HNO Dimerization as a Chemical Reference Standard for N₂O Isotopomer Ratio: Ab Initio Calculations, Formation...

Area of Science:

• Isotope Geochemistry
• Chemical Kinetics
• Spectroscopy

Background:

• The 15N-site preference (δ15NSP) of N2O is a key tracer for N2O sources and formation.
• Nitroxyl (HNO) dimerization is a proposed N2O formation pathway, potentially yielding a chemical reference standard.

Purpose of the Study:

• To measure the δ15NSP of N2O produced from HNO dimerization.
• To test the proposed HNO dimerization pathway for N2O formation.
• To establish a chemical synthesis of N2O as an absolute δ15NSP reference standard.

Main Methods:

• High-precision mid-infrared frequency comb spectroscopy to analyze N2O isotopomers.
• Chemical synthesis of N2O from HNO dimerization under acidic conditions (pH = 0.62).
• First-principles ab initio and transition state theory calculations.

Main Results:

• Measured δ15NSP(N2O) values decreased from 36.6‰ at 278 K to 23.4‰ at 336 K.
• Results align with a kinetic equilibrium model of the cis-hyponitrous acid/cis-hyponitrite acid-base system.
• Confirmed N2O formation is dominated by the decomposition of neutral cis-hyponitrous acid at low pH, excluding the trans-hyponitrite pathway.

Conclusions:

• The direct dimerization of HNO in acidic solution produces N2O primarily via the cis-hyponitrous acid decomposition pathway.
• The study provides the first step towards establishing HNO dimerization as an absolute δ15NSP reference standard for N2O.
• Accurate δ15NSP measurements using spectroscopy validate theoretical predictions and chemical pathway models.