New experiments and quantum-molecular mechanics model of isotopic fractionation in formaldehyde photolysis explains atmospheric dD-H2 anomaly and shows extreme isotopic fractionation in CO

Formaldehyde is a short-lived intermediate formed by the oxidation of virtually every VOC in the atmosphere. It is the source of half of atmospheric hydrogen, and a large source of CO and CO₂, and plays a role in particle growth. Efforts to better understand the remarkable transformations of formaldehyde are hindered due to lack of knowledge of some of the basic processes in formaldehyde photolysis. Here, we present a combined quantum and molecular mechanics, 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 using CCSD(T)/aug-cc-pVTZ, ωB97X-D/aug-cc-pVTZ and CASPT2/aug-cc-pVTZ levels of theory. Experimental internal conversion and intersystem crossing rates were used and modified with the density of states of the isotopologues based on Fermi’s ‘Golden Rule’. The following isotopologues of formaldehyde were investigated: HCHO, DCHO, DCDO, D¹³CHO, H¹³CHO, HCH¹⁷O, HCH¹⁸O, HC¹³H¹⁷O and HC¹³H¹⁸O. The method and mechanism were validated by comparison to all existing and newly obtained experimental data. The model was able to accurately replicate the experimental pressure trends of the kinetic isotope effects (KIEs) and was in excellent agreement. The model was used to predict the KIEs and the molecular hydrogen yields of the deuterated species at varying altitudes.