Application to liquid crystals of many spin dynamics in 13C-1H Cross-Polarization experiments.

A.K. CHATTAH

Strasbourg, France

Otro; Estadía Postdoctoral en el Institute Le Bel de la Universidad Louis Pasteur; 2002

Universidad Louis Pasteur - Proyecto ECOS-SUD

Esta charla se dió durante una visita de dos meses al Institute Le-Bel, Universidad Louis Pasteur de Strasbourg, France bajo el marco del convenio de colaboración Argentino-Francesa ECOS-SUD.

In this talk we discuss theoretical and experimental results about quantum dynamics in many spin system.

We study the Hamiltonian dynamics for a group of three spins 1/2 constituted by a carbon 13C coupled to two protons 1H under the conditions of a Cross- Polarization (CP) experiment, incorporating phenomenologically the relaxation effects produced for more remote spins.  These calculations allows us to analyze the behavior of the magnetization of 13C during the polarization transfer in a CP experiment with variable contact-time.

We perfom this experiment in the liquid crystal 4-n-octyl-4´-cyanobiphenil (8CB) which presents smectic and nematic mesophases. Groups of three spins coupled by effective (averaged by molecular tumbling) dipolar interactions can be found in the alkyl chain and also in the aromatic core of this molecule. The heteronuclear 13C-1H and homonuclear 1H-1H dipolar interactions have different influence in the polarization transfer and we can obtain them separately. It can also be noticed the influence of the relative sign of the dipolar interactions, in the frequency of the oscillations of 13C magnetization during CP. These results are compared with those obtained in a CP experiment where homonuclear interactions are canceled out (Lee-Goldburg condition).

In this talk we also mention a radio-frequency induced temperature increase observed during the CP contact-time (2-5 ms) in 8CB.

Our interest in liquid crystals arises because they are an intermediate step to study biological samples in natural conditions.