Orientation and low-temperature effects

At temperatures where the potential minimum, e, is comparable to kT, equation (18) is not valid because the method of treating the thermal velocity distribution is no longer appropriate. When e becomes comparable to kT, orientation effects are believed to become important because in general the depth of the potential minimum is dependent on the configuration of both molecules. The probability of a particular orientation can be derived from Boltzmann s equation. 

Nitric oxide exhibits a negative temperature coefficient for vibrational relaxation in self-collisions, below about 700° K. It has been suggested170 that this effect arises because the potential energy of the point of resonance, postulated by Nikitin, is strongly orientation dependent. (In this case the maximum depth of the potential minimum can be no greater than about 3 kcal. mole-1 which will not steepen the potential sufficiently to account for the observed relaxation rate, with l = 0.18 A.) 

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