Bond lengths, and beyond

Abstract

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-l,6:8,13 biscarbonyl[ 14]annulene, tricycio[8.4.1.13,8 ]hexadeca-2,4,6,8,10,12,14-heptaene-15,16-dione. Intensity data up to sin0/2 = 1.14,~, -I 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,~,. Root mean square (r.m.s.) amplitudes of vibration for C and O atoms have been determined to a precision of ca 0.0006 ,~. Based on the least-squares or's, uncertainties for the C--C and C--O bond lengths are 0.0005 ,~, 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 Vp = 0). Contour maps of ,Oex p and its negative Laplacian (--VZ,Oexp) 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.

A Topological Analysis of Charge Densities in Diamond, Silicon and Germanium Crystals

“…The field of its application to experimental (X-ray diffraction) charge densities is rapidly developing now (e.g. Kapphahn, Tsirelson & Ozerov, 1988;Craven & Stewart, 1990;Gatti, Bianchi, Destro & Merati, 1992;Destro & Merati, 1995;Iversen, Larsen, Souhassou & Takata, 1995;Tsirelson & Ozerov, 1996) owing to the possibility of analytically representing experimental p by various multipole techniques (Hirshfeld, 1971;Stewart, 1976;Hansen & Coppens, 1978), allowing analytical evaluation of the Laplacian, V2p, and gradient vector field of p. Although it seems that there is no fundamental restriction on the applicability of topological analysis to experimental p (Tsirelson, 1996), the numerical results in this case are affected by the thermal atomic motions in crystals (Tsirelson, 1996;Gatti, Bianchi, Destro & Merati, 1992) and in fact refer to the mean thermal nuclear distribution. In this situation, the only way to evaluate indirectly to what extent quantitative values of topological characteristics depend on the atomic thermal motion effects may be a comparative topological analysis of theoretical and experimental charge densities simultaneously.…”

Hirshfeld Atom Refinement

“…At 12 and 50 K, the su's are $0.0007 or $0.007 Å ; at the higher temperatures they are 50% larger. we mention again the precision of 0.007 Å quoted by Destro & Merati (1995) in a multipole analysis of X-ray data with a high resolution (d max ' 0.44 Å ). We achieve a comparable result with relatively low-resolution data (d max ' 0.65 Å ).…”

“…However, H-atom positions were not compared in this study. A later study of syn- 1,6:8,13-biscarbonyl[14]annulene at 19 K by Destro & Merati (1995) reported a precision in the positions of better than 0.0004 Å for non-H atoms and of $0.007 Å for H atoms. The average of the ten aromatic C-H bond lengths was found to be 1.087 (7) Å , indistinguishable from the average of neutron determined C ar -H distances of 1.083 (11) Å (Allen et al, 2004).…”

“…In the case of the [14]annulene mentioned above, H-atom ADPs were estimated from a TLS (translation/libration/screw coupling) analysis of the C atoms, supplemented with values estimated from spectroscopy for the C-H stretch and bend motions (Destro & Merati, 1995). Subsequently, several different and increasingly sophisticated methods for approximating H-atom ADPs have been proposed.…”

A Test of the Suitability of CCD Area Detectors for Accurate Electron-Density Studies

“…A least-square fit to the resulting 240 values of sin20 gave the following cell parameters: a = 29.831 (4), b = 15.505 (2), c : 11.985 (1) A and V 5543 (1) ,~3. The procedures for data collection and processing, which included a correction for scan-truncation effects, are similar to those recently described for syn-l,6:8,13-biscarbonyl-[14]annulene (Destro & Merati, 1995) and citrinin (Roversi et al, 1996).…”