The temperature dependence of the CH3...π host-guest interaction in the p-tertbutylcalixarene 1:1 toluene complex has been investigated by comparison of its known molecular structure at room temperature (RT) with that at 220K determined by single crystal X-ray diffraction. The diffraction pattern snowed a phase transition when the temperature was decreased from 298 to 220 K. The structure at 220 K could be solved assuming a twin by pseudo-merohedry with a fourfold twin axis  relating two monoclinic components with equal volumes of space group P 112/a a=b= 17.899(2), c=13.827(1) Å, V=4429.8(8) Å3, Z=2, mol. weight 741.06 a.m.u., Dcalc = 1.111 g·cm-3. The structure refinement converged to R1=0.103 and wR2=0.256 for 1655 unique observed data. The complex exists in two different conformations of the hosts which exhibit two different host-guest structural relationships both indicating that the most relevant differences induced by the low temperature are concerning the host-guest interaction mode. Particularly unexpected is the different temperature dependence of the CH3...π interactions between the tert-butyl of the host and the aromatic moiety of the guest with respect to that of the van der Waals interactions. The CH3...π interactions, which stabilizes the complex at RT, strongly decrease as the temperature decreases.
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