Spectroscopy of Organic Compounds: Concepts, Methods & Applications

Spectroscopy is defined as a general term for instrumental processes in which information about the molecular structure is obtained by careful examination of electromagnetic radiation scattering, absorption, or emission by compounds. 

Electromagnetic radiation is a continuous continuum of energy-bearing waves ranging from relatively short waves like high-energy X-rays (wavelengths of around 10 nanometres [nm]) to extremely long, low-energy waves like radio waves (with wavelengths of one metre [m] or even more).

Example of Spectroscopy

For example, visible light is the range of electromagnetic radiation detectable by human vision, having wavelengths of 400 to 700 nm roughly. Objects appear coloured when they absorb the visible light of certain wavelengths, and those absorbed wavelengths are consequently absent from the light, which passes from the coloured object to the eyes.

Molecules can absorb light of specific wavelengths, which is why the energy content of absorbed light is a precise value required to cause the molecule to be excited from one energy state to another. The myriad energy levels present in a molecule are said to be quantized due to each one different from another by a discrete, measurable energy value, simply as each step in a stairway is either a fixed height above or below all others. Therefore, by measuring the electromagnetic radiation wavelengths absorbed by a molecule, it is possible to gain information about different energy levels within it.

Then, this particular information can be correlated with the specific details of molecular structure. Instruments known as spectrometers measure the wavelengths of light, which are absorbed by molecules in different regions of the electromagnetic spectrum. Visible and ultraviolet spectroscopy, nuclear magnetic resonance spectroscopy, and infrared spectroscopy are the most important spectroscopic techniques for structure determination. Mass spectrometry is a fourth technique that does not rely on the absorption of electromagnetic radiation. However, it is valuable for the information that it provides about the type and number of atoms present in a molecule.

Including these, there are other examples of spectroscopy such as Silverstein spectroscopy, IR spectra of inorganic compounds, IR spectra of aromatic compounds, ultraviolet spectra of aromatic compounds, aromatic compounds IR spectra.

Introduction to Chemical Compounds 

Most organic compounds were differentiated from one another until the mid-twentieth century based on basic physical and chemical properties. However, knowledge of these properties yields only superficial clues about the molecular structure of a compound, and the determination of that structure was a complicated process (for the large molecules at least), which involved careful analysis of many reaction pathways. Also, chemists had no way to notice what molecules looked like due to the reason molecules are smaller that no device like a microscope could be developed, which would give a complete image of a molecular structure. A technique named X-ray crystallography may give precise structural data for some of the molecules, but only those, which can be obtained in solid, crystalline form.

Structure

Generally, a full X-ray structure determination is time-consuming, costly, which is applied to only the most puzzling structures. Sufficient information to decipher a structure of a molecule is much more easily obtained by the use of either one or more spectroscopic techniques.

The below figure shows the X-ray protein crystallography in molecular biology.

The structure of the cholera enterotoxin, represented in a false-colour image, which is obtained by X-ray protein crystallography.

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Visible (UV-visible) and Ultraviolet Spectra of Aromatic Compounds

Most of the organic compounds are transparent to the relatively high-energy radiation, which constitutes the visible (400–700 nm) and ultraviolet (200–400 nm) portion of the electromagnetic spectrum, and consequently, they appear to be colourless in solution. This is due to the electrons present in the σ bonds of organic molecules requiring the wavelengths of even higher energy (like those of X-rays) to excite them to the next accessible higher energy level.

However, electrons present in π bonds can be promoted to the higher energy levels by visible and ultraviolet light, and UV-visible spectroscopy (also called UV vis spectroscopy organic chemistry) consequently provides useful structural information for the molecules, which contain π bonds. When the multiple π bonds get separated from each other by the intervening single bonds, they are named conjugated.

The UV-visible molecule’s spectrum is dramatically affected by the presence of conjugation. As the conjugated π bonds count increases, the UV-visible spectrum exhibits the light absorption at a greater number of various wavelengths (it means the spectrum contains more absorption peaks), and the light of longer wavelengths (including the lower energy) is absorbed. The several individual peaks of UV-visible spectra generally coalesce to produce a continuous absorption spectrum, with a few of the strongest individual absorption peaks appearing as sharp spikes.

For example, azulene’s UV-visible spectrum is a molecule, which contains five conjugated π bonds, exhibits a strong absorbance in the visible region of the electromagnetic spectrum that correlates with its intense blue colour.

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The conjugated bonds in naturally occurring organic compounds, which are highly coloured, are extensive. The compound, which is largely responsible for the bright orange colour of carrots, β-carotene, holds 11 conjugated π bonds. Especially, UV-visible spectroscopy is informative for molecules, which contain conjugated π bonds.