The molecular structure of gauche-1,3-butadiene: Experimental proof of non-planarity

Joshua Baraban (1) , M. A. Martin-Drumel (2) , P. Bryan Changala (3) , Sandra Eibenberger (4) , Matthew Nava (5) , David Patterson (4) , John F. Stanton (6) , G. Barney Ellison (1) , Michael Mccarthy (7)

1 University of Colorado [Boulder]
2 ISMO - Institut des Sciences Moléculaires d'Orsay
3 JILA - Joint Institute for Laboratory Astrophysics
4 Harvard University
5 MIT - Massachusetts Institute of Technology
6 UF - University of Florida [Gainesville]
7 CfA - Harvard-Smithsonian Center for Astrophysics

The planarity of the second stable conformer of 1,3-butadiene-the archetypal diene for the Diels-Alder reaction, in which a planar conjugated diene and a dienophile combine to form a ring-is not established. The most recent high level calculations predict the species to adopt a twisted, gauche structure due to steric interactions between the inner terminal hydrogens rather than a planar, cis structure favored by the conjugation of the doubled bonds. Here we unambiguously prove experimentally that the structure cis-1,3-butadiene is indeed gauche with a substantial dihedral angle of 34° , in excellent agreement with theory. Observation of two tunneling components indicates that the molecule undergoes facile interconversion between two equivalent enantiomeric forms. Comparison of experimentally determined structures for gauche-and trans-butadiene provides an opportunity to examine the effects of conjugation and steric interactions.

Domaines: Chimie théorique et/ou physique

Joshua Baraban, M. A. Martin-Drumel, P. Bryan Changala, Sandra Eibenberger, Matthew Nava, et al.. The molecular structure of gauche-1,3-butadiene: Experimental proof of non-planarity. Angewandte Chemie International Edition, 2018, 57 (7), pp.1821-1825. ⟨10.1002/anie.201709966⟩. ⟨hal-02305403⟩