Investigating weak dipole interactions of cyclic nitriles in the gas phase: Theoretical views on molecular stability and reactivity

We investigated hydrogen bonding interactions between cyclic nitriles (C${}_3$H${}_3$CN, C${}_4$H${}_3$CN, C${}_5$H${}_5$CN, C${}_6$H${}_5$CN, C₁₀H${}_7$CN) and Lewis acids/bases (H${}_2$O, HCl, HCN, NH${}_3$, and C${}_2$H${}_2$), identifying N$\cdots$H$-$O, N$\cdots$H$-$Cl, N$\cdots$H$-$C, and N$\cdots$H$-$N bonds, with nitriles acting as proton acceptors. Among the proton donors, HCl exhibited the strongest interactions due to its highly acidic proton. The bimolecular complexes formed with C${}_3$H${}_3$CN, C${}_4$H${}_3$CN, and C${}_5$H${}_5$CN showed enhanced stability, attributed to enhanced electronic effects. Notably, the nitrile molecules examined in this study also possess astrochemical significance, as they have recently been detected in the interstellar medium. A comprehensive analysis of geometrical parameters, interaction energies, vibrational frequency shifts, hyperconjugation, and electron density offered deeper insights into the nature of these interactions and the resulting structural changes. We performed Atoms in Molecules (AIM) analysis to determine electron densities [$\rho \left(r_c\right)$] and the Laplacian of electron density [${\nabla }^2\rho \left(r_c\right)$] at bond critical point. Natural Bond Orbital (NBO) analysis was utilized to investigate charge transfer mechanisms, while Energy Decomposition Analysis (EDA) analyzed the energetic contributions to complex stability and destabilization.