Limiting behavior of2H spin lattice relaxation in molecules with multiple internal rotations

摘要

Expressions for the dependence of deuteron relaxation times for CD bond vectors on the number of internal rotations,Nthinsp;+thinsp;1, in polymeric chains and side chains of micelles and phospholipid vesicles have been derived for largeN. The expressions are valid for molecules in which : (i) the diffusion tensors have ellipsoidal symmetry,(ii) all angles between successive pairs of internal rotation axes are equal, and (iii) the internal rotations are independent and all diffusion coefficients for internal rotation are equal. For molecular entities with spherically symmetric rotational diffusion tensors, the asymptotic relation simplifies to the following relation: whereDOis the rotation diffusion coefficient,Dthe diffusion coefficient for internal rotation, ohgr;0is the magnetic resonance frequency, andQDis the quadropole coupling constant for the CD bond vector. For hydrocarbon chains the relaxation rate is within 10percnt; of its asymptotic value for relatively smallN(about 6) and converges rapidly to the asymptotic relaxation rate asNis increased.