NMR Studies of Ultrafast Intramolecular Proton Tautomerism in Crystalline and Amorphous N,N′-Diphenyl-6-aminofulvene-1-aldimine: Solid-State, Kinetic Isotope, and Tunneling Effects
Juan Miguel Lopez del Amo, Uwe Langer, Verónica Torres, Gerd Buntkowsky, Hans-Martin Vieth, Marta Pérez-Torralba, Dionísia Sanz, Rosa María Claramunt, José Elguero, and Hans-Heinrich Limbach*
Using solid-state NMR spectroscopy, we have detected and characterized ultrafast intramolecular proton tautomerism in the N−H−N hydrogen bonds of solid N,N′-diphenyl-6-aminofulvene-1-aldimine (I) on the microsecond-to-picosecond time scale. 15N cross-polarization magic-angle-spinning NMR experiments using 1H decoupling performed on polycrystalline I-15N2 and the related compound N-phenyl-N′-(1,3,4-triazole)-6-aminofulvene-1-aldimine (II) provided information about the thermodynamics of the tautomeric processes. We found that II forms only a single tautomer but that the gas-phase degeneracy of the two tautomers of I is lifted by solid-state interactions. Rate constants, including H/D kinetic isotope effects (KIEs), on the microsecond-to-picosecond time scale were obtained by measuring and analyzing the longitudinal 15N and 2H relaxation times of I-15N2, I-15N2-d10, and I-15N2-d1 over a wide temperature range. In addition to the microcrystalline modification, a novel amorphous modification of I was found and studied. In this modification, proton transfer is much faster than in the crystalline form. For both modifications, we observed large H/D KIEs that were temperature-dependent at high temperatures and temperature-independent at low temperatures. These findings are interpreted in terms of a simple quasiclassical tunneling model proposed by Bell and modified by Limbach. We obtained evidence that a reorganization energy is necessary in order to compress the N−H−N hydrogen bond and achieve a molecular configuration in which the barrier for H transfer is reduced and tunneling or an over-barrier reaction can occur.