Electronic effect and the nature of intramolecular H-bond. A theoretical approach based on NMR spectroscopy

MONTERO, MARCOS DAVID A.; MARTÍNEZ, FERNANDO; SANTA CRUZ, TERESITA; AUCAR, GUSTAVO ADOLFO; AUTSCHBACH, JOCHEN

Casa Piedra, Santiago

Congreso; World Association of Theoretical and Computational Chemists; 2014

During the last few decades there was an ever increasing interest on the more accurate description and deeper understanding of both, the new electronic effects that they generateand also the nature of the hydrogen bonds, HB; being them intra- or inter-molecular. There are still some broad questions to be more precisely answered, like, is the nature of HB ionic or covalent? Or, May it be dependent on the general molecular electronic structure? It is known that new physical insights may be acquired when theoretical models of NMR spectroscopy are applied to understand the electronic origin of its spectroscopic parameters. They are quite sensitive to local electronic effects, and also to non-local behavior of the electronic structure. Different electron-nucleus interactions contributing to J-couplings are strongly dependent on whether is the σ- or the π-electronic framework that mostly transmits the coupling. The NMR hydrogen shielding of the H-bonded proton is also largely modified in its components, diamagnetic and paramagnetic, as happens to heavier elements. In our group of research we are involved in the search of new insights that may highlight what makes the H-bonds be so special, by applying state of the art theoretical models to NMR spectroscopic parameters. In this poster we will show some of our own newest developments together with that in collaboration with Jochen Autschbach´s group, [2] concerning the analysis of electronic effects on the proton and other vicinal atoms (C, N, O) involved in hydrogen bondings. Some model compounds were chosen due to their special feature, e. g. malonaldehyde, nitromalonamide and nitromalonaldehyde; and also the two DNA pairs, i.e. AT and GC. We will show how NMR spectroscopic parameters are influenced by the degree of covalency of the H-bond as well as intra- and intermolecular resonance. Some analysis will be performed in terms of ?chemist?s orbitals?, i. e. localized molecular orbitals representing lone-pairs, atomic cores, and bonds. The effects on NMR parameters due to both, the charge assisted effect and the resonance assisted (π-electronic conjugation) effect will be discussed, together with a theoretical finding of measurable relativistic effects transmitted through H-bonds.