Comparative Analysis of Hydrogen–Tetrel and Halogen–Halogen Interactions in Halogen-Substituted Phosphinothioformamide–COX 2 Complexes: A Multilevel Theoretical Study

In this study, the nature and role of noncovalent interactions in complexes formed between phosphinothioformamide (PTF) and carbonyl dihalides (COX2; X = F, Cl, Br, or I) were systematically investigated using ab initio calculations at the MP2/aug-cc-pVDZ level of theory. The computational results indicate that in the gas phase, these systems give rise to two stable cyclic conformations (I1 and I2), each characterized by distinct bonding patterns and stabilization mechanisms. Conformation I1 is stabilized through the synergistic interplay of O···H (hydrogen bonding) and C···S (tetrel bonding), whereas in conformation I2, X···H (hydrogen bonds) and S···X (halogen bonds) play the dominant stabilizing roles. Binding energy analyses reveal that conformation I2 is significantly more stable than I1, a stability primarily attributed to the pronounced σ-hole strength, particularly in heavier halogens, which facilitates efficient electron density acceptance from nucleophilic sites of the PTF molecule. Energy decomposition analysis shows that electrostatic interactions constitute the principal stabilizing component in most complexes, while the contribution of dispersion interactions becomes increasingly significant in systems containing heavier halogens. Furthermore, natural bond orbital analysis demonstrates that the transition from conformation I1 to I2 is accompanied by a shift in the dominant orbital interaction character from hydrogen bonding to halogen bonding. Overall, the results of this study highlight halogen substitution as a key strategy for selectively tuning stability and binding motifs in molecular systems governed by noncovalent interactions.