We have investigated geometries, ionization potentials (IPs), and vertical
transition energies (VTEs) of NH4(NH3)(n) (n = 0-4) cluster radicals by ab
initio MO method at the correlated level. The structures in which NH4 donat
es as many NH bonds as possible to the hydrogen bonding with surrounding NH
3 molecules are the most stable for each n. The calculated IPs agree well w
ith experiment. The spatial expansion of the unpaired electron occurs with
stepwise solvation. The growing one-center Rydberg-like nature of the clust
er radicals results in the successive decrease in the transition energies t
o the low-lying excited states, which is responsible for the red shifts of
the electronic absorption bands. (C) 2001 Elsevier Science B.V. All rights
reserved.