Dynamics of ring cleavage and substitution in the reactive scattering of O(3P) atoms with C2H4S and C4H4S molecules

Article abstract of DOI:10.1021/jp962692c

Reactive scattering of ground state O(³P) atoms with C₂H₄S and C₄H₄S molecules has been studied at an initial translational energy E ～ 40 kJ mol^-1 using a supersonic beam of O atoms seeded in He buffer gas generated from a microwave discharge source. The center-of-mass angular distribution of SO scattering for O + C₂H₄S is cone-shaped in the backward hemisphere at a scattering angle θ ～ 120° with respect to the incident O atom direction, while the OC₄H₃S scattering for O + C₄H₄S is nominally isotropic. The O + C₂H₄S reaction disposes a lower fraction (f′ ～ 0.3) of the total available energy into product translation than the O + C₄H₄S reaction (f′ ～ 0.4), although the product translational energies are higher by a factor of ～2 for the more exoergic O + C₂H₄S reaction. The O + C₂H₄S reaction involves cleavage of the three-membered ring of the thiirane molecule, while the O + C₄H₄S reaction involves H atom displacement from the five-membered ring of the thiophene molecule. Both reactions occur on the triplet potential energy surface. The strongly exoergic concerted rupture of the three-membered thiirane ring disposes half the available energy into vibrational excitation of the ethene product molecule compared with only a fraction (～0.13) being disposed into SO product vibration. The displacement reaction of the five-membered thiophene ring involves O atom addition forming a persistent OC₄H₄S complex with a potential energy barrier to H atom displacement.

Full text of DOI:10.1021/jp962692c

J. Phys. Chem. A 1997, 101, 187-191
187
Dynamics of Ring Cleavage and Substitution in the Reactive Scattering of O(3P) Atoms with
C2H4S and C4H4S Molecules
X. Gao, M. P. Hall, D. J. Smith, and R. Grice*
Chemistry Department, UniVersity of Manchester, Manchester M13 9PL, U.K.
ReceiVed: September 3, 1996; In Final Form: October 16, 1996X
Reactive scattering of ground state O(3P) atoms with C2H4S and C4H4S molecules has been studied at an
initial translational energy E ∼ 40 kJ mol-1 using a supersonic beam of O atoms seeded in He buffer gas
generated from a microwave discharge source. The center-of-mass angular distribution of SO scattering for
O + C2H4S is cone-shaped in the backward hemisphere at a scattering angle θ ∼ 120° with respect to the
incident O atom direction, while the OC4H3S scattering for O + C4H4S is nominally isotropic. The O +
C2H4S reaction disposes a lower fraction (f ′ ∼ 0.3) of the total available energy into product translation than
the O + C4H4S reaction (f ′ ∼ 0.4), although the product translational energies are higher by a factor of ∼2
for the more exoergic O + C2H4S reaction. The O + C2H4S reaction involves cleavage of the three-membered
ring of the thiirane molecule, while the O + C4H4S reaction involves H atom displacement from the five-
membered ring of the thiophene molecule. Both reactions occur on the triplet potential energy surface. The
strongly exoergic concerted rupture of the three-membered thiirane ring disposes half the available energy
into vibrational excitation of the ethene product molecule compared with only a fraction (∼0.13) being disposed
into SO product vibration. The displacement reaction of the five-membered thiophene ring involves O atom
addition forming a persistent OC4H4S complex with a potential energy barrier to H atom displacement.
Introduction
Experimental Section
The nascent vibrational state distribution for SO(3Σ-) products
in the reaction of ground state O(3P) atoms with thiirane (C2H4S)
molecules has recently been determined in a laser pump-probe
experiment1 using laser-induced fluorescence detection. An
inverted vibrational distribution was observed, and a direct S
atom abstraction mechanism was proposed in accord with earlier
speculations based on O atom resonance fluorescence measure-
ments,2 which yield a rate constant k ) 7.2 × 109 dm3 mol-1
s-1 independent of temperature.
The apparatus was the same as that previously employed in
studies8,9 of O(3P) atoms with alkyl iodide molecules.
A
supersonic beam of O atoms seeded in He buffer gas was
produced from a high-pressure microwave discharge source.10
The thiirane and thiophene molecule beams issued from a glass
nozzle of diameter ∼0.25 mm using stagnation pressures ∼100
mbar maintained by a reservoir at ∼-15 and 25 °C. The
velocity distributions of the O atom beam measured by a beam
monitor quadrupole mass spectrometer and the molecule beams
by the rotatable mass spectrometer detector using cross-
correlation time-of-flight analysis11 yield the peak velocities Vpk,
full widths at half-maximum intensity Vwd, and Mach numbers
M quoted in Table 1. No evidence was found for any significant
dimerization of molecules in the reactant beams when monitor-
ing their mass spectra with the rotatable detector.
O(3P) + C2H4S f SO(3Σ-) + C2H4
(1)
The rate constant for the reaction of O(3P) atoms with thiophene
(C4H4S) molecules has been measured in a discharge flow
resonance fluorescence experiment,3 giving a preexponential
factor A ) 2.0 × 1010 dm3 mol-1 s-1 and an activation energy
Ea ) 9.4 kJ mol-1. On the basis of an observed break in the
Arrhenius plot for these measurements, it was suggested2 that
two reaction mechanisms corresponding to O atom addition to
the S atom and the CdC double bond of the thiophene ring
may be operative. However, the five-membered thiophene ring
with partial aromatic character4 is more stable than the strained
three-membered thiirane ring, and an H atom substitution
pathway analogous to that observed5,6 for O atoms with benzene
molecules might be anticipated
Results
Angular distribution measurements of SO reactive scattering
from thiirane molecules yield ∼35 counts s-1 against a
background ∼250 counts s-1, while OC4H3S reactive scattering
from thiophene molecules yields ∼40 counts s-1 against a
background ∼12 counts s-1. The laboratory angular distribution
of SO scattering in Figure 1 extends over a wide angular range
peaking close to the thiirane beam, while the OC4H3S scattering
in Figure 2 is confined close to the laboratory centroid
distribution shown by a broken curve. The number densities
of SO scattering were measured by counting selectively in those
short-time channels which contribute significantly to the SO
reactive scattering, in order to minimize interference from
background signals at low velocity arising from elastic scattering
of thiirane molecules. The laboratory velocity distributions of
SO flux in Figure 3 and OC4H3S in Figure 4 were measured
using integration times ∼3000 and ∼600 s, respectively, to gain
signal-to-noise ratios ∼11 and ∼14 at the peaks of the
distributions. Kinematic analysis of these data was performed
O(3P) + C4H4S f OC4H3S + H
(2)
Reactive scattering measurements have been undertaken to
investigate the dynamics of ring cleavage and the extent to which
reaction proceeds via the triplet potential energy surface7 rather
than undergoing intersystem crossing6 to the underlying singlet
potential energy surface.
X Abstract published in AdVance ACS Abstracts, December 15, 1996.
S1089-5639(96)02692-8 CCC: $14.00 © 1997 American Chemical Society

188 J. Phys. Chem. A, Vol. 101, No. 2, 1997
Gao et al.
distribution for SO scattering peaks at a fraction f ′ ∼ 0.2 of
the total available energy but disposes much higher energy into
translation than the OC4H3S scattering which peaks at a higher
fraction f ′ ∼ 0.4 of the much lower energy available to reaction
products in this case. The form of the product translational
energy distribution is also more poorly resolved for the O +
C4H4S reaction than for the O + C2H4S reaction. The peak
Ep′k and average E′ product translational energies are listed in
av
Table 2 together with the initial translational energies E and
reaction exoergicities ∆D0, calculated for eq 1 from heats of
formation given by Pedley et al.13 and for eq 2 being equated
to that of the benzene reaction.5
No evidence was found for H atom displacement occurring
in the thiirane reaction or for S atom abstraction occurring in
the thiophene reaction. However, S atom abstraction from
thiophene involves the formation of the very weakly bound
cyclobuta-1,3-diene molecule
Figure 1. Laboratory angular distribution (number density) of SO
reactive scattering from O + C2H4S at an initial translational energy E
∼ 41 kJ mol-1. Solid line shows the fit of the stochastic kinematic
analysis, and the broken line shows the distribution of laboratory
centroids.
O(3P) + C4H4S f SO(3Σ-) + C4H4
(4)
and is estimated13 to be strongly endoergic, ∆D0 ) -76 kJ
mol-1. Hence, the hypothetical pathway of eq 4 is energetically
inaccessible in these experiments.
Discussion
The approach of the O(3P) atom to the S atom of the thiirane
molecule involves overlap of the O atom p orbital with the lone
pair orbital of the S atom, corresponding to a pyramidal
configuration14 about the S atom as shown in Figure 7. On the
3
triplet A′′ potential energy surface, repulsion will be invoked
in both the C-S bonds as the π orbitals of the C2H4 moiety
overlap with the π orbitals of the SO moiety, leading to the
formation of C2H4 and SO(3Σ-) reaction products. However,
on the singlet 1A′ potential energy surface, the pyramidal
configuration15 corresponds to the thiirane 1-oxide molecule,
which is estimated16,17 to be stable by ∼90 kJ mol-1 with respect
to reaction products. Consequently, the triplet potential energy
surface which declines steeply in the exit valley is underlain
by the singlet potential energy surface which correlates with
the electronically excited singlet states of the reactant O(1D)
atom and the SO(1∆) product molecule.18 Hence, the triplet
potential energy surface is intersected by the singlet potential
energy surface in both the entrance and exit valleys as shown
in Figure 8. Indeed, infrared multiphoton dissociation of thiirane
1-oxide molecules follows a spin-forbidden pathway16,17 to form
the SO(3Σ-) product over the potential energy barrier shown in
Figure 8, corresponding19 to an activation energy Ea ∼ 150 kJ
Figure 2. Laboratory angular distribution of OC4H3S reactive scattering
from O + C4H4S at an initial translational energy E ∼ 37 kJ mol-1
.
TABLE 1: Beam Velocity Distributions, Peak Velocity Wpk,
Full Width at Half-Maximum Intensity Wwd, and Mach
Number M
beam
Vpk/m s-1
V
wd/m s-1
M
O(He)
C2H4S
O(He)
C4H4S
2450
800
2250
650
680
430
610
270
6.5
3.0
6.5
4.5
mol-1
.
This situation is illustrated by the molecular orbital diagram
for the thiirane ring of the O + C2H4S transition state shown in
Figure 7. Bonding is assumed to arise from the overlap of the
antibonding π* orbitals of the SO radical which are mainly
confined to the S atom and the C atom π orbitals of the C2H4
moiety. In the case of the singlet thiirane 1-oxide molecule
only the lowest two molecular orbitals are occupied, 1a′21a′′2,
resulting in two electron pair S-C bonds and no net C-C π
bonding. However the triplet transition state promotes one
electron to the higher 2a′ orbital 1a′21a′′12a′1, resulting in a
reduction of the C-S bonding by one half and the creation of
C-C π bonding of equal notional strength. Consequently, the
concerted rupture of the thiirane ring is expected to involve
simultaneous weakening of both C-S bonds and strengthening
of the C-C π bonding. The reaction of O atoms with thiirane
molecules results overall in the formation of a new π bond of
the C2H4 molecule as well as the bonding of the SO(3Σ-)
using the stochastic method12 with the differential cross sections
expressed as a product of an angular function T(θ) and a velocity
function U(u)
Icm(θ,u) ) T(θ) U(u)
(3)
The resulting angular and product translational energy distribu-
tions P(E′) are shown for the SO reactive scattering in Figure
5 and OC4H3S scattering in Figure 6. The SO angular
distribution for O atoms with thiirane molecules shows conical
scattering in the backward hemisphere, while the OC4H3S
angular distribution for O atoms with thiophene molecules is
nominally isotropic. The adverse kinematics of the O + C4H4S
reaction provide only poor resolution of the form of the center-
of-mass angular distribution which cannot be distinguished from
an isotropic distribution. The product translational energy

Reactive Scattering of O(3P) with C2H4S and C4H4S
J. Phys. Chem. A, Vol. 101, No. 2, 1997 189
Figure 3. Laboratory velocity distributions (flux density) of reactively scattered SO from O + C2H4S at an initial translational energy E ∼ 41 kJ
mol-1. The solid line shows the fit of the stochastic kinematic analysis.
Figure 4. Laboratory velocity distributions of reactively scattered
OC4H3S from O + C4H4S at an initial translational energy E ∼ 37 kJ
mol-1
.
molecule. Both the ground 1a′21a′′2 and first excited
1a′21a′′12a′1 singlet states correlate with the electronically
excited SO(1∆) state.
Figure 5. Angular function T(θ) and translational energy distribution
P(E′) for O + C2H4S at an initial translational energy E ∼ 41 kJ mol-1
.
The observed SO reactive scattering corresponds to direct
dynamics over the triplet potential energy surface arising from
collisions at small impact parameters with repulsion between
the reaction products,7 giving SO(3Σ-) molecules recoiling in
the backward hemisphere. The bent configuration of the
transition state on the triplet potential energy surface as shown
in Figure 7 results in the SO angular distribution peaking14 away
from the backward direction θ ) 180°, and repulsion between
reaction products results in a peak product translational energy
Ep′k ∼ 66 kJ mol-1. However, the average product transla-
tional energy corresponds to only a fraction f ′ ∼ 0.31 of the
total energy available to reaction products, while the laser-
induced fluorescence measurements1 show only a fraction
(∼0.16) being disposed into vibrational and rotational excitation
The dashed energy curve shows the distribution of initial translational
energy.
of the SO(3Σ-) product molecules. Hence, a major fraction
(∼0.5) of the total available energy must be disposed into
vibrational excitation of the C2H4 product molecules. This
corresponds to structural changes of the C2H4 moiety with the
H atoms moving from tetrahedral to planar configurations about
the C atoms and a contraction ∼0.15 Å in the C-C bond length,
as the three-membered ring of the thiirane reactant molecule
changes to the CdC double bond of the ethene product
molecule. The inverted SO(3Σ-) vibrational state distribution1
and high vibrational excitation of C2H4 products suggest a
concerted rupture of both C-S bonds of the three-membered

190 J. Phys. Chem. A, Vol. 101, No. 2, 1997
Gao et al.
Figure 7. Reaction intermediates on the triplet potential energy surfaces
for the O(3P) + C2H4S and C4H4S reactions. Schematic molecular
orbital diagram for the thiirane ring of the O + C2H4S transition state.
Orbital symmetries relate to reflection in the plane containing the O
and S atoms and the center of the C2H4 moiety.
Figure 6. Angular function T(θ) and translational energy distribution
P(E′) for O + C4H4S at an initial translational energy E ∼ 37 kJ mol-1
.
TABLE 2: Reaction Energetics (kJ mol-1), Initial
Translational Energy E, Peak Product Translational Energy
Ep′ k, Average Product Translational Energy E′av, and
Reaction Exoergicity ∆D0
reaction
E
E′pk
E′av
∆D0
O + C2H4S f SO + C2H4
O + C4H4S f OC4H3S + H
41
37
66
40
96
45
265 ( 10
67 ( 10
thiirane ring leading to a rapid recoil of the reaction products,
as indicated by the backward scattered SO angular distribution.
The mass spectrometric measurements of the present experi-
ments do not distinguish between the SO(3Σ-) ground state and
the SO(1∆) electronically excited state of the SO reaction
product. The low excitation energy Te ) 70 kJ mol-1 of the
SO(1∆) state18 implies that only the high-energy tail of the
product translational energy distribution of Figure 5 can be
identified with SO(3Σ-) on energetic grounds. However,
intersystem crossing to the singlet potential energy surface would
involve the formation of the stable thiirane 1-oxide molecule
and result in a longer-lived collision intermediate than is
indicated by the direct dynamics observed in these experiments.
Thus, reaction appears to be confined to the triplet potential
energy surface with little evidence for intersystem crossing to
the underlying singlet potential energy surface.
The SO reactive scattering from O + C2H4S closely resembles
that from the O + OCS reaction,7 where the SO product has a
conical angular distribution in the forward hemisphere peaking
at θpk ∼ 70°, but there is very little vibrational excitation of
the CO product molecule. DIPR model calculations7 indicate
that the SO angular distribution arises from broadside addition
of the O atom to the CdS double bond of the OCS molecule.
The more backward peaked angular distribution of the O +
C2H4S reaction in Figure 5 may be attributed to a less strongly
bent addition of the O atom to the electron lone pair orbital of
the S atom of the pyramidal OSC2H4 transition state of Figure
7. The vibrational excitation of the C2H4 product molecule
Figure 8. Potential energy profiles for the 3A′′ triplet and 1A′ singlet
potential energy surfaces of the O(3P) + C2H4S reaction. Symmetry
species refer to reflection of the electronic wave function in the plane
containing the SO bond and the center of the C2H4 moiety.
arises from the C-C π bond formation during rupture of the
C2H4S ring, in contrast to the CO bonding of OCS which
undergoes little change, with the CO bond length decreasing
by only ∼0.03 Å in forming the CO product molecule.
In contrast to the thiirane reaction, addition of the O(3P) atom
to the CdC double bond of the thiophene molecule results in
the formation of a stable triplet OC4H4S radical as shown in
Figure 7. This constitutes a persistent collision complex with
a lifetime longer than its rotational period before the H atom
adjacent to the O atom becomes displaced to form the reaction
products. The product translational energy distribution of Figure
6 indicates that the H atom displacement involves a potential
energy barrier, which disposes a substantial fraction of the total
available energy into product translation. Hence, the thiophene
reaction follows a similar mechanism to that of the benzene
molecule,5,6 but no evidence was found for the formation of a
stable OC4H4S adduct following intersystem crossing to the
underlying singlet potential energy surface, as has been ob-
served5 for the benzene reaction. Hence, the thiirane and

Reactive Scattering of O(3P) with C2H4S and C4H4S
J. Phys. Chem. A, Vol. 101, No. 2, 1997 191
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Acknowledgment. Support of this work by EPSRC and the
European Commission is gratefully acknowledged.
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