A statistical survey of the Cambridge Structural Database concerning density and packing

We have carried out a statistical survey of the Cambridge Structural Database concerning crystallographic density d, averaged atomic weight AA(w) (defined as molecular weight divided by the number of non-hydrogen 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 unit cell volume per molecule (or set of molecules in salts and co-crystals) and d(i) = M(w)/V-M the intrinsic density (the ratio of molecular weight over molecular volume). The molecular volume has been calculated 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 compound 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 density d. The tert-butyl subset has been selected to illustrate this analysis.