Лако-красочные материалы — производство

Pigments are comparatively large molecules. The computation time for a crystal structure calculation is approximately linearly proportional to the number of atoms per molecule (more accurately, per asymmetric unit), but depends exponen­tially on the number of degrees of freedom. However, pigment molecules are fairly rigid and have limited conformational flexibility, which reduces computa-

Figure 8.8 Crystal structures of pigments which have been solved from nonindexed X-ray powder diagrams by crystal structure prediction.

tional expenses if only one or a few conformations must be considered. In many cases, even distinct low energy conformations may be obtained from molecular or quantum mechanical calculations.

Crystal structure calculations have been successfully employed to obtain the structures of DPPI [46], BTI [47] (Figure 8.8), the а-form of quinacridone [11, 48], and the а-form of copper phthalocyanine [49] and a dioxazine pigment (see Chap­ter 20). Because of low crystallinity and excessive peak overlap, the powder pat­terns of these compounds could not be indexed. In all cases global minimization techniques yielded structure models, which subsequently could be refined by Riet — veld methods to solve the structures.

In the case of DPPI, two symmetrical rotamers were taken into account with dihedral angles of 60° and 90° between the perylene plane and the phenyl substit­uent. According to the expected probability of occurrence, centrosymmetric space groups (P 1, P 21/c, C 2/c) were selected for the calculation. Possible structures were generated systematically and the energy of each structure minimized con­sidering full molecular flexibility. Automated comparison of the resulting pack­ings to the experimental powder pattern [50] yielded a structure model in space group P 21/c suitable for refinement. However, this model, which could be refined to Rwp= 0.229, was only ranked fourth by packing energy.

A similar procedure was used to determine the structure of the a-form of cop­per phthalocyanine, P. B. 15 (Figure 8.9). This structure determination helped to clarify contradictory statements in the previous literature [51, 52], which have over time led to a somewhat enigmatic status of what is called an a-phase of phthalo — cyanines in the literature and in textbooks.

8.4.3