2 EXPERIMENTAL ELECTRON DENSITY STUDIES – THE BASICS
2.1 Principles of X‐ray Diffraction
2.1 Principles of X‐ray Diffraction
The basis of an X‐ray diffraction experiment is the interaction of X‐rays with crystalline matter.[3] In general, diffraction occurs when electromagnetic waves impinge on a periodic lattice of scattering material. To observe significant diffraction, the spacing d between the lattice planes of the scatterer and the wavelength λ of the incident beam should have the same order of magnitude. As the lattice spacing of single crystals is in the range of 1‐100 Å, X‐ray as well as neutron or electron beams are applicable for these kinds of diffraction experiments.
Whenever electromagnetic waves hit a three‐dimensional periodic scatterer, they are diffracted at discrete lattice points. The scattered waves, also called secondary waves, interact with each other. These interferences can be destructive and in their extreme example lead to a total extinction of the secondary wave. However, when the phase shift between the secondary waves is a multiple to 2π, the interference between the secondary waves is constructive (cf. Figure 2‐1). This is the case, when the diffraction condition, expressed by BRAGG's law, is fulfilled.[4]
2dsinθ =nλ (Eq. 2‐1)
Here, d is the spacing between the diffracting planes, θ the incident angle, and λ the wavelength of the beam. A diffraction pattern of distinct reflections is obtained by measuring the intensity of the scattered waves as a function of the scattering angle.
Figure 2‐1: Diffraction of waves at a three‐dimensional periodic lattice.
The arrangement of the reflections in the reflection pattern contains information on the crystal symmetry, i.e. its space group. The intensities of these reflections are linked to the electron density distribution in the crystal by a FOURIER transformation. This connection between the X‐ray intensities I and the elastic scattering amplitudes F(H) is provided by the kinematic theory of scattering by BORN[128]:
I∝F(H)2 (Eq. 2‐2)
H = ha* + kb* + lc* is defined as the scattering vector, with integral components concerning the reciprocal axis.[129] F(H) is also called the structure factor and can be expressed as the FOURIER transform of the static electron density distribution in the crystal.
F(H)=
∫
Vρ(r)⋅e2π i Hrdr (Eq. 2‐3)Reversely, the electron density of the crystal is the FOURIER transform of the structure factors. This coherence is used for the structure solution.
( )
r F( )
H iHrdrV
π
e−2
⋅
=
∫
ρ (Eq. 2‐4)
The integration can – to a good approximation – be replaced by a summation over all structure factors within the volume V of the unit cell.
( )
=∑
⋅ −H
H Hr
r F i
V
π
e 2
) 1 (
ρ (Eq. 2‐5)
Thus, theoretically, via a FOURIER transformation, the electron density ρ(r) can be calculated from the structure factors F(H), which are available from the diffraction experiment via the measured intensities. However, this direct evaluation causes some problems due to experimental shortcomings.
• The observed structure factors are affected by experimental errors. Even if these errors are corrected by different routines, they are nevertheless inherent in the data.
• As only a finite number of reflections can be collected, FOURIER truncation errors occur.
• The phase information is lost during the measurement. This is due to the fact, that the measured intensities are proportional to the squared structure factors.
Therefore, the electron density in the crystal can virtually not directly be calculated from the observed structure factors and has to be modeled. Fortunately, in practice the FOURIER transformation can be bypassed, because quantum mechanics facilitates the construction of a mathematical model of the electron density in a crystal. If the arrangement of the atomic nuclei in the crystal lattice is known, the structure factors can be calculated from a parameterized model. These calculated structure factors Fcalc are then compared to the observed structure factors Fobs. By optimizing the para‐
meters of the calculated model Fcalc is adjusted to Fobs in a least‐squares refinement.
Within the convolution approximation[130] the electron density can be formulated as the sum over the atomic fragments.
A FOURIER transformation leads to another description of the structure factor.
=
∑
⋅Here, fj(H) is the atomic form factor (or atomic scattering factor) of the atom j and tj(H) is the temperature factor. fj depends on two factors: the atom type and the scattering angle θ. The scattering power increases with the atomic number of the element due to the growing number of electrons and it decreases with an increasing
scattering angle. Therefore, each atom in a molecule contributes in a different way to the diffraction pattern, depending on its scattering power and its position in the unit cell. In return, the diffraction pattern contains information on the position of the atoms and the number of electrons, from which the type of the atom can be deduced.
Due to the thermal motion of atoms in a crystal the electron density – and that way, the positions of its maxima – can be assigned less accurately. Therefore, ρ(r) is the time‐average of the sum of atomic electron densities, which can be described as pure vibrational states.[131] This can be expressed within the convolution approximation, which assumes the total electron density to be composed of a superposition of density following the motion of the nucleus it is attached to.[130] In most cases, the thermal motion is sufficiently described by a probability function obeying a harmonic approximation.
u is the nuclear displacement vector and U the mean square displacement amplitude (MSDA), respectively. Sometimes, however, the thermal vibrational motion is described insufficiently by the harmonic approximation. In those cases the GRAM‐CHARLIER‐expansion can be applied to model small anharmonicities.[132]