As the singlet state is not an eigenstate of the considered Hamiltonian, zero-quantum coherences arise as a result of the transformation of Eq. ( ), and the initial polarization is redistributed in part. According to protocol 1 this occurs simultaneously with the hydrogenation reaction. Assuming that the reaction runs uniformly in time, we can average the density matrix over the bubbling period 𝜏 𝑏𝑢𝑏𝑙 :