Extreme Isotopic Fractionation in CO and H2 Formed in Formaldehyde Photolysis; Theory and Experiment

Formaldehyde is a common intermediate in reactions of atmospheric organic compounds, it is the most abundant atmospheric carbonyl, and its degradation is a large source of CO and H2. Isotopic analysis is valuable for characterizing molecular processes, not least because of the valuable insight into atmospheric transformations. Here, we present a combined Rice–Ramsperger–Kassel–Marcus (RRKM) and experiment-based model that significantly advances our ability to describe photolytic kinetic isotope effects and their pressure dependencies. RRKM theory was used to calculate the decomposition rates of the S0, S1 and T1 states with CCSD(T)/aug-cc-pVTZ, ωB97X- D/aug-cc-pVTZ and CASPT2/aug-cc-pVTZ levels of theory, respectively. The HCHO, DCHO, DCDO, D13CHO, H13CHO, HCH17O, HCH18O, H13CH17O and H13CH18O 1 species are considered. The method and mechanism were validated by comparison to all existing and newly obtained experimental data. The newly obtained experimental relative photolysis rates ranged from jHCHO/jHCH18O = 1.027 ± 0.006 for photolysis at 50 mbar to jHCHO/jDCDO = 1.418 ± 0.108 at 1000 mbar, using a xenon lamp. The model was able to accurately replicate the experimental pressure trends of the kinetic isotope effects (KIEs) and was in excellent agreement. The model shows that the measured altitude dependent deuterium enrichment in molecular hydrogen, cannot be explained solely by the effect of pressure, but must also include the wavelength dependence.