"Forbidden" Four-Center Reactions: Molecular Orbital Considerations for N2 + N2 and N2 + N2+

F.M. Bickelhaupt, R. Hoffmann, R.D. Levine

Research output: Contribution to JournalArticleAcademicpeer-review


The hypothetical four-center nitrogen exchange reaction of N2 + N2 is analyzed. We show that the three level crossings accompanying the least-motion nitrogen exchange reaction occur at different points along the reaction coordinate, leading to a mechanism requiring three “singly forbidden” reaction steps. Simple MO arguments show that the loss of one electron in N2 + N2+ reduces the energy demand associated with the energetically dominating first and third level crossing, suggesting that ionization of the reaction system lowers significantly the high activation barrier. This is supported by nonlocal density functional calculations on various N4 and N4+ structures, which, however, also indicate that the barrier still remains at high energy:  the tetraazacyclobutadiene intermediate involved in the neutral reaction is 166.7 kcal/mol higher in energy than N2 + N2; the corresponding radical cation is only 52.2 kcal/mol above N2 + N2+. The DFT results also indicate that the N2 + N2+ nitrogen exchange reaction, if it occurs at all, may also proceed via a competing mechanism involving a T-shaped transition state at 102.8 kcal/mol above N2 + N2+. Suggestions for further experimental investigations emerge from this analysis.
Original languageEnglish
Pages (from-to)8255-8263
Number of pages9
JournalJournal of Physical Chemistry A
Issue number44
Publication statusPublished - 1997
Externally publishedYes


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