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N ‐Methyl Inversion and Accurate Equilibrium Structures in Alkaloids: Pseudopelletierine
Author(s) -
VallejoLópez Montserrat,
Écija Patricia,
Vogt Natalja,
Demaison Jean,
Lesarri Alberto,
Basterretxea Francisco J.,
Cocinero Emilio J.
Publication year - 2017
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201702232
Subject(s) - conformational isomerism , isotopologue , rotational spectroscopy , ring flip , chemistry , fourier transform , spectroscopy , supersonic speed , population inversion , inversion (geology) , computational chemistry , crystallography , stereochemistry , molecule , molecular physics , physics , ring (chemistry) , thermodynamics , geology , optics , organic chemistry , quantum mechanics , paleontology , laser , structural basin
A rotational spectroscopy investigation has resolved the conformational equilibrium and structural properties of the alkaloid pseudopelletierine. Two different conformers, which originate from inversion of the N‐ methyl group from an axial to an equatorial position, have been unambiguously identified in the gas phase, and nine independent isotopologues have been recorded by Fourier‐transform microwave spectroscopy in a jet expansion. Both conformers share a chair–chair configuration of the two bridged six‐membered rings. The conformational equilibrium is displaced towards the axial form, with a relative population in the supersonic jet of N axial / N equatorial ≈2/1. An accurate equilibrium structure has been determined by using the semiexperimental mixed‐estimation method and alternatively computed by quantum‐chemical methods up to the coupled‐cluster level of theory. A comparison with the N‐ methyl inversion equilibria in related tropanes is also presented.