Pancake bonding

π-system radicals may interact with each other in a special way. An actual recombination of the radicals is not possible due to the delocalization in the π-system. But the binding between two such radicals is still a lot stronger than would be expected from pure van-der-Waals interactions. This phenomenon is called "pancake bonding".

There is a new paper out by us, studying this: "Study of the Diradicaloid Character in a Prototypical Pancake-Bonded Dimer: The Stacked Tetracyanoethylene (TCNE) Anion Dimer and the Neutral K₂TCNE₂ Complex" in ChemPhysChem.

Pancake bonding is quite demanding from a computational point of view, since you need to describe the static correlation of the radicals as well as dynamic correlation, which determines the dispersion interaction. We used the multireference averaged quadratic coupled cluster (MR-AQCC) method, which should be able to deal with these effects. MR-AQCC is very similar to multireference configuration interaction only that there is an additional correction for size-extensivity.

Ubuntu vs Windows 8

It is quite amazing how much trouble we had with Windows 8. We just got this new Asus Netbook (F201E). The pre-installed Windows 8 was not only completely non-intuitive to use, but so slow that we were close to just throwing the thing out of the window. The task manager showed that there were some processes running all the time on 50% CPU using up hundreds of megabytes RAM. Maybe you can get away with something like that on a workstation if it has multiple cores and lots of RAM. But it was defnitly the wrong thing for our netbook.

What fixed it? Ubuntu. That was my task for yesterday, which went really smooth. Just download the distribution. Then you need to create a bootable USB stick. Tweak the BIOS a little bit to make sure it reads the stick (activate CSM, deactivate SecureBoot). And even the Wifi worked pretty much right away (after I installed the updates).

And now everything runs smooth and as quick as you'd expect a modern computer to work (even if it's a low-end machine).

C++

If I wake up screaming tonight, it's because I have been programming C++ all day. I can see that it is a powerful programming language, but if you are not used to it, it just gives you a headache (and nightmares...).

Maybe I should try to understand how pointers work and not just put * and & signs at random until it compiles. That may make things easier ...

Phosphorescent Complexes

If you are into phosphorescent transition metal complexes here is a paper for you: The triplet state of organo-transition metal compounds. Triplet harvesting and singlet harvesting for efficient OLEDs by Yersin et al. It was very interesting to read this. They just give a relly nice picture of spin-orbit coupling effects and how they are affected by the ligand field. As a major effect: if the ligand field allows for quasi-degenerate states (e.g. octahedral complexes), then the zero-field splitting will be much stronger than in other cases (e.g square planar). And later on they show that the zero-field splitting shows a strong inverse correlation with the phosphoresence lifetime. But on the other hand, if the zero field splitting is too strong, then the highest triplet substate will no longer be efficiently thermally populated, which is a problem since this is usually the state with the strongest emission strength. For this reason there is as of yet no triplet harvesting complex with a phosphorescent lifetime below 1 μs.

Of course, I do not know if it is all correct. But it is nice to see that people are actually taking their time to produce realistic models to explain their results.

Density changes and flux

An often misunderstood concept (at least by me) is the relation between density (ρ) and flux (j), which is essential in many kinds of dynamical processes. In the absence of sources and sinks these are related according to the continuity equation

From this equation it obviously follows that

i.e. if the flux vanishes, the density has to be constant. The opposite is not true

Only the "divergence" of the flux has to vanish but there may still be a "rotation". Take the example of stirring a pot: there is motion inside the pot even though none of the water actually moves. The same idea holds for ring currents in a molecule like benzene.

The first time I came across this problem was in a somewhat different context, related to my diploma thesis. The question was whether the excited state intramolecular double proton transfer in bipyridyldiol occured in a concerted, sequential or mixed fashion.

We were claiming case (b) was correct, while (a) was the previously accepted view. I do not want to discuss the methodological details now, but there was one thing that bothered me: In our trajectory simulations, you could clearly see whether a single or double proton transfer occured. On the other hand there was a quantum dynamics paper, which showed the evolution of the probability density. And it was not clear to me at all, how one could interpret these density fluctuations in terms of single or double proton transfers. Did the classical trajectories show something that did not exist in the quantum world?

I would now claim the opposite is true: When you analyse quantum dynamics only in terms of probabilty densities, you miss important information (just as you would not know I was stirring the pot of water only from the density). The above continuity equation for quantum dynamics becomes

This means that the flux is coded in the phase of the wavefunction. For a full analysis of the dynamical process it is necessary to consider the phase as well as the amplitude. Another obvious consequence of this equation is that there is no density change in purely real wavefunctions (there may however be a phase change followed by a density change).

A similar question also arises in surface hopping dynamics: Is it enough to plot the state amplitudes against time or does it give additional information to consider also the state-to-state transition probabilities? You again need the transition probabilites for the whole picture, e.g. if you want to identify individual relaxation pathways as is done in this paper.