Macrocyclic Receptor Molecules for Guanidinium Cations. Preparation, X-Ray Structures, and Kinetic Stabilities of 1:1 Complexes of Guanidinium Perchlorate with Benzo-27-crown-9, Dibenzo-27-crown-9, and Dibenzo-30-crown-10

The complexation of guanidinium perchlorate (I) with 18-33 ring atom crown ethers and with different subunits (e.g., catechol and 1,3-xylyl moieties) in examd. by 2-phase extns. The 18-crown ethers are able to form perching complexes; crown ethers with >=27 ring atoms have a ring size capable of forming encapsulated complexes with I. Arom., catechol, and esp. 1,3-xylyl moieties have a destabilizing effect on complex formation. The crystal and the mol. structures of the 1:1 complexes of I with benzo-27-crown-9, dibenzo-27-crown-9, and dibenzo-30-crown-10, are detd. by x-ray crystallog. In these encapsulated complexes all H atoms of the guanidinium cation are used in H-bonds to the macrocyclic host. The 27-crown ethers show an optimal fit between cation and the macrocyclic host with a complementary binding scheme. Dynamic 500-MHz 1H NMR gave the kinetic stabilities of these complexes with Delta Gd.thermod. values of 11.5, 11.2, and 12.0 kcal mol-1 for the complexes with benzo-27-crown-9, dibenzo-27-crown-9, and dibenzo-30-crown-10, resp

Dispersion Properties of Explicit Finite Element Methods for Wave Propagation Modelling on Tetrahedral Meshes

We analyse the dispersion properties of two types of explicit finite element methods for modelling acoustic and elastic wave propagation on tetrahedral meshes, namely mass-lumped finite element methods and symmetric interior penalty discontinuous Galerkin methods, both combined with a suitable Lax–Wendroff time integration scheme. The dispersion properties are obtained semi-analytically using standard Fourier analysis. Based on the dispersion analysis, we give an indication of which method is the most efficient for a given accuracy, how many elements per wavelength are required for a given accuracy, and how sensitive the accuracy of the method is to poorly shaped elements.Applied Geophysics and Petrophysic