Matrix Isolation Ftir and Theoretical Study of ?-Pyrone Photochemistryelectronic Supplementary Information (Esi) Available: Tables S1, S2, S3, S12 and S14 Provide Internal Coordinates Used in the Normal Modes Analysis for ?-Pyrone, 2-Oxa-3-Oxobicyclo[2.2.0]Hex-5-Ene, Open-Ring Aldehyde?Ketene Isomers, Cyclobutadiene and 4-Formyl-2-Cyclobutene-1-One. Experimental Infrared Spectra of ?-Pyrone Isolated in Xe Matrix Are Compared with the Theoretical Spectra in Fig. S1. Table S4 Provides the Assignment of Infrared Observed Bands and the Calculated Vibrational Frequencies, Intensities and Potential Energy Distribution for ?-Pyrone Monomer. Calculated Vibrational Frequencies, Intensities and Potential Energy Distributions for All Open-Ring Aldehyde?Ketene Conformers and for 4-Formyl-2-Cyclobutene-1-One Monomer (Forms A and B) Are Listed in Tables S5?S11 and Table S15, Respectively. Table S13 Provides the Assignment of Infrared Observed Bands and the Calculated Vibrational Frequencies, Intensities and Potential Energy Distribution for Cyclobutadiene Monomer. Fig. S2 Presents Differential Spectra Showing the Progress of the Irradiation Experiments. The Optimized Structure for the ?-Pyrone Analogue of Benzvalene Is Shown in Fig. S3. Fig. S4 Illustrates the Observed Photochemistry of ?-Pyrone in an Argon Matrix. See Http://Www.rsc.org/Suppdata/Cp/B3/B309660B/