New type of chemical bonding discovered

On December 8, 2014 Angewandte Chemie published a paper “Fundamental Change in the Nature of Chemical Bonding by Isotopic Substitution” with confirmation of the existence of completely new type of attractive force that binds atoms together. We have asked one of the authors of this research, Prof. Jörn Manz from Freie Universität Berlin, to comment on this work.

The Study

Isotope effects are an important topic in chemistry, physics, biology, medicine, geology, cosmology, etc. Normally, isotopes are thought of as having the same nuclear charge but different masses and possibly with different nuclear spins. Usually different masses are due to different numbers of neutrons in the nucleus. But there are also other ways of forming nuclei with the same charge but different masses, e.g. nuclei which contain elementary particles different from protons and neutrons, for example muons. The most prominent example is hydrogen which has 5 isotopes with applications in chemistry: The lightest one is Muonium where the nucleus consists of a positive muon. The next heavier ones are the well known isotopes hydrogen H, deuterium D and tritium T. Recently, the muonic helium atom has been incorporated in this list – it may be denoted ⁴H. The mass ratios of these isotopes are essentially 1/9:1:2:3:4 for Mu:H:D:T:⁴H. Here, H and D are stable, whereas the others have finite lifetimes. For example, the lifetime of the muon (μ⁺ in Mu and μ^– in ⁴H) is only ca. 2 microseconds. At first glance, this appears to be rather short, but then this is about 10⁷ times longer than typical periods of molecular vibrations, much, much more than enough in order to employ Mu as chemical reactant and to observe it in products. In fact, there is a whole branch of chemistry called “Muonium chemistry” for applications of Mu in chemistry.

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Molecules consist of atoms. When one or more of these atoms are replaced by different isotopes, then the different molecules are called isotopomers. For example, the water molecule may be written as H₂O, whereas heavy water D₂O is a heavy isotopomer of water. In our publication, we consider alltogether five isotopomers of the radical BrHBr, specifically BrMuBr, BrHBr, BrDBr, BrTBr and Br⁴HBr.

Different isotopomers have different properties. For example, H₂O is essential to life, whereas D₂O is poisonous due to the effect of its heavier mass on biochemical processes. The different properties are called isotope effects. For example, different isotopomers may have different spectra, or they give rise to different reactivities. There is a huge literature on many isotope effects, from quantum mechanical fundamentals to applications.

Our paper discovers, for the first time, that different isotopomers can induce a fundamental change in the nature of chemical bonding. This is an entirely new isotope effect, different from all previous ones. Specifically, we show that BrHBr and the heavier isotopomers BrDBr, BrTBr and Br⁴HBr are van der Waals (vdW) bonded, whereas BrMuBr is vibrationally bonded. The two different types of chemical bonding give rise to different geometries, symmetries, and most importantly, to different energetics and mechanisms of chemical bonding. For example, the heavy isotopomers have linear or bent structures with C_∞v or C_s symmetries. In contrast, BrMuBr has a linear structure with D_∞h symmetry.

Concerning the energetics and mechanisms, the heavy isotopomers BrHBr etc. are formed from their constituents e.g. from Br + HBr by decreasing the mean potential energy (PE) while slightly increasing the vibrational zero point energy (VZPE). The VZPE is a quantum mechanical effect which implies that the atoms in a molecule are never at rest, but they vibrate even at zero absolute temperature. For the light isotopomer BrMuBr, the roles of PE and VZPE are completely reversed, i.e. when BrMuBr is formed from Br + MuBr, the PE increases, but this is overcompensated by an even larger loss of VZPE. This dominant effect of the VZPE is the reason for the term “vibrational bonding”. In a cartoon-type type description of this rather new type of chemical bonding of BrMuBr, the system is centered at the transition state (denoted by Eyring’s symbol ‡) of the Br + MuBr —> BrMuBr^‡ —> BrMu + Br reaction. In most cases, molecules do not like to sit at ‡ for long because they prefer to escape from ‡ to the their dissociated fragments at lower energies. But in the case of BrMuBr, the system is stabilized at ‡, essentially due to the vibrational zero point motion of the light Mu between its heavy partners. In a picture based on semiclassical theory, Mu may first move to one of the neighbouring Br atoms, say to the “left” one, on the way to forming BrMu + Br, but in the next instance, it vibrates back to the other, “right” Br atom on the way to forming Br + MuBr, and so on. This way, averaged over time, it forms “vibrational” bonds to both Br atoms, thus keeping them together.

Background

Proton Transport and Hydrogen Bond Chemist Mark Tuckerman on different viewpoints on proton, chemical bonds and first-principles calculations

The mechanism of vibrational bonding was discovered in the early 1980s, by a group of theoretical chemists which included me. At first, it was believed that vibrational bonding should exist in radicals such as IHI or BrHBr. In the late 1980s, these predictions induced experimental investigations called “transition state spectroscopy” which allowed, in principle, to observe reactive systems at their transition states ‡. The hope was, of course, to discover IHI or BrHBr at ‡ and thus to verify the mechanisms of vibrational bonding – but actually the experiments did not monitor neither IHI nor BrHBr at ‡, i.e. the predictions seemed to be false. Close analyses showed, however, that this failure was not a consequence of the mechanism of vibrational bonding, but it was due to inappropriate and inaccurate semi-empirical potential energies (PE) used in the 1980s with transition states that were too low in energy. So the mechanism of vibrational bonding remained, but there was no clear example, for about thirty years.

The present discovery of vibrational bonding in BrMuBr was triggered by recent (2012) experiments on the reaction Mu + Br₂ —> MuBr + Br (analogous to H + Br₂ —> HBr + Br). Specificly, one of the present coauthors (Donald G. Fleming, University of British Columbia and TRIUMF, Vancouver) recognized that BrMuBr might have been formed as an intermediate in this reaction. This result raised the profile once again about the possibility that BrMuBr might indeed be vibrationally bound, which alerted him to the need for new and much more accurate quantum calculations, leading to the collaboration that resulted in our paper published in Angewandte Chemie in 2014. The proof of the working hypothesis that BrMuBr is vibrationally bound depended on highly accurate quantum calculations based on ab initio quantum chemistry for the PE of the system, well beyond the semi-empirical ansatz which had been applied in the early 1980s. Its ultimate success was achieved by the two Japanese coauthors who carried out this formidable task (Toshiyuki Takayanagi toghether with his Masters student Kazumo Sato, Saitama University).

Future Direction

After our confirmation of the existence of vibrational bonding in BrMuBr, we understand the conditions which enable this new type of chemical bonding and the related isotope effect much better than before, which in turn enables us to search for new examples in similar systems. Two papers are already in preparation along this line. Ultimately, we hope to discover vibrational bonding in stable isotopomers of different molecules, which may even, in the fullness of time, be incorporated into the lexicon of chemical bonding, different from its well-established concepts of covalent, ionic, hydrogen and vdW bonding.