Observing electron density

An article by Zuo et al concerning charge density studies published in Nature (401, 21; 1999) was accompanied by comments on the significance of the work by Colin Humphreys (U. of Cambridge), past chairman of the IUCr Commission on Electron Diffraction. M.A. Spackman, J.A.K. Howard, and R. Destro wrote a letter to Nature expressing concern about Humphreys' comments. The Spackman letter, which Nature declined to publish, is printed here.

The news and views article by Humphreys (Nature 401, 21; 1999) commenting on the recent charge density work of Zuo et al. on Cu₂O was welcome in the pages of Nature, but one could be forgiven for gaining the impression that detailed imaging of bonding electrons in crystals is a rare feat. Fortunately, that is nowhere near the truth. Extraction of detailed electron density distributions of organic, organometallic, inorganic, ionic metallic and mineral crystalline systems from X-ray diffraction data is now a mature and highly productive field, and one of the most dynamic areas of modern X-ray crystallography. It embraces not only advances in single crystal X-ray diffraction (for example synchrotron radiation and high-speed data collection), but increasingly incorporates data from complementary methods, including powder diffraction, electron diffraction, polarized neutron diffraction, and theoretical and computational chemistry. An excellent introduction is afforded by recent monographs by Coppens (1) and Tsirelson and Ozerov, (2) and virtually all experimental work preformed in the field in the years 1992 - 1997 has been conveniently summarized in two review articles. (3, 4) Humphreys statement that X-ray diffraction “is normally unable to give details about the shape of the charge distribution, in particular the shape of the bonds” is completely untrue, and he goes on to erroneously attribute this to the fact that X-ray scattering from dislocations and defects “is greater than the scattering from bonding electrons”. In the overwhelming majority of modern experimental charge density studies, even those based only on X-ray diffraction; this is emphatically not the case. Such studies are now commonly performed at cryogenic temperatures, use synchrotron sources as well as more conventional laboratory X-ray sources, and CCD cameras as well as increasingly sophisticated detectors. These studies are now yielding not only the shape of the bonds, but a vast amount of chemical and physical insight based upon a three-dimensional picture of the electron distribution in the crystal. Dipole and higher moments of molecules in the crystal, electrostatic potentials, electric fields and electric field gradients at atomic nuclei, and even intermolecular interactions energies are common outcomes, and the total electron distribution is now almost routinely analyzed using Bader’s quantum theory of atoms in molecules, (5) affording additional detailed insight into bonding in crystals. The work reported by Zuo et al. is significant, in that it represents an excellent example of the synergistic interaction between X-ray diffraction, electron and modern theoretical methods as described above. But many more studies of this calibre have been published and are currently in progress. It is disappointing to us that Humphreys appears to have been unaware of them, and hence failed to put the work in the much broader perspective that the field deserves.

(1) P. Coppens, X-ray Charge Density and Chemical Bonding, Oxford Univ Press, NY, 1997;
(2) V.G. Tsirelson and R.P. Ozerov, Electron Density and Bonding in Crystals, Inst of Physics Publishing, Bristol, 1996;
(3) M.A. Spackman and A.S. Brown, Charge Densities from X-ray Diffraction Data, Ann. Rep. Prog.Chem., Sect. C, Phys. Chem., 91, 175-212, 1994;
(4) M.A. Spackman, Charge Densities from X-ray Diffraction Data, Ann. Rep. Prog. Chem., Sect. C, Phys. Chem., 94, 177-207, 1997;
(5) R.F.W. Bader, Atoms in Molecules - Quantum Theory, Oxford Univ Press, Oxford, 1990.