Molecular Structure, Infrared Spectrum, and Photochemistry of Squaric Acid Dimethyl Ester in Solid Argon

Squaric acid dimethyl ester (C6O4H6; 3,4-dimethoxycyclobut-3-ene-1,2-dione; DCD) was studied by matrix
isolation infrared spectroscopy and by density functional theory (B3LYP) and ab initio (MP2) calculations
with the 6-31++G(d,p) and 6-311++G(d,p) basis sets. Three conformers of the compound were theoretically
predicted. The two most stable conformers were identified in low-temperature argon matrixes and the energy
gap between them was determined. The trans−trans conformer (C2v) was found to be more stable than the
cis−trans form (Cs) by 4.2 kJ mol-1, in consonance with the theoretical predictions (MP2 calcd = 3.9 kJ
mol-1). In situ broadband UV irradiation (λ > 337 nm) of the matrix-isolated compound was found to induce
the ring-opening reaction leading to production of the bisketene, 2,3-dimethoxybuta-1,3-diene-1,4-dione as
well as the trans−trans → cis−trans conformational isomerization. The latter phototransformation allowed
separation of the infrared spectra of the two conformers initially trapped into a low-temperature matrix. Upon
higher energy irradiation (λ > 235 nm), the main observed photoproducts were CO and deltic acid dimethyl
ester (C5O3H6; 2,3-dimethoxycycloprop-2-en-1-one), the latter being obtained in two different conformations
(trans−trans and cis−trans). According to the experimental data, deltic acid dimethyl ester is produced by
decarbonylation of the initially formed bisketene and not by direct CO extrusion from DCD.