Nuclear Magnetic Resonance (NMR) spectroscopy is an useful analytical chemistry technique that used in quality control and research for determining the content and purity of a sample and also the molecular structure. For example, NMR can quantitatively analyze mixtures containing known compounds. For unknown compounds, NMR can either be used to match against spectral libraries or to infer the basic structure directly. It can be used to determine molecular conformation and also study physical properties at the molecular level such as conformational exchange, solubility, phase changes, and diffusion.
The principle of NMR is many nuclei having spin and all nuclei are electrically charged. If an external magnetic field is applied, an energy transfer is possible between the base energy to a higher energy level (generally a single energy gap). The energy transfer takes place at a wavelength that corresponds to radio-frequencies and when the spin returns to its base level, energy is emitted at the same frequency. The signal that matches this transfer is measured in many ways and processed in order to yield an NMR spectrum for the nucleus concerned.
In 1H-NMR spectroscopy, the experimental conditions are so designed that spin translation only hydrogen nuclei are noticed. The structural information can determined by:
a) Number of Signals
b) Position of Signals
c) Intensity of Signals
d) Splitting of Signals
2.1 Number of Signals
Each group of chemically equivalent protons gives rise to a signal. If the hydrogens are in equivalent, spectra show single peak. However, proton that are not equivalent, they will absorb at different frequencies and give separate signal on the NMR spectra. For example, ethane giving 1 single peak (as in Figure 2) whereas ethyl bromide giving 2 peaks (low resolution, no splitting) (as in Figure 3)
The positions of the signals in an NMR spectrum are based on how far they are from the signal of the reference compound on a delta value scale, it is depends on chemical shift.
Chemical shift is the difference in signal position of any nuclei in comparison to the signal position on a delta value scale (as in Figure 4) with the reference of tetramethylsilane (TMS) (as in Figure 5). The unit for measuring chemical shift is parts per million (ppm) and formula for chemical shift is shown in Figure 4.
Besides, TMS used as reference compound to set at 0.0 ppm because of its inert quality that prevents it from reacting with the sample and its highly volatile nature that makes it easy to evaporate out of samples.
Moreover, the nucleus of the molecule is found within a cloud of electrons that partly shields it from the applied magnetic field. The secondary field generate from electron circulation on a nuclei will oppose the applied field to resonate the nuclei under investigation, this effect is known as shielding effect or diamagnetism shielding. Higher the shielding, greater the energy needed to resonate nuclei and lead to signal at lower delta value to near the delta value of standard (TMS), which known as upfield region. For example, hydrogen attached to saturated carbon (-C-H).
When molecule having more electronegativity species which can withdraw the electron cloud density of nuclei, it can reduce the secondary field and thereby applied field can be easily resonate the nuclei, known as deshielding effect or paramagnetism effect. Same go for higher the deshielding effect, the lesser the energy needed to resonate nuclei and hence signal will show at higher delta value, also known as downfield region. For example, hydrogen attached to unsaturated carbon which attached to electronegative atom with double bond (X=C-H).
In addition, factors affecting chemical shift to downfield region are inductive effect (present of electronegative atom like halogen, oxygen and nitrogen), Vander Waal’s deshielding (present of bulky group to hinder sterically), anisotropic effect and hydrogen bonding. Nevertheless, the general region for chemical shift is shown in Figure 6.
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