Electronic Spin-Dependent Dynamics through the Lens of Angular Momentum Exchange. I. A Born–Huang Perspective

Ongoing research involving electronic spin-dependent dynamics, such as chiral- induced spin selectivity, is providing impetus to revise our understanding of nuclear en- tanglement with electronic spatial and spin degrees of freedom. In a spin-independent setting, non-adiabatic couplings are well-known mediators of electron–nuclear coupling within the adiabatic Born–Huang framework. These couplings become singular, and thus dominant, at conical intersections, and manifest in topological characteristics of the electronic states. However, their role extends further: the non-adiabatic couplings, appearing as pseudo-magnetic forces in a semi-classical setting, provide a mechanism that enables angular momentum exchange between electrons and nuclei in the adia- batic electronic basis that otherwise assumes infinite nuclear masses. In this two-part tutorial review, we explore the additional complexity that emerges by incorporating electronic spin and spin–orbit coupling into this framework. Given their role, it is perhaps not surprising that the non-adiabatic couplings become even more significant when also considering the intrinsic angular momentum of the electrons. Our aim is to offer a clear and structured introduction to the advanced topic of electronic spin- dependent dynamics, bridging foundational theory in Part I with detailed analysis of a model system in Part II.