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An
understanding of the energetics and dynamics of elementary reactions of
boron monoxide (BO) radicals with key unsaturated hydrocarbons [acetylene,
ethylene, methylacetylene allene),
diacetylene, and benzene] and the inherent formation of small boron-bearing
molecules is of considerable interest to understand boron combustion
processes, to the physical organic commu-nity due to the isoelectronic
nature of boron monoxide and the cyano radical, and to the reaction
dynamics community (reaction dynamics of transient radicals) both from the
experimental and theoretical viewpoints. Computations are conducted in
collaboration with Profs. Bartlett
(University of Florida), Mebel (FIU),
and Chang
(NDHU, Taiwan). Boron monoxide presents a crucial
transient radical in boron-based combustion processes, but the reaction
dynamics of this radical have not been investigated so far. Here, due to
its high energy density, boron has long been regarded as a good candidate
for rocket fuel additives. This interest has also been expanded recently to
ramjet and scramjet propulsion systems. The oxidation of boron is initially
unable to reach full energy release due to the formation of boron oxide (B2O3)
as an inert layer, which coats the non-reacted boron preventing further
reaction. Essentially, the carbon-based fuel ignites and reaches a high
enough temperature to remove the boron oxide layer, which, in turn, allows
clean boron to be accessible for the combustion phase. The very first
boron-bearing species formed in these processes is the boron oxide radical
(BO), which can either react to boron oxide (B2O3) or
with the hydrocarbon fuel to form carbon-, hydrogen-, oxygen-, and
boron-bearing molecules (CHOB molecules). Compared to
‘classical’ hydrocarbon flames, the incorporation of boron
results in a more complex, high temperature (1,800
– 4,000 K) organo boron chemistry. These considerations have led to
the development of boron-based combustion models involving detailed
experimental input parameters such as reaction products, intermediates, and
rate constants. Although the reaction dynamics of boron atoms with
hydrocarbon molecules have emerged during the recent years utilizing the
crossed molecular beam approach thus accessing the B/C/H system, dynamics
studies in the B/O/C/H system, in particular those involving reactions of
the boron monoxide radical (BO) with hydrocarbon molecules, have been
elusive to date. Therefore, a critical shortcoming of all boron-based
combustion models is the fact that elementary reactions in the B/O system
have never been coupled with those occurring in the B/C/H system. Crossed
beam studies of the reaction of boron monoxide (BO) with acetylene (C2H2)
showed that the HCCBO molecule plus atomic hydrogen can be formed.
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Boron
monoxide radicals are produced via laser ablation of boron at 266 nm and reaction
of the ablated boron atoms with carbon dioxide, which also acts as a
seeding gas. The beam is characterized by laser induced fluorescence (LIF).
Here, an LIF excitation spectrum of A
2Π – X 2Σ+
system in (0,0) vibrational band of boron monoxide
is shown. Experimental spectrum (upper curve) and best-fit simulation
(lower curve) corresponding to the rotational temperature Trot = 250 K. The double
headed appearance of the spectrum is caused by spin-orbit splitting of the
upper state.
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