the13C satellites in the19F NMR spectra of substituted difluorostyrenes permitted assignment of the13C NMR signals of the vinylic carbon atoms. Similarly, the signals in19F NMR spectra were assigned based on coupling constants of fluorine withipso-carbon. These assignments were found to be in good agreement with the data available from the literature (X=F, Cl). The developed approach was applied to the elucidation of the structure ofZ−PhCF=CClFe(CO)2Cp.

Structural Study of Methyl Acrylate in a Nematic Liquid Crystal by 1 H NMR with 13 C Satellites

The molecular structure and conformation of methyl acrylate have been determined by analyzing the 1H NMR spectrum with 13C satellites of the molecules dissolved in the liquid crystal ZLI 1132. In the structural analysis, direct coupling constants have been corrected for molecular vibrations and converted to those in the rα structure. The correlation between reorientational motion and internal rotation has been taken into account by using the mean external potential described in terms of bond interaction tensors. The potential for the internal rotation about the \(C{\kern 1pt} ---{\kern 1pt} C\) bond has been represented as V(Ψ) = (V1/2) (1 − cos Ψ) + (V2/2)(1 − cos 2Ψ), where Ψ is zero for the s-cis configuration, and the determined values of V1 and V2 are 2.7(1) and 8.4(13) kJ mol−1, respectively. The relative abundance of the s-cis conformer, 76(1)%, agrees with a gas-phase value, 67(11)%, determined by electron diffraction within experimental errors.

On a general Heisenberg exchange effective Hamiltonian

A general Heisenberg exchange effective Hamiltonian is deduced in a straightforward way from the elemental quantum mechanical principles for the case of magnetic ions with non-orbital degeneracy in a crystalline lattice. Expressions for the high order direct exchange coupling constants or parameters are presented. The meaning of this effective Hamiltonian is important because extracting information from the Heisenberg Hamiltonian is a difficult task and is however taken as the starting point for many quite profound investigations of magnetism in solids and therefore could play an important role in an introductory course to solid state physics.

Crystal and molecular structures of 1,4-dibromo- and 1,4-diiodocubane

The crystal structure of 1,4-dibromocubane has been determined by X-ray diffraction. The crystal is monoclinic, P21/n, a = 6.878(1), b = 6.960(7), c = 8.793(2) Å, β = 113.08(2)°, Z = 2, R = 0.055. The C—Br bond length, at 1.926 Å, is shorter than usual and the carbon framework is probably distorted by longer bonds along the edges. The direct dipole coupling constants between protons, obtained from the partially oriented NMR spectrum, show that a distortion of the proton positions from the ideal cube occurs in the bromo compound but not in diiodocubane, presumably because of the higher strain energy in the former. Keywords: halocubanes, crystal structures, NMR spectroscopy, molecular modelling calculations.

The Floquet theory of nuclear magnetic resonance spectroscopy of single spins and dipolar coupled spin pairs in rotating solids

The Floquet theory for the description of magic angle sample spinning (MASS) nuclear magnetic resonance (NMR) experiments is introduced. MASS NMR signals of single spin systems and homonuclear two-spin system in the presence of direct and indirect dipolar couplings are evaluated. The single spin system is utilized to develop our Floquet formalism for the MASS NMR experiments and the coupled spin system is discussed to demonstrate its methodology and its applicability. This theoretical approach enables the calculation of the positions, the amplitudes, and line shapes of MASS center and sideband. The diagonalization of the MASS Floquet matrices of the different spin systems provides eigenvalues and eigenvectors which in turn determine the frequencies and intensities of the various bands in the MASS spectra. Their frequency shifts can be explained in terms of anticrossing of Floquet states and their amplitudes can be calculated via the matrix elements of transition–amplitude Floquet operator. New Floquet operators are introduced and the commutation relations between them utilized to enable the evaluation of diagonalization matrices. The Floquet theory is applied to determine the rotational resonance conditions for homonuclear dipolar-coupled spin pairs. It is shown that significant band shifts are expected when degeneracies between Floquet states occur. These shifts endow the centerband and sidebands with powder line shapes from polycrystalline samples. Simulations of experimental line shapes can provide direct and indirect dipolar coupling constants, including their relative signs. This approach is valid for all possible dipolar coupling strengths as well as for all sample spinning speeds and isotropic and anisotropic chemical shift parameters. The decay of zero-quantum coherences in the coupled spin-systems is also considered in terms of the Floquet approach. The sensitivity of the MASS line shapes close to the rotational resonance conditions with respect to the relative magnitudes of the chemical shift anisotropies of the spins and their direct and indirect coupling constants is examined by the calculation of various MASS spectra. To do so the interaction parameters of the different 13C nuclei in glycine and alanine are used. Experimental results of the weakly dipolar coupled spin pairs in uniformly 13C labeled alanine are also presented and compared with theoretical results.

Determination of the orientation of the phenyl substituent in isomeric 1-ethyl-2,5-dimethyl-4-phenyl-4-piperidinols by two-dimensional nuclear overhauser effect (NOESY) spectroscopy. Conformation of the ? isomer of 1,2,5-trimethyl-4-phenyl-4-piperidinol

In the case of three isomers (α, Β, and γ) of 1-ethyl-2,5-dimethyl-4-phenyl-4-piperidinol it is shown that NOESY spectroscopy can be successfully used in the determination of the orientation of the phenyl substituent. It was observed that the chemical shifts of the protons and the carbon atoms in the ortho position and of the quaternary C atom of the phenyl substituent and the \(^1 J_{(C_{\left( 5 \right)} H_{(5a} )}\), direct spin-spin coupling constant (SSCC) satisfactory reflect the change in the orientation of the phenyl substituent on passing from the a isomer to the β and γ isomers. A chair 2a5e) ⇄ twist-boat (2e, 5e) conformational equilibrium in which the twist-boat conformation is stabilized by an intramolecular hydrogen bond is proposed for the Β isomer of 1,2,5-trimethyl-4-phenyl-4-piperidinol on the basis of the NMR data.