Improper hydrogen-bonded cyclohexane C-H_(ax)···Y_(ax) contacts: experimental evidence from~1H NMR spectroscopy of suitable axial cyclohexane models

摘要

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; it was predicted that the addition ofan appropriate bridging fragment X_(ax) between an axial substituent Y1 and a cyclohexane carbonwould strengthen the improper hydrogen-bonded contact C-H_(ax)···Y1 when the X_(ax)-Y1 bondvector bisects the cyclohexane ring. To support the theoretical predictions with experimental evidencefor this effect, several 2-substi-tuted adamantane analogues with suitable improper H-bonded C-H_(ax)···O contacts of different strength were synthesized, as models of the corresponding cyclohexanederivatives, and their ~1H NMR spectra were recorded at 298 K. The 'H NMR signal separation withinthe cyclohexane ring γ-CH2s 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.