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Nuclear Magnetic Resonance (Nmr) Spectroscopy

Nuclear magnetic resonance spectroscopy (NMR) is a type of analysis with which we can identify the positions of certain atoms in a compound. Only those atoms which possess nuclear spinning properties can exhibit NMR behavior. NMR spectroscopy is based on the magnetic properties exhibited by the nuclei of certain atoms. We will limit our discussion to the resonance spectroscopy (proton NMR) involving the nuclei of hydrogen atoms. With proton NMR, we can find or recognize the characteristic groups in the unknown compound, to which the specific protons are attached. In addition, we can also identify the number of different types of equivalent protons in a compound that we are analyzing.

The Basics of Proton NMR

The nucleus of an atom has an electric charge. Nuclear spin properties lead to nuclear magnetic moments. When hydrogen nuclei are exposed to a magnetic field, the nuclei will align themselves in coherence with the applied magnetic field as if they were tiny magnets. The alignment can be either along or opposite to the direction of the applied magnetic field. Among these two orientations, the one with the lower energy occurs when the alignment is along the direction of the field compared to the other orientation.
When sufficient energy (in the form of radio-frequency of the electromagnetic spectrum) is applied, the nuclei that are aligned along the magnetic field can reverse their orientation to align opposite to that of the applied magnetic field. The extent to which the nucleus is shielded from the magnetic field dictates the amount of energy required to change the orientation to a higher energy level. Based on this principle, we have to understand that depending on the extent of shielding they will absorb different frequencies. The energy that is absorbed is monitored and is recorded in the form of NMR spectrum. Thus protons in different regions of a molecule have different shifts (chemical shifts).
For the standard applied magnetic field, the absorption of frequencies differ only in terms of a few parts per million (ppm). In NMR spectroscopy, the relative standard molecule used is tetramethyl silane (TMS), and thus chemical shift (d) is measured in ppm relative to the proton signal of TMS. Note that all the protons of TMS are equivalent, and thus have the same frequency of absorption, and it is considered to have a standard arbitrary value of “0 ppm” – the reference standard.


Spin-Spin Coupling

Spin-spin coupling results from the possibility of the coupling of the neighboring protons. Signals often appear as groups of peaks as a result of spin-spin coupling. This results in what is called signal multiplicity. The number of peaks in a signal is dictated by the following means. Consider this hypothetical situation. Let’s say there are N1 equivalent protons in set 1, and N2 equivalent protons in the adjacent carbon (set 2).
(Note: Equivalent protons do not split themselves.)
If this is the case, signal 1 will have (N2 + 1) peaks, and signal 2 will have (N1 + 1) peaks. The intensities of a split pattern are symmetrical (bilaterally symmetrical). The relative intensities of the peaks in a signal follow a pattern. A singlet has only one peak. There is no splitting in singlets. So there is no relative ratio to compare with. A doublet has a 1 : 1 proportion of relative intensities. A triplet has 1 : 2 : 1 proportion of relative intensities. A quartet has 1 : 3 : 3 : 1 proportion of relative intensities, and so on.
The different types of equivalent protons reflect the total number of signals. In the NMR spectrum of a compound, the levels of intensities of the signals denote the proportion of equivalent protons present in the compound.
For example, the intensity of a signal resulting from six equivalent protons will be greater than the intensity of a signal resulting from two equivalent protons. The intensity is equivalent to the areas under the signals, and is proportional to the number of protons creating that particular signal.


Figure: Relative intensities of the peaks

Proton Chemical Shifts


*Approximate values with respect to TMS


Figure: The NMR spectrum of methyl propionate

Look at the NMR spectrum of methyl propionate given in Figure.


In the NMR spectrum, the signal at 0 ppm represents tetra methyl silane (standard). There are three distinct signals (sets of peaks). Altogether, there are 8 peaks – a set of three just after 1 ppm, a set of four just after 2 ppm, and a single peak around 3.5 ppm, totaling eight peaks. The set X is a triplet. The set Y is a quartet, and the set Z is a singlet. Sometimes it is hard to recognize the peaks distinctly because of interference between the nuclei, especially when there are multiple number of peaks.


In methyl propionate, there are three different types of protons denoted by X, Y, and Z. In the NMR spectrum, the set X is represented by the triplet in the nmr spectrum, the set Y is denoted by the quartet, and the set Z is denoted by the singlet.

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