Siddharth Srivastava posted an Question
July 04, 2021 • 10:43 am 30 points
  • IIT JAM
  • Chemistry (CY)

How can i rotate molecules and what is the concept of ia, ib, ic in microwave spectroscopy ? how to rotate a molecule ?

how can I rotate molecules and what is the concept of Ia, Ib, Ic in microwave spectroscopy ? how to rotate a molecule ? what is the role of moment of inertia in microwave spectroscopy ?

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    Paramjeet verma

    Every rotating molecule must have some moment of inertia (I = mr²) which can be different along different directions.

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    I still can't understand how a molecule could rotate when there is double bond or triple bond present ?

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    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)

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    Paramjeet verma

    Molecules are rotated along the principle axis ( axis with maximum no. of n where n = 360°/ꂦ ). Ia, Ib and Ic are the moment of inertia along three different axes mutualy perpendicular to each other. Ia is always taken as along the principle axis. If these Ia ,. Ib and Ic are are equal then there is no net dipole moment in the molecule and thus they are Microwave inactive. Linear molecules and Spherical top molecules (e.g. perfect tetrahedral and perfect octahedral molecules ) have Ia=Ib= Ic. So they are microwave inactive. symmetric top (e.g CH3F, CH3CN, allenes, BCl3, NH3, benzene, etc. ) and asymmetric top (e.g. H2O, H3BO3, CH3OH, NO2, etc) molecules do not equal momemt of inertia along three axes, so they are microwave active.

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    still , but I can't get ith either. how can you explain the rotation of molecule with moment of inertia with Ia,Ib and Ic ? can you show in rough , how does the rotation occur with examples.

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