A reappraisal of the structure of the second stable conformer of buta-1,3-diene

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Abstract
The structure of the second stable conformer of buta-1,3-diene was investigated by computing the torsional frequencies and the resulting variation of the heat capacity with temperature (220–320 K) from potential energy curves based (a) on calorimetric and spectroscopic data and (b) on a comprehensive set of ab initio calculations. In general, there was good agreement between the ab initio predictions and the experimental frequencies localized in the trans-well. However, the ab initio values for the torsional frequencies localized in the second well, and the temperature variation of Cp° based on the potential curves derived from the spectroscopic data, did not support the assignment of frequencies observed at 271.1 and 263.9 cm–1 to the second stable form. A new potential energy curve is proposed, which is in better agreement with the predictions of the ab initio calculations, and which accounts very well for the observed spectra and temperature variation of Cp°.

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