N+CN→C+N2: A global potential energy surface, entrance channel recrossing and the applicability of capture theory

Quantum and Semiclassical Dynamics of Nonadiabatic Electronic Excitation of C(3P) to C1D) by Hyperthermal Collisions with N2

The dynamics and kinetics of nonadiabatic excitation of C(3P) to C(1D) induced by hyperthermal collisions with N2 molecules are investigated using a quantum mechanical and two semiclassical nonadiabatic methods. The full-dimensional interaction potential energy surfaces and spin-orbit coupling, which facilitates the spin-forbidden process, are represented by a recently constructed diabatic potential energy matrix. The multistate quantum dynamics for selected partial waves found small transition probabilities due to the weak spin-orbit coupling. The spin-flip transition is the most favored near the threshold due to effective curve crossing. Strong oscillations are also found in the probabilities, which are attributable to resonances supported by the deep well in the singlet-state potential. Vibrational state-specified rate coefficients are reported from J-shifted quantum dynamics calculations, and they follow the Arrhenius form. Vibrational excitation in the N2 collision partner is found to increase the excitation rate at low temperatures, but the trend is reversed at high temperatures. The two semiclassical methods qualitatively reproduce the quantum rate coefficients.

Hyperthermal Dynamics and Kinetics of the C(3P) + N2(X1Σg+) → CN(X2Σ+) + N(4S) Reaction

The hyperthermal dynamics and kinetics of the title reaction, which plays an important role in hypersonic chemistry for atmospheric entry vehicles, are investigated using quasi-classical trajectory methods on a recently developed ground electronic state potential energy surface. The dynamics calculations indicated that the reaction follows a complex-forming mechanism, despite its large endoergicity. The calculated differential cross section is forward-backward symmetric, consistent with a long-lived reaction intermediate supported by the NCN potential well. The lifetime of the reaction complex is sufficiently long that the vibrational distribution of the CN product can be predicted by the phase space theory. The calculated vibrational state specific and thermal rate coefficients follow the Arrhenius behavior, and the agreement with existing low-temperature experimental thermal rate coefficients is satisfactory. Extrapolations to high temperatures relevant to hypersonic conditions are provided.

Benchmarking CASPT3 Vertical Excitation Energies

Based on 280 reference vertical transition energies of various natures (singlet, triplet, valence, Rydberg, n → π∗, π → π∗, and double excitations) extracted from the QUEST database, we assess the accuracy of third-order multireference perturbation theory, CASPT3, in the context of molecular excited states. When one applies the disputable ionization- potential-electron-affinity (IPEA) shift, we show that CASPT3 provides a similar accuracy as its second-order counterpart, CASPT2, with the same mean absolute error of 0.11 eV. However, as already reported, we also observe that the accuracy of CASPT3 is almost insensitive to the IPEA shift, irrespective of the transition type and system size, with a small reduction of the mean absolute error to 0.09 eV when the IPEA shift is switched off.

Study of the CN(X2Σ+) + N(4S) Reaction at High Temperatures: Potential Energy Surface and Thermal Rate Coefficients

Reactions involving C and N play an essential role in the chemistry around the surface of a hypersonic spacecraft during its atmospheric re-entry. The collision of CN with other molecules and atoms has particular interest in aerothermodynamic modeling. This work focuses on the study of the CN + N → N₂ + C reaction in the triplet manifold ³A″ of CN₂. A high-level full-dimensional potential energy surface for this system is developed from ab initio calculations at the MRCI-F12 + Q level of theory. This surface is employed in quasiclassical trajectory calculations, and thermal rate coefficients from 100 to 20,000 K are computed. The rates for the formation of N₂ are compared with the available experimental data, and good agreement is found. At low and intermediate temperatures, the N₂ formation is more efficient than the N-exchange process, while at high temperatures, the rates for both processes are comparable. Finally, analytically modified Arrhenius expressions for the reaction rates of N₂ formation and N-exchange are reported.

First-principles dynamics of collisional intersystem crossing: resonance enhanced quenching of C(1D) by N2

Intersystem crossing is a common and important nonadiabatic process in molecular systems. Revealed with a first-principles investigation, the quenching of C(¹D) by N₂ is efficient due to multiple passages via long-lived collisional resonances, despite relatively small spin–orbit couplings.

Thermal Decomposition of NCN: Shock-Tube Study, Quantum Chemical Calculations, and Master-Equation Modeling

The thermal decomposition of cyanonitrene, NCN, was studied behind reflected shock waves in the temperature range 1790-2960 K at pressures near 1 and 4 bar. Highly diluted mixtures of NCN3 in argon were shock-heated to produce NCN, and concentration-time profiles of C atoms as reaction product were monitored with atomic resonance absorption spectroscopy at 156.1 nm. Calibration was performed with methane pyrolysis experiments. Rate coefficients for the reaction (3)NCN + M → (3)C + N2 + M (R1) were determined from the initial slopes of the C atom concentration-time profiles. Reaction R1 was found to be in the low-pressure regime at the conditions of the experiments. The temperature dependence of the bimolecular rate coefficient can be expressed with the following Arrhenius equation: k1(bim) = (4.2 ± 2.1) × 10(14) exp[-242.3 kJ mol(-1)/(RT)] cm(3) mol(-1) s(-1). The rate coefficients were analyzed by using a master equation with specific rate coefficients from RRKM theory. The necessary molecular data and energies were calculated with quantum chemical methods up to the CCSD(T)/CBS//CCSD/cc-pVTZ level of theory. From the topography of the potential energy surface, it follows that reaction R1 proceeds via isomerization of NCN to CNN and subsequent C-N bond fission along a collinear reaction coordinate without a tight transition state. The calculations reproduce the magnitude and temperature dependence of the rate coefficient and confirm that reaction R1 is in the low-pressure regime under our experimental conditions.