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

Abstract

Stronger improper H-bonded cyclohexane C–H ax ⋯O contacts cause an increase in the 1 H NMR signal separation within the γ-CH 2 s of cyclohexane rings. B3LYP/6-31+G(d,p) calculations predicted the presence of improper hydrogen-bonded C–H ax ⋯Y ax contacts of different strength in cyclohexane derivatives; 1 it was predicted that the addition of an appropriate bridging fragment X ax between an axial substituent Y 1 and a cyclohexane carbon would strengthen the improper hydrogen-bonded contact C–H ax ⋯Y 1 when the X ax –Y 1 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–H ax ⋯O contacts of different strength were synthesized, as models of the corresponding cyclohexane derivatives, and their 1 H NMR spectra were recorded at 298 K. The 1 H NMR signal separation within the cyclohexane ring γ-CH 2 s is increased when the B3LYP/6-31+G(d,p)-calculated strength of the H-bonded C–H ax ⋯O=C ax contact interaction is increased.