Propiolic Acid in Solid Nitrogen: NIR- and UV-Induced cis → trans Isomerization and Matrix-Site-Dependent trans → cis Tunneling.

Propiolic acid (HCCCOOH, PA) was studied experimentally by infrared spectroscopy in a nitrogen matrix and by ab initio calculations. The vibrational spectra of the cis and trans monomers (O═C-O-H dihedral equal to 0 and 180°, respectively) were measured and assigned. The trans-PA monomer was produced by selective vibrational excitation of the cis-PA monomer molecules trapped in different matrix sites. Broadband in situ UV irradiation (λ > 235 nm) of matrix-isolated PA yielded as product the higher-energy trans conformer, with no other photoproducts being detected. Two cis- cis dimers were also identified in the matrixes and characterized structurally and vibrationally. trans-PA was found to decay back to cis-PA in the dark, by tunneling, and the different lifetimes of the higher-energy PA conformer resulting from pumping different matrix sites and different experimental conditions (using a filter blocking the higher-energy IR radiation of the spectrometer source and without using such a filter) were discussed.