The effect of guest inclusion on the crystal packing of p-tert-butylcalix[4]arenes.

The effect of guest inclusion in the crystal structures of p-tert-butylcalix[4]arene complexes has been investigated through a combination of molecular-mechanics-based solid-state calculations and statistical analysis, with a procedure previously developed and used to study a variety of classes of organic compounds. The results indicate that the general trends in the behavior of calixarene crystals are very similar, irrespective of the presence or the absence of a guest encapsulated in the calixarene cavity, and are similar to those obtained for most other organics. Some differences arise only when a statistical analysis of several descriptors is performed. The investigation of the description of endo calix[4]arenes is extended by calculating the packing coefficient of the guest inside the calixarenes cavity, C(cavity), which shows that most of them are well accommodated inside the host and have coefficients that are similar to those found in the liquid phase. Further evaluation of the interaction energies between guest and host shows that the coefficients tend to be smaller than 30 kcal mol(-1). The combination of small C(cavity) and low interaction energies suggests that guest mobility in the solid could be rather common in endo complexes of calixarenes.