Theoretical insights into the stability and nature of nonconventional C–H⋯Y (YN, P, As, Sb) hydrogen bonds in haloform–pnictogen trihydride complexes

Abstract

In this study, we investigate the stability and intrinsic nature of nonconventional C–H⋯Y hydrogen bonds (YN, P, As, Sb), with special attention given to the largely unexplored C–H⋯P interaction and the first theoretical observation of C–H⋯As/Sb interactions in complexes formed between haloforms (CHX₃, X = F, Cl, Br) and pnictogen trihydrides (YH₃, YN, P, As, Sb). The hydrogen-bond strength in the considered complexes increases in the order C–H⋯Sb < C–H⋯As ≈ C–H⋯P < C–H⋯N, with C–H⋯N being about two to three times stronger than C–H⋯Sb. The C–H stretching frequency shifts in the formed hydrogen bonds are predominantly red-shifting, increasing in the order C–H⋯As ≈ C–H⋯P < C–H⋯Sb < C–H⋯N. The larger red shifts observed for the C–H⋯Sb hydrogen bonds relative to the corresponding C–H⋯P/As ones arise from the high polarizability of the proton acceptor and the associated increase in the σ*(C–H) antibonding orbital population, whereas in the C–H⋯N hydrogen bonds the enhancement of the C–H stretching frequency red shift from X = F to Cl to Br is primarily driven by stronger electrostatic interactions between the interacting atoms. Symmetry-Adapted Perturbation Theory (SAPT) analyses reveal that the C–H⋯N hydrogen bonds are predominantly electrostatic in nature, whereas the C–H⋯P/As/Sb interactions exhibit a more balanced interplay of attractive contributions, with dispersion becoming increasingly important for heavier halogens and pnictogens. By extending the analysis as a function of the C⋯Y distance, we find that the C–H⋯N hydrogen bonds remain red-shifting and dominated by electrostatic interactions over the entire distance range, while the C–H⋯P/As/Sb interactions exhibit induction-driven blue shifts at short separations and dispersion-dominated red shifts at larger distances.