Impact of Nitrogen/Carbon Replacement on the Electronic Structure, Intramolecular Hydrogen Bonding, and π-Electron Delocalization in Malonaldehyde and Thiomalonaldehyde: A Quantum Chemical Study

Abstract

This work examines the effects of substituting nitrogen for carbon (N/C) in Malonaldehyde/Thiomalonaldehyde (MA/TMA) using the theoretical methods M06-2X, B3LYP, MP2, and CCSD with the 6-311++G(d,p) basis set. Key factors analyzed include the position of the nitrogen atom, lone pair-lone pair (Lp-Lp) repulsion, and resonance-assisted hydrogen bonding. The outcomes suggest that nitrogen positioning notably affects electronic energy and molecular stability, Following the sequence: N2 < N1 < N3 in terms of electronic energy and Lp-Lp repulsion. Generally, N/C substitution enhances RAHB stabilization, except in the N1 case. IMHB energies and various geometrical, spectral, and topological descriptors align well with the electronic energy analysis. Overall, the heteroatom position has the greatest influence on electronic energies that followed by lone pair-lone pair (Lp-Lp) repulsion and the effects of resonance-assisted hydrogen bonding. Likewise, the aromaticity indicators (HOMA, NICS, PDI, ATI, and FLUπ) show a small decrease in π-electron delocalization, contrasting with RAHB theory predictions.