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···HCX (M = Cu, Ag, Au; Y = HO, HCN, NH; X = F, Cl). When MCN formed a hydrogen bonding dimer with CHF or CHCl, the blue shift of C-H vibration frequency (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 (C-H). In CHF complexes, (C-H) undergoes a decrease in the blue shift, whereas that in CHCl exhibits an increase in the red shift. Especially, a transition from blue to red shift is observed within the AuCN···HCCl 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 NH > HCN > HO. 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 Δ(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 (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.