Introduction Nuclear Magnetic Resonance Spectroscopy: Basic Theory and Background

Abstract

Nuclear magnetic resonance (NMR) spectroscopy is one of the most powerful experimental methods available for atomic and molecular level structure elucidation. It is a powerful technique in that it is a noninvasive probe that can be used to identify individual compounds, aid in determining structures of large macromolecules, such as proteins, and examine the kinetics of certain reactions. NMR spectroscopy takes advantage of the magnetic properties of the observed nucleus that are influenced not only by its chemical environment, but also by physical interactions with its environment. Both can be examined by measuring specific NMR parameters such as coupling constants, relaxation times, or changes in chemical shifts. As NMR techniques and instrumentation advance, NMR spectroscopy is becoming more important in the environmental sciences, tackling problems and questions that previously were difficult to answer. For example, sensitivity enhancement techniques increase the ability to examine a sample without chemical or physical pretreatment. A sample examined in this manner is in its original state and is unaffected by chemical or physical reactions caused by the pretreatment procedure. Despite its increasing popularity and numerous advantages, NMR spectroscopy can be a mysterious, and at times daunting, technique. The purpose of this chapter is to provide an overview of basic NMR theory and background for the uninitiated. It is hoped that it will provide enough information to those unfamiliar with NMR and its terminology for them to find the remaining chapters understandable and interesting. Those who desire a greater understanding are referred to the many textbooks on solution-state NMR, solid-state NMR, and the application of NMR to geochemistry, soil chemistry, oils and coals, and carbonaceous solids. The advance that led to NMR spectroscopy came in 1939 with resonance experiments by Rabi and coworkers, who demonstrated the property of nuclear spin. In 1945, the research groups of Bloch and Purcell independently obtained the first nuclear resonance signals. For this they won the 1952 Nobel prize. The first application of NMR spectroscopy in the field of humic substance research was 1H NMR of liquids. González-Vila et al. were the first to apply 13C solution-state NMR to natural humic acids.