The possibility that aromatic nitration proceeds via the formation of an electron donor-acceptor complex and its possible evolution in a ''contact'' radical pair is discussed on the basis of ab-initio configuration interaction computations on benzene/toluene NO2+ systems. The analysis of the region of the potential energy hypersurfaces corresponding to the two reactants kept at van der Waals distances shows the existence of a conical intersection between the ground state and the first excited charge transfer singlet, leading to electron transfer from the aromatic substrate to the nitronium ion, The activated state for electron transfer (ET) might then be identified with the ''early'' transition state invoked by Olah to explain the high positional selectivity of substitution in spite of a low substrate selectivity. The retention of positional selectivity at encounter-limited rates may then be ascribed to the fact that the formation of a Wheland intermediate corresponds to a radical pair recombination and as such is spin density driven. The objection that ET is a kinetically difficult step is met, the computed upper limit of the barrier to ET being 13 kcal/mol for the toluene substrate.
On the occurrence of an electron-transfer step in aromatic nitration
PELUSO, Andrea;
1996-01-01
Abstract
The possibility that aromatic nitration proceeds via the formation of an electron donor-acceptor complex and its possible evolution in a ''contact'' radical pair is discussed on the basis of ab-initio configuration interaction computations on benzene/toluene NO2+ systems. The analysis of the region of the potential energy hypersurfaces corresponding to the two reactants kept at van der Waals distances shows the existence of a conical intersection between the ground state and the first excited charge transfer singlet, leading to electron transfer from the aromatic substrate to the nitronium ion, The activated state for electron transfer (ET) might then be identified with the ''early'' transition state invoked by Olah to explain the high positional selectivity of substitution in spite of a low substrate selectivity. The retention of positional selectivity at encounter-limited rates may then be ascribed to the fact that the formation of a Wheland intermediate corresponds to a radical pair recombination and as such is spin density driven. The objection that ET is a kinetically difficult step is met, the computed upper limit of the barrier to ET being 13 kcal/mol for the toluene substrate.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
