UV-INDUCED ISOMERIZATION OF (E)-CROTONIC ACID - COMBINED MATRIX-ISOLATED IR AND AB-INITIO MO STUDY

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 s hown that in both argon and krypton matrices monomeric (E)-crotonic ac id exists as a mixture of two conformers of similar energies, differin g by the relative orientation of the C=C-C=O axis (the s-cis and s-tra ns forms, having a C=C-C=O dihedral angle equal to 0 degrees and 180 d egrees, respectively). Upon UV-irradiation in the 240-250 nm region by a xenon lamp, photoisomerization reactions about both C-alpha-C and C =C bonds occur leading, respectively, to s-cis --> s-trans (E)-crotoni c acid rotamerization and (E)-crotonic acid --> (Z)-crotonic acid conv ersion. Results of ab initio SCF-MO calculations, in particular optimi zed geometries, relative stabilities, dipole moments and harmonic forc e fields, for the relevant conformational states of both (E) and (Z)-c rotonic acids are also presented and the conformational dependence of some relevant structural parameters is used to characterize the most i mportant intramolecular interactions present in the studied forms. Fin ally, results of a normal mode analysis based on the ab initio calcula ted vibrational spectra are used to help interpret the experimenal vib rational data, enabling a detailed assignment of the matrix-isolated s pectra and the characterization of the observed photoinduced isomeriza tion reactions.