Systematic errors in structure models obtained by X-ray diffraction

The bonding maxima on C-C bonds as found in a previous X-ray study of electron distributions on double and triple C-C bonds are approximately a factor of two smaller than the values expected theoretically. The present theoretical study on an ethyne model structure shows that the reduction of the maxima is mainly due to systematic errors in the structural parameters caused by the use of non-banded atoms during the X-ray diffraction refinement. The systematic errors in the structural parameters can be kept smaller than the standard deviations obtainable for accurate X-ray work, by high-order X-ray refinement based on reflexions with 2 sin θ/λ larger than approximately 1.4 Å^-1. The height of the maxima on the centres of the bonds is only slightly affected by the omission of weak or high-order reflexions from the difference synthesis.