Concluding Remarks

Jennifer C. Green

Inorganic Chemistry Laboratory, University of Oxford, South Parks Road, Oxford,

UK OX1 3QZ

Received 7th May 2003, Accepted 7th May 2003First published as an Advance Article on the web 11th June 2003

Professor Baerends agreed to give this lecture but unfortunately he was unable to come. I feelunqualified to carry the tablets down the mountainside, let alone write them in the first place. So Ithought a useful way of concluding the meeting was to look at what has been discussed in thecontext of the active areas of research in Inorganic Chemistry.I have drawn up a table of ‘‘What Inorganic Chemists Do’’ and ‘‘What Theory Can (or cannot)

Calculate ’’ (Table 1).At the heart of much inorganic research is the synthesis of new molecules. In this I would include

unstable species, which can be matrix isolated or detected in beams. Theory is very good at cal-culating structures of medium sized molecules, a not inconsiderable achievement. There have beenmany examples both in the talks and the posters of such successes. When molecules get larger weneed recourse to methods such as ONIOM techniques and QM/MM protocols and we have heardthis morning of developments along these lines.One of the drivers for synthesis is the search for new homogenous catalysts and the need to

understand the role of metals in organic transformations. In its ability to model reaction pathwaystheory can give unique insight into such processes, and aid in optimising stereo- and regio-selectivereactions. Rate data provide an experimental target for verifying calculations. Solvent effects can becritical in determining rate and several contributions have used methods such as COSMO1 fortreating them. Progress is still needed in computation in these areas; theory is already in a positionto rule out some mechanistic suggestions.One very active area of inorganic synthetic research is supra-molecular chemistry. This relies on

fortuitous combinations of many weak interactions to assemble an ordered array of molecules,which can, for example, bind ions selectively. Density functional theory is not very good at

Table 1

What Inorganic Chemists Do What Theory Can Calculate

Synthesis of new molecules Structures of medium-sized moleculesQM/MM methods

Homogenous catalysis, organic reactions Reaction pathwaysSolvent effects

Supramolecular chemistry, ion binding Weak interactionsMetalloenzymes Spin states

Spectroscopic propertiesNanoparticles Bulk properties, conductivity, magnetismSolid state chemistry Atomistic methodsSurface chemistry DFT with linear scalingHeterogenous catalysis Periodic methods

DOI: 10.1039/b305126a Faraday Discuss., 2003, 124, 453–455 453

This journal is # The Royal Society of Chemistry 2003

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modelling such weak interactions, and quantum theory is making very little contribution to thisarea.

We have had some excellent contributions detailing the challenge that metallo-enzymes provideto the quantum chemist. In some of the areas already discussed precision is not necessarily neededto answer interesting chemical questions but here it seems we have to be precise to compute energydifferences between close lying spin states. Though crystallography can often determine thestructure of a metallo-enzyme it does not necessarily determine the spin or oxidation state of themetal. This is the province of spectroscopy so theory needs to model spectroscopic properties.Development of time dependent density functional theory shows some promise in this direction.

I’d like to make a small diversion here. We have had appeals for experimental data in metalchemistry to serve as bench marks for calculations. I’d like to draw your attention to some data onmanganocene, Mn(Z-C5H5)2 (Fig. 1). Manganocene in the gas phase is high spin (S ¼ 5/2) witha thermally accessible low spin excited state (S ¼ 1/2). The separation is about 20 kJ mol�1.Standard density functional methods get the wrong ordering, BP86, for example, calculates the S ¼1/2 state as about 35 kJ mol�1 more stable than the S ¼ 5/2 state. The high spin molecule is muchmore ‘‘ ionic ’’ than the low spin. However, we cannot tackle this problem by having differentfunctionals, tuned for their degree of ionic character, for different states. The PE spectrum ofmanganocene is shown in stick-like form. The calculated IE come from a ADF/BP86 calculation.It is evident that the pattern of IE is reproduced very well by the calculation, but the absolutevalues are not precise. Three d ionizations are expected, the lower 5E

00

1 state of the cation and the5E

0

2 and 5A0

1 excited states. The third band between 9 and 11 eV was previously assigned to animpurity but the calculation shows that it represents a second 5E

00

1 ion state arising from a ligandionisation. It is strongly coupled to the lower state of the same symmetry so is displaced from themain band which comprises the other ligand ionizations. A small amount of the S ¼ 1/2 moleculeis also present in the gas phase and gives rise to a 3E

0

2 ion state which can be detected in thespectrum and is the ground state of the manganocene cation. The calculation gives a very goodnumber for this IE; this is more typical of the accuracy of IE calculations using this method. MayI suggest that here is a excellent data set on which to test the precision of open shell calculations?

To return to metallo-enzymes, they are of course not just large metal complexes, but areembedded in a protein which guides and orients the substrate for reaction at the active site. Hereagain weak interactions are critical and QM/MM methods are possible ways of modelling thewhole system.

Nano-particles are increasingly becoming a focus of technological research. Here we have aproblem of size but are no longer able to divide the problem into a quantum core and a MM

Fig. 1 Exchange splitting in PE spectra of open-shell molecules.

454 Faraday Discuss., 2003, 124, 453–455

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periphery, the whole particle needs to be treated at a high level. Also the properties of interest arebulk ones such as conductivity and magnetism. Atomistic methods coupled with dynamics can beof use, and we have heard that quantum methods have a role to play in developing and refining thepotentials used in such atomistic calculations. Also DFT codes with linear scaling, such asSIESTA2 and QUICKSTEP,3 seem to promise a useful approach.Solid state chemistry is a long established field of inorganic research of great technological

importance. With an ordered solid, periodic methods coupled with a quantum description of a cell,a defect or a grain boundary provide good descriptions of key properties. We have heard some niceaccounts of such methods.Closely allied to this are surface chemistry and heterogenous catalysis. Similar methods are used.

Good descriptions of surfaces have been achieved with investigations of catalysis yet to follow.Such synergies between theory and experiment will only bear fruit if communication between the

two communities is good. The onus is on the computational chemists to interpret and deconstructtheir calculations in terms and concepts familiar to the experimentalist. We have had contributionsshowing methods of such analysis. Also the nature of the experimental results must be fullyunderstood.Finally we have had a number of requests to the developers of density functionals to include

some transition metal systems in their test molecule sets. This is not going to happen unless thiscommunity gets together to draw up a list of benchmarks. It is no good sitting around wishing sucha thing would happen; we have to do something about it. I have drawn attention to a good data setfor testing spin state calculations, I am sure many here have similar favorite problems where goodexperimental data exists.

References

1 A. Klamt and G. Schuurmann, J. Chem. Soc., Perkin Trans., 1993, 799.2 J. M. Soler, E. Artacho, J. D. Gale, A. Garcıa, J. Junquera, P. Ordejon and D. Sanchez-Portal, J. Phys.

Condens. Matter, 2002, 14, 2745.3 M. Krack and M. Parrinello, Phys. Chem. Chem. Phys., 2000, 2, 2105.

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