INVESTIGADORES
ACOSTA Rodolfo Hector
congresos y reuniones científicas
Título:
Creation and relaxation of dipolar order in liquid crystals probed via the evolution of multiple quantum coherences
Autor/es:
L. BULJUBASICH; C. BONIN; C.E. GONZÁLEZ; G.A. MONTI; R.C. ZAMAR; R. H. ACOSTA
Lugar:
Tarragona, España
Reunión:
Congreso; 2nd Iberoamerican NMR Meeting; 2007
Resumen:
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. These techniques cannot be
used for 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 Z1.
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., 2003,
118, 3686-3691.
2. O. Mensio,
C.E. Gonzalez and R. C. Zamar, Phys. Rev. E, 2005
71 011704