The reaction of excited nitrogen atoms, N( 2 D), with the simplest alkyne, C 2 H 2 , was investigated for the first time under single-collision conditions in crossed beam experiments with mass spectrometric detection. The experimental results combined with electronic structure calculations and RRKM predictions allow us to identify cyanomethylene (HCCN) as the main primary reaction product and to establish its formation dynamics. The identification of the N/H exchange channel suggests that the title reaction is a good candidate to explain formation of nitriles in the upper atmosphere of Titan.

Ab initio molecular orbital calculations have been carried out on the N( 2 D) + CH 4 reaction in order to obtain information on possible reaction products. Stationary points associated with the product channels and their harmonic vibrational frequencies have been calculated at the MP2(full)/cc-pVTZ level of theory. Barrier heights and heats of reaction have been estimated at the projected MP4(full,SDTQ)/cc-pVTZ level of theory. Among the possible processes considered, the reaction pathways to produce CH 2 NH + H and CH 3 + NH have been found to be important. RRKM calculations have been performed to confirm this result. The saddle point structure in the entrance channel of the N( 2 D) + CH 4 reaction has also been calculated using the CASSCF method. It has been found that N( 2 D) inserts into the C-H bond of CH 4 , which is qualitatively consistent with recent experimental results.

Rate constants for the reactions N( 2 D, 2 P) + C 2 H 2 and C 2 D 2 have been measured using a technique of pulse radiolysis-resonance absorption between 220 and 293 K. Arrhenius parameters have been determined from the temperature dependence of the measured rate constants; the activation energies for the reactions of N( 2 D) were about 0.5 kcal/mol, while those for N( 2 P) were about 0.9 kcal/mol. The H/D isotope effect was found to be very small for both the N( 2 D) + C 2 H 2 and N( 2 P) + C 2 H 2 reactions. The rate constants for N( 2 D) + C 2 H 2 were found to be about 3 times as large as those for N( 2 P) + C 2 H 2 . To understand the overall reaction mechanism of the N( 2 D) + C 2 H 2 reaction, ab initio molecular orbital calculations of the lowest doublet potential energy surface have been performed. It has been found that the initial step of the reaction is the addition of the N atom to the π bond of acetylene. The rate constants have been calculated using conventional transitionstate theory and compared to the experimental results. Possible reaction pathways are discussed on the basis of the ab initio results.

The lowest doublet potential energy surface for the N(2D) + C2H4 reaction has been characterized using ab initio molecular orbital theory. The CASSCF/cc-pVDZ calculations predict that the dominant mechanism is the addition of N(2D) to the CC π-bond of C2H4 to form a cyclic three-membered intermediate radical rather than the insertion into the CH bond in C2H4. Reaction pathways have also been discussed on the basis of the PMP4(full,SDTQ)/cc-pVTZ//MP2/cc-pVDZ level calculations. The reaction is shown to have several possible products via somewhat complicated reaction mechanisms. The results of RRKM calculations predict that the main product channel is cyclic-CH(N)CH2 (azirine) + H under collision-free conditions.

Articles you may be interested inThe three-dimensional nonadiabatic dynamics calculation of D H 2 + and H D 2 + systems by using the trajectory surface hopping method based on the Zhu-Nakamura theory Accurate 3 dimensional quantum dynamical study of the Ne+H 2 + →NeH + +H reaction Three-dimensional quantum reactive scattering calculations have been carried out for the ͑DϩH 2 ͒ ϩ nonadiabatic ion-molecule collision. The calculations have been done using the time-independent close-coupling formalism with hyperspherical coordinates. The (3ϫ3) diatomics-in-molecule potential energy surfaces have been employed. The result of the accurate quantum scattering calculations have been compared to the results of the quasiclassical trajectory surface hopping method. Two versions of the method have been used; one uses Tully's fewest switches algorithm and the other is the trajectory surface hopping method of Tully and Preston, in which electronically nonadiabatic hopping is only allowed at the predefined crossing seams. We have found that the agreement between the quantum result and the result of Tully's method is generally good, but the Tully and Preston method significantly underestimates the nonadiabatic transition probability.

Thermal rate constants for the N(2D,2P) + CH4 (CD4) reactions have been measured using a technique of pulse radiolysis−resonance absorption in the temperature range between 223 and 298 K. Activation energies determined from the temperature dependence were about 1.5 and 1.0 kcal/mol for the reactions of N(2D) and N(2P), respectively. The H/D kinetic isotope effects were about 1.8 and 1.6 for N(2D) and N(2P), respectively. The rate constants for N(2P) + CH4 were much smaller than those for N(2D) by a factor of 40−60. Variational transition-state theory calculations of the rate constants for the N(2D) + CH4 (CD4) insertion reaction have been carried out using the reaction path information obtained from ab initio molecular orbital calculations. The comparison between the calculated and experimental rate constants shows that multiple surface coefficients are larger than the statistical value, meaning that nonadiabatic transitions are important for the N(2D) + CH4 reaction.

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A total of 100 trajectories for the photodissociation, CH3CHO → CH4 + CO, on the S0 potential surface have been calculated using the direct ab initio molecular dynamics method at the RMP2(full)/cc-pVDZ level of theory. The energy distributions for the relative translational energy, the CO internal energy, and the CH4 internal energy were calculated to be 28, 20, and 51%, respectively. It was predicted that the product CO is highly rotationally excited but vibrationally almost not excited; on average, the rotational and vibrational quantum numbers were 68.2 and 0.15, respectively, which qualitatively agrees with the recent observation of Gherman et al. (J. Chem. Phys. 2001, 114, 6128.)