TY - JOUR
T1 - "Forbidden" four-center reactions
T2 - Molecular orbital considerations for N2 + N2 and N2 + N2+
AU - Bickelhaupt, F. Matthias
AU - Hoffmann, Roald
AU - Levine, Raphael D.
PY - 1997/10/30
Y1 - 1997/10/30
N2 - 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 higherm 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 mechlism involving a T-shaped transition state at 102.8 kcaUmol above N2 + N2+. Suggestions for further experimental investigations emerge from this analysis.
AB - 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 higherm 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 mechlism involving a T-shaped transition state at 102.8 kcaUmol above N2 + N2+. Suggestions for further experimental investigations emerge from this analysis.
UR - http://www.scopus.com/inward/record.url?scp=0031250168&partnerID=8YFLogxK
U2 - 10.1021/jp971005u
DO - 10.1021/jp971005u
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AN - SCOPUS:0031250168
SN - 1089-5639
VL - 101
SP - 8255
EP - 8263
JO - Journal of Physical Chemistry A
JF - Journal of Physical Chemistry A
IS - 44
ER -