Comparative analysis of red and blue-shifting hydrogen bonds in 1:1 haloform complexes

The aim of our study is to provide a possible insight into blue-shifting and red-shifting hydrogen bonding complexes. For that, we have taken CHX₃ (X $=$ F, Cl, Br) with hydrogen bond acceptors such as H₂O, HCl, HCN, C₂H₂, NH₃, H₂S, PH₃, CH₃OH, C₆H₆ (Y $=$ O, N, Cl, P, S, $\pi$). In this work, we have optimized and computed the vibrational frequency by performing quantum chemical calculations on the systems utilizing B3LYP and MP2 levels of theory with 6-311++G(d, p) and aug-cc-pVDZ basis sets. Further, we compared the various calculated findings, such as geometrical parameters, interaction energies ($\Delta$E), hyper-conjugative interactions, second-order perturbation energies (E₂), Laplacian electron densities (${\nabla }^2\rho$) at the intermolecular bond critical point (BCP), vibrational shift ($\Delta \nu$). The dependence of vibrational frequency on bond length and Mulliken charge was studied. Since NH₃ has a highly negative Mulliken charge, it attracts the hydrogen (H) of CH having a positive Mulliken charge. This effect weakens the CH bond, leading to a red shift in the stretching frequency. In these complexes, NBO analysis revealed a higher value of second-order perturbation as a result of hyper-conjugative interaction from Lewis base to ${\sigma }^{\ast }$(CH), thus a higher redshift is observed.