Observing Transition States
Because of the rules of quantum mechanics, the transition state cannot be captured or directly observed; the population at that point is zero. This is sometimes expressed by stating that the transition state has a 'fleeting existence'. However, cleverly manipulated spectroscopic techniques can get us as close as the timescale of the technique will allow us. Femtochemical IR spectroscopy was developed for precisely that reason, and it is possible to probe molecular structure extremely close to the transition point. Often along the reaction coordinate reactive intermediates are present not much lower in energy from a transition state making it difficult to distinguish between the two.
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