Creation and relaxation of dipolar order in liquid crystals probed via the evolution of multiple quantum coherences

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

Tarragona, España

Congreso; EUROMAR Magnetic Resonance Meeting; 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 T2*, 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, T1D, 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 (T1D-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 T1D-inter free of Zeeman contamination is measured for the first time in a 7T magnetic field.   [1] H. Cho, D. G. Cory, and C. Ramanathan, J. Chem. Phys., vol. 118, 3686-3691 (2003). [2] O. Mensio, C.E. Gonzalez and R. C. Zamar, Phys. Rev. E, vol. 71 011704 (2005)