Infrared Multiple Photon Dissociation Spectroscopy of the H-H Stretching Mode and Low-Lying Electronic Transitions in...

Although iron is the most abundant transition metal in the interstellar medium, its interaction with hydrogen─by far the most abundant element─in small gas-phase molecules or complexes is poorly understood. Herein, we study the infrared spectroscopy of cationic iron complexes with one and two dihydrogen ligands, Fe⁺(H₂)_1,2, as well as their deuterated counterparts, Fe⁺(D₂)_1,2, using infrared multiple photon dissociation (IRMPD) spectroscopy. Quantum chemical calculations, including multireference configuration interaction (MRCI) with spin-orbit coupling, are used to simulate the electronic and vibrational contributions to the spectra. Broad electronic transitions are observed in the studied energy range of 2230-4000 cm^-1, which arise from d-d transitions at the metal center between states of quartet spin multiplicity. In the complex, the H-H stretching mode of the H₂ ligand becomes infrared active, and features arising from this mode are assigned with the help of quantum chemical calculations in the spectra of Fe⁺(H₂) and Fe⁺(D₂)₂. In Fe⁺(H₂), we assign a band with local maxima centered at ∼3138 cm^-1 and ∼3219 cm^-1 to the P and R branches of the H-H stretching mode, while the D-D stretch of Fe⁺(D₂)₂ has a band centered at 2448 cm^-1, with P and R branches not resolved. With a D/H wavenumber ratio of 0.726, the D-D stretch of Fe⁺(D₂) and the H-H stretch of Fe⁺(H₂)₂ are expected at 2309 cm^-1 and 3372 cm^-1, respectively. The rovibrational bands in Fe⁺(H₂) and Fe⁺(D₂)₂ exhibit pronounced broadening that cannot be explained by temperature. We assign the broadening to the strong dependence of the H-H and D-D stretching frequencies on the torsional motion of the complex, as shown by the calculations. The extreme redshift of the H-H and D-D stretching frequencies is caused by back-donation from iron d, d atomic orbitals into the σ* orbital of the H₂ molecule, which weakens the H-H bond.