Probing the conformational properties of guest molecules in solid inclusion compounds via EXAFS spectroscopy: bromine K-edge EXAFS studies of the bromocyclohexane/thiourea and trans-1-bromo-2-chlorocyclohexane/thiourea inclusion compounds

Abstract

Thiourea inclusion compounds comprise a thiourea host structure containing one-dimensional (1D) tunnels within which guest molecules are accommodated. Bromine K-edge EXAFS spectroscopy has been used to investigate local structrural properties of bromocyclohexane and trans-1-bromo-2-chlorocyclohexane guest molecules included within the thiourea tunnel structure.For the trans-1-bromo-2-chlorocyclohexane/thiourea inclusion compound, the intramolecular Br⋯Cl distance (ca. 4.50 A) determined from Br K-edge EXAFS spectra recorded between 295 and 80 K indicates that the trans-1-bromo-2-chlorocyclohexane guest molecules are in the diaxial conformation, and the intramolecular Br⋯C(3,5) distance (ca. 3.27 A) further confirms that the bromine substituent is in an axial position with respect to the cyclohexane ring. Furthermore, an intermolecular Br⋯Br distance is observed for this inclusion compound at 80 K, suggesting that there is head-to-head ordering of the guest molecules within the thiourea tunnel structure at this temperature.For the bromocyclohexane/thiourea inclusion compound, the intramolecular Br⋯C(3,5) distance (ca. 3.27 A) indicates that the major conformation of the bromocyclohexane guest molecules is the axial conformation. An intermolecular Br⋯Br distance is evident from the EXAFS data (at both 295 and 80 K), suggesting that there is head-to-head ordering of the bromocyclohexane guest molecules in this inclusion compound.