Short-Lived Radionuclides in Chemistry and Biology - American

6

Recoil Generated Radiotracers in Studies of Molecular Dynamics LEONARD D. SPICER

Downloaded by RUTGERS UNIV on May 29, 2018 | https://pubs.acs.org Publication Date: March 1, 1982 | doi: 10.1021/ba-1981-0197.ch006

Department of Chemistry, University of Utah, Salt Lake City, UT 84112 This chapter summarizes many of the contributions that the recoil technique of generating excited radiotracer atoms in the presence of a thermal environment is making to the field of chemical dynamics. Specific topics discussed critically include characterization of the generation and behavior of excited molecules including fragmentation kinetics and energy transfer, measurement of thermal and hot kinetic parameters, and studies of reaction mechanisms and stereochemistry as a function of reaction energy. Distinctive features that provide unique approaches to dynamical prob lems are evaluated in detail and the complementarity with more conventional techniques is addressed. Prospects for future applications are also presented.

Hphe recoil technique of generating excited radiotracer atoms in a thermal environment offers a unique approach to exploring many problems of modern chemical dynamics. From its inception the study of chemistry with recoil radiotracers has expanded rapidly to its current state of providing fundamental information about molecular interactions. Several areas where applications have been demonstrated include excited molecule production and characterization, hot atom and molecular kinetics, thermal bimolecular kinetics of fast reactions, and reaction stereochem istry and mechanisms. In addition, recent results have provided evidence that gas-phase thermal ion chemistry can be studied using this technique. In each case the recoil radiotracer approach has developed into a power ful tool that provides information complementary to and often unavail able via classical kinetic techniques. Several characteristics specific to recoil radiotracer methods provide significant advantages for dynamical studies. The most obvious is the large range of energies available, which, unlike the usual domain i n A

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Root and Krohn; Short-Lived Radionuclides in Chemistry and Biology Advances in Chemistry; American Chemical Society: Washington, DC, 1982.


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SHORT-LIVED RADIONUCLIDES

thermal kinetics, can include even the highest energies at which reaction cross sections are significant. Indeed, because translational recoil energy often is derived from nuclear sources, excess energy well above the range of normal chemical events must be degraded or moderated before results of chemical interest can be obtained. Nevertheless the two extremes of high-energy neutral atom or molecule processes and low-energy ion inter actions that are often inaccessible with standard kinetic procedures are available in recoil studies. The second principal advantage comes from the nature of radiotracer studies in general. Because the radiation emitted from radioisotopes as they decay can be detected with high sensitivity, ultratrace analysis is easily accomplished. This means that chemical phenomena involving labeled species can be studied essentially at infinite dilution. Thus, particularly in the gas phase, the chemical environment can often be preserved in its initial state during the entire course of the dynamical study and it will depend only on the certainty with which the initial state is known in carefully executed experiments. Finally, the recoil technique provides in situ generation of excited reactants in all phases. The uni formity of the distribution of reactant species is a function only of the homogeneity of the chemical sample and the stimulating radiation source, and in general because so few recoil events are initiated, no two nascent radiotracer containing reactants or intermediates interact with one another. A l l kinetic techniques have limiting constraints that determine their applicability to specific dynamical problems. Like most nonbeam methods, recoil studies are limited in principle by the fact that the experiment involves a statistical accumulation of collision data rather than a probe of single collision characteristics. Thus direct measurement of cross section data and studies of discrete state-to-state phenomena are not available. Nevertheless, cleverly designed experiments can provide good insight into both of these areas as will be discussed below particu larly for regions of phase space normally inaccessible to more refined kinetic techniques. The other principal constraint derives from the requirement of a radiation source for recoil stimulation. In the case of nuclear recoil this means that a high-energy accelerator or nuclear reactor must be available, often on site if the isotopes generated are short-lived. It should be pointed out, however, that while the cost factor to install such a major facility is prohibitive, the charges for the required routine use at centers where this nuclear equipment already exists can be comparable with other conventional instrumental techniques in physical chemistry. The effects of radiation on the chemical system being studied must also be considered in experimental design. Undesirable consequences



6.

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Radiotracers and Molecular Dynamics Studies

125

may result in either radiation damage causing molecular fragmentation or production of unwanted isotopes in the case of nuclear excitation. In general, the former problem can be controlled adequately using molecular scavengers so that radiation chemistry does not macroscopically perturb the system. The interference from concurrently produced isotopic species is most frequently eliminated or minimized either by avoiding source molecules with nucleogenic precursors or by discriminant isotope detec tion in the analysis of results. In this review a summary of some of the recent progress in applying recoil radiotracers to three areas of dynamics will be outlined. No attempt has been made to be encyclopedic but rather selected examples have been chosen, often from the author's own work. The areas into which this review is divided are: excited molecule dynamics; kinetic parameter characterization; and reaction mechanisms and stereochemistry. Excited Molecule Dynamics It has been well documented that hot atom replacement reactions deposit large amounts of internal energy in primary products (1,2,3). In cases where a heavy translationally excited reactant atom carries sizable momentum into a hot reaction, the products may also be generated with large excess kinetic energy. As a consequence of this excitation, the nascent molecules can undergo secondary processes that eventually lead to stable species. The most thoroughly studied secondary process is unimolecular decomposition resulting from excess vibrational and rota tional energy. Prospects for studying collisional dissociation and trans lationally hot, neutral polyatomic reactions are on the horizon and will be commented on at the end of this section. A systematic study of excited molecule dynamics begins by charac terizing the energy deposition and partitioning during hot reactions. The degradation of the energy deposited via collisional transfer or unimolec ular reaction provides a second avenue of investigation that is of fundamental importance. Data in several hot reaction systems that undergo unimolecular decomposition have been reported and used to obtain estimates for the average energy deposited. One of the most thoroughly studied systems is excited cyclobutane produced in the hot T-for-H replacement reaction (4,5,6). A discussion of recent published results on the cyclobutane reaction illustrates the kind of excited molecule dynamical information available from recoil techniques. The primary assumption in the approach reported is that the R R K M (Rice-Ramsperger-Kassel-Marcus) method for describing statistical energy redistribution and calculating decomposition rate constants is valid. Thus, deviations from expected R R K M behavior



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are considered to be symptomatic of nonstatistical intramolecular energy redistribution resulting from either intrinsic molecular properties or intermolecular encounters rather than errors in the R R K M method. The expected behavior of excited molecules is most often compared with experimentally measured yield results to characterize the molecular activation and deactivation processes. The initial detailed demonstration for this approach was pioneered by Bunker and his co-workers (7,8). The technique was applied to excited cyclobutane (9), and based on the results, methods have been developed to further explore the energetics of excited molecule formation (5) and the degradation of that energy by collisional transfer (6,10). Fortunately, results from independent ion beam studies of T-for-H replacement reactions with selective hydrocarbons are available (11-14), and these data have been used to approximate the required cross sections as a function of energy for hydrogen replacement reactions (15,16). The excitation functions developed in this way are consistent with the general features of trajectory calculations in simple hot reaction systems involving hydrogen atom replacement (17,18,19). With these excitation functions and the hot atom reaction data for cyclobutane in various bath gases, the first refined estimate for a secondary excitation function of a reaction product was published (5) based on the general R R K M approach (9). Thus, the partitioning of total available energy into internal modes of cyclobutane during hot hydrogen replacement reaction was detennined over the entire translational energy range for the replacement reaction cross section. Based on that work (5) it was possible to verify both that the average internal energy deposited in cyclobutane was about 100 kcal/mol and that the molecule exhibited statistical behavior in redistributing energy intramolecularly at least to the highest limits explored of 180 kcal/mol (20). It should also be noted that about 10% of the total energy generated in cyclobutane by recoil reaction appears as rotation. Such partitioning, of course, depends markedly on the mass of the hot atom that initiates reaction as well as the cross-section function, and a much higher rotational contribution can be expected in some recoil chemical activation processes. Another more direct approach has recently been used to define the excitation function for a reaction product from a recoil reaction (21). Instead of using an R R K M method for a single competitive unimolecular process, the sequential decomposition of excited CF4 was studied. The fragments formed were scavenged with C l to give direct yield data for each path as indicated below: 2

18

F +

CF -»*CF 4

3

1 8

F+F


(1)

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SPICER

»CF F -» F + C F 3

18

2

M CF

8

1 8

1 8

F -» F + C F F -»• F + C F -»• C +

Cl F

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Radiotracers and Molecular Dynamics Studies 1 8

2

18

Cl

2

CF FC1

1 8

CF FC1 18

2C1

2

2

C FC1 18

3

1 8

F CI

2

18

FC1


(2) Here M is any component of the reaction mixture that stabilizes vibrationally excited products on one or more collisions. Thermodynamic energy limits for each process were established. Thus, the integral of the C F excitation function could be estimated and used to obtain the initial internal energy distribution of the C F F formed in Equation 1. The qualitative features of both results (5,21) are the same, and, as expected, the total energy deposited by hot fluorine atoms (22) is somewhat greater than by hot tritium in these replacement reactions. Due to the extreme levels of internal energy available, chemically activated molecules produced by kinetic techniques can be used, in principle, to study inter- and intramolecular energy transfer in a region of energy space beyond that normally probed with standard methods. In this capacity, such kinetic chemical activation offers the distinct advantage of being free of the thermochemical constraints on the energy produced that accompany processes dependent on addition or insertion reactions. The controlling factors inherent in reactive stimulation via translational energy are momentum transfer and the efficiency with which a product molecule can distribute energy among its internal degrees of freedom. Recoil, kinetic chemical activation, as illustrated above, is charac terized by dynamical features that dictate a rather broad distribution of energies. In fact, the manifold of energies produced in a typical experi ment can be roughly divided into three main regions: (i) molecules with internal energy below that required for unimolecular reaction, that is, the decomposition threshold; (ii) molecules that have enough energy to decompose in the absence of collisions but that can be competitively stabilized by collisional energy transfer in the range of pressures con venient for recoil studies; and (iii) molecules with extreme internal energy, which decompose on a time scale that is fast compared with collision times even at the highest pressures explored in the experiment. This latter case may include processes that are direct in nature since all reaction channels that give the same product but are independent of pressure are classified together. A representation for the chemical activa tion of cyclobutane and its use in studies of energy transfer is shown in Equation 3. 4

3

1 8


128


[-H]

»C-C4H7T

(3a)

k,(E)[I] * c-C H T

(3b)

kAE)[R] » c-C H T

(3c)

4

T» + c - C H -+ c-C H T« 4

8

4

7

4

k (E) t


[-H]

7

7

•> C2H3T -(- C2H4

(3d)

+ C2H3T -f- C2H4

(3e)

Here I represents a reactively inert diluent, R is reactant cyclobutane, k and k are bimolecular rate constants for energy transfer, and k is the rate constant for unimolecular decomposition. This particular system exhibits a relatively straightforward chemistry giving ethylene as the only decomposition product of excited cyclobutane. The energy transfer information that can be obtained from recoil activation processes like those in Equation 3 depends on the level of sophistication used to represent the system quantitatively. The simplest approach provides relative average rate constants ki for various diluents, I, and is based on an algebraic analysis in which the energy-dependent rate constants fci(E), k (E) and k (E) are replaced with average values over the competitive stabilization-decomposition energy range. Using the pressure dependence at fixed composition and the concentration dependence at fixed pressure for two component systems, ratios for kJk and, hence, the relative energy transfer efficiencies have been obtained for several monatomic, diatomic, and polyatomic colliders (10,23,34). In this approach, processes represented in Equations 3a and 3e were identified by curve fitting a three-parameter equation to the pressure dependence over the range from a few Torr to 2 atm in several systems independently. The composition dependence for the competitive yields in Equations 3b, 3c, and 3d at a fixed pressure of 800 Torr were then determined experimentally by subtracting the noncompetitive contribu tions from the raw data. Extrapolation of the refined data to pure reactant and infinite dilution with bath gas provides intercepts whose ratio gives the desired relative rate constants. Correction for reduced mass and molecular size then provides relative energy transfer efficiencies on an equal collision basis. An assumption of the method is that the average internal excitation energies for the pure and diluted recoil systems are approximately the same. One-dimensional recoil trajectory calculations in the energy coordi nate for translational cooling of hot hydrogen atoms have focused on this x

T

d

r

a

T



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Radiotracers and Molecular Dynamics Studies

129

issue and verify that only minor changes in the average energy for reaction are evident for highly moderated or diluted cases compared with pure reactant if the reactive process has a threshold significantly above the thermal collision energy region (25). This condition is satisfied for the hydrogen replacement reaction in cyclobutane used in the above test case, and thus it is likely that the internal energy distribution for product molecules is also reasonably constant for the systems studied. A more carefully constructed approach to energy transfer explicitly accounts for the energy distribution of the chemically activated product (5,6,26,27). The initial distribution of energies upon production as well as the transient distributions formed by collisional relaxation of internal energy are used to calculate rate constants for unimolecular reaction. The formalism of Bunker (7,8,9) based on general R R K M theory is convenient for recoil chemical activation, since it explicitly accounts for both rotational and vibrational excitation in the product. In the cyclo butane model system reported, a stepladder approach to deactivation was incorporated, with the step size being a parameter determined by the best fit to the data (6). The overall processes considered are illustrated in Equation 4.

(4)

ME

Short-Lived Radionuclides in Chemistry and Biology - American

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