Stopped-Flow and DFT Studies of Proton Transfer and Isomerization of 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 in Water: A Completely Sorted out Square Scheme
Résumé
A complete kinetic study of pH-dependent isomerization of 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 orders 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 acid dissociation constants of HL (pKa) and HL‘ (pKa‘), as well as all of the individual forward and reverse isomerization rates of the acidic (HL/HL‘) and basic (L-/L‘-) forms. Although L‘- is the sole species present at pH > ca. 7, species HL and HL‘ establish an equilibrium favoring the former compound at low pH, with the distribution being ca. 79:21 at pH less than ca. 2. A slow decomposition of HL occurs at low pH 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 the experiment and indicate the structural nature of the new species L- and HL‘.
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