Proton relaxation measurement, interproton

Clearly though, even a purely qualitative evaluation of the geometric influences of the anomeric and exo-anomeric effects based on a new independent measurement tool, has some considerable relevance and further studies on this topic are being actively pursued at this time. And to leave the more sceptical readers of this article with some positive food-for-thought, we summarise below in Table V the interproton distances, determined (22.) by proton relaxation measurement, for an organic molecule, the bicycloheptene portion of XII which is, according to all presently available criteria, tumbling isotropically. 

This simple relaxation theory becomes invalid, however, if motional anisotropy, or internal motions, or both, are involved. Then, the rotational correlation-time in Eq. 30 is an effective correlation-time, containing contributions from reorientation about the principal axes of the rotational-diffusion tensor. In order to separate these contributions, a physical model to describe the manner by which a molecule tumbles is required. Complete expressions for intramolecular, dipolar relaxation-rates for the three classes of spherical, axially symmetric, and asymmetric top molecules have been evaluated by Werbelow and Grant, in order to incorporate into the relaxation theory the appropriate rotational-diffusion model developed by Woess-ner. Methyl internal motion has been treated in a few instances, by using the equations of Woessner and coworkers to describe internal rotation superimposed on the overall, molecular tumbling. Nevertheless, if motional anisotropy is present, it is wiser not to attempt a quantitative determination of interproton distances from measured, proton relaxation-rates, although semiquantitative conclusions are probably justified by neglecting motional anisotropy, as will be seen in the following Section. 

The second separation method involves n.O.e. experiments in combination with non-selective relaxation-rate measurements. One example concerns the orientation of the anomeric hydroxyl group of molecule 2 in Me2SO solution. By measuring nonselective spin-lattice relaxation-rat s and n.0.e. values for OH-1, H-1, H-2, H-3, and H-4, and solving the system of Eq. 13, the various py values were calculated. Using these and the correlation time, t, obtained by C relaxation measurements, the various interproton distances were calculated. The distances between the ring protons of 2, as well as the computer-simulated values for the H-l,OH and H-2,OH distances was commensurate with a dihedral angle of 60 30° for the H-l-C-l-OH array, as had also been deduced by the deuterium-substitution method mentioned earlier. 

This simple theoryis based on the expectation that, to a reasonable degree of approximation, proton-proton, dipolar contributions to the measured spin-lattice relaxation-rate are pairwise additive and decrease as a simple sixth power of the interproton distance. The simplified version of the dipole-dipole mechanism is summarized in the following two equations for spin i coupled intramolecularly with a group of spins j... 

Interproton distances of 0-ceIIobiose (see Ref. 49) error 0.01 A. Interproton distances of 1,6-anhydro- -D-glucopyranose (see Ref. 49) error 0.01 A. Interproton distances of -cellobiose octaacetate (see Ref. 49) error 0.05 A. Interproton distances of 2,3,4-tri-0-acetyl-l,6-anhydro- -D-glucopyranose (see Ref. 49) error 0.05 A. Error calculations based on the errors of the measured quantities in Eqs. 18 and 21. Interproton distances calculated from the relaxation parameters of the methylene protons. 

The significance of n.m.r. spectroscopy for structural elucidation of carbohydrates can scarcely be underestimated, and the field has become vast with ramifications of specialized techniques. Although chemical shifts and spin couplings of individual nuclei constitute the primary data for most n.m.r.-spectral analyses, other n.m.r. parameters may provide important additional data. P. Dais and A. S. Perlin (Montreal) here discuss the measurement of proton spin-lattice relaxation rates. The authors present the basic theory concerning spin-lattice relaxation, explain how reliable data may be determined, and demonstrate how these rates can be correlated with stereospecific dependencies, especially regarding the estimation of interproton distances and the implications of these values in the interpretation of sugar conformations. 

In spite of their common genesis, proton Rj -values provide quite different, albeit complimentary,structural information them those of 13c nuclei. We have developed (5) four different methods by which the magnitudes of Tndividual, interproton relaxation contributions may be determined accurately. It is obvious that, if one determines the relative magnitudes of the contributions which two donor protons Dl and D2 make to the relaxation of a common receptor proton, R, this gives a direct measure of the ratio of the interproton separations, as depicted in Equation Two. Clearly this equation will be valid only if both of the inter-nuclear vectors Dl- -R and D2-+ft have the same x -value,... 

It will be recalled from Equation One that the relaxation efficiency of a donor nucleus is proportional to the square of its magnetogyric ratio - which is why protons are the dominant source of relaxation. Since the magnetogyric ratio of deuterium is only one sixth that of the proton, it follows that replacement of a proton by a deuterium atom effectively eliminates the relaxation contribution from that site. (Because deuterium has a nuclear spin of unity whereas the proton has a spin of one-half, the relaxation contribution from a deuterium nucleus is 6% that of a proton located at precisely the same position.) Thus con5)ari-son of the proton R -values of any derivative with those of a specifically deuterated analog provides a direct measure of a specific interproton relaxation contribution. 

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