Significant conformational changes associated with molecular transport in a crystalline solid

Confocal Raman microspectrometry has been applied as an in situ probe of the transport of guest molecules along the one-dimensional tunnels in a crystalline urea inclusion compound, under conditions of guest exchange in which "new" guest molecules (pentadecane) are introduced at one end of the tunnel and displace the "original" guest molecules (1,8-dibromooctane). The Raman spectra, recorded as a function of position along the tunnel direction and as a function of time, demonstrate that the transport process is associated with a significant change in the conformational properties of the original (1,8-dibromooctane) guest molecules. In particular, in the boundary region between the original and new guest molecules, there is a substantial increase in the proportion of 1,8-dibromooctane guest molecules that have the gauche end-group conformation. The wider implications of this observation are discussed in relation to fundamental aspects of the molecular transport process in this material.