Fundamental change in the nature of chemical bonding upon isotopic substitution

October 28, 2014

Isotopes are atoms of the same element that have the same number of protons but a different number of neutrons. Because of the resulting differences in mass ratios, isotope effects can significantly influence the formation and breaking of chemical bonds in the course of chemical reactions. In extreme cases, isotopic substitution can even cause a fundamental change in the type of chemical bond, as reported by scientists in the journal Angewandte Chemie.

Such effects come to light most distinctly when the differences between the isotopes are particularly large, like those of hydrogen. In nature, hydrogen is found as 1H (protium, H), 2H (deuterium, D), and 3H (tritium, T). Additional isotopes have been synthesized in the lab, including some exotic atoms that contain other elementary particles. For example, muonium (symbol Mu) is made of an electron and a nucleus consisting of an antimuon. Chemically, it behaves like a , but it is nine times lighter than 1H.

Jörn Manz (Shanxi University Taiyuan and FU Berlin), Donald G. Fleming (University of British Columbia, Canada), Kazuma Sato, and Toshiyuki Takayanagi (Saitama University, Japan) have carried out novel quantum-mechanical ab initio calculations for the reaction between hydrogen bromide and bromine atoms to form the BrHBr radical. They were particularly interested in a comparison between the isotopes H, D, T, and Mu, as well as a heavy exotic isotope. "We found that the four heavy isotopomers behave essentially the same way," says Manz. " In contrast, the lightest isotopomer, BrMuBr, is held together by a completely different kind of bond."

BrHBr and its heavy analogues can adopt either a linear or a bent form. In the bent form, both bromine atoms are bound to each other. In the linear form, the two bromine atoms are bonded by way of the H atom, which is positioned very close to one of the two Br atoms while remaining significantly farther away from the other. Van der Waals bonds hold the whole thing together. These result from short-term charge transfers within a particle, which have an attractive effect.

In contrast, BrMuBr is bound together by a recently proposed, completely different type of bonding: vibrational bonding. In this type of bond, the molecular fragments are held together by their motion: the muonium vibrates in a transition state between the two bromine atoms. "Our calculations on BrMuBr are the first clear evidence for the existence of this new type of bonding," says Manz. "In addition, they are the first indication that isotopic substitution can change the nature of chemical bonding in a profound manner. The different isotopomers of the radical we have studied and compared here have completely different structures, symmetries, and, most importantly, energetics and mechanisms of chemical bonding."

Explore further: Tests confirm nickel-78 is a 'doubly magic' isotope

More information: Fleming, D. G., Manz, J., Sato, K. and Takayanagi, T. (2014), "Fundamental Change in the Nature of Chemical Bonding by Isotopic Substitution." Angew. Chem. Int. Ed.. doi: 10.1002/anie.201408211

Related Stories

Tests confirm nickel-78 is a 'doubly magic' isotope

September 5, 2014

The stability of atoms can vary considerably from one element to the next, and also between isotopes of the same element (whose nuclei contain the same number of protons but different numbers of neutrons). While many isotopes ...

Directly visualizing hydrogen bonds

July 15, 2014

Using a newly developed, ultrafast femtosecond infrared light source, chemists at the University of Chicago have been able to directly visualize the coordinated vibrations between hydrogen-bonded molecules—the first time ...

A new technique for understanding quantum effects in water

October 3, 2011

It covers over two thirds of our planet, is essential for life on Earth and its chemical formula is one of the few most people can name, but we still have much to learn about the structure of H2O. Now, scientists working ...

Recommended for you

Hydrogen from sunlight—but as a dark reaction

December 9, 2016

The storage of photogenerated electric energy and its release on demand are still among the main obstacles in artificial photosynthesis. One of the most promising, recently identified photocatalytic new materials is inexpensive ...

Cloud formation—how feldspar acts as ice nucleus

December 9, 2016

In the atmosphere, feldspar particles act as ice nuclei that make ice crystals grow in clouds and enable precipitation. The discovery was made by researchers of Karlsruhe Institute of Technology (KIT) and University College ...


Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.