The positive electrostatic potentials (σ-hole) have been found in ylides CH2XH3 (X = P, As, Sb) and CH2YH2 (Y = S, Se, Te), on the outer surfaces of group VA and VIA atoms, approximately along the extensions of the C-X and C-Y bonds, respectively. These electrostatic potentials suggest that the above ylides can interact with nucleophiles to form weak, directional noncovalent interactions similar to halogen bonding interactions. MP2 calculations have confirmed the formation of CH2XH3···HM complexes (X = P, As, Sb; M = BeH, ZnH, MgH, Li, Na). The interaction energies, interaction distances, topological properties (electron density and its Laplacian), and energy properties (kinetic electron energy density and potential electron energy density) at the X(1)···H(10) bond critical points are all correlated with the most negative electrostatic potential value of HM, indicating that electrostatic interactions play an important role in these weak X···H interactions. Similar to the halogen bonding interactions, weak interactions involving ylides may be significant in several areas such as organic synthesis, crystal engineering, and design of new materials.
Cooperativity between the H/Cl···C bonds of XH/XCl···CH2PH3 and the P···N interaction of CH2PH3···NH3 in XH/XCl···CH2PH3···NH3 (X = F, N3, CN, CCCN, CCF) was investigated by performing second-order Møller-Plesset perturbation theory (MP2) calculations and quantum theory of atoms in molecules (QTAIM) studies. The formation of a hydrogen/halogen bond greatly extends the scope and increases the most positive electrostatic potential of the σ-hole on the outer surface of the phosphorus atom. This increases the P···N interaction energy, the electron density at the P···N bond critical point, the electrostatic character of the P···N interaction, and it decreases the P···N interaction distance. The net result is that the formation of a hydrogen/halogen bond enhances the P···N interaction, and vice versa. However, the P···N interaction is enhanced by the presence of the hydrogen or halogen bond to a much greater degree than the hydrogen or halogen bond is enhanced by the P···N interaction.
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