Molecular Vibrations in IR Spectroscopy

[aiman3012]

Based on my reading, there are a few ways or modes of molecular vibration, like stretching and bending (wagging, rocking, scissoring, etc.). How do we know it vibrates or moves like that? Have we like seen it before?

 

[pines-demon]

Based on my reading, there are a few ways or modes of molecular vibration, like stretching and bending (wagging, rocking, scissoring, etc.). How do we know it vibrates or moves like that? Have we like seen it before?

The geometry of the molecule itself, we can do theory and if not computer simulations to see what are the forces and shape of the molecule. I think group/representation theory is used sometimes as some modes can be determined by symmetries alone (imagine HCl).