Ammonia is a chemical of great industrial importance as it is used in making the nitrogen based fertilizers. Its demand is so high that when Haber who developed the synthesis of ammonia by combining nitrogen and hydrogen using catalytic method, he was awarded Nobel prize.
Here we will discuss its peculiar structure. Nitrogen is the central atom to which 3 hydrogen atoms are attached. But due to sp2 hybridized bonds it is not flat but trigonal prism. A pair of electrons resides on the nitrogen giving it basicity.
The ammonia molecule readily undergoes nitrogen inversion at room temperature like an umbrella turning itself inside out in a strong wind. The energy barrier to this inversion is 24.7 kJ/mol, and the resonance frequency is 23.79 GHz, corresponding to microwave radiation of a wavelength of 1.260 cm.
The absorption at this frequency was the first microwave spectrum to be observed. This frequency of flipping is so constant at a given tempertaure that it was used to construct very accurate clock called ammonia clock. It was the first atomic clock constructed by NBS in 1949.
Its caption read as follows:
“The Bureau atomic clock program sought to provide a spectroscopic standard capable of being used as a new atomic standard of time and frequency to replace the mean solar day and so change the arbitrary units of time to atomic ones. With such a clock, new precise values might be found for the velocity of light; new measurements of the rotation of the earth would provide a new tool for geophysicists; and new measurements of the mean sidereal year might test whether Newtonian and atomic time are the same, yielding important results for relativity theory and cosmology”
It operated with an ammonia-regulated quartz crystal and ran with a constancy of one part in 20 million. Only limitation was its temperature dependence. So temperature was the deciding factor in its accuracy.