A chemical compound when seen at the very minute levels contain molecules. These molecules are in turn made of atoms. The nature of the molecule and consequently the nature of chemical compound is decided by the nature of atoms present in the molecule and also their arrangement in the molecule, termed as structure of the molecule. This relative arrangement is of utmost importance to a chemist. A group of atoms arranged in a particular way may form the antibiotic you consume to save your life and the same set atoms arranged slightly differently may end up as fatal poison. The antibiotic compound
may change to poison just by a simple chemical reaction initiated by room temperature.
Although expressed in hypothetical terms, it is an undeniable fact that the structure of the molecule makes the difference between life and death. So detecting the structure of a molecule is a challenge chemists face. A molecule is not like a bacterium that can be seen under the microscope. One cannot
directly see it and identify the placement of atoms in the structure. Although atomic force microscope technology which we briefly mentioned during our Physics tuition classes, can be used to identify structures, it doesn’t work with all types of molecules. Chemists use an indirect method to infer the structure of a molecule using the spin of an atomic nucleus. The technology is named Nuclear Magnetic Resonance (NMR).
The nuclei of atoms like hydrogen, carbon, etc (all the elements with odd number of neutrons or protons) show the properties of spin. The nuclei can assume two different spin states clockwise and anti-clockwise. There is an energy difference between the two spin states each of these nuclei can assume under applied magnetic field. The nuclei can be excited from one spin state to the other with the help of electromagnetic radiation. The energy difference between the two spin states changes with the intensity of magnetic field applied and the nature of the atoms surrounding an atom with nuclear spin.
For example when a carbon atom is surrounded by a hydrogen atoms placed at a particular distance then the resultant magnetic field on the carbon atom is combined effect of both applied magnetic field and the magnetic due to hydrogen atom. This effects the energy gap between the two spin states of the carbon nucleus. Consequently, the frequency of light required to excite the nucleus from the lower energy spin state to higher energy spin state changes. So based on the frequency at which excitation is facilitated one can predict structure of the molecule following a tedious deduction procedure.