A Matrix Isolation Spectroscopic and Quantum Chemical Study of Fumaric and Maleic Acid

Relative energies and vibrational spectra of buthenedioic acid [( E )- and ( Z )-HOOC−CH CH−COOH; fumaric and maleic acids] were calculated using Hartree−Fock (HF), Møller−Plesset to 2nd order (MP2), and B3LYP Density Functional Theory (DFT) levels of approximation. Computationally the lowest energy conformer of maleic acid has a nearly planar structure with an OH···O C intramolecular hydrogen bond closing a seven member ring. Second conformer of maleic acid adopts a cis arrangement around the C−O bonds and exhibits the two carbonyl bonds in a nearly orthogonal arrangement with one carboxylic group in the plane containing the carbon atoms. Fumaric acid has three planar conformers with relative energies within 5 kJ mol -1 . These conformers exhibit a cis arrangement around the C−O bond and differ by the relative conformation of their O C−C C axes. The infrared spectra of the studied compounds isolated in a solid argon matrix feature the most stable conformers. Narrowband tunable irradiation in the near-infrared region (6901 cm -1 ) was found to promote a conversion of the second most stable conformer of maleic acid into the most stable conformer.