On the actual accuracy of structural parameters in single-crystal diffraction studies and possible origins of their systematic errors
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The analysis of single crystal diffraction data for CH 3 CN acetonitrile and (CH 3 ) 2 CO acetone molecules, both as pure substances and solvates, retrieved from Cambridge Crystal Structure Database, confirmed the existence of previously identified ‘blind area’ in the X-ray geometric parameters. In our study, the geometric parameters of two small molecules in crystals were compared to gas-phase data for the individual molecules and high-level quantum chemical calculations of both individual molecules and their small clusters. The data analysis showed that experimental points of good crystal structures tightly fill the area of 0.02–0.03 Å in bond lengths and 1-2 o in bond angles irrespectively of the formal precision when R < 0.05. The average X-ray bond lengths are systematically shortened by ca. 0.02 Å compared to gas phase and quantum chemical data and diminish with an increase in the temperature of X-ray study. Possible reasons for the observed trends including intramolecular dynamics and intermolecular interactions, are discussed.
