Measurement of Electric Fields Experienced by Urea Guest Molecules in the 18-Crown-6/Urea (1:5) Host–Guest Complex: An Experimental Reference Point for Electric-Field-Assisted Catalysis

High-resolution
synchrotron and neutron single-crystal diffraction
data of 18-crown-6/(pentakis)­urea measured at 30 K are combined, with
the aim of better appreciating the electrostatics associated with
intermolecular interactions in condensed matter. With two 18-crown-6
molecules and five different urea molecules in the crystal, this represents
the most ambitious combined X-ray/synchrotron and neutron experimental
charge density analysis to date on a cocrystal or host–guest
system incorporating such a large number of unique molecules. The
dipole moments of the five urea guest molecules in the crystal are
enhanced considerably compared to values determined for isolated molecules,
and 2D maps of the electrostatic potential and electric field show
clearly how the urea molecules are oriented with dipole moments aligned
along the electric field exerted by their molecular neighbors. Experimental
electric fields in the range of 10–19 GV m–1, obtained for the five different urea environments, corroborate
independent measurements of electric fields in the active sites of
enzymes and provide an important experimental reference point for
recent discussions focused on electric-field-assisted catalysis.