The rotational spectrum of the tropane alkaloid scopine is detected by Fourier transform microwave spectroscopy in a pulsed supersonic jet. A nonconventional method for bringing the molecules intact into the gas phase is used in which scopine syrup is mixed with glycine powder and the solid mixture is vaporized with an ultrafast UV laser beam. Laser vaporization prevents the easy isomerization to scopoline previously observed with conventional heating methods. A single conformer is unambiguously observed in the supersonic jet and corresponds to the energetically most stable species according to quantum chemical calculations. Rotational and centrifugal distortion constants are accurately determined. The spectrum shows fine and hyperfine structure due to the hindered rotation of the methyl group and the presence of a quadrupolar nucleus (14 N), respectively. This additional information allows the angle of N-methyl inversion between the N-CH3 bond and the bicyclic C-N-C plane to be determined (131.8-137.8°), as well as the internal rotation barrier of the methyl group (6.235(1) kJ mol-1 ).
Keywords: alkaloids; conformation analysis; hydrogen bonds; laser vaporization; rotational spectroscopy.
© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.