Chemical dynamics simulations are performed to study the unimolecular dissociation of the benzene (Bz)−hexafluorobenzene (HFB) complex at five different temperatures ranging from 1000 to 2000 K, and the results are compared with that of the Bz dimer at common simulation temperatures. Bz−HFB, in comparison with Bz dimer, possesses a much attractive intermolecular interaction, a very different equilibrium geometry, and a lower average quantum vibrational excitation energy at a given temperature. Six low-frequency modes of Bz−HFB are formed by Bz + HFB association which are weakly coupled with the vibrational modes of Bz and HFB. However, this coupling is found much stronger in Bz−HFB compared to the same in the Bz dimer. The simulations are done with very good potential energy parameters taken from the literature. Considering the canonical (TST) model, the unimolecular dissociation rate constant at each temperature is calculated and fitted to the Arrhenius equation. An activation energy of 5.0 kcal/mol and a pre-exponential factor of 2.39 × 10 12 s −1 are obtained, which are of expected magnitudes. The responsible vibrational mode for dissociation is identified by performing normal-mode analysis. Simulations with random excitations of high-frequency Bz and HFB modes and lowfrequency inter-Bz−HFB vibrational modes of the Bz−HFB complex are also performed. The intramolecular vibrational energy redistribution (IVR) time and the unimolecular dissociation rate constants are calculated from these simulations. The latter shows good agreement with the same obtained from simulation with random excitation of all vibrational modes.

Chemical dynamics simulations are performed to study the association of benzene (Bz) and hexafluorobenzene (HFB) followed by the ensuing dissociation of the Bz–HFB complex. The calculations are done for 1000, 1500, and 2000 K with an impact parameter (b) range of 0–10 Å at each temperature. Almost no complexes are observed to form at b = 8 and 10 Å. Following three different methods of calculation of the temperature-dependent association rate constant k asso(T), the values obtained are 1.67 × 10–10, 1.86 × 10–10, and 2.05 × 10–10 cm3/molecule·s with a standard deviation of approximately 0.1 × 10–10 cm3/molecule·s for T = 1500 K. Among those values of k asso(T), the middle one is obtained by considering a relative translational energy of 3RT/2 at T = 1500 K, and the same is followed to calculate k asso(T) at 1000 and 2000 K. The Arrhenius parameters, using the k asso(T) values at three temperatures, are 0.203 × 10–10 cm3/molecule·s for the pre-exponential factor and −5.79 kcal/mol for the activation energy. The absolute value of the latter is similar to the Bz + HFB association energy of 5.93 kcal/mol. The ensuing dissociation dynamics of the complex is significantly different from the unimolecular dissociation dynamics, and an exponential function fits the N(t – t 0)/N(t 0) curves comparatively well. The ensuing dissociation is also observed to be independent of time for a statistically large sample size.

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The molecular structure, thermal stability, detonation properties and impact sensitivity of nitrogen-rich tetrazine-based designed high energy density molecules are examined. The effective stability of the designed molecules in terms of electronic structure, thermal excitation, photoexcitation and in presence of water are predicted. The topography of the excited and ground state (S 1 and S 0 ) of those molecules along the CÀNO 2 bond dissociation coordinate are studied. The existence of the double minima at the S 1 state and the S 1 -S 0 intersection produces a probabilistic path for molecules B1, B2 and A1, to revert back to their respective S 0 state from the S 1 state via both radiative and non-radiative deactivation mechanism. The energy content (in terms of heats of formation), detonation velocities and detonation pressure of tetrazine-based molecules are measured. Interestingly, the thermal stability of these designed molecules is higher than the two well-known high energy density molecules: RDX and HMX. The detonation velocities and the detonation pressure of these molecules are higher than RDX, however, are lower than HMX. In addition, the safety, reliability and stability of these high energy density molecules have been measured by formulating semi-empirical equations of impact sensitivity based on linear and multiple linear regression method and the present study is ended with the discussion of most probable synthetic routes to the designed molecules.