Central-barrier dynamics

Quasiclassical direct dynamics trajectories at the various levels of theory were later calculated to study the central barrier dynamics for the C1 I CH3C1, Cr + C2H5C1, C1- + CH3I, F +CH3C1, OH +CH3C1, and other Sn2 reactions.31,32,47,97 108 The effect of initial reaction conditions, such as energy injection, substrate orientations, and the mode of collision, on the fate of the reaction, product, and energy distribution, was analyzed. Some of these trajectory calculations required serious modification in RRKM and TST for... 

The above description assumes that an intermediate is formed with statistical classical dynamics and pooling of zero-point energy. If the dynamics of the intermediate is nonstatistical (i.e. as for Cl ---CH3C1 °), the intermediate s lifetime and product energy distribution may agree with experiment. A discussion of the applicability of classical mechanics for studying the central barrier dynamics of the [C1---CH3---C1] moiety is given below. 

Sn2 Nucleophilic Substitution. 8. Central Barrier Dynamics for Gas Phase CD + CH3CI. 

Direct dynamics trajectory calculations at the MP2/6-31-FG level of theory were then used to explore the reaction dynamics of this system [63]. Sixty-four trajectories were started from the central barrier shown at A in Fig. 11, with initial conditions sampled from a 300 K Boltzmann distribution. Of the 31 trajectories that moved in the direction of products, four trajectories followed the MEP and became trapped in the hydrogen-bonded [CH3OH ... 

E. Dynamical Model for Sn2 Substitution and Central Barrier Recrossing. 152... 

Adding quanta to the C-Cl bond promotes bond extension, so that the central barrier can be reached as Cl- approaches. This dynamical effect is in accord with the role of vibrational energy in A + BC -> AB + C triatomic displacement reactions.15 The plot in Figure 5 of the probability of directly attaining the central barrier versus Cl + CH3Clb collision impact parameter shows that direct substitution occurs at small impact parameters. In contrast, association extends to larger impact parameters. 

A dynamical model for SN2 nucleophilic substitution that emerges from the trajectory simulations is depicted in Figure 9. The complex formed by a collision between the reactants is an intermolecular complex CinterR. To cross the central barrier, this complex has to undergo a unimolecular transition in which energy is... 

Figure 9. Dynamical model for Sn2 nucleophilic substitution. The labels R and P denote the reactant and product sides of the central barrier, respectively.

The dynamical model described in Figure 9 indicates that the trajectories may recross the central barrier several times if the Cintra R Cintra p transition is faster... 

Additional experimental, theoretical, and computational work is needed to acquire a complete understanding of the microscopic dynamics of gas-phase SN2 nucleophilic substitution reactions. Experimental measurements of the SN2 reaction rate versus excitation of specific vibrational modes of RY (equation 1) are needed, as are experimental studies of the dissociation and isomerization rates of the X--RY complex versus specific excitations of the complex s intermolecular and intramolecular modes. Experimental studies that probe the molecular dynamics of the [X-. r - Y]- central barrier region would also be extremely useful. 

The MP2/6-31G direct dynamics simulation study was later extended to cover the dynamics from the central barrier for the SN2 reaction of Cl I C2H5CI.104 The majority of the trajectories starting from the saddle point moved off the central barrier to form the Cl- C2H5CI complex. The results were different from those obtained previously for the CH3C1 reaction, in which extensive recrossing was observed. The reaction of C2H5CI was, in this sense, consistent with the prediction by the RRKM theory. However, some of the... 

CU-CH3CI decomposition MP2/6-31G- [C1-CH3-C1] central barrier recrossing and intrinsic non-RRKM dynamics [121]... 

MP2/6-3H-G direct dynamics was used to study the actual exit-channel dynamics for this reaction. Initial conditions for the trajectories were selected by sampling the [HO-CH3-F] central barrier s 300 K Boltzmann distribution. The simulations showed... 

When the central barrier in a double energy minimum decreases below the lowest zero point vibrational level, the situation is equivalent to a true single minimum case (Forsen et al., 1978). A carbon tunnelling mechanism suggested to explain the dynamic behaviour in 2-norbornyl cations, (Fong, 1974 Dewar and Merz, 1986) has been shown to be unimportant (Yannoni etaL 1985). 

Both experiments and simulations have shown that the chemical dynamics of gas-phase X + CHjY XCH + Y Sn2 nucleophilic substitution reactions are non-statistical. Reactions, snch as C/ + CH Br CICH3 + Br, have X — CH3Y and XCHj — Y ion-dipole complexes separated by a central barrier (Figure 20.5) and the unimolecular dynamics of these complexes are intrinsically non-RRKM. These dynamics arise in part from weak couplings between the three low freqnency intermolecular modes of the complex and the complex s much higher frequency nine intramolecular modes. 

If a wavepacket remains localized in the central-barrier region, it may then be possible to resolve its vibrational dynamics and spectrum. A direct probe of non-RRKM dynamics for the Cl ---CH3CI complex would involve measuring the lifetimes of its resonance states. 

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