Proton Spin Coupling Research Articles

Power of Aerogel Platforms to Explore Mesoscale Transport in Catalysis.

We describe the opportunity to deploy aerogels-an ultraporous nanoarchitecture with co-continuous networks of meso/macropores and covalently bonded nanoparticulates-as a platform to address the nature of the electronic, ionic, and mass transport that underlies catalytic activity. As a test case, we fabricated Au||TiO2 junctions in composite guest-host aerogels in which ∼5 nm Au nanoparticles are incorporated either directly into the anatase TiO2 network (Au "in" TiO2, AuIN-TiO2 aerogel) or deposited onto preformed TiO2 aerogel (Au "on" TiO2, AuON/TiO2 aerogel). The metal-meets-oxide nanoscale interphase as visualized by electron tomography feature extended three-dimensional (3D) interfaces, but AuIN-TiO2 aerogels impose a greater degree of Au contact with TiO2 particles than does the AuON/TiO2 form. Both aerogel variants enable transport of electrons over micrometer-scale distances across the TiO2 network to Au||TiO2 junctions, as evidenced by electron paramagnetic resonance (EPR) and ultrafast visible pump-IR probe time-resolved absorption spectroscopy. The siting of gold nanoparticles in the TiO2 network more effectively disperses trapped electrons. Density functional theory (DFT) calculations find that increased physical contact between Au and TiO2, induced by oxygen vacancies, produces increased hybridization of midgap states and quenches unpaired trapped electrons. We assign the apparent differences in electron-transport capabilities to a combination of the relatively better-wired Au||TiO2 junctions in AuIN-TiO2 aerogels, which have a greater capacity to dilute accumulated charge over a larger interfacial surface area, with an enhanced ability to discharge the accumulated electrons via catalytic reduction of adsorbed O2 to O2- at the interface. Solid-state 1H nuclear magnetic resonance experiments show that proton spin-lattice relaxation times and possibly proton diffusion are strongly coupled to Au||TiO2 interfacial design, likely through spin coupling of protons to unpaired electrons trapped at the TiO2 network. Taken together, our results show that Au||TiO2 interfacial design strongly impacts charge carrier (electron and proton) transport over mesoscale distances in catalytic aerogel architectures.

Dynamics-based assessment of nanoscopic polymer-network mesh structures and their defects.

Polymer-network gels often exhibit complex nanoscopic architectures. First, the polymer-network mesh topology on scales of 1-10 nm is usually not uniform and regular, but disordered and irregular. Second, on top of that, many swollen polymer networks display spatial inhomogeneity of their polymer segmental density and crosslinking density on scales of 10-100 nm. This multi-scale structural complexity affects the permeability, mechanical strength, and optical clarity of the polymer gels, which is of central relevance for their performance in popular applications. As a result, there is a need to characterize the polymer network structures on multiple scales. On the scale of the spatial inhomogeneity of crosslinking, 10-100 nm, scattering of neutrons, X-rays, and light has extraordinary utility and is well established. On the scale of the mesh topology, 1-10 nm, in contrast, experimental techniques are less established. This review intends to close this gap by reviewing two intrinsically dynamic methods that yield information on polymer network mesh structures. First, NMR-based assessment of residual dipolar proton-spin couplings, which arise upon the introduction of crosslinks into a liquidlike polymer system to impart partial solidlike characteristics, is suitable to quantitatively assess network meshes and local network defects. Second, diffusive penetration of molecular, macromolecular, and mesoscopic colloidal probes through a polymer gel provides insight into its obstructing network mesh structure and its potential irregularity. Either method is highly synergistic to scattering-based assessment of the network structures on larger scales, and in concert, a rich picture on the nano- and mesoscopic gel topology is obtained.

Nuclear spin content and constraints on exotic spin-dependent couplings

There are numerous recent and ongoing experiments employing a variety of atomic species to search for couplings of atomic spins to exotic fields. In order to meaningfully compare these experimental results, the coupling of the exotic field to the atomic spin must be interpreted in terms of the coupling to electron, proton, and neutron spins. Traditionally, constraints from atomic experiments on exotic couplings to neutron and proton spins have been derived using the single-particle Schmidt model for nuclear spin. In this model, particular atomic species are sensitive to either neutron or proton spin couplings, but not both. More recently, semi-empirical models employing nuclear magnetic moment data have been used to derive new constraints for non-valence nucleons. However, comparison of such semi-empirical models to detailed large-scale nuclear shell model calculations and analysis of known physical effects in nuclei show that existing semi-empirical models cannot reliably be used to predict the spin polarization of non-valence nucleons. The results of our re-analysis of nuclear spin content are applied to searches for exotic long-range monopole–dipole and dipole–dipole couplings of nuclei leading to significant revisions of some published constraints.

BIRD‐J‐resolved HMBC and BIRD‐high‐resolution HMBC pulse sequences for measuring heteronuclear long‐range coupling constants and proton–proton spin coupling constants in complicated spin systems

Efficient pulse sequences for measuring long-range C-H coupling constants (J(C-H)) and proton-proton spin coupling constants (J(H-H)), named BIRD-J-resolved HMBC and BIRD-high-resolution HMBC, respectively, have been developed. In spin systems possessing a secondary methyl group positioned between protonated carbons (e.g. -CH(2)-CH(CH(3))-CH(2)-), the methine proton splits in a complicated fashion, resulting in difficulty in the determination of its spin coupling constants. For easy and accurate measurements of the long-range J(C-H) and J(H-H) in such a spin system, the BIRD pulse [90°x(H)-180°x (H/C)- 90° (-x)(H)] or [90°x(H)-180°x(H/C)-90° (-x)(H)180°x(C)] is incorporated into the J-resolved portion of the pulse sequence. As a result, the above secondary methyl group can be selectively decoupled, providing simplified cross-peak patterns, which are suitable for the accurate measurements of the long-range J(C-H) and J(H-H).

Spectroscopic Properties of Some Simple Esters. A Practical Application of Synthesis and Spectroscopy in the Undergraduate Organic Laboratory

This exercise, the synthesis and spectroscopic analysis of the allyl esters of some aromatic carboxylic acids, consists of a series of open-ended experiments. It was designed for the honors organic chemistry student, but can be easily adapted to fit the regular second-semester organic chemistry program. The primary objective of this exercise is to prepare students for research in organic synthesis. Thus, a series of synthetic transformations are described, incorporating the basic techniques of isolation, TLC and flash chromatographic purification, IR and NMR analyses, as well as the method of synthesis involving phase transfer catalysis. These experiments allow the students to monitor the effect of hydrogen bonding on the IR stretching frequencies for the hydroxyl and carbonyl groups, as well as to provide them with an excellent opportunity to examine the phenomenon of proton spin-coupling in the NMR spectra of simple organic systems.

Proton Spin−Spin Coupling and Electron Delocalization

The spin−spin coupling of protons measured by the constant JHH‘ in NMR experiments is dominated by a term proportional to the product of the electron spin densities at the two nuclei, the Fermi contact term. The probability of β electrons being in excess over α electrons at the position of nucleus n‘, given that there is an α electron at n, is proportional to the negative of the exchange density, the total Fermi correlation between n and n‘. Thus the delocalization of the Fermi hole between n and n‘ is the mechanism whereby the spin perturbation caused by the magnetic interaction of an electron with nucleus n is transmitted to n‘. The density and exchange density within the basin of a hydrogen atom are described primarily in terms of s-type basis functions, and one may approximate the exchange density between two protons at n and n‘ by the exchange between the two associated atomic basins determined by the delocalization index δ(A,B) within the quantum theory of atoms in molecules. It is shown that this m...

Determination of the relative signs of proton spin coupling constants by double irradiation

The method of Evans [1] for determining relative signs of nuclear spin coupling constants by double irradiation experiments has been extended to the case where the three coupled nuclei are all protons. It is thus possible to demonstrate from the high resolution proton magnetic resonance spectrum of the ring protons of 2-Furoic acid that the three spin coupling constants are all of like sign.

A new NMR method for measuring proton–proton spin coupling constants: J-resolved HMQC

A new NMR pulse sequence that incorporates J-scaling into HMQC enables easy analysis of strongly coupled proton spin systems.

Complete Assignment of1H and13C NMR Spectra and Conformational Analysis of Thioamide Cannabinoids

The complete assignment of the 1H and 13C NMR spectra of two carbothioamide- substituted meroterpenes is presented. Resonance assignments were achieved by the use of one- and two- dimensional NMR, NOED, selective decoupling measurements and the deuterium isotope effect on the 13C chemical shifts. Six-membered ring conformations were determined by analysis of proton spin coupling constants with the aid of spectral simulations in conjunction with molecular mechanics calculations and equations correlating coupling constants and dihedral angles. The anisotropy effect of benzene rings was used for structural assignments.

1H NMR structural analysis of isomeric tetrazole-substituted spirostan steroids: Shielding effects and conformational study

The structures of two pairs of tetrazole-substituted spirostan steroids were characterized by the use of one- and two-dimensional 1 H NMR measurements. The conformation of the seven-membered spirostan ring A of the steroid was determined by analysis of proton spin coupling constants and confirmed by 1D and 2D NOE experiments showing interactions between the methyl group and ring protons. The conformation of the tetrazole-substituted ring A was found to be the same for the four isomers, allowing them to be differentiated by the analysis of the effect of tetrazole-ring isomerization on the chemical shifts of neighbouring protons

The solution structure of mucous glycoproteins: Proton NMR studies of native and modified ovine submaxillary mucin

The solution structure of native and systematically modified ovine submaxillary mucin (OSM) has been probed by proton NMR spectroscopic methods. Most of the resonances in the spectra have been tentatively assigned to the peptide and O-linked disaccharide, α- N-acetylneuraminic acid 2 → 6 α- N-acetylgalactosamine protons. On the basis of the observed chemical shifts, spectral resolution, and behavior of the exchangeable protons it is concluded the mucin possesses internal segmental flexibility and exists in solution as a random coil peptide. No long-lived interresidue peptide or carbohydrate hydrogen bonds were detected. The removal of (i) the C8 and C9 carbons of the sialic acid residue, (ii) the entire sialic acid residue, and (iii) the complete disaccharide side chain resulted in no significant changes in peptide core conformation. A limited set of proton spin coupling constants and nuclear Overhauser enhancements has been obtained for the threonine glycopeptide side chains in native and modified mucin. The results are consistent with the previously reported conformations for the (1 → 6) linkage in oligosaccharides and the threonyl glycosidic linkage in glycopeptides. The OSM disaccharide may exist as a extended linear structure with rotational freedom about the GalNAc C5C6 bond, while the threonine glycosidic linkage appears to be sterically constrained, although multiple conformations about the threonine C βO γ bond may be allowed. The small chemical shift perturbations detected in the glycosylated threonine methyl protons and the GalNAc carbons upon removal of the terminal sialic acid residue are consistent with this model.

Tautomerism of pyrimidyl-2-methane derivatives: medium effects, thermodynamics, kinetics and mechanism

The Investigation of tautomeric equilibria among pyrimidyl-2-methanes has been undertaken. Pyrimidyl-pyrimidyledene equilibrium has been found in the case of 5-substituted 2-pyrimidyl-cyanoacetic esters. Unsymmetrically substituted pyrimidyl-2-methanes form two types (1-NH and 3-NH) of ylidene tautomers, which differ in ring proton spin coupling constants. The effect of substituents in the pyrimidine ring and of the solvent on tautomeric equilibrium are discussed. A drastic effect is produced by acids (CF 3 COOH) which shift the equilibrium toward the ylidene tautomer of higher basicity. The thermodynamic and kinetic characteristics and the mechanism of the tautomeric equilibrium were examined.

An ab initio Study of Butyrolactone and 3-Hydroxybutyrolactone. Semiempirical Calculations on Proton Coupling Constants in These Molecules

Optimization with the 3-21G basis set of the geometry of the envelope conformation of butyrolactone leads to a structure with an out-of-plane angle of 29.7° for the flap. Similar optimization of the twisted-ring conformation leads to a structure lying only 0.2 kJ mol-1 higher in energy and having a twist angle of 18.7°. These structures differ mainly in a vertical translation of C3 and C4 by c. 0.14 Ǻ with respect to the C2-C1-O2 plane. The ring may be twisted or bent at low energy- cost, and the barrier to inversion via the planar-ring (CS) structure is only 4.7 kJ mol-1. It is suggested that the envelope conformation is but one of many transient structures in a rapid ring-inversion process. For 3-hydroxybutyrolactone, the optimized twisted-ring conformation with twist angle 22.0° lies 0.3 kJ mol-1 below the optimized envelope conformation with out-of-plane angle 33.7° again these structures differ mainly in a vertical translation of C 3 and C 4 by c. 0.15 Ǻ with respect to the C 2-C 1-O2 plane. Moderate ring twisting and bending are rather more costly than in butyrolactone itself, and the hydroxy group clearly occupies the pseudo-axial position on C 3. Calculated bond lengths and angles for both molecules are mostly within the corresponding experimental ranges for a series of γ-lactones. Proton- proton spin coupling constants calculated by the FINITE/INDO method for the optimized 3-hydroxybutyrolactone structures are in moderate-to-good agreement with experimental values, and agreement is improved by application of a thermal population-averaging procedure. Thermally averaged values for butyrolactone are also presented.

Solvent effect calculation on the three-bonded fluorine—proton spin-coupling constants (3Jfh) of fluoroethylenes

The effect of solvent on 3JFHtrans and 3JFHcis for monofluoroethylene, trans-1,2-, cis-1,2- and 1,1-difluoroethylene and trifluoroethylene has been calculated by the FPT-INDO-solvaton method and compared with the experimental values in the literature. The effect of substituent groups and the dependence on the dielectric constant of the reported3JFH values have been reproduced by calculation. Examination of the electronic structures of solute molecules, taking into account the p electron component of F ⋯ H, leads to the conclusion that the effects of solvent on 3JFHtrans and 3JFHcis are caused by the same kinds of changes in electronic distribution.

An experimental study of resolution of proton chemical shifts in solids: Combined multiple pulse NMR and magic-angle spinning

High-resolution nuclear magnetic resonance spectra of protons in rigid, randomly oriented solids have been measured using combined homonuclear dipolar decoupling (via multiple pulse techniques) and attenuation of chemical shift anisotropies (via magic-angle sample spinning). Under those conditions, isotropic proton chemical shifts were recorded for a variety of chemical species, with individual linewidths varying from about 55 to 110 Hz (1–2 ppm). Residual line broadening was due predominately to (i) magnetic-field instability and inhomogeneity, (ii) unresolved proton–proton spin couplings, (iii) chemical shift dispersion, (iv) residual dipolar broadening, and (v) lifetime broadening under the multiple pulse sequences used. The magnitudes of those effects and the current limits of resolution for this experiment in our spectrometer have been investigated. The compounds studied included organic solids (4, 4′-dimethylbenzophenone, 2, 6-dimethylbenzoic acid, and aspirin), polymers (polystyrene and polymethylmethacrylate), and the vitrain portion of a bituminous coal.

A study of 14N relaxation and nitrogen–proton spin coupling in Watson–Crick base pair models through Fourier transform measurements on NH proton spin–lattice relaxation in the rotating frame

Indirect measurements of nitrogen-14 nuclear spin-lattice relaxation times and direct proton coupling constants are presented together with carbon-13 T1 data for a series of alkyl-substituted nucleic acid bases and mixtures thereof in DMSO-d6. With the exception of the guanine NH nitrogen, which possibly experiences a decrease in the electric field gradient upon complexation with cytosine, no indications of significant changes in the electronic environment around the nitrogen nuclei were found for any combination of bases. Forsen–Hoffman spin saturation transfer experiments on the NH and NH2 protons are also presented.

A study of 14N relaxation and nitrogen–proton spin coupling in nucleoside, indole, and ε-caprolactam systems through Fourier transform measurements of NH proton spin–lattice relaxation in the rotating frame

Measurements of nitrogen-14 nuclear spin–lattice relaxation times and direct proton coupling constants are presented for a series of nucleosides and nucleoside bases and indole derivatives in DMSO-d6 and for a series of ε-caprolactam concentrations in CDCl3. The data were obtained through NH proton T1ρ measurements, a potentially useful alternative to direct 14N and 15N nuclear magnetic resonance for the study of e.g. NH hydrogen bonding.

ChemInform Abstract: CONFORMATIONS OF PEPTIDES IN SOLUTION BY NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY. PART 5. HOMOALLYLIC PROTON SPIN COUPLING IN LINEAR PEPTIDES

Chemischer InformationsdienstVolume 8, Issue 46 Physical Organic Chemistry ChemInform Abstract: CONFORMATIONS OF PEPTIDES IN SOLUTION BY NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY. PART 5. HOMOALLYLIC PROTON SPIN COUPLING IN LINEAR PEPTIDES D. B. DAVIES, D. B. DAVIESSearch for more papers by this authorABU KHALED MD. ABU KHALED MD., ABU KHALED MD. ABU KHALED MD.Search for more papers by this authorD. W. URRY, D. W. URRYSearch for more papers by this author D. B. DAVIES, D. B. DAVIESSearch for more papers by this authorABU KHALED MD. ABU KHALED MD., ABU KHALED MD. ABU KHALED MD.Search for more papers by this authorD. W. URRY, D. W. URRYSearch for more papers by this author First published: November 15, 1977 https://doi.org/10.1002/chin.197746090AboutPDF 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. Volume8, Issue46November 15, 1977 RelatedInformation

Sigma- and pi-electron contributions to the contact proton spin—spin coupling by the finite perturbation theory

A new method of dissecting the contact proton—proton spin coupling into the σ- and π-electron contributions is proposed in the framework of the INDO finite perturbation theory. For benzene, trans- and cis-butadienes, the π contributions obtained by this method show the sign alternation with respect to the number of bonds between the coupled protons, and are almost independent of the geometrical arrangement of bonds between the coupled protons. Theoretical results for unsaturated and aromatic six-membered ring systems are compared with experiment, and the relative significance of the σ- and π-electron contributions is discussed. The influence of heteroatoms on the proton coupling constant is investigated. The effect of the electronegativity of the heteroatoms is found mainly in the σ contribution, and scarcely in the π contribution.

Conformations of peptides in solution by nuclear magnetic resonance spectroscopy. Part 5. Homoallylic proton spin coupling in linear peptides

Five bond proton spin coupling, 5J(HH), has been observed in some linear di- and tri-peptides with trans peptide bonds. Magnitudes of 5J(HH) were analysed in terms of homoallylic coupling using N-methylacetamide and NN-dimethylacetamide as standard compounds for groups antiperiplanar across peptide bonds. Together with 3J(HNCH) magnitudes the results for 5J(HH) can be used to limit the range of conformations (ϕ,ψ) for peptides in solution. Attention has been focused on two peptide conformations studied by 100 MHz 1H n.m.r. measurements of N-acetyl-L-alanyl-N-methylamide (C7 structure) and N-acetyl-L-valyl-glycyl-N-methylamide (β-turn) in different solvents. The conformational properties are compared with previous studies using X-ray crystallography, theoretical calculations, and spectroscopy (n.m.r., i.r.).