ELSEVIER
Journal of Analytical and Applied Pyrolysis
31 (1995) 169 175
JOURNALOI
ANALYTICALand
APPLIED PYROLYSIS
Study of the bimolecular pyrolysis of acetic acid by the
Austin Model 1 semi-empirical method
Abstract
A bimolecular
mechanism
for the dehydration
reactions
involved
in the pyrolysis of
aliphatic acids of low molecular
weight, e.g. acetic acid, was investigated theoretically by the
semi-empirical
AMI (Austin Model I) method. The AM I results provide an acceptable
description
of the energy-related
and structural
aspects of the processes involved in the
pyrolysis of acetic acid.
K~woI.L~.s: Acetic acid; Austin
Model 1 method: Pyrolysis
1. Introduction
The dehydration
reaction that yields water and ketene, and the decarboxylation
reaction that gives carbon dioxide and methane as products
in the pyrolysis of
acetic acid have been the subject of much kinetic study [I] over different temperature ranges. both in static vessels and flow systems, and by the use of shock tube
techniques.
As a rule, establishing
a molecular
reaction mechanism
from kinetic
data for this type of system is rather difficult owing to the concurrence
of processes
with similar activation
parameters.
Ideas and methods
of theoretical
chemistry
are very useful to avoid these
problems. Thus, the ab initio calculations
made by Ruelle [2] allowed the mechanism of both processes at a high temperature
to be elucidated. Theoretical
four-center transition
states in each unimolecular
process were determined,
as was a
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170
J.J. Quirunte
/J. And. Appl. Pyrolysis 31 (1995) 169-175
six-membered
ring transition
state for the decarboxylation
reaction catalyzed by a
water molecule formed in the parallel process, when the experimental
device was a
static vessel; the description
thus obtained was consistent with available experimental results.
More recently [3], the semi-empirical
AM1 method [4] was found to provide a
qualitative
description
consistent
with that of the best ab initio calculations
obtained by Ruelle and much better than those of its precursors (e.g. the MIND0/3
[ 51 and MNDO [6] methods).
In this work, the AM1 method was used to investigate
the mechanism
for the
bimolecular
dehydration
of acetic acid involving the formation
of acetic anhydride
as the rate-determining
step, which had never been considered in theoretical studies
reported to date, even though the mechanism
was put forward as early as 1968 by
Blake and Jackson [7] to account for kinetic data available for temperatures
below
600°C (flow; 460-600°C;
Kdeh = 109.52exp( -39.2
kcal moll’/RT)
SK’. At such
temperatures,
the acetic anhydride formed rapidly decomposes to ketene and acetic
acid.
2. Computational
method
Computations
were done using the micro-VAX
3800 implementation
of version
6.0 of the MOPAC (molecular
orbital package) semi-empirical
molecular
orbital
software package [8]. The key word “precise” was always included in the input data
files to increase the default convergence
criteria by a factor of (normally)
100.
The geometry of stationary
points (minima and saddle points) on the potential
energy hypersurface
was refined within the entire coordinate
space by minimizing
the Euclidean norm of the gradient energy [9] down to below 0.1 kcal mall’ A, and
then characterized
by calculating the associated force constants. All geometries were
optimized without any assumptions.
The electrostatic potential has been frequently used in the last few years to obtain
a static picture of some aspects of molecular
reactivity.
In order to derive a
reasonable
approximation
to the geometry of the transition
state for the bimolecular process as the starting point for direct potential hypersurface
search numerical
methods we calculated
and represented
the AMI electrostatic
potential
for the
acetic acid molecule in a suitable conformation,
by using the VSS program [lo]
within the molecular modelling software package CHEMX [ 1 I].
3. Results and discussion
The form of the electrostatic
potential
map obtained
for acetic acid (Fig. 1)
suggested a geometry of approach of the two molecules by which the positive zone
around one molecule approaches
the negative zone of the other in such a way that
the oxycarbonated
skeletons of both lie in the same plane. In this manner, after the
two molecules were approximated
to the reaction distance, the gradient norm was
J.J. Quirmte
Fig. 1. AMI
IO kcal mol-‘.
VSS
electrostatic
/ J. And. Appl. Pwol~xi?; 31 (1995) 169- I75
potential
for
acetic
acid:
(a) -- IO: (b) -5: (c) 0: (d)
171
and
minimized
and a stationary
point whose geometric features are shown in Table 1
was obtained. Other approach geometries for the two acetic acid molecules such as
those involving normal or parallel advance were tested without success.
An analysis of the sign distribution
for the eigenvalues
in the force constant
matrix at the stationary
point obtained confirmed it to be a first-order saddle point.
i.e. a transition
state (TS). Also, the representation
of the transition
vector (Fig. 2)
allowed such a TS to be assigned to the process by which an acetic anhydride
molecule is formed as a water molecule is eliminated.
Specifically,
the atomic
components
of the vector are indicative of the following shifts: the H,-0,.
CqPO,
and C, ,-O,,, intermolecular
distances are lengthened,
whereas the C,-0,.
C,, -0,
and O,,,-H,
distances are shortened.
Also, the leaving H,O molecule undergoes
simultaneous
rotation
and out-of-the-plane
bending. Therefore,
in the calculated
transition
state, one of the acetic acid molecules gradually transfers its acid proton
( H, ) to the hydroxyl group of the other, whose bond to C,, weakens progressively.
Simultaneously,
the oxygen atom in the carbonyl
group of the former molecule
(0,) gradually bonds to C,, . This latter atom (C,, ) possesses a virtually tetrahedral
structure at that point in the reaction coordinate,
even though it is an sp’ atom in
both the reactants and the products.
The calculated transition
state possesses a non-planar
six-membered
cyclic structure. The acid molecule that contributes
the -OH group to the water molecule
172
Table I
Bond lengths
J.J. Quirunte
/J.
And.
(A) and valence and dihedral
Reactants
HlLO2
HI -010
02kC3
c3-04
C3SC5
04LCll
H9%010
OIO~CI I
Cl1LOl2
CIILCI.1
02mHIL010
HlL02-C3
02-~(33-04
02SC3 -C5
04kC3
c5
c3-04-Cl
I
HI -0lOpH9
HI ~OIO~~CI I
H9SOlO~Cl
I
04 cllLolo
o4-cll~ol2
04kCll
Cl3
010.-Cl IL012
OIO-Cl ILCl3
o12pcl
I -Cl3
OIO~HlL02-C3
02-HlLOlO~H9
02ZHl-OIO-Cl
I
HlL02-C3S04
HI -02%C3 mC5
02LC3LO4LCl
I
c5~c3-04~cl
I
c3L04~c11L010
c3p04&cl
lpO12
c3m04Lcl
I -Cl3
Hl&OlO~Cl
I 04
HI-OIO-Cl
IL012
HILOIO~CIILCl3
H9%010-Cl
I-04
H9%010-Cl
I -012
H9%010-Cl
I -Cl3
0.971
%
1.364
1.234
1.486
0.971
I .364
I.234
I .486
109.65
116.55
114.08
129.37
109.65
116.55
114.08
129.37
0.00
180.00
0.00
180.00
Appl. Pyrol~xi.~ 31 (1995) 169- 175
angles
(deg) along
Transition
1.244
I.180
1.288
1.302
1.489
1.564
0.961
1.523
1.242
1.508
137.1 I
110.79
119.40
121.46
119.13
124.44
116.21
III.30
I I I .45
94.52
105.69
106.94
107.34
109.63
127.62
-9.02
174.29
45.28
- IO.10
169.39
~11.50
168.99
38.68
148.12
-73.43
-48.15
- 156.14
61.64
~ 179.64
72.37
-69.85
the reaction
state
coordinate
Products
I.229
1.373
1.490
I .393
0.961
I.228
I .483
I 19.47
128.73
III.80
124.45
I 10.09
121.96
127.93
-2.42
178.29
170.64
~ 10.78
formed must rotate about its carboxyl carbon atom. In the transition
state, atoms
O,“, H,, C2, C, and C, bound dihedrals that are tilted by only 10” relative to the
horizontal
plane, whereas the others (0,, C, , , 012 and C,,) delimit wider dihedrals
between themselves and the other atoms.
Q
a
I-Yg. 2. The AMI
acid. Arrows
transition
illustrate
state for the formation
the form of the normal
of acctic anhvdridc
mode which corrcspond~
from two molecules of acetic
to the reaction
coordinate.
Table 2 lists AM1 energy and electronic
parameters
obtained
throughout
the
dehydration
process. The calculated bond orders the net charges are those defined
in the MOPAC environment
[ 121. The degree of bond change was calculated from
the bond orders by using the formula of Nguyen et al. [ 131.The data in Table 2 also
reveal that the main structural changes undergone
by the system are in progress in
the transition
state, even though those affecting the parameters
for bonds H, 02,
C,--O, and O,~-C,, are at a slightly more advanced stage of change.
Activation
energies were calculated
by using the energy corresponding
to the
most stable conformation
of acetic acid as reference; consistent
with previously
reported theoretical results [2, 141, such a conformation
was the “cis-alternate”
one
(cisoidal on account of the relative position of the carbonyl group and the H atom
in the hydroxyl group, and alternate as regards the position of the methyl group
relative to the carbonyl
group). The AMI method provides quite an accurate
prediction for the energy difference between the cis and trans isomers of acetic acid
(experimental.
5.97 [15]; AMI, 5.84[3]; STO-3G, 5.11; 3 -2lG, 8.19; 6631G. X.36:
and 663lG**,
7.12 kcal molV [2]).
The AMI enthalpy of activation,
once corrected according to statistical thermodynamic criteria (zero-point
energy and the other thermal contributions).
turned
out to be 56.7 kcal mol- ’ at 500 C (i.e. somewhat higher than the experimental
value). However. such an energy is smaller than the AMI value for the four-center
monomolecular
dehydration
and decarboxylation
processes, as well as for the
six-center water-catalyzed
decarboxylation
process, so the bimolecular
mechanism
is that with the lowest activation
energy and hence that occurring at low temperatures, consistent
with experimental
data. Lee et al. [ 161 studied the gas-phase
thermal decomposition
of diacetyl compounds.
such as acetic anhydride,
by using
174
J.J. Quirunte
Table 2
AMI heat of formation,
bond
calculated at stationary
points
AHF(kca1
mol-‘)
at 298.15 K
Bond orders
HI-02
HI-010
02-C3
c3-04
c3-c5
044Cl1
H9-010
OIO~CI I
Cll-012
Cll~Cl3
( 12) and degrees
Net charges
HI
02
c3
04
c5
H9
010
Cl1
012
Cl3
( -e)
/J.
orders,
Anal. Appl. Pyrolysis
degrees
of bond
31 (1995) 169-175
evolution
Reactants
Transition
- 205.97
- 150.83
of bond evolution
0.910
0.000
1.045
1.795
0.949
state
and net changes
Products
- 190.43
( 13)
0.910
1.045
1.795
0.949
0.390
0.449
I ,416
1.326
0.955
0.547
0.899
0.608
I .664
0.916
0.243
-0.321
0.306
-0.361
-0.217
0.243
-0.321
0.306
-0.361
-0.217
0.354
-0.417
0.362
-0.327
-0.216
0.247
-0.41 I
0.406
-0.419
-0.257
0.000
(in parentheses)
(57.14)
(46.63)
(47.87)
(57.41)
(59.65)
(41.82)
0.000
0.963
1.820
0.978
0.945
0.917
0.963
0.000
I .877
0.949
0.191
-0.346
0.331
-0.301
-0.221
0.191
-0.383
0.323
-0.282
-0.241
the AM1 semi-empirical method. They found a six-membered ring TS involving the
keto form with an activation enthalpy of 52.02 kcal mol-‘. This result confirmed
that the formation of acetic anhydride is the rate-determining step of the bimolecular mechanism.
Therefore, even though the AM1 values for the activation energies of the
different processes involved in the pyrolysis of acetic acid represent an upper energy
limit [3], the AM1 method still provides a consistent qualitative description for such
processes as regards both purely mechanistic and energy-related aspects.
References
[I]
[2]
[3]
[4]
[5]
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F. Enriquez and J.M. Hernando,
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31 (1995) 169
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