Solvent effects on the electronic structure and spectroscopic properties of a binuclear rhodium carbonyl complex: a DFT and TDDFT study.

CONTEXT: The binuclear rhodium(I) complex H2Rh2(CO)6NMP, identified as a key species in previous cluster studies, was investigated to determine its full spectroscopic properties and stability across different solvent environments. Electronic absorption spectroscopy reveals solvent-dependent metal-to-ligand charge transfer (MLCT) transitions at 4.230 eV (ethanol), 2.372 eV (toluene), and 2.660 eV (chloroform), with complementary fluorescence emission behavior. Magnetic circular dichroism (MCD) and resonance Raman (RR) spectroscopy exhibit distinctive solvent-specific signatures and mode-selective enhancement patterns. Density of states (DOS) and photoelectron spectroscopy (PES) analyses establish a HOMO-LUMO gap of 0.86 eV with predominantly ligand character at the frontier orbitals. Natural bond orbital (NBO) analysis quantifies asymmetric charge distribution between the two rhodium centers, with π-backdonation occupancies of 0.53-0.91 electrons. Noncovalent interaction (NCI) and reduced density gradient (RDG) analyses visualize hydrogen bonding networks, van der Waals interactions, and steric repulsion that modulate molecular stability. METHODS: All calculations were performed with ORCA 6.1.1. Ground state geometry optimization used the r2SCAN-3c composite functional with a modified D4 dispersion correction and a small basis set correction term. Relativistic effects were treated via the zeroth-order regular approximation with DEF2-TZVPP basis sets; rhodium atoms employed the all-electron relativistic X2C-TZVPPall basis set. Exchange-correlation contributions used the chain-of-spheres resolution-of-identity (RIJCOSX) approximation. Solvent effects were incorporated using the SMD solvation model for ethanol, toluene, and chloroform. Excited state properties were computed using time-dependent DFT with the same functional/basis set combination. Excited state absorption, MCD (at 3000 G), fluorescence, and resonance Raman spectra were calculated using the ESD module. NBO analysis was performed with NBO6.0 following a single-point calculation (wb97xd/lanl2dz) on the optimized geometry. DOS and PDOS analyses were carried out; NCI and RDG analyses were performed with Multiwfn 3.8 and visualized with VMD 2.0.0a8.