NMR dipolar constants of motion in liquid crystals: Jeener–Broekaert, double quantum coherence experiments and numerical calculation on a 10-spin cluster

H.H. SEGNORILE; C.J. BONIN; C.E. GONZÁLEZ; R.H. ACOSTA; R.C. ZAMAR

SOLID STATE NUCLEAR MAGNETIC RESONANCE

ACADEMIC PRESS INC ELSEVIER SCIENCE

Año: 2009 vol. 36 p. 77 - 85

0926-2040

Two proton quasi-equilibrium states were previously observed in nematic liquid crystals, namely the Sand W quasi-invariants.Even though the experimental evidence suggested that they originate in a partition of the spin dipolar energy into a strong and a weak part, respectively, from a theoretical viewpoint, the existence of an appropriate energy scale which allows such energy separation remains tobe confirmed and a representation of the quasi-invariants is still to be given. We compare the dipolarNMR signals yielded both by the Jeener–Broekaert experiment as a function of the preparation time andthe free evolution of the double quantum coherence (DQC) spectra excited from the S state, withnumerical calculations carried out from first principles under different models for the dipolar quasi-invariants, in a 10-spin cluster which represents the 5CB (40-pentyl-4-biphenyl-carbonitrile) molecule.Knowledge of the nature of the dipolar quasi-invariants will be useful in studies of  dipolar-orderrelaxation, decoherence and multiple quantum NMR experiments where the initial state is a dipolar-ordered one.