title: Pseudorotational landscape of seven-membered rings: the most stable chair and twist-boat conformers of ε-caprolactone.
abstract: The conformational landscape and ring-puckering properties of ε-caprolactone have been analyzed by using microwave spectroscopy and quantum chemical calculations. Two conformers were detected in a supersonic jet expansion, the most stable form being a chair containing the ester group in its rectangular flap. This conformation benefits from reduced CH2 bond eclipsing and angle strain, while π-electron delocalization in the ester group is increased. The derived effective structure of the chair form satisfactorily agrees with the calculated near-equilibrium structure. A twist-boat conformer was also identified (9.4 kJ mol(-1) higher in energy at CCSD(T)/aug-cc-pVTZ level), and was located in the boat-twist-boat pseudorotation cycle of the seven-membered ring. Three other low-energy conformers were investigated and characterized in terms of the four puckering coordinates of the seven-membered ring. Potential interconversions in the four-dimensional conformation space are also discussed.
authors.affiliation: Institut für Physikalische Chemie & Elektrochemie, Lehrgebiet A, Gottfried-Wilhelm-Leibniz Universität, Callinstraße 3A, 30167 Hannover (Germany), Fax: (+49)511-7624009.
authors.forename: Michaela K
authors.initials: MK
authors.lastname: Jahn
authors.forename: David A
authors.initials: DA
authors.lastname: Dewald
authors.forename: Montserrat
authors.initials: M
authors.lastname: Vallejo-López
authors.forename: Emilio J
authors.initials: EJ
authors.lastname: Cocinero
authors.forename: Alberto
authors.initials: A
authors.lastname: Lesarri
authors.forename: Wenli
authors.initials: W
authors.lastname: Zou
authors.forename: Dieter
authors.initials: D
authors.lastname: Cremer
authors.forename: Jens-Uwe
authors.initials: JU
authors.lastname: Grabow
keywords: ab initio calculations; conformation analysis; lactones; medium-ring compounds; rotational spectroscopy