The molecular structure of gaseous 1,2 cyclohexanedione

The molecular structure of 1,2 cyclohexanedione has been investigated in the vapor at 295 K. The electron diffraction data were consistent with a model of pure enol form (2-hydroxy-2-cyclohexen-1-one) with the six-membered ring in the sofa conformation (with C5 out of the plane formed by the other carbon atoms). Quantum chemical [HF/MP2/B3LYP 6-311G(d,p)] calculations showed the enol form with hydrogen bond to be lower in energy than the keto form and the enol form without hydrogen bond to be higher in energy than the keto form. HF and B3LYP calculations estimated enol populations of over 99% at 295 K. The principal geometrical parameter values (rg and ∠α) obtained from least squares analysis are: r(CH) = 1.103(7) Å, r(CO) = 1.224(3), r(CC) = 1.357(2) Å [r(C–O) = r(CC)], r(C4C5) = 1.534(5) Å, [r(C5C6) = r(C4C5)], r(C1C6) = 1.511(3) Å, [r(C1C2) = r(C1C6) − 0.022 Å]; r(C3C4) = r(C6C1) − 0.004 Å], ∠C1C2C3 = ∠C2C3C4 = 121.2(2)° ∠C6C1C2 = 117.6(3)° [∠OC1C2 = ∠C6C1C2 + 1.7° and ∠OC2C1 = ∠OC1C2 − 4°], τHOC2C1 = 0.0°, τC3C2C1C6 = 2.4(assumed), τC4C3C2C1 = 2.8°(assumed), τC3C4C5C6 = 50.8°(16), τC4C5C6C1 = 55.3°(16) and τC2C3C4C5 = 22.8°(9).