NMR Studies of Ultrafast Intramolecular Proton Tautomerism in Crystalline and Amorphous N,N′-Diphenyl-6-aminofulvene-1-aldimine: Solid-State, Kinetic Isotope, and Tunneling Effect

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• 作者：Juan Miguel Lopez del Amo ; Uwe Langer ; Veró ; nica Torres ; Gerd Buntkowsky ; Hans-Martin Vieth ; Marta Pé ; rez-Torralba ; Dioní ; sia Sanz ; Rosa Marí ; a Claramunt ; José ; Elguero ; H
• 刊名：Journal of the American Chemical Society
• 出版年：2008
• 出版时间：July 9, 2008
• 年：2008
• 卷：130
• 期：27
• 页码：8620 - 8632
• 全文大小：453K
• 年卷期：v.130,no.27(July 9, 2008)
• ISSN：1520-5126

文摘

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.