Intrinsic reaction coordinate, single-product

The geometrical properties of the PES in the vicinity of a transition state mean that the steepest descent path down from the transition state (also generally calculated in mass-weighted coordinates, and called the intrinsic reaction coordinate or IRC) will usually lead only to a single reactant in one direction and a single product (or intermediate) in the other. However, a transition state can sometimes be shared by more than one reactant and/or product. One of these cases arises when the PES possesses a so-called valley-ridge inflection point (VRI). °... 

The energy minima were confirmed by the absence of imaginary frequencies, the transition states by the presence of a single one [40]. That the latter lead to the desired products was verified by following the intrinsic reaction coordinate [41]. 

The simplest chemical reactions correspond to overcoming a single reaction barrier on the way from reactants to products through a saddle point along the intrinsic reaction coordinate (IRC). The IRC corresponds to the steepest descent trajectory (in the mass-weighted coordinates) from the saddle point to configurations of reactants and products. 

The intrinsic reaction coordinate for neutral, base and acid catalyzed hydrolyses of formamide with a single H2O, HjO and OH reactant has been determined at MP2(full)/6-3 lG(d,p)//4-31G level [83]. Of the four pairs of reactants or protonated intermediates, N-protonated formamide is the most easily hydrolyzed with the calculated barrier for the breaking of the C-N bond being 6 Kcal-mol. Nucleophilic addition to the carbonyl carbon is barrierless for the reaction with hydroxide ion, but the resulting stable intermediate must overcome a barrier of 19 Kcal mol for scission of the C-N bond. While O-protonation is energetically favored over N-protonation by 14 Kcal-mof, nucleophilic addition to carbon in 0-protonated formamide faces a barrier of 24 Kcal-mof to yield a tetrahedral intermediate, which faces a further barrier of 16 Kcal-mof for the breaking of the C-N bond. The neutral reactants have to overcome a barrier of 44 Kcal-mof to yield products. 

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