RYDBERG BONDING IN (NH4)2

Chemical binding of two monovalent Rydberg species to form a singlet-state Rydberg dimer molecule is predicted to be possible. Ab initio electronic structure methods that include electron correlation (at levels up through QCISD(T)/6-31++G**// MP2(full)/6-31++G** + ZPE) are shown to be essential to achieving a proper description of such bonding. The (NH4)2 molecule, selected as the prototype for this study, is shown to be bound with respect to its Rydberg-species fragments, 2NH4, by 7.5-9.7 kcal/mol, depending on the level of treatment of electron correlation, and to be electronically stable (by ca. 4.3 eV) with respect to (NH4)2+ at the neutral's equilibrium geometry. The (NH4)2 Rydberg dimer is thermodynamically unstable with respect to 2NH3 + H-2 by 86-89 kcal/mol mol yet possesses all real vibrational frequencies; it is thus a metastable molecule held together by a weak Rydberg bond. The dissociation energy of the (NH4)2+ cation to form NH4+ + NH4 is found to be larger than that of the neutral (NH4)2.