Chemical reaction dynamics are studied to follow and understand the concerted motion of
several atoms while they rearrange from reactants to products. With the number
of atoms growing, the number of pathways, transition states, and product
channels also increases and rapidly presents a challenge to experiment and
theory. Here, we disentangle the competition between bimolecular nucleophilic
substitution (S
N
2) and base-induced elimination (E2) in the
polyatomic reaction F
-
+ CH
3
CH
2
Cl. We find
quantitative agreement for the energy- and angle-differential reactive
scattering cross sections between ion imaging experiments and quasi-classical
trajectory simulations on a 21-dimensional potential energy hypersurface. The
anti-E2 pathway is most important, but the S
N
2 pathway becomes more
relevant as the collision energy is increased. In both cases the reaction is
dominated by direct dynamics. Our study presents atomic level dynamics of a
major benchmark reaction in physical organic chemistry, thereby pushing the
number of atoms for detailed reaction dynamics studies to a size that allows
applications in many areas of complex chemical networks and environments.
The construction of high-dimensional global potential energy surfaces (PESs) from ab initio data has been a major challenge for decades. Advances in computer hardware, electronic structure theory, and PES fitting methods have greatly alleviated many challenges in PES construction, but building fitting sets has remained a bottleneck so far. We present the ROBOSURFER program system that completely automates the generation of new geometries, performs ab initio computations, and iteratively improves the PES under development. Unlike previous efforts to automate PES development, ROBOSURFER does not require any uncertainty estimate from the PES fitting method and thus it is compatible with the permutationally invariant polynomial (PIP) method. As a demonstration we have developed five related but different global reactive PIP PESs for the CH 3 Br + F − system and used them to perform quasiclassical trajectory (QCT) reaction dynamics simulations over a wide range of collision energies. The automatically developed PESs show good to excellent accuracy at known stationary points without any manual sampling, and QCT results indicate the lack of unphysical minima on the fitted surfaces. We also present evidence suggesting that the breakdown of single reference electronic structure theory may contribute significantly to the fitting errors of global reactive PESs.
Since the pioneering reaction dynamics
studies of H + H2 in the 1970s, theory increased the system
size by one atom in every
decade arriving to six-atom reactions in the early 2010s. Here, we
take a significant step forward by reporting accurate dynamics simulations
for the nine-atom Cl + ethane (C2H6) reaction
using a new high-quality spin–orbit–ground-state ab initio potential energy surface. Quasi-classical trajectory
simulations on this surface cool the rotational distribution of the
HCl product molecules, thereby providing unprecedented agreement with
experiment after several previous failed attempts of theory. Unlike
Cl + CH4, the Cl + C2H6 reaction
is exothermic with an adiabatically submerged transition state, allowing
testing of the validity of the Polanyi rules for a negative-barrier
reaction.
We develop the first accurate full-dimensional ab initio PES for the OH− + CH3I SN2 and proton-transfer reactions treating the failure of CCSD(T) at certain geometries.
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