X-ray studies of the terpenes. I. Crystallographic data for α- l -menthol

Abstract

268 ORIENTED OVERGROWTHS OF ALKALI HALIDES ON SILVER SUBSTRATES exposed even to dry air for more than about 10 hr. began to lose their orienting power on crystals grown from the vapor, but in growth from solution oriented overgrowths were obtained on films several weeks old. The induced polymorphism (Schulz, 1951b) of RbBr (which depends on good atomic matching) indicates that also in growth from solution the deposit probably contacts the silver. It might be objected that during the growth from solution the orientation was induced not by the sfl4er but by the substrate for the silver (the mica or NaC1) acting through imperfections in the silver film. Against such objections are these cases: (1) NaCI" on (Ag-NaC1). The orientation (Fig. 2(b)) is different from NaC1 directly on NaC1 (Fig. 2(a)). (2) CsC1 and CsBr on (Ag-mica). The orienting forces were sufficiently strong to induce the abnormal structure. Directly on mica these salts have the normal structure and a [100] fiber orientation. (3) CsC1 and CsBr on (Ag-NaC1). The orientation (Fig. 2(a)) is different from that of the direct :deposit on NaC1 which gives either the abnormal structure with the orientation of Fig. 2 (a) or the normal structure with the orientation in Fig. 2 (d). These examples indicate that the silver films are the effective orienting agents. Conclusion In addition to showing the actual orientation in a large number of particular cases (Tables 2 and 3) the experiments lead to the following more general conclusions: (1) Although an oriented silver substrate may have a very irregular microscopic geometric surface, it can, nevertheless, induce a definite orientation in a deposit. This indicates that the orienting influence of the substrate is propagated to the deposit across any crystallographic plane as interface. As a consequence it is rather artificial (even if convenient) to speak of matching onto the particular crystallographic plane which happens to be parallel to the macroscopic substrate surface. (2) Factors determining the orientation of a deposit would include (a) the relation of its lattice constants to those of the substrate, (b) the method of its growth (vapor or solution) and (c) electrical conditions at the interface. These results are of importance in the study of corrosion since a typical case is that of an ionic layer forming over a metal substrate (Cabrera & Mort, 1948-49). The author wishes to thank Prof. C. S. Barrett for helpful suggestions during discussion of the results. The c~ form of /-menthol has been found to be trigonal with space group C31, a-21.60±6, c = 6.11 q-1 /~ and with three molecules in the asymmetric unit. Previous powder work is thus shown to have been interpreted wrongly and is here corrected.