Barrierless association reactions and bond dissociation reactions play important roles in combustion and atmospheric chemistry, as well as being of fundamental importance in the theory of chemical kinetics. Calculating their rate constants is challenging for both electronic structure theory and kinetics theory. From the electronic structure viewpoint, it is difficult to obtain a qualitatively correct potential curve in the bond-breaking region where the variational transion state is located. In a paper that appeared today in the early edition of Proceedings of the National Academy of Sciences,
we validated an exchange–correlation functional against beyond-CSDT electronic structure theory, and we used the resulting density functional method for direct dynamics calculations of the pressure- dependent rate constants for the C=C bond dissociation of C2F4 by applying multifaceted variable-reaction-coordinate variational transition-state theory with the recently developed system-specific quantum Rice–Ramsperger–Kassel theory. Our computed dissociation rate constants agree well with the recent experimental measurements by Troe and coworkers. This work presents a prototypical example of the full treatment of the kinetics of barrierless reactions and their reverse reactions, including variational optimization of the transition state, multidimensional tunneling, and pressure effects. The supporting information of the article gives full details of the variable-reaction-coordinate variational transition state theory calculation of the high-pressure limiting rate constant.
We believe that the system-specific quantum RRK method employed in this work makes it much more convenient to accurately model pressure-dependent rate constants than was possible with previously available methods.