Excess Electrons Bound to Small Ammonia Clusters

Dipole-bound anions of small water clusters (H2O)(N)(-)(N >= 2) are well-known from experiment and theory. In contrast, the smallest ammonia cluster anion detected so far is the 13-mer (NH3)(13)(-). Here dipole-bound states of small ammonia clusters (NH3)(N)(-)(N = 2, 3, 4) are investigated using coupled-cluster ab initio methods. The trimer is found to be the smallest ammonia cluster able to form a dipole bound state, and its vertical detachment energy is predicted to be 27 meV. somewhat smaller than that of the water dimer. For the ammonia tetramer dipole-bound states with triple-acceptor monmers are identified akin to the well-studied double-acceptor binding motif of water cluster anions. Moreover, a (NH3)(6)(-) hexamer that has been considered as a model for a cavity-bound state is examined. Ab initio results for this system challenge the notion that an electron localized in an ammonia cavity can be thought of as a delocalized radical anion.