Kinetics of the tautomerization of thioimidic acids R−C(SH)NH → R−C(S)NH2: For R = H, F, HO, CN, NC, H2N, HC(O), HC(S), HC≡ C, CH3, CF3, H2C=CH, HOCH2, H2NCH2,CH3C(O), C2H5, (CH3)2CH, C6H5

Abstract

The kinetics of the tautomerization of thio-imidic acids RC(SH)NH were determined at low (50–300 K) and high (500–1500 K) temperatures as R was varied to encompass mono- and diatomic species H, F, HO, NC, CN through H₂N, HC(O), HC(S), HC≡ C, H₃C, F₃C, HOCH₂, H₂C=CH, CH₃C(O), H₂NCH₂ and including ethyl, isopropyl and phenyl groups. The presence of a labile H-atom on the R-group can give rise to an alternative reaction, as in, H₃CC(SH)NH → CH₂=C(SH)NH₂ but these encounter much higher barriers. At the lowest temperatures there is over a million-fold difference in the rate constants for the fastest, R = H₂N, and slowest, R = F, reaction with quantum mechanical tunneling playing a dominant role which is dealt with by canonical transition state and small curvature tunneling theory. The tautomerization of similar imidic acids proceeds at much slower rates due to higher energy barriers to reaction. Additionally basic thermochemical data such as formation enthalpy, entropy, isobaric heat capacity and an enthalpy function are provided for all the species which may be useful training sets for machine-learning/AI purposes.