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چکیده
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In this study, the nature and strength of intermolecular interactions in chloro (methylidene)phosphane···HOSX complexes, where X = F, Cl, Br, and I, were investigated using quantum chemical methods. Particular attention was paid to the formation and cooperative behavior of noncovalent interactions, including hydrogen bonds (HB), pnicogen bonds (PnB), and chalcogen bonds (ChB). Geometry optimizations and energetic analyses were performed to determine the most stable structures and to evaluate the effect of the halogen substituent on the stability of the complexes. The molecular electrostatic potential (MEP) analysis revealed that CH₂PCl possesses several positive electrostatic regions, including the σ-hole on the phosphorus atom, the σ-hole on chlorine, and the acidic hydrogen atoms of the CH₂ group. These regions enable CH₂PCl to act as both an electron acceptor and, in some interaction patterns, an electron donor. In the HOSX monomers, the acidic proton and the sulfur atom were identified as the main electron-accepting sites, while the oxygen atom and halogen substituents can function as electron-donating centers. The calculated interaction energies showed that the stability of the complexes depends strongly on both the nature of the halogen atom and the orientation of the interacting monomers. In most interaction patterns, increasing the atomic radius of X from F to Br enhanced the stability of the complexes, whereas an opposite trend was observed for some hydrogen-bonded structures. Natural Bond Orbital (NBO) analysis confirmed the presence of important donor–acceptor orbital interactions such as lp(Cl)→σ*(S–O), lp(X)→σ*(P–C), lp(O)→σ*(C–H), and lp(P)→σ*(S–O). These interactions demonstrate the orbital origin of the observed HB, PnB, and ChB interactions. The results indicate that chalcogen bonding often contributes significantly to the stabilization of the complexes, while hydrogen bonding and pnicogen bonding can either reinforce or compete with it depending on the geometry of the complex. Atoms in Molecules (AIM) analysis provided further topological evidence for the formation of the investigated noncovalent interactions through the presence of bond critical points between interacting atoms. The positive values of the Laplacian of electron density and total energy density, together with the small electron density values at the bond critical points, indicate that these interactions are weak closed-shell interactions with partial covalent character. Overall, the results show that substituent effects, σ-hole interactions, and cooperative orbital interactions play key roles in determining the stability and nature of CH₂PCl···HOSX complexes.
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