¹H-²H cross-relaxation study in a partially deuterated nematic liquid crystal.
A detailed study of the cross-relaxation effects between the ¹H and ²H spins systems is presented in the nematic phase of a 5-cyanobiphenyl (5CB) liquid crystal, partially deuterated at α position (5CB-αd₂). The proton spin-lattice relaxation time was measured at a frequency range from 5 kHz to 100 MHz at a temperature 5 K below the nematic-isotropic phase transition. In the low frequency domain, the spin-lattice relaxation rate (T₁⁻¹) dispersion clearly differs from that of the fully protonated 5CB homologue. At two distinct frequencies, T₁⁻¹ presents two distinct local maxima and for low frequencies T₁⁻¹ presents a stronger frequency dependence when compared with what is observed for 5CB. The T₁⁻¹ dispersion obtained for 5CB-αd₂ for frequencies above 60 kHz was interpreted in terms of the relaxation mechanisms usually accepted to interpret the spin-lattice relaxation in nematic phases in general and 5CB in particular. For lower frequencies it was necessary to consider cross-relaxation contributions between the proton and deuterium reservoirs. A detailed model interpretation of the deuterium quadrupolar dips with respect to the proton-spin relaxation is presented. The analysis of the quadrupolar relaxation independently confirms that the order director fluctuations is the dominant mechanism of proton relaxation in the low frequency domain.
Jožef Stefan Institute , Jamova 39, SI-1000 Ljubljana, Slovenia.
A
A
Gradišek
P J
PJ
Sebastião
S N
SN
Fernandes
T
T
Apih
M H
MH
Godinho
J
J
Seliger