Competitive photodissociation channels in jet‐cooled HNCO: Thermochemistry and near‐threshold predissociation

The photoinitiated unimolecular decomposition of jet‐cooled HNCO has been studied following S₁(¹A″)←S₀(¹A′) excitation near the thresholds of the spin‐allowed dissociation channels: (1) H(²S)+NCO(X²Π) and (2) NH(a¹Δ)+CO(X¹Σ⁺), which are separated by 4470 cm⁻¹. Photofragment yield spectra of NCO(X²Π) and NH (a¹Δ) were obtained in selected regions in the 260–220 nm photolysis range. The NCO(X²Π)yield rises abruptly at 38 380 cm⁻¹ and the spectrum exhibits structures as narrow as 0.8 cm⁻¹ near the threshold. The linewidths increase only slowly with photolysis energy. The jet‐cooled absorption spectrum near the channel (1) threshold [D₀(H+NCO)] was obtained using two‐photon excitation via the S₁ state, terminating in a fluorescent product. The absorption spectrum is similar to the NCO yield spectrum, and its intensity does not diminish noticeably above D₀(H+NCO), indicating that dissociation near threshold is slow. The NCO product near threshold is cold, as is typical of a barrierless reaction. NH (a¹Δ) products appear first at 42 840 cm⁻¹, but their yield is initially very small, as evidenced also by the insignificant decrease in the NCO yield in the threshold region of channel (2). The NH (a¹Δ) yield increases faster at higher photolysis energies and the linewidths increase as well. At the channel (2) threshold, the NH (a¹Δ) product is generated only in the lowest rotational level, J=2, and rotational excitation increases with photolysis energy. We propose that in the range 260–230 nm, HNCO (S₁) undergoes radiationless decay terminating in S₀/T₁ followed by unimolecular reaction. Decompositions via channels (1) and (2) proceed without significant exit channel barriers. At wavelengths shorter than 230 nm, the participation of an additional, direct pathway cannot be ruled out. The jet‐cooled photofragment yield spectra allow the determination, with good accuracy, of thermochemical values relevant to HNCO decomposition. The following heats of formation are recommended: ΔH⁰_f(HNCO)=−27.8±0.4 kcal/mol, and ΔH⁰_f(NCO)=30.3±0.4 kcal/mol. These results are in excellent agreement with recent determinations using different experimental techniques.