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Research has been conducted on the equatorial adamantyl and axial methyl groups. Results indicate that the chair conformation with these groups is the most stable and that the interactions between the groups is conformation-determining.

The structure of 111 cyclohexane derivatives bearing the axial substitution [Y.sub.aX]-C is optimized at the B3LYP/5-31+G(d,p) level. The calculations have shown that the C-[H.sub.aX]...[Y.sub.aX]-C contacts in common cyclohexane derivatives that are steric in nature include an improper hydrogen-bonding component.

C-Hax⋯Yax contacts are a textbook prototype of steric hindrance in organic chemistry. The nature of these contacts is investigated in this work. MP2/6-31+G(d,p) calculations predicted the presence of improper hydrogen bonded C-Hax⋯Yax contacts of different strength in substituted cyclohexane rings. To support the theoretical predictions with experimental evidence, several synthetic 2-substituted adamantane analogues (1-24) with suitable improper H-bonded C-Hax⋯Yax contacts of different strength were used as models of a substituted cyclohexane ring. The 1H NMR signal separation, Δδ(γ-CH2), within the cyclohexane ring γ-CH2s is raised when the MP2/6-31+G(d,p) calculated parameters, reflecting the strength of the H-bonded C-Hax⋯Yax contact, are increased. In molecules with enhanced improper H-bonded contacts C-Hax⋯Yax, like those having sterically crowded contacts (Yax = t-Bu) or contacts including considerable electrostatic attractions (Yax = O-C or O-C) the calculated DFT steric energies of the γ-axial hydrogens are considerably reduced reflecting their electron cloud compression. The results suggest that the proton Hax electron cloud compression, caused by the C-Hax⋯Yax contacts, and the resulting increase in Δδ(γ-CH2) value can be effected not just from van der Waals spheres compression, but more generally from electrostatic attraction forces and van der Waals repulsion, both of which are improper H-bonding components. [ABSTRACT FROM AUTHOR]

The structure of 111 cyclohexane derivatives bearing the axial substitution Yax-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 σ*(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-Hax⋯Yax-C contacts were retrieved from the Cambridge Crystallographic Database. Overall, the calculations predicted that the C-Hax Yax-C contacts in common cyclohexane derivatives that are generally thought to be steric in nature (Pauli repulsive forces) include an improper hydrogen-bonding component. [ABSTRACT FROM AUTHOR]

C-H(ax)···Y(ax) contacts are a textbook prototype of steric hindrance in organic chemistry. The nature of these contacts is investigated in this work. MP2/6-31+G(d,p) calculations predicted the presence of improper hydrogen bonded C-H(ax)···Y(ax) contacts of different strength in substituted cyclohexane rings. To support the theoretical predictions with experimental evidence, several synthetic 2-substituted adamantane analogues (1-24) with suitable improper H-bonded C-H(ax)···Y(ax) contacts of different strength were used as models of a substituted cyclohexane ring. The (1)H NMR signal separation, Δδ(γ-CH(2)), within the cyclohexane ring γ-CH(2)s is raised when the MP2/6-31+G(d,p) calculated parameters, reflecting the strength of the H-bonded C-H(ax)···Y(ax) contact, are increased. In molecules with enhanced improper H-bonded contacts C-H(ax)···Y(ax), like those having sterically crowded contacts (Y(ax) = t-Bu) or contacts including considerable electrostatic attractions (Y(ax) = O-C or O═C) the calculated DFT steric energies of the γ-axial hydrogens are considerably reduced reflecting their electron cloud compression. The results suggest that the proton H(ax) electron cloud compression, caused by the C-H(ax)···Y(ax) contacts, and the resulting increase in Δδ(γ-CH(2)) value can be effected not just from van der Waals spheres compression, but more generally from electrostatic attraction forces and van der Waals repulsion, both of which are improper H-bonding components.