Physical Chemistry Third Edition

1066 25 Equilibrium Statistical Mechanics. I. The Probability Distribution for Molecular States

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Born–Oppenheimer energy

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R

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(^) 0,el(2d choice)
(^) 1,el(2d choice)
(^) 0,el(1st choice)
(^) 1,el(1st choice)
Figure 25.3 The Electronic Energy (Born–Oppenheimer Energy) as a Function of
Internuclear Distance for Two Electronic States.
ordinary temperatures. The electronic partition function of O 2 at ordinary temperatures
is equal to 3.000 if the energy of the ground-level is chosen to equal zero.
The Rotational Partition Function
We treat diatomic molecules in the harmonic oscillator–rigid rotor approximation. The
rotational energy is given by Eq. (22.2-40):
εrotεJ
h ̄^2
2 Ie
J(J+1)hBeJ(J+1)hcB ̃eJ(J+1) (25.4-7)
where we have expressed the energy in terms of the rotational constant,Be, given by
Eq. (22.2-48)
Be
h
8 π^2 Ie



h

8 π^2 μre^2

(25.4-8)