화학공학소재연구정보센터
Journal of the American Chemical Society, Vol.122, No.15, 3708-3714, 2000
Solid-state C-13 chemical shift tensors in terpenes. 3. Structural characterization of polymorphous verbenol
Solid-state NMR analyses demonstrate that verbenol is a mixture of polymorphic materials containing two distinct lattice types. The major crystalline component, characterized by X-ray, consists of three molecules per asymmetric unit. The minor form contains four molecules per asymmetric unit but crystals suitable for X-ray analysis are unavailable. The minor polymorph is metastable and may be converted to the major form by melting and cooling to resolidify. Experimental and ab initio computed chemical shift tensors are compared to demonstrate that most of the structural variations in both polymorphs arise from differences in the orientation of the hydroxy hydrogen. Intramolecular steric interactions partially restrict allowed hydroxy hydrogen dihedral angle orientations. For the major form, comparison of experimental and various theoretical tensor principal values further narrows this dihedral angle to the range between 45 degrees and 80 degrees. Hence, tensor predictions generally agree with X-ray determined angles of 55 degrees, 69 degrees, and 74 degrees for the hydroxy dihedral angle. This tensor analysis is also applied to the minor form, but with less available information, only plausible structures can be proposed. Utilizing both the steric restrictions and principal and isotropic shift values, it is suggested that the minor form may involve molecules with dihedral angles near either 60 degrees or 180 degrees. Previous studies on Similar compounds show that such a lattice could consist of either a ring or a continuous repeating helical structure.