CONFORMATIONAL PROPERTIES OF GUEST MOLECULES IN CONSTRAINED SOLID-STATE ENVIRONMENTS - BROMINE K-EDGE X-RAY-ABSORPTION SPECTROSCOPY OF 2-BROMOALKANE UREA INCLUSION-COMPOUNDS/

Urea inclusion compounds containing 2-bromoalkane guest molecules are of considerable interest as they provide an opportunity to investigate host-guest chiral recognition between the chiral 2-bromoalkanes and t he chiral urea tunnel structure. To understand the conformational prop erties of the guest molecules in these solids, this paper reports brom ine K-edge X-ray absorption (EXAFS) studies of the 2-bromotridecane/ur ea and 2-bromotetradecane/urea inclusion compounds. There are two plau sible conformations for a 2-bromoalkane guest molecule within the urea tunnel structure in one conformation (denoted G), the bromine atom at tached to the chiral center is gauche and the methyl group attached to the chiral center is trans with respect to the alkane chain, whereas in the other conformation (denoted T), the bromine atom is trans and t he methyl group is gauche. The distance between the bromine atom and t he third carbon shell from the bromine atom is ca. 3.2 Angstrom for th e G conformation and ca. 4.3 Angstrom for the T conformation, allowing these conformations to be distinguished directly by bromine K-edge EX AFS spectroscopy. As a consequence of the incommensurate relationship between the host and guest substructures in the 2-bromoalkane/urea inc lusion compounds, no well-defined distances between the bromine atom a nd atoms of the host substructure are observed; the EXAFS data therefo re provide a selective probe of the local structural properties of the guest molecules. Analysis of the bromine K-edge EXAFS spectra indicat es that both G and T conformations of the 2-bromoalkane guest molecule s are present within the urea tunnel structure, with an excess of the T conformation. Considering both inclusion compounds over the range of temperatures (80-200 K) investigated, the proportion of guest molecul es in the T conformation is in the range 0.54-0.68. For an isolated 2- bromoalkane molecule, the G conformation is lower in energy than the T conformation, and tl-ius inclusion of the 2-bromoalkane within the ur ea tunnel structure promotes a relative stabilization of the conformat ion that is less favored in the isolated state. The more favorable hos t-guest interaction in the case of the T conformation provides a basis to rationalize the preference for this conformation within the urea t unnel structure.