The environmental effect on any type of hydrogen atom is expressed quantitatively in terms of a quantity called chemical shift. It is generally represented by the letter and is defined by the equation.
                                      
Where H_S is the magnetic field strength at which the maximum (peak) appears for a particular type of protons of the sample taken while H_r is the field strength at which absorption occurs for some reference substance such as water. For example, in case of ethyl alcohol, for protons in CH₃ group, 4.1 whereas for the proton in the OH group,-0.1.
      As H <V, equation can also be written as

                                     
However, the most common compound used as a reference is tetraethyl silane (TMS) i.e., (CH₃)₄ Si. It has many advantages. Firstly, from is structure
                                                                             
we see that all the 12 protons present in it are equivalent i.e. they have the same environment. Hence it gives a single well defined absorb piton peak in the NMR spectrum. Secondly, the protons are highly shielded by their electrons form the external magnetic field so that it shows NMR at a very height external magnetic field strength as compared to other proton. Thirdly, it is chemically, finery. Fourthly, its boiling point is low (27⁰ C)so that it evaporates off quickly anted the sample compound is easily recovered.

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