Infrared spectroscopy of alkali metal-solvent clusters

Infrared (IR) photodepletion spectroscopy coupled with mass spectrometry has been applied in the investigation of size-specific alkali metal-solute complexes. IR spectra have been recorded in the N-H stretching region for Li(NH3)n (4 = n = 7) and Na(NH3)n (3 = n = 8) and in the N-H and C-H stretching regions for Li(NH2CH 3)n (3 = n = 5), with supporting ab initio calculations. All clusters display a red-shift of the N-H stretching modes, consistent with partial electron transfer from the nitrogen to the alkali metal atom.;For Li(NH3)n, the IR spectra indicate that the first salvation shell is found to be completed with four ammonia molecules, which is in agreement with conclusions drawn from previous photoionisation studies. This finding is given credence from DFT and MP2 ab initio calculations carried out in the present work, where the lowest energy isomer for n = 4 is adopts a tetrahedral structure. The IR spectra for Na(NH3)n clusters are less definitive, but indicate a completed inner salvation shell with six ammonia molecules, a conclusion in disagreement with some previous experimental and theoretical investigations, but which is consistent with high-level ab initio calculations carried out in the present study.;Ab initio investigations into the localisation of the alkali metal valence electron in the three systems determined that not only are a critical number of solvent molecule required to permit formation of a solvated electron, but also a specific geometrical configuration is required. For lithium-ammonia and sodium-ammonia clusters, formation of the solvated electron was found to coincide with an ammonia molecule entering the second salvation shell, whereas for lithium-methylamine, electron salvation was not observed for the largest cluster studied, Li(NHCH3)4.