Infrared Multiple Photon Dissociation Spectroscopy of the H–H Stretching Mode and Low-Lying Electronic Transitions in Fe+(H2)1,2 and Fe+(D2)1,2

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_xz, d_yz atomic orbitals into the σ* orbital of the H₂ molecule, which weakens the H–H bond.