The Varian A-60 was the first NMR instrument to be popular with chemists, and particularly organic chemists. The power of nuclear magnetic resonance spectroscopy comes from the wealth of structural data that can be obtained in a short time from a relatively small amount of sample.

Nuclear magnetic resonance (NMR) spectroscopy began as a way to measure the magnetic properties of atomic nuclei in solids and liquids. How a nuclear magnetic resonance experiment works can be explained by thinking of an atomic nucleus as a magnetic top or gyroscope.

Most people know that a spinning top is very hard to tip over. When you push a top, it doesn't fall over, but tips a little and starts to rotate around the original axis of rotation. This is called precession, and it helps explain many other phenomenon such as why a bike is harder to tip over at high speeds and what makes a boomerang come back.

In an NMR experiment, each nucleus acts as a tiny magnetic top. For reasons that are not quite clear, atomic nuclei spin naturally. Because atomic nuclei have charged protons (in fact, a hydrogen nucleus is a proton), a magnetic field is generated as these charged particles spin. The magnetic field is oriented along the axis of rotation of the nucleus. Thus, we can think of nuclei, particularly hydrogen nuclei, as tiny magnetic tops.

When an NMR experiment begins, a magnetic field is turned on that induces all the magnetic nuclei to orient themselves in the same direction. Thus, just as a top starts to precess when pushed by some outside force, so do nuclei precess when pushed by a magnetic field. Next, a neat thing happens. If a second magnetic field is introduced perpendicular to the first, the nucleus can be induced to flip completely over. This flipping over only happens if the magnetic field is in resonance with the precessional frequency of the nucleus, hence the name nuclear magnetic resonance. (Pake, 1958)

The process of flipping over the nucleus also takes a certain amount of energy. As the perpendicular field is applied across the sample, the strength of the field is carefully monitored by a receiver after it passes through the sample. Any drop in field strength must be a result of different nuclei flipping over, and the decreases in field strength are recorded as peaks on an NMR spectrum.

If you understand that the precession frequency in a magnetic field depends on the electronic charge of the nucleus, it makes sense that nuclei in different electronic environments (shielded or deshielded in NMR language) will experience different resonance frequencies. The different resonance frequencies are represented in an NMR spectrum by peaks shifted more or less downstream from the TMS reference peak.

The Varian A-60 used a 6-inch electromagnet operating at 14,096 Gauss to produce the magnetic fields. (A Gauss is a measure of the strength of a magnetic field. By comparison, a refrigerator magnet puts out a field strength of approximately 100 Gauss.) Special electrical systems were employed to stabilize the magnetic field to the point where NMR peaks could be detected, and a sample spinner was used to decrease peak broadening due to random nuclear motion in three dimensions. Spectra on the A-60 were not recorded and calculated on a computer as is done today, but were traced in real-time on pre-calibrated sheets of chart paper inserted into a special slot on the instrument housing. At the time the A-60 was introduced, its resolution and power were unmatched for the price.

Note: The idea of explaining a spinning nucleus as a top or gyroscope is not new; this page borrowed the idea from Pake's 1958 review of magnetic resonance in Scientific American, referenced below. The idea of a magnetic top is my own.

REFERENCE

Pake, G. E. "Magnetic Resonance." Sci. Amer.(Reprint) 1958, 198(8), 1-7.