Analysis Of Coupling Constants Research Articles

Standard values of vicinal coupling constants in the assignment of relative configurations and conformational analysis of acyclic chiral carbinols and hydrocarbons

AbstractStandard values of vicinal coupling constants are proposed in order to assign relative configuratations and analyse the conformational distribution of acyclic chiral carbinols and hydrocarbons. Thus, the assignment of relative configurations has been achieved for the diastereomeric carbinols, (2R4R, 2S4S)‐ and (2R4S, 2S4R)‐5,5‐dimethyl‐4‐phenyl‐2‐hexanol and (1R3R, 1S3S)‐ and (1R3S, 1S3R)‐1‐mesityl‐4,4‐dimethyl‐3‐phenyl‐1‐pentanol, through the analysis of the respective vicinal coupling constants and the corresponding conformational distribution. The analysis of the vicinal coupling constants allows the establishment of the monoconformational character of the (2R4S, 2S4R) diastereomer in the first case and the (1R3R, 1S3S) diastereomer in the second case.

Nuclear Quadrupole Coupling Conrtants in Niobium Pentachloride and Related Compounds (II) Transition Metal Nuclei

93 Nb NQR spectrum in NbCl 5 has been investigated from 4.2 K to 480 K. The coupling constant shows an unusual positive temperature dependence at low temperatures. The EFG tensor is calculated with the d 2 sp 3 octahedral bond functions. The experimental data are analysed in terms of the contributions from the σ- and π-bonds. It is shown that the positive temperature dependence can be explained by the p π -d π bond mechanism. The theory is also applied to the related compounds and the consistency with the analysis of the halogen coupling constants is shown.

Conformations of cyclic peptide/calcium complexes in solution.

AbstractSynthetic cyclic octapeptides of general structure cyclo[Glu(γOBzl)‐Sar‐Gly‐(N‐R)Gly]2 (R = n‐hexyl and cyclohexyl) transport calcium ions selectively across organic phases and phospholipid membranes. We have now used proton nmr spectroscopy (360 MHz) to study the solution conformation(s) of their calcium complexes. When Ca(ClO4)2 was added to solutions of these peptides in CDCl3, nmr spectra of the resulting calcium complexes were characteristic of a single C2‐symmetric conformer. From a Karplus‐Bystrov analysis of vicinal coupling constants in both the peptide backbone and Glu side chain (treated as an ABCC′MX spin system), in conjuction with model‐building studies, a structure was proposed in which the calcium ion is bound in an octahedral‐type complex by the four (coplanar) carbonyl groups of the (all‐trans) Glu‐Sar and Gly‐(N‐R)Gly peptide bonds. Occurrence of preferred rotamers about Glu side chain Cα–Cβ bonds indicated that restricted rotation in peptide side chains arises upon calcium binding.

Addition compounds of antimony trichloride and tribromide: Crystal structure and charge transfer from NQR data

The correlation analysis of the quadrupcle coupling constants(QCC) for the antimony nuclei in the intermolecular compounds of SbCl 3 and SbBr 3 with aromatic hydrocarbons is carried out. The results confirm a supposition about the general similarity of the crystal structures of these complexes. The correlative dependence between the antimony QCC values and also the results of the chlorine and bromine NQR spectra Investigation permit the evaluation of the relative acceptor strength of the antimony trihalide molecules.

Stereospecificity of the15N?13C coupling constants and the chemical shifts in the13C NMR spectra of azobenzenes and nitrosobenzenes

In the molecules of azobenzenes and nitrosobenzenes at low temperatures in the restricted conformation, the ortho-carbon atoms is 18-32 ppm, depending on the structure of the compounds; the signal of the carbon atom occupying the cis-position of the unshared electron pair of nitrogen appears in a weaker field, and the signal of the carbon occupying the cis-position to the N=N or N=O bond appears in a stronger field. The assignment of the signals was based on an analysis of the spin-spin coupling constants J/sub /sup 15/N/sup 13/C/. It has shown that the stereospecificity of the geminal constants /sup 2/J/sub /sup 15/N/sup 13/C/ is a common property of two-coordinated nitrogen.

The polar group conformation of 1,2-dialkyl phosphatidylcholines An NMR study

The polar group conformation of 1,2- di-O- alkylglycerophosphocholine (dialkylphosphatidylcholine (ether phosphatidylcholine)) is investigated and compared to that of the corresponding diacylphosphatidylcholine (ester phosphatidylcholine). The motionally averaged conformation of the glycerophosphocholine group is similar in the two classes of compounds. The replacement of the ester linkages by ether bonds has therefore no significant effect on the average conformation and segmental motion of the polar group. This conclusion is based on 1H spin-spin coupling constant analysis which gives information about the conformation of the glycerol ( torsion angles θ 1 to θ 4 , Fig. 5) and the remainder of the polar group (torsion angles α 1, α 4, α 5) except for the conformation of the phosphodiester group (torsion angles α 2 and α 3). Comparison of 31P powder spectra of dialkyl- and diacylphosphatidylcholine shows that the conformation of the latter group is also very similar in the two classes of compounds. The 1H-NMR measurements were carried out with short chain compounds (dihexylphosphatidylcholine) which are present as monomers or small micelles in H 2O and long chain compounds (dioleylphosphatidylcholine) forming small micelles in CHCl 3/CH 3OH. The 31P-NMR work was carried out with dipalmitylphosphatidylcholine which forms multilamellar bilayer structures in H 2O. From NMR measurements using these different structures it is clear that for both ether and ester phosphatidylcholines the average conformation and segmental motion is independent of the state of aggregation. Furthermore, displacement of water of hydration by CHCl 3/CH 3OH has little effect on conformation and segmental motion. These results indicate that the motionally averaged conformation of the polar group is mainly determined by intramolecular energetics. 1H-NMR provides evidence that there is almost free rotation of the phosphocholine group about the C2-C3 glycerol bond. As a result there is no preferred conformation of the phosphocholine group with respect to the diacylglycerol part of the molecule. This rotation is probably responsible for the averaging of the chemical shielding tensor leading to axially symmetric 31P powder spectra as observed for aqueous phosphatidylcholine dispersions both below and above the transition temperature.

Rotational isomerism. A gas-phase 1H nuclear magnetic resonance study of 1-bromo-2-chloroethane

The average vicinal coupling constants of 1-bromo-2-chloroethane at different temperatures have been obtained in a gas-phase 1H NMR study of 1,2-disubstituted ethanes. Analysis of the experimental data, assuming a ”static” model with constant values for the vicinal coupling constants of the individual rotamers, yielded unacceptable results. A ”dynamic” model, which takes into account the torsional vibrations, has therefore been developed and used to analyse the gas-phase results. The values at 305 K for the vicinal coupling constants of the individual rotamers are: trans rotamer J T = 13.6 Hz, J' t = 4.9 Hz; gauche rotamer 1 2 ( J G1 + J G2) = 0.9 Hz, 1 2 ( J' G1 + J' G2) = 7.8 Hz. These couplings are temperature-dependent with J T changing about four times as much as the other coupling constants. Analysis of the average vicinal coupling constants, measured in different solvents, with the “dynamic” model gave values for the vicinal coupling constants of the individual rotamers in excellent agreement with those from the gas-phase data. The value of 1 2 ( J G1+ J G2) decreases noticeably with increasing dielectric constant of the solution. This can be explained by changes in the average dihedral angle between the coupling protons, in the gauche rotamers, which leads to changes of J G1 and J G2 in the same direction.

1H‐nmr study of protected methionine homo‐oligopeptides in helix‐supporting environment

Abstract1H‐nmr spectra for a series of Boc‐L‐(Met)n‐OMe (n = 2–9) homo‐oligopeptides have been observed in the helix‐supporting solvent trifluoroethanol (TFE) at millimolar concentrations. Interfering solvent peaks were eliminated using two decoupling frequencies to selectively remove the methylene and hydroxyl protons of the solvent. Comparisons with specifically α‐deuterated homo‐oligopeptides gave complete assignments of the NH region of the Boc‐Metn‐OMe oligomers up to the heptapeptide. Analysis of chemical shifts, coupling constants, and temperature dependence of chemical shifts suggests that up to the hexapeptide, similar structures exist in deuterochloroform and TFE. In contrast, nmr parameters at the heptapeptide for several internal residues differ in these solvents. These results suggest that a C7 to α‐helix transition may occur in TFE as the chain length of the methionine oligopeptides increases.

A high field proton magnetic resonance study of the solution conformation of thromboxane B2

The solution conformation of thromboxane B2 (TXB2) has been studied using high-field nuclear magnetic resonance techniques. In CDCl3, both anomers are present in solution with 76% 11α-OH TXB2 and 24% 11β-OH TXB2. In CD3OD, the predominant anomer is 11β-OH TXB2 (80%) while the concentration of 11α-OH TXB2 is 20%. The proton resonances were assigned at 400 MHz using double resonance techniques. The analysis of the vicinal coupling constants indicates that the six-membered ring is present in solution in a chair conformation with both of the aliphatic side chains equatorial. Proton longitudinal relaxation times were measured at 25 °C in CDCl3 for H-2, H-11β, H-12, H-13, H-14, and H-17 to H-19, both at 200 MHz and at 400 MHz. From the frequency dependence of these dipolar relaxation times, the rotational correlation times were evaluated. Within experimental error, all of the values are similar in magnitude (~ 2.0 × 10−10 s) indicating that this is the molecular rotational correlation time.

ChemInform Abstract: INTERNAL MOTIONS OF AROMATIC AMINO ACID SIDE CHAINS. SELECTIVE EXCITATION NUCLEAR RELAXATION AND SCALAR COUPLING CONSTANT ANALYSIS ON A MODEL SYSTEM

Chemischer InformationsdienstVolume 11, Issue 12 Physical Organic Chemistry ChemInform Abstract: INTERNAL MOTIONS OF AROMATIC AMINO ACID SIDE CHAINS. SELECTIVE EXCITATION NUCLEAR RELAXATION AND SCALAR COUPLING CONSTANT ANALYSIS ON A MODEL SYSTEM N. NICCOLAI, N. NICCOLAISearch for more papers by this authorE. TIEZZI, E. TIEZZISearch for more papers by this author N. NICCOLAI, N. NICCOLAISearch for more papers by this authorE. TIEZZI, E. TIEZZISearch for more papers by this author First published: March 25, 1980 https://doi.org/10.1002/chin.198012046AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume11, Issue12March 25, 1980 RelatedInformation

13C and some 15N N.M.R. spectra of heteroaromatic molecules containing bridgehead nitrogen atoms

The carbon-13 and (in part) nitrogen-15 chemical shifts of 1,2,3- triazaindolizine and seven three ringed analogues of indolizine have been analysed and enable deductions to be made concerning the extent that the free pair of electrons on the bridgehead nitrogen atom contributes to the delocalized electronic structures. Analogies between these molecules and indolizine are noted in that all the systems possess a high degree of aromaticity. Additivity relationships for nitrogen and methyl substitution are compared with those previously reported for other heteroaromatic molecules. Comparison of previously published 100-MHz and the present-work 270-MHz proton n.m.r. results enabled 13C spectral assignments to be confirmed by selective 1H irradiation experiments, since coupling constant analyses did not provide unambiguous assignments in all cases. One-, two-, three- and four-bond 13C-1H coupling constants are reported for most carbon atoms where first-order spectral analysis was possible. Several examples of non-first-order behaviour are presented and attempts to reconcile the occurrence of this behaviour are outlined.

Conformational preference of the fluoromethyl group in some benzyl fluorides: a 13C N.M.R. study

A number of model benzyl fluoride systems have been synthesized, and their proton-decoupled natural-abundance 13C n.m.r. spectra recorded and assigned. An analysis of the long-range five-bond coupling constants (5JCF) indicates that these parameters are predominantly a manifestation of hyperconjugation involving the CF σ bond. The utilization of these parameters as conformational probes is demonstrated.

Internal motions of aromatic amino acid side chains. Selective excitation nuclear relaxation and scalar coupling constant analysis on a model system

Internal motions of aromatic amino acid side chains. Selective excitation nuclear relaxation and scalar coupling constant analysis on a model system

Electron spin resonance studies of aryl-substituted cyclopolysilane anion-radicals

Several aryl-substituted cyclopolysilanes have been reduced to anion-radicals and have been studied by ESR spectroscopy. Compounds investigated were Si 5Me 9X (X  phenyl, biphenyl, p-Me 3SiC 6H 4, p-MeC 6H 4) and p-MeC 6H 4) and Si 5Me 9SiMe 2X (X  phenyl, p-Me 3SiC 6H 4 and p-Me 2NC 6H 4). The species have ESR spectra typical of delocalized cyclopentasilane anion-radicals except for biphenylnonamethylcyclopentasilane, which gives an ESR pattern indicating that the unpaired electron is mainly associated with the biphenyl ring system. An analysis of 1H and 13C hyperfine coupling constants for Si 5Me 9X −. suggests that the principal effect of the aryl substituents is inductive. The anion-radicals of Si 5Me 9SiMe 2X decay to Si 5Me 9H −., and the electronic nature of the aryl substituent has no major effect on the stability of the parent radical.

Conformation of mononucleotides and dinucleoside monophosphates. P[H] and H[H] nuclear Overhauser effects

The phosphorus-proton nuclear Overhauser effect (NOE) was used to investigate the quantitative distribution of rotamers about the C3'--O3' bond (phi') of 3'-AMP and 2',3'-cyclic-CMP and the C4'--C5', C5'--O5' bonds (psi, phi) of 5'-AMP. Phosphorus-proton and proton-proton NOE's were used to provide a qualitative insight into the backbone conformation and the glycosyl angle torsions of adenosylyl-(3' leads to 5')-adenosine (ApA). The major psi rotamer in 5'-AMP is the 60 degree (gg) form, while the major phi rotamer is the 180 degrees (g'g') form. The constrained model, 2',3'-cyclic-CMP, manifests the C3'endo furanose pucker predominantly. The results from these two models are consistent with nuclear magnetic resonance (NMR) J coupling analyses. The phi; distribution of 3'-AMP is dominated (77%) by the 180 degrees g- rotamer. The 3'-AMP results are consistent with phosphorus-hydrogen coupling constant analyses, but do not accord with phosphorus-carbon coupling constant results. The phosphorus-proton NOE reveals that the phosphorus of ApA occupies a region of conformation space not seen in 5'-AMP. The proton-proton NOE on APA shows a significant portion of syn rotamer in both X distributions and detects a cross-purine ring interaction consistent with base stacking known to exist in this system.

13C13C spin coupling constants in phenanthrene derivatives

AbstractThe synthesis of six 1‐X‐ (X = OH, OCH3, OCOCH3, CH3, CHO and CN) phenanthrene derivatives with a 13C label at C‐1 is described. An analysis of the 13C13C spin coupling constants shows the importance of π‐interaction for the coupling constant transmission. Small 13C13C spin coupling constants over 6 bonds are reported.

Conformational studies of some 2':3'-cyclic mononucleotides in solution by different nuclear-magnetic-resonance methods.

The conformations of the 2':3'-cyclic mononucleotides of adenosine and cytidine in deuterium oxide has been studied at pH 2.3, using lanthanide ions as paramagnetic nuclear magnetic resonance (NMR) probes. It was not possible to find any single conformation for these molecules which accounts for the observed shift and relaxation data. This situation is in agreement with the interpretation of vicinal 1H-1H and 1H-31P coupling constants, which indicate that the ribofuranose and cyclic phosphate rings are in rapid equilibrium between different puckered forms. The interpretation of the lanthanide data in terms of an equilibrium between different conformations give average rotamer populations in good agreement with the coupling constant analysis. The conformations of these systems in aqueous solutions were found to be more flexible than in the solid state, where rigid planar ribofuranose rings have been observed. Adenosine 2':3'-monophosphate differs from cytidine 2':3'-monophosphate at the glycosidic link.

Nucleotide torsional flexibility in solution and the use of the lanthanides as nuclear-magnetic-resonance conformation probes. The case of adenosine 5'-monophosphate.

In this article the single family of conformations of adenosine 5'-monophosphate (5'AMP) in aqueous solution previously obtained using the lanthanide ions as nuclear magnetic resonance (NMR) probes is shown to disagree with the conformational data available from independent NMR methods, such as coupling constant and nuclear Overhauser enhancement analysis. The original lanthanide experimental data is reinterpreted assuming the contribution of different conformers to the observed magnetic perturbations. The average conformation obtained for 5'AMP agrees quite satisfactorily with the other independent conformational information.

Shielding effects of the D-Phe aromatic ring in the 1H NMR spectrum of gramicidin S

Analysis of the side-chain coupling constants for the D-Phe residue of gramicidin S shows that the rotamer predicted for the minimum-energy structure is the predominant one. The aromatic ring of D-Phe shields the Pro C δ hydrogens, and the calculated shielding closely matches that found in the 1H NMR spectrum. As the temperature is raised, the rotamer population changes and the shielding is reduced accordingly. The shielding is absent in the saturated analogue of gramicidin S in which the aromatic rings were hydrogenated without changing the conformation of the rest of the molecule.

5,6,7,8-Tetrahydrofolic acid. Conformation of the tetrahydropyrazine ring.

It is suggested from analysis of proton spin-spin coupling constants that the tetrahydropyrazine ring of tetrahydrofolate is a roughly equal mixture of two half-chair conformations, one with the C-6 proton axial and the other with the C-6 proton equatorial. The chemical shifts and spin-spin coupling constants for the carbon-bound protons of (+/-)-L-, (-)-L-, and (-)-L-[6-2H] 5,6,7,8-tetrahydrofolate were measured at 25 degrees and at 300 MHZ. The resonances corresponding to the two C-7 protons in the deuterated compound constituted an AB quartet with JAB of 12 Hz and chemical shift difference of 92 Hz or 0.307 ppm; the C-7 protons are proposed to be a geminally coupled axial-equatorial pair whose rapid equilibration does not result in equivalence due to the adjacent chiral center at C-6. The spin-spin splitting in the C-7 resonances were 3.0 and 6.6 Hz for the low field and high field resonances, respectively, reflecting coupling to the C-6 proton. These coupling constants reflect the conformational equilibrium. The resonances assignable to C-9 protons are nearly equivalent in the 6-2H compound, but exhibit the resonances corresponding to a complex spin system in the 6-H compound.