Theoretical ab initio and semiempirical studies of biologically important di- and oligopyrrolic compounds: pyrromethene and protonated pyrromethene

Abstract

Comparative ab initio (HF/6-31G ∗ and MP2/6-31G ∗) and semiempirical (AM1 and PM3) studies of pyrromethene and its N-protonated form have been carried out. Structures, energies and charge distributions were calculated for conformational isomers formed by rotations around the single and double bonds at the methine bridge. Furthermore, transition structures were located and rotational barriers between conformers obtained. Using an empirical relationship between the NH stretching frequency and the hydrogen bond stabilization enthalpy, an experimental estimate of ΔH°(298 K) HB, solv = −(18–19) kJ mol −1 was determined for the intramolecular hydrogen bond in pyrromethene. Best agreement with this value is found with a theoretical estimate based on the HF/6-31G ∗ hydrogen bond energy, while it is overestimated at the MP2 and underestimated at the AM1 and PM3 levels. The activation barrier for intramolecular proton transfer was calculated to be about 40 kJ mol −1 at the MP2 level. The AM1 and PM3 methods predict the structures of the dipyrrolic frame with similar accuracy, but AM1 results conform better to MP2 results with respect to conformational energies and torsional angles at the methine bridge. The rotational barriers at the methine bridge are underestimated and the barrier for the proton transfer is overestimated at the AM1, PM3 and HF/6-31G ∗ levels, as compared with MP2 results.