Theoretical Insights into Regulation of Red/Blue-Shifting Hydrogen Bonds Through Cooperativity with Regium Bonds

To deeply understand the characteristics and regulation of red/blue-shifting hydrogen bonds (HBs), a theoretical investigation was conducted to explore the cooperativity between regium bonds and HBs in the complexes of Y···MCN···HCX₃ (M = Cu, Ag, Au; Y = H₂O, HCN, NH₃; X = F, Cl). When MCN formed a hydrogen bonding dimer with CHF₃ or CHCl₃, the blue shift of C-H vibration frequency v(C-H) decreases as the following sequence Au > Cu > Ag, and the red shift decreases following the order Ag > Cu > Au. Upon the formation of ternary complexes, the presence of regium bonding interactions exerts a positive synergistic effect, resulting in the strengthening of the HBs. This, in turn, leads to noticeable changes in the red and blue shifts of v(C-H). In CHF₃ complexes, v(C-H) undergoes a decrease in the blue shift, whereas that in CHCl₃ exhibits an increase in the red shift. Especially, a transition from blue to red shift is observed within the AuCN···HCCl₃ complex. As the strength of the regium bond increases, the trend of shifting from blue to red becomes more pronounced. For a given MCN, the changes occur in the order of NH₃ > HCN > H₂O. The interplay between two interactions was revealed by the molecular electrostatic potentials (MEP), the atoms in the molecule (AIM), and natural bond orbitals (NBO) analysis. It is revealed that Δv(C-H) is linearly correlated with a series of configuration and energy parameters. We explain the red- and blue-shifting HBs and their changes from the perspective of hyperconjugation and rehybridization. The presence of the positive synergistic effect enhances the hyperconjugation effect, thereby leading to a reduction in the blue shift and an increase in the red shift of v(C-H) within the complexes. This study enriches previous mechanisms regarding red- and blue-shifting HBs and introduces a novel idea to manipulate the characteristics of HBs, with the potential to impact the functioning of intricate systems.