HC Longuet-Higgins: The Man and His Science

From his publication in 1943, as an Oxford undergraduate, of the correct structure for diborane, to his explanation in 1967 of the anomalous chemical shifts of the protons in [4n]annulenes, H.C. Longuet-Higgins made many important contributions to theoretical chemistry. These contributions include: (a) showing how to find rapidly the non-bonding MOs of odd alternant hydrocarbons and demonstrating that these MOs can be used, in place of resonance structures, to make predictions about the m systems of organic molecules, (b) explaining the observed UV-Vis spectra of unsaturated hydrocarbons in terms of the interactions between transition diploes, (c) predicting that B6H6-2 and B12H12-2 each have a closed shell of electrons and (d) that cyclobutadiene should form stable complexes with transition metals, (e) investigating the intersections between potential energy surfaces, showing the limitations of the non-crossing rule, and discovering the change in the "Berry phase" around points at which two surfaces intersect, (f) predicting bond-length alternation in [4n+2]annulenes (but not in polyacenes) for sufficiently large values of n, (g) showing that the symmetry group for a non-rigid molecule can be generated from a combination of permutations of identical atoms and inversion of the molecule through its center of mass, (h) interpreting the ESR spectra of radical anions in terms of the nodal properties of the singly-occupied MOs, and (i) introducing the use of correlation diagrams for understanding and predicting the outcomes of electrocyclic reactions. This chapter presents a brief biography of Christopher Longuet-Higgins and a detailed discussion of each of the most important contributions that he made to theoretical chemistry, before he left the field in 1967, in order to begin research in artificial intelligence.