Abstract:A standard low-pressure limit Rice–Ramsperber–Kassel–Marcus rate constant is shown to significantly underestimate, by factors of three or more, the measured thermal dissociation rates for HCCH and HCN if the correct value of the bond-dissociation energy is used. An explanation for this discrepancy is sought by examining anharmonic effects due to isomerization. Classical expressions for the density of states and partition function are developed which include isomerization anharmonicity and can be substituted in… Show more
“… 8 Thermal rate coefficient for the Ar + HCN → Ar + H + CN reaction. Theoretical results: , this work ( k 1 + k 3 ); ···, this work ( k 3 ); --- and − − −, ref . Note that the calculated k 1 from the present work (which is shown by the long-dashed line), is nearly indistinguishable from the total rate constant (solid line).…”
Section: Dynamics Of the Reaction Ar + Hcn → Ar + H +
Cnmentioning
confidence: 57%
“…Note that the calculated k 1 from the present work (which is shown by the long-dashed line), is nearly indistinguishable from the total rate constant (solid line). Experimental data: (□) Tabayashi et al; (▵) Roth and Just; (○) Szekely et al; (◊) Szekely et al (as cited in refs and ); (---) recommended, ref . …”
Section: Dynamics Of the Reaction Ar + Hcn → Ar + H +
Cnmentioning
A dynamics study of the reaction Ar + HCN f Ar + H + CN for a wide range of initial vibrational and translational energies is reported. All calculations have been carried out with the quasiclassical trajectory method and a realistic potential energy surface for ArHCN. An attempt is made to reproduce the thermal rate coefficient for the reaction. Agreement with experiment is found to be good, and the limitations of the approach are stressed. A brief analysis of rotational effects, energy transfer, and unimolecular dissociation of highly excited HCN* molecules is also presented.
“… 8 Thermal rate coefficient for the Ar + HCN → Ar + H + CN reaction. Theoretical results: , this work ( k 1 + k 3 ); ···, this work ( k 3 ); --- and − − −, ref . Note that the calculated k 1 from the present work (which is shown by the long-dashed line), is nearly indistinguishable from the total rate constant (solid line).…”
Section: Dynamics Of the Reaction Ar + Hcn → Ar + H +
Cnmentioning
confidence: 57%
“…Note that the calculated k 1 from the present work (which is shown by the long-dashed line), is nearly indistinguishable from the total rate constant (solid line). Experimental data: (□) Tabayashi et al; (▵) Roth and Just; (○) Szekely et al; (◊) Szekely et al (as cited in refs and ); (---) recommended, ref . …”
Section: Dynamics Of the Reaction Ar + Hcn → Ar + H +
Cnmentioning
A dynamics study of the reaction Ar + HCN f Ar + H + CN for a wide range of initial vibrational and translational energies is reported. All calculations have been carried out with the quasiclassical trajectory method and a realistic potential energy surface for ArHCN. An attempt is made to reproduce the thermal rate coefficient for the reaction. Agreement with experiment is found to be good, and the limitations of the approach are stressed. A brief analysis of rotational effects, energy transfer, and unimolecular dissociation of highly excited HCN* molecules is also presented.
The unimolecular dissociation of the
C3H4 isomers allene and propyne has been
examined using two
complementary shock-tube techniques: laser schlieren (LS) and
time-of-flight (TOF) mass spectrometry.
The LS experiments cover 1800−2500 K and 70−650 Torr, in 1, 2,
and 4% propyne/Kr and 1 and 2%
allene/Kr, whereas the TOF results extend from 1770 and 2081 K in 3%
allene or propyne in Ne. The
possible channels for unimolecular dissociation in the
C3H4 system of isomers are considered in
detail, using
new density functional theory calculations of the barriers for
insertion of several C3H2 into H2
to evaluate the
possibility of H2 elimination as a dissociation route.
The dominant path clearly remains CH fission, from
either isomer, as suggested in earlier work, although some small amount
of H2 elimination may be possible
from allene. Rate constants for the CH fission of both allene and
propyne were obtained by the usual model-assisted extrapolation of LS profiles to zero time using an extensive
mechanism constructed to be consistent
with both the time variation of LS gradients and the TOF product
profiles. This procedure then provides rate
constants effectively independent of both the near-thermoneutral
isomerization of the allene/propyne and of
secondary chain reactions. Derived rate constants show a strong,
persistent pressure dependence, i.e., a quite
unexpected deviation (falloff) from second-order behavior. These
rate constants are nearer first than second
order even for T > 2000 K. They are also anomalously
large; RRKM rates using literature barriers and
routine energy-transfer parameters are almost an order of magnitude too
slow. The two isomers show slightly
differing rates, and falloff is slightly less in allene. It is
suggested that isomerization is probably slow enough
for this difference to be real. The anomalously large rates and
falloff are both consistent with an unusually
large low-pressure-limit rate in this system. Extensive
isomerization of these C3H4 is possible for
energies
well below their CH fission barriers, and this can become hindered
internal rotation in the activated molecule.
On the C3H4 surface we identify three such
accessible rotors. State densities for the molecule
including
these rotors are calculated using a previous general classical
formulation. Insertion of these state densities
into the RRKM model results in rates quite close to the measured
magnitudes, and showing much of the
observed falloff. The increase in the low-pressure rate is as much
as a factor of 8; a necessary but nonetheless
remarkable effect of anharmonicity on the unimolecular rate. This
again demonstrates the importance of
accessible isomerization and consequent hindered internal rotation on
the rate of dissociation of unsaturated
species.
“…The new methylvinoxy spectra also begin to address the unresolved issue of the barrier to internal rotation of a methyl group near a radical center. Better intuition about methyl rotor behavior is needed to build sensible statistical models of the unimolecular reactions of hot organic radicals in combustion systems . The density of rotation−torsion−vibrational states that enters statistical rate theory depends sensitively on whether each methyl rotor is free or experiences a substantial hindering potential.…”
Section: Introductionmentioning
confidence: 99%
“…Better intuition about methyl rotor behavior is needed to build sensible statistical models of the unimolecular reactions of hot organic radicals in combustion systems. 15 The density of rotation-torsion-vibrational states that enters statistical rate theory depends sensitively on whether each methyl rotor is free or experiences a substantial hindering potential. The same types of barriers are also important in mechanistic organic chemistry.…”
Jet-cooled laser-induced fluorescence spectra of the B̃ ←
X̃ electronic transition of 1-methylvinoxy and of a
cis/trans mixture of 2-methylvinoxy are presented. We observe some
50 vibronic bands in the spectrum of
1-methylvinoxy. The complexity is due to a change in the preferred
methyl rotor orientation on electronic
excitation. The B̃-state barrier to internal rotation is
approximately 750 cm-1. The
B̃-state fluorescence
lifetimes decrease from about 130 ns near the origin to 26 ns for
internal energy of 2700 cm-1. We
observe
some 22 vibronic bands in the spectrum of the cis/trans mixture of
2-methylvinoxy. The B̃-state fluorescence
lifetimes decrease gradually from about 190 ns near the origin to 140
ns for internal energy of 1170 cm-1
and
then very sharply at higher energy. The new spectra serve as
fingerprints for identification of specific
methylvinoxy isomers as products of chemical reactions of
O(3P) with alkenes, as recently observed by
Bersohn
and co-workers.
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