2 research outputs found
Exploring the Ring-Opening Pathways in the Reaction of Morpholinyl Radicals with Oxygen Molecule
Quantum chemistry calculations using hybrid density functional
theory and the coupled-cluster method have been performed to investigate
the ring-opening pathways in the oxidation of morpholine (1-oxa-4-aza-cyclohexane).
Hydrogen abstraction can form two different carbon-centered radicals,
morpholin-2-yl or morpholin-3-yl, or the nitrogen-centered radical,
morpholin-4-yl, none of which are found to have low-energy pathways
to ring-opening. Extensive exploration of multiple reaction pathways
following molecular oxygen addition to these three radicals revealed
two competitive low energy pathways to ring-opening. Addition of O<sub>2</sub> to either carbon-centered radical, followed by a 1,4-H shifting
mechanism can yield a long-lived cyclic epoxy intermediate, susceptible
to ring-opening, following further radical attack. In particular,
the second pathway begins with O<sub>2</sub> attack on morpholin-2-yl,
followed by a 1,5-H shift and a unimolecular ring-opening without
having to overcome a high barrier, releasing a significant amount
of heat in the overall ring-opening reaction. The calculations provide
valuable context for the development of mechanisms for the low temperature
combustion chemistry of nitrogen and oxygen-containing fuels
Atomic-Scale Structure and Stability of the Low-Index Surfaces of Pyrochlore Oxides
The
multifunctional properties of complex ternary oxides such as
pyrochlores are often influenced by surface structure. Optimizing
the surface-driven attributes of these materials necessitates a detailed
understanding of the structure and chemical composition of those surfaces.
Here we report atomistic simulations elucidating the diverse atomic-scale
structures of a set of low-index surfaces [(100), (110), (111), and
(112)] in pyrochlore compounds as a function of both A and B cation
chemistry. In pyrochlores, the low-index facets are all dipolar, requiring
the introduction of surface defects to eliminate the surface dipole.
We find that, due to the corresponding higher coordination of the
surface cations, the (110) facet is the most energetically stable
in all of the compounds considered, an interesting contrast to fluorite,
in which the (111) surface is most stable. We also reveal a correlation
between the surface energy and the energy to disorder the pyrochlore
as a function of B cation chemistry, implying a similar physical origin
for the two phenomena. Further, we find that surface rumpling is common
across all pyrochlore compounds. An even more interesting feature
emerging at these surfaces is the formation of extended structural
defects such as steps and trenches, which are found to be stable after
high-temperature annealing. As the formation of these features is
a consequence of surface defects introduced to eliminate the surface
dipole, we propose that the superior surface properties of materials
of pyrochlores are due to these extended structural features, which
are a direct consequence of the inherent dipole at the surfaces