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A simple program to determine the reaction rate and thermodynamic properties of reacting system
Transcript of A simple program to determine the reaction rate and thermodynamic properties of reacting system
A simple program to determine the reaction rate and
thermodynamic properties of reacting system
Patrıcia R.P. Barretoa,*, Alessandra F.A. Vilelab, Ricardo Garganob
aLaboratorio Associado de Plasma—LAP, Instituto Nacional de Pesquisas Espaciais—INPE/MCT, CP515,
Sao Jose dos Campos, SP, CEP 12247-970, BrazilbInstituto de Fısica, Universidade Brasılia, CP04455, Brasılia, DF, CEP 70919-970, Brazil
Received 14 April 2003; revised 14 April 2003; accepted 13 August 2003
Abstract
We developed a simple program to determine the reaction rate by using conventional transition state theory with the Wigner
transmission coefficient and also the thermodynamic properties of the species. The hydrogen abstraction in the reaction
NH3 þ H ! NH2 þ H2 is used as a model to demonstrate the program usage. The rate constants have been computed for the
gas-phase chemical reaction over the temperature range of 200–4000 K. Our data are in a good agreement with the published
data for this reaction.
q 2003 Elsevier B.V. All rights reserved.
PACS: 82.20.Pm; 82.20.Db; 82.20.Wt; 82.30.Hk; 82.30.Lp; 51.30. þ i; 82.60. 2 s
Keywords: Kinetics; Thermal rate constants; Conventional transition state theory; Direct dynamics; Bimolecular abstraction; GAUSSIAN 98;
Thermodynamic properties
1. Introduction
There has been considerable interest in recent
years, in the growth of boron nitride thin films. Like
carbon, boron nitride has different allotropes, the most
common of which are the hexagonal (hBN) and cubic
(cBN) phases. The hBN, although electrically insulat-
ing, has properties that are very similar to graphite
while the cBN has properties comparable to diamond.
At present there is little understanding of the
chemical process which are involved in and which
control the synthesis of either hexagonal or cubic
boron nitride from the vapor phase. Theoretical
research found in the literature includes thermodyn-
amic equilibrium calculations for mixtures involving
B/F/N/H [1–5], B/Cl/N/H [4,5], and B/N/H [4], as
well as limited kinetics studies of the reactions
between BCl3 and NH3 [6,7].
P.R.P. Barreto et al. proposed a kinetic mechanism
to describe the growth of boron nitride films, for the
Ar/B/F/N/H system, using both CVD and arcjet
reactor [8–12]. The gas-phase mechanism includes
35 species and 1012 reactions and also extends a
previous mechanism that contained 26 species and
67 elementary reactions. Rate constants for 117
elementary reactions were obtained from published
0166-1280/$ - see front matter q 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.theochem.2003.08.006
Journal of Molecular Structure (Theochem) 639 (2003) 167–176
www.elsevier.com/locate/theochem
* Corresponding author. Tel.: þ55-12-3945-6694; fax: þ55-12-
3945-6710.
E-mail address: [email protected] (P.R.P. Barreto).
experimental/theoretical data and those for the other
895 reactions should be estimated using transition
state theory (TST). As the number of reactions as
large we developed a simple code to determine the
reaction rate using conventional TST with the Wigner
transmission coefficient, since we normally work at
high temperature, 1000–3000 K, in which the tunnel-
ing effects decreases.
To show the capability of our code we applied it to
the well-known reaction NH3 þ H ! NH2 þ H2,
since it has been studies by several groups who
provided many theoretical [13–18] and experimental
results [19–22] in the temperature range of 500–
2000 K. Comparing the results obtained using our
code with published data, our data are in a good
agreement and now we can apply our code to the
reaction in a Ar/B/F/N/H system.
Following, we outline how the paper is organized.
In Section 2 we present the main characteristics of our
program. Our results and discussion are presented in
Section 3.
2. The program
Our program has basically two subroutines, one to
calculate the thermodynamic properties of the species
and the other one to determine the rate constants. The
input files are very easy to write down and the output
files are very simple to analyze, and they can be
exported, directly, to a graphical software. The
following sections will discuss the calculation of the
reaction rate using TST [23–26] and the determi-
nation of the thermodynamic properties [23,25].
2.1. Transition state theory
The TST was developed in the 1930s and has since
formed a framework for much of the discussion of rate
processes. It is a model to determine the rate constants
based in an interaction potential between reactant and
products with a statistical representation of the
dynamics. In addition to the Born–Oppenheimer
approximation it uses more three assumptions: there
is a surface in phase space that divides into a reactant
region and a product region; the reactant equilibrium
is assumed to maintain a Boltzmann energy distri-
bution and the transition state or activated complex is
assumed to have Boltzmann energy distribution
corresponding to the temperature of the reacting
system.
Considering a bimolecular reaction, such as:
A þ BC ! AB þ C ð1Þ
where A, B or C are atoms or group of atoms, the
thermal rate constants is given by:
kTST ¼kBT
h
QX‡
QAQBC
exp 2VG‡
a
RT
!ð2Þ
where QX‡ ; QA and QBC are the partition functions of
the transition state, X† (saddle point), and the reactant,
A and BC, respectively, kB Boltzmann constant, h
Planck constant, T temperature and R universal gas
constant, VG‡a is the barrier:
VG‡a ¼ V‡ þ 1ZPE ð3Þ
where V† is the classical potential energy of the saddle
point measured from the overall zero of energy and
1ZPE is the harmonic zero-point energy—ZPE.
The partition functions, QA; QBC and QX; are given
by:
Qj ¼ Qtrans £ Qrot £ Qvib £ Qelect;
j ¼ A; BC; X
ð4Þ
where Qtrans; Qrot; Qvib and Qelect mean the component
of the translation, rotation, vibration and electronic
partition functions.
The translation partition function is:
Qtrans ¼2pmkBT
h2
� �3=2
ð5Þ
where m gives the mass of the atom or group of atom.
The rotation partition function depends on the
molecule structure, and for a linear molecule the
rotation partition function, Qrot22D; is:
Qrot22D ¼8p2I kBT
seh2
!ð6Þ
whereas for a nonlinear molecule the rotation partition
function, Qrot23D; is
Qrot23D ¼p1=2
se
8p2ImkBT
h2
!3=2" #ð7Þ
P.R.P. Barreto et al. / Journal of Molecular Structure (Theochem) 639 (2003) 167–176168
where I and Im ¼ IAIBIC denote the moment of inertia
and the product of inertia, respectively, se is the
symmetry number of the molecule.
The vibrational partition function and zero-point
energy, in the harmonic approximation, are given by:
Qvib ¼Y
i
1 2 exp2hvi
kBT
� �� 2di
ð8Þ
1ZPE ¼1
2hX
i
divi ð9Þ
where vi; is the frequency, i ¼ 1;…;F and F is the
degree of freedom (F ¼ 3N 2 5 for linear molecule,
3N 2 6 for nonlinear molecule and F 2 1 for the
transition state) and di is the mode degeneracy.
The electronic partition function is given by:
Qelect ¼X
i
gi exp2e i
kBT
� �ð10Þ
where gi is the degeneracy, and e i is the energy of
electronic state g measured from the overall zero of
energy.
We have also included the transmission coefficient,
kðTÞ; in Eq. (2):
kWTSTðTÞ ¼ kðTÞkTSTðTÞ ð11Þ
where the transmission coefficient is used to account
for the tunneling effect along the reaction coordinate.
We have decided to estimate the transmission
coefficient by using the Wigner correction [13,23,
26] instead of some semi-classical tunneling approxi-
mation [26]. The Wigner correction for tunneling
assumes a parabolic potential for the nuclear motion
near the transition state and therefore cannot be
considered as an accurate correction. The Wigner
transmission coefficient is given by:
kðTÞ ¼ 1 þ1
24
"v‡
kBT
����������2
ð12Þ
where the imaginary frequency at the saddle point is
denoted by v‡:
For more precise rate constants calculation the
variational TST should be used and also semi-
classical methods for the tunneling effect [26], yet in
this case more information about the potential energy
surface is required, but it is not employed in the
present work. A useful way to verify the importance
of using the variational transition state instead of
conventional transition state is the curvature of the
reaction path. A constraint on reaction-path curvature
is provided by the skew angle [26]:
b ¼ Arc CosmAmC
ðmA þ mBÞðmB þ mCÞ
� 1=2
ð13Þ
where mA; mB and mC are the masses of the A, B and
C moieties, respectively, for the schematic reaction
shown by Eq. (1). Large reaction-path curvature is
often encountered in the tunneling region in system
with small skew angles, so the tunneling effects
should be smaller.
For a generic bimolecular reaction:
A þ B Okf
krC þ D ð14Þ
where the rate constant in the forward direction, kf ;
determined by TST, may be conveniently writing in
the Arrhenius form as:
kf ¼ ATn exp2Ea
RT
� �ð15Þ
where A is the pre-exponential factor, n is the
temperature power factor and Ea is the activation
energy.
The rate constants in the reverse direction, kr; can
be determined using the forward constant rate and the
equilibrium constant, Kc; as:
kr ¼kf
Kc
ð16Þ
Remembering:
Kc ¼ Kp
p
RT
� �Sns
ð17Þ
and:
DG ¼ 2RT ln Kp ð18Þ
so, we can rewrite the reverse reaction rate as:
kr ¼ kf
RT
p
� �Sns
expDS
R
� �exp 2
DH
RT
� �ð19Þ
where Sns is the stoichiometric factor that is defined
to be positive for products and negative for reactants
and p is the pressure.
P.R.P. Barreto et al. / Journal of Molecular Structure (Theochem) 639 (2003) 167–176 169
2.2. Thermodynamic properties
To determine the reverse rate constants, Eq. (16), it
is necessary to know the thermodynamic properties
such as entropy and enthalpy for the reactants and
products.
The thermodynamic properties, internal energy,
entropy, and heat capacity, can be written in terms of
the partition functions as:
E ¼ kBT› ln Q
›lnT
� �v
ð20Þ
S ¼ kB ln Q þ kB
› ln Q
› ln T
� �v
ð21Þ
cv ¼ kB
› ln Q
› ln T
� �vþkB
›2 ln Q
›ðln TÞ2
!v
: ð22Þ
The partition functions are given by Eqs. (4)–(10).
Others thermodynamic properties such as enthalpy,
Gibbs free energy, can be obtained combining
the previous properties as:
H ; E þ pV ð23Þ
G ¼ H 2 ST : ð24Þ
Our code prints out the entropy and sensible enthalpy,
Hsen ¼ H 2 DfH0; for all species, reactants, products
and saddle point, as a functions of temperature.
3. Results and discussion
To apply TST, we must know the geometries,
frequencies, and the potential energy for reactants and
saddle point. These properties are obtained from
accurate electronic structure calculation performed
using the GAUSSIAN 98 program [27]. First, we
determined the reactants and saddle point geometry
and frequencies using the second order Møller–
Plesset perturbation theory and the triple-zeta plus
polarization 6-31G(d). Furthermore, we calculated
Table 1
Geometrical parameters (bond distance in Angstroms and bond angle in degree) of all stationary points of test reaction NH3 þ H ! NH2 þ H2
Parameter This worka Reference
Bb Cc Dd Ee Ff
NH3
RNH 1.0168 (1.0124g) 1.007 1.012 1.01
AHNH 106.4004 (106.67g) 107.4 106.1 106.1
NH2
RNH 1.0283 (1.024g) 1.023 1.02
AHNH 103.3089 (103.4g) 102.7 102.7
H2
RHH 0.7375 (0.7414g) 0.734 0.73
TS
RNH 1.019 1.015 1.023 1.023 1.013 1.02
RNH11.290 1.247 1.305 1.305 1.286 1.30
RH1H2 0.920 0.944 0.869 0.869 0.861 0.87
AHNH 106.40 103.5 103.5 103.5
AHNH1100.20 100.2 99.1 99.1 99.1
ANH1H2 162.59 162.6 158.5 158.5 177.7 158.5
DHNH1H2 57.48 53.8 52.7 52.7
a MP2/6-31G(d).b HF/SSANO [13].c MP2/6-31G(d) [14].d MP2/6-31p [15].e J. C. Corchado et al. [16].f MP2/6-31G(d,p) [17].g JANAF Table [28].
P.R.P. Barreto et al. / Journal of Molecular Structure (Theochem) 639 (2003) 167–176170
the energies for all species, reactants and saddle point,
at the MP2/6-311þþG(3df,3pd), MP4(SDTQ)/6-
31G p p and B3LYP/6-311þþG(3df,3pd) level.
The geometries computed at MP2/6-31G(d) for
reactant, product and saddle point are shown in
Table 1. These geometries are also compared with
reference data. The interatomic distance and bond
angle absolute errors for the reactants and products
are smaller than 0.0044 A and 0.38, respectively,
when comparing with the experimental results and
0.0098 A and 1.08, respectively, for the others
calculated results. While the interatomic distance
and bond angle absolute errors for the transition
state are 0.058 A and 4.788, respectively, exception
for the angle ANH1H2where this error is 15.118,
between our data and the Corchado and Espinosa-
Garcia one [16]. Fig. 1 shows the optimized
geometry of the saddle point, and also the notation
used in Table 1.
The harmonic vibrational frequencies calculated
from ab initio methods are overestimated, in this case,
in about 8%, then we have used a scaling factor of
92%. The scaled frequencies are also compared with
published data, as shown in Table 2. The absolute
error among our calculated and experimental fre-
quencies is less than 118 cm21, except for the HH
stretching where this error is 235 cm21. Comparing
the absolute error among our calculated frequencies
with published calculated frequencies for the reactants
and products we observe for the NH stretching an
error of 480 cm21, when for the other frequencies this
error is less than 189 cm21. The absolute error in
frequency for the transition state is less than
435 cm21, except for the imaginary frequency
where the minimum error is 181 cm21 for the
Henon data [13] and the maximum error is
1119 cm21 for the Corchado data [16].
3.1. Thermodynamic properties calculations
The first subroutine in our code calculates the
thermodynamic properties, particularly the entropy
and enthalpy, Eq. (21) and (23). Fig. 2 compares the
calculated properties with the reference data from the
JANAF tables [28] for the reactants, NH3 and H,
products, NH2 and H2 and, also, gives the thermo-
dynamic properties for the transition state
(TS ¼ NH4).
Compared with reference data [28], the maximum
absolute errors in our calculation and the reference
data are 4.71 kcal K21 mol21 for the NH3 entropy at
4000 K and 2.41 kcal mol21 for the NH3 enthalpy at
298.15 K. Fig. 3 shows the maximum, minimum andFig. 1. Atomic numbering of the reaction system for reaction
NH3 þ H ! NH2 þ H2.
Table 2
Harmonic vibrational frequencies (cm21) and zero-point energy
(kcal mol21) of reaction NH3 þ H ! NH2 þ H2
This worka Reference
Bb Cc Dd Ee Ff
NH3
1068 (950g) 1146 1067 1067
1614(2) (1627g) 1803(2) 1642(2) 1642(2)
3224 (3337g) 3709 3386 3386
3368(2) (3444g) 3841(2) 3535(2) 3535(2)
NH2
1501 (1497g) 1516 1516
3147 (3219g) 3307 3307
3259 (3301g) 3417 3417
H2
4170 4301 4301
TS
622 695 683 723 563 683
652 726 694 732 598 694
1033 1209 1089 1258 1038 1089
1310 1408 1249 1485 1327 1249
1490 1633 1530 1541 1640 1530
1538 1684 1973 1731 1902 1973
3243 3666 3314 3647 3383 3314
3366 3761 3425 3750 3446 3425
2536i 2717i 1920i 2833i 1417i 1920i
a MP2/6-31G(d).b HF/SSANO [13].c MP2/6-31G(d) [14].d MP2/6-31 p [15].e J.C. Corchado et al. [16].f MP2/6-31G(d,p) [17].g JANAF Table [28].
P.R.P. Barreto et al. / Journal of Molecular Structure (Theochem) 639 (2003) 167–176 171
average error in the entropy and enthalpy for each
species analyzed here.
3.2. Rate constants calculations
The total energies for reactants, products and
saddle point as well as the reaction enthalpy
and potential barrier calculated by different methods
and basis set are shown in Table 3, in which we
applied the ZPE correction in the reaction enthalpy
and potential barrier. We found a large range of
values for the potential barrier, 14.39 –
25.37 kcal mol21, for the methods and basis set
used, as well as, for the reaction enthalpy, where
this range is 0.12–8.68 kcal mol21. The same
behavior, for the potential barrier, is found in the
literature, range of 7.6–22.6 kcal mol21 [13–15,17].
We would like to emphasize the calculation made in
Fig. 2. Comparisons of thermodynamic properties, enthalpy and entropy, obtained by our code (symbols) with experimental data from JANAF
Table [28] (lines).
P.R.P. Barreto et al. / Journal of Molecular Structure (Theochem) 639 (2003) 167–176172
MP4(SDTQ)/6-31G p p , where the absolute error
in the potential barrier was 0.1 kcal mol21 when
compared with the Garrett et al. [15] results for the
same methods and basis set.
The skew angle in this case is 46.78, as it is a mid-
value angle, we should expect same tunneling effects.
For reactions with intermediate/high-value of the
skew angle the Wigner transmission coefficient is not
the best correction. On the other hand, we are
interested in a high temperature range, 1000–
3000 K, where this problem is greatly reduced.
Fig. 4 compares the reaction rate obtained in this
work, considering conventional TST and also apply-
ing the transmission coefficient of Wigner (TST/W)
with data from the Refs. [13–22]. Several reaction
rates were calculated, each one considering the
different energy calculated by the four methods and
basis set. One can see from Fig. 4 that the rate
constants obtained for MP4/6-31G p p level both
agree with experimental and theoretical results, in the
temperature range, 500–2000 K.
Fig. 5 compares the reaction rate obtained in this
work considering the energy determined by MP4/6-
31G p p level, with the data from the POLYRATE 8.7
program [29], considering TST, TST/W, canonical
variational transition state theory (CVT) and also
CVT with zero curvature tunneling (CVT/ZCT). It is
possible to see that the POLYRATE curves from TST
and CVT are coincident. In the region of low
temperature, T , 500 K; the reaction rate obtained
by CVT/ZCT considerably differs from our calcu-
lation and also the TST and CVT calculation, since
the skew angle has a moderate-value, 46.78, it is
excepted a tunneling effects that is not well deter-
mined by the Wigner transmission coefficient. The
ratio between the POLYRATE rate constant (TST or
TST/W) and ours (TST or TST/W) are 22 times at
Fig. 3. Absolute error for calculates thermodynamic properties,
enthalpy and entropy, obtained by our code with experimental data
from JANAF Table [28].
Table 3
Total energy ðEaÞ; potential barrier and reaction enthalpy (kcal mol21) including ZPE correction, where A ¼ MP2/6-31g(d), B ¼ MP2/6-
311þþg(d,p), C ¼ MP4/6-31g p p , D ¼ B3LYP/6-311þþg(3df,3pd)
A B C D
NH3 256.354211 256.4567673 256.4013892 256.4284264
H 20.4982329 20.4998179 20.4982329 20.4998098
NH2 255.6908562 255.7728291 255.7298852 255.7544436
H2 21.1441408 21.1649448 21.1645717 21.1683161
TS 256.8096988 256.9196000 256.8693051 256.9030052
Reaction enthalpy 7.82 8.68 0.12 0.32
Potential barrier 25.37 21.76 17.58 14.39
B3LYP/6-311þþG [13] V‡ ¼ 7.6 kcal mol21; CAS(3,3)/SANO [13] V‡ ¼ 22.6 kcal mol21; PMP4/6-311 þ G(d,p) [14] V‡ ¼
15.7 kcal mol21; MP2/6-31p [15] V‡ ¼ 21.59 kcal mol21; MP4/6-31pp [15] V‡ ¼ 17.68 kcal mol21; MP2/6-31G(d,p) [17] V‡ ¼
10.8 kcal mol21.
P.R.P. Barreto et al. / Journal of Molecular Structure (Theochem) 639 (2003) 167–176 173
Fig. 4. Comparisons of Arrhenius plot of k against the reciprocal temperature ðKÞ in the range of 400–4000 K for reactions NH3 þ
H ! NH2 þ H2 obtained in this paper with reference data where C, D, H, J and K are theoretical data from Espinosa-Garcia [14], Corchado
[16], Marshall [18], Garrett [15] and Espinosa-Garcia [17], respectively, and E, F, G and I are experimental results from Marshall [21], Hack
[22], Michael [20] and Ko [19], respectively.
Fig. 5. Comparisons of Arrhenius plot of k against the reciprocal temperature ðKÞ in the range of 200–4000 K for the reactions NH3 þ
H ! NH2 þ H2 obtained in this paper with data from the POLYRATE program, considering TST, TST/W, TST/MEPSAG, CVT, CVT/ZCT.
P.R.P. Barreto et al. / Journal of Molecular Structure (Theochem) 639 (2003) 167–176174
500 K and drops for seven times at 800 K and drops
more while the temperature increases. It does not
differ considerable if we consider the TST or the TST/
W rate constants.
4. Conclusion
Our purpose was to develop a simple code to
determine the reaction rate for the B/F/H/N system
easily. We opted to write our own code instead of
using the available ones such as POLYRATE
program, for example, since in that way we can
simplify the inputs files and write output files that
can be used directly by some graphical software.
Our code allows us to determine several reaction
rates at once since we have one input file for all
the reactants, one input file for the transition state,
and one input file describing which and how many
reactions we want to analyze.
The analysis of reaction NH3 þ H ! NH2 þ H2
allowed us to check the capability of our code.
Our code reproduce the reaction rate from the
reference data and from the POLYRATE program in
a high temperature range, T . 500 K: Now we
can extend this methodology for other reactions
and systems.
Acknowledgements
The authors are grateful to the Centro Nacional
de Computacao Cientıfica de Alto Desempenho de
Campinas-SP (CENAPAD-SP) for the provision of
computational facilities. A.F.A. Vilela acknowl-
edges the Conselho Nacional de Desenvolvimento
Cientıfico e Tecnologico (CNPq) for the
fellowship which provided her the opportunity to
perform this work.
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