W. M. M. J. Bovee

Determination of molecular geometry, rotational diffusion constants and coupling constants between quadrupolar nuclei and protons by means of proton N.M.R.

Expressions are given for the longitudinal and transverse relaxation rates of the individual lines in a HRNMR proton AX2 spectrum. The relaxation mechanisms considered are dipolar and scalar interactions. Measurements are performed on the proton spectra of dilute degassed solutions of 2-amino pyrimidine in acetone-d6/D2O. The relative values are determined of the interproton distances in the AX2 system and of three rotational diffusion constants, Dxx , Dyy and Dzz . Two coupling constants between the ring protons and the 14N ring nitrogens are found to be 10·3 and 2·2 Hz.

Overall and internal motions of substituted benzenes studied by means of selective proton magnetic longitudinal relaxation time measurements

Selective T 1 measurements at different temperatures on seven substituted benzenes in CDCl3 solution show that for these molecules the rotational diffusion model applies. Anisotropic reorientation is important if the molecules contain two polar substituents para to each other. Anomalies in the temperature dependence of the T 1 values in a 2-methyl substituted phenol are explained by intermolecular hydrogen bonds, which strongly influence the molecular motion. In a 2,6-dimethyl substituted phenol this effect is absent. The correlation times and the energy barriers for methyl rotation of methyl and methoxy groups are determined. The reorientation of methoxy groups around the aryl oxygen bond is slower than the molecular motion. The T 1 values of ring protons and substituents can in some cases be used for spectral assignment. Expressions are given for the T 1 value of a ring proton relaxed by an ortho methyl or methoxy group and for the T 1 value of a reorienting methoxy group in the case of dipolar relaxati...

Synthesis and conformational analysis of the three 3,7-dimethylbicyclo[3.3.1]nonanes

Abstract The stereospecific synthesis of the three isomers of 3,7-dimethylbicyclo[3.3.1]nonane is described. The conformations of these compounds were studied with the use of 13C and 1H NMR spectroscopy. The 3β,7β-dimethyl derivative exists predominantly in the cc conformation, the 3α,7β-dimethyl derivative predominantly in the bc conformation, whereas 3α,7β-dimethylbicyclo[3.3.1]nonane exists as a mixture of the cb bc and the t-bb conformations, which rapidly interconvert. The ΔH and ΔS values for the conformational equilibrium in question, were determined by variable temperature13C NMR. The results of these experiments fit well with those of molecular mechanics. The enthalpies of activation of the interconversion of the various conformers of 3α,7α-dimethylbicyclo[3.3.1]nonane were also calculated and compared with those of the parent system bicyclo[3.3.1]nonane.

Selective proton magnetic T 1 measurements on 2,6-dimethyl-1,4-dimethoxybenzene

Selective T 1 measurements as a function of temperature on 2,6-dimethyl-1,4-dimethoxybenzene show that the dipolar interaction is the only effective relaxation mechanism. The rotational diffusion of the molecule is anisotropic, and the internal methyl group rotation of the 4-methoxy group is slower than that of the other methyl groups.

The structure of adducts of the three diastereoisomeric 1,4:3,6-dianhydrohexitols and lanthanide chelates in acetone

Abstract Gd(dpm)3 induced 1H and 13C relaxation rate enhancements showed that 1,4:3,6-dianhydro-D-glucitol (containing both a threo and an erythro -O-C-C-OH moiety) acts as a bidentate ligand for Gd(dpm)3. Coordination occurs selectively with the erythro -O-C-C-OH moiety. Involving the endohydroxyl group at C5 and the neighbouring ether oxygen. From a comparison of the Eu(fod)3 Induced 1H shifts of 1,4:3,6-dianhydro-D-glucitol with those of 1,4:3,6-dianhydro-D-mannitol and l,4:3,6-dianhydro-L-iditol it is concluded that the mannitol derivative (containing two erythro -O-C-C-OH moieties) forms analogous adducts, whereas the iditol derivative (containing two threo-O-C-C-OH moieties) has a low association constant.

Information obtainable from N.M.R. longitudinal relaxation as measured by a non-selective pulse sequence in an ABC system

From proton and carbon-13 longitudinal relaxation and 13C Overhauser enhancement measurements, the diffusion constants for the molecule acrylonitrile dissolved in carbon disulphide are found to be: Dxx =(0·11±0·06) × 1012 s-1; Dyy = (0·38 ± 0·12) × 1012 s-1; Dzz = (0·25 ± 0·02) × 1012 s-1 (the z axis is perpendicular to the molecular plane; the y axis is almost parallel to the C≡N bond). The diffusion and inertial principal axis systems are shown to be nearly coincident. From these diffusion constants, the nitrogen-14 quadrupolar coupling constant was calculated to be (4·9±0·8) MHz. Notwithstanding the fact that we can study more relaxation rates due to the presence of more lines in the proton ABC spectrum, as compared to an AB 2 or AX 2 system, we obtain less information. This is a result of the fact that the unknown spectral densities occur in linear combinations in the expression for the recovery rates. A procedure to obtain the maximum available information is described.

Proton nuclear magnetic longitudinal relaxation in a series of AB systems

The proton longitudinal relaxation rates of the lines of three AB systems, the methyl ester of cinnamic acid-D5 (A), the trideuteromethyl ester of cinnamic acid-D5 (B) and benzalacetone (C), in solution were determined. The following dipolar contributions to the relaxation were separated : intermolecular ones ; intramolecular ones between the A and B proton and between the A or B proton and other magnetic nuclei in the molecules. From the data it can be concluded that in compounds A and B the aryl and ethenyl planes are not parallel, while information is obtained about the position of the methoxy group. Molecule C occurs in two conformers between which an energy difference of at least 2·9 kJ/mol exists. Due to the inaccuracy in the experimental relaxation rates not all information is obtained that might in principle be extracted from them.