Structural chemistry in Helvetica Chimica Acta, 1917–1992

Abstract

What is Structural Chemistry? -Around 1959 the elder author of this paper happened to meet the famous, recently retired Professor Leopold Ruzicka on the steps of the ETH Chemistry building in Zurich. To make conversation, he remarked that he had just determined the structure of cyclododecane -what he meant was that he had just determined the crystal structure of this compound and established the conformation of the molecule [I]. 'Ahah' countered Ruzicka, 'Die kenne ich schon lange !'. Clearly, Ruzicka understood the word 'structure' to mean something quite different from that understood by his younger colleague. Structure -molecular structurehas always been the stuff that chemistry is built on, even when the very existence of molecules was conjectural, and it has meant quite different things at different times to different kinds of chemist. Here we take 'structural chemistry' to be concerned not just with the determination and consequences of molecular constitution but rather more with the detailed study of the metrical aspects of atomic arrangements in molecules and its consequences for chemical properties. In this century, many different physico-chemical techniques have been directed to the investigation of molecular structure -various kinds of spectroscopy, as well as diffraction methods applied to all states of matter, in addition to theoretical calculations at all levels of sophistication and credibility. Since we assume that most of these physico-chemical approaches, as far as Heloetica Chimica Acta ( H C A ) are concerned, are at least mentioned in the article by Gunthard and Heilbronner [2], we shall concentrate here on what is the most relevant for our purpose, the study of matter by X-ray crystallography; for this chapter, then, we define structural chemistry in a very limited sense as the study of structure, mainly in the solid state, and the implications of the results for chemistry as a whole.Historical Development. -The development of structural chemistry as seen through the pages of H C A closely follows the development of the subject in the world at large. The journal is almost the same age as X-ray crystal-structure analysis, give and take a few years. Laue's discovery that X-rays are diffracted by crystals was made in 1912 [3], and the atomic arrangements in several highly symmetrical crystals such as rock salt (NaCl), zincblende (ZnS), fluorspar (CaFJ, and diamond were determined by the Braggs, father and son, almost immediately afterwards in a series of fundamental studies [4] [5]. Nevertheless, in spite of important first steps made during the early years, the method was still in its infancy in 1917 and virtually unknown to chemists then and for many years to come.

Seeing Molecular Configuration in Twisted Crystal Form

“…With X‐ray crystallography, “structural chemistry” became the “study of the metrical aspects of atomic arrangements in molecules,” Jack's definition with Hans‐Beat Bürgi. Before the diffraction era, structural chemistry was grounded in qualitative symmetry arguments, the first of which followed from Pasteur's 1848 insight into the organization of atoms in tartrate salts.…”

“…In the mid‐1970s, Jack wrote a group of papers with Murray‐Rust and Bürgi that were meditations on the configuration of ideally T d symmetric chemical species, such as PO 4 , SO 4 , and AlCl 4 , that are frequently chiral in crystals, as they sit in general positions . Here, configuration is not seen in crystal form, but rather in the “metrical positions of the atoms” with respect to one another, as revealed by X‐ray crystallography. Configuration is more than a distinction between right and left.…”

Small Molecule Crystallography in the Laboratory of Organic Chemistry at ETH Zürich

“…The work on the conformations of medium‐sized rings spanned the entire 1960s to the early 1970s . A later review (1993) by Bürgi and Dunitz on Structural Chemistry in Helvetica Chimica Acta 1917–1992 , nicely puts the relevance of these studies into perspective . Predictions for the stable conformations of medium‐sized rings with the models available at that time were shown to be of very limited value, and X‐ray analysis finally demonstrated the regularities in these torsionally flexible molecules with respect to Baeyer and Pitzer strains, torsional angle preferences, and transannular interactions.…”

“…Predictions for the stable conformations of medium‐sized rings with the models available at that time were shown to be of very limited value, and X‐ray analysis finally demonstrated the regularities in these torsionally flexible molecules with respect to Baeyer and Pitzer strains, torsional angle preferences, and transannular interactions. Importantly, the now available knowledge about the stable conformations explained “puzzling aspects” of the chemical reactivity of these compounds. In the crystal structure of oxecane‐2,7‐dione (oxacyclodecane‐2,7‐dione, 6‐ketononanolide), a particularly short transannular distance between the dicoordinate oxygen atom of the ester and the keto C=O group was observed ( d (O⋅⋅⋅C)=2.83 Å) and explained by an n →π* interaction (Figure b) ,.…”

“…Bürgi and Dunitz wrote in 1993 that “repeating arrangements of molecules in crystals also tell us about details of the ways molecules can recognize and interact with one another – supramolecular chemistry” and that “careful study of the packing in molecular crystals should be a valuable source of information in supramolecular chemistry” …”