First principles model of isotopic fractionation in formaldehyde photolysis: wavelength and pressure dependence

Experimental studies show large isotope-dependent effects in the photolysis rates of formaldehyde isotopologues, that are both wavelength and pressure dependent. These effects are on the order of 10-20% for ¹³C and ¹⁸O (L. Feilberg et. al, J. Phys. Chem. A, 109, 8314-8319, 2004), and 60% for CHDO (E. J. K Nilsson et. al, ACP, 14, 551–558, 2014). We have made a model of the elementary processes involved in the photodissociation including unimolecular dissociation, collisional quenching and crossing between excited state surfaces. Computational chemistry is used to characterize some of these processes. The model is validated by comparison to all existing experimental data and is then used to make predictions about the isotopic fractionation in additional isotopicules (and for conditions not yet addressed by experiment) including fractionation in clumped molecules. The following isotopologues of formaldehyde have been investigated; HCHO, DCHO, DCDO, D¹³CHO, H¹³CHO, HCH¹⁷O, HCH¹⁸O, H¹³CH¹⁷O and H¹³CH¹⁸O. Rice–Ramsperger–Kassel–Marcus (RRKM) theory was used to calculate the rates for decomposition 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. Furthermore, the rates and likelihood of intersystem crossing were investigated by including the spin-orbit coupling between the excited states. The model was able to replicate the experimental pressure trends accurately, however, the kinetic isotope effect was one order of magnitude too small for the non-deuterated isotopologues. We predict a large clumped isotope anomaly in ¹³C¹⁸O produced by formaldehyde photolysis.