title: N-Methyl stereochemistry in tropinone: the conformational flexibility of the tropane motif.
abstract: The intrinsic conformational and structural properties of the tropinone azabicycle have been investigated in a supersonic jet expansion using rotational spectroscopy. The spectrum revealed the presence of equatorial and axial conformers originated by the inversion of the N-methyl group, with the tropane motif adopting a distorted chair configuration. The determination of substitution and effective structures for the two conformers reveals the flexibility and structural changes associated with the N-methyl inversion, mostly a flattening at the nitrogen atom and a simultaneous rising of the carbonyl group in the axial form. Relative intensity measurements indicate that the conformational equilibrium is displaced towards the equatorial form, with a relative population in the jet of N(eq)/N(ax) approximately 2/1, which would correspond to a relative energy of ca. 2 kJ mol(-1) in pre-expansion conditions. Supporting ab initio calculations of the molecular properties and inversion barrier complemented the experimental work.
authors.affiliation: Departamento de Química Física, Facultad de Ciencia y Tecnología, Universidad del País Vasco (UPV-EHU), Apartado 644, E-48080 Bilbao, Spain.
authors.forename: Emilio J
authors.initials: EJ
authors.lastname: Cocinero
authors.forename: Alberto
authors.initials: A
authors.lastname: Lesarri
authors.forename: Patricia
authors.initials: P
authors.lastname: Ecija
authors.forename: Jens-Uwe
authors.initials: JU
authors.lastname: Grabow
authors.forename: José A
authors.initials: JA
authors.lastname: Fernández
authors.forename: Fernando
authors.initials: F
authors.lastname: Castaño
mesh_terms: D008968:Molecular Conformation; D013055:Spectrophotometry, Infrared; D013237:Stereoisomerism; D014326:Tropanes / Q000737:chemistry*
chemical_list: D014326:Tropanes; C016808:3-tropinone