National Institute of Standards and Technology

Computational Chemistry Comparison and Benchmark DataBase
Release 22	May 2022
NIST Standard Reference Database 101

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XXI	Oddities

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VII.C.9.

Configuration changes and contributions to entropy

The rotational contribution to the entropy depends on the product of the moments of inertia. How much does this contribution change as the configuration of a molecule changes? We examine two molecules here: 1,2-dichloroethane (CH₂ClCH₂Cl) and hexane (C₆H₁₄). We use energies and rotational constants from HF/6-31G* calculations.

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1,2-dichloroethane (CH₂ClCH₂Cl) can be in a gauche form (Cl-C-C-Cl dihedral angle of +- 60 degrees) or a trans form (Cl-C-C-Cl dihedral angle of 180 degrees). The trans form has the Chlorine and Carbon atoms closer to collinear which results in the larger rotational constant and smaller product of the moments of inertia.

gauche		trans

Results from HF/6-31G* calculations for a temperature of 298.15 K

		gauche	trans	units
Rotational constants	A B C	0.34194 0.07273 0.06372	0.98541 0.05003 0.04847	cm-1
Product of moments of inertia		3.023	2.005	106 amu3Å6
Entropy	Srot	104.79	103.08	J K-1 mol-1
Relative energy		8	0	kJ mol-1

see section III.A.9 Barriers to internal rotation for more data on 1,2-dichloroethane.

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hexane (C₆H₁₄) can be in an extended form or various forms which are more curled up. There are three C-C-C-C dihedral angles in hexane. In the extended form all three dihedral angles are 180 degrees. In the curled (helical) form all three angles are 60 degrees. In the very curled form the dihedral angles are +60, +60, -90. The last angle is different from -60 due to steric repulsion.

extended		curled (helical)		very curled

Results from HF/6-31G* calculations for a temperature of 298.15 K

		extended	curled	very curled	units
Rotational constants	A B C	0.49165 0.03786 0.03655	0.19887 0.05790 0.05776	0.15470 0.07193 0.05803	cm-1
Product of moments of inertia		7.041748	7.202027	7.418987	106 amu3Å6
Entropy	Srot	108.31	108.40	108.52	J K-1 mol-1
Relative energy		0	12	21	kJ mol-1

see section III.A.9 Barriers to internal rotation for data on rotation about the central bond in butane (C₄H₁₀), which should have a potential energy surface for internal rotation similar to hexane.

For calculating ideal-gas thermochemical properties see section I.D. A brief description of the thermochemical quantities and methods.