Table of Contents
Introduction
Characterization of chemical compounds has been at the core of the chemical sciences since ages. With the advent of technology, instrumental techniques have been developed to identify the functional groups and the structure of molecules. Among the techniques available today, nuclear magnetic resonance (NMR) is the foremost to characterize the organic compounds, and in some cases, inorganic compounds too. If performed accurately, and the sample does not contain mixtures, proton NMR is sufficient to characterize an organic compound if the basic structure or the empirical formula is known. Even for unknown organic compounds, this technique goes a long way in establishing knowledge about the functional groups present and a good idea about the molecular structure. One of the major benefits of NMR is that it is a non-destructive technique, and excellent data may be produced from samples weighing less than 1 mg using contemporary tools. To be effective in utilizing NMR as an analysis tool, one must first comprehend the basic concepts behind the approach.
Some atomic nuclei behave as though they rotate on their axis. Because they are positively charged, they produce an electromagnetic field, and as a result, they function as little bar magnets. Not that all nuclei behave in this manner, but luckily, both 1H and 13C isotopes of hydrogen and carbon contain nuclear spins and respond to this approach. Since 1H is also known as a proton, the use of this isotope to detect the presence and position of hydrogen atoms in a molecule is called proton NMR spectroscopy. Spin orientation is the property that decides the suitability of a nucleus to be used in NMR. The nuclei's spin orientation is randomly orientated in the absence of an external magnetic field. The nuclear spins of a sample of these nuclei, however, take on particular orientations when exposed to an external magnetic field. These orientations can be either with the external field, i.e., parallel to and in the same direction as the external field, or against the field, i.e., antiparallel to the external field.
Choice of parameters for proton NMR spectroscopy
Since NMR is a highly sensitive technique, some parameters need to be strictly followed to obtain a good proton NMR.
- The first among these is the choice of solvent. Solvent protons must not be permitted to obstruct the recording of simple NMR spectra since they are done in solutions. Deuterated solvents, such as D2O, Acetone-D6, CD3OD, DMSO-D6, and CDCl3, are favored for use in 1H NMR, where Deuterium (2H), is typically represented by the letter D. However, it is also possible to utilize a solvent without hydrogen, such as carbon tetrachloride (CCl4) or carbon disulfide (CS2) since they don’t interfere in the proton NMR spectroscopy.
- The other most important thing is to ensure that the sample is not paramagnetic. Since the technique itself uses paramagnetism, such samples are not suitable for obtaining clear spectra.
- The next important factor in deciphering the molecular structure from the proton NMR is the frequency of the magnetic field applied. Typically, a higher frequency shows distinct peaks for different hydrogens in a molecule. In modern times, 500 MHz and 600 MHz are frequently used routinely in 1H NMR, although much higher frequency-enabled instruments have also been invented.
The reason for different behavior of different hydrogens in the proton NMR experiment
Electrons surround the hydrogen atoms bonded to other atoms in a molecule. Because electrons are charged particles, they move in reaction to the external magnetic field (Bo), generating a secondary field that counteracts the much greater applied field. Therefore, the external field must also be increased to increase the resonance. Thus, each hydrogen, being surrounded by electrons of different nature, experiences different shielding and appear at different positions in the proton NMR spectrum. The NMR spectrum is shown as a plot of the applied frequency vs absorption. Because the applied frequency rises from left to right, the left side of the plot represents the low field, downfield, or deshielded side, while the right side represents the high field, upfield, or shielded side. The more the applied frequency, the more distinct the peaks for each hydrogen are. A standard (usually TMS) with its peak at 0 ppm (the unit used in NMR) is used in all experiments by mixing it with the sample in the solution. It does not interfere with the sample and thus serves the purpose of a reference.
Structure elucidation through 1H NMR spectra
Three factors are crucial in identifying the nature of hydrogen for which a peak appears in a 1H NMR spectrum. The first one is the position, which signifies the nature of the functional group on which the hydrogen resides. This information enables us to understand how many hydrogens are present in a molecule and in which position. The integrated intensity of NMR signals should be ideally proportional to the nuclei ratio inside the molecule. The integrated intensities, in conjunction with the chemical shift and coupling constants, enable structural assignments. The signal strength thus enables us to group identical hydrogen atoms and hence the symmetry in the molecule.
Spin-spin couplings are in addition to chemical shift, the key signals in NMR spectra that allow structural assignments. Because nuclei have a weak magnetic field, they influence each other, altering the energy and, therefore, frequency of neighboring nuclei as they resonate. This phenomenon is referred to as spin-spin coupling. Due to this coupling, a single peak splits into multiple, and thus it enables us to determine the number of identical hydrogens and functional groups that neighbor particular hydrogen.
The “n+1” rule is typically followed in 1H NMR where “n” denotes the number of identical neighboring atoms. Thus, if two identical hydrogens are present of a carbon directly attached to the carbon containing the hydrogen under consideration, the number of splits in the peak would be 2+1 = 3, and the peak would be designated as a “triplet”. In molecules where atoms such as Boron is present, which itself responds to the proton NMR experimental conditions (because it also has the same nuclear spin of 1/2, and also 3/2, for another isotope), or those where a lot of hydrogens are similar in nature, or if hydrogen is bonded to some element other than carbon, the multiplicity rule is not often followed and the spectrum needs further study to elucidate the correct structure.