Intro to IR

Molecules vibrate and rotate in 3 dimensions at various discrete frequencies.  Non-linear molecules have 3N-6 normal vibrational modes, where N is the number of atoms in the molecule.  For example methane (CH4) has 9 normal vibrational modes.  The more atoms a molecule has the more complex the spectra.  There are different types of vibrations possible.  Some of the important ones for methanes C-H stretching and bending modes shown below.

stretching
symmetric

stretching
antisymmetric
gamess logJMOLSCRIPT{rotate y 45; model 12; vibration scale 0.5;vibration on;} gamess logJMOLSCRIPT{rotate y 45; model 13; vibration scale 0.5;vibration on;}
scissoring wagging
gamess logJMOLSCRIPT{rotate y 45; model 11; vibration scale 0.5;vibration on;} gamess logJMOLSCRIPT{rotate y 45; model 9; vibration scale 0.5;vibration on;}

Why 3N-6? 

Well each atom in a non-linear molecule can move in 3 dimensions (x,y,z) hence the 3N term.  However if I were to move every atom of the molecule in the x-direction that would just move or translate the molecule.  Thus we subtract -3 for translations in x,y and z direction.  We also must subtract the 3 possible rotations about x,y and z, hence another -3.  Linear molecules have 3N-5 vibrational modes.  Can you explain why a linear molecule has -5?

Infrared Radiation (IR)

These vibrational modes have quantized energy levels (frequencies of vibration).  If you shined light (radiation) of proper frequency at the molecule you can get the different normal modes to absorb the photons of light and vibrate faster (higher energy level).  The energy differences between the vibrational states of most molecules are on the order of infra-red (IR) radiation, hence the reason IR is sometimes called vibrational spectroscopy.

+ hν =