Behavior of Ylides Containing N, O, and C Atoms as Hydrogen Bond Acceptors

TLDR

The study examines the hydrogen‑bond basicity of ylides containing N, O, or C heavy atoms and how HB complex formation affects their structure. The paper proposes that ylides can form strong, low‑barrier hydrogen bonds with neutral molecules. The authors investigated a set of ylides (H3N+−N-H, Me3N+−N-H, H2O+−N-H, Me2O+−N-H, H2O+−O-, Me2O+−O-, Me3N+−C-H2) with classical donors HF, HCN, and HCCH to study HB interactions. The ylides proved to be excellent HB acceptors, forming stable complexes even with weak donors, while strong donors produced low‑barrier HBs with large interaction energies and short distances; the Laplacian and energy density at the bond critical point effectively distinguish weak, medium, and strong (LBHB) interactions.

Abstract

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 HB 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-H2. As HB 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 HB acceptors, forming stable complexes even with weak HB 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 (∇2ρBCP) and that of the energy density (HBCP) could characterize the strength of HBs. Thus, weak HBs (EI < 12.0 kcal/mol) show both ∇2ρBCP and HBCP > 0, and medium HBs (12.0 < EI < 24.0 kcal/mol) show ∇2ρBCP > 0 and HBCP < 0, while strong HBs (and therefore LBHBs; EI > 24.0 kcal/mol) show both ∇2ρBCP and HBCP < 0.