Description of natural abundance deuterium 2D-NMR experiments in weakly ordered liquid-crystalline solvents using a tailored cartesian spin-operator formalism

We report an appropriate cartesian spin-operator formalism which retains al l physical insights and provides a pictorial representation for I = 1 spin nuclei subjected to both a chemical shift and a small quadrupolar moment su ch as deuterons in liquid crystals with low order-parameters. Using a tree- like pictorial representation, this approach allows a straightforward descr iption of natural abundance deuterium autocorrelation 2D-NMR experiments in weakly ordered solvents. As illustrative examples we study the 2D Q-COSY a nd Q-resolved NMR sequences which have been recently developed to facilitat e the analysis of overcrowded proton-decoupled deuterium spectra in natural abundance of enantiomers oriented in liquid-crystalline solutions of poly- gamma-benzyl-l-glutamate (PBLG). These 2D-NMR experiments allow the correla tion between the two components of each quadrupolar doublet in the deuteriu m spectra to be obtained, and their assignment on the basis of chemical shi fts. Several improvements of the basic sequences are proposed and discussed . Applications of these sequences in natural abundance deuterium NMR spectr oscopy using a 9.4 T magnetic field and standard NMR equipment are examined .