Rydberg Bond. Rydberg-Covalent Bond. In 1992 Jack Simons proposed the concept of bonding between two NH4 Rydberg radicals . From following ab initio calculations we found that indeed two NH4 radicals can form a weak bond in (NH4)2 which was called a Rydberg bond. The bond strength was found to be about 10 kcal/mol. However, the neutral dimer is highly unstable towards dissociation into 2NH3 + H2. Moreover, the dissociation barrier was found to be about 2 kcal/mol and after inclusion of zero-point vibrational energy, this barrier disappears . Therefore, (NH4)2 could be only a short living species. The daughter cation (NH4)2+ with the single electron Rydberg bond  is however stable and its existence was proved by Garvey and Bernstein . The Na(NH4) and K(NH4) species with Rydberg-covalent bond have somewhat higher dissociation barriers but they are still not thermodynamically stable. We plan to design thermodynamically stable species with the Rydberg-covalent bond and explore a possibility of existence of other types of new chemical bond.
Complete list of our publications on Rydberg bond and Rydberg-covalent bond:
3. On the Possibility of Mixed Rydberg-Covalent Bonds. A. I. Boldyrev and J. Simons. J. Phys. Chem., 103, 3575 (1999).
2. Theoretical Search of the Large Rydberg Molecules: NH3CH3, NH2(CH3)2, NH(CH3)3 and N(CH3)4. A. I. Boldyrev and J. Simons. J. Chem. Phys. , 97, 6621 (1992).
1. Rydberg Bonding in (NH4)2. A. I. Boldyrev and J. Simons. J. Phys. Chem., 96, 8840 (1992). Additions and Corrections. Rydberg Bonding in (NH4)2. A. I. Boldyrev and J. Simons. J. Phys. Chem. 97, 1470 (1993).