This study systematically investigates the molecular design, stability, and spectroscopic characteristics of complexes formed between Selenium Ylide (SY) and HSX (X = F, Cl, Br, and I) using high-level quantum chemical methods. The research focuses on identifying the nature of noncovalent interactions, specifically halogen bonds (XB), chalcogen bonds (ChB), and hydrogen bonds (HB), which stabilize these molecular assemblies. Molecular Electrostatic Potential (MEP) analysis revealed that the carbon atom in the SY molecule serves as the primary nucleophilic site, while the selenium atom provides an electrophilic region. In the HSX monomers, three distinct positive potential regions associated with the S–H bond, the sulfur atom, and the S–X bond were identified, facilitating the formation of HB, ChB, and XB, respectively. The strength of the σ\sigmaσ-hole on the halogen atom was found to increase significantly from fluorine to iodine. Geometry optimizations identified three distinct stable configurations, denoted as S-I, S-II, and S-III. Among these, the S-III complexes, stabilized primarily by halogen bonding, exhibited the highest stability and the shortest intermolecular distances, following the order S-III > S-I > S-II. Interaction energy evaluations showed that the SY–HSI–III complex is the most stable structure due to the high polarizability and strong σ\sigmaσ-hole of the iodine atom. Vibrational frequency analysis indicated characteristic red shifts in the S–H and Se=C stretching modes, which were rationalized by Natural Bond Orbital (NBO) analysis as resulting from significant charge transfer from lone pairs to antibonding orbitals, such as lp(Se)→σ∗(S–H)lp(Se) \to \sigma^*(S–H)lp(Se)→σ∗(S–H) and lp(C)→σ∗(X)lp(C) \to \sigma^*(X)lp(C)→σ∗(X). Topological analysis via the Quantum Theory of Atoms in Molecules (QTAIM) confirmed the presence of bond critical points for all interactions. While S-I and S-II complexes were characterized as weak closed-shell interactions, the S-III complexes showed evidence of partial covalent character in the halogen bonds. Finally, Energy Decomposition Analysis (EDA) revealed that while S-I complexes are dominated by orbital interactions, the S-II and S-III structures are primarily stabilized by electrostatic contributions. These findings provide comprehensive insights into the role of σ\sigmaσ-hole interactions and substituent effects in tailoring the properties of selenium-containing ylide complexes.
