Molecular
Structure and Conformations of 1,2-Dimethoxycyclobutene-3,4-dione.
An Electron-Diffraction Investigation Augmented by Quantum Mechanical
and Normal Coordinate Calculations
posted on 2015-03-05, 00:00authored byLuke L. Costello, Lise Hedberg, Kenneth Hedberg
The
structure and conformations of 1,2-dimethoxycyclobutene-3,4-dione
in the vapor at a temperature of 185 °C have been measured by
gas-phase electron diffraction. The molecule exists in two forms,
one of symmetry C2v with
the methyl groups trans to the double bond, and one of Cs symmetry with a methyl group cis and the other trans
to this bond (these forms hereafter designated as trans and cis).
The molar ratio trans/cis is 68/32 with a 2σ uncertainty of
about 24. Many of the parameter values for the two forms are very
nearly alike and could not be measured experimentally. With the adoption
of parameter differences calculated at the B3LYP/cc-pVTZ level, the
following bond distances (rg/Å) and
bond angles (∠/deg) with estimated 2σ uncertainties were
obtained for trans/cis: C1C2 = 1.381(9)/1.381, C1C4
= 1.493(11)/1.495, C3C4 = 1.543(20)/1.545, CO = 1.203(4)/⟨1.200⟩,
C1O = 1.316(6)/⟨1.320⟩, OCH3 = 1.444(9)/⟨1.443⟩, CCC3 = 93.1(5)/⟨93.1⟩,
C3C4O = 136.7(29)/⟨136.9⟩, CCO
= 131.0(23)/137.5, and 131.8, COC = 117.2(12)/118.2
and 116.9; the individual angle values for the cis form listed as
averages differ very little. The bond distances and bond angles are
in excellent qualitative agreement with prediction based on conventional
ideas about the effects of conjugation and hybridization, and their
relative values agree very well with predictions from quantum mechanical
calculations.