Simultaneous measurement of J(HH) and two different nJ(CH) coupling constants from a single multiply edited 2D cross‐peak

Three different J‐editing methods (IPAP, E.COSY and J‐resolved) are implemented in a single NMR experiment to provide spin‐state‐edited 2D cross‐peaks from which a simultaneous measurement of different homonuclear and heteronuclear coupling constants can be performed. A new J‐selHSQMBC‐IPAP experiment is proposed for the independent measurement of two different ⁿJ(CH) coupling constants along the F2 and F1 dimensions of the same 2D cross‐peak. In addition, the E.COSY pattern provides additional information about the magnitude and relative sign between J(HH) and ⁿJ(CH) coupling constants. Copyright © 2013 John Wiley & Sons, Ltd.     

The absolute sign of J coupling constants determined using the order matrix calculation

A novel methodology using the order matrix calculation to determine the absolute sign of spin-spin couplings based on the structure of organic compounds is presented. The sign of the residual dipolar coupling (RDC) depends on the sign of corresponding scalar spin-spin coupling constant and the sign of the RDC has a dramatic influence on the order matrix calculation. Therefore, the sign of the spin-spin coupling constant can be obtained by an order matrix calculation through the corresponding RDC. Six types of spin-spin coupling constants, including 2J(H,H), 1J(C,F), 2J(C,F), 3J(C,F), 2J(F,H) and 3J(F,H), were obtained simultaneously. Except for 3J(C,F) where the measured RDCs have very small magnitudes, the signs were determined unambiguously.     

Indirect nuclear spin-spin coupling constants in 1,2-diboretane-3-ylidene, a homoaromatic system with pi and sigma 3c/2e bonds. Comparison of experimental data with calculations using density functional theory (DFT)

The non-classical 1,2-diboretane-3-ylidene 1a was studied by 13C and 29Si NMR spectroscopy in order to obtain coupling constants 1J(13C,11B) and 1J(29Si,13C). The magnitudes of 1J(13C,11B) were deduced from linewidth measurements in low-temperature 13C and 11B NMR spectra. Calculation of the coupling constants for model compounds related to 1a, using DFT methods based on optimized geometries [B3LYP/6-311+G(d,p)], gave data in agreement with the experiments. Furthermore, the calculations predict for the first time a negative sign of 1J(13C,11B) which mirrors the bonding situation in 1 as described by theory.     

Comparative analysis of hydrogen bonding with participation of the nitrogen, oxygen and sulfur atoms in the 2(2'-heteroaryl)pyrroles and their trifluoroacetyl derivatives based on the 1H, 13C, 15N spectroscopy and DFT calculations

The N-H...X (X = N,O,S) intramolecular hydrogen bond in the series of 2(2'-heteroaryl)pyrroles and their trifluoroacetyl derivatives is examined by the (1)H, (13)C, (15)N spectroscopy and density functional theory (DFT) calculations. The influence of the hydrogen bond on coupling and shielding constants is considered. It is shown that the N-H...N intramolecular hydrogen bond causes a larger increase in the absolute size of the (1)J(N,H) coupling constant and a larger deshielding of the bridge proton than the N-H...O hydrogen bond. The effect of the N-H...S interaction on the (1)J(N,H) coupling constant and the shielding of the bridge proton is small. The NMR parameter changes in the series of the 2(2'-heteroaryl)pyrroles due to N-H...X hydrogen bond and the series of the 1-vinyl-2-(2'-heteroaryl)-pyrroles due to C-H...X hydrogen bond have the same order. The proximity of the nitrogen, oxygen or sulfur lone pair to the F...H hydrogen bridge quenches the trans-hydrogen bond spin-spin couplings (1h)J(F,H-1) and (2h)J(F,N).     

Ab initio study of complexes with two cations as N-H donors to F-: structures and spin-spin coupling constants across N-H-F hydrogen bonds

A systematic ab initio study has been carried out to determine the MP2/6-31+G(d,p) structures and EOM-CCSD coupling constants across N-H-F-H-N hydrogen bonds for a series of complexes F(H(3)NH)(2)(+), F(HNNH(2))(2)(+), F(H(2)CNH(2))(2)(+), F(HCNH)(2)(+), and F(FCNH)(2)(+). These complexes have hydrogen bonds with two equivalent N-H donors to F(-). As the basicity of the nitrogen donor decreases, the N-H distance increases and the N-H-F-H-N arrangement changes from linear to bent. As these changes occur and the hydrogen bonds between the ion pairs acquire increased proton-shared character, (2h)J(F)(-)(N) increases in absolute value and (1h)J(H)(-)(F) changes sign. F(H(3)NH)(2)(+) complexes were also optimized as a function of the N-H distance. As this distance increases and the N-H...F hydrogen bonds change from ion-pair to proton-shared to traditional F-H...N hydrogen bonds, (2h)J(F)(-)(N) initially increases and then decreases in absolute value, (1)J(N)(-)(H) decreases in absolute value, and (1h)J(H)(-)(F) changes sign. The signs and magnitudes of these coupling constants computed for F(H(3)NH)(2)(+) at short N-H distances are in agreement with the experimental signs and magnitudes determined for the F(collidineH)(2)(+) complex in solution. However, even when the N-H and F-H distances are taken from the optimized structure of F(collidineH)(2)(+), (2h)J(F)(-)(N) and (1h)J(H)(-)(F) are still too large relative to experiment. When the distances extracted from the experimental NMR data are used, there is excellent agreement between computed and experimental coupling constants. This suggests that the N-H-F hydrogen bonds in the isolated gas-phase F(collidineH)(2)(+) complex have too much proton-shared character relative to those that exist in solution.     

Characterizing hydrogen bonding and proton transfer in 2:1 FH:NH3 and FH:collidine complexes through one- and two-bond spin-spin coupling constants across hydrogen bonds

Ab initio equation-of-motion coupled cluster singles and doubles calculations have been carried out on a variety of 2:1 FH:NH(3) complexes (F(b)H(b):F(a)H(a):NH(3)) to investigate the effects of structural changes on one- and two-bond spin-spin coupling constants across F(a)-H(a)-N and F(b)-H(b)-F(a) hydrogen bonds and to provide insight into experimentally measured coupling constants for 2:1 FH:collidine (2:1 FH:2,4,6-trimethylpyridine) complexes. Coupling constants have been computed for 2:1 FH:NH(3) equilibrium structures and proton-transferred perpendicular and open structures at 2:1 FH:NH(3), FH:pyridine, and FH:collidine geometries. (2h)J(Fa)(-)(N), (1)J(Fa)(-)(Ha), and (1h)J(Ha)(-)(N) exhibit expected dependencies on distances, angles, and the nature of the nitrogen base. In contrast, one- and two-bond coupling constants associated with the F(b)-H(b)-F(a) hydrogen bond, particularly (2h)J(F)()b(-)(F)()a, vary significantly depending on the F-F distance, the orientation of the hydrogen-bonded pair, and the nature of the complex (HF dimer versus the anion FHF(-)). The structure of the 2:1 FH:collidine complex proposed on the basis of experimentally measured coupling constants is supported by the computed coupling constants. This study of the structures of open proton-transferred 2:1 FH:NH(3), FH:pyridine, and FH:collidine complexes and the coupling constants computed for 2:1 FH:NH(3) complexes at these geometries provides insight into the role of the solvent in enhancing proton transfer across both N-H(a)-F(a) and F(b)-H(b)-F(a) hydrogen bonds.     

Determination of (3)J((1)H3'-(31)P) couplings in a DNA oligomer with enhanced sensitivity employing a constant-time TOCSY difference experiment

A constant-time TOCSY difference experiment for the determination of (3)J((1)H3'-(31)P) coupling constants in non-isotope-labelled DNA oligonucleotides is presented. The method is tested on a DNA octamer and compared with the established constant-time NOESY difference method. Each (3)J((1)H3'-(31)P) coupling constant is determined from amplitude changes caused by phosphorous decoupling, which are observable on multiple cross-peaks, thus leading to a high accuracy of the value of the (3)J((1)H3'-(31)P) coupling constant. The new experiment delivers up to three times the sensitivity compared with previously reported methods.     

Characterization of monofluorinated polycyclic aromatic compounds by 1H, 13C and 19F NMR spectroscopy

Monofluorinated polycyclic aromatic hydrocarbons (F-PAHs) have attracted much attention in analytical, environmental, toxicological and mechanistic studies because of their physico-chemical properties, which are closely similar to those of the parent PAHs. Because of this, full NMR characterization has become of interest. Complete 1H, 13C and 19F NMR chemical shifts, and also 1J(H,C), (n)J(C,F), (n)J(H,F) and (n)J(H,H) coupling constants, have been assigned for the F-PAHs 1-fluoronaphthalene, 2-fluorofluorene, 5-fluoroacenaphthylene, 2-fluorophenanthrene, 3-fluorophenanthrene, 3-fluorofluoranthene, 1-fluoropyrene, 1-fluorochrysene, 2-fluorochrysene, 3-fluorochrysene and 9-fluorobenzo[k]fluoranthene. To allow comparison with the corresponding parent PAHs, the 1H and 13C chemical shifts of acenaphthylene, phenanthrene, fluoranthene, pyrene and benzo[k]fluoranthene were determined. Chemical shift increments and the effects on the coupling constants from the fluorine substitution are discussed.     

1H and 13C NMR chemical shifts and spin-spin coupling constants in trans- and cis-decalins

The NMR parameters characterizing the spectra of trans- and cis-decalins were determined from theoretical calculations and experimental spectra. The calculated values of the shielding constants are in good agreement with the measured chemical shifts, with a small but noticeable difference in accuracy for the bridgehead atoms. Of all the spin-spin coupling constants, only most of (1)J(C,C) and (1)J(C,H) values could be extracted from the spectra, and the corresponding computed values are in good agreement with experiment. It appears that the applied density functional theory (DFT) approach overestimates slightly the J(C,C) coupling and underestimates the differences between one-bond (1)J(C,H) coupling constants. For all these constants [J(C,C), J(C,H) and J(H,H)] through one to three bonds, which could not be obtained experimentally, the predicted values are in good agreement with the general rules relating spin-spin coupling to the number and spatial arrangement of the intervening bonds.     

Time-dependent quantum study of the kinetics of the H(2S) + FO(2II) OH(2II) + F(2P) reaction

Quantum mechanical wave packet calculations are carried out for the H((2)S) + FO((2)II) --> OH((2)II) + F((2)P) reaction on the adiabatic potential energy surface of the ground (3)A' triplet state. The state-to-state and state-to-all reaction probabilities for total angular momentum J = 0 have been calculated. The probabilities for J > 0 have been estimated from the J = 0 results by using J-shifting approximation based on a capture model. Then, the integral cross sections and initial state-selected rate constants have been calculated. The calculations show that the initial state-selected reaction probabilities are dominated by many sharp peaks. The reaction cross section does not manifest any sharp oscillations and the initial state-selected rate constants are sensitive to the temperature.     

Suppression of phase and amplitude J(HH) modulations in HSQC experiments

The amplitude and the phase of cross peaks in conventional 2D HSQC experiments are modulated by both proton–proton, J(HH), and proton–carbon, ¹J(CH), coupling constants. It is shown by spectral simulation and experimentally that J(HH) interferences are suppressed in a novel perfect‐HSQC pulse scheme that incorporates perfect‐echo INEPT periods. The improved 2D spectra afford pure in‐phase cross peaks with respect to ¹J(CH) and J(HH), irrespective of the experiment delay optimization. In addition, peak volumes are not attenuated by the influence of J(HH), rendering practical issues such as phase correction, multiplet analysis, and signal integration more appropriate. Copyright © 2014 John Wiley & Sons, Ltd.     

Structures, energies, and spin-spin coupling constants of fluoro-substituted 1,3-diborata-2,4-diphosphoniocyclobutanes: four-member B-P-B-P rings B2P2F(n)H(8-n) with n = 0, 1, 2, 4

An ab initio study has been carried out to determine the structures, relative stabilities, and spin-spin coupling constants of a set of 15 fluoro-substituted 1,3-diborata-2,4-diphosphoniocyclobutanes B(2)P(2)F(n)H(8-n), for n = 0, 1, 2, 4, with four-member B-P-B-P rings. Except for B(2)P(2)F(4)H(4) with four fluorines bonded to two borons, these rings are puckered in a butterfly conformation. For a fixed number of fluorines, the isomers with B-F bonds are significantly more stable than those with P-F bonds. As the number of fluorines increases, the energy difference between the most stable isomer and the other isomers increases. Transition structures which interconvert axial and equatorial positions present relatively small inversion barriers. Coupling constants involving (31)P, namely, (1)J(B-P), (1)J(P-F), (2)J(P-P), (2)J(P-F), and (3)J(P-F) are large and are capable of providing structural information. They are sensitive to the number of fluorines present and can discriminate between axial, equatorial, and geminal B-F and P-F bonds, although not all do this to the same extent. (1)J(B-P) and (2)J(P-P) are similar in equilibrium and transition structures. Although transition structures no longer discriminate between axial and equatorial bonds, (1)J(P-F) and (3)J(P-F) remain sensitive to the number of fluorine atoms present.     

Theoretical prediction and assignment of vicinal 1H-1H coupling constants of diastereomeric 3-alkoxy-6,7-epoxy-2-oxabicyclo[3.3.0]octanes

Spin-spin coupling constants between nuclei in NMR spectroscopy reflect their spatial arrangement. A number of calculation methods, applying different levels of theory, have been developed to support the stereochemical assignment of novel compounds. Nevertheless, revisions of the assignment of structures in the literature are not rare. In the present work, the reliability of the calculation methods amenable for a theoretical prediction of spin-spin coupling constants of vicinal protons to support correct stereochemical assignment of substitution at five-membered rings of 3-alkoxy-6,7-epoxy-2-oxabicyclo[3.3.0]octanes was studied. Experimental (3)J(H,H) coupling constants were compared with the coupling constants calculated for all possible diastereomers. The fully quantum chemical approach provided theoretical (3)J(H,H) coupling constants with an absolute deviation of no more than 1.1 Hz for 91% of the experimentally studied coupled spins, whereas the methods without quantum chemical geometry optimization resulted in completely unreliable predictions. Consequently, for a reliable stereochemical assignment of small and medium size molecules, the protocol for calculating the coupling constants based on the results of the quantum chemical geometry optimization is recommended.     

二[氧合-二(3-二茂铁基丙烯酸二正丁基锡(Ⅳ))]配合物的合成及谱学表征

由FcCH=CHCO2H和(n Bu)2SnO反应合成了{[(FcCH=CHCO2)Sn(n Bu)2]2O}2(A)新配合物[其中Fc=(η5 C5H5)Fe(η5 C5H4)].经红外光谱和核磁共振(1H、13C、119Sn)谱学研究, 确定其组成和结构.提出 [(RCOOSnR′2)2O]2类化合物中一一指认与内环锡、外环锡键连的两类烷基R′中各个碳峰的原则.     

Do traditional, chlorine-shared, and ion-pair halogen bonds exist? An ab initio investigation of FCl:CNX complexes

Ab initio MP2/aug'-cc-pVTZ calculations have been carried out to determine the structures, binding energies, and bonding of complexes FCl:CNX, with X = CN, NC, NO(2), F, CF(3), Cl, Br, H, CCF, CCH, CH(3), SiH(3), Li, and Na. Equation-of-motion coupled cluster calculations have also been carried out to determine the coupling constants (1)J(F-Cl), (1X)J(Cl-C), and (2X)J(F-C) across these halogen bonds. As the strength of the base is systematically increased, the nature of the halogen bond changes from traditional, to chlorine-shared, to ion-pair. The type of halogen bond present in a complex can be readily determined from its structure, binding energy, AIM bonding analyses, and spin-spin coupling constants. Coupling constants across halogen bonds are compared with corresponding coupling constants across traditional, proton-shared, and ion-pair hydrogen bonds.     

Spin-spin coupling tensors in fluoromethanes

All spin-spin coupling tensors J of the fluoromethanes CH3F, CH2F2, and CHF3 are obtained theoretically by multiconfiguration self-consistent field linear response (MCSCF LR) ab initio calculations. Furthermore the principal values and the orientation of the principal axis systems of each theoretical J tensor are specified. Experimental liquid crystal NMR (LC NMR) data on the tensorial properties of the CF spin spin coupling in CH3F and CH2F2, and the FF spin-spin coupling in CHF3 are also reported. In the analysis of the experiments, the contributions from molecular vibrations, as well as that of the correlation of vibrational and rotational motion to the experimental anisotropic couplings, D(exp), are taken into account. The information of the anisotropic indirect coupling, 1/2J(aniso), is detected as the difference between D(exp) and the calculated dipolar coupling, D(calc). The extracted indirect contributions, 1/2J(aniso), are in fair agreement with the ab initio results. All relative (experimental and theoretical) CF and FF indirect contributions, 1/2J(aniso)/D(exp), are negative and under 1.7% in magnitude, when the observed molecular orientations are used. Therefore, in the one bond CF couplings and in the two bond FF couplings, the indirect contribution can normally be ignored without introducing serious error to the determination of molecular orientation and/or structure. However, a more accurate method is to partially correct for the indirect contribution by utilising the transferability of the spin-spin coupling tensors in related molecules. This is due to the fact that even small contributions may be significant, if the order parameter of the internuclear direction is negligibly small, leading to dominating indirect contributions. The very good agreement of the experimental values with the calculated coupling constants and the reasonable agreement in the anisotropic properties, which are experimentally much more difficult to define, indicates that the MCSCF LR method is capable of producing reliable J tensors for these systems, contrary to the case of density-functional theory.     

19F and 1H NMR spectra of halocarbons

19F NMR chemical shifts and coupling constants are reported for 215 compounds. For 77 of these compounds, 1H NMR spectral data are also given. Long-range couplings, including 8J(F,F) and 5J(F,H), are reported. The complexity of halocarbon spectra owing to the presence of rotational isomers, asymmetric centers, long-range couplings, and chlorine isotope effects are illustrated, and the methods used for analyzing such complex spectra are briefly discussed.     

Efficient measurement of the sign and the magnitude of long‐range proton‐carbon coupling constants from a spin‐state‐selective HSQMBC‐COSY experiment

A spin state‐selective Heteronuclear Single‐Quantum Multiple‐Bond Connectivities (HSQMBC‐COSY) experiment is proposed to measure the sign and the magnitude of long‐range proton‐carbon coupling constants (ⁿJ(CH); n > 1) either for protonated or for non‐protonated carbons in small molecules. The simple substitution of the selective 180° ¹H pulse in the original selHSQMBC pulse scheme by a hard one allows the simultaneous evolution of both proton‐proton and proton‐carbon coupling constants during the refocusing period and enables a final COSY transfer between coupled protons. The successful implementation of the IPAP principle leads to separate mixed‐phase α/β cross‐peaks from which ⁿJ(CH) values can be easily measured by analyzing their relative frequency displacements in the detected dimension. Copyright © 2012 John Wiley & Sons, Ltd.     

Indirect nuclear 57Fe-13C and 57Fe-1H spin-spin coupling in ferrocenes and cyclopentadienyliron complexes: measurements and DFT calculations

Several 57Fe-labeled ferrocene derivatives and other cyclopentadienyliron complexes were studied by 57Fe and 13C NMR with respect to isotope-induced chemical shifts 1Delta12/13C(57Fe) and the magnitude and sign of coupling constants 1J(57Fe,13C) and 2J(57Fe,1H). The geometries of the complexes were optimized by DFT methods [B3LYP/6-311+G(d,p)] and chemical shifts (GIAO) and coupling constants were calculated at the same level of theory. The trends in calculated 57Fe nuclear shielding agree well with the experimental data and, in the case of coupling constants 1J(57Fe,13C) and 2J(57Fe,1H), both sign and magnitude are correctly reproduced.     

Porphyrin-based peptide receptors: syntheses and NMR analysis

The synthesis and purification of a water-soluble host compound that contains three pyridinium units and one spacer-connected benzocrown ether unit in the meso-positions of porphyrin and of its Zn(II) or Cu(II) complexes is described. Metalation leads to small (compared to the apo-derivative) changes of selectivities with different peptides, with complexation constants in water of above 10(5)M(-1). One complex containing the tripeptide Gly-Gly-Phe is analyzed in detail by COSY, HSQC, HMBC, and NOESY NMR experiments. Temperature-dependent spectra show activation energies for a intramolecular hydrogen exchange of amide protons with valence isomerization of the porphyrin ring, in accordance with the literature. Sharp signals for the spin system are only found at elevated temperature. Vicinal coupling constants within the crown ether moiety indicate stronger puckering than that reported for benzocrowns. All NMR signals of the complexed peptide are shielded, in particular those of the terminal phenylalanine unit, in line with its stacking on the porphyrin surface. A corresponding structural model, obtained by CHARMm simulation, is also in line with the observed intermolecular NOE cross peaks.