A combined experimental and theoretical study of the homogeneous, unimolecular decomposition kinetics of 3-chloropivalic acid in the gas phase

Article abstract of DOI:10.1021/jp003560t

Decomposition kinetics of 3-chloropivalic acid in the gas phase were determined in a static system over the temperature and pressure ranges of 380.5-430.1 °C and 43-120 Torr, respectively. The reaction, in vessel seasoned with allyl bromide, and in the presence of free-radical suppresser toluene, is homogeneous, unimolecular, and follows a first-order rate law. The rate coefficients are given by the following equation: log k₁ (s^-1) = (12.42 ± 0.36) - (205.8 ± 4.7) kJ mol^-1(2.303RT)^-1. The reaction mechanism for the formation of isobutene, hydrogen chloride, and carbon dioxide has been theoretically characterized. The theoretical study, at MP2/6-31G** computational level, points out that the molecular mechanism corresponds to a concerted and highly synchronous process yielding the products. An analysis of bond orders and NBO charges shows that the polarization of the C-Cl breaking bond can be considered the driving force for this fragmentation process. The rate coefficients obtained from experimental data and theoretical calculations are in good agreement.

Full text of DOI:10.1021/jp003560t

J. Phys. Chem. A 2001, 105, 1869-1875
1869
A Combined Experimental and Theoretical Study of the Homogeneous, Unimolecular
Decomposition Kinetics of 3-Chloropivalic Acid in the Gas Phase
Gabriel Chuchani,† Alexandra Rotinov,† Juan Andre´s,*,‡ Lu´ıs R. Domingo,§ and
V. Sixte Safont‡
Centro de Qu´ımica, Instituto Venezolano de InVestigaciones Cient´ıficas (I.V.I.C.), Box 21827, Caracas 1020-A,
Venezuela, Departament de Cie`ncies Experimentals, UniVersitat Jaume I, Box 224, 12080 Castello´, Spain, and
Departament de Qu´ımica Orga`nica, UniVersitat de Vale`ncia, Dr. Moliner 50, 46100 Burjassot, Vale`ncia, Spain
ReceiVed: September 29, 2000; In Final Form: December 7, 2000
Decomposition kinetics of 3-chloropivalic acid in the gas phase were determined in a static system over the
temperature and pressure ranges of 380.5-430.1 °C and 43-120 Torr, respectively. The reaction, in vessel
seasoned with allyl bromide, and in the presence of free-radical suppresser toluene, is homogeneous,
unimolecular, and follows a first-order rate law. The rate coefficients are given by the following equation:
log k1 (s-1) ) (12.42 ( 0.36) - (205.8 ( 4.7) kJ mol-1 (2.303RT)-1. The reaction mechanism for the formation
of isobutene, hydrogen chloride, and carbon dioxide has been theoretically characterized. The theoretical
study, at MP2/6-31G** computational level, points out that the molecular mechanism corresponds to a concerted
and highly synchronous process yielding the products. An analysis of bond orders and NBO charges shows
that the polarization of the C-Cl breaking bond can be considered the driving force for this fragmentation
process. The rate coefficients obtained from experimental data and theoretical calculations are in good
agreement.
1. Introduction
of the R-propiolactone intermediate. The first-step transition
structure was associated with a five-member ring with participa-
Kinetic studies on the decomposition of different 2-substituted
carboxylic acid derivatives have been carried out by some of
us.1-5 The results show that the reaction is homogeneous,
unimolecular and obeys a first-order rate law. Two different
molecular mechanisms have been proposed: (i) a hydrogen
chloride elimination, and (ii) a pathway conducted by the
neighboring group participation of the carbonyl oxygen of the
COOH group coupled with the C-Cl bond polarization (Scheme
1). Both proposals take place along a two-step mechanism: the
first step is the formation of the R-lactone intermediate with
concomitant HCl elimination. The second and common step is
the cleavage of the lactone ring with formation of CO and the
corresponding aldehyde. In addition, some theoretical studies
have been conducted with the aim of complementing the
understanding of the molecular mechanisms associated with
these decomposition processes.6-10 The combination of experi-
mental and theoretical results yields a precise idea of the nature
for the molecular mechanisms associated with this reaction
pathway.
In 1987, Chuchani and Rotinov studied the kinetics for the
pyrolysis of 2-chloropropionic acid,11 in which acetaldehyde,
carbon monoxide, and hydrogen chloride were obtained. The
mechanism was believed to proceed via a polar five-member
cyclic TS as described in Scheme 1. This unusual HCl
elimination led to a theoretical study of the 2-chloropropionic
acid decomposition by using ab initio calculations at MP2/6-
31G** level.6 The theoretical result agrees with the experimental
data describing a two-step mechanism involving the formation
tion of leaving chloride and hydrogen, assisted by the carbonyl
oxygen of the COOH group, as shown in Scheme 1. In the
second step, the unstable R-propiolactone intermediate decom-
poses to acetaldehyde and CO.
As an extension to the 3-substituted carboxylic acids,
Chuchani et al. have studied the decomposition reaction of
3-chloropropionic acid.4 However, it was found very difficult
to make a complete kinetic work: the pyrolytic elimination just
showed a first-order parallel decomposition at only one tem-
perature (Scheme 2). Then, to procure a more reliable inter-
pretation concerning the mechanistic elimination of the chloride
leaving group at the position-3 of propionic acid, and avoid
any possible parallel elimination, the present work aimed at
examining the pyrolysis kinetics of 3-chloropivalic acid (3-
chloro-2,2-dimethylpropionic acid), and we report a combination
of kinetic experimental and theoretical studies of this decom-
position process. The main objectives are to obtain the kinetic
parameters and to characterize the stationary points on reactive
PES in order to describe the nature of the molecular mechanism
for the decomposition process.
The present paper is organized as follows: in Sections 2 and
3 the experimental details and the computational method and
model are explained, respectively; in Section 4 the experimental
and theoretical results are presented and discussed. Section 5
presents conclusions and closes the paper.
2. Experimental Section
3-Chloropivalic acid of 99.0% purity was bought from
Aldrich. The pyrolysis product isobutene was analyzed by GLC,
in a column of Porapak Q 80-100 mesh, while the HCl gas
was quantitatively estimated by titration with a solution of 0.04
* Author to whom correspondence should be addressed.
† Instituto Venezolano de Investigaciones Cient´ıficas (I.V.I.C.).
‡ Universitat Jaume I.
§ Universitat de Vale`ncia.
10.1021/jp003560t CCC: $20.00 © 2001 American Chemical Society
Published on Web 02/13/2001

1870 J. Phys. Chem. A, Vol. 105, No. 10, 2001
Chuchani et al.
SCHEME 1: 2-Chloropropionic Acid Pyrolysis Mechanisms
SCHEME 2: 3-Chloropropionic Acid Pyrolysis
Mechanism
substrate in toluene was injected directly into the reaction vessel
with a syringe through a silicone rubber septum.
3. Computational Method and Model
The transition state theory (TST)14,15 was devised to facilitate
the interpretation of rate coefficients, and is used almost
universally by chemists interested in reaction mechanisms.16,17
We have selected this method to calculate the kinetic parameters
in the present study.
Calculations at MP2 level of theory with the 6-31G** basis
set18 have been performed with the Gaussian9819 program. The
Berny analytical gradient optimization routines20,21 were used
for optimization. The requested convergence on the density
matrix was 10-9 atomic units; the threshold value of maximum
displacement was 0.0018 Å, and that of maximum force was
0.00045 hartree/bohr. The nature of each stationary point was
established by calculating and diagonalizing the Hessian matrix
(force constant matrix). The transition structures were character-
ized by means of a normal-mode analysis. The unique imaginary
frequency associated with the transition vector (TV),22 i.e., the
eigenvector associated with the unique negative eigenvalue of
the force constant matrix, has been characterized. The intrinsic
reaction coordinate (IRC)23 path was traced in order to check
and obtain the energy profiles connecting each transition
structure to the two associated minima of the proposed mech-
anism, by using the second-order Gonza´lez-Schlegel integration
method.24,25
N NaOH. The identities of the substrate and product were
additionally verified with a mass spectrometer and NMR
spectroscopy.
The chloropivalic acid substrate was dissolved in a quantity
of toluene at least equal to that of the substrate. The pyrolysis
experiments were carried out in a static reaction apparatus as
previously described,12,13 but with some modifications and
additions of modern electronic and electrical devices. The
reaction vessels were seasoned with allyl bromide. The presence
of toluene served as free radical suppresser. The concentration
of the substrate is described as P0 in pressure units of Torr.
This P0 is obtained by introducing 0.05-0.15 mL of the solution
into the reaction vessel. These amounts, when applying the
molar fraction correction, give pressures between 40 and 120
Torr. Consequently, tables describing P0 are the initial concen-
tration of the substrate in pressure units. The rate coefficients
were determined by HCl titration. The temperature was con-
trolled by a resistance thermometer controller type SHINKO
DIC-PS 25TR maintained within (0.2 °C and measured with
a calibrated platinum-platinum-13% rhodium thermocouple. No
temperature gradient was found along the reaction vessel. The
Each stationary point in the potential energy surface (PES)
is characterized by an index which is equal to the number of
negative eigenvalues of the Hessian matrix (0 for a minimum,
1 for a saddle point). This index is also the number of imaginary
wavenumbers obtained in a normal-mode analysis of the
corresponding molecular structure. This analysis also provides

Decomposition Kinetics of 3-Chloropivalic Acid
J. Phys. Chem. A, Vol. 105, No. 10, 2001 1871
TABLE 1: Ratio of Final to Initial Pressure (pressures in
Torr)a
TABLE 2: Stoichiometry of the Reaction at 419.2 °Ca
time, min
2.5
5
8
10
15
temperature (°C)
P0
Pf
Pf/P0
average
reaction (%)(pressure)
HCl (%)(titration)
19.4
19.2
30.1
29.5
40.0
39.1
45.5
43.8
51.8
49.5
389.0
399.5
409.7
419.2
430.1
61.6
71.7
72.8
86.2
75.8
174
2.58
2.52
2.86
2.56
2.65
a Vessel seasoned with allyl bromide, in the presence of toluene
inhibitor. The % reaction pressure is estimated by using equation (PT/
P0 - 1) × 100.
180.7
208.3
220.7
200.9
2.63
TABLE 3: Homogeneity of the Reaction at 419.2 °C
a Seasoned vessel and in the presence of toluene.
a
b
c
S/V (cm-1
)
104k1 (s-1
)
104k1 (s-1
)
thermodynamic quantities such as zero-point vibrational energy
(ZPVE), temperature corrections (E(T)) and the absolute entropy
(S(T)),26 and consequently the rate coefficient can be estimated.
Temperature corrections and absolute entropies were obtained
assuming ideal gas behavior, from the harmonic frequencies
and moments of inertia by standard methods.27 A temperature
of 673.15 K (400 °C), inside the experimental range was taken
in the calculations.
1
6
1.21d
0.23d
7.59
7.49
a S ) surface, V ) volume. b Clean Pyrex vessel. c Seasoned vessel.
d Several runs gave irregular k values.
TABLE 4: Effect of the Inhibitor Toluene on Rates at 419.2
°C
P0
Pi
Pi/P0
104k1 (s-1
)
The first-order rate coefficient (k(T)) was computed using
70.8
86.7
65.5
74
47.7
151.5
177
219
219.5
0.7
1.8
2.7
3.0
6.2
8.01
7.34
7.59
7.40
7.65
14,28
the TST
and assuming that the transmission coefficient is
equal to 1, as expressed by the following relation:
k(T) ) (kT/h) exp(-∆Gq/RT)
(1)
43
Pressure of the substrate (torr, P0), pressure of the inhibitor toluene
(torr, Pi).
where ∆Gq is the Gibbs free energy change between the reactant
and its corresponding transition structure, and k and h are the
Boltzmann and Planck constants, respectively. ∆Gq was calcu-
lated as follows:
TABLE 5: Invariability of the Rate Coefficients with Initial
Pressure at 419.2 °C
P0 (Torr)
104k1 (s-1
43
7.65
65.5
7.59
70
7.64
86.4
7.34
120.5
7.50
∆Gq ) ∆Hq - T∆Sq
(2)
)
surements with the quantitative titration analysis of HCl with a
solution 0.04 N of NaOH (Table 2).
and
∆Hq ) PEB + ∆ZPVE + ∆E(T)
(3)
The effect of the surface on the rate of decomposition was
examined by carrying out several runs in a vessel with a surface-
to-volume ratio of 6.0 cm-1 relative to that of the normal vessel
equal to 1.0 cm-1. The packed and unpacked Pyrex vessels,
seasoned with allyl bromide, had no effect on rates. However,
the packed and unpacked clean Pyrex vessels gave a small
heterogeneous effect (Table 3).
The pyrolysis experiments were carried out in the presence
of an amount of free radical suppresser toluene, at least equal
to that of chloropivalic substrate, to prevent any possible chain
reactions. The effect of different proportions of toluene in the
decomposition process is shown in Table 4. No induction period
was observed. The rate coefficients were reproducible with a
relative standard deviation not greater than 5% at a given
temperature.
where PEB is the potential energy barrier and ∆ZPVE and
∆E(T) are the differences of ZPVE and temperature corrections
between the TS and the corresponding reactant, respectively.
∆Sq is obtained directly as the entropy (taken from the normal-
mode Gaussian98 outputs) difference between the TS and its
corresponding reactant.
4. Results and Discussion
4.1. Experimental Results. The products of the gas-phase
decomposition of 3-chloropivalic acid, in a static system
seasoned with allyl bromide and in the presence of the free-
radical inhibitor toluene, are isobutene, HCl, and CO2.
ClCH2C(CH3)2COOH f CH2dC(CH3)CH3 + HCl + CO2
The first-order rate coefficients of this chloroacid, calculated
from k1 ) (2.303/t) log [2Po/(3Po - Pt)], were independent of
its initial pressure (Table 5). Plotting log(3Po - Pt) against time
t gave a good straight line up to 50% decomposition (Figure
1). The approximate straight line is an indication of a first-
order reaction. The temperature dependence of the rate coef-
ficients and the corresponding Arrhenius equation are described
in Table 6 (90% confidence coefficient from least-squares
procedure).
The replacement of the two hydrogens at carbon-2 of
3-chloropropionic acid by methyl groups, i.e., 3-chloropivalic
acid, shows a significant decrease in the rate of decomposition
(Table 7). This result may infer that pathways different from
those of the 2-, 4-, and 5-chlorocarboxylic acids4,11 pyrolysis
may occur. Consequently, to establish the most adequate
mechanisms of the 3-chloroacids elimination processes, a
theoretical study has been conducted.
(4)
The stoichiometry (eq 4) required that, for very long reaction
time, the final pressure, Pf, should be three times the initial
reactant pressure, P0. The average experimental Pf/P0 value at
five different temperatures and 10 half-lives was 2.63 (Table
1). The small departure from the theoretical stoichiometry was
due to the little polymerization of isobutylene in the presence
of HCl gas, as was checked by introducing a sample of
isobutylene in the reaction vessel (and in the presence of HCl
gas): a decrease of the olefin pressure is observed. In addition,
a compound of relatively high molecular weight, containing only
C and H, product of the isobutylene polymerization, was found
by using our GC/MS. Verification of the above stoichiometry,
up to 50% reaction, was possible by comparing the percentage
decomposition of the substrate calculated from pressure mea-

1872 J. Phys. Chem. A, Vol. 105, No. 10, 2001
Chuchani et al.
Figure 1. Plot of log(3P0 - Pt) against t (min) at T ) 409.7 °C. P0
and Pt are given in Torr. r ) 0.9999.
Figure 3. Arrhenius plots obtained from experimental and theoretical
data. Log k (s-1) versus 103/T (K-1). The experimental value of log A
is 12.42 ( 0.36 s-1, while the theoretical value is 13.71 s-1. The
experimental value of activation energy is 205.8 ( 4.7 kJ‚mol-1, while
TABLE 6: Variation of Rate Coefficients with Temperature
T (°C) 380.5
104k1 0.93
389.0
1.51
399.5
2.74
409.7
4.93
419.2
7.59
430.1
13.42
the theoretical value is 227.1 kJ‚mol-1
.
(s-1
)
log k1 (s-1) ) (12.42 ( 0.36) - (205.8 ( 4.7) kJ mol-1 (2.303RT)-1
TABLE 7: Comparative Rate Coefficients (k1, s-1) at 340.0
°C, and Relative Rate of Olefin Formationa
substrate
105 k1
relative rate olefin formation
ClCH2CH2COOH
14.7
16.3
CH2)CH2
1.0
ClCH2C(CH3)2COOH
0.90
CH2)C(CH3)2
a Data for 3-chloropivalic acid from ref 4.
Figure 2. Optimized geometry of 3-chloropivalic acid, including atom
numbering.
4.2. Theoretical Results. The equilibrium geometry of
3-chloropivalic acid is depicted in Figure 2, including atom
numbering. An exhaustive exploration of PES related with the
decomposition of this system renders two TSs (see Figure 4):
TS1 is associated with the decomposition process, and the TV
is mainly controlled by the simultaneous C3-Chl, O2-H1 and
C1-C2 breaking bonds, yielding directly the products in one
step. Another TS, corresponding to the butyrolactone formation
with HCl elimination (TS2) has also been found with a large
value of relative energy (see Table 9).
4.2.1. Energetics and the Kinetic Parameters. The thermo-
chemical data of products relative to the reactants, (∆H, ∆S,
and ∆G) obtained from the normal mode Gaussian98 outputs,
as explained, are reported in Table 8. The activation parameters
(∆Hq, ∆Sq, ∆Gq) together with the calculated elementary first-
order rate constants, corresponding to the decomposition reaction
of the 3-chloropivalic acid through TS1 and TS2, and experi-
Figure 4. Optimized geometries of the TS1 and TS2.
mental values of k1 are reported in Table 9. It can be seen that
the decomposition process occurs through TS1 rather than
through TS2: TS1 is ca. 90 kJ mol-1 more stable.

Decomposition Kinetics of 3-Chloropivalic Acid
J. Phys. Chem. A, Vol. 105, No. 10, 2001 1873
TABLE 8: Relative Enthalpies (∆H, kJ mol-1), Entropies
(∆S, J mol-1 K-1) and Gibbs Free Energies (∆G, kJ mol-1
TABLE 10: Imaginary Frequency (Freq, cm-1), Hessian
Unique Negative Eigenvalue (Eig, a. u.), Main Components
of the Transition Vector (C, a. u.), and Corresponding
Geometric Parameters (G, bonds in Å, Bond and Dihedral
Angles in Degrees) for the TS1 and TS2
)
∆H
∆S
∆G
products
-13.45
333.12
-237.69
TS1
TABLE 9: Activation Enthalpies (∆Hq, kJ mol-1), Entropies
(∆Sq, J mol-1 K-1) and Gibbs Free Energies (∆Gq, kJ
Freq
Eig
1166.46i
-0.15055
mol-1 a
)
∆Hq
∆Sq
∆Gq
k
C
G
TS1
TS2
221.5
304.68
10.92
-0.47
214.15
305.01
3.4 × 10-4
C1-C2
C2-C3
0.443
-0.135
0.425
-0.161
0.644
-0.220
-0.141
0.127
-0.153
0.134
1.978
1.392
2.451
1.255
1.159
3.0 × 10-11
a Theoretically calculated (at 673.15 K) first-order rate coefficients
(k, s-1). The interval (calculated taking into account the experimental
uncertainties) for the observed rate coefficient at 673.15 K is from 5.357
C3-Chl
C1-O2
H1-O2
O1-C1-C2
O2-C1-C2
H1-O2-C1
C5-C2-C4-C3
H3-C3-C2-C1
× 10-5 to 1.507 × 10-3 s-1
.
107.72
114.01
115.35
-137.77
-97.98
The decomposition of chloropivalic acid is an exothermic
process; the ∆H value is negative. The global process is
spontaneous, the ∆G value is -237.69 kJ mol-1
.
An analysis of the experimental and theoretical values of ∆Hq
and ∆Sq for TS1, show discrepancies, mainly regarding ∆Sq.
However, a comparison between the values for the calculated
and experimental k1 rate constant shows a good agreement. The
experimental interval for the observed rate coefficient at 673.15
K is 5.357 × 10-5 to 1.507 × 10-3 s-1. The theoretical value
falls into the experimental range.
TS2
Freq
Eig
796.82i
-0.98053
C
G
C1-C2
0.206
-0.197
-0.338
-0.260
-0.268
-0.184
0.460
0.522
0.156
0.212
0.179
1.528
2.746
1.068
C3-Chl
H1-O2
In Figure 3, the experimental and theoretical Arrhenius plots
are presented. It can be seen that, although the rate constants
agree, some divergences were found between the theoretical
and experimental values for the A-factors and the slope
(activation energy) of both experimental and theoretical Arrhe-
nius plots. Probably, the use of more sophisticated treatments
of internal rotors and tunneling or the use of more demanding
quantum mechanical calculations (a large basis set representa-
tion, higher calculation level) could improve the theoretical
results.
C4-C2-C3
C1-C2-C4
O1-C1-C2
O2-C1-C2
C1-C2-C4-C3
H2-C3-C2-C1
H3-C3-C2-C1
O2-C1-C2-C3
Chl-C3-C2-C1
H1-O2-C1-C2
109.98
108.44
127.49
106.49
106.81
126.87
-71.77
-4.48
65.02
-0.115
0.116
-70.70
simultaneous character can be better shown by an analysis of
the bond order evolution and the synchronicity values (see
below).
4.2.2. Geometries and Transition Vectors. TVs and geometries
for the transition structure corresponding to the decomposition
process (TS1) and for TS2 are reported in Table 10. These
stationary points are depicted in Figure 4. Further information
on TSs (the whole set of geometric parameters, the force
constants, all positive eigenvalues, or all vibrational frequencies)
is available from the authors on request (andres@exp.uji.es;
safont@exp.uji.es).
For TS2 (see Table 10), the bond distances that participate
significantly in the TV are the H1-O2 and C3-Chl breaking
bonds, in antiphase with the C1-C2 distance. However, the
main components of the TV correspond to a bond and a dihedral
angle (O2-C1-C2 and C1-C2-C4-C3, respectively). These
components are related with the closure of the lactone. This
reaction pathway corresponds to the ring closure of the lactone
with formation of HCl. An analysis of the geometric parameters
values shows an asynchronous character for this TS. The C3-
Chl bond distance is 2.75 Å, larger than the preceding case.
The H1-O2 bond distance is 1.07 Å, and the O2-C3 distance
(data not shown in Table 10) is 2.15 Å.
These data seem to indicate that the carbon-chloride bond
is polarized before the leaving of the hydrogen and the closure
of the lactone ring. Again, an analysis of the bond order
evolution and synchronicity value will shed more light on this
issue (see below).
For TS1 (see Table 10), the main component of TV is the
H1-O2 distance, in phase with the C1-C2 and C3-Chl
distances. These components correspond to the decomposition
process. In addition, the O1-C1-C2 and O2-C1-C2 bond
angles in antiphase with the aforementioned distances compo-
nents have also a significant participation in the TV; the opening
of the CO2 angle and the elimination process are coupled. The
C2-C3 and C1-O2 motions slightly participate in the TV: the
C2-C3 and C1-O2 bonds are somewhat shortened while the
elimination process takes place, because they evolve from single
to double bonds.
The geometric parameters of TS1 displayed in Table 10
correspond with a concerted molecular mechanism: the elimi-
nation processes appear to be roughly simultaneous. Thus, the
distances for the breaking bonds C3-Chl, H1-O2, and C1-
C2 are 2.45, 1.16, and 1.98 Å, respectively, while the H1-Chl
forming bond distance (data not shown in Table 10) is 1.53 Å.
The distances for the bonds that evolve from single to double,
C2-C3 and C1-O2, are 1.39 and 1.26 Å, respectively. The
The imaginary frequency values are 1166i cm-1 for TS1 and
796i for TS2; indicating that these stationary points are
associated with the heavy atoms motions, mainly TS2.
4.2.3. Bond Order Analysis. To obtain a deeper analysis of
the extent of bond or bond forming/breaking along the reaction
pathway, the bond order (B) concept has been used.29-31 The
Wiberg bond indices 32 have been computed by using the Natural
33,34
Bond Orbital (NBO)
analysis as implemented in Gaussi-

1874 J. Phys. Chem. A, Vol. 105, No. 10, 2001
Chuchani et al.
TABLE 11: Wiberg Bond Indices (Bi), and Percentage of
Evolution through the Chemical Process (%Ev) of the Bond
Indices at the TS1 and TS2 Calculated by Means of Eq 8a
An analysis of the NBO charges at reactant shows a large
positive charge at C1 (1.01) and at H1 (0.53). Negative charges
appear at C2 (-0.18), C3 (-0.40), C4 (-0.65), C5 (-0.65),
O1 (-0.71), O2 (-0.80), and Chl (-0.10) centers. At TS1, the
charge at C1 has slightly increased to 1.16, while the charge
on H1 has slightly decreased to 0.46. On C2, an important
increase of the positive charge, up to 0.33, takes place, and on
C3, a decrease of negative charge, down to -0.03, is sensed.
The negative charges at C4 and C5 are the same (-0.65), and
a slight decrease of negative charges on the two oxygens can
be sensed, down to -0.66 and -0.77 for O1 and O2,
respectively. The chlorine atom supports now a negative charge
larger than before (-0.47). Then, from an electronic point of
view, the negative charge mainly moves from C2 and C3 to
Chl. This also indicates that the extension and polarization of
the C3-Chl bond can be seen as the driving force for the
decomposition process.
C1-C2 C2-C3 C3-Chl Chl-H1 H1-O2 C1-O2
TS1 BiR
0.950
0.525
0.985
1.320
1.012
0.312
0.000
0.438
0.728
0.342
0.978
1.302
BiTS
%Ev 44.7
Sy
35.3
69.2
47.5
0.89
53.0
38.2
O2-C3
C3-Chl
Chl-H1
H1-O2
TS2
BiR
0.002
0.183
21.2
1.012
0.164
83.8
0.000
0.290
31.5
0.728
0.440
39.6
BiTS
%Ev
Sy
0.70
a The synchronicity value (Sy) obtained from eq 5 is also included.
an98.19 Several breaking/forming bond processes are involved
in the fragmentation, and the global nature of the decomposition
reaction can be monitored by means of the synchronicity (Sy)
concept proposed by Moyano et al.,35 using the following
expression:
5. Conclusions
This work can be considered a collaborative project between
experimental and theoretical research groups. The 2-chloropiv-
alic acid has been pyrolyzed, and the corresponding rate
coefficient has been determined by HCl titration. The kinetic
parameters and the reactive potential energy surface, at MP2/
6-31G** calculation level, have been theoretically obtained. The
first-order rate coefficient was then evaluated in terms of
transition state theory and the theoretical results have been
compared with the experimental data. From the combination
of experimental and theoretical studies, the following conclu-
sions can be drawn:
n
|δBi - δBav|
∑
δBav
Sy ) 1 - i)1
(5)
2n - 2
In eq 5, n is the number of bonds directly involved in the
reaction, and the relative variation of the bond index is obtained
from
(1) The experimental data show that the decomposition is
homogeneous, unimolecular, and follows a first-order rate law,
which has been determined.
(2) The decomposition process, via a six-member cyclic
transition structure, TS1, yields the corresponding products in
one step. The molecular mechanism can be described as
concerted and highly synchronous.
(3) An analysis of bond orders and NBO charges suggests
that the extension and polarization of the C3-Chl bond with
initial migration of the chlorine atom seems to be the driving
force for the decomposition process at TS1.
(4) An alternative TS2, associated with a reaction pathway
including butyrolactone formation, has also been found, although
its high energy makes it kinetically irrelevant.
BiTS - BiR
BiP - BiR
δBi )
(6)
where the superscripts TS, R, and P refer to the TS, reactant,
and product, respectively. The average value is therefore
n
δBav ) n-1 δB
(7)
∑
i
i)1
Synchronicities calculated from expression 5 and the Wiberg
32
bond indices
of the breaking/making bond processes are
reported in Table 11. The percentage of evolution of the bond
order through the chemical step has been calculated by using
(5) An analysis and comparison of the calculated and
experimental kinetic parameters and Arrhenius plots point out
the validity of the theoretical approaches. A good agreement is
achieved between the theoretical and experimental results.
% Ev ) δBi × 100
(8)
The results are also included in Table 11. For TS1, it can be
seen that all the percentages of evolution of the bond order are
within a range of 35% to 69%, thus describing a highly
synchronous TS. The displacement of the chloride atom is the
more advanced motion (69.2% evolution), while the C2-C3
and C1-O2 bonds, that evolve from single to double bonds,
are late, displaying a small evolution percentage. Then, the
extension of the C3-Chl bond with initial migration of the
chloride atom can be seen as the driving force for the
decomposition process. The synchronicity value of 0.89 shows
that the reaction pathway can be described as concerted and
highly synchronous.
TS2 has a larger asynchronous character than TS1; the
corresponding Sy value is 0.70. The O2-C3 and the Chl-H1
forming bond processes are late, 21% and 31.5% evolution,
respectively, while the C3-Chl is very advanced, 84% evolu-
tion.
Acknowledgment. This work was supported by research
funds of the Ministerio de Educacio´n y Ciencia of the Spanish
Government by DGICYT (project PB96-0795-C02-02) and of
CONICIT (Consejo Nacional de Investigaciones Cient´ıficas y
Tecnolo´gicas) of Venezuela (project S1-97000005). All calcula-
tions were performed on two Silicon Graphics Power Challenge
L of the Servei d'Informa`tica of the Universitat Jaume I. We
are most indebted to this center for providing us with computer
capabilities.
References and Notes
(1) Chuchani, G.; Rotinov, A. Int. J. Chem. Kinet. 1989, 21, 367.
(2) Chuchani, G.; Dom´ınguez, R. M.; Rotinov, A. Int. J. Chem. Kinet.
1991, 23, 779.
(3) Chuchani, G.; Mart´ın, I.; Rotinov, A.; Dom´ınguez, R. M. J. Phys.
Org. Chem. 1993, 6, 54.

Decomposition Kinetics of 3-Chloropivalic Acid
J. Phys. Chem. A, Vol. 105, No. 10, 2001 1875
(4) Chuchani, G.; Mart´ın, I.; Rotinov, A.; Dom´ınguez, R. M.; Pe´rez I.
M. J. Phys. Org. Chem. 1995, 8, 133.
(5) Chuchani, G.; Dom´ınguez, R. M. Int. J. Chem. Kinet. 1995, 27,
85.
(6) Safont, V. S.; Moliner, V.; Andre´s, J.; Domingo, L. R. J. Phys.
Chem. 1997, 101, 1859.
(7) Domingo, L. R.; Andre´s, J.; Moliner, V.; Safont, V. S. J. Am. Chem.
Soc. 1997, 119, 6415.
(8) Domingo, L. R.; Picher, M. T.; Andre´s, J.; Safont, V. S.; Chuchani,
G. Chem. Phys. Lett. 1997, 274, 422.
(9) Domingo, L. R.; Picher, M. T.; Safont, V. S.; Andre´s, J.; Chuchani,
G. J. Phys. Chem. 1999, 103, 3935.
(10) Rotinov, A.; Chuchani, G.; Andre´s, J.; Domingo, L. R.; Safont, V.
S. Chem. Phys. 1999, 246, 1.
(11) Chuchani, G.; Rotinov, A. Int. J. Chem. Kinet. 1987, 19, 789.
(12) Swinbourne, E. S. Aust. J. Chem. 1958, 11, 334.
(13) Maccoll, A. J. Chem. Soc. 1965, 965.
(14) Glasstone, K. J.; Laidler, K. J.; Eyring, H. The Theory of Rate
Processes; McGraw-Hill: New York, 1941.
(15) Laidler, K. J. Theories of Chemical Reaction Rates; McGraw-Hill:
New York, 1969.
(16) Petersson, G. A. Theor. Chem. Acc. 2000, 103, 190.
(17) Garrett, B. C. Theor. Chem. Acc. 2000, 103, 200.
(18) Frisch, M. J.; Pople, J. A.; Binkley, J. S. J. Chem. Phys. 1984, 80,
3265.
(19) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.;
Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A.
D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi,
M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.;
Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick,
D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.;
Ortiz, J. V.; Baboul, A. G.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz,
P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-
Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe,
M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.;
Gonzalez, C.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian98;
Gaussian, Inc.: Pittsburgh, PA, 1998.
(20) Schlegel, H. B. J. Comput. Chem. 1982, 3, 214.
(21) Schlegel, H. B. J. Chem. Phys. 1982, 77, 3676.
(22) McIver, J. W., Jr. Acc. Chem. Res. 1974, 7, 72.
(23) Fukui, K. J. Phys. Chem. 1970, 74, 4161.
(24) Gonza´lez, C.; Schlegel, H. B. J. Phys. Chem. 1990, 94, 5523.
(25) Gonza´lez, C.; Schlegel, H. B. J. Chem. Phys. 1991, 95, 5853.
(26) Hehre, W. J.; Radom, L.; Schleyer, P. v. R.; Pople, J. A. Ab Initio
Molecular Orbital Theory; John Wiley & Sons: New York, 1986.
(27) McQuarrie, D. Statistical Mechanics; Harper & Row: New York,
1986.
(28) Benson, S. W. The foundations of chemical kinetics; McGraw-
Hill: New York, 1960.
(29) Varandas, A. J. C.; Formosinho, S. J. F. J. Chem. Soc., Faraday
Trans. 2 1986, 282.
(30) Lendvay, G. J. Mol. Struct. THEOCHEM 1988, 167, 331.
(31) Lendvay, G. J. Phys. Chem. 1989, 93, 4422.
(32) Wiberg, K. B. Tetrahedron 1968, 24, 1083.
(33) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. ReV. 1988, 88,
899.
(34) Reed, A. E.; Weinstock, R. B.; Weinhold, F. J. J. Chem. Phys.
1985, 83, 735.
(35) Moyano, A.; Perica`s, M. A.; Valent´ı, A. J. Org. Chem. 1989, 54,
573.