Molecular vibrational energy

[Benzeen]

when a substance cools, its molecules vibrate at a slower speed and when a substance is heated its molecules vibrate quicker, right?. Is the increase in the molecules vibration a result of the heat energy being applied to it, or is it the increase in the vibrational energy of the molecule what causes the substance to heat up? do they both occur simultaneously? thanks

 

[Rajini]

Hi,
When a molecule cools or heated up, the atoms vibrates. Only its displacement changes but not frequency. And these fundamental frequencies are independent of temperature. To find these fundamental frequencies, you scan the molecule with a range of energies [you are not applying heat diretly!], say, from 0 to 4000 cm^{-1. Now molecule's vibrational fundamental frequency matches with a frequency of the applied energy and so you get a peak...sometimes fundamental frequency are also called resonance frequencies (because molecule's vibrational frequency coincides with applied energy)..Moreover, in the simple frequency formula there is nothing like temperature..it mainly depends on force constant.}