Conformational Transformation in Squaric Acid Induced by Near-IR Laser Light

Abstract

Two conformers of monomeric squaric acid (3,4-dihydroxy-3-cyclobutene-1,2-dione) were studied using the matrix-isolation method. Both forms of the compound, differing in rotation of one of the OH groups by 180°, were trapped from the gas phase into a low-temperature nitrogen matrix, whereas only the lowest-energy conformer was trapped in solid argon and in solid neon. Narrowband near-infrared laser light was used to induce transformation of the most stable form of squaric acid (having C2v symmetry) into the higher-energy conformer (Cs symmetry). Effective stabilization of the photogenerated species occurred only for the compound isolated in a nitrogen matrix. Moreover, the stabilization of the higher-energy Cs conformer of squaric acid by the solid nitrogen environment was found to strongly depend on the trapping site in the matrix. The spectroscopic characterization of the higher energy Cs conformer is reported here for the first time.