Stopped-Flow and DFT Studies of ProtonTransfer and Isomerisation Rate Constants in Water for 5-Amino-3-imino-1,2,6,7-tetracyano-3H-pyrrolizine and Its Related Base 2-(5-Amino-3,4-dicyano-2H-pyrrol-2-ylidene)-1,1,2-tricyanoethanide:a Complete Square Scheme

A complete kinetic study of pH-dependent isomerization of species 5-amino-3-imino-1,2,6,7-tetracyano-3H- pyrrolizine (HL) and 2-(5-amino-3,4-dicyano-2H-pyrrol-2- ylidene)-1,1,2-tricyanoethanide (L'-) in the pH range from 0.5 to 13 in aqueous solution, which spans over 7 order of magnitude for the pseudo-first order rates (log k from 4.8 to 2.5) has revealed for the first time the existence of the corresponding conjugate species L- and HL'. The study has allowed the determination of the pKa values of HL and HL', as well as all the individual forward and reverse isomerization rates of the acidic (HL/HL') and basic (L-/L'-) forms. While L'- is the sole species present at pH > ca. 7, species HL and HL' establish an equilibrium favoring the former compound at low pH, the distribution being ca. 79:21 at pH < ca. 2. At pH < 2, a slow decomposition of HL occurs via the initial formation of an unobserved and unstable protonated form H2L+. Geometry optimizations of all species at the B3LYP/LANL2DZ level, including water solvation energy, afford results in qualitative agreement with experiment and indicate the structural nature of the new species L- and HL'.