VII.C.9. |
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 (CH_{2}ClCH_{2}Cl) and hexane (C_{6}H_{14}). We use energies and rotational constants from HF/6-31G* calculations.
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 | 10^{6} amu^{3}Å^{6} | |
Entropy | S_{rot} | 104.79 | 103.08 | J K^{-1} mol^{-1} |
Relative energy | 8 | 0 | kJ mol^{-1} |
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 | 10^{6} amu^{3}Å^{6} | |
Entropy | S_{rot} | 108.31 | 108.40 | 108.52 | J K^{-1} mol^{-1} |
Relative energy | 0 | 12 | 21 | kJ mol^{-1} |
For calculating ideal-gas thermochemical properties see section I.D. A brief description of the thermochemical quantities and methods.