How NMR Spectroscopy is different from UV- Visible Spectroscopy?

How NMR Spectroscopy is different from UV- Visible Spectroscopy?
Structure elucidation – key role of NMR Spectroscopy

The key objective of both the spectroscopic techniques is identification and quantification of the components of samples. However, there are intrinsic differences between the two which need to be clearly understood before deciding between them for your analysis requirements.

UV-Visible Spectroscopy

UV-visible spectroscopy is based on the interaction of the sample with the electromagnetic radiation in the form of light over the wavelength region 190 – 900 nm. Such interaction leads to excitation of electrons from ground state to unstable higher energy states. The extent of absorption is in direct proportion to the concentration of the absorbing species present in the beam path.

NMR Spectroscopy

Nuclear magnetic resonance on the other hand does not involve any transitions of the electronic levels. The magnetic properties of the nuclear particles are exploited to seek information on the structure of molecules. Transitions between spin states of nuclear particles are achieved by placing the sample in a uniform magnetic field.

Basis of Absorption

UV-visible spectroscopy is dependent on presence of chromophoric groups in molecules, degree of conjugation in molecular structure or complexation reaction between metal ions and ligands which tends to produce coloured compounds.

On the other hand nuclear magnetic resonance involves transitions between spin levels of nuclear particles under the influence of an external magnetic field. The spin angular momentum of the nucleus is a vector sum of spin angular momenta of the neutrons and protons present in the nucleus. Nuclei having odd sum of protons and neutrons have half integral or multiples thereof of angular momentum and give rise to NMR absorption

Sample Handling

Samples for UV- visible analysis are taken in the cuvettes or cells made of quartz with general path length of 1 cm. On the other hand NMR samples are contained in glass tubes of 5 mm outer diameter and 15 – 20 cm length.

Any solvent in which the sample is soluble is used for UV – visible analysis and a blank solution is taken as reference in the beam path. In case of NMR the solvent needs to be aprotic and carbon tetrachloride is a convenient choice. However, due to limited solubility of analytes other deuterated solvents such as deuterated chloroform,deuterated benzene, deuterated acetone and deuterated dimethylsulfoxide are commonly used.

There is no need for a reference standard tube. An internal standard such as tetramethylsilane (TMS) can be used.

Speed of Response

UV – Vis spectroscopy is a fast technique in comparison to NMR. Excitations are of the order of femto seconds in UV -Visible spectroscopy whereas in NMR analysis the response times are much longer,even upto a minute or longer

Sample Excitation

Sample and reference are placed inside the spectrometer compartment along the beam path in case of UV –visible analysis. However, in case of NMR the sample tube is exposed to a magnetic field in a cavity between the poles of a magnet. Permanent magnets, electro magnets and superconducting magnets have been in use in NMR spectrometers. Modern high-resolution spectrometers use superconducting magnets.

The sample tube inside the magnet cavity is rotated using an air turbine so that the sample experiences a uniform magnetic field. In addition a coil around the tube generates radio frequency for excitation and detection of NMR signals.

Spectral Response

The UV – visible spectra are recorded as absorption bands on a scale with absorbance along Y-axis and wavelength along the X-axis. On the other hand NMR spectra are recorded with X-axis representing chemical shift in ppm units and Y-axis as absorption intensity. The reference compound TMS has zero value and chemical shifts increase from right to left.

The line widths decrease sharply from UV–Vis to NMR. The comparatively long life times of excited nuclear spin states helps NMR provide a wealth of information about the structure of molecules. NMR finds several applications in analysis of multi component mixtures of organic compounds and structure elucidation through identification of common functional groups.

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