Divalent carbon atom as the proton acceptor in hydrogen bonding

Area of Science:

• * Quantum Chemistry
• * Supramolecular Chemistry
• * Organic Chemistry

Background:

• * Carbodiphosphoranes and carbenes feature a divalent carbon atom (C L2, where L is a sigma electron donor).
• * This divalent carbon can possess lone electron pairs, enabling it to act as a proton acceptor.
• * Understanding the proton-accepting capabilities of these carbon centers is crucial for predicting their role in hydrogen bonding interactions.

Purpose of the Study:

• * To investigate the proton-accepting properties of divalent carbon atoms in carbodiphosphoranes and carbenes.
• * To analyze the formation and strength of hydrogen bonds involving these carbon centers.
• * To compare the proton-accepting abilities of these carbon atoms with traditional proton acceptors like oxygen and nitrogen.

Main Methods:

• * High-level quantum chemical calculations, including Density Functional Theory (DFT) and second-order Møller-Plesset perturbation theory (MP2).
• * Utilized the aug-cc-pVTZ basis set for accurate electronic structure calculations.
• * Analyzed electron density distribution using the atoms in molecules (AIM) procedure.

Main Results:

• * Divalent carbon atoms in carbodiphosphoranes and carbenes function as effective proton acceptors, forming hydrogen bonds of the type D-H...CL2.
• * These hydrogen bonds were found to be relatively strong, comparable to those involving more electronegative atoms.
• * The highest proton-accepting ability was observed in the C(NH3)2 carbodiphosphorane derivative, explained by the Leffler-Hammond postulate. Imidazol-2-ylidene showed the strongest hydrogen bonds among carbenes due to its aromaticity.

Conclusions:

• * Divalent carbon atoms in the studied compounds are confirmed as normal proton acceptors.
• * The strength of hydrogen bonds formed by these systems is significant.
• * The proton-accepting ability is influenced by the electronic environment, with aromaticity enhancing it in carbenes.