Potential surfaces ethane rotation

The new results by Buenker et al. [87] constitute a challenge to the interpretation of the ethane spectrum what became traditional in the last 30 years. It clearly indicates the need for more theoretical work with an even more extended basis set and a computation of the potential surfaces for all the relevant vibrational motions, implying a closer look at the consequences of the Jahn-Teller effect in the degenerate states as well as possible vibronic couplings between close-lying excited states. Equally needed are experimental works carried out at even higher resolution than has hitherto been possible more information on the vibrational and rotational structures are badly needed. This concerns not only the absorption spectra but also the electron-impact and photoelectron spectra. 

Looking more globally at the potential surface, we encounter phenomena such as rotational barriers and inversion barriers. The barriers to rotation in ethane and many other compounds are described well by any of the basis sets so far considered (STO-3G, 3-21G, 6-31G ), even at the HF level. However, when the barriers are small, as in the case of methanol ( 1.1 kcal/mol), the HF method tends to overestimate A rot. 

The origin of a torsional barrier can be studied best in simple cases like ethane. Here, rotation about the central carbon-carbon bond results in three staggered and three eclipsed stationary points on the potential energy surface, at least when symmetry considerations are not taken into account. Quantum mechanically, the barrier of rotation is explained by anti-bonding interactions between the hydrogens attached to different carbon atoms. These interactions are small when the conformation of ethane is staggered, and reach a maximum value when the molecule approaches an eclipsed geometry. 

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