Longuet-Higgins, Hugh Christopher (1923– )

English theoretical chemist whose main contributions have involved the application of precise mathematical analyses, particularly statistical mechanics, to chemical problems.

Life Longuet-Higgins was born in Kent and studied at Oxford. He was professor of theoretical physics at King's College, London, 1952–54, and professor of theoretical chemistry at Cambridge 1954–67. He then went to Edinburgh University to study artificial intelligence and information-processing systems, which he thought had a closer bearing on true biology than purely physiochemical studies. In 1974 he moved to Sussex University, where he expanded this field into studies of the mechanisms of language and the perception of music.

Work Longuet-Higgins successfully predicted the structures of boron hydrides and the then unknown beryllium hydride, and the existence of the ion (B12H12)2−.

In 1947 Longuet-Higgins developed the orbital theory of conjugated organic molecules, deriving theoretically results that had been known experimentally for decades. He showed how the properties of conjugated systems can be derived by combining into molecular orbital theory a study of nonbonding orbitals.

He formulated a theory to describe the thermodynamic properties of mixtures, which he later extended to polymer solutions. He also investigated the optical properties of helical molecules and worked on electronic spectra.

From 1954, he used mathematical techniques to make theoretical chemical predictions. He predicted, for example, that cyclobutadiene (which had defeated all attempts to prepare it) should exist as a ligand attached to an atom of a transition metal; such a compound was successfully prepared three years later.

In a larger piece of work, he applied group theory to define the elements of symmetry of nonrigid molecules, such as hydrazine (N2H4), and thus was able to classify the individual quantum levels of the molecule.