Proton Transfer and Chirality of Ammonia-HX (X = F, Cl, Br) Complexes Confined in Carbon Nanotubes

Abstract

Proton transfer is of fundamental importance in chemistry and biology, yet achieving precise control in confined environments remains challenging. In this work, we systematically investigate the encapsulation of NH3·HX complexes (X = F, Cl, Br) into chiral carbon nanotubes (CNT(6, m), m = 2–5) to elucidate how the nanoconfinement influence proton transfer behavior. The results indicate that CNT(6, 3) provides a suitable confinement size for NH3·HX, and that the encapsulation effect strongly depends on the halide species. In which, NH3·HCl undergoes a complete proton transfer to form NH4+Cl– inside the CNT(6, 3), as evidenced by significant elongation of the Ha–Cl bond (from 1.38 to 1.86 Å) and shortening of the N–Ha bond (from 1.60 to 1.10 Å). Energy decomposition analysis reveals that orbital interactions and dispersion are the dominant driving forces for the host–guest interactions, with NH4+Cl–@CNT(6, 3) exhibiting the strongest binding. Further chiral spectroscopic analyses (ECD and VCD) reveal that the composite systems exhibit pronounced mirror-symmetric optical responses. Notably, the absorption asymmetry factor |gabs| is significantly enhanced upon encapsulation (from 0.088 to 0.856) of NH3·HCl. The independent gradient model based on Hirshfeld partition (IGMH) analysis indicates that the three hydrogen atoms of NH3·HCl engage in N–H···π interactions with the inner wall of the nanotube, adopting a C3-symmetric arrangement─a structural motif that proves crucial for the amplification of chiral signals. Our findings demonstrate that CNTs can act as efficient nanoreactors to modulate proton transfer through confinement and host–guest interactions, and provide a theoretical foundation for the design of chiral nanomaterials for applications such as asymmetric catalysis.