Neutral products from cation-molecule reactions in the gas phase

Radiat. Phys. Chem. Vol. 20, No. 1, pp. 29--40~ 1982 Printed in Great Britain.

0146-5724/821070029--12503.00/0 Pergamon Press Ltd.

NEUTRAL PRODUCTS FROM CATION-MOLECULE REACTIONS IN THE GAS PHASE THOMAS HELLMAN MORTON Department of Chemistry, University of California, Riverside, CA 92521, U.S.A. (Received 11 December 1980)

AMtraet--The use of neutral product analysis for examining ionic reaction pathways from electron impact is described. This approach merges techniques of mass spectrometry with those of radiation chemistry. Comparisons are made between experimental results and predictions based on density-ofstates arguments using RRKM microscopic rate coefficients. The importance of examining isomer distributions is stressed with special attention given to the question of the mechanism of bimolecular proton transfer in the gas phase. MASS SPECTROMETRYmeasures the mass to charge ratios of ionic products of chemical reactions caused by ionizing radiation. Much of radiation chemistry focuses on the neutral products that result from the action of ionizing radiation. These two disciplines are complimentary, but few investigators have sou~ght to collect neutral products from radiolysis experiments using conditions that duplicate the ionizing conditions of most mass spectrometers (namely, ionizing gases at pressures
29

A MASS SPECTROMETRIC EXPERIMENT Several years ago we examined the ion chemistry of tetramethylallene, 1, using drift-mode ion cyclotron resonance (ICR). We were particularly interested in reactions of the molecular ion, 1 "+, formed at low ionizing energies, and we hoped to measure the rate of hydrogen transfer, reaction (1), for the lowest energy state of 1 "+. At ionizing energies below 10 eV, 1 "+ is the only ion produced by electron impact. At a pressure of 1 x 10-4 torr, products of reaction of 1 .+ with neutral 1 are seen in the ICR. At 10 eV ionizing energy, two types of products are seen from this reaction, the protonated parent ion, m/z 97, and io~ns of greater mass. As the electron energy is lowered further to 9eV, the protonated parent ion is no longer detectable, and only the higher mass products, CloH15+, C,H~7 + and C~3H2~÷ (m/z 135, m/z 149 and m/z 177 in the ratio 1:8:1.5) are seen. This phenomenon is exemplified in Fig. 1, which shows abundances of 1 .+ (m/z 96), the protonated parent (m/z 97), and the major high mass ion (m/z 149) as a function of ionizing energy, v~ We drew two inferences from this result. First, there are two low lying states of 1 "+, which have different reactivities. The higher energy state reacts with 1 to form protonated parent, while the lower energy state produces only the higher mass ions. Second, the lower energy state of 1"+ reacts via formarion of a dimer ion, m/z 192, which subsequently decomposes, as shown in reaction (2). At 10-4 tort the lifetime of the dimer ion is too short for it to be observed directly by ICR, and it either reverts to reactants or forms the product ions that are observed. The formation of dimer ions from unsaturated molecular ions reacting with their neutral

30

T.H.

MORTON How can we justify this pathway? We discovered that it is possible to dimerize neutral I thermally to neutral 2, °o) and the mass spectrum of 2, shown in Fig. 2, exhibits the same three m a j o r ions as are seen in the reaction of 1 with 1 "+ in the ICR at the lowest ionizing energies. My purpose in describing this experiment is to point out that identification of the neutral product--in this case (CH3)2CH.--is often the crucial clue in understanding the pathway of a gas phase ion reaction. In addition, it illustrates three of the experimental approaches available to the mass spectrometrist: (1) Alteration of ionizing conditions (e.g. variation of ionizing electron energy). A number of useful techniques probe a variety of interesting ways of imparting an electric charge to neutral molecules, among them chemical ionization, ('') field ionization kinetics: n) and PIPECOY 3) (2) Examination of subsequent reactions of ions under study (e.g. ion-molecule reactions). Many

IO ]

.g

o~ g -

nvz 9 7

~

0

:

i

,o

.

~,

,3

~

,~

~

,'~

"

IONIZING ELECTRON ENERGY (eV)

FIO. 1. Abundances of 1"* and its ion-molecule reaction products as a function ionizing electron energy at a total pressure of 1 of I x 10-4 torr in the ICR. The fragment ion (m/z 79) formed at ionizing energies > 12 eV was ejected simultaneous with observation of the m/z 97 ion by applying a double resonance (f=/f, = 1.225, rf power = 40 mV/cm) to the drift plates of the ICR cell parents is well precedented in radiation chemistry,(s) moreover, comparable dimerizations of neutral allenes are well characterized in the literature of organic chemistry. (')

(CH,)2C-C-CtCH3)2

(1) (CH3hC=C--C(CH3h

>-~-v ) [1"+]*

) C?H,; +

1 m/z 96

mlz

Why do we require an intermediate dimer ion for reaction (2)? One reason is that the major ion-molecule reaction product, m/z 149, demands the concurrent formation of neutral C3H?. Although it is conceivable that the reaction could have proceeded via formation of two (or more) neutral fragments with the same total mass, the low energy of the reaction would seem to require that the most stable neutral fragment, isopropyl radical, (CH3hCH', be formed. However, there is no C3H7 structural unit in 1. Therefore, some sort of rearrangement must be taking place. Happily, there is a reasonable pathway for production of (CH,)2CH', as shown in reaction (2): 1 .÷ and 1 attach, and the resulting m/z 192 ion rearranges via a hydrogen shift to form the isomeric ion 2 "÷.

40 CII HI~ t-

f-~

~o~ o

%%

, 20

40

.... 60

•+

hydrogen

=/= z92

I00

120

[ 140

160

P80

h 200

-CH"

C13H21

(2)

)

~

LI$1.. 80

FIG 2. Nominal 10 eV mass spectrum of compound 2 (recorded on a Hitachi-Perkin-Eimer RMU6-D). The major fragment ions are labelled and correspond to species produced by the ion-molecule reaction of 1 with its molecular ion at 9 eV in the ICR.

Internal 1 + 1 "+

97

2

31

Neutral products from cation-molecule reactions in the gas phase instrumental techniques have become available for examining unimolecular and bimolecular reactions of ions, among them pulsed ICR, °'~ metastable ion spectroscopy, <"> collision-induced fragmentation, "6~ and SIFT. <'~ (3) Preparation of new molecules (e.g. compound 2) whose ionization permits various hypotheses to be tested. This includes not only new molecular structures, but also specific isotopic substitution of well known molecules. NEUTRAL PRODUCT STUDIES: COMPLEX FORMATION From this mass spectrometric study (and from numerous others in the literature) it is clear that ion-molecule collisions (even at low pressures) can lead to covalently bonded complexes, which have finite lifetimes even in the absence of collisional stabilization. In the investigation described above, the intermediacy of a covalent complex was inferred on the basis of expulsion of a neutral fragment corresponding to a structural unit that was not present in the reactant species. The reactions of many complexes, however, do not necessarily give such clear-cut evidence of their intermediacy. How, then, do we identify the characteristics of such a complex if we cannot examine its structure directly by spectroscopy? Ausloos and coworkers have, over the past two decades, developed one set of approaches, which are discussed elsewhere in this issue. (~s> Five years ago we reported the development of an apparatus that permits us to explore questions like this by collecting neutral products from radiolysis experiments that duplicate the conditions of ionization in a mass spectrometer. This apparatus is an Electron Bombardment Flow (EBFlow) reactor, shown schematically in Fig. 3. Our current apparatus has a horizontally mounted stainless steel reaction vessel, 1.3 m long by 6 cm dia., enclosed in a solenoid. A differentially pumped electron gun with a filament source is mounted at one end, and a liquid nitrogen-cooled trap is mounted at the other. Reactant gases are introduced through variable leak valves and flow into the trap, where they are condensed. Typical throughput at a steady state pressure of 10-3 torr is on the order of 10-~mol/sec, and, with ionizing electron currents 5-100/~A, -< 1% conversion is achieved. In a run of approximately 1 h, yields of as much as a few micromoles of radiolysis product can be collected for subsequent analysis. One of our studies has probed the nature of the complex that is formed by reaction of isopropyl

ELECTRON

BOMBARDMENT

FLOW REACTOR

COLD TRAP TOROIDAL MAGNETS ~ I T O BARATRON MANOMETER ~

/

:+OLENOD

"

/////]/]////////~//]A

TO

LL |11[ [

REACTANT GASES IN CTRON

FILAMENT

~ COLLECTOR

TO VACUUM PUMP

Fro. 3. EBFIow reactor, schematic diagram. Electron filament is a directly heated thoria-coated iridium ribbon immersed in a bank of 500 G toroidal parament magnets (axial field). Products of EBFIow radiolysis are transferred from the cold trap after completion of the radiolysis and are sealed in a glass tube for subsequent analysis.

cation with ethylene. Lampe et ai. have shown that small cations react with ethylene to form stable complexes on the microsecond time scale, c~s> We wanted to probe the structural nature of these complexes, particularly to find out if it was reasonable to represent them as covalently bound intermediates. We first considered the possibility on theoretical grounds by using RRKM calculations to estimate relative rates of decomposition of such a complex to its reactant species vs rearrangement to a more stable isomer of the complex. For the complex of isopropyl cation and ethylene we chose to model the initially formed

%.+. CH3

L'" ~°' \

FIG. 4. Potential energy diagram used for RRKM calculations of microscopic rate constants for reversion (k,) and rearrangement (k2) of an initially formed complex of isopropyl cation and ethylene. This complex is modelled as a corner-protonated cyclopropane (Ref. 27), with heat of formation estimated from S. L. CMONOand J. L. FRANKLIN,]. Am. Chem. Soc. 1972, 94, 6347. Other beates of formation based on Refs. 22 and 31.

32

T. H. MORTON

structure as a protonated cyclopropane, as shown in Fig. 4. To describe the vibrational characteristics of this complex we used spectroscopic frequencies reported for 1, 1-dimethylcyclopropane, <2°) with additional frequencies for vibrations of the extra proton. This intermediate can revert in a straightforward fashion to isopropyl cation and ethylene, and we modeled the transition state for this decomposition by using the vibrational frequencies for isopropyl cation and ethylene plus five additional torsional modes based on rigid vibrations of those two species (the structure of such transition states discussed further on).

For rearrangement of the protonated cyclopropane, we chose a transition state that resembles the primary isoamyl cation and assigned an energy to that transition state corresponding to our estimate of the heat of formation of the primary cation. A potential energy diagram is shown in Fig. 4. The frequencies that we chose for the intermediate and the two transition states are shown in Table 1. When we allow the intermediate cation to have an energy content corresponding to the energy of the separated reactants, rearrangement to t-amyl cation competes efficiently with reversion reaction. As we increase the energy content

TABLE l. VIBRATIONALFREQUENCIESANDTORSIONALMOMENTSOFINERTIAUSEDFOR RRKMcALCULATIONS OF MICROSCOPICRATE CONSTANTSIN FIG. 4. NUMBERSIN PARENTHESES REPRESENT DEGENERACIESUSED(21)

INTERMEDTATES (42 internal degrees of freedom)

TRANSITION STATES (41 internal degrees o f freedom),

Corner-Protonated Cyclopropane

t-Amyl~Catlon

[C2H4 + (CH3)2CH+]

(CH3)2CHCH2CH2 +

(CH3)2CCH2CH3

3066

1298

2958(8)

3272

1343

2915(11)

3022

1240

2855(3)

3105

1156

2890

3000(2)

1153

1450(7)

3019

1127

1471(5)

2964

1129

1305(3)

2990

1071

1387(6)

1050

1335(2)

2946

1113

1300(3)

2863

2932

1060

1200

2800(3)

995

1200(2)

2897

1041

1050

2799

974

1095

2870

931

970

2737

964

794(4)

2770

839

906(3)

1623

943(2)

459(3)

2740

781

860(2)

1449

892

368(4)

1493

764

768

1446

825

2 methyl r o t o r s

1472

738

347(2)

1443(3)

670

1460

684

305(3)

1388

420

1444

550(2)

1362

180(2)

1432

498

1 ethyl rotor

1405

394

Ired-33.9 amu-~2~-l)

1388

358

1342

335

!322

300

3 methyl rotors fred-3.0 amu-~2(o-3~

1348 2 methyl rotors fred-3.0 amu-A2(~'3) ' 5 rigid rotations treated as internal rotors Ired" 24.2 amu-~2~=2)

2 methyl rotors

amu-:2(o'3 "A) Ired'3,0

Ire d- 25.4 amu-A2~ -2) Ire d- 41.8 amu-~ 2 ~ =2)

Ired=268.9 amu-~ 2 ~ "i) Ired-136.1 amu-~ 2 ~-i)

fred-3.0 amu-~2~ -3)

33

Neutral products from cation-molecule reactions in the gas phase TABLE 2. UNIMOLECULAR RRKM MICROSCOPIC RATE CONSTANTS (USING ANHARMONIC OSCILLATORS) CALCULATED FOR REACTIONS SHOWN IN FIG. 4 Combtned I n t e ~ n a l Energy o f C2H4 p l u s (CH3) 2Ch'~ (~eal/mol) 0.05

0.6

J

Rate c o e f f i c i e n t s

(sec -1)

kl

k2

k3

5

1.3 x 108

4.1 x 107

6.6 x 102

20

2.0 x 108

2.8 x 107

4.4 x 102

50

1.8 x 109

6.2 x 105

62

5

5.0 x 1010

.5.4 x 107

9.4 x 102

20

7.1 x 1010

3,8 x 107

6.0 x 102

50

6.2 x 1011

2.0 x 106

23

of the isopropyl cation in this reaction, rearrangement becomes less probable in competition with reversion. Absolute reaction rates for several different energies and rotational states of the protonated cyclopropane intermediate are shown in Table 2.c~" Our conclusion based on these calculations is that we should be able to observe products from Cs cations when C3H~ cations are generated in the presence of ethylene at a total pressure on the order of l0 -3 tort. However, it is clear that the efficiency of trapping should be low. For instance, when the total vibrational energy of ethylene and (CH3)~CH÷ is of the order of kT (0.6 kcal/mol), the efficiency of complex formation (estimated as k2lk,) will be only 0.001 for J = 5 (and substantially smaller for larger J). Thus, we estimate that only a small fraction of collisions between (CH3)2CH÷ and ethylene will product t-amyl cations. Once formed, however, the t-amyl cations have very slow rates (k3) of reversion. For our experiments we generated propyl cations by 70eV electron impact on di-n-propyl ether. The partial pressure of ethylene in the EBFlow reaction vessel was 1 × 10-3 torr, and the partial pressure of di-n-propyl ether was 3 × 10-4 torr. Bombardment of this mixture with 70eV electrons afforded a yield of 2-methyl-l-butene corresponding to approximately 1 molecule produced for every 100 electrons entering the EBFlow reaction vessel. A slightly lower yield of 2-methyl-2-butene was recovered, and the ratio of these two isomers was 1.2: l, as noted in reaction (5).(4) To be sure, many radical pathways may compete in this radiolysis (as evidenced by the other CsH,o isomers, which are recovered in comparable yields). But we can conceive of no nonionic pathway that would produce the 2methylbutenes, since comparable rearrangements

of simple C.H2,+~ radicals are without precedent. What is the sequence of events that happens in the reaction of a propyl cation with ethylene? This is shown in reactions (3)-(5). Let us assume that the C3H7÷ species initially formed from 70eV bombardment of di-n-propyl ether rearranges to its most stable form, isopropyl cation, (CH3)2CH÷, as represented in reaction (3). This cation then forms a complex with ethylene to yield a CsH,, ÷ species that rearranges, as shown in reaction (4). The CsH,, ÷ cation lives long enough to undergo collisions with neutral molecules, among them din-propyl ether, which is an excellent base (gas phase basicity equals 195 kcal/mol(22)). Thus, the expected ultimate products are protonated di-npropyl ether and the neutral CsH,o isomers shown in reaction (5). Our results, then, are consistent with the rearrangement of the CsH. + complex of propyl cation and ethylene to t-amyl cation, which is the most stable isomer. The abundance in which this cation appears to be formed is low, as we might have expected based on our RRKM calculations. Since the rate of collisional stabilization of initially formed CsH, + complexes cannot exceed the collisional rate between the complex and neutral ethylene (which at a pressure of 10-3 torr is of the order of 3 × l04 sec -t) the isopropyl cation must be thermalized before it can form a complex that will live long enough to rearrange. We presume that this thermalization occurs in one or two collisions with ethylene, since the concentration of di-npropyl ether is sufficiently great that it would intercept virtually all propyl cations if, say, ten collisions were required to deactivate the propyl cation. Therefore, we believe we have a rather consistent picture for complex formation (and oligomerization) of cations with olefins in the gas phase.

34

T. H. MORTON

(3)

CH3CH2CHzOCHzCH2CH 3 7OoV, CH3CH2CH:

(4)

(CH3)2CH+

i H2 ,i

• ](CH3)zClq%

~

(CH3)zCH +

I

CH2

(5)

~'k

Pr20 1.2

NEUTRAL

PRODUCTS FROM PROTON TRANSFER Given this result, which strongly indicates that covalent complexes are formed in reactions of unsaturated ions with molecules, we may now ask whether it is obligatory for covalent complexes to be formed in such ion molecule reactions. For example, does Lewis acid-base association precede proton transfer reactions between electron deficient carbocations and N-donor bases? To answer this kind of question, we have analysed neutral products from proton transfer, since they contain information about isomerism that can tell us a great deal about the choice of reaction channels. We have focused interest on the reaction of tertiary alkyl cations with amines. This reaction has been discussed theoretically by Chesnavich, Su and Bowers for the case of t-butyl with ammonia: ~> +

'~

:~

1

surface connecting the complex to the product is not monotone increasing but possesses an energy maximum, which defines the position of the transition state. In both cases, the branching ratio for alternative channels of reaction can be described using RRKM theory. We have explored the applicability of RRKM theory by considering a cation, as proton donor, which has different varieties of acid protons. Although microscopic rate constants calculated by RRKM theory are very sensitive to rotational state and energy, the ratio of two rate constants is far less sensitive. More important, to compute the ratio of two product channels, one need only describe vibrational structure for the two transition states, since reactant state contributions cancel.

NH4 (6)

In their treatment Bowers et al. consider the formation of a covalent Lewis acid-base complex, namely the t-butylammonium ion, in a vibrationally excited state, which subsequently decomposes in a unimolecular fashion to isobutene and ammonium ion, as shown in reaction (6). They have considered two possible transitional states for the unimolecular decomposition. The first model, an orbiting transition state, depicts the potential energy as monotonically increasing from the intermediate to the products. The vibrational structure of this transition state is well represented as consisting of the fundamental frequencies of the products plus five torsional degrees of freedom that are composed of combinations of rigid rotations of the product molecules (as will be discussed below). The second model is for a saddle point transition state, in which the potential energy

3

4

5

For example, consider the methylcyclopentyl cation, 3. The three methyl protons are all equivalent, and donation of any one of them to a base will lead to the same product, methylenecyclopentane, 4. The four methylene hydrogens adjacent to the positively charged carbon are equivalent to one another, as well, and donation of any one of these protons to a base will lead to 1-methylcyclopentene, 5. Using RRKM theory and an orbiting transition state model, we have calculated the expected product ratio if there is a single covalent complex that lies intermediate be-

Neutral products from cation-molecule reactions in the gas phase tween reactants and products as shown in Fig. 5. The vibrational frequencies chosen for the transition states are based upon vibrational spectra of the products and are listed in Table 3. Let us pause for a moment to describe in further detail our RRKM calculations for an orbiting transition state (which are the same sort of calculations used for k~ in Fig. 4). Consider the dissociation of an ion to two fragments, one charged and the other neutral. If the potential energy increases monotonically in passing from the intermediate complex to the products as shown in Fig. 5, the position of the transition state cannot be inferred from the potential energy surface above. Therefore, we must also consider the repulsive centrifugal potential, which is a consequence of overall rotation of the intermediate when Y~0. If we add the attractive

35

potential that exists between the products (e.g. the solid curve shown in Fig. 6) to this repulsive potential (the dashed line in Fig. 6), we get an effective potential function with a local maximum (shown in greatly exaggerated form as the dotted line in Fig. 6). This local maximum is taken to be the transition state, and, for realistic values of Y, it greatly resembles the products, with a distance of several ~ between them. (~) For the reaction shown in Fig. 5, decomposition of the Lewis acid-base complex of methylcyclopentyl cation and a base, each of the two product channels will have a transition state that resembles its respective products. Suppose the base is NH3. The intermediate complex has 57 internal degrees of freedom, and each transition state should have 56. However, for either product channel, C~H,o has 42 and NH4÷ has 9 internal degrees of

TABLE 3. VIBRATIONALFREQUENCIES AND TORSIONAL MOMENTS OF INERTIAOF TRANSITION STATES FOR R R K M CALCULATIONS OF PRODUCT RATIOS FROM DECOMPOSITION OF |-METHYLCYCLOPHENYLAMMONIUM CATION TO C6Hio PLUS NH4 +. NUMBERS IN PARENTHESES REPRESENT DF~3ENERACIES Production of 4

3084

"1459

P r o d u c t i o n of

977(2)

3134(3)

1372

896

3134(3)

1430(2)

927

3044

1334

890

3033

1405

892(2)

3033

1298

880

3005

1309

880

2961(3)

1204

830

2972

1305

842

2912(3)

1170

822

2950

1288

821

2857(3)

1152

780

2925

1261

789

1685(2)

1270

695

2908

1241

764

1658

1180

579

2907

1218

673

1465

1133(2)

432

2883

1212

570

1460

1026

387

2850

1221

418

1455

967

360

2842

1149

363

1445

933

240

1685(2)

1397(3)

240

1397

905

1662

1015

160

1477 3 NH4+ r o t o r s

I x-" ~ -

2.6 ~ _ ~ 2 (a-- 3)

I z (reduced) = 2.5 ainu-A2 (o = I) 2 r o t a t i o n s of 4

3 NH4÷ r o t o r s I x - l y - 2.6 smu-A 2 (o = 3) I z (reduced) - 2.5 mu.-,~2 (o .. 1) 2 r o t a t i o n s of 5

Ix = 167 asu-,~2 (a'= 1)

I x = 185 amu'-~.2 (o - 1)

Zy - 85 -,,',u-.~2 (o - 2)

ly - 81 --u-~2 (a - I)

36

T.H. MORTON

STRUCTURE

+ Bose

OF O R B I T I N G

TRANSITION

STATE

® FIG. 7. Torsional degrees of freedom (represented by solid arrows) in an orbiting transition state corresponding to the upper pathway in Fig. 5 when the base is ammonia. Of 6 rigid rotations of the product molecules, only 5 are used as internal degrees of freedom; the 6th (represented by the dashed arrow) is an overall rotation of the transition state.

kcol) FIG. 5. Potential energy curves for a single intermediate decomposing to two different products via two different orbiting transition states. In the example shown, the nonfixed energy of the higher transition state, Et, is equal to the difference in proton affinity between the corresponding product and the base. In the RRKM expression for the branching ratio, Wt represents integrated densities of states (sums of states) and L * pathway degeneracies. ION-INOUCED

)0 r," hi Z LU

DIPOLE

ATTRACTIVE,

~r

I

/

J Z 0 el

0

"~'~ ~

CENTRIFUGAL (ROTATIONAL

REPULSIVE)' ~r -

DISTANCE

Fro. 6. A simple model for attractive (solid curve) and effective (dotted curve) potentials for separation of an ion and a polarizable neutral (after Ref. 23). freedom. In order to describe each transition state, we need 5 more degrees of freedom in addition to those contributed by the products' vibrational degrees of freedom. These 5 degrees of freedom are taken to be torsional modes of the transition state, which are based on the moments of inertia of the product molecules themselves, as illustrated in Fig. 7. Of 6 rigid rotations of the product

molecules, 5 are used as internal degrees of freedom of the transition state. These are represented by the solid arrows in Fig. 7. In calculating relative rates of production of 4 and 5 from the same intermediate, the ratio of microscopic rate constants is given by the expression shown in Fig. 5, where E t represents the disposable energy in the transition state for production of 4, Wt the sum of states for a transition state and L" the path degeneracy for a transition state. Product 5 is 3.9kcal/mol (0.17 eV)(25) more stable than product 4. Therefore, there is that much more disposable energy in the transition state for production of 5 than there is in the transition state for production of 4. The path degeneracies are equated to the number of chemically equivalent protons whose removal can give the product. As discussed above, removal of any of the three methyl protons yields 4, thus, that path degeneracy is taken to be 3. For production of 5, the path degeneracy should have a value of 2, since there are only two protons cis to the base in the intermediate whose removal would yield 5. Needless to say, absolute rate coefficients for the two channels depend greatly on the energy content and rotational state of the intermediate. Nevertheless, the ratio of rate coefficients is comparatively insensitive to these variables: for J = 1 and E t = 2 k c a l / m o l , W4t/Wst = 7 . 5 x 10-4; for J = 1 and E t = 20 kcal/mol, W4t/Wst = 1.0 x 10-2. For J = 100, these values change only in the second or third decimal place. (2~> The structure of the base has a detectable effect on the ratio. For a base with no internal degrees of freedom, the W4t/Wst ratio is 1.6 x 10-3 for E t = 2 kcal/mol and 1.7 x 10-2 for E t = 20 kcal/mol. For

Neutral products from cation-molecule reactions in the gas phase a base with more internal degrees of freedom than ammonia the ratios become smaller than for ammonia] 2" Our conclusion, based on RRKM calculations, is summarized in reaction (7). If a single intermediate (regardless of its structure) decomposes to give 4 and S via different orbiting transition states, then product ratio 4/S should be ~<0.01. For small ./, E t is equal to the exothermicity of the proton transfer from cation 3 to the base, provided that 3 has been thermalized. In other words, E t is the difference in proton affinity between product 4 and the base. Suppose, however, that 3 is vibrationally excited. Then E t is increased by this excitation energy. How much would vibrational excitation alter the ratio expected for 4/S? From the 10 energies for which we have calculated W4t/Wst, we find that they fit (ra>0.998) an exponential function, namely W,t -- 1 - e -°'°°°5°~E*.

Wst

Using this expression, we note that E would have to exceed 200 kcal/mol for W,~'/Ws'f to becomes as great as 0.1. These calculations make an unambiguous and testable prediction regarding the outcome of a proton transfer from 3. We examined this reaction in the gas phase using 70 eV electron bombardment of a mixture of 4 x 10-' torr bromocyclohexane and 2 x l0 -4 torr of triethylamine. The C6H, ÷ cation produced by electron impact on the bromide rapidly rearranges to 3 (which is a thermodynamic sink, as shown by solution NMR studies (2s~)and mass spectrometric experiments,~:9)and the sequence shown in reaction (7) takes place. In our EBFlow experiment, products 4 and 5 are recovered in nearly equal yields,~4~ as noted in

37

reaction (7). We have run several experiments to validate the origin of these products. One of our most important controls is to vary the base. For instance, we have radiolysed bromocyclohexane together with ammonia. The recovered 4:5 ratio is 0.3-0.4. °°~ Although other processes may be giving rise to the recovered products, any difference between radiolysis in the presence of ammonia, on the one hand, and radiolysis in the presence of triethylamine, on the other hand, must be ascribed to the variation in base. Therefore, the observed 4 :$ ratio in reaction (7) represents a lower bound on the isomer distribution for deprotonation of 3 by triethylamine. Our results with methylcyclopentyl cation clearly disagree with the prediction based upon the intermediacy of a single complex that yields both 4 and 5. That is, our data show that channel selection in gas phase proton transfer reaction is not adequately described by a single intermediate decomposing via different orbiting transition states. We have probed the mechanism of proton transfer further by examining the role of thermochemistry in the selection of transition states. As discussed above, the t-amyl cation yields the two products 2-methyl-l-butene and 2-methyl-2butene when deprotonated by di-n-propyl ether. This proton transfer is only a few kilocalories exothermic, and one might predict, on the basis of RRKM theory, that the thermodynamically more favored product should be produced in greater abundance. In this case, 2-methyl-2-butene is thermodynamically more stable than 2-methyl-1butene by 1.5 kcal/mol. Statistically, however, the thermodynamically less stable product would be favored as there are only two protons that can be removed to give 2-methyl-2-butene, while there are 6 protons that can be removed to give 2-methyl-1butene.

-Br 70eVp

:5

4

Calculated for orbiting transition state Observed

5

CLO01- 0.01 0.8

T. H. MORTON

38

[(CH3)2CHCH2CH2 +] primary cation

7-I

(CH3)2CHCH2CH2Br

'

+ ÷ [(CH3)2CCH2CH 3]

base~

(8)

(CH3)2C=CHCH 3 + H2C=C(CH3)CH2CH 3

C5HI0 .+ (CH3)2C-CHCH 3

What are the relative roles of thermodynamics and statistical probabilities in very exothermic proton transfers? From the methylcyclopentyl cation result we infer that statistical considerations dominate: a product ratio of 0.8 is observed where a statistical ratio of 0.75 would be expected. In order to generate t-amyl cation in the presence of a powerful base (such as triethylamine), we chose to generate the t-amyl cation by 70 eV electron impact on the primary isoamyl bromide (CH3)2CHCH2CH2Br, as shown in reaction (8). In the mass spectrum of this bromide, CsH, + constitutes one-sixth of the total ionization. Unfortunately, there is a prominent CsH,o÷ cation, as well, in the mass spectrum, with an abundance comparable to that of CsHH +. Therefore, it was necessary to find a method to distinguish neutral CsH,o that was a product of proton transfer (from CsH, +) from C5H1o that was a product of neutralization of CsH,o "+ by charge exchange. To do this, we observed that the 7deuterated bromide shown in reaction (10) gives a CsH,o D + ion in the mass spectrum, but eliminates only DBr to form the odd-electron fragment ion. We assume that CsH. + cations generated by electron impact on (CH3)2CHCH2CH2Br are, at least initially, in the primary cation form, as shown in reaction (8). This primary cation rearranges to form the tertiary cation extremely rapidly, and the t-amyl cation thus formed should be vibrationaUy excited. We estimate the energy difference between the primary cation and the tertiary cation to be the same as the energy difference between isobutyl cation and t-butyl cation, approximately 32 kcal/mol. (31' In solution phase, t-amyl cation is well known to scramble all of its hydrogens rapidly, and the activation barrier for this process is E,=18.8kcal/mol. (2s) We have performed RRKM calculations using this activation barrier for scrambling of all the hydrogens in gaseous

D~k

I

t-amyl cation, using as our transition state a protonated cyclopropane. Frequencies used for the reactant state were the same as used for t-amyl cation in Table 1, while frequencies used for the transition state were varied in order to determine the importance of the structure chosen. Table 4 shows typical values of scrambling rate coefficients for several transition state structures and energies. Each of the three sets of vibrational frequencies was chosen to give the same partition function, Q~b, which when substituted into the expression for the Arrhennius preexponential factor,

A=L#k_~ Qtlamyl Qt reproduces the experimental value of A in solution, 1.6 x 1013 set-1. (27) A value of L"= 6 and a value of Q~o,t/Qrot= 1.7 were used for this computation. The transition state was taken to have only two free internal rotators (2 methyl groups), as opposed to the four internal rotors (3 methyl groups and an ethyl group) in t-amyl cation. °2) It is clear from Table 4 that hydrogen scrambling should be essentially complete on the microsecond time scale. Therefore we expect that CsH1oD+ produced by electron impact on the deuterated primary bromide scrambles the deuterium label equally among all five carbons, as indicated in reaction (9). The product of a Bronsted acidbase reaction of this deuterated cation should produce mostly deuterated neutrals. In our EBFlow radiolysis experiment, we examined the yield of neutral CsHgD from 70 eV electron bombardment of a mixture of the deuterated primary bromide and triethylamine. There was a significant yield of undeuterated 2-methyl-l-butene, and we attribute this to neutralization of the CsH,o "+ cation by charge exchange. The ratio of deuterated

EtsN

70 eV D 8r

D

scrombled deuterium

2.5

:

I

(9)

Neutral products from cation-molecule reactions in the gas phase

39

TABLE 4. VIBRATIONAL FREQUENCIES FOR TRANSITION STATES USED TO COMPUTE HYDROGEN SCRAMBLING RATES OF t-AMYL CATIONS A N D CORRESPONDING R R K M MICROSCOPIC RATE CONSTANTS. N U M B E R S IN PARENTHESES REPRESENT DEGENERACIES

GROUP Z

G~OUP I I

GROUP 1'r1

3065

1066

2865

866

3065

666

3001

1018

2801

818

3001

618

2954(2)

923

2754(2)

723

2954(2)

523

2926(2)

869

2726(2)

669

2926(2)

469

29OO(2)

780

2700(2)

580

2900(2)

380

2872(2)

765(3)

2672(2)

565(3)

2872(2)

365(3)

i/~54(4)

429

1254(4)

229

1054(4)

300

1404(2)

412

1204(2)

212

1004(2)

250

1378(4)

358

1178(4)

158

978(4)

158

1216

335

1016

135

816

135

1175

300

975

100(2)

775

129

1110

23

910

710

112

1095

20

895

695

100

E t" ( ~ a l / m o D 2.05

thltmolecttla: ~ 1.3 x 108

70

H£¢ros¢oplc ~ t e

Constants (se¢ "1)

1.2 x 108

3.9 x 106

14.05

5.6 x 109

3.0 x 1010

7.7 x 1010

20.05

2.5 x 1010

2.2 z 1011

8.4 x 1011

2-methyl-2-butene to deuterated 2-methyl-l-butene was 0.4, as compared to a purely statistical ratio that we would expect to be 0.33.(4) After our publication of these results, Pierre Ausloos pointed out that the same ratio of these isomers was observed in radiolyses of neopentane and was ascribed to the reaction of t-amyl cation with an electron.°3) This raises the interesting possibility that neutralization of even-electron cations by electron attachment may proceed in a fashion analogous to an exothermic proton transfer reaction. Our studies show that Lewis acid-base complex formation does not, in general, precede proton transfer. Subsequent to the publication of our experiments, Michael Mautner reported a mass spectrometric investigation that provides an independent demonstration of this.(u) Recently, Cacace et al. °s~ published a 7-radiolysis study that also supports our conclusion. Now, we are embarking on a detailed study of product ratios as a function of base strength and complexity, in order to probe further the dynamics of proton transfer. Our RRKM calculations suggest that if

any kind of intermediates are formed in proton transfer, two bases (e.g. ammonia and di-n-butyl ether, whose proton affinities lie within 0.5 kcal/mol of one another) with different numbers of internal degrees of freedom should give different product distributions. It is conceivable, however, that proton transfer is a direct reaction (with no intermediates). If that is so, then product channel selection will depend only on the exothermicity of the proton transfer and not upon the structure of the base. We are examining this possibility. CONCLUSION

In the first section of this paper we describe three techniques used for elucidation of reaction pathways in mass spectrometric experiments; variation of ionizing conditions, examination of ion reactivity, and synthesis of new molecules. We use all of these methodologies in our neutral product studies, thus extending mass spectrometric techniques to what are essentially radiation chemical experiments. We have elsewhere described the first kind of technique (variation of ionizing

40

T.H. MORTON

energy): z) In this paper we have discussed the latter two techniques, and it is our expectation that, though similar studies, the separate disciplines of mass spectrometry and radiation chemistry will continue to influence one another. Acknowledgement--The work described here was performed at Brown University and was supported by grants from the Research Corporation, the Petroleum Research Fund (administered by American Chemical Society) and by the National Science Foundation. REFERENCES 1. F. B. BURNSand T. H. MORTON,J. Am. Chem. $oc. 1976, 98, 7308. 2. T. H. MORTON,J. Am. Chem. Soc. 1980, 102, 1596. 3. D. G. HALL and T. H. MORTON,.Z Am. Chem. Soc. 1980, 102, 5686. 4. W. J. MAmNELL;and T. H. MORTON,J. Am. Chem. Soc. 1978 100, 3536; 1979, 101, 1908. These experiments were run in a different, earlier version of the apparatus described herein. 5. D. G. HALL, C. GUPTAand T. H. MORTON,J. Am. Chem. Soc. 1981, 103, 2416. 6. For the purposes of calculating unimolecular microscopic rate constants, RRKM therory is substantially the same as the Quasiequilibrium Theory (QET) widely used by mass spectroscopists. Calculations reported herein were performed using the computer program RRKM (QCPE #234) by W. L. Hase and D. L. Bunker, with semiclassicai state counting. For a recent discussion of these types of calculations, see O. I. AsumoJo and J. I. BRAUMAN,J. Am. Chem. Soc. 1979 101, 3715. 7. D. S. BOMSE,T. H. MORTON,D. D. BRIGLIA,and R. D. WXLCOX,Paper presented at the 6th Northeast Regi. Meet. Amer. Chem. Soc. August, 1974. 8. S. G. LtAS and P. AUSLOOS,3". Res. NBS, 1971, 75A, 591. 9. W. R. DOLBIER,Jr. and S. H. DAI, J. Am. Chem. Soc. 1970, 92, 1774. 10. D. S. BOMSEand T. H. MORTOn, Tetrahedron Lett. 1974, 3481. 11. K. R. JENN1NGS,In Gas Phase Ion Chemistry (Edited by M. T. Bowers), Vol II, p 124. Academic Press, New York, 1979. 12. P. J. DERIUCK,A. M. FALICK, S. LEWIS and A. L. BURLINGAME,J. Phys. Chem. 1979 83, 1567. 13. J. J. BUTLERand T. BAER, J. Am. Chem. Soc. 1980 102, 6764; G. D. Willett and T. BAER, J. Am. Chem. Soc. 1980, 102, 6769; 1980, 102, 6774.

14. J. F. WOLF, R. H. STALEY, I. KOPPEL, M. TAAGEPERA, R. T. MCIvER,Jr., J. L. BEAUCHAMPand R. W. TAFr, J. Am. Chem. $oc. 1977 99, 5417. 15. R. G. COOKS,J. H. BEV~ON, R. M. CAPmOLIand G. R. LESTER, Metastable Ions. Elsevier, Amsterdam, 1973. 16. W. WAGNER, H. HEIMBACHand K. LEVSEN, Int. Z Mass Spectrom. Ion Phys., 1980 36, 125. 17. D. SMITHand N. G. ADAMSIn Gas Phase Ion Chemistry (Edited by M. T. Bowers), Vol. I, p. 2. Academic Press, New York, 1979. 18. S. G. LIASand P. AusLoos, Radiat. Phys. Chem. 1982, 20, 87. 19. W. N. ALLENand F. W. LAMPE,J. Am. Chem. Sac. 1977, 99, 6816. 20. V. T. ALEKSANYAN,M. R. ALIEV, M. Y. LUKINA,O. A. NESMEYANOVAand G. A. KHOTIMSKAYA,IZV. Akad. Nauk SSR, Ser. Khim. 1968, 807. 21. M. KLINE and T. H. MORTON,Unpublished results, 1978. 22. D. H. AUE and M. T. BOWERS In Gas Phase Ion Chemistry. (Edited by M. T. Bowers) Vol. II pp. 1-51, Academic Press, New York, 1979. 23. W. J. CHESNAVlCN,T. SU, and M. T. BOWERS,J. Am. Chem. Sac. 1978,100, 4362; W. J. CHESNAVICHand M. T. BOWERSIn Gas Phase Ion Chemistry, (Edited by M. T. Bowers) Vol. II, pp. 119-151. Academic Press, New York, 1979. 24. W. FORST, Theory of Unimolecular Reactions. Academic Press, New York, 1973. 25. J. Cox and G. PILCHER, Thermochemistry of Organic and Organometailic Compounds. Academic Press, New York, 1970. 26. N. CASTERand T. H. MORTON,Unpublished resutls, 1978. 27. J. P. GRANDY,N. CANTERand T. H. MORTON,Unpublished results, 1978. 28. M. SAUNDERS, P. VOGEL, E. L. H A G E N

and J.

ROSENFELD,Accts. Chem. Res. 1973, 6, 53. 29. C. WESDEMIOTIS,R. WOLFSCHOTZand H. SCtnVARZ, Tetrahedron 1980, 36, 275. 30. D. G. HALLand T. H. MORTON,Unpublished results, 1980. This experiment was run in the apparatus described herein, 31. H. M. ROSENSTOCK,K. DRAXL,B. W. STEINERand J. T. HERRON, J. Phys. Chem. Ref. Data 1977, 6, Supplement 1. 32. E. TALLMAN and T. H. MORTON, Unpublished results, 1978. 33. R. E. REBaERT and P. AUSLOOS,J. Res. NBS 1972 76A, 329. 34. M. MAUTNERJr. Am. Chem. Sac. 1979, 101, 2389. 35. M. ATTINA, F. CACACE, P. GIACOMELLO and M. SPERANZA,J. Am. Chem. Sac. 1980, 102, 6896.