FTIR Spectroscopic and Theoretical Study of the Photochemistry of Matrix-isolated Coumarin

• Published in: Photochemistry and photobiology Vol. 83; no. 5; pp. 1237 - 1253
• Main Authors: Kuş, Nihal, Breda, Susana, Reva, Igor, Tasal, Erol, Ogretir, Cemil, Fausto, Rui
• Format: Journal Article
• Language: English
• Published: Oxford, UK Blackwell Publishing Ltd 01.09.2007
• Subjects: Comparative analysis; Coumarins - chemistry; Crystal lattices; Matrix; Methods; Molecular Structure; Molecules; Natural products; Photochemistry; Solvents; Spectroscopy, Fourier Transform Infrared - methods; Ultraviolet Rays
• ISSN: 0031-8655; 1751-1097
• DOI: 10.1111/j.1751-1097.2007.00152.x

The infrared spectrum of monomeric unsubstituted coumarin (C9H6O2; 2H‐1‐benzopyran‐2‐one), isolated in solid argon at 10 K is presented and assigned. The UV‐induced (λ > 200 nm) unimolecular photochemistry of the matrix‐isolated compound was studied experimentally. Three main photoreactions were observed: (a) decarboxylation of the compound and formation of benzocyclobutadiene and CO2, with the Dewar form of coumarin as intermediate; (b) isomerization of the compound, leading to production of a conjugated ketene; and (c) decarbonylation, leading to formation of CO and benzofuran complex. Further decomposition of benzofuran to produce ethynol is suggested. Photochannels (a) and (b) correspond to those previously observed for matrix‐isolated α‐pyrone and its sulfur analogs (Phys. Chem. Chem. Phys. 2004, 6, 929; J. Phys. Chem. A 2006, 110, 6415), while route (c) is similar to the UV‐induced photochemistry of coumarin in the gaseous phase (J. Phys. Chem. A 2000, 104, 1095). Interpretation of the experimental data is supported by extensive calculations performed at the B3LYP/6‐311++G(d,p), MP2/6‐31G(d,p) and MP2/6‐311++G(d,p) levels.