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Ab initio study of rotational isomerism in 1,3-butadiene. Effect 21 of geometry optimization and basis set size on the barriers to rotation and on the stable rotamers

1984; Elsevier BV; Volume: 109; Issue: 1-2 Linguagem: Inglês

10.1016/0166-1280(84)80138-4

1872-7999

George R. De Maré, Daniel Neisius,

Organic Chemistry Cycloaddition Reactions

Analytic gradient (force) methods at the STO-3G, 3-21G, 6-31G and 6-31G* basis set levels have been used to optimize the geometry of 1,3-butadiene at each critical point (minima, maxima) in the torsional potential energy curves for rotation about the central CC bond (dihedral angle θ). Each CHCH2 group was constrained planar and the effect of this geometry constraint was investigated by additional STO-3G and 3-21G optimizations. The planar trans conformation (θ = 180°) is predicted to be most stable, in agreement with experiment. Small variations in the predicted relative energy (ΔE) and in the position of the transition state (TS) for rotation from the trans position are observed: ΔE = 23.5, 23.6, 25.9 and 25.4 kJ mol−1 and θ = 95.0, 101.8, 101.6 and 101.5° for the STO-3G, 3-21G, 6-31G and 6-31G* basis set optimizations, respectively. The STO-3G optimizations predict an energy minimum for the planar cis conformation (θ = 0°), 7.7 kJ mol−1 above the trans minimum. All three larger basis sets predict a maximum for the cis structure, with small gauche—gauche rotational barriers (2.0–4.1 kJ mol−1, the largest value corresponding to optimizations with the 3-21G basis set and non-planar CHCH2 groups). The gauche minimum is predicted to lie 11.3–13.2 kJ mol−1 above the trans, at θ's in the range 35.5–38.5°. The positions and relative energies of the critical points in the torsional potential energy curves predicted by the 3-21G, 6-31G and 6-31G* optimizations are in good agreement with those proposed by Durig, et al. [1] to reproduce the Raman overtone spectrum of 1,3-butadiene for a gauche—trans rotamer equilibrium. A series of 6-31G** computations with the optimized geometries from the smaller basis sets indicates that optimization at the 6-31G** basis level would give results similar to those obtained with the above three split-valence basis sets.