PCCP Blog

When I was perusing the ‘recently published’ pages for something to write about, I was moderately surprised to stumble on this article. It immediately caught my eye as being unlike anything I’d ever seen in PCCP before and I was curious.  My curiosity was well rewarded. Cardenas and Ayers have taken one of the more infamously qualitative principles of synthetic chemistry and relatively successfully attempted to quantify and assess its validity, something I have no doubt that the synthetic community will be thrilled to hear.

Although the broad concept of acid-base chemistry is familiar to anyone with a science GCSE, it’s more subtle nuances tend only to be revealed at a higher level. Acids and bases are described as having both a strength and an independent hardness, both of which are purely qualitative. Traditionally, strong acids and bases displace their weak counterparts, and hard acids and bases stick together. So far so good, but because these two traits are independent of one another, the two principles can sometimes be conflicting. Strong acids tend to be hard, and strong bases tend to be soft, but this does not always hold true.

Cardenas and Ayers characterise all acid-base reactions into four cases. In two of these, the hardness and strength principles reinforce each other, and in the other two they are opposed. They investigate which rule wins out in each case, and to what extent it dominates. Their findings are surprising and exciting, and could really have a serious impact as they provide a good deal of clarity to this issue. Reading this is highly recommended.

How reliable is the hard–soft acid–base principle? An assessment from numerical simulations of electron transfer energies
Carlos Cárdenas and Paul W. Ayers
Phys. Chem. Chem. Phys., 2013, 15, 13959-13968
DOI: 10.1039/C3CP51134K

by Victoria Wilton

Researchers in the group of Professor Martina Havenith at the Ruhr-University Bochum have shown that terahertz spectroscopy can be used, employing meticulous physical chemical considerations, to open a window into the solvation of ions in solution. In this paper they join the discussion regarding the solvation of trivalent lanthanide ions in water, by studying lanthanum chloride and bromide at different concentrations. They determine association constants for the ion pairs and investigate the nature of the lanthanide solvation—adding the experimental support to the proponents of outer-sphere interaction between anions and trivalent lanthanide ions in solution.

Sharma and co-workers investigate the ion pairing of lanthanum halides from a true physical chemical approach. A puritan approach that to me, make this paper a pivotal example of what a PCCP paper should be. Is should not necessarily following the most recent trend, but answer or address an important question. Here, the hydration of lanthanide ions. We do not know the structure of solvation lanthanide ions, even the stoichiometry is unknown. Except for an assumption that lanthanum has nine water molecules in the ligand sphere forming a symmetrical tricapped trigonal prism, the paper attacks the question of lanthanide solvation from a refreshingly new angle.

The image attached to this post shows how well terahertz spectroscopy can describe a solution of ions. Remembering that this image is generated from several layers of a priori information, such as the behavior of neat water, simple electrolytes, solutions of alkali halides etc. I strongly recommend that you read the paper is you are interested in the rare earths, electrolytes or specific ion effects.

by Dr Thomas Just Sørensen

Read this exciting PCCP article today:

From solvated ions to ion-pairing: a THz study of lanthanum(III) hydration
Vinay Sharma, Fabian Böhm, Michael Seitz, Gerhard Schwaab and Martina Havenith
Phys. Chem. Chem. Phys. 2013, 15, 8383-8391
DOI: 10.1039/C3CP50865J

It’s no secret that I find rare gas chemistry very exciting, so when I came across this article by Jien-Lian Chen et al. I knew immediately that it would form the subject of my next blog post. The group, based in Taiwan, have performed calculations that predict a new series of rare gas molecules. Whilst this is, in itself, an exciting achievement, what really grabbed me was that they predict these molecules to be relatively stable – stable enough to be detected experimentally.

As far as I am aware, that is the holy grail of theoretical rare gas chemistry, as there is still very little experimental data on molecules of this kind. The group’s calculations were rigorous, comparing a variety of high-level computational methods and finding good agreement between them. They predicted the strength of the bonds to be significant, and examined the most likely unimolecular dissociation pathways and found sizable barriers to each of them for this class of molecule. All this adds up to a real possibility for generating some new, experimental rare gas data.

So what are these fantastic new molecules? They take the general form F–RG–BNR, where RG can be Argon, Krypton or Xenon, and R can be quite a variety of small groups. Interestingly, they found that the stability of the molecule was largely unaffected by the identity of the R group, indicating that it may be possible to study even more of them than were covered in this investigation.

All we need now is for somebody to work out how to synthesise these molecules in a spectroscopically useful environment, and perform the experiments to confirm the theoretical predictions. Experimental spectroscopists, consider the challenge issued!

By Victoria Wilton

Read the full details of this PCCP article:

Theoretical prediction of new noble-gas molecules FNgBNR (Ng = Ar, Kr, and Xe; R = H, CH₃, CCH, CHCH₂, F, and OH)
Jien-Lian Chen, Chang-Yu Yang, Hsiao-Jing Lin and Wei-Ping Hu
Phys. Chem. Chem. Phys., 2013, 15, 9701-9709
DOI: 10.1039/C3CP50447F

Hydrogen is the only element in the periodic table that is not truly part of a group, although it is often nominally assigned to group 1. All chemists are familiar with the concept of the hydrogen bond, but how many think of the halogen bond in the same light? How many are even aware of the halogen bond as a special entity, less still that the two interactions are to all intents and purposes the same thing?

In his recent paper, Grabowski uses theoretical techniques to show that both interactions are ruled by the same electrostatic mechanism. He also provides an excellent summary and comparison of the information currently known about the two interactions that indicates a clear progression in some bonding properties from hydrogen through to the heavy halogens.

He describes how the atomic volume of the halogen decreases as the positive charge on it increases, and that this effect is magnified by shortening the internuclear distance. This information accompanies the observation that the strength of the Lewis acid-base interaction increases with the increasing atomic number of the halogen involved, although with some exceptions, hydrogen bonds are generally stronger still.

This discovery clearly has important implications for our understanding of non-covalent molecular interactions, and our understanding of how best to classify hydrogen based on its bonding properties.

by Victoria Wilton

Read this HOT PCCP article today:

Hydrogen and halogen bonds are ruled by the same mechanisms
Sławomir J. Grabowski
DOI: 10.1039/C3CP50537E

The fact that modern life relies heavily on fossil fuels is a environmental and a political problem, which will become your problem come when our supplies of fossil fuels run out. Solutions that deal with our need for domestic and industrial energy demands are—more or less—readily available. Our main problem is how to find transportable energy materials to fuel our cars, planes and ships, when no oil-based fuels are available. In the group of Tejs Vegge they have focused on ammonia as a possible mobile energy material.

The main issue in generating next-generation fuels is to remove the energy cost associated with transforming electricity into chemical energy. The solution is catalysts. The manufacture of ammonia, through the Haber–Bosch process, is enabling planet Earth to sustain the 7 billion humans that inhabit it today. If the process were stopped a couple of billion humans would die. Research has made the process of making ammonia very efficient, and has revolutionized our understanding of heterogeneous catalysis. Even so, the Haber–Bosch process is a high-energy process, consuming approximately 2 % of our total energy production.

The paper titled ‘DFT based study of transition metal nano-clusters for electrochemical NH₃ production’ is focusing on finding a process where electricity can be used to generate ammonia in an electrochemical cell. That is, how to make ammonia efficiently in a small scale, low temperature process on site at/in wind turbines and solar power plants. Using computational chemistry several catalysts are screened.

by Dr Thomas Just Sørensen

If you want to learn more see the paper, which was published in PCCP:

DFT based study of transition metal nano-clusters for electrochemical NH₃ production
J. G. Howalt, T. Bligaard, J. Rossmeisl and T. Vegge
Phys. Chem. Chem. Phys., 2013, 15, 7785-7795
DOI: 10.1039/C3CP44641G

The complex electronic structure of the lanthanides and actinides is largely a mystery. The f-shell is not readily explained in our theoretical treatment of atoms and molecules, and coupled with the difficulty of obtaining readily interpreted experimental data. The result is that we are still struggling to understand the elements at the bottom of the periodic table. Xu and co-workers conduct a good and meticulous computational study of lanthanide trifluorides, and do significantly increase our understanding of the rare earth elements, albeit in simple molecules in the gas phase. My problem is that I work in aqueous solution, where lanthanide ions have not just three ligands—as the molecules investigated in this paper have—they have eight, nine or ten.

The bonding orbitals in the lanthanide series can, in theory, be 4f, 5d, and 6s orbitals. Traditionally, the contracted f-shell orbitals have been viewed as shielded, and the interaction between lanthanide ions and ions of the opposite charge has to be regarded as purely ionic in nature. This work shows that our current computational models include a significant covalent contribution in the bonding of fluoride to trivalent lanthanide ions.

I look forward to the paper where lanthanide ions with a full coordination sphere are treated, although I fear it will be a long wait. Luckily, the papers reporting the progress towards a full computational treatment of lanthanide in solution are also worth reading, even for experimentalists like me.

On structure and bonding of lanthanoid trifluorides LnF₃ (Ln = La to Lu)
Wei Xu, Wen-Xin Ji, Yi-Xiang Qiu, W. H. Eugen Schwarz and Shu-Guang Wang
Phys. Chem. Chem. Phys., 2013, 15, 7839-7847
DOI: 10.1039/C3CP50717C

by Dr Thomas Just Sørensen