Rydberg bonding in ammonium dimer ((NH4)2)

Abstract

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(fu11)/6-31++GS* + 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 + H2 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.

Theoretical Search for Anions Possessing Large Electron Binding Energies

“…We believe that our VDE values that exceed the 12 eV limit by 0.2Ϫ0.6 eV, found for the two negatively charged systems As 3 F 16 Ϫ and Ta 3 F 16 Ϫ may be approaching the largest possible VDE values because: (i) the largest known vertical electron detachment energies described in the literature thus far are those calculated by Boldyrev and Simons for TeF 7 Ϫ (11.9 eV), [14] and those obtained (also theoretically) by Schaefer's group for AsF 6 Ϫ (10.5 eV), [11] and (ii) the limit for the maximum possible excess electron binding energy is thought to be about 12 eV. [12,13] Although the extremely large VDE (12.63 eV) calculated by us for Ta 3 seems to be the largest excess electron binding energy reported in the literature thus far, we believe the maximum possible VDE value has not yet been achieved.…”

“…Bond lengths (R) in Å , valence angles (Є) in degrees, vertical electron detachment energies in eV has been found thus far is TeF 7 Ϫ , for which the VDE of 11.9 eV was estimated (at the KT level) by Boldyrev and Simons. [14] Since the VDE of the superhalogen anion increases with the number of halogen ligands involved, one could explore this route in order to search for systems that maximise the VDE. However, one should recall that the…”

“…In particular, the early calculations (using discrete-variational X a -method) performed on As 2 F 11 and P 2 F 11 resulted in an EA of 8.6 eV (for both cases). [6] To the best of our knowledge, the only known VDE that approaches the 12 eV limit is that estimated by Boldyrev and Simons [14] for TeF 7 Ϫ (11.9 eV); however, it corresponds to the result calculated at the Koopmans' Theorem (KT) [15] level, which is not very accurate. [16] Eur.…”

New anionic states of the lithium trimer

“…For the ground 2ju+ state, the vertical detachment energy (VDE) is 1.309±0.005 and 1.158±0.010 eV, for Li 3 and Na3, respectively [7,8]. Upon bending, the 21u+ state correlates with the lowest 2 B2 state (a local minimum on the pseudo-rotating ground state surface of the neutral).…”

“…An excellent review on the neutral, cationic, and anionic alkali metal clusters has recently been published by Bonacic-Koutecky et al [1]. Our interests in double-Rydberg anions [21 as well as in chemical bonds between Rydberg radicals [3] led us to this study on anionic states of the alkali metal trimers.…”