Charge separation in a molecule consisting of two identical atoms: Size matters

Physicists from the University of Stuttgart show the first experimental proof of a molecule consisting of two identical atoms that exhibits a permanent electric dipole moment. This observation contradicts the classical opinion described in many physics and chemistry textbooks.

The work was recently published in the journal Science.

A dipolar molecule forms as a result of a charge separation between the negative charged electron cloud and the positive core, creating a permanent electric dipole moment. Usually this charge separation originates in different attraction of the cores of different elements onto the negative charged electrons. Due to symmetry reasons homonuclear molecules, consisting only of atoms of the same element, therefore could not possess dipole moments.

However, the dipolar molecules that were discovered by the group of Prof. Tilman Pfau at the 5th Institute of Physics at the University of Stuttgart do consist of two atoms of the element rubidium. The necessary asymmetry arises as a result of different electronically excited states of the two alike atoms. Generally this excitation will be exchanged between the atoms and the asymmetry will be lifted. Here this exchange is suppressed by the huge size of the molecule, which is about 1000 times larger than an oxygen molecule and reaches sizes of viruses. Therefore the probability to exchange the excitation between the two atoms is so small that it would statistically only happen once in the lifetime of the universe. Consequently, these homonuclear molecules possess a dipole moment. A permanent dipole moment additionally requires an orientation of the molecular axis. Due to their size the molecules rotate so slowly that the dipole moment does not average out from the viewpoint of an observer.

Physicists from the University of Stuttgart succeeded in experimentally detecting the dipole moment. They measured the energy shift of the molecule in an electric field by laser spectroscopy in an ultra cold atomic cloud. The same group caused worldwide a stir when they created these weakly bound Rydberg molecules for the first time in 2009. The molecules consist of two identical atoms whereof one is excited to a highly excited state, a so-called Rydberg state. The unusual binding mechanism relies on scattering of the highly excited Rydberg electron of the second atom. So far theoretical descriptions of this binding mechanism did not predict a dipole moment. However, the scattering of the Rydberg electron of the bound atom changes the probability distribution of the electron. This breaks the otherwise spherical symmetry and creates a dipole moment. In collaboration with theoretical physicists from the Max-Plank-Institute for the Physics of Complex Systems in Dresden and from the Harvard-Smithonian Center for Astrophysics in Cambridge, USA, a new theoretical treatment was developed that confirms the observation of a dipole moment.

The proof of a permanent dipole moment in a homonuclear molecule not only improves the understanding of polar molecules. Ultra cold polar molecules are also promising to study and control chemical reactions of single molecules.

Story Source:

The above story is reprinted from materials provided by University of Stuttgart.

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Journal Reference:

W. Li, T. Pohl, J. M. Rost, S. T. Rittenhouse, H. R. Sadeghpour, J. Nipper, B. Butscher, J. B. Balewski, V. Bendkowsky, R. Low, T. Pfau. A Homonuclear Molecule with a Permanent Electric Dipole Moment. Science, 2011; 334 (6059): 1110 DOI: 10.1126/science.1211255

First chemical complex consisting of rare earth metals and boron atoms produces unexpected results

Boron is an intriguing member of the periodic table because it readily forms stable compounds using only six electrons—two fewer than most other main-group elements. This means that chemists can easily add boron to unsaturated hydrocarbons, and then use electron-rich atoms, such as oxygen, to change organoborons into versatile units such as alcohols and esters. Recently, researchers found that combining transition metals with boron ligands produces catalysts powerful enough to transform even fully saturated hydrocarbons into new organic functionalities with high selectivity.

Now, Zhaomin Hou and colleagues from the RIKEN Advanced Science Institute in Wako have made another breakthrough in this field: they have created the first-ever complexes between boron ligands and rare earth metals1. Because these novel chemical combinations display a surprising ability to incorporate molecules such as carbon monoxide into their frameworks, they have potential applications that range from synthesizing organic substrates to controlling noxious emissions.

Rare earth metals are hot commodities because they are vital for products in high demand such as smartphones and electric cars (Fig. 1). However, full chemical studies of these elements are only in their infancy since they are difficult to handle under normal conditions. 

According to Hou, typical methods to prepare transition metal–boron complexes—halogen or metal exchange reactions, for example—seemed unsuitable for rare earth metals. Instead, the team used a vigorous lithium–boron compound to handle the reactive rare earth precursors, producing previously unseen scandium–(Sc–B) and gadolinium–boron (Gd–B) complexes in good yields, but not without difficulty. “Rare earth–boron compounds are air- and moisture-sensitive and sometimes thermally unstable,” says Hou. “They therefore require great care in isolation and handling.”

To determine whether or not the Sc–B complex could act as a nucleophile—an important electron-donating reagent in organic chemistry—the team reacted it with N,N,-diisopropylcarbodiimide, a molecule that easily accepts electrons to change into an amidinate salt. X-ray analysis revealed that initially, the carbodiimide became incorporated between Sc and carbon ligands on the rare earth metal, but extra quantities of the reagent became incorporated between the Sc–B bond. Furthermore, adding carbon monoxide to this mixture also caused a rare earth–boron insertion, accompanied by an unexpected rearrangement into a cyclic structure. 

Because chemists rely on insertion reactions to efficiently transform ligands into a diverse range of products, these findings should enable development of brand new synthetic techniques—opportunities that Hou and his team are actively pursuing.

More information: Li, S., et al.  Rare earth metal boryl complexes: Synthesis, structure, and insertion chemistry. Angewandte Chemie International Edition 50, 6360–6363 (2011). 

Provided by RIKEN (news : web)