Experimental and theoretical study of the homogeneous, unimolecular gas-phase elimination kinetics of 2-furoic acid

Article abstract of DOI:10.1002/kin.20241

The kinetics of the gas-phase elimination kinetics of CO₂ from furoic acid was determined in a static system over the temperature range 415-455°C and pressure range 20-50 Torr. The products are furan and carbon dioxide. The reaction, which is carried out in vessels seasoned with allyl bromide and in the presence of the free-radical suppressor toluene and/or propene, is homogeneous, unimolecular, and follows a first-order rate law. The observed rate coefficient is expressed by the following Arrhenius equation: log k₁ (s^-1) = (13.28 ± 0.16) - (220,5 ± 2.1) kJ mol^-1 (2.303 RT)^-1. Theoretical studies carried out at the B3LYP/6-31++G** computational level suggest two possible mechanisms according to the kinetics and thermodynamic parameters calculated compared with experimental values.

Full text of DOI:10.1002/kin.20241

Experimental and
Theoretical Study of the
Homogeneous,
Unimolecular Gas-Phase
Elimination Kinetics
of 2-Furoic Acid
1
2
2
2
´
´
´
BEATRIZ C. RAMIREZ, ROSA M. DOMINGUEZ, ARMANDO HERIZE, MARIA TOSTA,
TANIA CORDOVA,1 GABRIEL CHUCHANI2
1Laboratorio de Fisico-Qu´ımica Orga´nica. Escuela de Qu´ımica, Facultad de Ciencias, Universidad Central de Venezuela,
Caracas 1020-A, Venezuela
2Centro de Qu´ımica, Instituto Venezolano de Investigaciones Cient´ıficas (IVIC), Apartado 21827, Caracas 1020-A,
Venezuela
Received 26 September 2006; revised 5 December 2006; accepted 13 December 2006
DOI 10.1002/kin.20241
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: The kinetics of the gas-phase elimination kinetics of CO2 from furoic acid was
determined in a static system over the temperature range 415–455◦C and pressure range
20–50 Torr. The products are furan and carbon dioxide. The reaction, which is carried out
in vessels seasoned with allyl bromide and in the presence of the free-radical suppressor
toluene and/or propene, is homogeneous, unimolecular, and follows a first-order rate law. The
observed rate coefficient is expressed by the following Arrhenius equation: log k1(s−1) = (13.28
0.16) − (220.5 2.1) kJ mol−1 (2.303 RT)−1. Theoretical studies carried out at the B3LYP/6-
31++G∗∗ computational level suggest two possible mechanisms according to the kinetics
and thermodynamic parameters calculated compared with experimental values. ꢀ 2007 Wiley
C
Periodicals, Inc. Int J Chem Kinet 39: 298–306, 2007
INTRODUCTION
Both experimental and theoretical studies on the
gas-phase elimination of several α- or 2-substituted
aliphatic carboxylic acids suggest the mechanism of
decarbonylation [1–7], as described in reaction (1)
Correspondence to: G. Chuchani; e-mail: chuchani@ivic.ve.
2007 Wiley Periodicals, Inc.
c
ꢀ
(1)

HOMOGENEOUS, UNIMOLECULAR GAS-PHASE ELIMINATION KINETICS OF 2-FUROIC ACID
299
It is interesting to note that the atom of the sub-
stituent directly attached at the α- or 2-carbon of
the COOH group is a heteroatom. However, when
a nitrogen atom is at the 2-position of the car-
boxylic acid, for example, an α-amino acid (such as
N,N-dimethylglycine [8] or N-phenylglycine [9]), a
gas-phase decarboxylation process takes place
[reaction (2)].
and the purity of this substrate were verified by GC-
MS (Saturn 2000, Varian, with a DB-5MS capillary
column 30 m × 0.25 mm i.d., 0.25-µm film thick-
ness). The quantitative analysis of the product CO2
was carried out using a gas chromatograph (Varian
3600X) with a thermal conductivity detector (cap-
illary column: GS-Q, 30 m × 0.53 mm i.d., Helium gas
carrier).
(2)
Kinetic Studies
Consequently, the mechanism described in reac-
tion (2) differs both experimentally and theoreti-
cally from that reported for the gas-phase elimi-
nation of several types of 2-substituted carboxylic
acids.
The effect of a heteroaromatic group attached at the
acid side of a carboxylic acid in the gas-phase elimi-
nation reaction has scarcely been reported, particularly
when the heteroatom is located at the 2-carbon with re-
spect to the COOH group. In this respect, a few years
ago, the homogeneous, unimolecular gas-phase elim-
ination kinetics of picolinic acid was found to decar-
boxylate at very low temperature [10]. The mechanism
of decomposition of this acid was thought to be polar
in nature, as depicted in reaction (3):
The kinetic experiments were performed in a static re-
action system previously reported [11–13]. The rate co-
efficients were determined manometrically. The tem-
perature was controlled by a Shinko DC–PS resistance
thermometer controller and maintained with 0.2◦C
and measured with a calibrated iron–constantan ther-
mocouple. No temperature gradient was found along
the reaction vessel. The substrate was dissolved in
dioxane and injected (0.05–0.1 mL) directly into the
reaction vessel with a syringe through a silicone rubber
septum.
COMPUTATIONAL METHOD AND MODEL
The kinetics and mechanisms of 2-furoic acid gas-
phase elimination to give furan and carbon dioxide
were investigated by means of electronic structure cal-
culations using DFT B3LYP/6-31++G(d,p) as imple-
mented in Gaussian 98W [14]. Structures of the reac-
tants and products were optimized at each theory level,
and transition states were searched and optimized us-
ing synchronous transit (QST2, QST3) methodologies.
The Berny analytical gradient optimization routines
were used. The requested convergence on the density
matrix was 10−9 atomic units, the threshold value for
(3)
Some heteroaromatic molecules have double bond
characteristics and are less aromatic than pyridine, for
example, furan. To understand the double bond char-
acteristics of these compounds, the present work was
aimed at examining the gas-phase elimination kinet-
ics of 2-furoic acid and to report on a combination
of kinetic experiments and theoretical studies of the
said substrate. The intention was to obtain the kinetic
parameters and to characterize the potential energy sur-
face (PES) in order to establish the molecular mecha-
nism of the elimination reaction.
˚
maximum displacement was 0.0018 A, and that for the
maximum force was 0.00045 Hartree/Bohr. The nature
of the stationary points was established by calculating
and diagonalizing the Hessian matrix (force constant
matrix). Transition state (TS) structures were charac-
terized by means of normal-mode analysis. The transi-
tion vector (TV) associated with the unique imaginary
frequency, that is, the eigenvector associated with the
unique negative eigenvalue of the force constant ma-
trix, has been characterized.
EXPERIMENTAL
The substrate 2-furoic acid acquired from Aldrich (of
98.8% purity) was used. The identification of products
Thermodynamic quantities such as the zero-point
vibrational energy (ZPVE), temperature corrections
International Journal of Chemical Kinetics DOI 10.1002/kin

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RAMIREZ ET AL.
300
E(T ), and absolute entropies S(T ) were obtained by
frequency calculations and, consequently, the rate co-
efficient was estimated assuming the transmission co-
efficient is equal to 1. Temperature corrections and
absolute entropies were obtained assuming ideal gas
behavior from the harmonic frequencies and moments
of inertia by standard methods [15] at average tem-
perature and pressure values within the experimental
range. Scaling factors for frequencies and zero-point
energies are taken from the literature [16].
(4)
stoichiometry (4), up to 85% reaction, was verified by
comparing between the carbon dioxide formation with
the pressure increase of the substrate decomposition
(Table I). To determine the influence of the surface area
on the rate of elimination (4), several runs were carried
out in a vessel with a surface-to-volume ratio six times
greater than that of the normal vessel (Table II). The
packed and unpacked clean Pyrex vessels had a marked
effect on the rates. However, the packed and unpacked
Pyrex vessels seasoned with allyl bromide appears to
have no effect on rates.
The first-order rate coefficient k(T ) was calculated
using the TST [14] and assuming that the transmission
coefficient is equal to 1, as expressed in the following
relation:
k(T ) = (KT/h) exp (−ꢀG#/RT)
Different proportions of the free-radical inhibitor
toluene and/or propene in the elimination process are
shown in Table III. 2-Furoic acid, in seasoned vessels,
had to be carried out in the presence of at least twice the
amount of the inhibitor toluene and/or propene in order
to prevent any radical reaction. No induction period
was observed and the rates were reproducible with a
standard deviation <5% at a given temperature. (The
precision of manometric measurements is 0.5 mm.)
The first-order rate coefficients of this elimination
calculated from k1 = (2.303/t) log P0/(2P0 − Pt) were
found to be independent to initial pressures (Table IV).
A plot of log(2P0 − Pt ) against time t gave a good
straight line up to 85% reaction. The variation of the
rate coefficients with temperature and the correspond-
ing Arrhenius equation is given in Table V (90% con-
fidence coefficient from least-squares procedure). The
Arrhenius plot is described in Fig. 1.
where ꢀG# is the Gibbs free energy change between
the reactant and the transition state and K and h are
the Boltzmann and Planck constants, respectively.
ꢀG# was calculated using the following
relations:
ꢀG# = ꢀH# − T ꢀS#
and
ꢀH# = V # + ꢀ(ZPVE) +ꢀE(T )
where V # is the potential energy barrier and ꢀ(ZPVE)
and ꢀE(T ) are the differences of ZPVE and temper-
ature corrections between the TS and the reactant, re-
spectively.
Frequency calculations were made at average ex-
perimental conditions (T = 435◦C, P = 0.105 atm),
at the same level of optimization as used in the theo-
retical calculations. Thermodynamic quantities such as
the zero-point vibrational energy (ZPVE), temperature
corrections (E(T )), energy, enthalpy, and free energies
were obtained from vibrational analysis. Entropy val-
ues were calculated from vibrational analysis using the
factor Cexp of Chuchani–Cordova [17].
With these available experimental kinetic and ther-
modynamic parameters (Table VI), together with a the-
oretical study, it may be possible to consider a rea-
sonable mechanism for the gas-phase elimination of
2-furoic acid.
RESULTS AND DISCUSSION
The elimination reaction of 2-furoic acid in a static sys-
tem, seasoned with allyl bromide and in the presence
of the free-radical inhibitor toluene and/or propene
was determined over the temperature range 415–455◦C
and the pressure range 20–120 Torr. The decomposi-
tion products are furan and CO2 [reaction (4)]. The
Figure 1 Arrhenius plot of the elimination kinetics of 2-
furoic acid. Slope = −11519.4, intercept = 13.28, SD =
0.006, r = 0.9999.
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HOMOGENEOUS, UNIMOLECULAR GAS-PHASE ELIMINATION KINETICS OF 2-FUROIC ACID
301
Table I Stoichiometry of the Reaction
Substrate
Temperature (◦C)
Parameter
Value
2-Furoic acid
435
Time (min)
5
10
15
20
26
Reaction (%) (pressure)
CO2 (%) (GLC)
28.5
28.2
45.1
46.0
62.1
60.0
74.4
74.6
85.3
84.1
Table II Homogeneity of the Reaction
Substrate
Temperature (◦C)
S/V (cm−1 a
)
104 k (s−1 b
)
104 k (s−1 c
)
1
1
2-Furoic acid
415.6
1
6
12.96 0.30
13.27 0.33
3.58 0.10
3.42 0.21
a S = surface area (cm2); V = volume (cm3).
b Clean Pyrex vessel.
c Vessel seasoned with allyl bromide.
Table III Effect of the Free-Radical Suppressor Toluene and/or Propene on Rates
Substrate
Temperature (◦C)
P (Torr)
0
Pi (Torr)
Pi/P
104k (s−1
)
0
1
2-Furoic acida
435.7
40.0
56.5
43.0
34.0
38.5
42.0
36.0
—
40.0
60.0
74.0
108.0
134.0
130.5
—
0.7
1.4
2.2
2.8
3.2
3.6
9.52
9.45
10.42
10.52
10.48
10.69
10.46
P0 = pressure of the substrate; Pi = pressure of the inhibitor.
a Vessel seasoned with allyl bromide.
Table IV Variation of the Rate Coefficients With Initial Pressurea,b
Substrate
Temperature (◦C)
Parameters
Value
2-Furoic acid
445.2
P (Torr)
22
17.49
25
17.68
29.5
17.67
38.5
17.68
46
17.69
0
104 k (s−1
)
1
a Vessel seasoned with allyl bromide.
b In the presence of the inhibitor toluene and/or propene.
Table V Temperature Dependence of the Rate Coefficients
Substrate
Parameters
Temp. (◦C)
Value
2-Furoic acid
415.9
3.58 0.10
425.6
6.31 0.09
435.7
10.60 0.12
445.2
455.1
104 k (s−1
)
17.64 0.09
28.53 0.06
1
Rate equation log k1 (s−1) = (13.28 0.16) − (220.5 2.1) kJ mol−1 (2.303RT)−1; r = 0.9998.
Table VI Kinetic and Thermodynamic Parameters at 435.0◦C
Substrate
104 k (s−1
)
E (kJ mol−1
)
log A (s−1
)
ꢀS‡ (J mol−1 K−1
−6.2
)
ꢀH‡ (kJ mol−1
)
ꢀG‡ (kJ mol−1
219.0
)
1
a
2-Furoic acid
10.27
220.5 2.1
13.28 0.16
214.6
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RAMIREZ ET AL.
302
Scheme 1 Two mechanisms were found for 2-furoic acid thermal decomposition.
THEORETICAL RESULTS
Transition States and Mechanisms
The gas-phase elimination reaction of 2-furoic acid can
occur through two transition states. Transition states
were verified by means of IRC calculations. The tran-
sition state for mechanism 1 (TS-I) is a cyclic four-
membered structure as suggested by values of log A of
13.07–13.64 [18]. TS-I geometries are almost identi-
cal for all levels of theory, both in distances and angles
between the atoms involved in the reaction (O1, C2,
C3, C9, O11, and H12), with the hydrogen being trans-
ferred (H12) midway between the carbon C2 and the
oxygen O11 (Fig. 2, Scheme 2). The four atoms in-
volved in the TS structure lie in a different plane with
respect to the furan ring plane. Structural parameters
for the four-membered ring TS are shown in Table IX.
The TS-I dihedral angle C2 C9 O11 H12 shows
that the four atoms involved in the TS are in a plane
Kinetic and Thermodynamic Parameters
The gas-phase thermal decomposition of 2-furoic acid
was studied by theoretical calculations of the electronic
structure. Two possible transition states, four- and five-
membered ring structures, were found connecting the
reactant and products. The two mechanisms were ex-
amined to establish the kinetic and thermodynamic pa-
rameters to be compared to the experimental coun-
terparts. The two possible mechanisms are shown in
Scheme 1.
Results from B3LYP/6-31++G∗∗ calculations for
mechanisms 1 and 2 are shown in Tables VII and VIII.
Calculated parameters of the two mechanisms were
compared with experimental values, showing better
accord for mechanism 1.
Table VII Kinetics and Thermodynamic Parameters for Thermal Decomposition of 2-Furoic Acid by Mechanism 1 at
435◦C and 0.105 atm
Level of Theory
ꢀH=(kJ mol−1
)
ꢀG=(kJ mol−1
)
ꢀS=(J K−1 mol−1
)
E
a
=(kJ mol−1
)
log A k (s−1
)
B3LYP/6-31G++G∗∗
232.2
214.6
232.4
219.0
−0.1
−6.2
238.1
220.5
13.60
13.28
1.08
10.27
Experimental
Table VIII Kinetics and Thermodynamic Parameters for Thermal Decomposition of 2-Furoic Acid by Mechanism 2 at
435◦C and 0.105 atm
Level of Theory
ꢀH=(kJ mol−1
)
ꢀG=(kJ mol−1
)
ꢀS=(J K−1 mol−1
)
Ea=(kJ mol−1
)
log A
k (s−1
)
B3LYP/6-31++G∗∗
242.0
214.6
241.1
219.0
1.3
−6.2
247.9
220.5
13.67
13.28
0.242
10.27
Experimental
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HOMOGENEOUS, UNIMOLECULAR GAS-PHASE ELIMINATION KINETICS OF 2-FUROIC ACID
303
Figure 2 Two views of the four-membered ring TS structure found for mechanism 1, showing the transition vector. [Color
figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]
Scheme 2
Scheme 3
while the dihedral angle O1 C2 C9 O11 indicates
that the four-membered ring structure is at approx-
imately 120◦ from the plane of the furan ring. The
process is concerted in the sense that O11 H12 and
C2 C9 bond distances increase, showing breaking of
C9, O11, H12, and C3, are almost in a plane (dihedral
angle C2 C9 O11 H12, Table X) at about 32◦ from
the furan ring plane (dihedral C2 C9 O11 H12). The
process is concerted in the sense that O11 H12 and
C2 C9 bond distances increase showing breaking of
these bonds, with more progress in the O11 H12 bond
˚
˚
these bonds (0.970–1.441 A in TS-I and 1.472–1.630 A
in TS-I, respectively). The C9 O11 and C2 O1 bond
˚
distances reveal changes of bond order (1.211–1.288 A
˚
in TS-I and 1.377–1.404 A in TS-I, respectively), while
˚
the C2 H12 distance decreases, indicating bond for-
mation. The transition vector TV associated with the
imaginary frequency characteristic of the TS shows
that the process is dominated by the elongation of the
O11 H12 bond.
The transition state for mechanism 2 (TS-II), which
is a cyclic five-membered structure (Fig. 3, Scheme 3),
is also in accord with the log A values (12.70–13.94)
[19]. Intrinsic reaction coordinate (IRC) calculations
were performed to verify that this TS connects the
reactants and products of the reaction. The IRC plot
for the TS-II reaction path is shown in Fig. 4.
breaking compared to mechanism 1 (0.970–1.580 A in
˚
TS-II for the O11 H12 bond and 1.472–1.550 A in
TS-II for the C2 C9 bond). The C9 O11 and C2 O1
bond distances reveal changes of bond order (1.361–
˚
˚
1.280 A in TS-II and 1.377–1.329 A in TS-II, respec-
tively). The distance between C3 H12 decreases in-
dicating bond formation. The transition vector TV as-
sociated with the imaginary frequency characteristic
of the TS shows that the process is dominated by the
elongation of the O11 H12 bond while hydrogen H12
is being transferred from O11 to C3.
Natural bond orbital (NBO) charges for mechanism
1 are shown in Table XI. There is a redistribution of
electron density in the TS compared to the reactant
The structure of TS-II is similar to a bicycle, in
which the atoms involved in the reaction changes, C2,
Figure 3 Two views of the five-membered TS structure found for mechanism 2, showing the transition vector. [Color figure
can be viewed in the online issue, which is available at www.interscience.wiley.com.]
International Journal of Chemical Kinetics DOI 10.1002/kin

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RAMIREZ ET AL.
304
density, while O1 suffers small changes, implying that
participation of the oxygen in the furan ring is not
important.
NBO charges for mechanism 2 (TS-II) are shown
in Table XII. In this case, for the analysis of NBO
charges and orbital occupation for bond order cal-
culations, we considered the zwitterion intermediate
proposed for mechanism 2 (Scheme 1) as the prod-
uct formed in a rate-determining step. This unstable
intermediate quickly decomposes to furan and carbon
dioxide.
Figure 4 IRC plot for TS-II reaction path at B3LYP/6-
31++G∗∗ level of theory.
NBO charges show a decrease in positive charge in
H12 (from 0.517 to 0.429 in TS-II), increase in nega-
tive charge in C3 (from −0.204 to −0.605 in TS-II),
increase in positive charge in C2 (from 0.149 to 0.484
in TS-II), and a small decrease in negative charge in
O1 (from −0.473 to −0.407 in TS-II), as the zwitte-
rion species forms while there is a almost no change for
O11 in TS-II. The most significant change is the polar-
furoic acid, a decrease in negative charge in O11 (from
−0.705 to −0.689 in TS-I), decrease in positive charge
δ+ in H12 (from 0.517 to 0.431 in TS-I), an increase in
negative charge in C2 (from 0.149 to −0.100 in TS-I),
a decrease in negative charge in C3 (from −0.204 to
−0.139 in TS-I), and an increase in positive charge in
C9 (from 0.759 to 0.811 in TS-I) as the C2 C9 bond
breaks and C2 H12 bond forms. The C3 atom is also
involved as can be seen from the changes in electron
ization of the C2 C3 bond in the sense C2δ+ C3δ−
which increases in TS-II.
,
Table IX Structural Parameters of 2-Furoic Acid and the Four-Membered Ring TS-I for 2-Furoic Acid Thermal
Decomposition, From B3LYP/6-31++G∗∗ Calculations
˚
Distances (A)
C2 H12
O11 H12
C2 C9
C9 O11
C2 O1
R
TS-I
2.379
1.274
0.970
1.441
1.472
1.630
1.211
1.288
1.377
1.404
Dihedral Angles (deg)
O1 C2 C9 O11
118.65
C2 C9 O11 H12
−6.03
TS-I
TS-I
Imaginary Frequency (cm−1
1530.1
)
Table X Structural Parameters of 2-Furoic Acid and the Five-Membered Ring TS-II for 2-Furoic Acid Thermal
Decomposition, From B3LYP/6-31++G∗∗ Calculations
˚
Distances (A)
C3 H12
O11 H12
C2 C9
C9 O11
C2 O1
C2 C3
R
TS-II
2.476
1.250
0.970
1.580
1.472
1.550
1.361
1.280
1.377
1.329
1.374
1.438
Dihedral Angles (deg)
Bond Angles (deg)
C9 C2 C3 H12
31.90
C2 C9 O11 H12
2.04
C3 H12 O11
131.86
C2 C3 H12
85.64
TS-II
TS-II
Imaginary Frequency (cm−1
)
1330.9
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HOMOGENEOUS, UNIMOLECULAR GAS-PHASE ELIMINATION KINETICS OF 2-FUROIC ACID
305
Table XI NBO Charges for Reactant R, TS, and
Table XIII NBO Analysis for Furoic Acid Thermal
Products P, From B3LYP/6-31++G∗∗ Calculations for
Mechanism 1, Which Involves a Four-Membered Ring
TS-I
Decomposition From B3LYP/6-31++G∗∗ Calculations
Structure O11 H12 H12 C2
C2 C9
O11 C9
δBR
δBTS
δBP
0.7072
0.2204
0.0000
0.0009
0.4802
0.9179
1.0370
0.7542
0.0002
27.3
1.0332
1.3398
1.8888
35.8
Structure
C2
C3
H12
O11
C9
O1
R
ET-I
P
0.149 −0.204 0.517 −0.705 0.759 −0.474
−0.100 −0.139 0.431 −0.689 0.811 −0.419
0.090 −0.335 0.234 −0.516 1.030 −0.479
%E
68.8
52.3
v
δB = 0.461
Sy = 0.790
av
Table XII NBO Charges for Reactant R, TS, and
Products P From B3LYP/6-31++G∗∗ Calculations for
Mechanism 2, Which Involves a Four-Membered Ring
TS-II
Wiberg bond indexes (Bi ), % evolution through the reaction
coordinate (%Ev), are shown for reactants R, TS-I, and products P.
Average bond index variation (δBav) and synchronicity parameter
(Sy) are also reported.
Structure C2
C3
H12
O11
C9
O1
Table XIV NBO Analysis for Furoic Acid Thermal
Decomposition From B3LYP/6-31++G∗∗ Calculations
R
ET-II
INT
0.149 −0.204 0.517 −0.705 0.759 −0.473
0.484 −0.605 0.429 −0.702 0.726 −0.407
0.297 −0.646 0.287 −0.499 0.287 −0.518
Structure O11 H12 C2 C9 O11 C9 C2 C3 C3 H12
δBR
δBTS
δBP
0.7072
1.037 1.0332 1.5693 0.0009
0.1797 0.8852 1.3340 1.2376 0.4963
0.0000 0.0006 1.8701 1.0610 1.8681
Bond Order Analysis
%E
74.6
14.6
35.9
65.3
Sy = 0.755
Wiberg bond indexes (Bi ), % evolution through the reaction
57.1
v
δB = 0.495
Bond order calculations (NBO) were performed [20–
22]. Wiberg bond indexes [23] were computed using
the NBO program [24] as implemented in Gaussian
98W to further investigate the nature of the TS along
the reaction pathway. Bond breaking and making pro-
cesses involved in the reaction mechanism can be mon-
itored by means of the synchronicity (Sy) concept pro-
posed by Moyano et al. [25], which is defined by the
expression
av
coordinate (%Ev), are shown for reactants R, TS-II, and products
P. Average bond index variation (δBav) and synchronicity parameter
(Sy) are also reported.
H12 C2, C2 C9, and O11 C9 (TS-I, Scheme 2,
Fig. 2), and (b) mechanism 2, bonds O11 H12,
C2 C9, O11 C9, C2 C3, and C3 H12 (TS-II,
Scheme 3, Fig. 3). All the other bonds do not un-
dergo important changes during the process. Calcu-
lated Wiberg indexes Bi for the reactant, TS, and prod-
ucts for mechanisms 1 and 2 are shown in Tables XIII
and XIV.
Bond order analysis revealed more progress in
O11 H12 bond breaking (68%), intermediate advance
in H12 C2 bond formation, and lesser progress in
C2 C9 and O11 C9 bonds. A synchronicity parame-
ter Sy of 0.79 implies a polarized asynchronic process,
with late TS in the sense of H12 C2 bond formation,
for mechanism 1.
Mechanism 2 is described by five reaction coordi-
nates. In this case, Wiberg bond order indexes show
more progress in O11 H12 (≈75%), followed by the
bond order changes in C2 C3 (≈65%) and C3 H12
(≈57%), moderate advance for O11 C9 (≈36%), and
little progress in C2 C9 bond breaking (≈15%). This
process is more asynchronic than mechanism 1 (Sy =
0.755), and it is also dominated by O11 H12 bond
breaking.
ꢀ
ꢂ
ꢃ
n
ꢁ
Sy = 1 −
|δBi − δBav|/δBav
2n − 2
i=1
where n is the number of bonds directly involved in
the reaction. The relative variation of the bond index is
obtained from
ꢄ
ꢅꢆꢄ
ꢅ
δBi = BiTS − BiR BiP − BiR
where the superscripts R, TS, and P represent reactant,
transition state, and product, respectively.
The evolution in bond change is calculated as
%Ev = δBi × 100
The average value is calculated from the equation
n
ꢁ
δBav = 1/n
δBi
i=1
Bonds indexes were calculated for bonds involved in
the reaction changes: for (a) mechanism 1, O11 H12,
International Journal of Chemical Kinetics DOI 10.1002/kin

´
RAMIREZ ET AL.
306
6. Chuchani, G.; Dom´ınguez, R. M.; Rotinov, A.; Mart´ın,
I. J Phys Org Chem 1999, 12, 612.
CONCLUSIONS
7. Chuchani, G.; Dom´ınguez, R. M.; Herize, A.; Romero,
R. J Phys Org Chem 2000, 13, 757.
The experimental data show that the elimination pro-
cess of 2-furoic acid in the gas phase is homogeneous,
unimolecular, and follows a first-order rate law. Theo-
retical calculations suggest that the reaction proceeds
in a concerted asynchronous mechanism. Two con-
certed mechanisms were found for the thermal decom-
position of 2-furoic acid. Calculated activation param-
eters are in better accord with mechanism 1 at the
B3LYP/6-31++G∗∗ level of theory. The TS structure
for mechanism 1 is a four-membered ring, where the
four atoms lie in a plane forming 120◦ with the furan
ring plane. The process is concerted polar asynchronic,
dominated by the acid hydrogen transfer to the car-
bon bearing the carboxylic moiety in the furan ring.
Mechanism 2 is described as a cyclic TS structure in
which the five atoms involved are in a plane that is at
about 30◦ to the furan ring. This process is also dom-
inated by the breaking of the O H bond of the acid
moiety and concerted polar in nature. An experimen-
tal log A of 13.28 suggests that the reaction is likely
to occur through a four-membered-ring type of mech-
anism. Additionally, the formation of the zwitterion
species in the gas phase is unlikely because it cannot be
stabilized by solvent interactions. The small negative
value of the entropy of activation suggests loose po-
lar TS. These arguments favor mechanism 1; however,
mechanism 2 is not ruled out. NBO analysis suggests
that the polarization of the O H bond is the determin-
ing factor in the decomposition process and implies
a polar asynchronic process. A reasonable agreement
is achieved between the theoretical and experimental
results.
8. Ensuncho, A.; Lafont, J.; Rotinov, A.; Dom´ınguez,
R. M.; Herize, A.; Quijano, J.; Chuchani, G. Int J Chem
Kinetic 2000, 33, 465, and references cited therein.
9. Dominguez, R. M.; Tosta, M.; Chuchani, G. J Phys Org
Chem 2003, 16, 869.
10. Lafont, J.; Ensuncho, A.; Dominguez, R. M.; Rotinov,
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Chem 2003, 16, 84.
11. Maccoll, A. J Chem Soc 1955, 965.
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