International
Tables for Crystallography Volume B Reciprocal space Edited by U. Shmueli © International Union of Crystallography 2006 |
International Tables for Crystallography (2006). Vol. B. ch. 2.1, p. 191
Section 2.1.2.3. An approximation for organic compounds^{a}School of Chemistry, Tel Aviv University, Tel Aviv 69 978, Israel, and ^{b}St John's College, Cambridge, England |
In organic compounds there are very many interatomic distances of about 1.5 or 1.4 Å. Adoption of the preceding criterion would mean that the inner portion of the region of reciprocal space accessible by the use of copper radiation is not within the sphere of intensity statistics based on fixed-index (first process) averaging. No substantial results are available for fixed-parameter (second process) averaging, and very few from the approximation to it (third process).
To the extent to which the third process is acceptable, an approximation to the variation of with is obtainable. The exponent in equation (2.1.2.2) can be written as where s is the radial distance in reciprocal space, is the distance from the jth to the kth atom and is the angle between the vectors s and r. Averaging over a sphere of radius s, with treated as the co-latitude, gives This is the familiar Debye expression. It has the correct limits for s zero and s large, and is in accord with the argument from resolution.