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“Vibrational bonding”: a new type of chemical bond is discovered

Posted on 12. March, 2015.

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Chemical bonding is the attractive force between atoms that causes them to form into aggregates such as molecules or solids. At its foundation, a chemical bond is always a result of the summed attractive and repulsive electrostatic interactions between a number of positively charged nuclei and a number of negatively charged electrons, but a hierarchy of different bond types of varying strength can be identified.

The strongest chemical bonds are those described as covalent, metallic, or ionic; in the first two of these, electrons are shared between multiple charged cores, while in the last, electron transfer between atoms leads to oppositely charged ions which attract one another directly. Weaker types of bonding occur when electrically neutral molecules interact with one another; such interactions, which include hydrogen bonds, dipole–dipole interactions, and the London dispersion force, are 10 – 100 times weaker than covalent bonding, but are responsible for the existence of the liquid and solid states in molecular substances and noble gases. The net effect of forming a chemical bond is to lower the total energy of the system from that of the initially separated combining species.

A long-sought but elusive new type of chemical bond, occurring on a minimum-free, purely repulsive potential energy surface, has recently been convincingly shown to be possible on the basis of high-level quantum-chemical calculations. This type of bond, termed a vibrational bond, forms because the total energy, including the dynamical energy of the nuclei, is lower than the total energy of the dissociated products, including their vibrational zero-point energy. For this to be the case, the ZPE of the product molecule must be very high, which is ensured by replacing a conventional hydrogen atom with its light isotope muonium (Mu, mass=1/9 u) in the system Br –H–Br, a natural transition state in the reaction between Br and HBr. A paramagnetic species observed in the reaction Mu+Br2 has been proposed as a first experimental sighting of this species, but definitive identification remains challenging.

Read the full article in  Science Progress, Volume 98, Number 1, March 2015, pp. 12-33.

Authors: Christopher J.Rhodes and Roderick M.Macrae

Keywords: vibrational bonding, muonium, quantum chemistry

DOI:10.3184/003685015X14232279560803

Image: Spin density distribution of the Br –H–Br complex in the symmetrical collinear geometry with rH–Br=1.64 Å. Blue and red correspond to positive and negative values.