Author Archive

PCCP–UCL symposium

PCCP journal cover imageWe invite you to join us for a joint PCCP–UCL symposium on Tuesday 15th October 2013.

Following a welcome by Professor Helen Fielding and brief introduction of PCCP by Editor Philip Earis, the following international speakers, including members of the journal’s Editorial Board, will talk about some of their exciting research:

14:10
Katsuhiko Ariga
, NIMS, Japan
Manual nanotechnology: a new page in the nanotechnology manual

14:40
Cyrus Hirjibehedin
, London Centre for Nanotechnology
Tunable single molecule magnetoresistance driven by magnetically sensitive negative differential resistance

15:10
Gaoquan Shi
, Tsinghua University, China
Graphene based electrochemical devices

15:50
Daniela Goldfarb
, Weizmann Institute of Science, Israel
Gd3+ spin labeling for structural interrogation of biomolecules

16:20
David Rueda
, Imperial College London
Watching AID scan ssDNA and transcribed dsDNA with single molecule resolution

Location: Chemistry Lecture Theatre, Department of Chemistry, UCL, 20 Gordon Street, London WC1H 0AJ.

No registration necessary.

**** The lectures will be followed with a wine reception ****

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The surprising truth about the dominance of hardness in acid-base reactions

Table of contents imageWhen 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


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Poster prizes: Theoretical Chemistry Group Conference 2013

Luke Crawford and Martina Stella with their PCCP poster prize certificates

Luke Crawford and Martina Stella with their PCCP poster prize certificates

PCCP was delighted to sponsor prizes for the two best graduate student poster presentations at the Theoretical Chemistry Group Conference 2013, which took place 24-26 June in Nottingham.

The prizes were awarded to Martina Stella (Bristol) for her poster entitled “A simple exact density functional theory embedding scheme” and Luke Crawford (St Andrews) for a poster entitled “New DFT insights into methyl propanoate formation at a palladium centre”.

Congratulations to them both!

Keep up to date with the latest PCCP articles and news: sign up to receive our free table of contents e-alerts and follow us on twitter.

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From solvated ions to ion-pairing: a THz study of lanthanum(III) hydration

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

Table of contents image

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Un-solvated photophysics

When investigating dyes, we almost always address the molecular system experimentally in a solution, and theoretically in the gas phase. Greisch and co-workers circumvent this problem by doing both in condensed phases as well as the gas phase. To me, it explained a phenomenon that I have come across several times: the >1800 cm-1 shift between ab initio calculated transition energies and the observed spectra. A difference e.g. ZINDO methods is compensated for, and why these are the tinctorial chemist’s choice when predicting spectra from molecular structures. With this paper it is demonstrated that if you want to design gas phase dyes, DFT calculations can be used.

The focus of the paper is the triplet energy, triplet lifetime and triplet deactivation of rhodamine dyes, addressing their use in STED super-resolution microscopy and the triplet state mediated photodegradation. While that motivation is good, I am too excited about the gas phase fluorescence spectra and their use in benchmarking computational approaches. Emission spectra of isolated ionic dyes, can also answer some of the questions regarding counter ion effects, symmetry and solvation that has been around for half a century. I am eagerly awaiting the next installment promised at the end of this paper.

By Dr Thomas Just Sørensen

Read the full details of this PCCP paper today:

Intrinsic fluorescence properties of rhodamine cations in gas-phase: triplet lifetimes and dispersed fluorescence spectra
Jean-François Greisch, Michael E. Harding, Mattias Kordel, Wim Klopper, Manfred M. Kappes and Detlef Schooss
Phys. Chem. Chem. Phys., 2013, 15, 8162-8170
DOI: 10.1039/C3CP44362K

Table of contents image

This article is part of a collection to coincide with the theme of Bunsentagung 2013: ‘Theory meets Spectroscopy’. You may be interested in the other articles in this collection too.

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New, Stable Rare Gas Molecules Predicted!

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

F Ar B N H

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Spot the difference: hydrogen and halogen bonds

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

Table of contents image

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Finding the catalyst for next-generation fuels

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

Table of contents image

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The mysterious f-block

Table of contents imageThe 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

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Understanding defects in graphene

The products of making graphene by thermally exfoliating graphite oxide are much more complex than previously thought, new research shows.

The most common way to prepare graphene is by thermally reducing – or ‘exfoliating’ – graphite oxide. But the graphene produced in the process often contains defects and lacks the perfect ‘honeycomb’ structure. One explanation is that these defects may be the result of organic by-products forming and escaping as gases during the reaction.

Scientists from Singapore and the Czech Republic allowed exfoliation to take place in an autoclave at 500 degrees for two hours then analysed the gases produced using gas chromatography–mass spectrometry (GC-MS). They detected many other volatiles in addition to H2O, CO and CO2, including polycyclic aromatic molecules, and those containing sulphur and nitrogen heteroatoms that are present as contaminants in the graphite oxide.

Moreover, the nature of the volatiles released varies hugely depending on pressure (2 bar versus 100 bar) and the gaseous atmosphere in which the exfoliation was carried out (hydrogen versus inert argon). The method by which the graphite oxide itself was prepared also had an effect – the Hummers method yielded the highest number of volatiles.

Understanding these by-products is crucial as they can affect the structure of the resultant graphene which influences its future use. The team suggest that measuring the volatiles produced during exfoliation could help determine the nature of defects.

Read this HOT PCCP article in full today:

Complex organic molecules are released during thermal reduction of graphite oxides
Z Sofer, P Šimek and M Pumera
DOI: 10.1039/C3CP51189H

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