Quasi-equilibrium 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

AIP

Año: 2009 vol. 130 p. 1 - 10

0021-9606

Previous work showed that by means of the Jeener–Broekaert (JB) experiment, twoquasiequilibrium states can be selectively prepared in the proton spin system of thermotropicnematic 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 and the 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 a reference 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 densityoperators 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 TWaccurately, so avoiding experimental difficulties thatusually impair dipolar order relaxation measurement such as Zeeman contamination at high fieldsand also superposition of the different quasi-invariants. This procedure opens the possibility ofmeasuring 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 moleculardynamics in mesophases. In fact, we report the first measurement of TWin 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 the W-quasi-invariant is also governed by the cooperative molecular motions.