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

Abstract

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 $j_{HCHO}/j_{HC{H}^{18}O}$ = 1.027 ± 0.006 at 50 mbar to j_HCHO/j_DCDO = 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.