Double quantum coherence study of the dipolar-ordered states in liquid crystals

C. BONIN; H. SEGNORILE; C.E. GONZÁLEZ; R. H. ACOSTA; R.C. ZAMAR

Mainz, Alemania

Congreso; 27th Annual Meeting of the Magnetic Resonance Discussion Group of the Society of German Chemists; 2005

Asociación Alemana de Química

A relevant question is related to the applicability of spin thermodynamics to the small spin system of magnetically isolated molecules as is the case found in ordinary nematic liquid crystals (LC). In these mesophases there is a strong residual intramolecular spin-spin dipolar energy due to the high degree of orientational molecular order while the fast molecular motion averages out the intermolecular dipolar spin interactions.  It is possible to bring the spin system of LC into states of dipolar order by means of the Jeener-Broekaert pulse sequence. Recently it was shown that, by a suitable setting of the two preparation pulses, proton intrapair or interpair dipolar quasi-invariants can be created in the nematic phase. These states relax exponentially to thermal equilibrium with the molecular environment with different spin-lattice relaxation times T1D. The existence of two dipolar constants of the motion relies on the posiblity of classifying the proton interactions into “strong” and “weak”. For example, in 5CB and PAAd6  it is possible to explain the dipolar FIDs by assuming that each proton is strongly coupled only to its nearest neighbor, that is, forming a set of weakly interacting spin pairs [1]. However, this rough classification of the dipolar interactions into weakly interacting, uncorrelated spin pairs, that would attain the semi-equilibrium through fluctuations of the “local field” produced by the interpair interactions, does not seem consistent with the fact of characterizing the whole molecular spin system by a single dipolar temperature. To inquire further into this problem, we  analyze the transient dynamics of the spin system after preparing it in a non equilibrium state. In this regard, we studied the time evolution of the double quantum coherence (DQC) in 5CB using the pulse sequence designed by Emid, Smidt and Pines for selective DQC in solids [2].