Spin-orbit coupling in molecular complexes beyond van der Waals regime: Key factors for further splitting of ²P_3/2 ground state^†

We report a joint spectroscopic and theoretical study probing spin-orbit coupling (SOC) in a variety of molecular complexes between an iodine atom and a ligand (L) with L ranging from Ar, HF to formic/acetic acids, and glycine/N-methylated glycine derivatives. Cryogenic photoelectron spectroscopy of L·I^- (L=HCOOH, CH₃COOH) reveals three distinct peaks, identified as three SOC states, denoted as X(1/2), A(3/2), and B(l/2) for the corresponding neutrals. The X and A separation ΔE_XA is measured to be 0.10 eV for both, whereas the X and B gap ΔE_XB is 0.98 and 0.97 eV for formic and acetic acid, respectively. These new ΔE_XA values are compared with the previously reported values for the molecular complexes L·I^· with L=Ar, HF, glycine, and A-methylated glycines. All together these complexes encompass a diversity of intermolecular interactions, from van der Waals to weak and strong hydrogen bonding. While the ΔE_XB remains similar, the ΔE_XA is shown to be extremely sensitive to the type of ligands and interactions, spanning from 5 meV to 150 meV. High-level relativistic quantum calculations including explicit SOC formulism nicely reproduce all experimental SOC splitting. A direct correlation between the magnitude of ΔE_XA with the intermolecular interaction strength or bond distance of the neutral complexes—the stronger interaction (shorter bond length), the greater splitting, is established.