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–1. 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.