@article { ISI:A1997YE04100017,
	title = {{\textquoteleft}{\textquoteright}Forbidden{\textquoteright}{\textquoteright} four-center reactions: Molecular orbital considerations for N-2+N-2 and N-2+N-2(+)},
	journal = {JOURNAL OF PHYSICAL CHEMISTRY A},
	volume = {101},
	year = {1997},
	month = {OCT 30},
	pages = {8255-8263},
	abstract = {The hypothetical four-center nitrogen exchange reaction of N-2 + N-2 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 {\textquoteleft}{\textquoteright}singly forbidden{\textquoteright}{\textquoteright} reaction steps. Simple MO arguments show that the loss of one electron in N-2 + N-2(+) 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 N-4 and N-4(+) 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 N-2 + N-2; the corresponding radical cation is only 52.2 kcal/mol above N-2 + N-2(+). The DFT results also indicate that the N-2 + N-2(+) 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 N-2 + N-2(+). Suggestions for further experimental investigations emerge from this analysis.},
	issn = {1089-5639},
	doi = {10.1021/jp971005u},
	author = {Bickelhaupt, FM and Hoffmann, R and LEVINE, RD}
}
