Conformational Characteristics of Feet-to-Feet-Connected Biscavitands.

X-ray crystallography of an acetoxy-protected bisresorcinarene and biscavitands possessing phosphonate and dialkylsilyl bridges revealed that the bisresorcinarene and the biscavitands adopt helical forms in the solid state. Helical conformations were also found in solution. The helix-helix interconversions of the biscavitands occurred with high activation barriers of more than 50 kJ mol. The activation parameters of the helix-helix interconversions were determined using exchange spectroscopy (EXSY). The positive activation enthalpies and the negative activation entropies suggest that the transition states of the helix-helix interconversion process are most likely more strained and symmetric than the ground states. The compensatory enthalpy-entropy correlation is found in the series of activation parameters, giving rise to a compensation temperature of 254 K.