Quasi-Classical Trajectory Study of NH(³∑^–) + NH(³∑^–) Reactive Collisions

Abstract

A full-dimension quasi-classical trajectory study of collisions between two NH radicals is presented. Interatomic interactions are represented by a previously reported global six-dimensional potential energy surface for singlet electronic state of the N2H2 system. This study suggests that the formation of N2 from the collision of two NH radicals may occur via a one-step (NH + NH → N2 + H + H) or two-step (NH + NH → N2H + H → N2 + H + H) microscopic reaction mechanism. A fast vibrational energy redistribution is observed in the four-body complex in the latter mechanism. Excitation functions are presented and discussed. A variant of the vibrational energy quantum mechanical threshold method was used to correct the zero-point energy leakage in the classical calculations. The influence of reactant’s rotational and vibrational energy on reactivity was investigated by state-specific calculations with one or both reactants excited. Reaction rate constants for the ground and some rotationally excited states are presented using an Arrhenius–Kooij-like functional form.

A Theoretical Study of the N2 + H2 Reactive Collisions for High Vibrational and Translational Energies

“…This function, depending upon the six interatomic distances, was constructed within the double many-body expansion [17] and calibrated to multireference configuration interaction ab initio energies [16,18]. Such a PES was previously used in dynamic studies of the NH + NH reaction [19] and the vibrational relaxation in nonreactive collisions of H 2 and N 2 [15]. For the interest of this work, from the N 2 H 2 DMBE PES, the reaction enthalpies are:…”

“…For molecular dynamic calculations, the quasiclassical trajectories method was used [20,21]. In trajectory calculations, we utilized an adapted version of the VENUS96 [22] code, coupled to the N 2 H 2 PES and making the appropriate assignment of all possible reactive channels (see Reference [19]). For the problems of interest, mentioned in the Introduction, the chosen relative diatom-diatom translational energy covered the range 20.0 ≤ E tr /kcal mol −1 ≤ 120.0; while the selected initial vibrational quantum numbers for N 2 and H 2 were v = 0, 4, 6, 8, 10, 12 and v = 4, 6, 8, 10, 12, respectively.…”

“…In turn, the diatomic-diatomic initial separation was fixed at 15 Å, a value sufficiently large to make the interaction negligible. In turn, the maximum impact parameter b max , which leads to the r reactive channel [23], was obtained following the usual procedure by computing batches of 800 trajectories for fixed values of b [15,19,24] and decreasing its value until a reactive trajectory was obtained. This procedure should allow for accuracy in b max of about 0.1 Å; the calculated values are reported in Table 1.…”

On apparent barrier-free reactions

“…The reactive collisions between two NH radicals were recently studied [12]. There, the quasi-classical trajectory methodology was used combined with a previously developed potential energy surface (PES) for the ground electronic state of N2 H2 [13].…”

“…Both collision NH+NH and SO+OH are dipole-dipole dominated processes [11,12]. For them, orientation effects are essential in the formation of the four bodies' moiety.…”