Vibrational mode selectivity in the unimolecular decomposition of CH2NNO2

Abstract

Vibrational mode selectivity has been investigated for the unimolecular decomposition of methylene nitramine (MN), CH2NNO2, by using classical trajectories computed on a (previously reported) potential-energy surface (PES) that is based on ab initio results. The PES allows for the two known primary decomposition pathways: (I) N–N bond scission to form H2CN and NO2 and (II) concerted molecular elimination giving HONO and HCN. Of particular interest in this study is the influence of selective vibrational mode excitations on the rates and branching of the decomposition via the competing reaction channels, which have barrier heights that differ by only 2 kcal/mol. Rates were computed for 57.7 kcal/mol above the zero-point energy for initial conditions corresponding to overtone excitations of each of the 15 normal modes. In each case, zero-point energy was assigned to each of the normal modes, and then one of the modes was assigned 57.7 kcal/mol excitation energy giving a total of 85.0 kcal/mol. One calculation was done at this energy with initial conditions corresponding to a microcanonical (statistical) distribution in which the energy was randomly distributed among all of the vibrational modes. Comparisons of the mode selective results with this statistical rate show that there is substantial enhancement of the decomposition rates for the two reactions for excitation of three of the normal modes (those with frequencies of 275, 442, and 753 cm−1). Excitation of the other 12 modes yielded rates in accord with the statistical rates for the 2 reactions. The eigenvectors of the normal modes of the molecule that give mode selectivity in the reaction rates project onto the reaction coordinates for the two pathways. One other mode (with frequency 606 cm−1) also projects strongly onto the reaction path for II), but there is no enhancement of the rate when it is excited. However, we found that energy transfers out of this mode to other modes not coupled to either reaction coordinate. The remaining 11 modes do not project to any significant extent onto either reaction coordinate. The results show that projection of a vibrational mode onto the reaction coordinate is necessary for mode selective reaction, but is not sufficient since energy transfer into the reaction coordinate must compete with energy flow to other modes that do not project onto (i.e., are uncoupled from) the reaction coordinates.

Ab Initio Study of Reactions of sym-Triazine

“…The studies by Miller and Waite 16 and Rice et al. have clearly shown a correlation between projection of vibrational modes onto the reaction coordinate and coupling with the reaction coordinate. Thus, similar analyses based on projections of the vibrational modes of sym -triazine and the (HCN) 3 cluster onto the reaction coordinate should verify whether the intramolecular motions of these molecules are coupled with the reaction path.…”

“…Vibrational Coupling. It has been shown that projection of the eigenvector of a vibrational mode onto the eigenvector corresponding to the direction along the reaction path is related to the coupling of that vibrational mode with the reaction coordinate. , Waite and Miller showed that the unimolecular decay rates of the Henon−Heiles model behave statistically for all energies, even though this system has quasiperiodic classical motion at low energies, conditions under which mode specificity might be expected . They suggested that the statistical behavior was due to coupling of the intramolecular motions of the model with the dissociative reaction coordinates for the system, since there was some degree of projection of all of the vibrational modes onto the reaction coordinates for the Henon−Heiles model.…”

“…Rice et al . showed that rates of unimolecular decomposition through competing channels were enhanced and branching ratios changed upon excitation of vibrational modes of a reactant that project strongly onto the reaction coordinates of the system …”

“…As set forth in Rice et al , we calculated local normal modes 18 for points along the reaction path for (III) at the MP2/6-31G** level. The infinitesimal translations and rotations were projected out, leaving 3 N − 7 vibrational modes of the molecule and the eigenvector corresponding to the direction along the reaction path .…”

Ab Initio and DFT Potential Energy Surfaces for Cyanuric Chloride Reactions

“…In our previous study, we used a method of projecting the vibrational eigenvectors of sym -triazine and the symmetric (HCN) 3 cluster onto the reaction path for the concerted triple-association reaction . Our premise in doing so was based on studies that indicate a correlation between the magnitude of the projection of a vibration onto the reaction coordinate and the coupling of that mode with the reaction path. , For modes that project strongly onto the reaction coordinate, we concluded that these vibrations are possible efficient energy transfer routes between the molecules and the reaction coordinate.…”