Proton Nuclear Magnetic Resonance Shifts Research Articles

Cooperativity in a cycloalkane-1,2/1,3-polyol corona: Topological hydrogen bonding in 1,2-diol motifs.

A corona, consisting of 18 carbon atoms bearing 12 hydroxy groups in a continuous hydrogen-bonded chain, is built up by alternating degenerate conformations of alternating alkane-1,2-diol and 1,3-diol motifs. Geometries, proton nuclear magnetic resonance shifts and interaction energies for the dodecahydroxycyclo-octadecane and selected fragments are determined by density functional calculations at the B3LYP/6-311+G(d,p) level. Cooperative effects of O-H⋯O-H bonding are evident from the simple juxtaposition of these two motifs with a common OH group in butane-1,2,4-triol conformers. Bracketing a 1,2-diol motif with two 1,3-diol motifs in hexane-1,3,4,6-tetrol leads to a structure in which the 1,2-diol motif displays a bond critical point for hydrogen bonding. This is associated with enhancement of the shift of the hydrogen-bonded OH proton and of the corresponding H⋯O interaction energy. The full corona has a complete outer ring of O-H⋯O-H bond paths, and an inner ring of bond paths, due to C-H⋯H-C hydrogen-hydrogen bonding, which result in a central ring critical point. The topological O-H⋯O-H hydrogen bond, never seen in simple alkane-1,2-diols, is associated with cooperative enhancement of the H⋯O interaction energy, but this is not a necessary condition for a bond path: values for topological C-H⋯H-C hydrogen-hydrogen bonds can be as low as -0.4 kcal mol-1 .

DNP-NMR of surface hydrogen on silicon microparticles

Dynamic nuclear polarization (DNP) enhanced nuclear magnetic resonance (NMR) offers a promising route to studying local atomic environments at the surface of both crystalline and amorphous materials. We take advantage of unpaired electrons due to defects close to the surface of the silicon microparticles to hyperpolarize adjacent 1H nuclei. At 3.3 T and 4.2 K, we observe the presence of two proton peaks, each with a linewidth on the order of 5 kHz. Echo experiments indicate a homogeneous linewidth of ∼150−300 Hz for both peaks, indicative of a sparse distribution of protons in both environments. The high frequency peak at 10 ppm lies within the typical chemical shift range for proton NMR, and was found to be relatively stable over repeated measurements. The low frequency peak was found to vary in position between −19 and −37 ppm, well outside the range of typical proton NMR shifts, and indicative of a high-degree of chemical shielding. The low frequency peak was also found to vary significantly in intensity across different experimental runs, suggesting a weakly-bound species. These results suggest that the hydrogen is located in two distinct microscopic environments on the surface of these Si particles.

1H NMR spectra of alcohols and diols in chloroform: DFT/GIAO calculation of chemical shifts.

Proton nuclear magnetic resonance (NMR) shifts of aliphatic alcohols in chloroform have been computed on the basis of density functional theory, the solvent being included by the integral-equation-formalism polarisable continuum model of Gaussian 09. Relative energies of all conformers are calculated at the Perdew, Burke and Ernzerhof (PBE)0/6-311+G(d,p) level, and NMR shifts by the gauge-including atomic orbital method with the PBE0/6-311+G(d,p) geometry and the cc-pVTZ basis set. The 208 computed CH proton NMR shifts for 34 alcohols correlate very well with the experimental values, with a gradient of 1.00 ± 0.01 and intercept close to zero; the overall root mean square difference (RMSD) is 0.08 ppm. Shifts for CH protons of diols in chloroform are well correlated with the theoretical values for (isotropic) benzene, with similar gradient and intercept (1.02 ± 0.01, -0.13 ppm), but the overall RMSD is slightly higher, 0.12 ppm. This approach generally gives slightly better results than the CHARGE model of Abraham et al. The shifts of unsaturated alcohols in benzene have been re-examined with Gaussian 09, but the overall fit for CH protons is not improved, and OH proton shifts are worse. Shifts of vinyl protons in alkenols are systematically overestimated, and the correlation of computed shifts against the experimental data for unsaturated alcohols follows a quadratic equation. Splitting the 20 compounds studied into two sets, and applying empirical scaling based on the quadratic for the first set to the second set, gives an RMSD of 0.10 ppm. A multi-standard approach gives a similar result.

Diels-Alder Adducts of 3,6-Dibromophencyclone with Short-Chain N-n-Alkylmaleimides: 1H and 13C Nuclear Magnetic Resonance Studies of Hindered Rotations and Magnetic Anisotropy, and Ab Initio Calculations for Optimized Structures

3,6-Dibromophencyclone, 2, reacted with N-ethylmaleimide, 3a; N-n-propylmaleimide, 3b; and N-n-butylmaleimide, 3c; to form the corresponding Diels-Alder adducts, 4a, 4b, and 4c. The nuclear magnetic resonance (NMR) spectra of the adducts were studied at ambient temperatures at 300 MHz for proton and 75 MHz for carbon-13. Full proton assignments were achieved by high-resolution COSY45 spectra for the aryl proton regions. Rigorous assignments for protonated carbons were obtained with the heteronuclear chemical shift correlation spectra (HETCOR). Slow exchange limit (SEL) spectra were observed for both proton and carbon-13 NMR for each adduct, with slow rotation on the NMR timescales for the unsubstituted bridgehead phenyl groups. Endo Diels-Alder adduct stereochemistry was supported by substantial magnetic anisotropic shielding effects in the 1H NMR spectra of the alkyl groups. Proton NMR shifts are compared with those previously reported for the corresponding adducts, 5b and 5c, obtained from 3b and 3c, respectively, with the parent compound, phencyclone, 1. Results of ab initio molecular modeling calculations at the Hartree-Fock level using the LACVP* basis set for conformers of the dibrominated adducts, 4a-4c, are presented, together with HF/6-31G* results for the non-brominated adducts, 5a, 5b, and 5c. Novel aspects of this present work include: (a) attempts to quantitatively evaluate alkyl proton NMR shielding magnitudes in the adducts, relative to maleimide precursors, and (b) use of ab initio Hartree-Fock level calculations to try to reconcile adduct geometries with the observed shielding magnitudes. Our results here complement and extend studies of: (a) adducts of the parent phencyclone with straight-chain N-n-alkylmaleimides, and (b) adducts of 3,6-dibromophencyclone with other symmetrical dienophiles.

Configuration analysis of unsaturated hydroxy fatty acids

Stereoisomers of methyl ricinoleate, methyl isoricinoleate, and related methyl esters of bis-homoallylic hydroxy fatty acids were reacted with ( R)- and ( S)-α-naphthylethyl isocyanate to form diastereomeric carbamates that separated well on silica gel high-performance liquid chromatography. Proton nuclear magnetic resonance shifts of the carbomethoxymethyl protons were used to assign configuration to the alcohol component of the carbamate derivative. Combining high-performance liquid chromatography with proton nuclear magnetic resonance spectroscopy may allow one to determine the configuration of naturally occurring hydroxy fatty acids as well as to determine configurational purity.

Determination of the absolute configuration of chiral secondary alcohols from tetra-O-acetylglucosidation-induced proton nuclear magnetic resonance shifts

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTDetermination of the absolute configuration of chiral secondary alcohols from tetra-O-acetylglucosidation-induced proton nuclear magnetic resonance shiftsRamine Faghih, Catherine Fontaine, Isao Horibe, Paulo M. Imamura, Gabor Lukacs, Alain Olesker, and Shujiro SeoCite this: J. Org. Chem. 1985, 50, 24, 4918–4920Publication Date (Print):November 1, 1985Publication History Published online1 May 2002Published inissue 1 November 1985https://doi.org/10.1021/jo00224a053RIGHTS & PERMISSIONSArticle Views137Altmetric-Citations11LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (400 KB) Get e-Alerts Get e-Alerts

Isotropic proton nuclear magnetic resonance shifts of nickel(II) complexes of β-thioketoesters

Isotropic proton nuclear magnetic resonance shifts in pyridine adducts of nickel(II) complexes of β-thioketoesters were investigated. The paramagnetic mono-pyridine adducts which have different spin delocalization mechanism from paramagnetic di-pyridine adducts were detected when the pyridine-complex ratio was low. The five-coordinated complex was also detected by the spectrophotometric method. It was suggested that π delocalization mechanism plays a significant role for the delocalization of unpaired electron.

Isotropic proton nuclear magnetic resonance shifts of adducts of bis(β-ketoesterato)nickel(II) complexes

The isotropic proton magnetic resonance shifts of the pyridine base adducts of bis(β-keto-esterato)nickel(II) complexes were measured. The slow exchange of γ-picoline was observed at room temperature. When a temperature was raised, the relative shift ratio among three signal of γ-picoline closely resembled to that in the case of bis(acetylacetonato)nickel(II) complex. With regard to the spin delocalization onto β-ketoesters, it was suggested by INDO calculations that ligand-to-metal β spin transfer was mainly responsible for it.

External transverse magnetic field effect on electrolyte diffusion in alkali chloride-water solutions

The observed external transverse magnetic field effect on electrolyte diffusion in diamagnetic alkali chloride (LiCl, NaCl; KCl and CsCl)-water solutions, expressed as the fractional arithmetic average integral diffusion coefficient, D *=[(⪡ D 0 H > -< D 0 > )/< D 0 > ;]×10 2; has been correlated with structural (hydration number, viscosity and ionic limiting equivalent conductance) and microdynamical (proton nuclear magnetic relaxation times and proton nuclear magnetic resonance shifts) parameters expressing the configurational rearrangement modes of the solvent and solute molecules forming the aqueous solution.

Proton Nuclear-Magnetic-Resonance Study of Low-Spin Ferriprotoporphyrin(IX) Dimethyl Ester

Proton nuclear magnetic resonance shifts, spin-lattice and spin-spin relaxation times have been measured of low-spin bis-pyridine ferriprotoporphyrin(IX) dimethyl ester in chloroform. From the relaxation behavior the hyperfine coupling constant has been obtained and the contact term of the chemical shift was calculated. Deviations between measured and calculated chemical shifts may be attributed to second-order Zeeman interactions. The geometry of pyridine coordinated to the fifth and sixth position of ferriprotoporphyrin(IX) dimethyl ester was estimated from measured relaxation rates. From the non-exponential decay of the Mz magnetization a mean lifetime of taub = 50 ms for pyridine attached to low-spin ferriprotoporphyrin(IX) dimethyl ester was found at 253 K.

Lanthanide-induced shifts in proton nuclear magnetic resonance spectra. VI. 3-(.alpha.-Naphthyl)-5,5-dimethylcyclohexanone and derived alcohols. Synthesis and stereochemical studies by means of lanthanide-induced proton nuclear magnetic resonance shifts

Lanthanide-induced shifts in proton nuclear magnetic resonance spectra. VI. 3-(.alpha.-Naphthyl)-5,5-dimethylcyclohexanone and derived alcohols. Synthesis and stereochemical studies by means of lanthanide-induced proton nuclear magnetic resonance shifts

Proton nuclear magnetic resonance shifts for water containing alkylammonium ions

In contrast with previous results, proton chemical shifts for water are found to move to lower applied fields as the size of the alkyl group in symmetrical tetra-alkylammonium ions is increased for temperatures below 300 K, although the trend is reversed at higher temperatures.

Anomalous temperature dependence of isotropic proton nuclear magnetic resonance shifts in paramagnetic chromium(II) and cobalt(II) complexes

Anomalous temperature dependence of isotropic proton nuclear magnetic resonance shifts in paramagnetic chromium(II) and cobalt(II) complexes

Paramagnetic proton nuclear magnetic resonance shifts of metmyoglobin, methemoglobin, and hemin derivatives.

Paramagnetic proton nuclear magnetic resonance shifts of metmyoglobin, methemoglobin, and hemin derivatives.

Concerning the Electron-Withdrawing Power and the Electronegativity of Groups

The polar substituent constants, σ*, obtained recently by Taft from reactivity considerations are discussed. Several qualitative relationships of σ* values to substituent constitution are established. These relationships relate quantitative correlations of the effect of structure on reactivity to concepts of bonding and electronegativity. The Mulliken concept of electronegativity is supported. The relatively large electron-withdrawing powers of Cl, Br, and I in bonds to carbon suggest the participation of the d orbitals of the valence shell. A discrepancy is noted between σ* values (and Mulliken's electronegativities) and the group electronegativities proposed by Dailey and Shoolery on the basis of proton nuclear magnetic resonance shifts in ethyl and methyl derivatives.