UV-induced isomerization of (E)-crotonic acid. Combined matrix-isolated IR and ab initio MO study

Abstract

The results of a combined vibrational and structural study of the (E)-crotonic acid monomer undertaken by matrix-isolated low-temperature IR spectroscopy and ab initio SCF-MO calculations are presented. It is shown that in both argon and krypton matrices monomeric (E)-crotonic acid exists as a mixture of two conformers of similar energies, differing by the relative orientation of the CC—CO axis (the s-cis and s-trans forms, having a CC—CO dihedral angle equal to 0° and 180°, respectively). Upon UV-irradiation in the 240–250 nm region by a xenon lamp, photoisomerization reactions about both C_α—C and CC bonds occur leading, respectively, to s-cis→ s-trans(E)-crotonic acid rotamerization and (E)-crotonic acid →(Z)-crotonic acid conversion. Results of ab initio SCF-MO calculations, in particular optimized geometries, relative stabilities, dipole moments and harmonic force fields, for the relevant conformational states of both (E) and (Z)-crotonic acids are also presented and the conformational dependence of some relevant structural parameters is used to characterize the most important intramolecular interactions present in the studied forms. Finally, results of a normal mode analysis based on the ab initio calculated vibrational spectra are used to help interpret the experimenal vibrational data, enabling a detailed assignment of the matrix-isolated spectra and the characterization of the observed photoinduced isomerization reactions.