The geometries and energies of the iminopnictoranes
(H3MNH; M = P, As, Sb, and Bi) and their
reaction
paths with formaldehyde are predicted by means of ab initio
calculations. The multiplicity of MN bonds is
discussed by comparing the MN and M−H bond lengths, the bond
length ratios MN:M−N, the bond angles
of M−N−H, and the barrier to internal rotation about the MN bond
with those of the ylides, and it was concluded
that the contribution of the ionic canonical form
M+−N- is much more important than that of
the MN form.
For the two reaction routes of the iminopnictoranes with
formaldehyde (aza-Wittig and Corey−Chaykovsky-type
reactions), all of the stationary points and transition states were
fully optimized by using an analytical gradient
with the LANL1DZ and the 9s5p-d/[3s2p-d] basis set (DZ-d) at the MP2
level. For M = P and As, the aza-Wittig reaction is more favorable than the Corey−Chaykovsky-type
reaction from both the thermodynamic and
the kinetic viewpoint. In the case of M = Sb and Bi, the
Corey−Chaykovsky-type reaction takes place
predominantly. The higher level calculations, such as
MP4(SDTQ)/DZ-d//MP2/DZ-d and QCISD(T)/DZ-d//MP2/DZ-d, were performed to get accurate energies of these intermediates
and transition states.
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