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

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 degrees, 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 C-2v, symmetry) into the higher-energy conformer (C-s symmetry). Effective stabilization of the photogenerated species occurred only for the compound isolated in a nitrogen matrix. Moreover, the stabilization of the higher-energy C-s 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 C-s conformer is reported here for the first time.