NMR proton spin dynamics in thermotropic liquid crystals subject to multipulse excitation

R. H. ACOSTA; R.C. ZAMAR; G.A. MONTI

PHYSICAL REVIEW E - STATISTICAL PHYSICS, PLASMAS, FLUIDS AND RELATED INTERDISCIPLINARY TOPICS

Año: 2003 vol. 68 p. 41705 - 41705

1063-651X

Previous experiments of NMR spin-lattice relaxation times as a function of the Larmor frequency, asmeasured with the field-cycling technique ~FC!, were shown to be very useful to disentangle the variousmolecular motions, both local and collective, that dominate the relaxation in different time scales in liquidcrystals. However, there are many examples where the known theoretical models that represent the molecularrelaxation mechanisms cannot be fitted to the experimental trend in the region of low fields, making it difficultto obtain reliable values for the spectral densities involved, especially for the cooperative motions whichdominate at low frequencies. In some cases, these anomalies are loosely ascribed to ‘‘local-field’’ effects but,to our knowledge, there is not a detailed explanation about the origin of these problems nor the range offrequencies where they should be expected. With the aim of isolating the dipolar effects from the influence ofmolecular dynamics, and taking into account the previous results in solids, in this work we investigate theresponse of the proton spin system of thermotropic liquid crystals 4-pentyl-48-cyanobiphenyl ~5CB! and4-octyl-48-cyanobiphenyl ~8CB! in nematic and smectic A phases, due to the NMR multipulse sequence90y+ -( t- ux- t)N . The nuclear magnetization presents an early transient period characterized by strong oscillations,after which a quasistationary state is attained. Subsequently, this state relaxes towards internal equilibriumover a time much longer than the transverse relaxation time T2. As occurs in solids, the decay time of thequasistationary state T2e presents a minimum when the pulse width ux and the offset of the radiofrequency areset to satisfy resonance conditions ~spin-lock!. When measured as a function of the pulse spacing t in ‘‘onresonance’’experiments, T2e shows the behavior expected for cross relaxation between the effective Zeemanand dipolar reservoirs, in accordance with the thermodynamic theory previously developed for solids. Particularly,for values of t comparable with T2, the relaxation rate follows a power law T2e} t22, in all the observedcases, for the resonance conditions ux5 p/3 and equivalent frequency ve5 p/3 t. When t is similar to orgreater than typical dipolar periods, the relaxation rate becomes constant and for t much shorter than T2, thethermodynamic reservoirs get decoupled. These experiments confirm that the thermodynamic picture is validalso in liquid crystals and the cross relaxation between the reservoirs can be detected without interference withspin-lattice relaxation effects. Accordingly, this technique can be used to estimate the frequency range, wherecross-relaxation effects can be expected when Zeeman and dipolar reservoirs are put in thermal contact witheach other and with the lattice, as in FC experiments. In particular, the present results allow us to associate theanomalies observed in low-field spin-lattice relaxation with nonadiabatic energy exchange between the reservoirs.