Hydrogen Coupling Constants Research Articles

Isolation and detailed 1H and 13C NMR structural assignment for three trachylobanes from Psiadia punctulata (Asteraceae) grown in Africa

• New NMR data for three trachylobanes, are presented for the first time in Literature. • The presented data completely clarified the 1H and 13C NMR data assignments for these trachylobanes. • A new methodology, recently published, was applied to clarify all multiplicities, measuring all J values for all substances. • A stereochemistry clarification including W-coupling measuring is provided for each studied compound. The first isolation of a trachylobane from an African specimen of Psiadia punctulata (Asteraceae) is presented in this paper. A complete 1 H and 13 C NMR spectral analysis of this compound and two other trachylobane diterpenes, previously isolated from the same plant, are also provided. The use of NMR techniques such as g COSY, g HSQC, g HMBC and 2D- J -resolved, in combination with a software-assisted methodology, led to a complete and unequivocal assignment of 1 H and 13 C signals. This was achieved together with the measurement of all homonuclear hydrogen coupling constants. The presented detail level of the assignment data has never been published before for trachylobanes. Furthermore, with all determined NMR experimental data from the spectra and to obtain a reliability assessment, signals were simulated in the FOMSC3 and NMR_MultSim software.

Zwitterionic Trivalent (Alkyl)lanthanide Complexes in Ziegler-Type Butadiene Polymerization

The organolanthanide complexes Ln{C(SiHMe2)3}3 (Ln = La, 1a; Ce, 1b; Pr, 1c; Nd, 1d) react with 1 or 2 equiv of B(C6F5)3 to yield the well-defined zwitterionic species Ln{C(SiHMe2)3}2HB(C6F5)3 (Ln = La, 2a; Ce, 2b; Pr, 2c; Nd, 2d) or LnC(SiHMe2)3{HB(C6F5)3}2 (Ln = La, 3a; Ce, 3b; Pr, 3c; Nd, 3d), respectively. These complexes are shown to contain labile, bridging Si–H⇀Ln and o-F→Ln interactions based upon the observation of low one-bond silicon–hydrogen coupling constants (1JSiH) in 1H NMR spectra of 2a and 3a, the presence of one set of C6F5 signals in the 19F NMR spectra of 2a and 3a, the detection of only m-F and p-F resonances in the 19F NMR spectra of 2b–d and 3b–d, two νSiH bands in the IR spectra of 2a–d, and X-ray crystallography analyses of 2b and 3d. In addition, a hexahaptotoluene molecule is coordinated to neodymium in 3d. Reactions of 1a and (AlMe3)2 yield labile adducts. With greater than 3 equiv of AlMe3, exchange between free and bound AlMe3 groups was observed in EXSY NMR experiments. Com...

Rare Earth and Main Group Metal Poly(hydrosilyl) Compounds

The potassium poly(hydrosilyl) compound KSi(SiHMe2)3, which contains three β-SiH groups, is synthesized by the reaction of Si(SiHMe2)4 and KOtBu. A single-crystal X-ray diffraction study reveals chains composed of head-to-tail KSi(SiHMe2)3 monomers. This potassium poly(hydrosilyl) anion reacts with divalent metal halide salts in THF to form the bis(silyl) compounds Mg{Si(SiHMe2)3}2THF2 (1·THF2), Ca{Si(SiHMe2)3}2THF3 (2·THF3), and Yb{Si(SiHMe2)3}2THF3 (3·THF3). A trivalent yttrium bis(silyl), as part of a KCl-containing polymeric chain, is supported by K↼H–Si bridging interactions. The N donors pyridine (py) and dimethylaminopyridine (DMAP) readily substitute THF, giving tetrahedral magnesium and octahedral calcium silyl compounds. The one-bond silicon–hydrogen coupling constants (1JSiH), infrared stretching frequencies (νSiH), and solid-state structures of 2·py4 and 3·THF3 indicate classical two center-two electron bonding between the metal center and the silyl ligand, as well as terminal (nonbridging) Si...

A theoretical study of high-pressure-induced phases of LiAlH4 using calculated NQCC parameters

Quadrupolar parameters of nuclei can be used as a tool to understand the electronic structure of compounds. Lithium alanate (LiAlH4) is a potential hydrogen storage material because of its high capacity of 10.5 wt % H2. However, the drawbacks of its dehydrogenation process are the relatively high temperatures and the slow dehydrogenation kinetics; furthermore, its reversibility is rather poor. Understanding the bonding nature of Al and H is essential for improving its dehydrogenation performance. In this work the charge density distribution in LiAlH4 is studied. Thus using calculated nuclear quadrupole coupling constants of hydrogens (2H-NQCCs), the electronic structure of α-LiAlH4 with high pressure forms of LiAlH4, (β- and γ-LiAlH4) were compared. The results show that easier condition for dehydrogenation is expected in β-LiAlH4. Comparison of calculated dehydrogenation enthalpies of LiAlH4 phases verifies this prediction. The electric field gradient (EFG) of quadrupolar nuclei were calculated to obtain NQCC parameters. All calculations performed using Gaussian 03 at B3LYP/6-31G* level of theory.

Investigation of two‐ and three‐bond carbon–hydrogen coupling constants in cinnamic acid based compounds

Two- and three-bond coupling constants (2 JHC and 3 JHC ) were determined for a series of 12 substituted cinnamic acids using a selective 2D inphase/antiphase (IPAP)-single quantum multiple bond correlation (HSQMBC) and 1D proton coupled 13 C NMR experiments. The coupling constants from two methods were compared and found to give very similar values. The results showed coupling constant values ranging from 1.7 to 9.7 Hz and 1.0 to 9.6 Hz for the IPAP-HSQMBC and the direct 13 C NMR experiments, respectively. The experimental values of the coupling constants were compared with discrete density functional theory (DFT) calculated values and were found to be in good agreement for the 3 JHC . However, the DFT method under estimated the 2 JHC coupling constants. Knowing the limitations of the measurement and calculation of these multibond coupling constants will add confidence to the assignment of conformation or stereochemical aspects of complex molecules like natural products. Copyright © 2016 John Wiley & Sons, Ltd.

Correlations among experimental and theoretical NMR data to determine the absolute stereochemistry of darcyribeirine, a pentacyclic indole alkaloid isolated from Rauvolfia grandiflora

Darcyribeirine (1) is a pentacyclic indole alkaloid isolated from Rauvolfia grandiflora. Stereochemistry of 1 was previously proposed based on 1D (coupling constant data) and 2D (NOESY correlations) NMR techniques, having been established a configuration 3R, 15S, and 20R (isomer 1a). Stereoisomers of 1 (i.e., 1a–1h) can be grouped into four sets of enantiomers. Carbon chemical shifts and hydrogen coupling constants were calculated using BLYP/6-31G* theory level for the eight isomers of 1. Calculated NMR data of 1a–1h were correlated with the corresponding experimental data of 1. The best correlations between theoretical and experimental carbon chemical shift data were obtained for the set of enantiomers 1e/1f to structures in the gaseous phase and considering solvent effects (using PCM and explicit models). Similar results were obtained when the same procedure was performed to correlations between theoretical and experimental coupling constant data. Finally, optical rotation calculations indicate 1e as its absolute stereochemistry. Orbital population analysis indicates that the hydrogen bonding between N–H of 1e and DMSO is due to contributions of its frontier unoccupied molecular orbitals, mainly LUMO+1, LUMO+2, and LUMO+3.

Detailed1H and13C NMR Spectral Data Assignment for Two Dihydrobenzofuran Neolignans

In this work we present a complete proton (1H) and carbon 13 (13C) nuclear magnetic resonance (NMR) spectral analysis of two synthetic dihydrofuran neolignans (±)-trans-dehydrodicoumarate dimethyl ester and (±)-trans-dehydrodiferulate dimethyl ester. Unequivocal assignments were achieved by 1H NMR, proton decoupled 13C (13C{1H}) NMR spectra, gradient-selected correlation spectroscopy (gCOSY), J-resolved, gradient-selected heteronuclear multiple quantum coherence (gHMQC), gradient-selected heteronuclear multiple bond coherence (gHMBC) and nuclear Overhauser effect spectroscopy (NOESY) experiments. All hydrogen coupling constants were measured, clarifying all the hydrogen signals multiplicities. Computational methods were also used to simulate the 1H and 13C chemical shifts and showed good agreement with the trans configuration of the substituents at C7 and C8.

A Non-Karplus Effect: Evidence from Phosphorus Heterocycles and DFT Calculations of the Dependence of Vicinal Phosphorus–Hydrogen NMR Coupling Constants on Lone-Pair Conformation

In contrast to literature reports of a Karplus-type curve that correlates (3)J(PH) with phosphorus-hydrogen dihedral angle, a recently reported glycine-derived 1,3,2-oxazaphospholidine (7c) has two hydrogen atoms on the ring with identical PNCH dihedral angles but measured coupling constants of ∼6 and 1.5 Hz. DFT calculations were in accord with these values and suggested that the smaller coupling constant is negative. Experimental evidence of the opposite signs of these coupling constants was obtained by analysis of the ABX NMR spectrum of the new glycine-derived N-p-toluenesulfonyl phosphorus heterocycle 6c. DFT calculations on 6c and on Me(2)NPCl(2) and t-BuPCl(2) were also in accord with NMR data and allowed confirmation of unusual features including a lone pair effect on (3)J(PH), the negative coupling constant, temperature-dependent chemical shifts due to rotation about the sulfonamide S-N bond, and vicinal phosphorus-hydrogen coupling constants over 40 Hz. Calculation of phosphorus-hydrogen coupling constants both as a function of PYCH dihedral angle θ (Y = O, N, C) and lone pair-PYC dihedral angle ω shows similar θ,ω surfaces for (3)J(PH) with a range of (3)J(PH) from -4.4 to +51 Hz and demonstrates the large non-Karplus effect of lone-pair conformation on vicinal phosphorus-hydrogen coupling constants.

Detailed 1H and 13C NMR structural assignment and relative stereochemistry determination for three new and one known semi-synthetic sesquiterpene lactones

► 1 H and 13 C NMR data for four semisynthetic sesquiterpene lactones. ► Full assignment of NMR data, including chemical shift and constant couplings. ► NOE, constant couplings and theoretical calculations to determine stereochemistry. In this work is described a complete 1 H and 13 C NMR analysis for a group of four sesquiterpene lactones, three previously unknown. The unequivocal assignments were achieved by 1 H NMR, 13 C{ 1 H} NMR, J-resolved , gCOSY, gHMQC, gHMBC and NOESY experiments and no ambiguities were left behind. All hydrogen coupling constants were measured, clarifying all hydrogen signals multiplicities.

Monte Carlo-based searching as a tool to study carbohydrate structure

A torsion angle-based Monte Carlo searching routine was developed and applied to several carbohydrate modeling problems. The routine was developed as a Unix shell script that calls several programs, which allows it to be interfaced with multiple potential functions and various utilities for evaluating conformers. In its current form, the program operates with several versions of the MM3 and MM4 molecular mechanics programs and has a module to calculate hydrogen–hydrogen coupling constants. The routine was used to study the low-energy exo-cyclic substituents of β- d-glucopyranose and the conformers of d-glucaramide, both of which had been previously studied with MM3 by full conformational searches. For these molecules, the program found all previously reported low-energy structures. The routine was also used to find favorable conformers of 2,3,4,5-tetra- O-acetyl- N, N′-dimethyl- d-glucaramide and d-glucitol, the latter of which is believed to have many low-energy forms. Finally, the technique was used to study the inter-ring conformations of β-gentiobiose, a β-(1→6)-linked disaccharide of d-glucopyranose. The program easily found conformers in the 10 previously identified low-energy regions for this disaccharide. In 6 of the 10 local regions, the same previously identified low-energy structures were found. In the remaining four regions, the search identified structures with slightly lower energies than those previously reported. The approach should be useful for extending modeling studies on acyclic monosaccharides and possibly oligosaccharides.

1H and 13C NMR spectral data of bioactive cage‐like polycyclic compounds

Bioactive cage-like polycyclic compounds have attracted the attention of several research groups because of their unique appearance and their biological activities. Their structures were established on the basis of (1)H NMR and (13)C NMR spectroscopic data. The (1)H and (13)C signal assignments and most homonuclear hydrogen coupling constants were assigned by use of techniques such as 1D (1)H and (13)C NMR and 2D gCOSY, non-edited gHSQC and gHMBC. The gNOESY experiments proved the endo-stereochemistry.

Accurate Measurement of Vicinal Carbon–Hydrogen Coupling Constants via Ammonium Nitrogen Based on HMBC Experiments

Abstract Vicinal spin–spin coupling constants frequently used for conformation analysis were determined for ammonium-containing 13C–N–C–1H systems by using HMBC. 1-Deoxynojirimycin hydrochloride provided an appropriate system for measuring the antiperiplanar and gauche interactions, which were determined to be 7.3 and 1.6 Hz, respectively.

15N NMR Chemical Shifts for the Identification of Dipyrrolic Structures

A variety of dipyrromethanes and dipyrromethenes have been prepared, and their 15N NMR chemical shifts have been measured by two-dimensional correlation to 1H NMR signals. The nitrogen atoms in five examples of dipyrromethanes consistently exhibit chemical shifts around -231 ppm, relative to nitromethane. Seven examples of hydrobromide salts of meso-unsubstituted dipyrromethenes consistently display 15N chemical shifts around -210 ppm, while their corresponding zinc(II) complexes exhibit chemical shifts around -170 ppm. The presence of electron-withdrawing substituents on one of the pyrrolic rings of dipyrromethenes affects the chemical shifts of both of the nitrogen nuclei in the molecule. Boron difluoride complexes of meso-unsubstituted dipyrromethenes display 15N chemical shifts around -190 ppm. Two examples of free-base dipyrromethenes bearing substituents at the meso-position exhibit 15N chemical shifts at approximately -156 ppm, and for the zinc complexes of these compounds at -162 ppm. One-bond nitrogen-hydrogen coupling constants, when measurable, were consistently in the range of -96 Hz. Since the measured 15N chemical shifts have such a high regularity correlated to structure, they can be used as diagnostic indications for identifying the structure of dipyrrolic compounds.

1H and 13C NMR spectral data for a tricyclic derivative of a Diels–Alder adduct

A complete NMR analysis with full assignment for (1)H and (13)C NMR spectral data for 5-(acetyloxy)-3-hydroxy-9,10-dimethoxy-6-oxo-11-oxatricyclo[6.2.1.0(2,7)]undec-2-yl acetate is described. This compound was prepared by rapid hydrogenation of the unstable Diels-Alder adduct obtained from the reaction between 3,4-dimethoxyfuran and 2,5-diacetoxy-1,4-benzoquinone. Full homonuclear hydrogen coupling constants measurements and molecular mechanics calculations were performed for the determination of the relative stereochemistry.

Benzamidoximes. II. A 15N NMR study

A 15N NMR study of a series of arylamidoximes was performed at natural isotopic abundance in acetone- d 6 and CDCl 3 solutions. The chemical shifts and nitrogen–hydrogen coupling constants of these compounds are reported. The influence of the substituents on the aromatic ring has also been analyzed. A comparison of the nitrogen chemical shift data of these compounds in three solvents, viz., CDCl 3, acetone- d 6 and DMSO- d 6 has shown a good correlation of the hydrogen bonding between the compound and the solvent. The electrostatic attraction between two molecules also explains the 15N chemical shifts.

Total assignment of 1H and 13C NMR data for the sesquiterpene lactone 15-deoxygoyazensolide.

We describe a complete analysis of the 1H and 13C spectra of the anti-inflamatory, schistossomicidal and trypanosomicidal sesquiterpene lactone 15-deoxygoyazensolide. This lactone, with a structure similar to other important ones, was studied by NMR techniques such as COSY, HMQC, HMBC, Jres and NOE experiments. The comparison of the data with some computational results led to an unequivocal assignment of all hydrogen and carbon chemical shifts, even eliminating some previous ambiguities. We were able to determine all hydrogen coupling constants (J) and signal multiplicities and to confirm the stereochemistry. A new method for the determination of the relative position of the lactonization and the position of the ester group on a medium-sized ring by NMR was developed.

Conformational analysis of trinucleotides containing a single abasic residue; mimics of DNA damage

AbstractThe conformations of three deoxytrinucleotide analogues [d(TpXpT), where X = T, tetrahydrofuran (THF) or propyl (Pr)] were investigated using 1H NMR spectroscopy as part of our studies of DNA‐base damage. The phosphorus‐decoupled 1H NMR spectrum of each compound was simulated and values for the vicinal proton–proton coupling constants of the sugar ring hydrogens were extracted at several different temperatures, for use in conformational analyses. It was found that the south‐pucker preference of the sugar 3′ to the modification is increased whereas that of the 5′ is decreased relative to the puckers observed for the non‐modified system. The conformational change is <25%. This subtle effect may be sufficient for recognition by DNA repair enzymes. Copyright © 2003 John Wiley & Sons, Ltd.

Conformational analysis of 3′-S-PO3-linked ribo- and deoxyribodinucleoside monophosphates

The conformations of two phosphodiester-linked dinucleotides and their analogues containing a 3′-S-phosphorothiolate linkage were investigated using 1H NMR spectroscopy. The phosphorus decoupled 1H NMR spectrum of each compound was simulated and values for the vicinal proton–proton coupling constants of the sugar ring hydrogens abstracted at several different temperatures, for use in full conformational analyses. The UV absorbance temperature profiles of the dimers were also analysed. It was found that in both the deoxyribo and ribo analogues the sugar ring conformations are affected by the phosphorothiolate modification, with a large increase in the percentage of north-type puckering. This change is mainly due to the decrease in the influence of the gauche effect between the 3′-substituent and the O4′-ring oxygen experienced when substituting an oxygen atom for the less electronegative sulphur atom. Copyright © 1999 John Wiley & Sons, Ltd.

Synthesis of a New Spin Trap: 2-(Diethoxyphosphoryl)-2-phenyl-3,4-dihydro-2H-pyrrole 1-Oxide.

Recently, the synthesis of a new phosphorylated nitrone, 2-(diethoxyphosphoryl)-2-methyl-3,4-dihydro-2H-pyrrole 1-oxide (DEPMPO) (2) was described. The presence of the phosphorylated group strongly stabilized the DEPMPO-superoxide spin adduct. To understand the role of the diethoxyphosphoryl group in this stabilization, a new phosphorylated nitrone, 2-(diethoxyphosphoryl)-2-phenyl-3,4-dihydro-2H-pyrrole 1-oxide (DEPPPO) (7), was prepared through a four-step synthetic pathway, and its ability to trap free radicals was investigated. Data obtained from spin trapping experiments of a wide variety of free radicals generated in situ showed the formation of two diastereoisomers spin adducts with different phosphorus and hydrogen coupling constants. Superoxide trapping by DEPPPO gave a persistent nitroxide spin adduct, and its half-time life was measured and compared to that of the DEPMPO analogue.

A theoretical study on the internal rotation and hyperfine structures of the ethyl radical (CH 3-CH 2)

It is confirmed by the UMP2/6-31G** level of calculation, that the zero-point vibrational energy dominates in determining the conformational preference and the extremely low barrier to internal rotation in the ethyl radical. The isotropic hyperfine coupling constants of hydrogens and carbons of the ethyl radical have been calculated using an extended ab initio single- or multi-reference CI wavefunction (SR-CI or MR-CI). The spin density at each nucleus of interest has been calculated as a function of the basis sets and the CI-dimensions. The influence of the internal rotation of the methyl group and the inversion of the methylene group on the hyperfine coupling constants has also been studied. Furthermore, the anisotropic hyperfine coupling constants (dipolar term) have been calculated and compared with available experimental values.