Reaction rate prediction trajectory calculation

Rice et al. [99] developed a global potential energy surface based on the Mowrey et al. [103] results and performed extensive classical trajectory calculations to study the dynamics of the CH2NN02 dissociation reactions. They calculated rates for reactions (III) and (IV) with classical barriers of 35 and 37 kcal/mol, respectively. They found that N-N bond fission dominates at low energy but that HONO elimination is competitive. Chakraborty and Lin [104] predict the opposite on the basis of their ab initio barriers and RRKM theory calculations. The two dissociations channels are closely competitive and it is not clear that ab initio methods are sufficiently reliable to distinguish between two reactions that have such similar energy requirements. Also, the Zhao et al. results [33] are not in accord with the theoretical predictions. 

The simplest way of taking aceount of vibrational effeets is to assume vibrational adiabatieity during the motion up to the eritieal dividing surface [27]. As mentioned aheady in the Introduetion, mueh of the earlier work on vibrational adiabatieity was concerned with its relationship to transition-state theory, espeeially as applied to the prediction of thermal rate constants [24-26], It is pointed out in [27] that the validity of the vibrationaUy adiabatie assumption is supported by the results of both quasielassieal and quantum seattering ealeulations. The effeetive thresholds indicated by the latter for the D -I- H2(v =1) and O + H2(v =1) reactions [37,38] are similar to those found from vibrationaUy adiabatic transition-state theoiy, which is a strong evidence for the correctness of the hypothesis of vibrational adiabatieity. Similar corroboration is provided by the combined transition-state and quasielassieal trajectory calculations [39-44]. For virtually all the A + BC systems studied [39-44], both collinearly and in three... 

The precise knowledge of reaction kinetics furnished by ACOMP allows predictive control of reactions, via calculable flow rates of reagents into the reactor, to yield desired molecular weight and composition trends, with subsequent online verification of the actual reaction trajectory. In this sense, the predictive approach is a prelude to full feedback control of the reactor, where the predicted trajectory can serve as an Ansatz, and deviations from the desired trajectory can then be corrected by small changes to the feed pumps and other variables. 

Two points are less settled and merit further investigation. The first concerns the vibrational excitation of bonds other than that broken in the reaction. It has usually been assumed that this will make little difference to reaction rates, but some recent experiments indicate significant effects. Secondly, information-theoretic analyses of trajectory calculations predict that as a reaction becomes more exothermic, its energy requirements become... 

The reactions of the bare sodium ion with all neutrals were determined to proceed via a three-body association mechanism and the rate constants measured cover a large range from a slow association reaction with NH3 to a near-collision rate with CH3OC2H4OCH3 (DMOE). The lifetimes of the intermediate complexes obtained using parameterized trajectory results and the experimental rates compare fairly well with predictions based on RRKM theory. The calculations also accounted for the large isotope effect observed for the more rapid clustering of ND3 than NH3 to Na+. 

An alternative way to obtain the spectral density is by numerical simulation. It is possible, at least in principle, to include the intramolecular modes in this case, although it is rarely done [198]. A standard approach [33-36,41] utilizes molecular dynamics (MD) trajectories to compute the classical real time correlation function of the reaction coordinate from which the spectral density is calculated by the cosine transformation [classical limit of Eq. (9.3)]. The correspondence between the quantum and the classical densities of states via J(co) is a key for the evaluation of the quantum rate constant, that is, one can use the quantum expression for /Cj2 with the classically computed J(co). This is true only for a purely harmonic system [199]. Real solvent modes are anharmonic, although the response may well be linear. The spectral density of the harmonic system is temperature independent. For real nonlinear systems, J co) can strongly depend on temperature [200]. Thus, in a classical simulation one cannot assess equilibrium quantum populations correctly, which may result in serious errors in the computed high-frequency part of the spectrum. Song and Marcus [37] compared the results of several simulations for water available at that time in the literature [34,201] with experimental data [190]. The comparison was not in favor of those simulations. In particular, they failed to predict... 

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