The proton affinities (PA), energies of protonation by hydronium ion (EpT) and energies of hydration (EH) of acetylene and several substituted acetylenes have been investigated by means of LCAO-MO-SCF calculations, mainly at the 3-21G level. Calculations were performed on the ground and low lying excited states of each species, and in the case of substituted acetylenes, the results of Markovnikov (M) and anti-Markovnikov (AM) protonation and hydration were compared. The calculated PA values show that the excited state cations are all significantly red shifted compared with the ground state cations, indicating strongly enhanced basicity, of the acetylene group on electronic excitation. The results lead to the conclusion that hydration of both the lowest singlet and low lying triplet states should be much more facile than the corresponding ground state reactions, in accord with experimental observation on analogous systems. The results also lead to the conclusion that the S0 and S1reactions should both be strongly regioselective in either the M or AM sense, depending on the nature of the substituent, but that the T2reactions should show little preference for either mode of addition, independent of substitution. The geometries and electronic distribution of the various excited species will be discussed, and the theoretical results will be compared with available experimental data on the rate profiles for this type of photohydration.
ASJC Scopus subject areas
- Chemical Engineering(all)