Preface

Extracting chemical information from modern wave functions and from experimental high-resolution electron densities is a challenging but most valuable task. Many chemical concepts have been introduced at an intuitive level, often in the distant past, and hence need to be continuously confronted and scrutinised against a wealth of data obtained from modern computer calculations and increasingly automated X-ray equipment. Some concepts are widely used in the primary chemical literature but their often vague and intuitive definition turns out to be the source of confusion and vigorous debate. They include: hydrogen bonding, electronegativity, covalence, hardness/softness, aromaticity and the origin of rotation barriers. It is important to bridge the expanding gap between widely used chemical concepts and modern accurate physical and computational data on molecules.

Ron Gillespie and I had entertained the idea for holding a Faraday Discussion on this topic for some time. However, it was only during a dinner, hosted by Bernard Silvi and Andreas Savin in a quaint Paris restaurant, that the broad format and exact title of this Discussion was established. Little did I realize at the time how rewarding it would be to see this idea unfold into a real event, attended by so many fine scientists, and keen and talented students.

This Discussion is not about ultimate agreement between theory and experiment. As chemists we can only envy the current agreement between theory (quantum electrodynamics) and experiment of the electron’s gyromagnetic ratio to 10 digits after the decimal point. Quantum mechanics, in the shape of computational quantum chemistry, is also increasingly successful in matching theory and experiment, be it in a less spectacular fashion. However, in spite of its remarkably accurate predictions, quantum mechanics is still not properly understood. The link between daily life intuition and the measurement of physical properties is not clear cut. I would argue that chemical concepts, although intuitive and used on a daily basis, are also not as clearly linked to the underlying physics as one might hope.

It is not easy to define chemical concepts from an underlying physical reality, particularly when not everybody wants to, sitting beneath the banner of “if it isn’t broke, don’t fix it”. Fortunately, there are a handful of “gatekeepers” in the works who do attempt to clearly define these concepts, for example the participants of Faraday Discussion 135. This group takes to heart the conceptual problems that still face present day chemists, whether it be via teaching a class of receptive undergraduate minds or undertaking research programmes on nanomaterials. As chemists we are facing the situation where concepts elude final pinpointing or, as was once written about aromaticity, where the definition changes every thirty years!

Plate1 Participants at the Faraday Discussion 135 Meeting: Chemical Concepts from Quantum Mechanics, held at Hulme Hall, University of Manchester, 4–6 September 2006. Photograph taken by Ms Morwenna Gilbert and provided courtesy of Ivan Glukhov.

During David Clary’s wonderful dinner speech at the Discussion banquet I learnt that the last Faraday Discussion of substantial overlap with the current one was held in 1923, entitled “The electronic theory of valency”. In that meeting, scientists such as J. J. Thomson (who discovered the electron), N. V. Sidgwick (a pioneer of the VSEPR model) and G. N. Lewis (proponent of the electron pair) and others discussed issues that are so commonplace and fundamental to chemistry, that one may forget that they were ever a matter of debate. This meeting took place prior to the establishment of (quantum) “spin” and the mature and final formulation of quantum mechanics as a whole. In that context, it is almost heart-breaking to see G. N. Lewis struggle to understand the chemical bond with phrases like “When two molecules, (…) combine with one another, it is as though the two previously unpaired electrons were clamped together by some powerful mechanism. Quantum theory, so far as it has been developed hitherto, offers no interpretation of this fact, unless it is to be found in the very recent work of Sommerfeld in connection with his inner quantum number.” What is clear is that Lewis looked for a rigorous link between his own strong (and correct) chemical intuition and physics. How can this desire be better summarised than in one of the subtitles of his own address, “Reconciliation of the physicist’s and chemist’s views of the atom?” Is this an example of reductionism?

I prefer to use the term “emergence”, which Wikipedia defines as “the process of complex pattern formation from more basic constituent parts or behaviours”. I believe that emergence applies very well to the relationship between chemistry and quantum mechanics. Chemistry emerges from quantum mechanics. A useful analogy with the game of chess can be drawn. If the rules of chess correspond to the rules of quantum mechanics, then chemistry corresponds to the study of the enormous variety of games, strategies and playing styles (e.g. aggressive, sly, cautious). Perhaps naïve reductionism attempts in vain to reduce an aggressive playing style to, say, the agile use of the queen. Keeping up the analogy, this could be as futile as explaining hydrogen bonding by means of kinetic energy density only. Emergence, on the other hand, recognises upfront the complex interplay of many (simpler) elements, which together form new patterns at a higher level. Pinpointing these patterns and how they arise is the true and feasible challenge.

Finally, when it is said that the Royal Society of Chemistry staff are with you every step of the way, I can only wholeheartedly confirm this. They, as well as the organising committee, have made my work an order of magnitude more manageable. Many thanks to you all! Many thanks also to the contributors who delivered their work against a tight deadline and to the participants who took time to file a sizeable number of questions, replies and comments. Finally, I thank, somewhat presumptuously, the reader for taking time to dive into this volume.

Paul Popelier

Chair

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