A STATISTICAL-SURVEY OF THE CAMBRIDGE-STRUCTURAL-DATABASE CONCERNING DENSITY AND PACKING

We have carried out a statistical survey of the Cambridge Structural D atabase concerning crystallographic density d, averaged atomic weight AA(w) (defined as molecular weight divided by the number of non-hydrog en atoms) and packing coefficient C. For 97 658 hits we have found the following average values: d = 1.6, AA(W) = 18.2 and C = 19.4 (instead of 18). We have introduced two other new parameters V-1 = V/Z the uni t cell volume per molecule (or set of molecules in salts and co-crysta ls) and d(i) = M(w)/V-M the intrinsic density (the ratio of molecular weight over molecular volume). The molecular volume has been calculate d for 24 496 hits (compounds containing H, Li, Be, B, C, N, O, F, Si, P, S, Cl, Br, I atoms). The equations d = 1.66 (d(i) x C-K) for a comp ound and (d(A)/d(B)) = (d(i)(A)/d(i)(B)) x (C-K(A)/C-K(B)) for a pair of compounds allows to understand the effects of atom nature (light vs . heavy atoms) and shape (smooth vs. rough) on the experimental densit y d. The tert-butyl subset has been selected to illustrate this analys is.