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Improper Hydrogen Bonded Cyclohexane C–Hax···Yax Contacts: Theoretical Predictions and Experimental Evidence from 1H NMR Spectroscopy of Suitable Axial Cyclohexane Models

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journal contribution
posted on 03.06.2011, 00:00 by Antonios Kolocouris, Nikolaos Zervos, Frank De Proft, Andreas Koch
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 (124) 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.