Electron Density Analysis on the Alpha Acidity of Nitriles

Abstract

24 substituded cyanocompounds and the corresponding anions obtained upon H +-abstraction from diverse positions were subjected to an electron density analysis with the Quantum Theory of Atoms in Molecules (QTAIM). All the electron densities were obtained at the B3LYP/6-31++G(2d,2p) level on completely optimized geometries. In accordance to experimental evidence, α-H + abstraction is found as the most favored one (by at least 100 kJ mol-1 in all the tested compounds). The presence of additional resonance electron attractors reduces significantly the α-deprotonation energy, whereas this magnitude is quite insensitive to the inclusion of resonance electron donors. The electron density rearrangement accompanying the deprotonation is apparently in line with the predictions of the resonance model (RM). In fact, a significant part of the electron density gained by expelling the proton is transferred to cyano N and to other groups where significant resonance structures delocalize the negative charge. Nevertheless, some significant modifications have to be introduced on the RM picture when the QTAIM results are studied in detail.