Spin-orbit coupling in molecular complexes beyond van der Waals regime: Key factors for further splitting of 2P3/2 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<middle dot>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 Delta E-XA is measured to be 0.10 eV for both, whereas the X and B gap Delta E-XB is 0.98 and 0.97 eV for formic and acetic acid, respectively. These new Delta E-XA values are compared with the previously reported values for the molecular complexes L<middle dot>I-<middle dot> 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 Delta E-XB remains similar, the Delta E-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 Delta E-XA with the intermolecular interaction strength or bond distance of the neutral complexes-the stronger interaction (shorter bond length), the greater splitting, is established.