Friedif Calculator
The Friedif value of Flack and Shmueli: how much resonant-scattering contrast a composition offers, and therefore how well its absolute structure can be determined with a given radiation.
- You supply
- A chemical formula — brackets and charges included, as on the CHN page — and one or more radiations. It is a property of the composition, so no cell or density is required.
- Reading it
- 80 or more indicates few absolute-structure problems; 34 or lower means difficulty reaching an e.s.d. on the Flack parameter below 0.1. The references below give the thresholds in full.
Worked examples: a nickel complex · an organic cation
Input
Results
| Formula as entered | C32H26N2O2Ni |
|---|---|
| Read as | C32H26N2O2Ni |
| Friedif(Cu): | 223 |
| Friedif(Mo): | 534 |
Notes and references
- This is an implementation of an Excel spreadsheet calculation which was created by Flack/Shmueli: H. D. Flack and U. Shmueli, Acta Crystallogr. A, 2007, 63, 257–265.
- A Friedif value of 80 or more indicates only few absolute structure determination problems. A Friedif value of 34 or lower indicates difficulties to reach an e.s.d. on the Flack parameter less than 0.1. (see H. D. Flack, Acta Chim. Slov., 2008, 55, 689–691.)
- The quality of an absolute structure determinations is often derived from the Flack parameter x along with its associated e.s.d. u, thus x(u). u < 0.04 is associated with a strong inversion-distinguishing power, 0.04 < u < 0.1 means enantiopure-sufficient inversion-distinguishing power. A reliable absolute structure determination requires u < 0.04 and |x| < 2u. If there is a priori biological, chemical, or physical evidence for true enantiopurity of the compound of interest, 0.04 < u < 0.1 and |x| < 2u is also acceptable. see H. D. Flack and G. Bernardinelli, J. Appl. Cryst., 2000, 33, 1143–1148.