Vibrational Couplings in the NO Stretch Region in Diiron Trinitrosyl Complexes

Two-dimensional infrared spectroscopy offers unique capabilities for probing vibrational coupling in complex metal--ligand systems. In this paper, we combine two-dimensional infrared spectroscopy with vibrational perturbation theory to investigate vibrational coupling in a diiron trinitrosyl complex across three stable redox states. Although these systems are challenging for electronic structure methods, we demonstrate that key features of experimental 2D IR spectra can be accurately reproduced using reduced-dimensional anharmonic calculations with physically motivated parameter adjustments, including harmonic frequency scaling and dephasing time optimization. Analysis reveals that N--O stretching modes maintain high locality across all redox states, with coupling patterns that directly reflect variations in Fe--N bond strength. Using curvilinear coordinate analysis, we demonstrate these differences result from systematic changes in cubic anharmonic force constants rather than mode delocalization. Our results establish N--O stretches as sensitive probes of metal--ligand bonding strength, expanding the toolkit for studying biologically relevant nitrosyl complexes.