title: 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.
abstract: 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. (15)N cross-polarization magic-angle-spinning NMR experiments using (1)H decoupling performed on polycrystalline I- (15)N 2 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 (15)N and (2)H relaxation times of I- (15)N 2, I- (15)N 2- d 10, and I- (15)N 2- d 1 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.
authors.affiliation: Institut für Chemie and Biochemie, Takustrasse 3, Freie Universität Berlin, D-14195 Berlin, Germany.
authors.forename: Juan Miguel
authors.initials: JM
authors.lastname: Lopez del Amo
authors.forename: Uwe
authors.initials: U
authors.lastname: Langer
authors.forename: Verónica
authors.initials: V
authors.lastname: Torres
authors.forename: Gerd
authors.initials: G
authors.lastname: Buntkowsky
authors.forename: Hans-Martin
authors.initials: HM
authors.lastname: Vieth
authors.forename: Marta
authors.initials: M
authors.lastname: Pérez-Torralba
authors.forename: Dionísia
authors.initials: D
authors.lastname: Sanz
authors.forename: Rosa María
authors.initials: RM
authors.lastname: Claramunt
authors.forename: José
authors.initials: J
authors.lastname: Elguero
authors.forename: Hans-Heinrich
authors.initials: HH
authors.lastname: Limbach