Magnetic Resonance in Chemistry

doi: 10.1002/mrc.1260240701pmid: N/A

Ando, I.; Webb, G. A.

doi: 10.1002/mrc.1260240702pmid: N/A

Recent advances in the field of high‐resolution NMR spectroscopy in both pure liquids and solutions, under high pressure, are reviewed. Experimental techniques employed for making the NMR measurements at high pressures are discussed. An overview of high‐pressure NMR studies as applied to intramolecular dynamics, structures of liquids, molecular conformations and intermolecular interactions is presented.

Alvarez‐Ibarra, C.; Arias‐Pérez, M. S.; de Andrés, A; Balcázar, J. L.

doi: 10.1002/mrc.1260240703pmid: N/A

The conformational analysis of a chiral acyclic ketone with one asymmetric carbon, (±)‐2,2,6,6‐tetramethyl‐5‐phenylheptan‐3‐one, has been achieved for the first time by means of an x‐ray diffraction study and the analysis of 1H1H and 13C1H geminal and vicinal coupling constants and 13C1H direct coupling constants. The application of the Durette‐Horton, Pachler and Altona empirical equations supports the hypothesis of a marked predominance in solution of the conformation observed in the solid state.

Holak, Tadeusz A.; Aksnes, Dagfinn W.; Grahn, Hans; Edlund, Ulf

doi: 10.1002/mrc.1260240704pmid: N/A

A 13C electron–nuclear spin–lattice relaxation study showed that the interaction between alkyl‐substituted aromatics and a paramagnetic relaxation reagent, tris(acetylacetonato)chromium(III), is purely electrostatic, i.e. of a dipole–dipole or dipole–induced dipole type. Except for aromatics having bulky alkyl groups, the preferred chelate position is close to the positive end of the dipole, i.e. close to the α‐position. However, the major electron–nuclear mechanism is non‐specific, and the relaxation rate is mainly controlled by the mutual translational/rotational diffusion of the two species. Paramagnetic shifts induced on addition of Cr(acac)3 support the suggested interaction mechanism.

Grindley, T. Bruce; Lu, Mu‐Jian; Colpa, Johannes P.; Wu, Shi‐Ming; Buncel, Erwin

doi: 10.1002/mrc.1260240705pmid: N/A

The 1H and 13C NMR spectra of 4,4′‐bis(N‐isopropyl‐N‐nitrosoamino)biphenyl at 22°C were consistent with the presence of three isomers due to slow rotation about the N—N bonds of the two N‐nitroso groups. The structures of the three isomers were assigned by means of their symmetries and from the known effects of the two orientations of the N‐nitroso group on 1H and 13C NMR chemical shifts. Each conversion of an N‐nitroso group from anti to syn to an aryl group was associated with the same free energy difference (0.26 kcal mol−1) after symmetry corrections were made. The changes in the 13C NMR spectra recorded from 95 to 150°C were simulated using a program for classical four‐site exchange. The potential barrier to rotation was calculated to be 20.76±0.06 kcal mol−1 at 400 K.

Laatikainen, Reino

doi: 10.1002/mrc.1260240706pmid: N/A

1H NMF spectra of 2‐hydroxy‐ and 2‐methoxy‐benzyl methy1 ethers were measured in various solvents and solvent mixtures. Precise nJ(CH2,H), where n = 4, 5 or 6, and 5J(OH,H) long‐range couplings were applied to the characterization of the conformational behaviour of the systems. Qualitatively, the compounds appear to exist in two Principal conformations: in a closed syn with an internal hydrogen‐bonded form, and/or in an open anti conformation with the CH2OR group tilting fairly freely in the benzene plane. The open form is favoured in acetone and dioxane. The computational strategies used in the spectral analysis of the long‐range couplings and in the analysis of the solvent‐solute association equilibria are described.

Laatikainen, Reino; Tuppurainen, Kari

doi: 10.1002/mrc.1260240707pmid: N/A

A procedure for combining experimental NMR information and, for example, molecular mechanics is described. An iterative computer program and a strategy based on the approach are applied for the determination of the torsional behaviour of the CH2 group in 2‐hydroxy‐ and 2‐methoxy‐benzyl methyl ethers. The analysis gives fairly well defined potentials of the torsion and angular density functions. The least‐squares model obtained for the 5J(CH2,H) coupling is in accordance with previous observations. STO‐3G level ab initio calculations for 2‐hydroxybenzyl methyl ether are reported and are in reasonable agreement with the dynamic model.

Laschi, Franco; Rossi, Claudio

doi: 10.1002/mrc.1260240708pmid: N/A

The X‐band ESR spectra of manganese(II) complexes with nucleobase constituents such as pyridine, imidazole and purine have been analysed at various metal to ligand molar ratios and pH values. Mixed complexing Mn(II)H2ON base, together with the direct N base coordination of the metal ion, have been characterized on the basis of the ESR parameters a, ΔH, intensity and line shape. The ESR evidence for a direct interaction of pyridine and imidazole with Mn(II) is reported.

Sándor, Péter; Radics, Lajos

doi: 10.1002/mrc.1260240709pmid: N/A

Isotropic carbon–carbon coupling constants over one and three chemical bonds have been measured in ortho‐, meta‐ and para‐substituted anilines, C6H4NH2X (X = F, Cl, Br, I, OCH3, NH2, NO2, CN, COOCH3, CH3), in natural abundance. The experimental data are compared with J(CC) values calculated by assuming additivity of substituent coupling increments. Additivity conditions are discussed. Non‐additivities are interpreted, on a qualitative level, in terms of mesomeric and inductive interactions between the substituents.

Pons, Miquel; Chapman, Dennis

doi: 10.1002/mrc.1260240710pmid: N/A

The conformational analysis of short polymer chains involves the use of statistical models, and their predictions must be checked against experimental information. Ytterbium‐induced shifts in the 13C NMR spectra of fatty acids provide a non‐perturbing, straightforward method of simultaneously obtaining positional information about all the carbon atoms up to nine bonds away from the comuplexation point. Experimental results for saturated and unsaturated C‐18 fatty acids are compared with the predictions of a restricted isomeric states (RIS) model.