title: Isotope and phase effects on the proton tautomerism in polycrystalline porphycene revealed by NMR.
abstract: Using high resolution solid state (15)N and (2)H spectroscopy and longitudinal relaxometry we have studied the tautomerism of porphycene in the solid state, corresponding to a double proton transfer in two cooperative hydrogen bonds. The tautomerism is degenerate above 225 K but the degeneracy is lifted below this temperature, indicating a phase transition. Thus, the high-temperature phase is characterized by a dynamic proton disorder and the low-temperature phase by a dynamic proton order. (15)N magnetization transfer experiments obtained under cross polarization (CP) and magic angle spinning (MAS) conditions reveal the presence of two nonequivalent molecules A and B in the unit cell of phase II, exhibiting slightly different equilibrium constants of the tautomerism. Rate constants of the tautomerism in phase I could be obtained by the analysis of the longitudinal (15)N and (2)H relaxation times. The former, obtained at 9.12 MHz, exhibit a T(1) minimum around 270 K and are consistent with proton transfer induced dipolar (1)H-(15)N relaxation mechanism. The latter, obtained at 46.03 MHz, exhibit a minimum around 330 K and arise from quadrupole relaxation. Within the margin of error, the rate constants of the HH and of the HD/DD tautomerism are the same, exhibiting a barrier of about 30 kJ mol(-1), as expected for an overbarrier reaction in a configuration with two compressed hydrogen bonds. By contrast, in the low-temperature phase a switch of the DD transfer kinetics into the nanosecond time scale is observed, exhibiting a non-Arrhenius temperature dependence which is typical for tunneling. This increase of the rate constants by lowering the temperature is discussed in terms of a switch from a concerted HH transfer in phase I to a stepwise transfer in phase II, where intermolecular interactions lower the energy of one of the cis-intermediates.
authors.affiliation: Institut für Chemie und 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: Mariusz
authors.initials: M
authors.lastname: Pietrzak
authors.forename: Gerd
authors.initials: G
authors.lastname: Buntkowsky
authors.forename: Hans-Martin
authors.initials: HM
authors.lastname: Vieth
authors.forename: Mohamed F
authors.initials: MF
authors.lastname: Shibl
authors.forename: Oliver
authors.initials: O
authors.lastname: Kühn
authors.forename: Martin
authors.initials: M
authors.lastname: Bröring
authors.forename: Hans-Heinrich
authors.initials: HH
authors.lastname: Limbach