High-level ab initio calculations have been used to determine the
minimum energy structures of N,N‘-diformylhydrazine,
N-methyl-N,N‘-diformylhydrazine, and
N,N‘-dimethyl-N,N‘-diformylhydrazine.
These calculations
show that the global minimum is a nonplanar structure in which the
nitrogen lone pairs are essentially perpendicular
to one another. However, the energy required for
(Z,Z)-diformylhydrazine to adopt a planar
structure is less than 1
kcal/mol (MP2/6-31+G**). This is due to attractive
intramolecular hydrogen bonds between the N-hydrogens
and
the carbonyl oxygens in the planar geometry. When one or both
amide configurations are inverted (Z,E;
E,E), or
when the nitrogens are substituted, with methyl for example, these
hydrogen bonds are lost and the planar structure
becomes much less stable relative to the twisted rotamer. Thus, we
conclude from these calculations that
diacylhydrazines are intrinsically nonplanar with respect to the
CO−N−N−CO torsion, and that with the exception
of (Z,Z)-diformylhydrazine the rotational
barriers are large. The observation of a planar crystal structure
for
diformylhydrazine is due to additional intermolecular hydrogen bonds
which are available to planar diformylhydrazine
in the crystal lattice. Finally, these calculations have
significant implications for the structure and dynamical
properties
of nonsteroidal ecdysone agonists, azapeptides, and azatides which
incorporate the diacylhydrazine structure.