To perform estimation, firstly, we need to construct a library, that is, a set of spectra representing individual reagents. An element of the library does not need to be a full spectrum of the reagent; it can be a spectrum cut down to regions or even one area involving a single peak. For instance, such an object can be obtained by choosing chemical shift intervals corresponding to substrates from the very first spectrum, measured when there is still a considerable amount of substrates in the reaction mixture. Similarly, for the product, it can be retrieved from the spectrum measured for t = T . An alternative approach would be to provide the spectra of reagents measured separately outside the reaction mixture, as in our previous work, but we do not use this method in the present analysis. For all the experiments described, the library was constructed by cutting spectra to regions marked in Fig. -3, i.e. the same regions as those manually integrated using Mnova software. The details are presented in Supporting Information.