title: C-H(ax)...Y(ax) contacts in cyclohexane derivatives revisited-identification of improper hydrogen-bonded contacts.
abstract: The structure of 111 cyclohexane derivatives bearing the axial substitution Y(ax)-C was optimized at the B3LYP/6-31+G(d,p) level. The natural bond orbital analysis revealed the presence of overlap interactions between the axial substituent and the antibonding sigma*(C-Hax) orbitals; these calculated hyperconjugative interactions suggest the presence of improper H-bonded contacts. The addition of an appropriate bridging fragment between the axial substituent and cyclohexane carbon strengthens significantly the hydrogen-bonding component of the contact and several structures of axially substituted cyclohexane derivatives including such hydrogen-bonded C-H(ax)...Y(ax)-C contacts were retrieved from the Cambridge Crystallographic Database. Overall, the calculations predicted that the C-H(ax)...Y(ax)-C contacts in common cyclohexane derivatives that are generally thought to be steric in nature (Pauli repulsive forces) include an improper hydrogen-bonding component.
authors.affiliation: Faculty of Pharmacy, Department of Pharmaceutical Chemistry, University of Athens, Panepistimioupolis-Zografou, 15771 Athens, Greece. ankol@pharm.uoa.gr
authors.forename: Antonios
authors.initials: A
authors.lastname: Kolocouris
mesh_terms: D003198:Computer Simulation; D003510:Cyclohexanes / Q000737:chemistry*; D006860:Hydrogen Bonding; D008956:Models, Chemical
chemical_list: D003510:Cyclohexanes