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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.


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.
gauchetrans
gauche dichloroethane trans dichloroethane

Results from HF/6-31G* calculations for a temperature of 298.15 K
gauchetransunits
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 amu36
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.



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
linear hexane curved hexane very curved hexane

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 amu36
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.