STRUCTURE AND DYNAMICS OF t-BUTYLCALIX[4]ARINE INCLUSION COMPOUNDS. G.D. Enright, E.B. Brouwer, E.J. Gabe, C.I. Ratcliffe and J.A. Ripmeester, Steacie Institute for Molecular Sciences, National Research Council of Canada, Ottawa K1A OR6, Canada

We have carried out an extensive study of the structure and dynamics of several t-butylcalix[4]arine inclusion compounds with toluene, nitrobenzene, benzene, pyridine, cyclohexane, pentane, chlorobutane, xenon and methylcyclopentanone as guests. Overlap of the partially occupied symmetry-related guest sites is common in these compounds and often leads to difficulties in fully resolving the guest. By employing solid state NMR (a technique that is particularly sensitive to dynamics and microscopic structure) together with single crystal X-ray diffraction a greatly improved understanding of these systems has been obtained. We have clearly resolved a guest-induced distortion of the host lattice that is present in almost all of these compounds. The prevalence of a similar distortion, which can be described as a slight bending outward of the calix phenyl unit accompanied by a fixed rotation of the t-butyl group, with such a wide variety of guest molecules suggests that the interaction between the guest and host is dominated by steric effects. No contact distances significantly less than that determined from Van der Waals radii are observed. A comparison of the structure of the benzene and pyridine compounds reveals that the in-plane guest rotation observed in benzene is absent in pyridine despite the similar positions occupied by the guests. Although it has been suggested that weak N...H-C hydrogen bonding may stabilize the pyridine guest in such compounds, we believe that steric effects can explain these observations as well.