Proton NMR Relaxation Study on the Nematic-Nematic tic Phase Transition in A131 Liquid Crystal

ALINA ALUCULESEI; FABIAN VACA CHAVEZ; CARLOS CRUZ; PEDRO J. SEBASTIÃO; N. NAGAVENI; VEENA PRASAD; RONALD DONG

JOURNAL OF PHYSICAL CHEMISTRY B - (Print)

AMER CHEMICAL SOC

Lugar: Washington; Año: 2012 vol. 116 p. 9556 - 9563

1520-6106

A study of the proton NMR spin−lattice relaxation time, T1, of the A131 liquid crystal compound as a function of temperature and Larmor frequency, using a combination of fast field-cycling and standard NMR techniques, is presented. The frequency dispersion in a wide range (from 10 kHz to 300 MHz) at different temperatures and the temperature variation of T1, in several frequency conditions, were analyzed considering the contributions of the molecular movements generally detected in liquid crystals. In the case of nematic phases of calamitic liquid crystals, the nuclear spin relaxation is dominated by collective movements and local molecular reorientations. The experimental results clearly show a transition within the nematic range of this compound, previously identified as one from the uniaxial to the biaxial phase. This transition can be associated with a slowing down of the molecular rotations around the long molecular axis, where the preferred orientation defines the principal director as detected in the T1 dispersion analysis.