Calculation of average values for CC bond lengths and CCC bond angles. PCILO conformational study of n-butane with the newly found and other molecular geometries

The best average values for CC bond lengths and CCC bond angles were calculated for fragments in which the CC bond was at least one CC bond distant from heavy atom substituents by using the results of searches of the Cambridge Structural Database. The values found, 1.51 Å and 113° for a CCCH3 fragment, and 1.52 Å and 114° for a fragment inside a carbon chain, differ significantly from the values commonly used in theoretical calculations which are generally based on a limited set of crystallographic data or on the diamond structure. To test the sensitivity of quantum-chemical energy calculation methods to different input geometries, the PCILO method was taken as an example and checked for reproducibility of experimental conformational energy for the n-butane molecule. Energy minimization with respect to the CC bond length and the CCC bond angle led to values of 1.49 Å and 115°, respectively. For this geometry the gauche conformation was found to be preferred by 1.4 kJ mol−1 over the trans conformation. This result is inconsistent with experimental data and indicates that the proper choice of geometrical parametrization is of the greatest importance in energy calculations.