1 UV-Induced Unimolecular Photochemistry of Diketene Isolated in Cryogenic Inert Matrices S. Breda, * I. Reva and R. Fausto Department of Chemistry, University of Coimbra, 3004-535 Coimbra, PORTUGAL Supplementary Material Figure S01 shows the experimental infrared spectra of diketene monomers isolated in argon matrix at 15 K and xenon matrix at 30 K compared with the theoretical infrared spectra of DK, 3MO and 2MO calculated at the B3LYP/6-311++G(d,p) level. Scheme S01 provides the optimized structure and atom labelling for all considered molecules. Tables S01-S08 provide internal coordinates used in the normal mode analysis. The theoretically calculated spectra of isomers 3-methylene-2- oxetanone (3MO) and 2-methylene-3-oxetanone (2MO) are given in Tables S09 and S10, respectively. In Tables S11, S12, S13, S14 and S15 are the theoretically predicted spectra of ketene, cyclopropanone, cyclobutan-1,3-dione, allene and ethylene (Et). Table S16 shows the experimental infrared combination bands and overtones appearing in the 2300-1930 cm -1 region of the infrared spectrum of DK. * Corresponding author (e-mail: sbreda@qui.uc.pt)