Accurate determination of interference terms between carbon-proton dipolar interactions and carbon or proton chemical shift anisotropy from longitudinal carbon-13 relaxation studies

Abstract

CSA (chemical shift anisotropy)-dipolar cross-correlation densities (also named CSA-dipolar interference terms) may couple longitudinal nuclear magnetizations of a two spin 1/2 system (in the present case, carbon-13 <I ^C _z > and proton <I ^H _z > magnetizations) with the magnetization mode <2I ^H _z I ^C _z >. A pulse sequence has been devised for converting <2I ^H _z >, created by the relaxation arising from CSA-dipolar interference, into observable transverse carbon-13 magnetization. Because of the weakness of interference terms, drastic precautions have been taken to eliminate all other contributions. An account of the theory underlying these experiments is given. It includes aspects of spin dynamics and the derivation of cross-correlation spectral densities expressed within the ‘two-step model’ (or the ‘model free approach’), which is particularly well suited for treating dynamics of large molecules or of organized systems, A micellized surfactant, which possesses an aromatic ring bearing the polar head, provides a good example since it involves a slow motion capable of enhancing effects of these CSA-dipolar interference terms; one of the aromatic CH bonds constitutes the two spin system under investigation. Experimental results are discussed in terms of carbon and proton chemical shift anisotropies and of order parameters.