1,4-Cyclohexanedione. Composition, Molecular Structures, and Internal Dynamics of the Vapor: An Electron Diffraction Investigation Augmented by Molecular Orbital Calculations

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• 作者：Matthew Frogner ; Robert D. Johnson ; Lise Hedberg ; Kenneth Hedberg
• 刊名：Journal of Physical Chemistry A
• 出版年：2013
• 出版时间：October 31, 2013
• 年：2013
• 卷：117
• 期：43
• 页码：11101-11106
• 全文大小：367K
• 年卷期：v.117,no.43(October 31, 2013)
• ISSN：1520-5215

文摘

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 C_2h 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掳 intervals along a pseudorotational coordinate that began at the D₂ position and that reflected the angle between the C鈺怬 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 (r_g/脜; _伪/deg) r(C鈥揌) = 1.115(11) [1.124(11)], r(C鈺怬) = 1.211(3) [1.233(6)], r(C1鈥揅2) = 1.524(5) [1.526(5)], and r(C2鈥揅3) = 1.533(11) [1.539(11)]. The corresponding ring angle values are (C1C2C3) = 111.1(5) [111.0(4)] and (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鈺怬 bond vectors, 螖(CO,CO), equal to 21.3掳 from the 180掳 value in the D₂ form corresponding to an average angle between the CO bond vectors of 158.7(1)掳. The amount of the chair form in the gas at 435 K is 24(10)%. The listed uncertainties are estimated at 2蟽.