Raman microspectrometry as a new approach to the investigation of molecular recognition in solids: Chloroform-cryptophane complexes

Information about the orientation and dynamics of a chloroform molecule encaged in cryptophane-A and cryptophane-E is obtained by means of Raman microspectrometry. The microscopy technique allows us to study the two observed crystalline structures of the CDCl3-cryptophane-A complexes separately. The spectra, recorded under different polarization conditions, are compared with those of CHCl3-D-30-cryptophane-A and analyzed in light of the crystalline structures determined by X-ray diffraction. The spectral analysis is improved by ab initio calculations performed for the isolated guest and host molecules. The orientation of the encaged chloroform with the C-H bond pointing toward the center of the cyclotriveratrylene structures is confirmed. The vCD (or vCH) stretching wavenumber of chloroform is red shifted upon complexation as compared to the wavenumber of the gaseous isolated state. This red shift, larger for the CDCl3-cryptophane-E complex than for the CDCl3-cryptophane-A complex, is the signature of stronger binding host-guest interactions with cryptophane-E. In addition, the analysis of the band shape of the vCD stretching vibration of chloroform suggests different rotational dynamics for the guest molecule encaged in cryptophane-A or in cryptophane-E. This work illustrates the capability of the Raman microspectrometry technique to investigate molecular recognition phenomena in solids.