Introduction to NMR Spectroscopy
NMR spectroscopy or nuclear magnetic spectroscopy is a spectroscopic technique used in quality control and research to determine the molecular structure and purity of a sample. This analytical method is used to know the quality of a known product or the quantity of certain product in a reaction mixture. For an unknown product NMR spectroscopy can be used to determine the molecular structure of the product by analyzing the spectra of the compound. Before using NMR as a successful analytical tool, we need to know the physical principles on which this spectroscopy is based on.
Principles of NMR Spectroscopy
Many elemental isotopes poses nuclei with characteristic spin, which is denoted by l. Some of them have integral spin (such as l = 1, 2, 3 etc.), some of them have fractional spins (such as l = 1/2, 3/2, 5/2 etc.) and few of them have no spin value i.e. l = 0 (such as 12C, 16O, 32S). Organic chemists are interested on the nuclei poses l = 1/2 spins such as 1H, 13C, 19F and 31P. Spinning charge generates a magnetic field around it with a magnetic moment proportional (μ) to the spin. In presence of external magnetic field (B0), two spin states generates one is +1/2 and one is -1/2. The nucleus of lower energy +1/2 spin state generates magnetic field same as the direction of external magnetic field. And the nucleus of higher energy -1/2 spin state generates magnetic field opposite to the direction of external magnetic field. The energy difference between these two energy levels depends on the external magnetic field applied and is always very small. Thus an energy transfer is possible between these two energy levels. An energy transfer to a higher energy level takes place at a particular wavelength that corresponds to a particular radio-frequency, the energy with the same radio-frequency will be emitted when the spin returns to its lower level. The Signal that matches with this energy transfer is then measured and processed to show the NMR spectra of that nuclei.
Basic NMR techniques
An NMR spectroscopy consists of a sample-holder surrounded with a very strong magnet, a radio-frequency emitter, a radio-frequency receiver and a display to show the spectrum. The basic techniques of an NMR spectroscopy is as follows:
At first a sample need to be prepared in an NMR tube by taking a small amount of a particular compound dissolving in a suitable deuterated solvent. As most solvents are hydrocarbons and hydrogens or protons are NMR active, we need to use deuterated solvent to avoid getting many peaks for the protons of the solvents. The sample need to be dissolved completely into the solvent before placing the tube in the spectrometer.
Shim and lock
Modern NMR machines shim automatically, but in old machines shim need to be done manually. Machine also maintain lock system before showing any spectrum.
At last spectrum will be shown on the display to analyze its purity or to identify in case of a new compound. As for example below is a spectrum of ethyl acetate. The spectrum is plotted as intensity vs. chemical shift in ppm. We will need both the intensity and the chemical shift to analyze an unknown or a known compound.