Termolecular collision

The third-order process presumably involves reaction of a complex formed between the alkene and hydrogen halide with the second hydrogen halide molecule, since there is little likelihood of productive termolecular collisions. 

We conclude that the rds transition state includes the elements of one cinnamoyl-imidazole and two butylamine molecules, but we do not know anything about their assembly. However, because a termolecular collision is very improbable, we are justified in supposing that this is a complex reaction, the three molecules having been brought together in stepwise fashion. 

This need not necessarily involve a termolecular collision, since it may be preceded by the equilibrium... 

C15-0045. The reaction of NO with CI2 is 2 NO + CI2 2 NOCl Use reactant molecules to write appropriate elementary reactions that satisfy the following criteria (a) a unimolecular decomposition that generates Cl (b) a bimolecular collision in which a Cl atom is transferred between reactants and (c) a termolecular collision leading to the observed products. 

Termolecular Reactions. If one attempts to extend the collision theory from the treatment of bimolecular gas phase reactions to termolecular processes, the problem of how to define a termolecular collision immediately arises. If such a collision is defined as the simultaneous contact of the spherical surfaces of all three molecules, one must recognize that two hard spheres will be in contact for only a very short time and that the probability that a third molecule would strike the other two during this period is vanishingly small. 

In order to have a finite probability that termolecular collisions can occur, we must relax our definition of a collision. We will assume that the approach of rigid spheres to within a distance of one another constitutes a termolecular collision that can lead to reaction if appropriate energy and geometry requirements are met. This approach is often attributed to Tolman (41). The number of ternary collisions per unit volume per unit time between molecules A, B, and C such that A and C are both within a distance of B is given by ZABC. 

One may estimate the relative frequency of bimolecular and termolecular collisions using equations 4.3.1 and 4.3.11. 

Zabc termolecular collision frequency ev fraction of surface sites that are vacant... 

In fact, although termolecular collision numbers are certainly much smaller than bimolecular collision numbers, they are sufficient to ensure the reversibility of the reactions. Following Tolman s15 approach,16,17 for a reaction... 

Concerted 2 + 2 + 2 cycloadditions are thermally allowed as n2s + n2s + n2s or n2s + n2a + 7i2a processes. The termolecular collisions necessary for these cycloadditions are very unlikely and the only examples known are those where at least two of the component n bonds are held together in one reactant. 

In Table 1 (pp. 251-254), IM rate constants for reaction systems that have been measured at both atmospheric pressure and in the HP or LP range are listed. Also provided are the expected IM collision rate constants calculated from either Langevin or ADO theory. (Note that the rate constants of several IM reactions that have been studied at atmospheric pressure" are not included in Table I because these systems have not been studied in the LP or HP ranges.) In general, it is noted that pressure-related differences in these data sets are not usually large. Where significant differences are noted, the suspected causes have been previously discussed in Section IIB. These include the reactions of Hcj and Ne with NO , for which pressure-enhanced reaction rates have been attributed to the onset of a termolecular collision mechanism at atmospheric pressure and the reactions of Atj with NO and Cl with CHjBr , for which pressure-enhanced rate constants have been attributed to the approach of the high-pressure limit of kinetic behavior for these reaction systems. 

The case for a two-step mechanism for the Aac2 reaction involving a tetrahedral addition intermediate, has already been discussed (p. 104), and has been widely accepted. It is not possible, however, to exclude completely the possibility that a concerted nucleophilic displacement is involved, and it is possible to write such mechanisms involving transition states of lhe correct composition. All such mechanisms necessarily involve a termolecular collision and are therefore not readily reconciled with the observed entropies of activation. 

This mechanism has been suggested by Lane38 as the best rationalization of the available evidence. It has the merit that the second water molecule plays a simple, integral part in the transition state, and is consistent with as many of the observed facts as is any other mechanism. In particular it is a symmetrical mechanism, applicable equally to acid-catalyzed hydrolysis and ester formation. It does, however, involve a termolecular collision, and this fact is at variance with the observed entropy of activation, as discussed above. 

A bimolecular reaction which would proceed with comparable velocity at the same temperature as this reaction would have a heat of activation of about 60,000 calories, as may be inferred from the table on page 96. Now termolecular collisions are about 1,000 times less frequent than bimolecular collisions at atmospheric pressure. Thus if we have a bimolecular reaction and a termolecular reaction with equal heats of activation, the rate of the latter should be at least 1,000 times smaller than that of the former at the same temperature. It will probably be more nearly 10,000 times slower, since a larger proportion of the ternary collisions are likely to be ineffective on account of unfavourable orientation of the molecules during impact. Conversely, if a termolecular reaction and a bimolecular reaction are to take place at equal rates at the same temperature, then the heat of activation of the termolecular reaction would need to be the smaller by an amount AE, such that e ElRT = 1,000 to 10,000. Thus, other things being equal, the heats of activation of termolecular reactions ought to be about 5,000 calories less at the ordinary temperature, and about 15,000 calories less at 1,000° abs., than those of bimolecular reactions. We have also to allow for the diminished duration of collisions at higher temperatures, which we can do by comparison with the nitric oxide oxidation. 

The simple intermediate steps that make up a reaction mechanism invariably involve (a) spontaneous decomposition of one molecule, (b) most commonly a bimolecular collision between two molecules, or (c) an unlikely termolecular collision between three molecules. From a practical standpoint, nothing more complicated is ever observed. 

Additional experimental investigations and theoretical treatments of collisional deactivation processes have recently been reported from several laboratories,250 253 Temperature effects on the lifetimes of intermediate adducts formed in the 0 -C02 interaction and in other relatively simple processes have been examined by Meisels and co-workers.252 254 Here the theoretical treatment involves application of a modified RRKM approach to the unimolecular dissociation of the adduct and/or of the termolecular collision complex consisting of the adduct plus the deactivating species M,. 

A termolecular collision involving two activated molecules and one normal molecule is very improbable and the following steps were offered as best describing the mechanism of the decomposition. 

H. Termolecular Collisions Collision Complexes. Having defined a collision radius o-a for the hard sphere molecule M of core diameter o-r, it is possible to define a collision complex M M as a pair of molecules whose centers are a distance o- apart, where o-a [Pg.156]

In this last expression, the preexponential factors are all similar in containing a product of two collision frequencies, a steric factor, and a mean lifetime. The latter may be approximated in a number of ways, each of which yields about 10 sec. Since bimolecular collision frequencies are about 10 liters/mole-sec, this would make Z V about 10 liters /mole -sec. The collision theory thus leads to a frequency of termolecular collisions of about 10 liters /mole -sec, which as we shall see from Table XII.9, is about the order of magnitude observed for the fastest reactions. 

We can see qualitatively that fis. Rb has almost no activation energy and a steric factor of about >4, while Rg at best goes at every termolecular collision. Thus, even at 1 atm, fcqM < 10 cc/mole-sec, while /ks = 2 X 10 cc/mole-sec. On the other hand (H)/(Br2) 10 , so that Rg/Rb 1. 

If we assume that 10 goes on every termolecular collision and set fcio lO ccVmole> -sec, from Table XII.5, h =... 

The kinetic molecular theory of gases predicts that an increase in temperature increases molecular velocities and so increases the frequency of in-termolecular collisions. This agrees with the observation that reaction rates are greater at higher temperatures. Thus there is qualitative agreement between the collision model and experimental observations. However, it is found that the rate of reaction is much smaller than the calculated collision frequency in a given collection of gas particles. This must mean that only a small fraction of the collisions produces a reaction. Why ... 

As pointed out by Pasto et al. the Ad 3 mechanism does not necessarily imply a termolecular collision in the transition state. One can in fact envisage the formation of an acid-olefin complex producing the necessary positive polarisation on the carbon atom prior to the rate-determining step consisting in the formation of the activated complex with the in-coming nucleophile. Such complexes have been identified in various investigations ... 

The general form of eqn. (4), as well as the lack of dependence of the limit on vessel surface and diameter, is readily understood if the gas phase deactivation term in the denominator of eqn. (1) is dominant. Let it be assumed that the reaction of a chain carrier X with Oj can give rise to different products according to whether it reacts in a bimolecular or termolecular collision thus... 

The rate was judged to be too large for a termolecular collision process furthermore, the activation energy —Rd nk/d l/T) was found to be negative. Therefore, these workers preferred the two-step mechanism... 

Both values are in excess of ozone yields actually achieved in ozonizers. If we assume that Reaction 9 proceeds at every termolecular collision, we can show that it will be faster than Reaction 2 as long as the ratio O/O2 1/100. 

Termolecular collision were also studied. Such collisions may be regarded as a sequence of two binary collisions. In the first, a single Ar atom collides with a benzene molecule and in the second the binary collision complex collides with an additional Ar atom. The beginning and the end of each collision was determined by FOBS. The starting distance between the centers of mass of the binary complex, BAr and the second atom Ar , R n, of the second collision is chosen randomly Irom the Iree paths probability density function... 

The collision lifetime of benzene with Ar increases as the relative kinetic energy of the colliding partners decreases [8]. The long lifetimes of the collision complex at low kinetic energies enable stabilizing termolecular collisions to occur and this fact explains the formation of clusters in low-temperature molecular beams. 

We have studied termolecular collisions between a benzene moleeule and two Ar atoms as a function of pressure, temperature and intermolecular potential without presupposing a given mechanism but, instead, letting the calculations dictate the final results [31]. The results show that termolecular collisions form termolecular complexes and oeeur by three mechanisms a) the Chaperon meehanism, in which the first Ar in is the first Ar out of the termolecular complex, is the dominant one at high pressures. Two thirds of ah termolecular collisions go by this mechanism, b) The Energy Transfer mechanism, in which the first Ar in is the last Ar out of the termolecular complex. 

Bemshtein, V. and Oref, 1. (in press), Termolecular collisions between benzene and Ar, J. Chem. Phys. 

Elementary reactions involving collisions of four or more molecules are not observed, and even termolecular collisions are rare if other pathways are possible. 

One possible mechanism would be a single-step termolecular reaction of two NO molecules with one O2 molecule. This would be consistent with the form of the rate expression, but termolecular collisions are rare, and if there is an alternative pathway it is usually followed. 

---