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

Formaldehyde (HCHO) is an important intermediate in the breakdown of organic molecules in the atmosphere. It is the most abundant atmospheric carbonyl, and a major source of CO and H2 upon degradation. Isotopic analysis offers valuable insights into molecular processes, deepening our understanding of atmospheric transformations. We present a model of the isotope-dependent photolytic isotopic fractionation of formaldehyde incorporating Rice-Ramsperger-Kassel-Marcus (RRKM) analysis, validate the model with new and pre-existing experimental data, and use it to describe photolytic kinetic isotope effects (KIEs) and their pressure dependencies. RRKM theory was used to calculate 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, respectively. We considered isotopologues HCHO, DCHO, DCDO, D13CHO, H13CHO, HCH17O, HCH18O, H13CH17O, and H13CH18O. We find that isotopic substitution notably affects the density of states, influencing rates of unimolecular decomposition and collisional energy transfer. Experimental photolysis rates ranged from jHCHO/jHCH18O = 1.027 ± 0.006 at 50 mbar to jHCHO/jDCDO = 1.418 ± 0.108 at 1000 mbar using a xenon lamp. The model accurately reproduced experimental pressure trends in KIEs, revealing that altitude-dependent deuterium enrichment in H2 cannot be explained by pressure effects alone and must also consider wavelength dependence.