1,4-Cyclohexanedione. Composition, molecular structures, and internal dynamics of the vapor: an electron diffraction investigation augmented by molecular orbital calculations.

Electron diffraction experiments on the vapor of 1,4-cyclohexanedione have been carried out at a nominal temperature of 435 K. The results are consistent with the presence of a mixture of a chair form of C2h symmetry and a twisted boat form of D2 symmetry. The former has the familiar dynamic properties of a semirigid molecule, but the D2 form undergoes a large-amplitude twisting motion (pseudorotation) that degrades the symmetry to C2. The analysis was designed to elucidate parameter values and internal dynamics of each conformer and the composition of the system. The large-amplitude motion of the twisted boat form was modeled by placement of 10 pseudoconformers at approximately 5° intervals along a pseudorotational coordinate that began at the D2 position and that reflected the angle between the C═O bond vectors. A Gaussian weighting of the pseudeoconformers centered on the (lowest-energy) D2 position was assumed. Differences in the interatomic distances and bond angles of these pseudoconformers were calculated via B3LYP/cc-pVTZ theory and introduced as constraints. The bond length averages over the twisted boat forms followed by values for the chair in square brackets are (rg/Å; [angle]α/deg) r(C-H) = 1.115(11) [1.124(11)], r(C═O) = 1.211(3) [1.233(6)], r(C1-C2) = 1.524(5) [1.526(5)], and r(C2-C3) = 1.533(11) [1.539(11)]. The corresponding ring angle values are [angle](C1C2C3) = 111.1(5) [111.0(4)] and [angle](C6C1C2) = 116.3(8) [115.7(8)]. In the twisted boat form, pseudorotation leads to a weighted average displacement of the angle between the C═O bond vectors, [angle]Δ(CO,CO), equal to 21.3° from the 180° value in the D2 form corresponding to an average angle between the CO bond vectors of 158.7(1)°. The amount of the chair form in the gas at 435 K is 24(10)%. The listed uncertainties are estimated at 2σ.