Chemical reasonableness in Rietveld analysis; organics

Abstract

We know a lot about normal values of bond distances, bond angles, torsion angles, and other molecular parameters. This knowledge can be incorporated into the structure solution process and into Rietveld refinement through the use of restraints and rigid bodies. An important measure of the quality of the refined model is the chemical reasonableness of molecular geometry. Refinement of the structures of calcium tartrate tetrahydrate and guaifenesin is used to illustrate the importance of chemical reasonableness in determining the quality of a Rietveld refinement.

Racemic calcium tartrate tetrahydrate [form (II)] in rat urinary stones

“…5). In spite of the coordination differences, the sum of the Ca-O bond valences (Brown & Altermatt, 1985) are similar, 2.22 and 2.19 in forms (I) (Kaduk, 2007) and (II), showing that the Ca is 'crowded'. As stated by Kaduk (2007), the large amount of energy gained by the formation of the hydrogen bonds in both forms helps explain the high Ca bond valence and the relatively high density of these compounds.…”

“…In spite of the coordination differences, the sum of the Ca-O bond valences (Brown & Altermatt, 1985) are similar, 2.22 and 2.19 in forms (I) (Kaduk, 2007) and (II), showing that the Ca is 'crowded'. As stated by Kaduk (2007), the large amount of energy gained by the formation of the hydrogen bonds in both forms helps explain the high Ca bond valence and the relatively high density of these compounds. Thermodiffractometry would be needed to confirm if the multiple events seen on the DSC, TGA and DTA of CaT forms (I) and (II) FULLPROF (Rodriguez-Carvajal, 1993) Rietveld plot obtained after the ab initio structure solution and refinement from powder data.…”

“…The crystal structure of the previously known form of CaT, denoted form (I), which frequently grows in bottles of wine, has been reported four times (Ambady, 1968;Hawthorne et al, 1982;Boese & Heinemann, 1993;Kaduk, 2007) in the P2 1 2 1 2 1 space group. A racemic CaT [form (II)] has been mentioned (Mastai et al, 2002), but the crystal structure of this thin-needle-shaped form was not reported, although it was said that Frolow, Zimmerman and Addadi (unpublished) have studied and solved it.…”

Utilizing organic and organometallic structural data in powder diffraction

“…Missing elements of a model can lead to other regions of a model being compromised in refinement, and thus when unusual geometry is observed, the structural model deserves some scrutiny. Additional examples of the use of the CSD in Rietveld analysis can be found in the paper of Kaduk (2007).…”

Chemical reasonableness in Rietveld analysis: Inorganics

“…In the early stages of refinement, the two crystallographically independent BF 4 anions were described as rigid bodies (Table I and Figure 5). The GSAS (Larson and Von Dreele, 2000) definitions and conventions described previously (Kaduk, 2007) were followed. The central atom of the tetrahedral group is located at the center of a cube, and the outer atoms occupy alternate corners of the cube.…”