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During a chemical reaction, atom distances, bond angles and dihedral angles constantly change until the reacting molecules have formed the reaction products. In most cases, bonds have to be loosened or bond angles have to be twisted at the start of the reaction requiring energy. Consequently, the curve of the reaction energy profile points upward. At the end of the reaction, new bonds are formed and atom distances and angles approach the new equilibrium distances and angles. During this process energy is released, i.e. the curve now points downward. Between up- and downward pointing parts of the energy profile the curve has to pass a peak representing the energy barrier. This peak is termed the transition state. It can be thought of as a peak in the mountains connecting two valleys with the curves of the energy profile representing a street which in one valley goes uphill and after going over the pass leads downhill to the opposite valley.

From a mathematical point of view, the transition state is the saddle point on a potential energy surface. Like any other point on this surface, the transition state represents a chemical structure which is characterized by atom distances and bond and dihedral angles.

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