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

Abstract

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