Models for spin relaxation in dilute solutions of randomly coiled polymers

Models for the interpretation of magnetic relaxation in a randomly coiled macromolecule in dilute solution are considered. Expressions are given for the spin‐lattice relaxation time and the nuclear Overhauser enhancement resulting from modulation of the dipole–dipole interaction by three specific independent and mutually competitive types of macromolecular motion. Overall molecular rotation is characterized by a single exponential correlation time and internal anisotropic rotation by the well‐known Woessner theory. A simple description of backbone motion is presented based upon a three‐bond crankshaft motion and the concept of limited coupling among backbone bonds. The resulting correlation function is numerically similar to that recently proposed by Valeur, Jarry, Gény, and Monnerie from a somewhat different standpoint. Representative calculations for appropriate ranges of the model parameters are made for a 13C nucleus relaxing by heteronuclear dipole–dipole interaction with a single 1H nucleus.