Electron density map

Carte de densité électronique (Fr). Elektronendichte-Karte (Ge). Mappa di densità elettronica (It). 電子密度図 (Ja). Mapa de densidad electrónica (Sp).

Definition

A three-dimensional description of the electron density in a crystal structure, determined from X-ray diffraction experiments. X-rays scatter from the electron clouds of atoms in the crystal lattice; the diffracted waves from scattering planes [math]\textstyle{ h, k, l }[/math] are described by structure factors [math]\textstyle{ \mathbf{F}_{hkl}. }[/math] The electron density as a function of position [math]\textstyle{ x, y, z }[/math] is the Fourier transform of the structure factors:

[math]\textstyle{ \rho(xyz) = {1\over V}\sum_{hkl} F(hkl)\exp[-2\pi i(hx + ky + lz)] }[/math].

The electron density map describes the contents of the unit cells averaged over the whole crystal and not the contents of a single unit cell (a distinction that is important where structural disorder is present).

Three-dimensional maps are often evaluated as parallel two-dimensional contoured sections at different heights in the unit cell.

Units

Electron density is measured in electrons per cubic ångström, e Å⁻³.