ABSTRACT
This chapter reviews NMR theory by beginning with magnetic properties of nuclei. The data acquisition topic outlines both the classical mechanical description of NMR spectroscopy using a vector representation and the quantum mechanical description that invokes energy transitions. Relaxation of nuclei and properties contributing to spin-lattice relaxation time T1 (longitudinal) and spin–spin relaxation time T2 (transverse) are included. The chemical shift, which is a function of the nucleus and its environment, is described. Measurement relative to a reference compound is explained. The importance of spin coupling in structure elucidation is also included. The nuclear Overhauser effect (NOE), a phenomenon resulting from the transfer of nuclear spin polarization via cross-relaxation, is described with emphasis on effects of molecular weight and mixing time.
Challenges in LC-NMR are summarized using four main issues. These include limits on the dynamic range of the receiver; selecting the appropriate volume of the LC-NMR flow probe; accurate synchronization of HPLC and NMR systems; and concerns regarding low sensitivity of the NMR technology. Because the primary nuclei in structure elucidation are 1H and 13C, the properties of these nuclei and their differences and similarities with respect to NMR detection are discussed.
