Conformational analysis and near-infrared-induced rotamerization of malonic acid in an argon matrix

Relative energies and vibrational spectra of the conformational states of a malonic acid monomer (HOOCCH2COOH) were calculated using various levels of approximation [Hartree-Fock (HF), Moller-Plesset to second order (MP2), an d B3LYP density functional theory (DFT)I. The calculations predict the exis tence of six different conformers according to skeletal C-C bond and O-H bo nd rotation. Three conformers are found with energies close enough to enabl e their spectroscopic observation. The lowest-energy conformer (I) shows a nearly planar structure with an OH . . .O=C intramolecular hydrogen bond cl osing a six-member O=C-CH2-C-O-H ring. The second- and third-lower energy c onformers (LI and III) differ from the conformational ground state by less than 5 kJ mol(-1). Conformers II and III adopt a cis arrangement around the C-O bonds: conformer II exhibits the two carbonyl bonds in a nearly orthog onal arrangement, with one carboxylic group in the plane containing the car bon atoms, whereas in conformer III (C-2 symmetry), the carbonyl bonds make a dihedral angle of ca. 120 degrees, and both carboxylic groups are placed out of the plane containing the skeletal carbons. Their relative energy is predicted by the highest-level calculations to be ca. 1 kJ mol(-1). The th eoretical predictions agree with the analysis of the infrared spectra of mo nomeric malonic acid isolated in a solid argon matrix, where the presence o f the three above-mentioned conformers was unequivocally identified. Narrow band tunable irradiation in the near-infrared region was found to promote e fficiently the interconversion between the experimentally observed conforme rs.