Molecular Structure, Equilibrium Conformation, and Ring-Puckering Motion in 1,1,3,3-Tetramethylcyclobutane. An Electron-Diffraction Investigation Augmented by Molecular Orbital and Normal Coordinate Calculations.

The molecule cyclobutane (CB) has a nonplanar carbon skeleton folded around a line connecting diagonally opposite atoms. The puckering angle (the change from planarity) of ∼30° is generally attributed to steric repulsion between the four sets of adjacent methylene groups that would be opposed in a planar ring and is relieved by the puckering. According to this criterion, a similar molecule, 1,1,3,3-tetramethylcyclobutene (TMCB), in which adjacent methylene groups do not exist, would be expected to have a planar ring in the equilibrium form. We have investigated the structure of TMCB to test this expectation. Two models were designed for the tests: one having D2h symmetry (planar ring) and one of C2v symmetry (nonplanar ring). Each model incorporated the dynamics of large-amplitude bending around a line joining the methylene groups. Our results suggest the D2h model is to be preferred. Dynamic averages (rg /Å; ∠g /deg) of the more important distances and angles in the D2h model with estimated 2σ uncertainties, are as follows. <C-H> = 1.105 (5), C1-C5 = 1.524 (10), C1-C2 = 1.559 (11), C2-C1-C4 = 87.4 (8), C1-C2-C3 = 92.0 (7), C5-C1-C6 = 109.0 (13), and C2-C1-C5 = 115.8 (8). The large-amplitude bending of the ring leads to a thermal average value of the folding angle equal to 177.1°. The results, including the differences between TMCB and CB, are discussed.