Multiple Dissociation Pathways in HNCO Decomposition Governed by Potential Energy Surface Topography

Area of Science:

• Molecular photochemistry
• Chemical dynamics
• Quantum chemistry

Background:

• Understanding chemical processes requires knowledge of potential energy surfaces (PESs).
• Non-adiabatic transitions between coupled PESs are known to influence dissociation pathways.
• The role of a single PES in determining reaction outcomes remains less understood.

Purpose of the Study:

• To investigate the influence of a single potential energy surface (PES) on HNCO photodissociation dynamics.
• To elucidate the state-specific correlations between NH and CO products.
• To explore the relationship between PES topography and reaction pathways.

Main Methods:

• Detailed experimental photodissociation of HNCO.
• Acquisition of state-specific correlations for NH (a1Δ) and CO (X1Σ+) products.
• Dynamics simulations utilizing a full-dimensional, machine-learning-based PES of HNCO.

Main Results:

• A trimodal CO rotational distribution was observed in HNCO photodissociation.
• Three distinct dissociation pathways were identified exclusively on the S1 excited electronic state.
• Two pathways deviating from the minimum energy path (MEP) were found to yield cold and hot CO rotational populations, respectively.

Conclusions:

• The topography of the S1 state PES, specifically a narrow acceptance cone near the transition state, governs the observed HNCO photodissociation dynamics.
• These findings provide a clear example of how PES topography dictates molecular photochemistry outcomes.
• The study highlights the critical role of single PES features in understanding complex chemical reactions.