O-H⋅⋅⋅N and C-H⋅⋅⋅O hydrogen bonds control hydration of pivotal tropane alkaloids: tropinone⋅⋅⋅H(2)O complex

Abstract

The effect of monohydration in equatorial/axial isomerism of the common motif of tropane alkaloids is investigated in a supersonic expansion by using Fourier-transform microwave spectroscopy. The rotational spectrum reveals the equatorial isomer as the dominant species in the tropinone⋅⋅⋅H2 O complex. The monohydrated complex is stabilized primarily by a moderate OH⋅⋅⋅N hydrogen bond. In addition, two CH⋅⋅⋅O weak hydrogen bonds also support this structure, blocking the water molecule and avoiding any molecular dynamics in the complex. The water molecule acts as proton donor and chooses the ternary amine group over the carbonyl group as a proton acceptor. The experimental work is supported by theoretical calculations; the accuracy of the B3LYP, M06-2X, and MP2 methods is also discussed.

Keywords: alkaloids; dissociation energy; hydrogen bonds; microwave spectroscopy; quadrupole coupling constants.