Scientists began to wonder about the energetic of cluster formation early in the 19th century. Henry Eyring developed the activated-complex theory describing kinetics of reactions. Interest in studying the weak interactions of molecules and ions(e.g. van der Waals) in clusters encouraged gas phase spectroscopy, in 1962 D.H. Rank studied weak interactions in the gas phase using traditional infrared spectroscopy. D.S. Bomse used IRPD with an ICR to study isotopic compounds in 1980 at California Institute of Technology. Spectroscopy for weak bonding clusters was limited by low cluster concentration and the variety of accessible cluster states. Cluster states vary in part due to frequent collisions with other species, to reduce collisions in gas phase IRPD forms clusters in low pressure ion traps (e.g. FT-ICR). Nitrogen and water were one of the first complexes studied with the aid of a mass spectrometer by A. Good at University of Alberta in the 1960s.