title: Tautomerism in Reduced Pyrazinacenes.
abstract: Monoprotic and diprotic NH tautomerism in reduced oligoazaacenes, the pyrazinacenes, was studied by using first principles simulations. Stepwise reductions in the metadynamics-sampled free energy profile were observed during consecutive monoprotic tautomerizations, with energy barriers gradually reducing with increasing proton separation during monoprotic processes. This is accompanied by an increasing contribution from the quinoidal electronic structure, as evidenced by the computed highest occupied molecular orbital (HOMO) structure. An unusual odd-even effect in the free energy profiles is also observed upon changing the length of the pyrazinacene. Calculated HOMO structures reveal an increasing tendency for delocalization of pyrazine lone pairs with an increasing number of ring annelations. The influence of tautomerism on the pyrazine lone pair delocalization was also observed. Tautomers with protons situated centrally on the pyrazinacene backbone are predicted to be more stable due to a combination of (enamine) delocalization and a loss of Clar sextet resonance stabilization in tautomers with protons at terminal pyrazine rings. Experimental evidence suggesting the structure of pyrazinacene tautomers is included and discussed as a support to the calculation.
authors.affiliation: WPI Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science, Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan, Max Planck Institute for Polymer Research, Mainz, Ackermannweg 10, 55124, Germany, Institut de Physique et Chimie des Matériaux de Strasbourg (IPCMS), UMR 7504 CNRS-University of Strasbourg, 23 rue du Loess, F-67034 Strasbourg, France, Computational Materials Center, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047 Japan, and Fuel Cell Materials Center, Nanoionics Group, National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki, 305-0044, Japan.
authors.forename: Roberto
authors.initials: R
authors.lastname: Scipioni
authors.affiliation: WPI Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science, Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan, Max Planck Institute for Polymer Research, Mainz, Ackermannweg 10, 55124, Germany, Institut de Physique et Chimie des Matériaux de Strasbourg (IPCMS), UMR 7504 CNRS-University of Strasbourg, 23 rue du Loess, F-67034 Strasbourg, France, Computational Materials Center, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047 Japan, and Fuel Cell Materials Center, Nanoionics Group, National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki, 305-0044, Japan.
authors.forename: Mauro
authors.initials: M
authors.lastname: Boero
authors.affiliation: WPI Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science, Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan, Max Planck Institute for Polymer Research, Mainz, Ackermannweg 10, 55124, Germany, Institut de Physique et Chimie des Matériaux de Strasbourg (IPCMS), UMR 7504 CNRS-University of Strasbourg, 23 rue du Loess, F-67034 Strasbourg, France, Computational Materials Center, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047 Japan, and Fuel Cell Materials Center, Nanoionics Group, National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki, 305-0044, Japan.
authors.forename: Gary J
authors.initials: GJ
authors.lastname: Richards
authors.affiliation: WPI Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science, Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan, Max Planck Institute for Polymer Research, Mainz, Ackermannweg 10, 55124, Germany, Institut de Physique et Chimie des Matériaux de Strasbourg (IPCMS), UMR 7504 CNRS-University of Strasbourg, 23 rue du Loess, F-67034 Strasbourg, France, Computational Materials Center, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047 Japan, and Fuel Cell Materials Center, Nanoionics Group, National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki, 305-0044, Japan.
authors.forename: Jonathan P
authors.initials: JP
authors.lastname: Hill
authors.affiliation: WPI Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science, Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan, Max Planck Institute for Polymer Research, Mainz, Ackermannweg 10, 55124, Germany, Institut de Physique et Chimie des Matériaux de Strasbourg (IPCMS), UMR 7504 CNRS-University of Strasbourg, 23 rue du Loess, F-67034 Strasbourg, France, Computational Materials Center, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047 Japan, and Fuel Cell Materials Center, Nanoionics Group, National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki, 305-0044, Japan.
authors.forename: Takahisa
authors.initials: T
authors.lastname: Ohno
authors.affiliation: WPI Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science, Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan, Max Planck Institute for Polymer Research, Mainz, Ackermannweg 10, 55124, Germany, Institut de Physique et Chimie des Matériaux de Strasbourg (IPCMS), UMR 7504 CNRS-University of Strasbourg, 23 rue du Loess, F-67034 Strasbourg, France, Computational Materials Center, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047 Japan, and Fuel Cell Materials Center, Nanoionics Group, National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki, 305-0044, Japan.
authors.forename: Toshiyuki
authors.initials: T
authors.lastname: Mori
authors.affiliation: WPI Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science, Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan, Max Planck Institute for Polymer Research, Mainz, Ackermannweg 10, 55124, Germany, Institut de Physique et Chimie des Matériaux de Strasbourg (IPCMS), UMR 7504 CNRS-University of Strasbourg, 23 rue du Loess, F-67034 Strasbourg, France, Computational Materials Center, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047 Japan, and Fuel Cell Materials Center, Nanoionics Group, National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki, 305-0044, Japan.
authors.forename: Katsuhiko
authors.initials: K
authors.lastname: Ariga