Nuclear magnetic resonance, or NMR as it is abbreviated by scientists, is a phenomenon which occurs when the nuclei of certain atoms are immersed in a static magnetic field and exposed to a second oscillating magnetic field. Some nuclei experience this phenomenon, and others do not, dependent upon whether they possess a property called spin. You will learn about spin and about the role of the magnetic fields in Chapter 2, but first let's review where the nucleus is.
Most of the matter you can examine with NMR is composed of molecules. Molecules are composed of atoms. Here are a few water molecules. Each water molecule has one oxygen and two hydrogen atoms. If we zoom into one of the hydrogens past the electron cloud we see a nucleus composed of a single proton. The proton possesses a property called spin which:
can be thought of as a small magnetic field, and
will cause the nucleus to produce an NMR signal.
Not all nuclei possess the property called spin. A list of these nuclei will be presented in Chapter 3 on spin physics.
Spectroscopy is the study of the interaction of electromagnetic radiation with matter. Nuclear magnetic resonance spectroscopy is the use of the NMR phenomenon to study physical, chemical, and biological properties of matter. As a consequence, NMR spectroscopy finds applications in several areas of science. NMR spectroscopy is routinely used by chemists to study chemical structure using simple one-dimensional techniques. Two-dimensional techniques are used to determine the structure of more complicated molecules. These techniques are replacing x-ray crystallography for the determination of protein structure. Time domain NMR spectroscopic techniques are used to probe molecular dynamics in solutions. Solid state NMR spectroscopy is used to determine the molecular structure of solids. Other scientists have developed NMR methods of measuring diffusion coefficients.
The versatility of NMR makes it pervasive in the sciences. Scientists and students are discovering that knowledge of the science and technology of NMR is essential for applying, as well as developing, new applications for it. Unfortunately many of the dynamic concepts of NMR spectroscopy are difficult for the novice to understand when static diagrams in hard copy texts are used. The chapters in this hypertext book on NMR are designed in such a way to incorporate both static and dynamic figures with hypertext. This book presents a comprehensive picture of the basic principles necessary to begin using NMR spectroscopy, and it will provide you with an understanding of the principles of NMR from the microscopic, macroscopic, and system perspectives.
Before you can begin learning about NMR spectroscopy, you must be versed in the language of NMR. NMR scientists use a set of units when describing temperature, energy, frequency, etc. Please review these units before advancing to subsequent chapters in this text.
Units of time are seconds (s).
Angles are reported in degrees (o) and in radians (rad). There are 2 radians in 360o.
The absolute temperature scale in Kelvin (K) is used in NMR. The Kelvin temperature scale is equal to the Celsius scale reading plus 273.15. 0 K is characterized by the absence of molecular motion. There are no degrees in the Kelvin temperature unit.
Magnetic field strength (B) is measured in Tesla (T). The earth's magnetic field in Rochester, New York is approximately 5x10-5 T.
The unit of energy (E) is the Joule (J). In NMR one often depicts the relative energy of a particle using an energy level diagram.
The frequency of electromagnetic radiation may be reported in cycles per second or radians per second. Frequency in cycles per second (Hz) have units of inverse seconds (s-1) and are given the symbols or f. Frequencies represented in radians per second (rad/s) are given the symbol . Radians tend to be used more to describe periodic circular motions. The conversion between Hz and rad/s is easy to remember. There are 2 radians in a circle or cycle, therefore
2 rad/s = 1 Hz = 1 s-1.
Power is the energy consumed per time and has units of Watts (W).
Finally, it is common in science to use prefixes before units to indicate a power of ten. For example, 0.005 seconds can be written as 5x10-3 s or as 5 ms. The m implies 10-3. The animation window contains a table of prefixes for powers of ten.
In the next chapter you will be introduced to the mathematical beckground necessary to begin your study of NMR.