Bond lengths, and beyond

The degree of precision and accuracy in molecular geometry attainable with modem X-ray diffraction and cryogenic techniques is documented by presenting the results of a low-temperature study of syn-1,6:8,13 biscarbonyl[14]annulene, tricyclo[8.4.1.1 3,8 ]hexadeca-2,4,6,8,10,12,14-heptaene-15,16-dione. Intensity data up to sin θ/λ = 1.14 A -1 have been measured from a spherical crystal at T = 19 K. Their multipolar analysis, up to the octopole level for C and O atoms, and to quadrupoles for anisotropically described H atoms, has led to precisions in the coordinates of the heavy atoms which are better than 0.0004 A. Root mean square (r.m.s.) amplitudes of vibration for C and O atoms have been determined to a precision of ca 0.0006 A. Based on the least-squares 's, uncertainties for the C―C and C―O bond lengths are 0.0005 A, and those of the C―H bond distances 0.006-0.007 A. The topological properties of the experimentally derived charge density of the annulene molecule in the crystal have been determined by evaluating the location and nature of its critical points (points where ⊇ρ = 0). Contour maps of ρ exp and its negative Laplacian (-⊇ 2 ρ exp ) are presented. The occurrence of a bond critical point midway between the two C atoms of the carbonyl bridges seems to indicate the existence of a chemically unexpected weak bond between the two C atoms.