EXPERIMENTAL CHARGE-DENSITY STUDY ON AN IONIC CROWN-ETHER COMPLEX - [KC18-CROWN-6]N-+(3)-CENTER-DOT-H2O

The low-temperature (120 K) structure experimental charge density distribution and electrostatic potential of the title complex were determined from high-resolution X-ray diffraction intensities. The molecular structure and packing is in accord with previous observations on analogous compounds. The nuclear-centered spherical harmonics expansion technique was applied in the treatment of the data to extract the static electron density. Deformation electron density maps in nonbonded regions of the oxygen atoms show lone-pair polarization toward the cation. This valence deformation is compared to that found in the neutral host-guest complex of 18-crown-6.2cyanamide. The anisotropic atomic displacement parameters are tested against the rigid-body motion model, and the residuals for both structures are analyzed with computer graphics. The degree of transferability in the topology of static density deformations is examined in connection with the transferability in the pattern of residual root-mean-square surfaces representing atomic displacements due to internal motion.