Intramolecular surfaces for vicinal proton–proton coupling constants 3JHH

Equations for the intramolecular surfaces of the ³J_HH coupling constants in ethane, ethylene, and acetylene are formulated, and the corresponding coefficients are estimated from calculations at the DFT/B3LYP level. The chosen variables are changes in bond lengths, in the torsion angle φ between the coupled protons H_a and H_b, in bond angles, and in dihedral angles. The ³J_HH surface of ethane is formulated as an extended Karplus equation with the coefficients of a truncated Fourier series on the torsion angle φ expanded as second-order Taylor series in the chosen variables taking into account the invariance of ³J_HH under reflections and rotations of nuclear coordinates. Partial vibrational contributions from linear and square terms corresponding to changes in the geometry of the H_a ? C_a ? C_b ? H_b fragment are important while those from cross terms are small with a few exceptions. The ³J_HH surface of ethane is useful to predict contributions to ³J_HH from changes in local geometry of derivatives but vibrational contributions are predicted less satisfactorily. The predicted values at the B3LYP/BS2 level of the ³J_HH couplings (vibrational contributions at 300 K) from equilibrium geometries are 9.79 (?0.17) for acetylene, and 17.08 (1.93) and 10.73(0.93) for the trans and cis couplings of ethylene.     

P.E.HSQC: a simple experiment for simultaneous and sign-sensitive measurement of (1JCH+DCH) and (2JHH+DHH) couplings

The angular information content of residual dipolar couplings between nuclei of fixed distance makes the accurate and sign-sensitive measurement of (¹J_CH + D_CH) and (²J_HH + D_HH) couplings highly desirable. Experiments published so far are typically highly specialized for the effective measurement of a subset of couplings. The P.E.HSQC presented here, is an E.COSY based experiment which allows the simultaneous measurement of all heteronuclear and homonuclear couplings within CH, CH₂, and CH₃ groups in a single spectrum with the necessary precision and sign information. The simplicity of the approach and the absence of artefacts like phase distortions due to antiphase evolution make it ideally suited for coupling determination of organic molecules at natural abundance.     

HfV2·Hx中扩散导致的1H核自旋弛豫 近邻无规行步处理

对氢合金HfV₂·H_x中¹H核自旋弛豫起主导的因素是偶极相互作用,研究的核心是如何理论计算¹H在基质晶格中扩散造成偶极相互作用涨落的谱密度。我们把Torrey同核偶极作用近邻无规扩散行步理论推广到同核、异核偶极作用并存的情况;并就中子衍射提供的HfV₂·H_x结构(C-15)参数计算了确定谱密度J_q^HH(ω)和J_q^HV(ω)(q=0,1,2)所需的G_HH(k,y)和G_HV(k,y)。引用以前别人的核磁共振实验数据,依照我们的理论分析计算扩散激活能和H在间隙位上的振动频率;与他们用唯象理论获得的相应数值对比,结果是满意的。 关键词：     

Prediction of vicinal proton–proton coupling constants 3 J HH from density functional theory calculations

Vicinal coupling constants ³ J _HH have been calculated at the optimized geometries for a series of selected molecules with the aim of developing a practical procedure for predicting this kind of coupling. Calculations of couplings and optimizations of molecular geometries have been carried out at the DFT/B3LYP level using a moderate sized basis set. When the Fermi contact contributions to ³ J _HH calculated for 25 mono- and 23 1,1-di-substituted ethanes are multiplied by a factor of 0.904, the corresponding predicted couplings J ^pre are in good agreement with the experimental J ^exp couplings, with standard deviation σ of 0.10?Hz. When such a comparison is carried out for the remaining sets of molecules the σ deviation increases to 0.26?Hz for a dataset of 21 couplings from 11 monosubstituted cyclohexanes, to 0.19?Hz for a dataset of 40 couplings from 6 norbornane type molecules and to 0.25?Hz for a dataset of 54 couplings from 14 three-membered rings. For the complete dataset of 163 couplings the?σ?deviation amounts to 0.20?Hz. This figure is further reduced to 0.17?Hz by adding to the J ^pre coupling a small correction given by the term ?0.15cos?, depending on the dihedral angle ? between the coupled protons. A larger σ deviation of 0.31?Hz was reported for the best empirically parameterized extended Karplus equation. DFT J ^pre values could be further improved by more accurate calculations for the pertinent substituted ethane constituents of the molecule in question by applying a substituent effect model.     

Sensitivity- and Gradient-Enhanced Heteronuclear Coupled/Decoupled HSQC–TOCSY Experiments for Measuring Long-Range Heteronuclear Coupling Constants

A pulsed field gradient version of the sensitivity-enhanced 2D HSQC–TOCSY experiment is proposed for measurement of long-range heteronuclear coupling constants. The coupling constants are obtained by computer-aided analysis of mixed-phase multiplets with and without the heteronuclear splitting. Generation of pure phase data is not required. Since large¹J_XHandJ_HHcouplings are used for coherence transfer, smallⁿJ_XHcan be measured accurately, which could be difficult to obtain from purely heteronuclear polarization transfer experiments.     

Measurement of Magnitude and Sign of Heteronuclear Coupling Constants in Transition Metal Complexes

Sets of specifically tailored E.COSY-type correlation experiments and double-quantum/zero-quantum (DQ/ZQ) experiments are presented which enable the determination of sign and size of small heteronuclear coupling constants across the metal center of transition metal complexes. For the octahedrally coordinated complexes, [Ru(TPM)(H)(CO)(PPh₃)]⁺[BF₄]⁻(1) and [Ir(TPM)(H)(CO)(CO₂CH₃)]⁺[BF₄]⁻(2), 14 of 15 and 15 of 15 possible two-bond scalar coupling constants across the metal center were measured, respectively, using¹⁵N and¹⁵N/¹³C enriched samples (TPM = tris(1-pyrazolyl)methane)). The reduced coupling constants²K_X-M-Y= 4π²²J/(hγ_Xγ_Y) were found to be positive when the coupled nuclei X and Y weretranswith respect to the metal center, and negative when the coupled nuclei were incisposition. The validity of this sign rule was verified forJ_CC,J_NN,J_PN,J_PC,J_CN,J_HP,J_HC, andJ_HNcouplings. Idiosyncracies associated with 2D NMR spectra for the sign determination of coupling constants with¹⁵N which lead to corrections for the signs ofJ_HN,J_PN, andJ_CNcouplings reported previously are discussed.     

Measurement of Homonuclear J-Couplings from Spin-State Selective Double-/Zero-Quantum Two-Dimensional NMR Spectra

¹H-detected two-dimensional double-/zero-quantum experiments are described for measurement of homonuclear ²J_HH-couplings of NH₂ or CH₂ groups in proteins. These experiments utilize multiple-quantum coherence for determination of the size and the absolute sign of the geminal scalar and dipolar couplings in the presence of broad lines. Spectra are simplified by gradient selection and spin-state selective filters.     

A theoretical and experimental 1H NMR spectroscopy study of the stereoelectronic interactions that rule the conformational energies of alanine and valine methyl ester

Amino acids exhibit a bipolar zwitterionic structure (⁺H₃N‐CHR‐COO^?) in solution; hence, conformational studies of these compounds have been limited to the gas phase. The conformational preferences of amino acids have been widely attributed to intramolecular hydrogen bonding, despite steric and hyperconjugative effects. In this work, we propose the conformational study of alanine and valine methyl esters, which do not show zwitterionic structures in solution, by¹H NMR and theoretical calculations. The ³J_HH spin–spin coupling constants and theoretical calculations were found to be in agreement, showing that the interplay between steric hindrance and hyperconjugation is the forces that are responsible for determining the conformational preferences of alanine and valine methyl esters. Copyright © 2013 John Wiley & Sons, Ltd.     

Note on the theory of dissociation pressures of gas hydrates

Couplings over four bonds, ⁴ J _HH, are discussed for compounds where stereochemistry is thought to be the dominant factor determining their values. A simple MO theory [3, 4] provides calculated coupling constants in agreement with experiment. The form of the theory permits an analysis of the mechanisms responsible for long-range coupling. The approach is generalized to include distorted cyclohexane fragments and these results are presented in a graphical form.     

Effect of protonation on individual hydrogen bonds in the 8-oxoguanine-cytosine base pair: NMR,NBO and AIM analyses

Protonation increases the total binding energy of the 8-oxoguanine-cytosine (8OG:C) base pair by 60–70% at the B3LYP/6-311++G(d,?p) level of theory. It changes the individual H-bond energies, estimated from electron charge densities at bond critical points, by 1.16 to ?16.41?kcal?mol^?1. The individual H-bond energies and the two bond X–Y spin–spin coupling constants (^2hJ_X–Y) increase with protonation where 8OG behaves as an H-bond donor; the reverse is true for the H-bonds in which the 8OG unit acts as an H-bond acceptor. Similar to ^2hJ_X–Y, the value of ^1hJ_O–H (a one-bond H?···?Y spin–spin coupling constant) is distance dependent and in linear correlation with the O?···?H distance, but the ^1hJ_N–H values are independent of the N–H distance and the PSO term is the predominant portion in it. The ¹J_X–H spin–spin coupling constant is dominated by the negative FC term for all hydrogen bonds, although the PSO term is the best to investigate the behaviour of ¹J_X–H across the X–H?·?Y H-bond.     

Calculated spin-spin coupling surfaces in the water molecule; prediction and analysis of J (O,H), J (O,D) and J (H,D) in water isotopomers

Ab initio symmetry and internal valence coordinate oxygen–proton and proton–proton spin–spin coupling surfaces for the water molecule have been computed for the first time. Calculations have been performed at the SOPPA (CCSD) level using a large basis set and a grid of forty-nine geometries on the two surfaces. Equilibrium values differ significantly from some other calculated values especially for the Fermi contact terms. The bond length dependence of J(O, H) is ‘normal’ i.e. J(O, H₁) is much more sensitive to stretching the O–H₁ bond than the O–H₂ bond. This contrasts greatly with the corresponding situation in methane.

The surfaces have been averaged over the nuclear motion using a recent highly accurate force field to give values of J (O, H) and J (O, D) for H₂ ¹⁷O, HD¹⁷O and D₂ ¹⁷O and J(H, D) for HD¹⁶O, HD¹⁷O and HD¹⁸O over a range of temperatures. For J (O, H) and J (O, D) bond stretching at first order is the dominant part of the nuclear motion correction with second order bending making an important contribution. For J (H, D) the second order bending is by far the largest contribution to the nuclear motion corrections although the other terms partially cancel this contribution. Non-additivity can be largely attributed to the bending term for J (O, H). As expected, the bending terms also contribute relatively more to the temperature dependence of the couplings for J (O, H), J (O, D) and J (H, D). Our calculated J (O, H) in H₂ ¹⁷O of -77.22Hz at 293K is in very good agreement with Wasylishen and Friedrich's observed value of -78.70 (±0.02) Hz in cyclohexane at this temperature. Our calculated J(H, D) in HD ¹⁶O at 323K of -1.233Hz is close to a recent experimental value of -1.114 (±0.003) Hz in nitromethane-d ₃ observed by Sergeyev et al. at that temperature.     

Long Range Proton Couplings in Estrones

^4,6J_HH-long range couplings in an estrone-type molecule are observed and demonstrated to be helpful for the complete proton chemical shift assignment.     

The 2D {P} Spin-Echo-Difference Constant-Time [C, H]-HMQC Experiment for Simultaneous Determination of JH3′P and JC4′P in C-Labeled Nucleic Acids and Their Protein Complexes

A two-dimensional {³¹P} spin-echo-difference constant-time [¹³C, ¹H]-HMQC experiment (2D {³¹P}-sedct-[¹³C, ¹H]-HMQC) is introduced for measurements of ³J_C4′P and ³J_H3′P scalar couplings in large ¹³C-labeled nucleic acids and in DNA–protein complexes. This experiment makes use of the fact that ¹H–¹³C multiple-quantum coherences in macromolecules relax more slowly than the corresponding ¹³C single-quantum coherences. ³J_C4′P and ³J_H3′P are related via Karplus-type functions with the phosphodiester torsion angles β and ε, respectively, and their experimental assessment therefore contributes to further improved quality of NMR solution structures. Data are presented for a uniformly ¹³C, ¹⁵N-labeled 14-base-pair DNA duplex, both free in solution and in a 17-kDa protein–DNA complex.     

Natural J coupling (NJC) analysis of the electron lone pair effect on NMR couplings: 2. The anomeric effects on 1 J (C,H) couplings and its dependence on solvent

The electronic origin of the influence of the anomeric effect (negative hyperconjugative interaction, NHI) on the Fermi contact (FC) term of ¹ J(C, H) couplings has been studied from a theoretical point of view at the DFT-B3LYP level. The HN=CH₂, molecule was chosen as the primary model compound, in which both FC ¹ J(C, H) couplings were decomposed into bond contributions with the natural J coupling dissection approach (NJC). Differences between the ¹ J (C, H)^FC couplings for C——H bonds in synperiplanar and antiperiplanar orientations with respect to the nitrogen non-bonding electron pair closely follow the experimental trend. They are made up chiefly of three NJC contributions: ‘bond’, ‘direct lone pair’ and the ‘carbon-core orbitals’. The NHI influence on these terms was studied by applying the natural bond orbital (NBO) deletion procedure to the charge transfer interaction into the antiperiplanar (C——H) antibond (n(N)→(C——H)?) prior to the NJC dissection calculation. The dielectric solvation effect on both the total FC terms and the respective NJC contributions was estimated by carrying out the calculations using the polarization continuum model. Inhibition of the anomeric effect is evident when the solvent polarity is increased. NHI saturates rapidly with increasing solvent dielectric. Specific solute-solvent interaction effects on ¹ J(C, H) couplings were estimated by evaluating molecular complex models of the form CH₂=HN…S (S = H₂O and DMSO).     

Bi-2223银夹板厚膜的载流特性与应变的关系

测量了Bi_1.8Pb_0.4Sr₂Ca₂Cu₃O_y银夹板厚膜的应力与应变、应变和临界电流密度关系,及其在不同应变下的J_c(H)特性.结果表明,在应力-应变曲线上存在两个应变转变,分别对应于Bi_1.8Pb_0.4Sr₂Ca₂Cu₃O_{y关键词：}     

Triple resonance three-dimensional protein NMR: before it became a black box

Three-dimensional triple resonance experiments have become an integral part of virtually every solution NMR study of proteins. The approach relies on uniform isotopic enrichment of proteins with ¹³C and ¹⁵N, and establishes the scalar connectivity pathway between nuclei through the large ¹J_NH, ¹J_CH^{, 1}J_CC, and ¹J_CN couplings. The magnetization transfer process takes place through multiple, efficient one-bond magnetization transfer steps, rather than a single step through the smaller and variable ³J_HH couplings. The relatively large size and good uniformity of the one-bond couplings allowed the design of efficient magnetization transfer schemes that are effectively uniform across a given protein, nearly independent of conformation. Although conceptually straightforward, practical implementation of three-dimensional triple resonance experiments on proteins originally posed serious challenges. This account provides a personal perspective on some of the historical background to this work, the problems encountered as well as their solutions, and their evolution into today’s standard arsenal of experiments.     

An Example of Crossed Ship Conformation: The Tetrahydropyran Ring in Trioxa-bis-Spirocital Series

Abstract

The three ³J_HH coupling constants observed in the ¹H NMR spectra (500 MHz) of trioxa-bis-spirocital 1 and 2 serve to calculate the dihedral angles Ψ(8,9) and Ψ(9,10) of these two molecules. The values obtained, 54° for 1, 25° for 2, reflect the change in conformation: chair → crossed ship when passing from 1 to 2. This change in conformation corresponds in ¹³C NMR to strong shielding (?3.6 to ?5.5 ppm) of the lines of the carbons C₈, C₉ and C_10.     

Observation of Through-Hydrogen-Bond JHC′ in a Perdeuterated Protein

It is demonstrated that J connectivity between amide protons and hydrogen-bond-accepting carbonyl carbons can be observed in perdeuterated human ubiquitin. A selective pulse scheme is used to detect these small ^2hJ_HC′ interactions in the presence of the much larger through-covalent-bond ²J_HC′ and ³J_HC′ couplings. The ratio of the observed through-H-bond correlation intensity and the ²J_HC′ connectivity observed in a reference spectrum indicates ^2hJ_HC′ values of ca. 0.4–0.6 Hz, which are only slightly smaller than the corresponding ^3hJ_NC′ values. However, for technical reasons, ^2hJ_HC′ couplings are more difficult to measure than ^3hJ_NC′.     

Accurate and Precise Measurement of Heteronuclear Long-Range Couplings by a Gradient-Enhanced Two-Dimensional Multiple-Bond Correlation Experiment

We propose a phase-sensitive gradient-enhanced two-dimensional heteronuclear multiple-bond correlation (psge-2D HMBC) experiment for speedy, accurate, and precise measurement of²J_CHand³J_CH. The experiment does not suppress one-bond correlations. Rather, the value of a desired long-rangeJ_CHis obtained from the pertinent cross-peak pattern in the HMBC spectrum, using the corresponding¹J_CHcorrelation pattern as reference. The application of the proposed experiment is illustrated for the trisaccharide raffinose.     

General Features of the X Part of an ABX Spin System in Isotropic and Liquid Crystalline Phases as Illustrated by theC–{H} Spectra of 2,2′-Difluorobiphenyl

The¹³C–{¹H} NMR spectra of 2,2′-difluorobiphenyl dissolved in isotropic and liquid crystalline solvents have been obtained and analyzed. They are examples of the X part of an ABX spectrum. It is shown that the spectrum of the isotropic solution yieldsJ_AX,J_BX,J_AB, δ_AB, and δ_X, but only if all the transitions are detected, and that intensities as well as frequencies of the transitions are used in the analysis. It is demonstrated that for 2,2′-difluorobiphenyl this requires that for some of the carbons it is necessary to detect very weak transitions. For the spectra of liquid crystalline solutions of ABX systems it is shown that the dipolar couplingsD_AX,D_BX, andD_ABare obtained only if these couplings are in a certain sensitive range of relative values. The sensitive range can be adjusted by using variable angle sample spinning (VASS). It is demonstrated that VASS spectra taken near the magic angle can be used to obtain the absolute signs of the scalar couplings.