Infrared absorption spectrum of the simplest deuterated Criegee intermediate CD₂OO

We report a transient infrared (IR) absorption spectrum of the simplest deuterated Criegee intermediate CD₂OO recorded using a step-scan Fourier-transform spectrometer coupled with a multipass absorption cell. CD₂OO was produced from photolysis of flowing mixtures of CD₂I₂, N₂, and O₂ (13 or 87 Torr) with laser light at 308 nm. The recorded spectrum shows close structural similarity with the spectrum of CH₂OO reported previously [Y.-T. Su et al., Science 340, 174 (2013)]. The four bands observed at 852, 1017, 1054, and 1318 cm⁻¹ are assigned to the OO stretching mode, two distinct in-plane OCD bending modes, and the CO stretching mode of CD₂OO, respectively, according to vibrational wavenumbers, IR intensities, rotational contours, and deuterium-isotopic shifts predicted with extensive quantum-chemical calculations. The CO-stretching mode of CD₂OO at 1318 cm⁻¹ is blue shifted from the corresponding band of CH₂OO at 1286 cm⁻¹; this can be explained by a mechanism based on mode mixing and isotope substitution. A band near 936 cm⁻¹, observed only at higher pressure (87 Torr), is tentatively assigned to the CD₂ wagging mode of CD₂IOO.