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Abstract: (Chemical Abstract 2001, 134: 131605)
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A theoretical investigation (AIM and ELF analyses together with
NMR chemical shifts)
has been conducted for three-membered heterocycle (N, P and As) oxides.
An aromatic stabilization was found for the P and As rings. However, the
N derivatives displayed a net negative hyperconjugation in the N-O bond formation,
without ring aromaticity observed for their electronic properties.
The calculated dC and
dH shifts also supported the ring delocalization for the
P and As unsaturated heterocycle oxides (dC ca. 165
and dH ca. 9 ppm).
In addition, these values for 1H-azirine oxide resembled standard C=C double
bond values (dC ca. 130 and
dH ca. 7 ppm).
The different behavior for the N oxides
was also observed in their Meisenheimer rearrangement (MR).
All the reaction paths, yielding the corresponding hydroxyl structures,
were exothermic (G2 method). However, the N derivatives showed the lowest
values for activation enthalpy, DH#.
The C=C bond influence in the MR was slight,
with the same DH values for the saturated and unsaturated paths.
This rearrangement for the P and As oxides yielded TSs closer to the reactives;
however, the corresponding TSs resembled the products for the N-derivatives.
The different reaction paths have been examined by their corresponding AIM and
ELF analyses at the B3LYP/6-311G* level.ding a net electron delocalization
compatible with the 3c-4e bond model.
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