Question

List the different modes of dichloro methane. Which modes are IR active and which are Raman active. It would be grateful if you could help me answer this question in an essay format with a comprehensive three paragraph. Thanks in advance.

Answer 1

dichloromethane has 9 vibrational modes as it is non linear milecule (3N-6=3*5-6=9).First is the symmetric C-H stretching in which the H atoms moves symmetrically of both the C-H bonds , C displaces less in comparison to H ,, it causes a net change in dipole momemnt and well as the polarisability ellipsoid is deformed.So, C-H stretching is both IR (3077cm^-1)and Raman active.Second is the C-H asymmetric stretching mode , it occurs when the two C-H bonds stretch in the plane in opposite directions , that means H atoms moves in opposite directions in the two bonds comparitively , this causes a net increase in dipole moment along the bond which is stretched more, also the polarisability ellipsoid is deformed here corresponding to the change in shape of the molecule and dipole moment along the three axis.It is both IR (3037cm^-1) and raman active.Third is the in plane bending motion called scissoring where the two C-H bonds moves like two flaps of a scissors inwards .The polarisability ellipsoid changes its shape and there is a change in net dipole moment.It is both IR(1327cm^-1) and raman active.

Fourth is the out of plane bending motion called wagging in which the two C-H bonds move out of the plane parallel to each other like the tail of a dog keeping both the bonda on the tail and moving to and fro on both sides.This also causes a change in net dipole moment and polarisability ellipsoid.It is both IR (1218cm^-1) and raman active.Fifth is the out of plane motion i opposite directions called twisting , here the two C-H bonds moves out of plane in completely opposite directions , here the dipole moments cancel out each ither as they are in opposite directions so the mode is IR inactive , but it is raman active as it still changes the polarisability ellipsoid.Sixth is the asymmetric C-Cl stretching in which both molecules moves and the bond stretches in opposite directions as one elongates and the other one is compressed.There is quite a change in the dipole moment and the IR band is intense(874cm^-1) and also there is change in polarisability ellipsoid and it becomes Raman active too.

Seventh is the in plane rocking motion of C-Cl bond as both c-H bonds move in the same direction in the same plane, causing a net change in dipole moment and also a change in polarisability ellipsoid.So its is IR active(863cm^-1) and Raman active.Eighth is the symmetric C-Cl stretching in which both C-Cl bonds move symmetrically in the plane in same directions.C atoms displace comparitively less and both Cl move in the same direction as in both out or in.It is both IR and raman rctive with ir band at 771 cm^-1 .Ninth and last is the C-cl in plane scissoring , here both the c-Cl bonds moves inside like the flaps of a scissor and thereby causing a change in the net dipole moment and also causing a change in polarisability along different axis..

So, eight modes are both IR and raman active and one mode is only raman active out of the nine vibrational modes of dichloromethane