Following photodissociation at 248 nm of gaseous methyl formate (HC(O)OCH 3 , 0.73 Torr) and Ar (0.14 Torr), temporally resolved vibration−rotational emission spectra of highly internally excited CO (ν ≤ 11, J ≤ 27) in the 1850−2250 cm −1 region were recorded with a step-scan Fourier-transform spectrometer. The vibration−rotational distribution of CO is almost Boltzmann, with a nascent average rotational energy (E R 0 ) of 3 ± 1 kJ mol −1 and a vibrational energy (E V 0 ) of 76 ± 9 kJ mol −1 . With 3 Torr of Ar added to the system, the average vibrational energy was decreased to E V 0 = 61 ± 7 kJ mol −1 . We observed no distinct evidence of a bimodal rotational distribution for ν = 1 and 2, as reported previously [Lombardi et al., J. Phys. Chem. A 2016, 129, 5155], as evidence of a roaming mechanism. The vibrational distribution with a temperature of ∼13000 ± 1000 K, however, agrees satisfactorily with trajectory calculations of these authors, who took into account conical intersections from the S 1 state. Highly internally excited CH 3 OH that is expected to be produced from a roaming mechanism was unobserved. Following photodissociation at 193 nm of gaseous HC(O)OCH 3 (0.42 Torr) and Ar (0.09 Torr), vibration− rotational emission spectra of CO (ν ≤ 4, J ≤ 38) and CO 2 (with two components of varied internal distributions) were observed, indicating that new channels are open. Quantum-chemical calculations, computed at varied levels of theory, on the ground electronic potential-energy schemes provide a possible explanation for some of our observations. At 193 nm, the CO 2 was produced from secondary dissociation of the products HC(O)O and CH 3 OCO, and CO was produced primarily from secondary dissociation of the product HCO produced on the S 1 surface or the decomposition to CH 3 OH + CO on the S 0 surface.

The spin-forbidden and spin-allowed reactions of the excited and ground electronic state O( 1 D, 3 P) + N 2 O(X 1 Σ + ) systems have been studied theoretically. Quantum calculations at the UCCSD(T)/CBS(T, Q, 5)//CCSD/aug-cc-pVTZ level have located two crossing points, MSX1 and MSX2, with energies of 11.2 and 22.7 kcal mol −1 above O( 3 P) + N 2 O, respectively. The second-order P-independent rate constants for the adiabatic and non-adiabatic thermal reactions predicted by adiabatic TST/VTST and nonadiabatic TST, respectively, agree closely with the available literature results. The secondorder rate constant, k 2a = 9.55 × 10 −11 exp(−26.09 kcal mol −1 /RT) cm 3 molecule −1 s −1 , for the O( 3 P) + N 2 O → 2NO reaction, contributed by both the dominant MSX2 and the minor TS1-a channels, is in reasonable accord with prior experiments and recommendations, covering the temperature range of 1200−4100 K. The calculated rate constant, k 2b = 4.47 × 10 −12 exp(−12.9 kcal mol −1 /RT) cm 3 molecule −1 s −1 , for the O( 3 P) + N 2 O → N 2 + O 2 (a 1 Δ g ) reaction, occurring exclusively via MSX1, is also in good agreement with the combined experimental data measured in a shock tube study at T = 1940−3340 K (ref 16) and the result measured by Fourier transform infrared spectroscopy in the temperature range of 988−1083 K (ref 17). Moreover, the spin-allowed rate constants predicted for the singlet-state reactions, k 1a = (7.06−7.46) × 10 −11 cm 3 molecule −1 s −1 for O( 1 D) + N 2 O → 2NO and k 1b = (4.36−4.66) × 10 −11 cm 3 molecule −1 s −1 for O( 1 D) + N 2 O → N 2 + O 2 (a 1 Δ g ) in the temperature range of 200−350 K, agree quantitatively with the experimentally measured data, while the total rate constant k 1 = k 1a + k 1b was also found to be in excellent accordance with many reported values.

The spin‐forbidden dissociation reaction of the N2O(X1Σ+) ground state has been investigated by both quantum calculations and experiments. Ab initio prediction at the CCSD(T)/CBS(TQ5)//CCSD(T)/aug‐cc‐pVTZ+d level of theory gave the crossing point (MSX) energy at 60.1 kcal/mol for the N2O(X1Σ+) → N2(X1normalΣg+) + O(3P) transition, in good agreement with published data. The T‐ and P‐dependent rate coefficients, k1(T,P), for the nonadiabatic thermal dissociation predicted by nonadiabatic Rice‐Ramsperger‐Kassel‐Marcus (RRKM) calculations agree very well with literature data. The rate constants at the high‐ and low‐pressure limits, k1∞ = 1011.90 exp (−61.54 kcal mol−1/RT) s−1 and k1o = 1014.97 exp(−60.05 kcal mol−1/RT) cm3 mol−1 s−1, for example, agree closely with the extrapolated results of Röhrig et al. at both pressure limits. The second‐order rate constant (k1o) is also in excellent agreement with our result measured by FTIR spectrometry in the present study for the temperature range of 860‐1023 K as well as with many existing high‐temperature data obtained primarily by shock‐wave heating up to 3340 K. Kinetic modeling of the NO product yields measured at long reaction times in the present work also allowed us to reliably estimate the rate constant for reaction (3), O + N2O → N2 + O2, based on its strong competition with the NO formation from reaction (2) which has been better established. The modeled values of k3 confirmed the previous finding by Davidson et al. with significantly smaller values of A‐factor and activation energy than the accepted ones. A least‐squares analysis of both sets of data gave k3 = 1012.22 ± 0.04 exp[− (11.65 ± 0.24 kcal mol−1/RT)] cm3 mol−1 s−1, covering the wide temperature range of 988‐3340 K.