Tautomerism in reduced pyrazinacenes

Roberto Scipioni, Mauro Boero, Gary J. Richards, Jonathan P. Hill, Takahisa Ohno, Toshiyuki Mori, Katsuhiko Arig

Research output: Contribution to journalArticlepeer-review

14 Citations (Scopus)


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.

Original languageEnglish
Pages (from-to)517-525
Number of pages9
JournalJournal of Chemical Theory and Computation
Issue number2
Publication statusPublished - 2010 Feb 9
Externally publishedYes

ASJC Scopus subject areas

  • Computer Science Applications
  • Physical and Theoretical Chemistry


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