179
J. Phys. Chem. 1991, 95, 179-183
Catalysis of Friedel-Crafts Reactions by Electric Fields Emili Carbonell, Miquel Duran, Agusti Lledbs, and Juan Bertrin* Department of Chemistry, Unioersitat Autbnoma de Barcelona, 081 93 Bellaterra, Catalonia, Spain (Received: March 27, 1990)
The effects of uniform and nonuniform electric fields on the HF + CH4 reaction have been investigated by using an ab initio R H F method with the 3-21+Gbasis set. It is found that barrier heights and positions of stationary points along the reaction coordinate are dramatically altered upon introduction of such electric fields. The changes are analyzed in terms of modifications in atomic charges and variations of electron densities at the main bond critical points. The electric fields are found to belong to the reaction coordinate of this chemical process.
Introduction The Friedel-Crafts reaction is one of the most important examples of catalytic processes in organic chemistry. This process, where alkylation of aliphatic and aromatic compounds by alkyl halides is carried out in the presence of acid catalysts, has become a subject of great interest. Among the few theoretical studies devoted to the Friedel-Crafts reaction, some deal with catalysis by the Lewis acid BF3. In particular, Alagona et. a1.I have studied, with the STO-3G basis set, the electrophilic attack step of the reaction between H F (taken as a model for an alkyl halide) and benzene catalyzed by BF3. In this reaction, a large decrease in potential barrier is found owing to a bifunctional mechanism where a six-membered ring involving two fluorine atoms is formed, so the coordinates of the catalyst intervene in the internal reaction coordinate. Further, Branchadell et ale2have studied the H F CH4 reaction catalyzed by BF3 through use of the 3-21G basis set. These authors find that, besides the bifunctional mechanism proposed previously by Alagona et. aL,I there can be an acid catalysis by BF3, in such a way that the internal coordinate of the reaction remains essentially unaffected. This second type of catalysis is less efficient in the gas phase than the bifunctional mechanism. However, they find that when the acid strength is very large, e.g., for a proton, the acid catalysis turns out to be very efficient. It is well-known that electric fields can change dramatically the rates of chemical processes. Several theoretical studies3-I5 have attempted to interpret the modifications of chemical reactions caused by electric fields. Some studies refer to proton transfers under the effect of an electric field generated by an ion3" or a uniform electric field?-I0 Other works deal with the effect of both kind of fields on the Walden inversion reaction."J2 In addition, uniform electric fields have been shown to dissociate ethylene into a hydride ion and protonated acetylene,I3 and to dissociate alkyl
+
~
~~
( I ) Alagona, G.;Scrocco, S.;Silla, E.; Tomasi, J. Tfieor.Cfiim. Acta 1977, 45, 127. (2) Branchadell, V.; Oliva, A.; Bertrin, J. J . Mol. Caral. 1988, 44, 285. (3) Cernusik, 1.; Urban, M. Collec?. Czech. Cfiem. Commun. 1978, 43, 1956. (4) Sanhueza, J. E.; Tapia, 0. J. Mol. Srruct. 1982, 89, 131. (5) Rode, B. M. Theor. Cfiim. Acta 1980, 56, 262. (6) Scheiner. S.; Redfern, P.; Szczesniak, M. M. J. Pfiys. Cfiem. 1985.89, 282. (7) Parker, B. R. Cfiem. Pfiys. Lett. 1974, 24, 22. (8) Zundel, G.; Eckert, M. J. Pfiys. Cfiem. 1987, 91, 5170. (9) Eckert, M.; Zundel, G.J . Phys. Cfiem. 1988, 92, 7016. (10) Andrts. J. L.; LledBs, A.; Duran, M.; Bertrin, J. Reu. Port. Quim., in press. ( I I ) Carbonell, E.; AndrCs, J. L.; Lledb, A.; Duran, M.; Bertrin, J. J. Am. Cfiem.SOC.1988, I IO, 996. (12) Andrts, J. L.; LledBs, A.; Duran, M.; Bertrln, J. Cfiem.Phys. Let?. 1988, 153, 82. (13) Pancir, J.; Zaharadnik, R. Heh. Cfiim. Acta 1978, 61, 59. (14) Carbonell, E.; LledBs, A.; Duran, M.; Bertrln, J. To be published. (15) Nakatsuji, H.: Hayakawa, T.; Yonezawa, T. J . Am. Cfiem.Soc. 1981, 103, 7426.
0022-365419 1/2095-0119$02.50/0
halides into halide anions and alkyl cations.I4 Finally, it has been shown that the potential energy profile of the NH3 umbrella inversion is affected by uniform electric fields.Is One can think that the large acid catalysis caused by a proton on the HF CHI reaction mentioned above may be due in large part to the electric field generated by the proton. The purpose of this paper is to study how nonuniform electric fields created by a positive charge placed at several distances from the fluorine atom and uniform electric fields with different intensities will influence the H F CHI reaction. Furthermore, understanding of this influence may allow to find new efficient ways to carry out the Friedel-Crafts process.
+
+
Methodology This theoretical work has been carried out by means of ab initio calculations using the gradient techniques which are commonplace in studies of potential energy surfaces.I6 Stationary points of perturbed surfaces have been located directly starting from the unperturbed stationary points and characterized by the correct number of negative eigenvalues of their Cartesian second derivative matrix. The a b initio calculations have been performed at the oneconfiguration self-consistent-field level of theory. Selected calculations have been carried out using perturbation theory up to second order to evaluate correlation energy. Given that the studied process exhibits a large charge separation, the diffuse function augmented 3-21+G basis has been used to describe adequately the electron density about negatively charged atoms. Some calculations have also employed the split-valence 3-2 1GI7 or the polarized 6-3 lG**19,20basis sets. The positive charge has been simulated by a hydrogen atom having a very small exponent and carrying no electron, whereas the presence of a uniform electric field has been considered through addition of the electron-fieldinteraction term into the one-electron Hamiltonian. Energy second derivatives for wave functions under the effect of a uniform electric field have been computed analytically by means of the formalism developed by Duran et aL2' The remainder of calculations have been carried out with the help of the GAUSSIAN .3622 and MONSTERGAUSS23 programs. ~
(16) Schlegel, H. B. J . Comput. Cfiem. 1982, 3, 214. (17) Binkley, J. S.; Pople, J. A.; Hehre, W. J. J . Am. Cfiem. Sot. 1980, 102, 939. ( 1 8 ) Clark, T.; Chandrasekhar, J.; Spitznagel, G. W.: Schleyer, P. von R. J. Comput. Cfiem. 1983,4, 294. (19) Hehre, W. J.; Ditchfield, R.; Pople, J. A. J. Cfiem. Pfiys. 1972, 56, 2257. (20) Hariharan. P. C.; Pople, J. A. Theor. Cfiim. Acta 1973, 28, 213. (21) Duran, M.; Andrts, J. L.;Lledb, A.; Bertrln, J. J . Cfiem.Pfiys. 1989, 90, 328. (22) Frisch, M. J.; Binkley, J. S.; Schlegel, H. B.; Raghavachari, K.; Melius, C. F.; Martin, R. L.; Stewart, J. J. P.; Bobrowicz, F. W.; Rohlfing, C. M.; Kahn, L. R.; Defrees, D. J.; Seeger, R.; Whiteside, R. A.; Fox, D. J.; Fleuder, E. M.; Pople, J. A. GAUSSIAN 86, Carnegie-Mellon Quantum Chemistry Publishing Unit: Pittsburgh, PA, 1984.
0 1991 American Chemical Society
180 The Journal of Physical Chemistry, Vol. 95, No. I, 1991
Carbonell et al.
n
Figure 1. 3-2 1 +G optimized geometries for the reactant complex (a, upper) and transition state (b, lower) of the HF + CH4 reaction. Distances are given in A.
Results and Discussion We will present first the results found for the uncatalyzed reaction using the 3-21+G basis set. Second, we will present the calculations for different nonuniform and uniform electric fields using the same basis set. Finally, we discuss the results found in the present work. A. Uncatalyzed Process. In Figure 1 we depict the optimized geometries of the reactant complex (or intermediate) (a) and the transition state (b) of the HF + CH, reaction. There is another energy minimum for the reactant complex where the C, H, and F atoms are not collinear, and whose energy is higher by 0.73 kcal/mol than that of the minimum presented in Figure la. However, even though larger basis sets or correlation energy might interchange the relative ordering of the two minima, only the first complex has the atoms placed adequately for the reaction to proceed. The stabilization energy of the optimized complex is I .23 kcal/mol relative to infinitely separated reactants. The geometrical parameters of each fragment of this structure are very similar to those of separated reactants. The same happens with its harmonic vibrational frequencies, with the addition of a new small frequency corresponding to the FH-C bond (88 cm-I), and two doubly degenerate low-frequency bendings (1 33 and 268 cm-I). The transition state (Figure 1b) shows a symmetrical structure involving a planar four-membered ring, with the two F-H distances longer than the two H-C distances. This transition state lies 84.45 kcal/mol above the intermediate of Figure la. This structure exhibits a large imaginary frequency (23601’cm-I), which is related to the sharp curvature characterizing a high-barrier process. The main components of the associated eigenvector show that in the transition state one of the two ring hydrogens is getting closer to the carbon atom, whereas the other hydrogen is approaching the fluorine atom. It is interesting to compare the results found in this work for the transition state using the 3-21+G basis set with that found in a previous work using the 3-21G basis set.2 The most important change appears in the different lengths of the C-H and H-F bonds on the ring: whereas the 3-21G basis set gives 1.37 and 1.26 A, respectively, the 3-21 +G basis set gives 1.26 and 1.42 A. Thus, the ordering in bond lengths has been interchanged, so with the 3-2 I +G basis set the hydrogens are closer to the carbon atom than they are to fluorine. This difference can be seen more clearly by looking at the electron density plots. Whereas with the 3-21G basis set (Figure 2a) a ring point appears, with the 3-21+G basis set (Figure 2b) two different fragments are observed (CH5+and F),and the two C-H bond paths show the curvature visually presented by nonclassical ions2, The interaction between those two fragments has an electrostatic nature, as confirmed by a Mulliken population analysis showing the charge of the CH5 fragment to be 0.65 au. On the contrary, the charge on this (23) Peterson, M.R.; Pokier, R.A. Program MONSTERGAUSS, Department of Chemistry, University of Toronto, 198 1 . (24) Bader, R. W. F.; Tal, Y.; Anderson, S. G.; Nguyen-Dang, T. T. Zsr. J . Chem. 1980, 19, 8, and references therein.
Figure 2. Electron density plots for the transition state of the HF + CH4 reaction computed with the 3-21G basis set (a, upper) and the 3-21+G basis set (b, lower). Bond critical points ( 0 )and bond paths are also shown.
fragment for the 3-21G basis set is only 0.52 au. The reason for the increase in polar character of the transition state upon extension of the basis set lies in the fact that the additional basis functions favor the ionic structure CH5+=F.Given that the studied reaction seems to proceed through a polar transition state, it is clear from the above results that it is necessary to include diffuse functions to compute a good wave function.and thus to optimize correct geometries for transition states. In order to test further the quality of the 3-21+G basis set on the present calculations, we have reoptimized the transition state using the 6-31G** basis set. The C-H and H-F bond lengths become 1.29 and 1.32 A, respectively. Regarding fragment charges, the charge on CH5 is 0.63 au. Finally, no ring point appears. One observes thus that the differences between the 3-21G and 3-21+G basis sets are repeated to some extent between the 3-21G and 6-31G** basis sets. Therefore, the 3-21+G basis set, although being computationally much cheaper than the 6-3 1G** basis set, does already yield the correct trends for this structure. As to correlation energy, we have reoptimized the transition state at the MP2/3-21+G level of theory. The new C-H and H-F bond lengths turn out to be 1.30 and 1.43 A respectively; further, the charge on the CH5 fragment is 0.64 au. These results do not differ substantially from those lacking inclusion of correlation energy, yet it is very interesting to note that the position of this stationary point in the total potential energy hypersurface does change to a small extent. Given the similarity between the results including and excluding correlation energy, we have restricted our calculations to the one-configuration level. B. Effect of Electric Fields. In a series of proton-transfer reaction~~”9~~ and in the Walden inversion reaction” it has been shown that the effect of a positive ion is represented mainly by the effect of a positive charge. Of course there is some effect owing to the charge transfer to the ion. The goal of this section is to carry out an insight into the effect produced in Friedel-Crafts reactions by a positive charge placed at several distances from the fluorine atom, which gives rise to a nonuniform electric field. Likewise, we shall study the effect of a uniform electric field with different intensities on the same reaction. As mentioned above, the 3-21+G basis set will be used for all computations. In all cases the complete set of geometrical parameters of the perturbed systems have been reoptimized to obtain a stationary point, so the systems have aligned freely with respect to the electric field. In particular, the second reactant complex mentioned in section A is aligned inadequately by electric fields for the reaction to proceed, so it is no longer considered in this work. (25) Sol& M.; Lledbs, A.; Duran, M.; Bertrln, J. To be published.
The Journal of Physical Chemistry, Vol. 95, No. I, 1991 181
Catalysis of Friedel-Crafts Reactions TABLE I: Effect of a Positive Charge on the Reactant Complex and on the Transition State of the Reaction Studied"
TABLE II: Effect of a Uniform Electric Field on the Reactant Complex and on the Transition State of the Reaction Studied"
Reactant Complex
Reactant Comolex d m
10.00 5.00 4.00 3.00 2.00 0.94
rl
4
r2
0.94 0.94 0.95 0.95 0.95 0.96 1.03
2.50 -1.56 2.47 -1.53 2.40 -1.45 2.37 -1.42 2.29 -1.34 2.12 -1.16 1.66 -0.63
E -139.48676 -139.48946 -139.49661 -139.50157 -139.511 73 -139.53897 -139.59864
QF
un-c
-0.50 -0.51 -0.53 -0.54 -0.57 -0.65 -0.80
88 92 103 111
126 165 314
Transition State m
10.00 5.00 4.00 3.00 2.00 0.94
1.42 1.44 1.49 1.52 1.57 1.68 1.75
r2
4
1.26 1.25 1.24 1.23 1.22 1.21 1.20
0.16 0.19 0.25 0.29 0.35 0.43 0.55
E -1 39.352I6 -1 39.35758 -139.37230 -139.38273 -139.40449 -139.46267 -139.58031
QF
hmag barrier
-0.65 -0.68 -0.70 -0.76 -0.81 -0.90 -0.97
23603 84.45 2224i 82.75 19583 78.01 17231 72.56 1371i 66.29 7733 47.87 458' 11.50
" d , rl. and r2 are defined in Figure 3, whereas q is defined in the
text. E stands for absolute energy, QF stands for the total charge on fluorine, and u H X stands for the stretching frequency corresponding fundamentally to H-C bond. uimpBstands for the imaginary frequency of the transition state.
F 0.000 0.005 0.010 0.020 0.030
rl
r2
4
0.94 0.94 0.95 0.95 0.96
2.50 2.47 2.38 2.27 2.18
-1.56 -1.49 -1.43 -1.32 -1.22
E -139.48676 -139.49250 -139.49876 -139.51286 -139.52916
un-c 88 99 112 136 162
QF
-0.50 -0.52 -0.53 -0.57 -0.60
Transition State
F
rl
r2
4
0.000 0.005 0.010 0.020 0.030
1.42 1.47 1.53 1.66 1.95
1.26 1.24 1.23 1.23 1.22
0.16 0.23 0.30 0.43 0.73
E -139.35216 -139.36399 -139.37727 -139.40897 -139.44852
QF -0.65 -0.70 -0.75 -0.84 -0.94
uimg
barrier
23603 20893 1730i 1073i 4751'
84.45 80.70 76.24 65.19 50.60
F is the intensity of the applied field, r , and r, are defined in Figure 3, whereas q is defined in the text. E stands for absolute energy, QF stands for the total charge on fluorine, and uH-c stands for the
stretching frequency corresponding fundamentally to H-C bond. uiml stands for the imaginary frequency of the transition state. 1 au of electric field = 5.1422 X IO" V/m. -139.3
Y
Figure 3. Schematic definition of geometrical parameters involved in the
HF + CHI reaction catalyzed by a positive charge (e). (a, upper) reactant complex; (b, lower) transition state.
-139.6
I
0.94
-1
i
0
In Tables I and I 1 we present the geometrical parameters and absolute energies of the intermediates and transition states catalyzed by a positive charge or by a uniform electric field, where rl and rz have been defined according to Figure 3a (intermediate) and Figure 3b (transition state), d is the distance between the positive charge and fluorine, whereas F stands for the intensity of the uniform electric field. Q F is the charge on the fluorine atom. The reaction coordinate q has also been defined as the difference between r, and rz. For intermediates, the stretching frequency corresponding fundamentally to the bond between the two fragments is also given, whereas for the transition states the energy relative to the intermediate with the same perturbation and the value of the imaginary frequency are also shown. In Figure 4 we present the plots of the potential energy curves corresponding to the reaction catalyzed by the positive charge (a) and by the uniform electric field (b). We have only plotted the curve from the intermediate to the transition state. Obviously, the evolution of the system past the transition state would lead to a structure similar to the initial intermediate. From the values of r , and r, in Tables I and I1 and more clearly from Figure 4,one can see that when the strength of the perturbation is increased the intermediate is found later along the reaction coordinate. This means that the transfer process of a hydrogen to the alkyl group is more advanced the stronger the perturbation, given that the hydrogen-carbon distance is shorter. This also means that the FH.CH4 complex becomes stronger upon increase of the field intensity. This is reflected as well in the increase of the frequency corresponding to the H-C stretching which increases from a very low value in the field-free intermediate (88 cm-I) to much higher values for strong fields (e.g., 162 cm-l for F = 0.03 au). Further, the ion-pair character of the inter-
E l
9
- 139.3
0.000
0.030
- 139.5. - 139.6 -1
0
i
9
Figure 4. Potential energy schemes for the HF + CHI reaction catalyzed by a nonuniform electric field created by a positive charge placed at several distances from the fluorine atom (a, upper) and by a uniform electric field (b, lower). q (in A) stands for the reaction coordinate (see text). Energies are given in au.
mediate increases with the strength of the perturbation, as seen from the values of QF in Tables I and 11. Finally, it must be remarked that the stabilization energy at the intermediate by the effect of the electric field applied is far higher than the thermal agitation energy, so the chemical system will align itself spontaneously along the direction of the electric field.
Carbonell et al.
182 The Journal of Physical Chemistry, Vol. 95, No. 1, 1991 TABLE 111: Energy Gradients Induced by the Electric Fields, and Electron Densities at Bond Critical Points for the F-H and H-C Bonds in the Reactant Complex (at the Field-Free Geometry) of the Studied Reaction
d
QF
Positive Charge bcp(F-H) bcp(H-C) grad(F-H)
grad(H-C)
0.0073 0.0073 0.0073 0.0072 0.0071 0.0060 0.0068
0.0000 0.0001 0.0005 0.0007 0.001 1 0.0022 0.0055
10.00 5.00 4.00 3.00 2.00 0.94
-0.50 -0.51 -0.53 -0.54 -0.57 -0.63 -0.76
0.3165 0.3157 0.3133 0.31 16 0.3081 0.2978 0.1900
F
QF
bcp(F-H)
bcp(H-C)
grad(F-H)
grad(H-C)
0.3165 0.3147 0.3129 0.3089 0.3047
0.0073 0.0074 0.0075 0.0077 0.0079
0.0000 -0.0022 -0.0047 -0.0102 -0.0166
0.0000 0.0003 0.0006 0.0013 0.0021
m
0.0000 -0.0009 -0.0029 -0.0042 -0.0068 -0.0140 -0.0356
Uniform Electric Field 0.000 0.005 0.010 0.020 0.030
-0.50 -0.52 -0.53 -0.56 -0.58
"he nonuniform electric field is created by a positive charge placed at a distance d (in A) from the system. The uniform electric field has an intensity F. QF stands for the total charge on fluorine. 1 au of electric field = 5.1422 X IO" V / m .
Paying attention to the transition state, one can see in Tables I and I 1 that upon increase of the field strength the transition state is found later along the reaction coordinate. This behavior emerges even more clearly from Figure 4. In parallel, an increase in the ion-pair character of the transition state is found. Given that the transition state exhibits a higher charge separation than the intermediate, its stabilization by electric fields is also larger. Thus, when the distance between the positive charge and the fluorine atom is shortened or when the intensity of the uniform electric field is augmented, the energy barriers relative to intermediates decrease. For instance, the field-free barrier of 84.45 kcal/mol becomes barely 1 1 S O kcal/mol when a positive charge is placed at the normal distance of proton from fluorine (0.94 A), and to 50.60 kcal/mol when a uniform electric field of 0.03 au is applied. Related to the height of the barrier is the value of the imaginary frequency of the transition state, which is lower when the strength of the perturbation increases, thus showing a smaller curvature of the potential energy profile, i.e., a flatter potential energy surface. The electron density maps on the plane containing the fourmembered ring (Figure 2b) are not altered to a great extent when the perturbations increase. No ring point appears either, whereas the values of the electron density at the H-C bond critical points increase, and those at the F-CH5+ bond critical point decrease with the field strength. Since a diminution in the value of the density at a bond critical point means an increase in the electrostatic character of such a bond, the electrostatic interaction between the considered fragments increases with the field strength. This can be confirmed also from the values of the charges in both fragments for the different field intensities. For instance, when the positive charge is placed at the normal distance of a proton from fluorine (0.94 A),the charge of the CH5 fragment is 0.97 au. I n addition, for a uniform electric field having an intensity of F = 0.03 au, the same fragment has a charge of 0.94 au. C. Discussion. We have seen in the preceding section that the positions of the stationary points along the reaction coordinate and the barrier heights are dramatically altered upon increase of the intensity of a uniform electric field or a shortening of the fluorine-positive charge distance. Therefore, the external perturbation, in this case an electric field, belongs to a correct representation of the reaction coordinate. To get a deeper insight into the participation of this external perturbation, we have applied the uniform electric field with various intensities and placed the positive charge at several distances from the fluorine atom at the field-free optimized intermediate, without allowing for any geometry relaxation. Thus, we collect in Table 111 the charge on
(eF),
the fluorine atom the F-H and the H-C bond critical point densities, and the derivatives of the total energy with respect to the F-H and H-C bond lengths, for all values of the fluorinepositive distance and uniform electric field intensities. The values for the the fluorine atom show that its negative charge increases with the strength of the perturbation, thus augmenting the ionic character of the structure. In this reaction, given that the transition state has an important ionic character, this increase in polar character of the field-free intermediate represents an advance in the reaction coordinate from an electronic point of view. If attention is paid to the values of the electron density at the F-H and H-C bond critical points, it can be seen that, when the positive charge gets closer to fluorine, the value at the F-H bond critical point is decreased. When the charge is placed at the normal distance a proton would be (0.94 A),the decrease in electron density is so strong that the minimum in density along the fluorine-hydrogen bond is found on the very hydrogen, with a value much lower than those found for longer distances. The changes produced on the H-C bond critical point are much smaller. If attention is turned to the effect of a uniform electric field on the same critical points, one can see that the density at the F-H critical point decreases and the density at the H-C bond critical point increases. Thus, according to the relationship between electron densities at bond critical points and bond strengths,24the C-H bond strengthens and the H-F bond weakens. A similar modification in electron density could have been produced by a lengthening of the F-H distance and a shortening of the H-C distance. These two distances obviously belong to the reaction coordinate. Thus, since the electric field causes an effect identical with that obtained by those variations, one can say that the electric field belongs to the reaction coordinate, which must be understood in a less restrictive sense than usual. In order to understand the reasons for the evolution of the intermediate along the reaction coordinate, we must look at the forces acting on the nuclei of the intermediate. Obviously the forces acting on the nuclei of the field-free intermediate are zero in the absence of perturbation. However, in Table I11 one can see that, upon increase of the field strength, without geometry change, a force (negative gradient) appears trying to lengthen the F-H distance and to shorten the H-C distance. The origins of these forces have been pointed out by N a k a t s ~ j ias ' ~ owing to the polarization of the electronic cloud. The force acting on a nucleus will follow the direction of the field if the electron population around the nucleus is larger than the nuclear charge, and follow the opposite direction otherwise. In the present system, the total charges on fluorine and carbon are negative, whereas the hydrogen charges are positive. Thus, one can understand that the induced forces will cause the transfer of the hydrogen atom from fluorine to carbon, thus leading to an advance of the intermediate in the reaction coordinate and originating a decrease in energy barrier. Conclusions
In the reaction studied in this paper the H F molecule is taken as a model for an alkyl halide. This notwithstanding, the polar character of the transition state we have found would be common to Friedel-Crafts reactions with alkyl halides. This strong polar character means that any electric field will align adequately the chemical system in a favorable way for the process to take place. Likewise, the applied field will dramatically lower the energy barrier of the process. The calculations presented here lack inclusion of correlation energy owing to practical methodological reasons. Although the introduction of correlation energy might change slightly the quantitative results, the overall qualitative conclusions would not be modified. The effect of an electric field favoring the H F + CHI reaction is one of the reasons for the efficiency of the acid catalysis in Friedel-Crafts reactions. Furthermore, it leads us to better understand the reason why a Friedel-Crafts reaction is largely accelerated in solution. In this case, the polar character of the intermediate and the transition state creates a reaction field. Given that the polar character at the transition state is higher than at
183
J . Phys. Chem. 1991, 95, 183-191 the intermediate, the former will be more stabilized than the latter, because the field will be more intense and the interaction between the electric field and the chemical system will be larger. Finally, it is worth mentioning that this reaction is especially suited to be catalyzed by strong uniform electric fields, because the chemical system aligns itself in a suitable way. Such field are nowadays
attainable by means of different experimental techniques and may provide a new way to carry out an efficient Friedel4rafts reaction.
Acknowledgment. This work has been supported by the Spanish Direccidn General de Investigacidn Cientifica y TEcnica under Project No. PB86-0529.
A Shock Tube Study of Reactions of C Atoms with H, and 0, Using Excimer Photolysis of C,O, and C Atom Atomic Resonance Absorption Spectroscopy A. J. Dean, D. F. Davidson,* and R. K. Hanson High Temperature Gasdynamics Laboratory, Department of Mechanical Engineering, Stanford University, Stanford, California 94305 (Received: April 2, 1990; In Final Form: July 11, 1990)
The reactions of C(3P) with H2 and O2were studied at high temperature in reflected shock wave experiments. C atoms were detected at 156.1 nm by atomic resonance absorption spectroscopy (ARAS). Controlled levels of C atoms for calibrating the ARAS diagnostic were made by high-temperature pyrolysis of highly dilute CH4/Ar mixtures. For kinetics experiments, C atoms were formed by pyrolysis of C302above 2300 K and by ArF excimer photolysis of C302below 2050 K. Formation of C atoms in the presence of excess H2or O2permitted first-order rate coefficient determinations of the reactions C(,P) + H2 CH H ( I ) and C('P) O2 CO 0 (2), yielding k , = 4.0 X IOL4exp(-l1700 KIT) (*50%) cm3 mol-' s-I, over the temperature range 1525-2540 K and pressure range 0.5-1.2 atm, and k , = 1.2 X I O l 4 exp(-2010 KIT) (*50%) cm3 mol-' s-I, over the temperature range 1500-4200 K. The rate coefficient of the reverse (exothermic) reaction, k-', is I . I X I O l 4 cm3 mol-' s-l, constant with temperature. A rate coefficient for the reaction C C3O2 products (6) of k6 = 2.4 X 1 019Tl.75 cm3 mol-' s-I, over the temperature range 1450-1 900 K, was determined from photolysis experiments. High-temperature absorption coefficient data for C302at 156 and 193 nm (1500-1800 K) are also presented.
-
+
+
-
+
-
+
-
Introduction
s-' between IO00 and 1500 K, by comparison with low-temperature
Atomic carbon is an important reactive component of hydrocarbon flames. Although a minor species in most combustion situations, C atoms play a role in hydrocarbon oxidation and in NO, formation and removal. The reactions C(3P) H2 C H H AH298 = 22.8 kcal/mol (1)
rate coefficient measurements of CH2 H CH H2. Becker et aL7 recently determined k-l = 8.4 X 10I2cm3 mol-' s-l in a photolysis flow reactor at room temperature. Reaction 2 has previously been measured at room temperature. Braun et aL8 and Husain and Kirsh9 both report values of 2.0 X 1OI3 cm3 mol-' s-' from experiments in which C atoms were formed by photolysis of C302. A slightly lower value of 1.6 X IOl3 cm3 mo1-I s-l is reported by Husain and Younglo using a similar technique. Recently, Becker et al." determined a value of 2.8 X I O l 3 cm3 mol-' s-I from low-pressure experiments in which C atoms were formed from KrF excimer laser photolysis of CH2Br2. The study of C atom reactions at combustion temperatures requires a controlled source of C atoms. C atoms can be generated by either photolysis or pyrolysis. Pyrolysis of highly dilute mixtures of simple hydrocarbons such as ethane or methane above 3000 K rapidly leads to the formation of steady levels of C atoms (Le., C atom plateaus) which correspond to almost complete conversion of the parent molecule to C atoms. Similarly, the pyrolysis of highly dilute mixtures of C3O2 above 2300 K also yields C atom plateaus. In this experiment, the presence of an excess of H2 or O2(compared to the C atom concentration) results in a first-order decay in the C atom concentration-time profile. The time constant of the decay is inversely proportional to the rate coefficient. Photolysis of C 3 0 2in the vacuum-ultraviolet region can also serve as a direct source of C atoms.12 C 3 0 2has a very strong
- +
+
C(3P)
+ O2
CO
+
0
AH298 = -138.1 kcal/mol
(2)
are of particular interest in hydrocarbon flames. Several workers'" have postulated reaction -1 to be the principal source of C atoms in rich hydrocarbon flames. Peeters and Vinckier4 measured C atoms in a rich C2H4/02/Ar flame with 4 = 1.06. They found the ratio [C]/[CH] to be 0.3, indicating rapid conversion of C H to C atoms. Reaction -1 was proposed as the likely channel for C atom formation, since k-, is large and the concentration of H atoms is relatively high in rich flames. Reaction 2 is an important C atom removal channel in hydrocarbon-air flames. There are no high-temperature measurements of the rate of reaction 1 in either direction. Grebe and Homann5 include this reaction in a mechanism which successfully models the C2* chemiluminescence from a discharge flow reactor at 298 K. They infer k-, = 3 X I O l 3 cm3 mol-' s-l. Thorne et aL6 include this reaction in their mechanism for hydrocarbon/NO interaction in a low-pressure flame. They estimate k-, as 1.5 X IOI4 cm3 mol-' ( I ) Gaydon, A. G. The Spectroscopy of Flames, 2nd ed.; Chapman and Hall: London, 1974. (2) Becker, K. H.; Kley, D.; Norstrom, R. J. Symp. ( f n t . ) Combust. [Proc.],12th 1969, 405. ( 3 ) Peeters. J.; Lambert, J. F.;Hertoghe, P.; van Tiggelen, A. Symp. (fnt.) Combust. [Proc.],13th 1971, 321. (4) Peeters, J.; Vinckier, C. Symp. (fnt.) Combust. [Proc.],15th 1974,969. (5) Grebe, J.; Homann, K. H. Ber. Bunsen-Ges. Phys. Chem. 1982, 86, 587. ( 6 ) Thorne, L. R.; Branch, M. C.; Chandler, D. W.; Kee, R. J.; Miller, J. A. Symp. (Int.) Combust. [Proc.],2/st 1986, 965.
0022-3654/9 1/2095-0l83$02.50/0
+
+
(7) Becker, K. H.; Engelhardt, B.; Weisen, P. Chem. Phys. Lett. 1989, 154, 742.
(8) Braun, W.; Bass, A. M.;Davis, D. D.; Simmons, J. D. Proc. R. Soc. London, A 1969, 312, 417. (9) Husain, D.; Kirsh, L. J. Trans. Faraday SOC.1971, 67, 2025. (10) Husain, D.; Young, A. N. J . Chem. Soc., Faraday Trans. 2 1975, 71, JLJ.
( 1 I ) Becker, K. H.; Brockmann, K. J.; Wiesen, P. J . Chem. Soc., Faraday Trans. 2 1988, 84. 455.
0 1991 American Chemical Society
