A mechanism for unimolecular chemical reactions put forward by the British physicist Frederick Lindermann (1886–1957) in 1921 and examined in more detail by the British chemist Sir Cyril Hinshelwood (1897–1967) in 1927. The mechanism postulates that a molecule of A becomes excited by colliding with another molecule of A, and that having been excited there is a possibility that it undergoes unimolecular decay. If the process of unimolecular decay is sufficiently slow, the reaction has a first-order rate law, in agreement with experiment. The Lindemann-Hinshelwood mechanism predicts that if the concentration of A is reduced, the reaction kinetics become second order. This change from first to second order agrees with experiment qualitatively, although it does not do so quantitatively. The mechanism fails quantitatively because the molecule has to be excited in a specific way for a reaction to take place. The RRK and RRKM theories improve on this deficiency of the Lindemann-Hinshelwood mechanism.