Structure and solvation in nickel cation complexes determined by infrared photodissociation spectroscopy
Walters, Richard Sheridan
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++Nickel cation complexes, Ni(H2O)n and Ni(C2H2)n, are produced in a laser vaporization pulsed nozzle source, size-selected, and studied by infrared photodissociation spectroscopy. Beginning with n = 3, the ion-molecule complexes fragment by ligand elimination in the region of the OH and CH stretching fundamentals, respectively. Rare gas atoms are attached to the small complexes (n = 1-3) to enhance photodissociation, and these mixed clusters fragment by losing the rare gas atoms. Dissociation is more efficient on resonance, thus monitoring the fragment ion yield as a function of laser wavelength produces the infrared photodissociation (IRPD) spectrum of the complex that has been size-selected. Vibrational bands shifted away from the free molecule fundamentals in both systems are attributed to the nickel cation-ligand interactions. The structures of the small complexes (n = 1-4) are determined by comparing the experimental vibrational spectra to the predictions of Density Functional Theory (DFT). New red-shifted bands appearing at specific cluster sizes are attributed the onset of the second solvation sphere. Therefore, coordination numbers are determined for these gas phase metal ion systems. For the hydrated nickel ions, the free OH stretches are no longer observed by n = 7, indicating complex hydrogen-bonded networks have +formed. In the IRPD spectra of the larger Ni(H2O)n complexes, the separation between dangling OH vibrations suggest that inductive forces from the nickel cation continue to perturb the water monomers, even at the largest cluster size studied (n = 30). Comparison of IRPD spectra to DFT results confirms that À-complexes are +formed for the Ni(C2H2)n complexes. Jahn-Teller effects are observed for the n > 2 cluster ions. Condensation reaction products are investigated with theory for the n = 4 and 5 complexes and found to be more stable than the unreacted species. The +vibrational spectrum of the Ni(C2H2)4 complex is consistent with an unreacted structure where all four acetylenes are intact and À-bonded to the nickel cation. The spectroscopy of the n = 5, 6 complexes seems to be consistent with the simple solvation of the n = 4 core. However, absolute structures cannot be assigned due to congestion in the CH region.