title: Molecular structure, infrared spectrum, and photochemistry of squaric acid dimethyl ester in solid argon.
abstract: Squaric acid dimethyl ester (C(6)O(4)H(6); 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 (C(2)(v)) was found to be more stable than the cis-trans form (C(s)) by 4.2 kJ mol(-1), in consonance with the theoretical predictions (MP2 calcd = 3.9 kJ mol(-1)). In situ broadband UV irradiation (lambda > 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 (lambda > 235 nm), the main observed photoproducts were CO and deltic acid dimethyl ester (C(5)O(3)H(6); 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.
authors.affiliation: Department of Chemistry, University of Coimbra, P-3004-535 Coimbra, Portugal.
authors.forename: S
authors.initials: S
authors.lastname: Breda
authors.forename: I
authors.initials: I
authors.lastname: Reva
authors.forename: L
authors.initials: L
authors.lastname: Lapinski
authors.forename: R
authors.initials: R
authors.lastname: Fausto
mesh_terms: D001128:Argon / Q000737:chemistry*; D003503:Cyclobutanes / Q000737:chemistry*; D004952:Esters / Q000737:chemistry*; D007700:Kinetics; D008745:Methylation; D008958:Models, Molecular; D015394:Molecular Structure; D010777:Photochemistry; D013055:Spectrophotometry, Infrared; D014732:Vibration
chemical_list: D003503:Cyclobutanes; D004952:Esters; D001128:Argon; C030042:squaric acid