Quasiequilibrium States in Thermotropic Liquid Crystals Studied by Multiple-Quantum NMR

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

JOURNAL OF CHEMICAL PHYSICS

AMER INST PHYSICS

Año: 2009 vol. 130 p. 24501 - 24510

0021-9606

Previous work showed that by means of the Jeener–Broekaert (JB) experiment, two quasi-equilibrium states can be selectively prepared in the proton spin system of thermotropic nematic liquid crystals (LCs) in a strong magnetic field. The similarity of the experimental resultsobtained in a variety of LC in a broad Larmor frequency range, with crystal hydrates, supports theassumption that also in LC the two spin reservoirs, into which the Zeeman order is transferred,originate in the dipolar energy and that they are associated with a separation in energy scales: Aconstant  of  motion  related  to  the  stronger  dipolar  interactions (S) ,  and  a  second  one (W)corresponding to the secular part of the weaker dipolar interactions with regard to the Zeeman andthe   strong   dipolar   part.   We   study   the   nature   of   these   quasi-invariants   in   nematic   5CB(4´ -pentyl-4-biphenyl-carbonitrile) and   measure   their   relaxation   times   by   encoding   themultiple-quantum coherences of the states following the JB pulse pair on two orthogonal bases, Zand X. The experiments were also performed in powder adamantane at 301 K which is used as areference compound having only one dipolar quasi-invariant. We show that the evolution of thequantum  states  during  the  buildup  of  the  quasiequilibrium  state  in  5CB  prepared  under  the S condition  is  similar  to  the  case  of  powder  adamantane  and  that  their  quasiequilibrium  density operators have the same tensor structure. In contrast, the second constant of motion, whose explicit operator form is not known, involves a richer composition of multiple-quantum coherences of even order on the X basis, in consistency with the truncation inherent in its definition. We exploited the exclusive presence of coherences of 4, 6, 8, besides 0 and  2 under the W condition to measure the spin-lattice relaxation time TW  accurately, so avoiding experimental difficulties that usually impair dipolar order relaxation measurement such as Zeeman contamination at high fields and also superposition of the different quasi-invariants. This procedure opens the possibility of measuring  the  spin-lattice  relaxation  of  a  quasi-invariant  independent  of  the  Zeeman  and S reservoirs, so incorporating a new relaxation parameter useful for studying the complex molecular dynamics in mesophases. In fact, we report the first measurement of TW in a LC at high magnetic fields. Comparison of the obtained value with the one corresponding to a lower field (16 MHz) points  out  that  the  relaxation  of  the W -order  strongly  depends  on  the  intensity of the external magnetic field,  similarly  to  the  case  of  the S reservoir,  indicating  that  the  relaxation  of  theW -quasi-invariant  is  also  governed  by  the  cooperative  molecular  motions.