Creation and Relaxation of Dipolar Order in Liquid Crystals Probed via the Evolution of Multiple Quantum Coherences

L. BULJUBASICH; C. J. BONÍN; C. E. GONZÁLEZ; G. A. MONTI; R. C. ZAMAR; R. H. ACOSTA

Conferencia; Euromar Magnetic Resonance Conference; 2007

Multiple Quantum (MQ) coherence excitation and detection has been generally used for cluster  size  determination  and  for  probing  dynamics  in  many  body  systems  in  solids. However, these techniques cannot be used for the study of the spin dynamics under the action of a Hamiltonian that conserves the coherence number. For instance, during the free  evolution  of  the  NMR  signal  the  system  evolves  under  the  truncated  dipolar Hamiltonian  and  the  coherence  number  remains  as  +1  and  -1,  while  the  correlated number  of  spins  changes.  Another  situation  where  standard  MQ  techniques  are  of limited use is on the study of dipolar order. Recently a method was proposed in which it is  possible  to  measure  the  evolution  of  MQ  coherences  during  the  creation  of  dipolar order  in  solids  recorded  in  two  non-commuting  bases,  X  and  Z  [1].  The  creation  of dipolar order is clearly identified by the stabilization, after a transitory period of the order of 5 times T 2 *, via a relation of 1.5 between the intensity of +2 and -2 coherences with respect to order zero coherence on the X basis. The decay of these coherences can be then monitored in order to obtain a measure of the dipolar relaxation time, T 1D , which is free  of  the  Zeeman  contamination  usually  present  in  the  determination  of  these relaxation times with the Jeener-Broekaert (JB) pulse sequence.  In the present work we apply this method to the study of the creation and relaxation of dipolar  order  in  thermotropic  liquid  crystal  5CB  in  the  nematic  phase.  This  system presents a distribution of protons on its molecule that enables the identification of strong interactions  between  pairs  of  spins  (intrapair)  and  other  much  weaker  interactions (interpair) [2]. The transitory to the establishment of the intrapair dipolar order in 5CB is compared with that obtained in Adamantane powder, which presents comparable dipolar interactions  between  equivalent  spin  pairs.  The  relaxation  time  (T 1D-intra )  obtained  with the method is contrasted with the usual JB sequence. Creation of interpair dipolar order is  observed  to  be  slower  than  the  intrapair  and  composed  of  higher  order  coherence numbers. The relaxation time T 1D-inter  free of Zeeman contamination is measured for the first time in a 7T magnetic field.