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Nilanjan Bhowmick AIR 3, CSIR NET (Earth Science)
Paramjeet verma
Every rotating molecule must have some moment of inertia (I = mr²) which can be different along different directions.
I still can't understand how a molecule could rotate when there is double bond or triple bond present ?
okay! I got your doubt. See, when I am talking about rotation of molecule , it is the rotation of whole molecule take place including all the atoms present in the molecule. (and not along any particular double bond or triple bond). Rotation is not possible when we fix one atom and rotate the other atom having double bond in between them. For example O2 has one double bond ( O=O ) if we fix one oxygen atom and rotate the other then rotation is not possible. But we can rotate the whole O2 molecule (both O atoms are rotated together) along the axis containing both O atoms (Ia). Similarly HCN molecule consists one triple bond and one single bond. If we fix the CN group then roatation of sigma bond (H-C) is possible but if we fix the H-C group and rotate the N atom then rotation is not possible. However, the whole HCN molecule can be rotated along the axes (Ia, Ib,Ic or x, y, z axes). And this rotation is considered in the microwave spectroscopy (and not the rotation of any paricular bond between the two atoms)