Structure of guest-host complexes of β-cyclodextrin with arenes:: a quantum-chemical study

The structure of beta-cyclodextrin (beta-CD), as well as the structure and energetics of beta-CD-naphthalene, beta-CD-fluorene, beta-CD-phenanthrene, beta-CD-cyclohexane (1 : 1), and beta-CD-naphthalene (2 : 2) inclusion complexes was studied by the semiempirical MNDO/PM3 method. Calculations of a beta-CD-naphthalene-cyclohexane (1 : 1 : 1) complex were also performed. The minimum heat of formation was found for the symmetric beta-CD conformation with C-7 symmetry axis. The structure is stabilized by the ring of interunit H-bonds formed by the protons of the 2-OH groups and the O atoms of the 3'-OH groups of the glucose units, preferableness of this orientation of interunit H-bonds was confirmed by ab initio calculations of the molecule of alpha-(1-4)-glucobiose (maltose) in the MP2/6-31G(d,p)//6-31G(d,p) approximation. The formation of any inclusion compounds of beta-CD with arenes is energetically favorable: the complexation energy varies in the range -9 to -12 kcal mol(-1). Among complexes with naphthalene, that of composition 2 : 2 is the most energetically favorable, which is in agreement with experimental data. In this complex, beta-CD exists as a dimer of the "head-to-head" type, in which both partners are linked by a system of H-bonds. The structure of the "head-to-head" dimer of beta-CD was simulated by ab initio calculations of the H-bonded dimer of alpha-D-glucose in the RHF/6-31G(d,p) approximation. In the dimer, both components are linked by a pair of H-bonds formed by the protons of the 3-OH groups and the O atoms of the 2-OH groups. The dimerization energies obtained from ab initio and semiempirical MNDO/PM3 and AM1 calculations differ by about 2.5 times (8.6 vs. 3.2 and 3.8 kcal mol(-1), respectively).