8768 J. Phys. Chem. A, Vol. 105, No. 38, 2001
Wierzbicki et al.
HOMO-4 of TMDDD (extending over 3.3 Å) is less than that
of its counterpart. Other orbitals show significant delocalization
of the lone pairs in both molecules. In TMDDD, the nitrogen
lone-pairs are involved in the carbonyl π bond; substitution of
the carbonyl causes loss of planarity in computational models.
We conclude that the cagelike structure of TMDDD permits
the formation of extended delocalized bonding orbitals similar
to those seen in HMTD.
Given the softness of the peroxide bonds, an investigation
of possible thermal interconversion between the right- and left-
handed forms of HMTD at or below room temperature seemed
appropriate. Attempts to find a single transition state in which
the orientations of all three peroxide bonds reverse simulta-
neously were not successful. However, a multistep pathway
illustrated in Figure 5 was revealed.
In M-HMTD (I), all three peroxide bonds have a sinister
orientation about the N-N axis. The reversal of one peroxide
bond to the dextral orientation occurs in two steps: first, to
intermediate III via transition state II, and second, to what we
call M-iso-HMTD (V) via transition state IV. V is a stable
structure only about 0.3 kcal/mol higher in energy than the P-
and M-HMTD enantiomers (I and VII). One of the two
remaining sinister peroxide bonds of V is reversed to yield the
enantiomer of V, P-iso-HMTD (not shown), via transition state
VI. Finally, the orientation of the remaining sinister peroxide
bond is reversed to obtain P-HMTD (VII) in two steps,
mirroring the I to III and III to V steps. The barriers of about
16-17 kcal/mol suggest that thermal interconversion may occur
at room temperature.
though extended orbitals (Figure 4). Also, DFT calculations were
used to investigate the nature of the transition states that connect
the chiral forms of HMTD. We find the barriers to intercon-
version using the B3LYP/6-31+G(d) model low enough (16-
17 kcal/mol) to allow racemization at room temperature, and
we conclude that the synthesis of a single enantiomer is unlikely.
Acknowledgment. We thank the Alabama Supercomputer
Authority and Nichols Corporations for providing computer time
on Cray SV1.
Supporting Information Available: CIF files pertaining to
the HMTD and TMDDD. This material is available free of
References and Notes
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