Spectroscopy: An Important Tool for Structural Illustration of Heterocyclic Drugs

Spectroscopy is the investigation and measurement of spectra produced by matter interacting with or emitting electromagnetic radiation. Originally, spectroscopy was defined as the study of the interaction between radiation and matter as a function of wavelength. Now, spectroscopy is defined as any measurement of a quantity as a function of wavelength or frequency. During a spectroscopy experiment, electromagnetic radiation of a specified wavelength range passes from a source through a sample containing compounds of interest, resulting in absorption or emission. During absorption, the sample absorbs energy from the light source. During emission, the sample emits light of a different wavelength than the source’s wavelength.
In absorption spectroscopy, the sample’s compounds are excited by the electromagnetic radiation provided by a light source. Their molecules absorb energy from the electromagnetic radiation, become excited, and jump from a low energy ground state to a higher energy state of excitation. A detector, usually a photodiode, on the opposite side of the sample records the sample’s absorption of wavelengths, and determines the extent of their absorption. The spectrum of a sample’s absorbed wavelengths is known as its absorption spectrum, and the quantity of light absorbed by a sample is its absorbance.
Each molecule within a sample will only absorb wavelengths with energies corresponding to the energy difference of the present transition. In simpler terms, this means that a molecule that jumps from ground state 1 to excited state 2, with an energy difference of ΔE, will allow other wavelengths to pass through until it can absorb radiation from a wavelength that corresponds to ΔE. Light that passes through to the photodiode without any absorption is called Stray Radiant Energy, or stray light. Absorption that occurs due to an energy difference between the two states is called an absorption line, and a collection of absorption lines creates an absorption spectra. The frequency of each absorption line in an absorption spectra tells us about the sample’s molecular structure, and can be influenced by factors such as stray light, environmental temperature, and electromagnetic fields.