"Vibrational Bonding": A New Type of Chemical Bond Is Discovered

ABSTRACT 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 + Br} has been proposed as a first experimental sighting of this species, but definitive identification remains challenging. Keywords: vibrational bonding, muonium, quantum chemistry 1. The nature of a chemical bond 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. The formation of a typical covalent bond can be visualised as follows. If two atoms approach one another sufficiently closely for a bond to form, their outermost--most weakly bound--electrons are attracted towards both nuclei with comparable force. (These electrons are known as the valence electrons--the ones responsible for bonding. The innermost electrons--the core electrons--remain overwhelmingly associated with their own nucleus and are not changed much during molecule formation.) Correspondingly, both nuclei are attracted to the electron, and this effect serves to draw the nuclei closer together. At some characteristic distance, this force of attraction is balanced by the repulsive forces between the nuclei, and the equilibrium between these forces holds the nuclei in a relatively fixed configuration, about which they undergo vibration. As energy is now required to separate the atoms from one another, we can say that they are held together by a chemical bond. Electrons that are confined within the electrostatic potential of atomic nuclei behave as standing waves, and occupy a far greater volume than do the nuclei; in the context of a covalent bond, the valence electrons extend through a region of space encompassing both nuclei. The lengths of chemical bonds are set by the scale of spatial delocalisation of valence electrons, and are of the order of hundreds of picometres. 1.1 van der Waals Forces. The van der Waals force (or van der Waals interaction) is named after Johannes Diderik van der Waals, and is a generic term to describe the weaker forces between molecules (or between parts of the same molecule), excluding those from covalent bonds, from electrostatic attractions between pairs of ions, and from attractions between ions and neutral atomic or molecular species. …