Rigid-rotor rotational partition function
A linear rigid rotor has rotational levels proportional to J(J+1), with magnetic degeneracy 2J+1.
Key knowledge
A linear rigid rotor has rotational levels proportional to J(J+1), with magnetic degeneracy 2J+1. In the classical high-temperature limit, the rotational partition contribution is approximately T divided by the rotational temperature and molecular symmetry number. Low-temperature homonuclear populations need the allowed nuclear-spin and rotational-state weights rather than a universal symmetry division.
Values and equations
EJ = ℏ²J(J+1)/(2I); θrot = ℏ²/(2IkB); qrot ≈ T/(σθrot)
- EJ
- rigid-rotor level energy above J=0 J
- J
- nonnegative integer rotational quantum number 1
- ℏ
- reduced Planck constant J s
- I
- moment of inertia about an axis perpendicular to the bond kg m²
- θrot
- rotational temperature K
- kB
- Boltzmann constant J K⁻¹
- T
- absolute temperature K
- σ
- appropriate rotational symmetry number in the classical limit 1
- qrot
- rotational partition contribution 1
Conditions and scope
- Use the rigid linear-rotor approximation and kBT large relative to rotational level spacing for the displayed high-temperature expression.
- Electronic angular momentum, centrifugal distortion and nuclear-spin restrictions require additional treatment.
Sources · 2
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
Massachusetts Institute of Technology2 sources
- Lecture 12: Rotational Partition Function, pp.1–3Robert W. Field and Robert G. Griffin — MIT OpenCourseWare, Physical Chemistry II (Spring 2008; some notes retain 2007 headers)Textbook chapter · PDF
- Lecture 17: Rigid Rotor, pp.3–6Robert W. Field — MIT OpenCourseWare, Physical Chemistry (Fall 2017)Textbook chapter · PDF
Evidence and review
Read level sum, high-temperature integral and symmetry discussion; excluded notes' extra c in moment-of-inertia conversion and inaccurate finite-temperature correction.
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Entry details
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