[rhoark]

I was hoping to find a reference for the temperatures where rotational and vibrational degrees of freedom stop being "frozen out" for various substances. All I've found on the net are isolated references to the temperatures for one or two substances, stuck deep in an article, and usually not even both of them for the same substance. Surely there's a consolidated reference somewhere.

[tomothy]

The temperatures at which vibrational modes or rotational modes for gas phase molecules 'freeze out' are characterised by the rotational and vibrational temperatures which are tabulated in standard physical chemistry textbooks like Atkins.

From the Boltzmann disbrution, you know the population of an energy level relative to the ground state is given by

[itex] \frac{n_j}{n_0}=exp(\frac{-(E_j - E_0)}{kT}) [/itex]

From this the temperature at which rotational modes freeze out can be calculated to be

[itex]\theta^R=\frac{\hbar^2}{2kI}[/itex] where [itex]I[/itex] is the moment of inertia of the rotational mode.

Similarly the vibrational temperature is given by

[itex]\theta^V=\frac{h\nu}{k}[/itex] where [itex]\nu[/itex] is the vibrational frequency of the mode.

These formulae are derived using results from quantum mechanics (rigid rotor and simple harmonic oscillator) and statistical mechanics.

Hope this helps.