Application of 13C–13C Spin–Spin Couplings in Structural Studies on Organic Compounds

Abstract

It is already well recognized that crucial information on the electron structure of a chemical bond and the structure of organic compounds including those of biological origin is encoded in the Js. Therefore, the knowledge of trends governing them provides a unique opportunity for the creative designing of new chemical reactions, leading to unknown molecular structures and/or the elucidation of biosynthetic pathways. This chapter is devoted to the CC couplings across one, two, and three bonds. However, it is not our goal to provide a comprehensive review on all couplings published in the literature; the references to the relevant reviews published so far on this topic are included in the list of the literature [1–14]. We shall rather concentrate our attention on the factors which influence the magnitude of a given coupling. The J data presented in the subsequent parts of the chapter have been collected and arranged with the thought of showing how hybridization of the orbitals of the atoms involved in the coupling, substituent electronegativity, the complex and hydrogen bond formation and geometry of the compound bear on the JCC magnitude and which range of changes can be expected for a given type of coupling when all these effects are taken into account. The changes caused by the abovementioned factors are in many cases very strong and by no means can be neglected when either structural studies or measurements of Js are carried out. With the introduction of high magnetic fields and of cryogenic probes, the measurements of JCCs became routine even for comparatively large molecules and for a small amount of the sample. The latter is especially important in the case of compounds of biological origin which are often only accessible in the quantity of several milligrams. Another valuable source of information on Js is provided by ab initio and density functional theory (DFT) quantum mechanical calculations. In particular, DFT calculations of spin–spin couplings for molecules of moderate sizes can currently be performed on modest computer systems, providing an access to large sets of data in a reasonable time [15,16]. A comparison of the experimental couplings with calculated ones is very interesting and useful since, on the one hand, one can evaluate the validity of the theoretical approach employed and, on the other hand, it provides a deeper insight into the coupling mechanism. It is also important from a practical point of view since it allows one to trace possible errors in the experimental JCCs reported. However, it should be stressed at this point that it is crucial to have large sets of data for such comparisons, as otherwise any linear regressions of J(expl.) versus J(calcd.) obtained are likely to be accidental. We would like to mention at the end that generally, the IUPAC nomenclature has been applied throughout the whole chapter. However, in some cases, the arbitrary numbering of the substituents has been necessary in order to account for their strong effect.