Stability of the Rydberg Dimer (NH₄)₂

Abstract

In this paper we reexamine the stability of the dimer of the ammonium radical NH 4 reported recently by Boldyrev and Simons (J. Phys. Chem. 1992, 96, 8840). More flexible diffuse basis sets are introduced designed to give a better representation of the important 3s orbital on N, overlap from which leads to the weak Rydberg bond. At the MP2(full) level of calculation the energetic results, molecular geometry, and vibrational frequencies are generally in good agreement with Boldyrev and Simons, except for the very low-frequency modes. However, the essentially free rotation of the NH 4 monomer within the weak Rydberg two-electron bond requires further analysis of the dimer stability. Following a reaction coordinate toward the (strongly exothermic) ground state products 2NH 3 + H 2 shows that there is a very low barrier of about 2.0 kcal/mol, which disappears when corrected for zero-point energy differences. This will destabilize the dimer and make its observation improbable.

On the Possibility of Mixed Rydberg-Valence Bonds

“…We repeated the study of Wright and McKay at the MP2(full)/6-31++G** level (which was used in our previous calculations 1 on (NH 4 ) 2 ) and found a barrier to dissociation of 3.43 kcal/mol. After considering ZPE corrections, we found (NH 4 ) 2 to lie 3.2 kcal/mol above this barrier, similar to what was found in ref . Even when H is replaced by D or T to form (ND 4 ) 2 or (NT 4 ) 2 , the barrier to dissociation is not large enough to cause the neutral dimer, with its ZPE, to lie below the barrier.…”

“…Although the (NH 4 ) 2 dimer is stable with respect to dissociation into the two Rydberg radicals, Wright and McKay observed that it is highly thermodynamically unstable with respect to dissociation into 2 NH 3 + H 2 and the barrier connecting the locally stable (NH 4 ) 2 to these products is too small to render the Rydberg-bound dimer long-lived. That is, as shown in ref , the geometrically and electronically stable (NH 4 ) 2 local minimum is separated from the 2NH 3 + H 2 products by a very small barrier (1.8−2.4 kcal/mol), and, when zero-point energy corrections (ZPE) are taken into account, (NH 4 ) 2 lies above (by about 5 kcal/mol) this barrier. As a result, Wright and McKay 12 concluded that the lifetime of the (NH 4 ) 2 Rydberg dimer would be of the order of 1 ns as a result of which its experimental observation would be very difficult.…”

Is Electronegativity a Useful Descriptor for the Pseudo‐Alkali Metal NH4?

“…For example, it was predicted by Boldyrev and Simons [14] that the ammonium radical would form a dimer similar in properties to the alkali metal dimers. Wright and McKay indicated that spontaneous decomposition to ammonia and molecular hydrogen would occur when the ammonium molecules rotated in such a way that two hydrogen atoms were in proximity, [15] but a later study found that mixed alkali metal-ammonium dimers would be stable. [16] The ammonium radical can bind a second electron in its fully-symmetric orbital to form an ammonium anion [17] dubbed the "double-Rydberg anion" NH 4 À .…”

Long-Range N–N Bonding by Rydberg Electrons

“…36 The Rydberg dimer, (NH 4 ) 2 , is bound by about 8 kcal/mol with respect to the two NH 4 moieties, but it is metastable with respect to exothermic dissociation into ammonia and molecular hydrogen. The estimated 37 barrier height is less than 2 kcal/mol, which makes its experimental isolation challenging. In contrast, the corresponding radical cation, (NH 4 ) 2 + , in which the unpaired electron occupies a bonding Rydberg orbital, was predicted to be more stable.…”

“…We also estimated the barrier heights for H 2 elimination in NH 2 (CH 2 CH 2 ) 2 NH 2 using transitionstate calculations. Considering that the simplest Rydberg diradical, (NH 4 ) 2 , was deemed to be kinetically unstable because of a low (less than 2 kcal/mol ≈ 1000 K) barrier for the dissociation into ammonia and molecular hydrogen, 37 we were particularly interested to see whether NH 3 CH 2 NH 3 can survive at excess energies above this value. The AIMD trajectories executed with excess energies of 350−5000 K showed that NH 3 CH 2 NH 3 remained stable in the course of dynamics with excess energy up to 500 K (∼1 kcal/mol), but fell apart when the energy was higher (see Table S7 for details of the dissociation channel).…”