Infrared Active Molecules?

Question

Which of the following molecules are IR active and why?
H2O, MeCN, Cl2, CH3Br

Is C triple bond C more likely to have a higher frequency than a C double bond C and again why?
(NOTE: Please answer thoroughly!! Thanks)

Answer 1

Unfortunately, Andrew H IS wrong in what he says. Not all covalent compounds are IR active for the following reason...to be IR active they need to undergo a net change in dipole moment when they stretch or bend under the influence of IR radiation. Of the list you give, water, methyl cyanide and bromomethane are all IR active, but chlorine is not (since it has no dipole moment).
The stretching of a C=C bond occurs at around 1650 wavenumbers and that of C(triplebond)C at around 2150 wavenumbers. This is because of the additional energy needed to cause a triple bond to stretch (it being stronger than a double bond). Hope that helped

Answer 2

Ok, I may be wrong on this, but if I remember correctly all covalent bonds are IR active to a certain degree.

I don't know what MeCN is, but the water, Chlorine and Methyl Bromide are all covalently bonded and should vibrate to an Infrared wave which passes them and excites them.

This process of IR absorbtion causes electrons to jump to higher energy states and will cause the molecule to stretch, vibrate, rotate, rock or bend which causes problems when using IR Spectroscopy to identify large molecules, as the dance can become quite complex.

The energy is inversly proportional to the wavelength, and it takes more energy to excite a double bond than a single bond etc.

More energy = Longer Wavelength = Lower Frequency