Rotational barriers tabulated

Table A4.6 gives the internal rotation contributions to the heat capacity, enthalpy and Gibbs free energy as a function of the rotational barrier V. It is convenient to tabulate the contributions in terms of VjRTagainst 1/rf, where f is the partition function for free rotation [see equation (10.141)]. For details of the calculation, see Section 10.7c.

The correction 8 is calculated using equation (38) and the values of the barrier corrections 8S (RH) and 5S ( R) are tabulated as functions of the rotation barrier V and of the partition function q of the internal rotation, which is assumed to be free (eqn 16). 

The preferred conformations of carbonyl compounds, like 1-alkenes, are eclipsed rather than bisected, as shown below for ethanal and propanal. The barrier for methyl group rotation in ethanal is 1.17kcal/mol. Detailed analysis has indicated that small adjustments in molecular geometry, including a-bond lengthening, must be taken into account to quantitatively analyze the barrier. The total barrier can be dissected into nuclear-nuclear, electron-electron, nuclear-electron, and kinetic energy (At), as described in Topic 1.3 for ethane. MP2/6-311+G (Mf,2p) calculations lead to the contributions tabulated below. The total barrier found by this computational approach is very close to the experimental value. Contributions to the ethanal energy barrier in kcal/mol are shown below. 

Ethane.—The tabulated thermodynamic functions for ethane are based on a value of (12.03 0,52) kJ mol for the 3-fold barrier to internal rotation derived from entropy measurements. This value also gave a good overall fit to the measured vapour heat capacity. A relatively early microwave value for the barrier height is (12.7 1.3) kJ mol. ... 

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