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Spin-lattice relaxation : a probe for molecular geometry and motion in solution

University of British Columbia

The relevant theory and experimental protocol whereby proton spin-lattice relaxation rates (R₁-values) can be used to assign quantitatively the geometry of molecules in solution are described. It is shown that proton R₁-values can provide an accurate measure of solution geometry for rigid molecules which have well dispersed proton n.m.r. spectra, and for which the interproton distances do not span an excessive range. The method is based on identifying the magnitudes of specific, dipole-dipole interproton relaxation contributions (p-values) from measured initial slope proton R₁-values. These p-values can be obtained from either (a) a combination of non-selective, single-selective, double-selective and triple-selective pulse experiments, or (b) selective deuteration. The interproton distances (r) can then be explicitly calculated via the dipole-dipole formalism (ρ ~ r⁻⁶). The interproton distances between the three bicycloheptene ring protons of 1,2,3,4,7,7-hexachloro-6-0-exo-benzoyl-bicyc1o[2.2.1]hept-2-ene (1), determined by method (a), were in excellent agreement with those obtained from computer simulation and Dreiding molecular models. Initial slope, non-selective R₁ values for the protons of 1,2,3,4-tetra-O-trideuterioacetyl-β-D-arabinopyranose (2_), its 5,5-dideuterio-(3) and both isomeric 5-deuterio-derivatives (;4,5_) were determined at 400 MHz. Simple intercomparisons between these data {method (b)} gave ρ-values, from which interproton distances were calculated; these values were in very close agreement with those obtained by neutron diffraction. Method (a) was also used to evaluate the ρ-values for 2; these data were essentially the same as those obtained by (b). The quality control experiments necessary for quantitative conformational analysis are illustrated. These includes (a) a practical definition of an initial slope R₁-value, (b) an evaluation of the extent of dipole-dipole relaxation contributions, and (c) the use of carbon-13 R₁-values to detect the presence or absence of anisotropic motion. It is demonstrated that a combination of non-selective and single-selective relaxation experiments provides quantitative information concerning the validity of the initial slope approximation and the extent to which each proton is relaxed via the dipole-dipole mechanism. A comprehensive evaluation of the applicability and limitation of carbon-13 R₁-values along with deuterium R₁ values as "probes" for molecular motion is also presented. Finally, preliminary surveys of the application of the method to systems having substantial anisotropic motion, systems having complex, second order spectra, and systems with conformational time-averaging are given to place the methodology into chemical contexts which were not a priori designed to be optimized for the approach described herein.

Text

2010-03-18

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http://hdl.handle.net/2429/22070

Chemistry

University of British Columbia

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