Improper hydrogen-bonded cyclohexane C-Hax⋯Yax contacts: experimental evidence from 1H NMR spectroscopy of suitable axial cyclohexane models

B3LYP/6-31+G(d,p) calculations predicted the presence of improper hydrogen-bonded C-Hax⋯Yax contacts of different strength in cyclohexane derivatives;1 it was predicted that the addition of an appropriate bridging fragment Xax between an axial substituent Y1 and a cyclohexane carbon would strengthen the improper hydrogen-bonded contact C-Hax⋯Y1 when the Xax-Y1 bond vector bisects the cyclohexane ring. To support the theoretical predictions with experimental evidence for this effect, several 2-substituted adamantane analogues with suitable improper H-bonded C-Hax⋯O contacts of different strength were synthesized, as models of the corresponding cyclohexane derivatives, and their 1H NMR spectra were recorded at 298 K. The 1H NMR signal separation within the cyclohexane ring γ-CH2s is increased when the B3LYP/6-31+G(d,p)-calculated strength of the H-bonded C-Hax⋯O=Cax contact interaction is increased.

Stronger improper H-bonded cyclohexane C-Hax⋯O contacts cause an increase in the 1H NMR signal separation within the γ-CH2s of cyclohexane rings.