ABSTRACT
This chapter deals with the theory of time-correlation functions and spectral densities for different motional models, starting again with rigid, spherical objects and proceeding toward more complex systems. Real molecules are of course not rigid bodies but are subject to both small amplitude vibrations and more extensive internal motions. Nuclear spin relaxation properties reflect that fact and their measurements provide an excellent tool for studying molecular flexibility with atomic resolution. The calculation of the intramolecular correlation functions for the internal rotational diffusion model follows a similar approach to that used in preceding sections for the rigid body motion. In Woessner's original work, the orientation was specified in terms of directional cosines rather than Euler angles, which were employed by Luginbühl and Wuthrich and are used in this presentation. The vertical transitions represent the local motion, changing the orientation of a vector in the molecular frame.
