Behavior of ylides containing N, O, and C atoms as hydrogen bond accepters

The hydrogen bond (HB) basicity of a series of ylides containing nitrogen, oxygen, or carbon as heavy atoms, as well as the influence of the formation of the HE complexes on their structure, has been studied. In addition, in this paper we propose the formation of some rather strong HBs (that could be considered low-barrier hydrogen bonds, LBHBs) between ylides and different neutral molecules. The ylides chosen for the study were H3N+-N-H. Me3N+-N-H. H2O+-N-H, Me2O+-N-H, H2O+-O-, Me2O+-O-, and Me3N-C-H-2 As HE donors, classical donors such as HF, HCN, and HCCH were used. The analysis of the protonation energies of the ylides and the optimized geometries, interaction energies, and characteristics of the electron density of the complexes shows that these ylides are very good HE accepters, forming stable complexes even with weak HE donors. With strong donors, when the proton transfer did not take place, very strong HBs were formed with quite large interaction energies and very short HB distances which could be considered as LBHBs. Moreover, we have found that the sign of the Laplacian of the electron density at the bond critical point ((V) over bar rho (BCP)) and that of the energy density (H-BCP) could characterize the strength of HBs. Thus, weak HBs (E-1 < 12.0 kcal/mol) show both (V) over bar rho (BCP) and H-BCP > 0, and medium HBs (12.0 < 24.0 kcal/mol) slow <(V)over bar>(2)rho (BCP) > 0 and H-BCP < 0. while strong HBs land therefore LBHBs: E-1 > 24.0 kcal/mol) show both (V) over bar rho (BCP) and H-BCP < 0.