Spin Coupling Effect on Geometry-Dependent X-ray Absorption of Diradicals

TL;DR

This study theoretically explores how electron spin coupling in diradicals influences geometry-dependent X-ray absorption spectra during photochemical ring opening, revealing non-local probes of chemical dynamics linked to spin states.

Contribution

It introduces a theoretical analysis of spin coupling effects on X-ray spectra, highlighting geometry-dependent signals and a novel non-local X-ray probe based on spin-induced spectral features.

Findings

Spin coupling affects core-to-LUMO transition energies and intensities.

Geometry dependence observed in spectral features related to ring opening.

Identification of a non-local X-ray probe sensitive to electron spin states.

Abstract

We theoretically investigate the influence of diradical electron spin coupling on the time-resolved X-ray absorption spectra of the photochemical ring opening of furanone. We predict geometry dependent carbon K-edge signals involving transitions from core orbitals to both singly and unoccupied molecular orbitals. The most obvious features of the ring opening come from the carbon atom directly involved in the bond breaking, through its transition to both the newly formed SOMO and the available LUMO state. In addition to this primary feature, the singlet spin coupling of four unpaired electrons that arises in the core-to-LUMO states creates additional geometry dependence in some spectral features, with both oscillator strengths and relative excitation energies varying observably as a function of the ring opening. We attribute this behavior to a spin-occupancy-induced selection rule, which occurs when singlet spin coupling is enforced in the diradical state. Notably, one of these geometry-sensitive core-to-LUMO transitions excites core electrons from a backbone carbon not involved in the bond breaking, providing a novel non-local X-ray probe of chemical dynamics arising from electron spin coupling.