RESEARCH
Shock Tube Reveals Reaction Mechanisms Rate-determining steps established for homogeneous isotope exchange reactions between deuterium and ammonia, acetylene New significant information on reaction mechanisms has been obtained by Cornell University chemists using shock-tube techniques [/. Am. Chem. Soc, 87, 143, 150 (1965)]. One study by Dr. S. H. Bauer and his coworkers shows that the homogeneous isotope exchange reaction between deuterium and ammonia (diluted with 96 to 987c argon) proceeds via a mechanism in which the vibrational excitation of deuterium molecules by argon determines the rate. This exchange is the model reaction for the general class, AB + C D -> AC + BD. The Cornell workers have also found, in another study, that the homogeneous gas-phase reaction between acetylene and deuterium goes via a different mechanism. This reaction is also a model for atom displacement, but more complicated than the ammonia-deuterium case. By studying the NH 3 -D 2 exchange reaction (at 1300° to 1700° K.) behind reflected shocks in a single-pulse shock tube, Dr. Bauer and his coworkers, Dr. Assa Lifshitz and Dr. Chava Lifshitz (both now at Hebrew University, Jerusalem, Israel), have found that the exchange rate is first order with respect to deuterium and argon and almost zero order with respect to ammonia. The major advantage of the shocktube technique is the direct gas dynamic heating ,of the sample, which allows the walls of the reaction vessel to remain at room temperature. Under these conditions all of the reactions being studied are homogeneous and not wall-catalyzed. The heating is extremely rapid (10 8 second), and reaction times are very short (500 to 800 microseconds). Thus, the sequence of reaction steps in a particular reaction can be followed as a function of time. By contrast, previous kinetic studies of D-H exchange had the drawback that the thermal energy came through the walls of the reaction vessel. The walls thus took part in 40
C&EN
JAN.
2 5,
1965
the kinetic processes, Dr. Bauer says. This heterogeneous process is particularly critical in exchange reactions where the homogeneous process is far too slow to account for the exchange rates found. The Cornell chemists show that the ammonia-deuterium exchange reaction in the presence of argon is neither simple bimolecular nor a radical chain. (Kineticists generally agree that these two mechanisms are probable in most
cases.) The rate-controlling step is not the formation of a bimolecular four-centered transition state because the reaction rate is independent of the ammonia concentration. Furthermore, the activation energy found at Cornell for the homogeneous exchange (40 kilocalories per mole) is considerably lower than it would be if the rate-limiting step were the formation of a four-centered transition state (about 60 kilocalories per mole).
Shock-Tube Techniques Well-Suited for Study of Reaction Steps Chemical kinetics investigations fall into two broad categories. A kineticist must first establish the mechanism for the over-all reaction as it's observed. Then he may undertake to probe in detail the individual reaction steps. Shock-tube techniques are particularly useful for the latter type of study and, in general, can be applied to the solution of some problems that can't be handled using other methods. The most generally useful tool for chemical kineticists is the single-pulse shock tube. Relative rates can be measured with precision, even if the exact temperature is not known (as Dr. Wing Tsang at the National Bureau of Standards, Washington, D.C., has shown). Some of the special features of shock-tube experiments are: • The ranges of temperature and density in which experiments can be made are wider than with conventional techniques. • Heterogeneous reactions are eliminated. • Reactions which appear to be concurrent using other techniques often can be separated and individually studied as a function of time. The resolution in time of a sequence of reaction steps is possible because of the initial very rapid and homogeneous heating of the sample by the shock wave. But the shock-tube technique also has some limitations: • It isn't possible to use gas densities below 1/200 atmosphere (as measured at normal temperature and pressure) unless the shock tube has a very large diameter. (For example, a 50-cm. diameter tube is being used for low-density studies at Avco Corp.'s Everett Research Laboratory, Everett, Mass.) • The purity of samples is sometimes in question because of the large "test tube" and other metal gas-handling equipment generally used. (Dr. Bauer at Cornell is designing a glass-Teflon shock tube which will have no metal parts, will be free of grease, and heatable to 200° C.) • Shock temperatures aren't known accurately enough. • Rapid spectrochemical analysis (while desirable) is limited to a few reaction species. (However, mass spectrometric analyses seem feasible with a time resolution of 10 to 15 microseconds.)
Rate Control. In fact, the rate is controlled by the collision frequency between D 2 and argon. At first, Dr. Bauer says, it may seem that the rate is determined by a homogeneous dissociation of D 2 into atoms, followed by a fast reaction with ammonia. However, he and his co-workers calculated (using the measured dissociation rate of D 2 with argon) that the number of D atoms that could be formed is 10~7 of the number of NH 2 D molecules generated during the specified time interval. Thus, dissociation can't be the rate-determining first step with an atom displacement exchange as the second step. A chain mechanism, another possibility, can also be ruled out. In the first place, the extremely long chain required to account for the observed exchange rate should depend on ammonia concentration. However, this is contrary to the Cornell data. But there is a second, much stronger
SHOCK TUBE. Dr. Peter Borrell (left) and Dr. William S. Watt use a single-pulse shock tube to study homogeneous isotope exchange reactions such as NH3-D2 and CH4-D2
Rate of Deuterium-Ammonia Exchange Is Probably Limited by the Population of Vfbrationally Excited Deuterium Molecules Three alternative mechanisms have been proposed for the deuterium-ammonia exchange by Dr. Bauer and his co-workers at Cornell. Each mechanism involves an unusual feature, but all reduce to the observed rate law if sufficient special assumptions are made. The most probable of the three alternatives is the one where the rate of exchange is limited by the population of vibrationally excited deuterium molecules (in the fifth and higher levels). In this case: P 2 +Mj"^ZP z +Mj
P< V W' NH/V)+D2^
(M. represents the reaction partners; i = 1 for Ar; i = 2 for NH:5; i = 3 for Do) In general, k\ < < kx. In addition, for the NH:5-D^ exchange, k€i [Ar] > > (k€2 [NH3] + k63 [D2]) and k x [NH 3 ] > > k- € i Mi. The first inequality infers that vibrationally excited D2 molecules react very rapidly with ammonia molecules, but vibrationally excited NH3 molecules react comparatively slowly with deuterium molecules. This also holds when H2S or CH. react with D2. However, data on the H2-D2 exchange show that the H2
+ H2. In preliminary studies of the CH4-D2 reaction, Dr. Bauer (with Dr. Peter Borrell, now at the University of Keele, Staffordshire, England, and Dr. William S. Watt, now at the Cornell Aeronautical Laboratory, Buffalo, N. Y.) found that k el /k €2 ~ 1; k el /k c3 ~ 1/100; and k x / k - € l ~ 300. Thus the general vibrational excitation mechanism is further confirmed. The alternative possibilities, that deuterium forms an electronically excited complex with argon or that deuterium forms a relatively long-lived complex with the exchange partners, would require unreasonable assumptions to permit reduction of the kinetic equations to the observed rate law. Therefore, such mechanisms are not likely.
argument to exclude a chain mechanism, Dr. Bauer believes. The chain mechanism is based on an atom displacement reaction: D + NH 3 -^ NHoD + H. Since the activation energy for this reaction should be about 10 kilocalories per mole, the rate constant would have to have a pre-exponential factor higher than 10 18 cc. iniole -1 sec. - 1 to account for the observed exchange rate. This is too high for a simple bimolecular process. The mechanism favored by Dr. Bauer and his co-workers revolves around the production of a steadystate concentration of vibrationally excited deuterium molecules. The probability of reaction between the vibrationally excited D 2 [D 2 (v) ] and NH.} (or HoS or H 2 ) is independent of the relative translational energy of the colliding pair. But the probability is greatly influenced by v (the vibrational energy level); it is quite low when v is less than 5 and becomes large when v exceeds 6. This implies that the activation energy for the exchange reaction must be available in an appropriate molecular mode. Hard collisions with large amounts of momentum normal to the direction of separation are not particularly effective. But large vibrational amplitudes which provide momentum along the direction of separation are. Acetylene. In studying the homogeneous gas-phase reaction between JAN.
2 5, 1965
C&EN
41
Exchange mechanism for D2-C2H2 reaction involves two types of unstable intermediates
and Ho. The transition state (in the acetylene reaction) has the structure
H— C=
c—H
*
*
«
0
i>—x> rather than
H-cssC—-H D
o
which would be expected by analogy with the transition state of the N H ; r D 2 exchange reaction:
H /*
x>—v
Molecular complex intermediate is particularly sensitive to dissociation by collision with another acetylene molecule
Excited "ethylene" intermediate has Vd symmetry and is about 3 e.v. above the ground state
acetylene and deuterium, Dr. Bauer and Dr. Kenji Kuratani (now at the Aeronautical Research Institute, University of Tokyo, Japan) used incident shocks in samples diluted (to 17 to 20%) with argon. They measured the substitution rate of D for H at 1300° to 1665° K. by recording the infrared emission intensity of C 2 HD (at 2555 cm.- 1 ) and of C 2 H 2 (at 3195 cm." 1 ). The total order for the C 2 H 2 -D 2 substitution reaction is 1.24 ± 0.05. The effective activation energy for the substitution reaction is 32.5 kilocalories per mole. Dr. Bauer believes that two types of transient species are involved in the reaction mechanism. One is a "loose" complex (C 2 H 2 -D 2 ) which is readily destroyed by collisions with C 2 H 2 molecules. The other is a "tight" complex (C 2 H 2 D J ) which resembles highly excited ethylene. The Cornell chemists find intriguing the fact that, during their shock-tube experiments, "unsuspected" reactions
took place. These reactions occurred at high temperatures but under conditions they had previously assumed weren't severe enough for appreciable exchange to take place. For instance, when acetylene and hydrogen are mixed at temperatures up to 1700° K., no change in the concentration of acetylene can be detected for as long as 1 millisecond. During this interval, however, each acetylene molecule is subjected to an enormous number of collisions with argon (the ambient gas), acetylene, and hydrogen molecules. Acetylene and hydrogen are actually involved in an extensive association-dissociation reaction, with the acetylene rapidly replenished by the reverse process. This becomes evident, Dr. Bauer says, when deuterium is used in place of hydrogen. From the empirical rate expressions derived from their shock-tube data, Dr. Bauer and his co-workers have established that the mechanism for PI-D substitution in C 2 H 2 differs significantly from that in NH 3 , H 2 S, HCl,
42
C&EN
JAN.
25,
1965
This deduction is based on the relative amounts of C 2 H 2 lost and of C 2 HD and C2Do generated, and on the overall kinetic rate law. Two factors prompted the Cornell chemists to rule out the possibility of chain reactions in the C 2 H 2 -D 2 exchange. They found that the activation energy for the exchange reaction is low (about 33 kilocalories per mole), and that the rate depends on the first power of the deuterium concentration. Indeed, Dr. Bauer says, the observed rates are much too high to be consistent with the known or estimated rates of homogeneous dissociation of D 2 and of C 2 H 2 (C 2 H 2 - * C 2 H + H ) . The fact that the production rate of C 2 D 2 is about half that of C 2 HD suggests that these species are derived from the same transition state. The rate's dependence on a small fractional power of the acetylene concentration indicates a quenching reaction. That's why Dr. Bauer postulates an exhange mechanism involving two types of unstable intermediates. The loose molecular complex between acetylene and deuterium, C 2 H 2 D 2 , is particularly sensitive to dissociation by collision with another acetylene molecule. The tight complex, C 2 H 2 D *, has Vd symmetry (an ethylene structure with the —CH2 groups in planes at 90° to each other), and its energy is about 3 e.v. above the ground state. Species in this state have a much greater probability for dissociation than for internal conversion to stable dideuteroethylene.
