Stability and Electronic Features of Calcium Hexa-, Hepta-, and Octa-Coordinated Ammonia Complexes: A First-Principles Study.

Neutral and positively charged calcium ammonia complexes are investigated by means of high-level quantum chemical calculations. We report optimal structures, binding energies, and vibrational spectra for Ca(NH3)1-80,+. The bigger Ca(NH3)6-80,+ complexes can be classified as solvated electron precursors (SEPs) and are best described as a Ca(NH3)6-82+ core with two or one peripheral electrons. In their ground state, only ∼10% of the outer electron density is estimated to be within the calcium van der Waals radius. For these systems, we calculated several low-lying electronic states, where electrons populate diffuse outer orbitals. The Aufbau principle for the outer electrons is found to be identical to previously studied SEPs: 1s, 1p, 1d, 1f, 2s, and 2p. We show that going from Ca(NH3)5, which has an incomplete first coordination shell and the two valence electrons that are mainly in the valence sphere of calcium, to Ca(NH3)6, both the vibrational and electronic features change abruptly. Infrared, visible, and ultraviolet spectroscopy can be used to identify and characterize calcium SEPs.