Infrared spectroscopy and density functional theory of transition metal - nitrogen complexes
Pillai, Emmanuel Dinesh
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+Transition metal (TM) cation - nitrogen complexes of the form TM(N2)n are investigated via infrared photodissociation spectroscopy (IRPD) and density functional theory (DFT). The bonding characteristics of a single nitrogen molecule to first row transition metal cations from ++Sc to Zn are explored with DFT. The computed geometries and energetics indicate nitrogen prefers to bond in the end-on configuration to the cations. All the complexes are predominantly bound by charge-quadrupole forces with some exhibiting partial covalent character. The degree of covalent character is determined by the extent of 3d-4s orbital hybridization of the metals. The early transition metals cause a greater perturbation of the N-N bond than the late row metal ions. However, the dissociation energies of the late row metal cations are larger than the early metal ions. The calculated energetics are compared to experimental data to test the accuracy of DFT methodologies. ++Complexes of the form V(N2)n and Nb(N2)n are generated in a pulsed nozzle laser vaporization cluster source, size selected and studied by IRPD spectroscopy by probing the N-N stretch of nitrogen. In both systems, the IR forbidden N-N stretch of free nitrogen becomes active upon binding to the metal ions. Vibrational excitation of the complexes near 2200 - 2300 -1cm leads to photodissociation. The complexes fragment by the loss of intact N2 molecules and the IRPD spectra are obtained by monitoring the fragmentation yield as a function of the IR ++wavelength. The fragmentation mass spectra indicate that V and Nb have a coordination number of six. The IRPD spectra for both complexes have resonances red-shifted with respect to the N-N stretch in free nitrogen. The red-shifts are consistent with the Dewar-Chatt-Duncanson complexation model that is used to model such TM-ligand systems. DFT calculations are used +to predict structures, energetics and ground electronic states of the small TM(N2)n complexes. The IRPD spectra in conjunction with DFT calculations suggest that the metal ions undergo a spin-state change due to progressive ligand binding. The spectroscopy and theoretical predictions of these metal ion-ligand systems offers new insight into their chemistry.