Developed VSEPR (Valence Shell Electron Pair Repulsion) Theory
Gillespie explored the chemistry behind the brightly coloured solutions produced when certain elements (such as iodine, sulfur. selenium, and tellurium) were dissolved in concentrated sulfuric acid. In the process, he developed superacids, such as HSO3F, which became widely used in organic and inorganic chemistry. This development led to the production of non-metallic elements in the form of polyatomic cations (I2+, S82+) a discovery that opened up a whole new field of chemistry.
Gillespie also studied the shapes of molecules and, along with R. S. Nyholm, developed the widely-used Valence Shell Electron Pair Repulsion (VSEPR) theory. VSEPR can be used to describe and predict the shapes of molecules (e.g. linear, cubical, pyramidal, etc.) based on the number of electron pairs in the outer shells.
Gillespie wrote Molecular Geometry (1972), Chemistry (1986, 2nd edition 1989) and The VSEPR Model of Molecular Geometry (1991). He has written over 370 academic articles and received many honours, among which are several teaching awards.