Abstract
AbstractWe have studied the system NH3…H+…HNCH‐CHCH2, which is a model for an enzymatic site characterized by an important pi‐electron contribution, with the aim of contributing to the molecular orbital theory of H‐bridge formation and proton transfer. Ab initio canonical orbitals from STO‐3G and 4.31G bases have been determined for various geometrical configurations, and analyzed in terms of Mulliken's Modified Valence Atomic Orbitals obtained by suitable contraction. Also, the orbital correlation diagram has been obtained and discussed. At least two sigma MO's can be considered specific of the H‐bond: they involve the expected hybrids giving rise to stereoelectronic effects associated with the lone pairs of the two heavy atoms. Population analysis suggests that the bridge hydrogen changes its population very little during its motion along the bridge, although its in situ atomic orbital does not change: this means that it acts essentially as a relay center in charge transfer between the two moieties of the H‐bonded complex. As expected on intuitive grounds, charge transfer involves essentially the sigma cores.
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