HNO Dimerization as a Chemical Reference Standard for N2O Isotopomer Ratio: Ab Initio Calculations, Formation Kinetics, and Frequency Comb Spectroscopy.

The 15N-site preference of N2O, δ15NSP = [14N15NO]/[15N14NO] - 1 ≈ δ15Nα - δ15Nβ = SP, quantifies the relative enrichment of 15N at the central (α) versus terminal (β) position in nitrous oxide and serves as a robust isotopic fingerprint for tracing N2O sources and formation pathways. One such pathway involves dimerization of nitroxyl (HNO), which can occur directly or enzymatically. The direct dimerization of HNO in aqueous solution has been suggested to proceed via acid-base equilibria, forming cis-hyponitrite or cis-hyponitrous acid intermediates that decompose to N2O. Measuring δ15NSP(N2O) from HNO dimerization would not only test the postulated formation pathway but also offer an easily reproducible chemical reference standard for isotopic studies. Using high-precision mid-infrared frequency comb spectroscopy, we determine the 15N-site preference by analyzing the absorption ratio of multiple rovibrational line pairs of the α and β isotopomers. At pH = 0.62, δ15NSP(N2O) decreases with synthesis temperature from 36.6‰ at T = 278 K to 23.4‰ at T = 336 K, in very good agreement with prediction based on a kinetic equilibrium model of the strongly pH-dependent cis-hyponitrous acid/cis-hyponitrite acid-base system. These results confirm N2O formation via the cis pathway is dominated─at the low synthesis pH─by the decomposition of the neutral cis-hyponitrous acid. Alternative formation of N2O from trans-hyponitrite, predicted to yield δ15NSP ≈ -7‰, can be excluded. Our work, combining high-precision spectroscopic measurements with first-principles ab initio and transition state theory calculations, is the first step toward establishing the chemical synthesis of N2O from HNO dimerization under acidic conditions as an absolute δ15NSP reference.