Let's consider the simple functional group C=O (carbon double bonded to oxygen). There is only one type of vibration for this group and that's the stretching mode, where C<=>O oscillates back and forth. What exactly is happening during the stretching? Are electrons, and therefore electron density, shifting from side to side, going from the carbon atom to the oxygen atom? Also, I don't think I'm grasping the idea of an electronic vibrational state. Could someone help me understand this in terms of what I stated above? Thanks.