Molecular Structure and Conformational Composition of 3,4-Epoxy-1-butene As Determined by ab Initio Molecular Orbital Calculations, Microwave Spectroscopy, and Gas-Phase Electron Diffraction

Abstract

The structure and conformational composition for 3,4-epoxy-1-butene have been studied by ab initio molecular orbital calculations at the HF/6-31G* level and a combined least-squares analysis of electron diffraction data and microwave rotational constants. Ab initio calculations showed the presence of three stable conformers, anti, gauche-1, and gauche-2 with the anti form being lowest in energy. The gauche-1 and gauche-2 forms were 0.51 and 2.32 kcal mol-1 higher in energy, respectively. Using calculated values for entropies and zero-point and electronic energies of these three forms, a conformational mixture of 76% anti, 22% gauche-1, and 2% gauche-2 was obtained at the experimental temperature of 298 K. The combined electron diffraction and microwave data analysis indicated the presence of both the anti and gauche-1 forms, with a preference for the anti form [64(17)%]. For this model, the experimental values for the geometrical parameters (rg, ∠α) are as follows (values given for the anti form): r(C−H)av = 1.097(5) Å, r(C4C5) = 1.331(4) Å, r(C3−O1) = 1.436(2) Å, r(C2−O1) = 1.429(2) Å, r(C2−C3) = 1.477(7) Å, r(C3−C4) = 1.500(7) Å, ∠C4C3O1 = 115.2°(19), ∠C5C4C3 = 123.5°(21), φCCCC = 146(12)°.