The characteristic rotational temperature (θR or θrot) is commonly used in statistical thermodynamics, to simplify the expression of the rotational partition function and the rotational contribution to molecular thermodynamic properties. It has units of temperature and is defined as
,
where B is the rotational constant, and is a molecular moment of inertia. Also h is the Planck constant, c is the speed of light, ħ = h/2π is the reduced Planck constant and kB is the Boltzmann constant.
The physical meaning of θR is as an estimate of the temperature at which thermal energy (of the order of kBT) is comparable to the spacing between rotational energy levels (of the order of hcB). At about this temperature the population of excited rotational levels becomes important. Some typical values are 88 K for H2, 15.2 K for HCl and 0.561 K for CO2.
Famous quotes containing the word temperature:
“The bourgeois treasures nothing more highly than the self.... And so at the cost of intensity he achieves his own preservation and security. His harvest is a quiet mind which he prefers to being possessed by God, as he prefers comfort to pleasure, convenience to liberty, and a pleasant temperature to that deathly inner consuming fire.”
—Hermann Hesse (1877–1962)