Nonconventional hydrogen and halogen bonds in the binary complexes of haloform with hydrogen cyanide and methylidynephosphane derivatives.

This study presents a comprehensive analysis of the nature and stability of the nonconventional hydrogen bonds in the complexes formed between haloforms (CHX3, X = F, Cl, and Br) and hydrogen cyanide (HCN) or methylidynephosphane (HCP) derivatives, denoted as MCZ (M = H, F, Cl, Br, CH3, C2H, CN, CP, CAs, Li, and Na and Z = N and P), with particular emphasis on the rarely investigated C-H⋯P and C-H⋯C hydrogen bonds. The more stable MCN complexes are governed by C-H⋯N hydrogen bonds, while the MCP systems show C-H⋯C/P/Cl hydrogen bonds and C-X⋯X/C/P halogen bonds. Complex stability increases with C-H donor polarity (CHF3 < CHCl3 < CHBr3) and the electron-donating/metallic character of M, while the strength of the hydrogen bond follows the order of C-H⋯N ≫ C-H⋯C > C-H⋯Cl ≈ C-H⋯P. SAPT2+ analysis identifies electrostatics as the main stabilizing term in C-H⋯N complexes, but dispersion is dominant in most MCP complexes. It is found that the C-H stretching frequency shifts change from blue to red as X varies from F to Br and M changes from electron-withdrawing substituents to electron-donating groups and subsequently to alkali metals. The magnitude of the frequency shifts is significantly smaller for C-H⋯P, C-H⋯C, and C-H⋯Cl than for C-H⋯N, with the blue shifts of the former being approximately 2.0-3.5 times weaker. Remarkably, the observed red and blue shifts in the C-H stretching frequencies in the C-H⋯N hydrogen bond are suggested to originate from coulombic interactions between the H and N atoms.