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MASS SPECTROMETRY of VERY FAST CHEMICAL REACTIONS

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MASS SPECTROMETRY of VERY FAST CHEMICAL REACTIONS MASS SPECTROMETRY of VERY FAST CHEMICAL REACTIONS s. N. Foner and R. L. Hudson mass spectrometer employing a crossed atomic chlorine with butane (which we shall dis­ A molecular beam reactor has been developed cuss in detail later) one confidently expects the for studying very fast elementary gas phase Cl-atom to abstract a hydrogen atom from the reactions. With this system it has been possible to butane to form HCI and leave a butyl (C4H 9 ) obtain direct identification of free radicals and radical, according to the equation unstable molecules formed in hydrogenation, oxidation, and chlorination reactions. CI + C4 H IO ----> HCI + C4 H 9 (1) Although it has been established that chemical Although the rate constants for this reaction are reactions usually proceed by a series of elementary quite well known from competitive chlorination steps involving atoms and free radicals as inter­ experiments, 1 direct observation of the butyl mediates, information on these elementary steps is radical product of the reaction had not been far from complete. Indeed, most of the informa­ reported prior to our study, and, therefore, was tion on the elementary reaction steps has been one of the targets of the experimental program. indirectly deduced from kinetic data on overall On the other hand, there are many reactions reaction rates, without the benefit of direct obser­ that have been proposed to explain experimental vational evidence that the free radicals supposed results that, to state it mildly, are highly specu­ to be participating in the reactions are actually lative. In complex reactions it is not too difficult present. to suggest several alternative mechanisms that In many simple reactions, the deductive evi­ dence for the radicals produced is so strong that lC.C . Fettis and J.H. Knox, " The Rate Constants of Halogen Atom there is no reason to doubt the correctness of the Reactions," Progress in Reaction Kinetics, Volume 2, Macmillan Company, assignments. For example, in the reaction of New York, 1966, 1-38. 2 APL Technical Digest Studies of very fast bimolecular reactions have been carried out with a mass spectrometer incorporating a high intensity crossed molecular beam system. At the low pressures involved, the products observed are the result of single molecular collisions. Free radicals formed in a variety of elementary reactions have been directly observed, and an unusual reaction has been discovered in which an oxygen atom, in a single step, removes two hydrogen atoms from opposite ends of a molecule. could possibly explain the results, and in the temperature and one atmosphere pressure, the absence of direct observations of the intermediates molecular collision rate would be about 5 x 109/sec, it is not possible to establish which one is the so that a molecule would undergo several million correct mechanism. collisions before sampling. Marked improvement One effective approach that has been used to in resolution can be obtained by reducing the elucidate mechanisms of chemical reactions has pressure from one atmosphere (760 Torr) to about been to analyze the reacting systems by mass 1 Torr, in which case only several thousand colli­ spectrometry, first , to see if the theoretically postu­ sions would occur before analysis. Further signifi­ lated radicals and stable molecules are actually cant reduction in pressure to limit the number of present and second, to measure their concentra­ collisions runs into two problems: (1) diffusion to tions. Mass spectrometer sampling times have the walls of the apparatus becomes important and ranged from many seconds for static systems, in wall reactions have to be considered, and (2) the which case the more interesting highly reactive mass spectrometer sensitivity decreases mono­ components have completely disappeared, down to tonically along with the reactor pressure, so that the order of a millisecond for fast flow reactors, detection of radicals becomes more difficult. Some in which case free radicals and other intermediates of the reactions we have been interested in study­ can be readily observed. ing take place within 10 to 100 collisions, so that a For studying the details of very fast elementary different approach is needed. reactions, a fast flow reactor with a millisecond A potentially powerful technique for studying sampling system does not provide adequate time very fast reactions is to employ crossed molecular resolution, as can be seen from the following con­ beams. Crossed molecular beams have been used siderations. If the reaction takes place at room to study elastic scattering and reactive scattering July - August 1968 3 of alkali atoms with halogen compounds2 (for vibration or about 10-13 sec, while for a more example, the reaction of potassium atoms ~ith complicated molecule the decomposition time 3 methyl iodide has been studied in great detaII ) . would depend in a complex way on the number of In a typical experiment, collimated molecular degrees of freedom of the system and the amount beams of the reactants are directed at right angles of excitation energy, but in any event decomposi­ to each other and the molecules interact in the tion would occur long before the molecule could region where the beams intersect. There is a finite be detected in the mass spectrometer. probability that a molecule will suffer a single In an exchange reaction, which can be written collision in the region where the beams cross, but in most general form as negligible likelihood for two or more collisions occurring. The characteristic time resolution for a A+B ~ C+D, (3) crossed beam experiment is the duration of a bimo­ 12 where A and B are the reacting molecules and C lecular collision, which is typically less than 10- and D are the product molecules, there is no sec (for an assumed chemical interaction distance 4 problem in satisfying momentum and energy con­ of 2 A and a mean relative velocity of 4 x 10 servation requirements, and collisional stabiliza­ cm/sec, one obtains a collision time of 5 x 10-usec). tion of the products to prevent back reaction is Almost all of the previous reaction studies with unnecessary. All of the bimolecular reactions crossed molecular beams have been done with observable with crossed molecular beams are alkali atoms as one of the reactants, principally of this type. because these atoms can be detected readily with a A very important subclass of exchange reactiolls surface ionization detector consisting of a heated is the metathetical reaction of atom transfer, in tungsten wire or ribbon that converts the imping­ which one of the colliding molecules snatches an ing alkali atoms into positive ions. For other atoms atom away from the other molecule. The most or molecules there are no high efficiency detectors common type encountered is that in which a available, so that achieving adequate beam hydrogen atom is transferred. Atom transfer intensity becomes a very serious problem in the reactions in which one of the reacting molecules design of experiments. is an atom or free radical occur very frequently in Since we shall be concerned with elementary chain reactions. Some examples of atom transfer bimolecular reactions, some discussion is in order reactions involving atoms and free radicals are : on the general characteristics of bimolecular reactions and the rate constants associated with H + D2 --> HD + D very fast reactions. H + 02 ~ OH + 0 Bimolecular Reactions 0+H2--> OH+H Bimolecular reactions can be classified into two o H + H 2 --> H 20 + H general types: (1) association reactions, in which H + Br2 ~ HBr + Br the two colliding molecules combine to form a new H + C 2H 6 --> H 2 + C 2H 5 molecule, and (2) exchange reactions, in which the N+NO~N2+0 two colliding molecules react to produce two new product molecules. CI + H2 ~ HCI + H The association reaction: CH3+H2 ~ CH4+H CH + C H ~ CH + C H S A + B~C . (2) 3 2 6 4 2 In most of the reactions listed above, a radical in which the molecules (or atoms) designated by reacts with a stable molecule to produce a different A and B combine to form the molecule C , is only radical and molecule. Exceptions are the second of academic interest for the very low pressure and third reactions, in which a radical reacts with experiments which we are concerned with because a stable molecule to produce two radicals. These the molecule C, in the absence of a stabilizing two chain branching reactions, incidentally, are collision with another molecule to remove excess responsible for the rapid proliferation of radicals energy, is not a persistent entity and will decom­ in mixtures of hydrogen and oxygen leading pose back into A and B. The time for unimolecular to explosions. decomposition of molecule C is extremely short; for a diatomic molecule, decomposition would take Rate Constants and Steric Factors place in the time needed to execute a molecular The rate equation for the bimolecular reaction A + B --> C + D can be written as 2D. R. Herschbach, " Reactive Scattering in Molecular Beams," Advances in Chemlcal Physics, Volume 10, Interscience Publishers, New York, 1966, _ d[A] = k[A] [B) = _ d[B] = d[C] = d[D] , (4) 319-393. ~ ~ ~ ~ 3E.F. Greene andJ. Ross, "Molecula r Beams and a Chemical Reaction," Science 159, 1968,587-595. where k is the rate constant, and [A], [B], [C], 4 APL Technical Digest and [D) are the concentrations of A, B, C , and D , respectively. Bimolecular reactions are usually interpreted either in terms of classical collision theory or the transition-state theory of chemical kinetics. While the transition-state theory is more •E sophisticated than the collision theory and should, A + B (REACTANTS) _1. ___ _ in principle, allow one to calculate rate constants from detailed knowledge of the partition functions t of the activated complex, a more descriptive picture emerges from the collision theory.
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