Electronic spectroscopy and photochemistry of cis and trans-HNNO

Diazene oxide (HNNO) is a chemically strategic transient that links N₂O to more reactive nitrogen reservoirs and can control branching in H + N₂O networks under pressure- and radiation-dependent conditions. The electronic structure, spectroscopy, and photodissociation mechanisms of the cis and trans isomers of the HNNO radical have been investigated using high-level multireference (MRCI+Q) and single-reference (EOM-CCSD) methods in conjunction with augmented correlation-consistent basis sets. Vertical excitation spectra and photoabsorption cross sections simulated via Wigner sampling of ground-state vibrational wavefunctions reveal that both isomers absorb appreciably only in the near-UV region, with negligible visible absorption. For trans-HNNO, the dominant near-UV band arises from the 2² A' ← X²A' transition near 320-325 nm. One-dimensional potential energy cuts along key stretching and bending coordinates demonstrate that photodissociation to NH(X (Prather et al., 2015³⁾Σ^-) + NO(X²Π) is the most plausible product channel for both isomers, accessed through nonadiabatic couplings within the doublet manifold and spin-orbit-mediated intersystem crossing to dissociative quartet states, rather than by direct bond cleavage. This work establishes a comprehensive spectroscopic and photochemical baseline for the HNNO radical and provides a mechanistic framework for understanding its UV-driven chemistry, highlighting the important role of nonadiabatic and spin-orbit coupling effects that will require explicit dynamical treatment in future studies.