Comparison of temperature effect on electroreduction of tert-butyl chloride and tert-butyl bromide-Theoretical study

Graphical abstract: Display Omitted Highlights: → MD simulations are performed for electrochemical reduction of t-BuCl and t-BuBr. → Reaction rates, transfer coefficients and saddle point avoidance are compared. → For both molecules the transfer coefficient α decreases with temperature T. → At the same T α(t-BuCl) is lower than α(t-BuBr) for both viscosities studied. → In similar conditions the saddle point avoidance for t-BuBr is larger than for t-BuCl. - Abstract: An effect of temperature on kinetics and mechanism of the reaction RX + e → R· + X- is studied using molecular dynamics simulations method for two molecules: t-BuCl and t-BuBr in two different solvent viscosities γx = 2 ps-1 and γx = 40 ps-1. The potential surfaces used in the simulations are two-dimensional and depend on the solvent generalized coordinate x and the C-X bond elongation y, the latter part based on quantum calculations. For both systems the transfer coefficient α is found to decrease with temperature and this effect is stronger in the more viscous solvent. For γx = 40 ps-1 the following α values are reported: at T = 278 K 0.234 (t-BuCl) and 0.264 (t-BuBr), while at T = 398 K 0.208 (t-BuCl) and 0.222 (t-BuBr). In identical conditions and for the same height of the energy barrier, α(t-BuCl) is always lower than α(t-BuBr). The results are interpreted as an effect of the saddle point avoidance phenomenon.