Infrared spectroscopy of Li(NH[subscript 3])[subscript n] clusters for n=4-7

Infrared spectra of Li(NH[subscript 3])[subscript 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(NH[subscript 3][subscript 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(NH[subscript 3])[subscript n] is complete at n=4. They are also consistent with neutron diffraction studies of concentrated lithium/liquid ammonia solutions, where Li(NH[subscript 3][subscript 4] is found to be the basic structural motif.