A Data Set of Plausible Proton Transfer Steps For Arrow-Pushing Mechanisms

A large data set of kinetically plausible proton transfer steps was created. A set of over 48 million proton transfer steps, between heteroatoms, was generated combinatorially from a set of about 8,000 acids and conjugate bases for which experimental aqueous pKas around room temperature were available. The set was augmented with about 100 estimated pKas of highly reactive species important for reaction mechanisms. The resulting set of pKas span a range from -15 to +37. Rate constants were estimated at 25 °C using the pKas and utilizing a simplified Eigen equation without statistical factors. Steps with estimated rate constants ≥ 10^3 M^-1 s^-1 – a conservative boundary – were included in the data set. An additional set of 15,138 proton transfer steps were estimated using the Eigen-Bernasconi equation for proton transfers from carbon acids to heteroatom bases for which intrinsic rate constants and Brønsted 𝛽 values were known. Steps for proton transfers from carbon with estimated rate constants ≥ 10^-1 M^-1 s^-1 were added to the data set. Each entry was encoded in SMIRKS format, which is commonly used for machine learning, with electron-flow specification. The objective of this work was the creation of a structurally rich data set rather than accurate calculation of rate constants.