Stereochemistry of crown ether analogs in the solid state and in solution. X-ray crystal structure, molecular mechanics calculations and nuclear magnetic resonance studies of benzo-16-crown-5

The X-ray crystal structure of the title molecule indicates that there is one 'unusual' transoid O-C-C-O torsional network in the macrocyclic ring. C-13 NMR chemical shift differences between solution and solid stare spectra are discussed in light of the solid state geometry. In the gas phase, calculations indicate that the X-ray crystal structure is not the most stable conformation. Rather, an optimized X-ray structure is favoured which retains the one trans O-C-C-O network, but has substantially expanded geometries for the remaining three O-C-C-O moieties. The total energy of this structure is 0.83 kcal/mol lower than that obtained for a structure whose geometry was obtained via preliminary MM + optimization, followed by further minimization via the AM1 method. (C) 1998 Elsevier Science B.V. All rights reserved.