Scopine isolated in the gas phase

Abstract

The rotational spectrum of the tropane alkaloid scopine has been detected by Fourier transform microwave spectroscopy in a pulsed supersonic jet. A non-conventional method for bringing the molecules intact into the gas phase was used, mixing scopine syrup with solid glycine powder and vaporizing the solid mixture with an UV ultrafast laser beam. Laser vaporization prevented the easy isomerization to scopoline previously observed when conventional heating. A single conformer was unambiguously observed in the supersonic jet, which corresponds to the energetically most stable species according to quantum chemistry calculations. Rotational and centrifugal distortion constants have been accurately determined. The spectrum reveals hyperfine splittings due to the combined effect of hindered rotation of the methyl group and the presence of a quadrupolar nucleus (14N). This additional information allowed determining the angle of N-methyl inversion between the N- CH3 bond and the bicyclic C-N-C plane (135o), as well as the internal rotation barrier of the methyl group (6.24(1) kJ mol-1).