Contribution of lone-pair electrons to the electronic circular dichroism of chiral aliphatic amines

To clarify the role of nitrogen lone-pair electrons in the electronic circular dichroism (ECD) of chiral aliphatic amines, a series of cyclic amine models was investigated. Combined experimental far-UV ECD spectroscopy, quantum chemical calculations, and hole-electron analysis showed that the characteristic bands in the 180-220 nm region arise predominantly from nitrogen lone-pair excitations into diffuse acceptor orbitals with Rydberg-type character. The preferred orientation of the lone pair depends on molecular substitution and ring conformation, whereas the solvent environment significantly modulates the ECD signals. Protonation suppresses lone-pair activity and the associated ECD responses. Polyhydroxylated derivatives exhibit distinct low-energy excited-state transitions pathways because of altered orbital ordering and mixing. This work establishes a mechanistic foundation for lone-pair-driven chiroptical responses and provides a systematic framework for the configurational analysis of chiral aliphatic amines, with potential applications in synthetic chemistry and pharmaceutical analysis.