Infrared spectra of Li(NH3)(n) clusters as a function of size are reported for the first time. Spectra have been recorded in the N-H stretching region for n=4-->7 using a mass-selective photodissociation technique. For the n=4 cluster, three distinct IR absorption bands are seen over a relatively narrow region, whereas the larger clusters yield additional features at higher frequencies. Ab initio calculations have been carried out in support of these experiments for the specific cases of n=4 and 5 for various isomers of these clusters. The bands observed in the spectrum for Li(NH3)(4) can all be attributed to N-H stretching vibrations from solvent molecules in the first solvation shell. The appearance of higher frequency N-H stretching bands for n > or =5 is assigned to the presence of ammonia molecules located in a second solvent shell. These data provide strong support for previous suggestions, based on gas phase photoionization measurements, that the first solvation shell for Li(NH3)(n) is complete at n=4. They are also consistent with neutron diffraction studies of concentrated lithium/liquid ammonia solutions, where Li(NH3)(4) is found to be the basic structural motif.
The first mass-selective vibrational spectra have been recorded for Na(NH3)n clusters. Infrared spectra have been obtained for n = 3-8 in the N-H stretching region. The spectroscopic work has been supported by ab initio calculations carried out at both the DFT(B3LYP) and MP2 levels, using a 6-311++G(d,p) basis set. The calculations reveal that the lowest energy isomer for n or= 7 is indicative of molecules entering a second solvation shell, i.e., the inner solvation shell around the sodium atom can accommodate a maximum of six NH3 molecules.
Photoionization threshold measurements have been carried out for small Li(NH3)n clusters (n = 1-5) and have been combined with ab initio calculations to determine structural information. The calculated adiabatic ionization energy for the lowest-energy isomer of each cluster is found to be in excellent agreement with the corresponding experimental photoionization threshold, providing evidence that the calculated structures are correct. The combination of the photoionization efficiency curve and the calculated adiabatic ionization energies also confirms the tentative assignment of the infrared spectrum of Li(NH3)4 reported by Salter and co-workers (J. Chem. Phys. 2006, 125, 34302); i.e., the 3 + 1 isomer does not contribute and the spectrum is due solely to the 4 + 0 isomer. The findings are consistent with an inner solvation shell that can hold a maximum of four ammonia molecules around the central lithium atom.
Spectra of clusters formed between lithium atoms and methylamine molecules are reported for the first time. Mass-selective infrared spectra of Li͑NH 2 CH 3 ͒ n have been recorded in both the N-H and C-H stretching fundamental regions. The infrared spectra are broadly in agreement with ab initio predictions, showing redshifted N-H stretching bands relative to free methylamine and a strong enhancement of the N-H stretching fundamentals relative to the C-H stretching fundamentals. The ab initio calculations suggest that, for n = 3, the methylamine molecules bunch together on one side of the lithium atom to minimize repulsive interactions with the unpaired electron density. The addition of a fourth methylamine molecule results in closure of the inner solvation shell and, thus, Li͑NH 2 CH 3 ͒ 5 is forced to adopt a two-shell coordination structure. This is consistent with neutron diffraction studies of concentrated lithium/methylamine solutions, which also suggest that the first solvation shell around the lithium atom can contain a maximum of four methylamine molecules.
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