Analysis Of Coupling Constants Research Articles

Defects in hydrogenated SrO

The paramagnetic ${V}_{\mathrm{OH}}$ and ${V}_{\mathrm{F}}$ defects generated by $\ensuremath{\gamma}$ irradiation at 77 K have been observed in hydrogenated SrO crystals. These defects are not stable at room temperature. EPR and electron-nuclear double-resonance measurements have been performed at low temperature in order to identify the impurity nucleus. In both cases, the defect is axially symmetric about a crystalline 100> axis. An analysis of the anisotropic hyperfine coupling constants of these centers in MgO, CaO, and SrO shows that there is an approximate linear dependence between the distance from the trapped hole (oxygen-fluorine for the ${V}_{\mathrm{F}}$ and oxygen-oxygen for the ${V}_{\mathrm{OH}}$ and the crystal lattice parameter. A complex structure in the vicinity of $g\ensuremath{\approx}2$, probably due to aggregate defects, is shown to be associated with hydrogen.

Multiple regression analysis of carbon-13 chemical shifts and carbon-13 proton coupling constants in ortho-substituted aromatics

Multiple regression analysis of carbon-13 chemical shifts and carbon-13 proton coupling constants in ortho-substituted aromatics

Conformation of hydroxy-L-proline in aqueous solution. Comparison of results from lanthanide ion probe measurements and proton coupling constants

The conformation of hydroxy-L-proline in D2O solution has been studied by analysing its 1H chemical shifts and spin-lattice relaxation times perturbed by the binding of lanthanide(III) ion. Spin couplings, including a large long-range coupling constant (2.22 Hz) between the protons bonded to the β- and δ-carbons, remain unaltered by the binding of LaIII. Thus, conformational changes upon complex formation are absent. The assignment of protons in this study which utilizes the lanthanide probe is in agreement with previous result from analysis of the coupling constants. The conformations estimated independently by these two approaches are similar, although from a more quantitative point of view the puckering angles ω1, (between the planes formed by Cα, Cβ, and N and by Cβ, Cδ, and N) and ω2(between the planes formed by Cβ, Cδ, and N and by Cβ, Cγ, and Cδ) differ by ca. 20 and 15°, respectively, between the two conformations.

Orbital Valence, Urey-Bradley and Approximate General Valence Force Fields and Vibrational Mean Amplitudes of Some Spherical Top M(CH3)4Type Molecular Model (M=C, Si, Ge, Sn and Pb)

Abstract The intramolecular force fields of tetramethylcarbon, tetramethylsilane, tetramethylgermanium, tetramethyltin and tetramethyllead have been examined using OVFF, UBFF and GVFf potential functions under the approximation of ‘point-mass’ for methyl group. Approximate force fields for the (2x2)F2 species of each molecule have been examined using L12=O, L21=O and P.E.D., methods. Transferability of OVFF into GVFF has also been observed. An analysis of the Coriolis coupling constants has been presented in the light of the force fields of the molecules. Vibrational mean amplitudes for bonded and non-bonded distances have been reported and analysed. The Bastiansen-Morino shrinkage effedt has been evaluated. The results have been discussed in the light of the relative behaviour of the molecules.

Conformational consequences of metallo-methyl interactions: A 13C nuclear magnetic resonance study

A number of model benzyl-organometallic systems have been synthesized and characterised, and their proton-decoupled natural-abundance 13C NMR spectra measured and assigned. A comparison of the 13C chemical shifts of carbons formally para to the carbonmetal (CM) bond strongly supports the importance of hyperconjugative interactions in the neutral ground state for metallomethyl substituents. Further, an analysis of the carbonmetal coupling constants has defined unambiguously the generality and origin (hyperconjugation) of a new concept, namely, large five-bond carbonmetal coupling in benzylic systems. The utilization of 5 J( 13C 117,119Sn) for structural and stereochemical assignments is demonstrated.

Intramolekulare Cycloadditionen in der Reihe der Binaphthyle

Intramolecular cyloadditions of binaphtyl compoundsThree new bridged ketones, 7,8 and 9, have been isolated in 44%, 3% and 19% yields respectively (Scheme 2) by heating 2,2′‐bis‐allyloxy‐1,1′‐binaphthyl (5) at 215° for 16 hours. These compounds could be epimerized about C(16) by bases, and in particular 9 yielded the new epimer 10. The structures of the alcohols obtained by reduction of the keto group are also given (Scheme 2). The constitution of all compounds was derived from spectroscopic data, chiefly from their 1H‐NMR, spectra (tab. 2, 3 and fig. 1). The assignments were based on the observed long‐range coupling constant between H(endo)‐C(16) and H(endo)‐C(5) in 7 and 10 and on the analysis of chemical shifts and coupling constants in both the ketones and their derivatives. Moreover, the structures of the compounds investigated have been proved by x‐ray analysis of ketone 8 (chap. 3, fig. 2). The thermal conversion of binaphthylether 5 to the bridged ketones proceeds via an intramolecular Diels‐Alder reaction, followed by Claisen rearrangement (Scheme 8). On heating, the bis‐beta‐methylallyl ether 20 yielded the ketone 21 and a small amount of the ether 23 (Schemes 5 and 7). Ether 23 and binaphthyl monoallyl ether 26 were converted thermally to the bridged ketones 31 (Scheme 7) and 27 (Scheme 6) respectively. In addition, 26 underwent an intramolecular ene‐reaction to give the spiroketone 28 (Schemes 6 and 9). The structures of these compounds were also established, mainly by analysis of their 1H‐NMR. spectra.

Electronic structure of arsabenzene: Microwave spectrum, dipole moment, and nuclear quadrupole coupling constants

The microwave spectrum of arsabenzene was analyzed; a dipole transitions were observed. The following rotational constants were obtained; A = 4871.03 ± 0.18 MHz, B = 2295.87 ± 0.01 MHz, C = 1560.10 ± 0.01 MHz. The dipole moment was 1.10 ± 0.04 D. The nuclear quadrupole coupling constants due to the 75As nucleus were χ aa = −186.4 ± 0.1 MHz, χ bb = 43.5 ± 0.2 MHz, χ cc = 142.9 ± 0.2 MHz, and the asymmetry parameter, η = 0.533 ± 0.002. Analysis of the quadrupole coupling constants indicated that the population of the 4 p orbitals on arsenic decrease in the order n a > n b > n c .

Analysis of the conformation of polypeptides : the combined use of energy computations and nuclear magnetic resonance studies

This review surveys current approaches to the problem of determining the solution conformation of polypeptides. The basic principles of energy computations are described. The utility, problems, and limitations of various theoretical methods are summarized: conformational energy mapping, energy minimization, scanning of selected local conformations, statistical predictive schemes. The need for combining the calculations with experimental studies is pointed out. The information content of various physico-chemical methods is compared for this purpose. The analysis of nmr coupling constants is discussed in more detail. In combination with energy computations, it can furnish specific information on local aspects of the conformation. Examples of such combined studies on small peptides are summarized.

Conformation of the common purine (beta) ribosides in solution: further evidence for a correlation between N-S state of the ribose moiety and syn-anti equilibrium.

The solution conformations of adenosine, guanosine and inosine in liquid ND3 have been determined by NMR. Comparison of the Karplus analysis of the proton HR spectra of the ribose moiety obtained in this solvent with the data from aqueous solutions of A and I proves that the conformations of the nucleosides are very similar in both liquids. From the analysis of the vicinal coupling constants of the ring protons it has been deduced that the S state C(2')-endo is slightly preferred. The mole fraction in S approximates 0.6 for all three nucleosides. C-13 relaxation measurements have been applied in the determination of the correlation times for rotational diffusion. Only at temperatures below - 40 degrees C is the pseudo-rotation of the furanoside ring slowed down sufficiently for it not to contribute to the measured relaxation rates. From NOE studies and T1 measurements on the individual protons it is derived that the N, C(3')-endo, form of the ribose is correlated with an anti conformation of the base (Y approximately 210 degrees to 220 degrees) and the S, C(2')-endo, form of the ribose with a syn conformation of the base (Y approximately 30 degrees to 50 degrees). The glycosyl torsion angles derived for the two conformations of A, G, and I are equal within the limits of accuracy.

Determination of the absolute stereospecificity of the enzymatic reduction of NAD + analogs by means of the primary acid cyclization reaction

NADH, (3-acetylpyridine)ADH, (3-thionicotinamide)ADH and (pyridine-3-aldehyde)ADH were labeled with deuterium by the B specific enzyme lipoyl dehydrogenase. The primary acid rearrangement products, α-O 2′-6B cyclotetra-hydropyridine adenine dinucleotides, of the respective reduced coenzyme analogs were synthesized by incubation at pH 2–2.5. Analysis of the spin-spin coupling constants of the 220 MHz proton magnetic resonance spectra establishes that the structure of the ribose-tetrahydropyridine linkage of these analogs is the same as that of the primary acid product of NADH. All four specifically labeled dinucleotides possess the identical absolute configuration at the C4 position with respect to the deuterium label, establishing that the three coenzyme analogs are reduced by lipoyl dehydrogenase with the same absolute stereospecificity as NAD +. The primary acid reaction is described as a general method for the determination of the absolute configuration at the C4 position of specifically labeled β-1,3-substituted, 1,4 dihydropyridine nucleotides.

Bond orbital analysis of the deuteron quadrupole coupling constants in MD4 molecules

The deuteron quadrupole coupling constants were used as constraints on the bond orbital functions of a series of MD4 molecules (M=B, C, N, Al, Si, P). It was demonstrated that a minimal basis set may be sufficient to describe first row deuterates, but not for second row deuterates, where double zeta orbitals are needed.

A 13C and 1H NMR investigation of the bonding in norbornenyl complexes of palladium(II) and platinum(II)

The modes of bonding in several norbornenyl complexes of palladinm(II) and platinum(II) has been investigated by both 1H and 13C NMR spectroscopy. Analysis of internal 1H-1H, 13C-1H, and 195Pt-13C coupling constants indicate considerable distortion of the norbornenyl ligand in these complexes relative to organic norbornenes. The nature of this distortion is compatible with a π-homoallylic bonding scheme.

ENDOR of VF and VOH centers in CaO

ENDOR (electron-nuclear double resonance) studies on CaO single crystals positively identify the VF center and resolve a discrepancy in the reported g values of the VOH center. Data for the same two centers in MgO were also obtained and compared with previous work. An analysis of the anisotropic hyperfine coupling constants shows that the relative lattice relaxation for both centers is roughly the same in these hosts. As expected, no motional averaging of the anisotropic resonance spectra was observed for either center.

Conformational analysis—XV : Force field calculations and NMR determination of conformational equilibria of organosilicon compounds

Force field calculations of the conformational energies of fifteen silanes are described. The calculated structures of silaethane, 2-silapropane and 2-methyl-2-silapropane are in acceptable agreement with available microwave data. The calculated torsional barriers of silaethane 2-silapropane; 2-methyl- 2-silapropane and 1-silapropane are consistent with reported barriers. In 2-silabutane and related compounds, the gauche conformation is more stable than the anti conformation as a result of attractive van der Waals energy terms. The strain energies of the two eclipsed conformations of 2-silabutane are identical and substantially lower than the strain energies of the two eclipsed conformations of butane which are of unequal energy. The shape of the torsional curve for 2-silabutane differs dramatically from that of butane. 1-Silabutane is stable in the anti conformation and the gauche-anti energy difference is similar to butane. The two eclipsed conformations of 1-silabutane stand in the same order as those for butane but are of higher energy. A comparison of the torsional curve for 1-silabutane with butane illustrates the steeper barriers for the former compound. Conformational equilibrium constants for 2-silabutane, 1-silabutane, and several compounds containing the 2-silabutane structure are obtained by NMR analysis of vicinal coupling constants and are in agreement with the calculated force field values. The conformational preferences of SiH 3, SiH 2, SiH(CH 3) 2, and Si(CH 3) 3 on cyclohexane are calculated. Unique features of silacyclohexane and the conformational preferences of hydrogen, methyl, and t-butyl on this ring are discussed.

π-complexes of 1 H-1,2-diazepines. Synthesis and characterization of tricarbonyliron and -ruthenium derivatives

The synthesis of a number of tricarbonyliron and -ruthenium derivatives of 1 H-1,2-diazepines are described. The complexes have been characterised by infrared, mass, Mössbauer and nuclear magnetic resonance spectra. The 7-membered heretocycles are coordinated to the metal atoms via two olefinic double bonds in typical diene fashion. A complete analysis of NMR chemical shifts and coupling constants in (1-acetyl-1 H-1,2-diazepine)Fe(CO) 3 has been carried out with the aid of a computer simulated spectrum.

Interplay of direct and indirect coupling contributions in proton hyperfine splitting constants of alkyl radicals

An ab initio calculation on the CH 3-CH 2 radical suggests that the spn-polarization and spin-delocalization contributions are both equally important and should be simultaneously considered in the theoretical analysis of proton hyperfine coupling constants of alkyl radicals.

Electron spin resonance evidence for the cations Cd2 3+, AgCd2+, and AgHg2+

Analysis of the hyperfine coupling constants for these radicals show that the unpaired electrons are in σ-bonds comprising largely the outer s-orbitals of the atoms: these atoms have equal spin-densities in Cd23+ and AgCd2+, but mercury is favoured in AgHg2+.

Hyperfine Coupling Constants and Covalency in Sandwich Complexes

An analysis of the isotropic proton hyperfine coupling constants of V(C5H5)2, Ni(C5H5)2, and Cr(C6H6)2+ is given. The unpaired electrons in the e1g* molecular orbitals in Ni(C5H5)2 are found to be rather strongly delocalized into the carbon 2pz–π framework of the ligands. In agreement with bonding theories this indicates that the interaction between the metal 3dxz and 3dyz orbitals and the ligand e1g–π molecular orbitals is very important for the chemical bonding in sandwich complexes. In Cr(C6H6)2+ and V(C5H5)2 the proton coupling constants are determined mainly by σ delocalization of the unpaired electron in the a1g(3dz2) orbital into the hydrogen 1s orbitals. In V(C5H5)2, also, delocalization into the ligand π framework is present and this is thought to be due to the e2g orbitals. It is concluded that the interaction of the metal 3dxy and 3dx2−y2 orbitals with the ligand e2g–π molecular orbitals is appreciable.

Electron spin resonance measurements of dibenzenechromium cation

ESR spectra of liquid and solid solutions of Cr(C 6H 6) 2I have been measured. Even in solid solution the spectra show a well resolved proton hyperfine structure which is explained by rotation of the Cr(C 6H 6) + 2 cation around its main axis. An analysis of the 53Cr hyperfine coupling constants indicates that the unpaired electron is in the A 1g (3d z2 ) orbital. The experimental results agree fairly well with the results of an extended Hückel MO calculation.

One-to-Two Millimeter Wave Spectroscopy. II.H2S

Pure rotational transitions of ${\mathrm{H}}_{2}$${\mathrm{S}}^{32}$, ${\mathrm{H}}_{2}$${\mathrm{S}}^{33}$, and ${\mathrm{H}}_{2}$${\mathrm{S}}^{34}$ have been measured with high precision in the region of one to two millimeters. From these measurements the SH bond length is found to be 1.3226A and the bond angle to be 92\ifmmode^\circ\else\textdegree\fi{} 6\ensuremath{'}. The ${\mathrm{S}}^{33}$ nuclear quadrupole coupling observed along the principal inertial axes are $\mathrm{eQ}{q}_{\mathrm{aa}}=\ensuremath{-}32$ Mc/sec, $\mathrm{eQ}{q}_{\mathrm{bb}}=\ensuremath{-}8$ Mc/sec, and $\mathrm{eQ}{q}_{\mathrm{cc}}=+40$ Mc/sec. Approximate analysis of the quadrupole coupling constants shows that the bonding orbitals of S are $\mathrm{spd}$ hybrids and yields a value of -0.06\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}24}$ ${\mathrm{cm}}^{2}$ for the quadrupole moment of the ${\mathrm{S}}^{33}$ nucleus. From the known ratio $\frac{{Q}^{33}}{{Q}^{35}}$ this gives ${Q}^{35}=0.04\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}24}$ ${\mathrm{cm}}^{2}$. Zeeman studies show that the molecular ${g}_{J}$ factor is approximately, but not exactly, equal for different rotational states. A mean value of 0.24 nuclear magneton is found for ${g}_{J}$.