Possible rearrangement mechanisms of hydrogen-bond arrays formed at the lower rim of tetrahydroxyc
alix[4]arene and tetrahydroxythiac
alix[4]arene were studied by means of density function
al theory and the resolutionidentity approximation modification of M
ller-Plesset perturbation theory (RI-MP2). Influence of solvent toheight of energy barriers was quantified by use of the conductorlike screening model (COSMO) of implicitsolvent (chloroform). Gener
ally, two types of mechanisms were investigated. The first is represented by asynchronous single-step jump of
all four hydroxyl protons. Pathways of the second mechanism include therotation of one or more hydroxyl groups around the C
Ar-O bond. Theoretic
al results, in agreement withrecently published experiment
al data (Lang et
al.
J. Chem. Phys. 2005,
122, 044056), prefer a jump mechanismfor the methylene-bridged c
alix[4]arene. Concerning the thiac
alix[4]arene, results obtained by COSMO aswell as RI-MP2 c
alculations show that the rotation
al mechanism is very competitive and it could even bemore favorable.