The electron affinity of cyclopropyl radical measured by the kinetic method

ELSEVIER

and Ion processes

International Journal of Mass Spectrometry and Ion Processes 167/168 (1997) 541-557

The electron affinity of cyclopropyl radical measured by the kinetic method 1 Randal A. Seburg, Robert R. Squires* The Department of Chemistry, Purdue University, West Lafayette, IN 47907, USA Received 3 January 1997; accepted 29 April 1997

Abstract The electron affinity of cyclopropyl radical (c-C3H5) has been derived using a single reference variant of the Cooks kinetic method. Thiocarboxylate anions (RCOS: R = PhCH2, CH2=CH, CHz=CHCH2, CH3C(O), and c - C 3 H D were synthesized in a helium flow reactor by the addition of OCS to alkyl anions or by reaction of SH- with acyl chlorides. Collision-induced dissociation of these ions in a triple quadrupole mass analyzer generated, inter alia, R- and O C S - fragments. Plots of the electron affinities of R against the natural logarithms of the yield ratios of R- to O C S - were linearly correlated. From these calibration lines, the electron affinity of c-C3H5 was determined to be 0.397 -+ 0.069 eV. This value is slightly lower than, but in good agreement with, the electron affinity of c-C3H5 predicted by G2 theory. Deleterious effects due to structural dissimilarity in the fragment ions and to multiple fragmentation channels are minimal. © 1997 Elsevier Science B.V. Keywords: Cyclopropyl radical; Electron affinity; Kinetic method; Collision-induced dissociation; Triple quadrupole

1. Introduction

The cyclopropyl ring system represents a fascinating structural archetype that has attracted much interest for many years [1]. The constraint of a three-membered ring induces unique bonding in cyclopropanes [2] and a great deal of strain [3]. As a result, cyclopropanes exhibit nucleophilic reactivity that is in many ways similar to alkenes [4], as well as other interesting ringopening processes [5]. The physical properties of cyclopropyl sytems are also significantly affected by the unusual bonding. The C - H * Corresponding author. t Dedicated to Professor Chava Lifshitz in recognition of her many inspiring contributions to gas-phase ion chemistry.

bond dissociation enthalpy of cyclopropane is exceptionally high [6], and the singlet-triplet energy gap in the three-membered ring carbene c y c l o p r o p y l i d e n e , c - ( C H 2 ) 2 C : , favors the singlet state disproportionately relative to other alkyl substituted carbenes [7]. Sequential C - H bond homolyses connect cyclopropane and cyclopropylidene via the cyclopropyl radical. The thermochemistry of the cyclopropyl radical is thus important and is the focus of this investigation. The cyclopropyl radical (1) was first observed by ESR spectroscopy upon radiolysis of liquid cyclopropane [8]. In this and subsequent ESR studies [9], examination of the ill- and ~3C-hyperfine coupling constants revealed that 1, unlike other cycloalkyl radicals, is a

0168-1176/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved PH S 0 1 6 8 - 1 1 7 6 ( 9 7 ) 0 0 1 0 1 - 8

542

R.A. Seburg, R.R. Squires~International Journal of Mass Spectrometry and Ion Processes 167/168 (1997) 541-557

nonplanar, a-radical. No estimation of the out-ofplane angle of the a-hydrogen was made from the ESR data. However, ab initio theoretical computations generally agree that this angle is about 40 ° [10]. Chemical trapping experiments placed an upper limit of 3 kcal tool -l on the inversion barrier of 1 and in fact implicated a tunneling process [9b]. Although the thermal ring opening of cyclopropyl to allyl radical is orbital- and state-symmetry forbidden [11], it readily occurs because of the substantial relief of ring strain [12]. Theory states that the ring-opening proceeds nonsynchronously through a transition state having one methylene group rotated ca. 25 °, while the other remains nearly orthogonal to the C - C - C plane, and that this transition state is common to both the disrotatory and conrotatory pathways [11,13]. Ab initio and density functional theories compute the ring-opening barrier as ca. 22 kcal mol -l [11,14], signicantly higher than the corresponding ring-opening processes in cyclopropyl cation ( 0 - 2 k c a l m o l -j) [15] and cyclopropylidene (11 kcal mo1-1) [16]. The infrared spectrum of 1 has been observed by photolysis of allyl radical trapped in argon [ 17] or xenon matrices [18]. The photoelectron spectrum of cyclopropyl radical has been reported, and the vertical ionization potential has been estimated to be 8.18 eV [19]. Much less is known about the corresponding gaseous negative ion, cyclopropyl anion (1-). Unlike most simple alkyl anions, cyclopropyl anion is bound with respect to electron detachment. This was predicted by ab initio calculations [20] and subsequently verified experimentally by the observation of 1- in the gas phase as a product of collision-induced CO2-1oss from cyclopropanecarboxylate [21,22]. The barriers for inversion (16-18 kcal mol-l) and ring-opening (29 kcal mo1-1) of cyclopropyl anion predicted by theory [21,23] are significantly higher than those for the radical. Although 1- is a stable species in the gas phase, its electron binding energy [i.e. the electron affinity of cyclopropyl radical, EA(1)] has never been measured directly.

Many techniques are available for determining electron affinities of molecules [24]. Chief among these are laser photoelectron spectroscopy (LPES) [25] and laser photodetachment spectroscopy (LPD) [26]. Both techniques allow measurement of EAs of molecules, radicals and other open-shell species with high accuracy and precision, provided that the corresponding anion is available as a precursor. However, the cyclopropyl anion is notoriously difficult to synthesize in the gas phase. As with other unsubstituted alkyl anions, 1- is not accessible by electron impact (EI) ionization, for most alkanes do not yield abundant carbanion species upon EI [27]. In addition, generation of cyclopropyl anion by proton transfer reactions is not feasible because the gas-phase acidity of cyclopropane (z~r'/acid = 411.5 kcal m o l -l [28]) is much higher than that of the conjugate acid of NH~( N H 3 ; AHacid = 403.6 kcal mo1-1 [29]), or other compounds typically used for proton transfer reactions. Even strong-base anions translationally excited in a drift tube do not generate 1from cyclopropane [30]. Deliberate generation of cyclopropyl anion has been accomplished only by collision-induced dissociation of cyclopropanecarboxylate [21,22], a method that does not produce sufficient yields of 1- for LPES or LPD experiments. Thus, a method for measuring electron affinities that does not require formation of the carbanion precursor is desirable. The Cooks kinetic method provides an alternative means for measuring electron affinities. It is a thermokinetic method founded on the rates of competitive dissociation of mass-selected molecular cluster ions [31]. For estimation of electron affinities by this method, the dissociation of an activated negative ion-molecule complex is examined, see Scheme 1. The relative rates of the two fragmentation pathways, that is, the relative abundances of the dissociation products M [ and M2, correlate with the difference in electron affinities (AEA) of M ~and M2. In fact, the logarithm of the yield ratio of ionic

R.A. Seburg, R.R. Squires~International Journal of Mass Spectrometry and Ion Processes 167/168 (1997) 541-557

M1- + M2 MI-"M2

,,,

= MI""M2-] M1 +

M2-

products is proportional to AEA, see Eq. (1): 1n /I[M~-]'~ ~ ) ~AEA

(1)

The description of the system as two competitive reaction channels relies on the nonadiabaticity of the system, that is, that the Born-Oppenheimer approximation becomes invalid at some point during dissociation. A linear calibration plot is constructed from compounds with known EA, and the unknown EA of a compound can then be determined from its measured yield ratio. The kinetic method has been applied extensively in measurements of gas-phase acidities and proton affinities and, to a lesser extent, in measurements of metal ion affinities and polyatomic ion affinities [31 ]. Relatively few reports of electron affinities measured by the kinetic method exist. Burinsky et al. determined the EA ofp-tert-amylnitrobenzene by characterizing the dissociation of negative cluster ions of variously substituted nitrobenzenes [32]. Chen and Cooks applied this methodology to mixed cluster ions of polycyclic aromatic hydrocarbons [33]. Cben et al. evaluated the EAs of coronene, corannulene and C60 by the kinetic method, finding an anomalously low value for C60 that was interpreted in terms of a 'local' EA [34]. In each of these investigations, the dissociating species was formally an electrostatically bonded cluster ion composed of a molecular radical anion and a neutral molecule. As such, the EAs measured are those of closed-shell, stable molecules. One would like to extend the method to open-shell, reactive species, such as radicals, carbenes and biradicals. In these cases, the precursor negative ions may be covalently bonded species. We recently reported one such

543

application: determination of the EAs of m- and p-benzyne by CID of the corresponding dehydrosulfinate anions [35]. Several groups have applied dissociative methods to covalently bound precursor anions as a means for estimating acidities. The silane cleavage method, pioneered by DePuy and coworkers, involves the competitive dissociation of pentavalent hydroxysiliconate anions for measuring the acidities of alkanes [28] and haloarenes [36]. Acidities have been estimated for ketene from dissociation of ester-enolate ions [37], for amino acids from dipeptides [38], and for nucleic acid bases from deprotonated dinucleotides [39]. In this paper, we extend the kinetic method and further demonstrate its utility for the measurement of electron affinities of free radicals, where the precursor anion is a covalently bonded species. Specifically, we determine the EA of cyclopropyl radical by collision-induced dissociation of thiocarboxylate ions, RCOS- (Scheme 2).

2. Experimental The gas-phase experiments described in this paper were performed at room temperature in a flowing afterglow-triple quadrupole instrument kl

j

R--CO S-

\

~

k2

R- +

OCS

R" +

OCS'-

0

O

3

s-

~'~s4 O

-~'~S

-

5 Scheme 2.

CH3

S-

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described in detail elsewhere [40]. Helium buffer gas was maintained in the 1 m x 7.3 cm flow reactor at 0.4 Torr, with a flow rate of 200 STP cm3s -l. Hydroxide ions are generated by electron impact (El) ionization of a N20/CH4 mixture in the upstream ion source. These ions are swept down the flow reactor by the helium buffer gas and react with gaseous neutral reagents introduced into the flow tube via leak valves. Reaction of O H - with propene or toluene produces allyl or benzyl anions, respectively, which form adducts with OCS added further downstream, yielding thiocarboxylate ions, RCOS-. Reaction of O H - with CS2 generates SH- ions. Acyl chlorides react with SH- ions to yield the corresponding thiocarboxylates. The ions in the flow tube are cooled to room temperature by up to 105 collisions with the buffer gas. The negative ions in the flowing plasma are extracted from the flow reactor through a 1.0 mm orifice and focused into an EXTREL triple quadrupole mass analyzer for single-stage or tandem mass spectrometry. For the collision-induced dissociation (CID) experiments, ions mass-selected in the first quadrupole (Q1) collide with argon target gas in the r.f. only, gas-tight central quadrupole (Q2). The ionic fragments are extracted with an electrostatic lens into the third quadrupole (Q3) for mass analysis and detected with a conversion dynode/electron multiplier operated in pulsecounting mode. The argon collision gas pressure was 0.200 mTorr for all CID experiments. The axial kinetic energy of the reactant ion is defined by the pole offset voltage applied to Q2. The laboratory collision energy (Elab) is converted to the center-of-mass energy (EcM) by the expression, EcM = Elab[m/(m + M)], where m and M are the masses of the neutral target and the reactant ions, respectively. Energy-resolved ion appearance curves were obtained by monitoring the product ion yield as a function of the axial kinetic energy of the reactant ion. Appearance curves were collected for each ionic fragment produced upon CID of the thiocarboxylate ions.

Measurements of the CID yield ratios of fragment ions R- and O C S - were performed at EcM values of 10, 11 and 12 eV by the following procedure. First, the high- and low-mass resolution of Q3 were adjusted such that the full widths at half maximum (FWHM) of each fragment ion peak were equivalent. This adjustment is crucial to prevent mass discrimination, which would lead to erroneous yield ratios. Each individual yield ratio comprises the average of 10 successive readings of R- ion intensity divided by the average of 10 subsequent O C S - ion intensities. The ion intensities were read from a digital counter, operating with a gate time of 10 s. For all reactant ions except 2-phenylethanethioate, four individual yield ratios were measured at each collision energy and averages were calculated. Three yield ratios were measured for 2-phenylethanethioate. Uncertainties in the yield ratios were calculated by noting that ion intensity measurements follow a Poisson distribution, and thus the uncertainty in an individual reading of the counter is the square root of the intensity. Error propagation was carried out in the standard manner [41 ]. Ab initio molecular orbital calculations were preformed using an IBM RISC/6000 39H computer using the Gaussian 94 suite of programs [42]. G2 energies and enthalpies were obtained by using the G2 keyword in the input files [43]. 2.1. Materials

All solution-phase reactions were carried out under a nitrogen atmosphere in oven-dried glassware. Cyclopropanecarbonyl chloride was synthesized by refluxing a solution of SOC12 and cyclopropanecarboxylic acid for 30min [44]. The acid chloride was obtained pure by distillation, b.p. 112°C (750Torr). Pyruvoyl chloride was synthesized following a different procedure [45]: dichloromethyl methyl ether, CHC12OCH3, was added dropwise over 20 min at room temperature to pyruvic acid, and the solution was stirred at 60°C for 35 min. The

R.A. Seburg, R.R. Squires/International Journal of Mass Spectrometry and Ion Processes 1671168 (15’97) 541-557

pyruvoyl chloride was isolated as a light yellow liquid by reduced pressure distillation through a Vigreaux column, b.p. 40°C (120 Torr). Acryloyl chloride, toluene, propene and CS2 were commercial reagents and were used without further purification. All liquid samples were degassed prior to use. Gas purities were as follows: He (99.995%); argon (99.998%); N20 (99%); CH4 (99%); ocs (97.5%).

3. Results and discussion 3.1. Theory of the kinetic method The working equation for the kinetic method can be derived in many ways. Applying absolute rate theory, the rate of a unimolecular dissociation can be represented by Eq. (2) [46]:

k= y$exp(-ee/RT) where Q is the partition function of the reactant ion, Q* is the partition function of the transition state, and e. is the activation energy. For two competing dissociations (Scheme l), the ratio of rate coefficients is simply Eq. (3):

ki= Q;Qz exp( - A&e/RT) kz QIQ; which can be Aeo = el - Ed:

rewritten

(3) as

Eq.

(4), where

(4) For a ‘loosely bound’ negative ion-molecule complex in which the barrier to electron transfer between MI and M2 is small compared to the dissociation energy, the cluster ion can then be thought of as a single species, and the two dissociations effectively proceed from the same reactant; therefore, Qt = Q2. The thiocarboxylate ions used for this study can be described in terms of a single well potential surface; thus, Q, = Q2 is rigorously true. One assumes that the partition

545

functions for the transition states differ significantly only in the vibrational modes of the reaction coordinate, v1 and v2. Thus, Q;/Q; = vJv2. For two competing channels, the ratio of rate coefficients is equivalent to the ratio of the product abundances. Thus, Eq. (4) reduces to Eq. (5): ln(z~)=ln(~)=ln(~)-~

(5)

When using the kinetic method for measuring Bronsted acidities, v, and v2 are the vibrational frequencies of the hydrogen bonds in the dissociated products, and these are often assumed to be equivalent [47,48]. For dissociating negative cluster ions or covalently bound anions, the interpretation of v, and v2 or, more appropriately, the partition function ratio in Eq. (4) is not as straightforward (vide infra). The difference in activation energies Aeo is equivalent to the negative of the difference in electron affinities (AEA = EA[M,] - EA[M,]) of the dissociating species2, provided that no reverse activation barriers exist for the dissociations or that they are unequal. Thus, Eq. (5) becomes Eq. (6), and Fig. 1 describes the potential surface for the system. In(z)

=ln(:)+g

(6)

A plot of ln(Z[M~]/Z[M~]) vs AEA will be linear with a slope of l/RT and an intercept of ln(v ,Iv2). The temperature T is the effective temperature a measure of the internal energy of the parent ions. For many applications of the kinetic method, entropy effects are assumed to cancel, leading to Eq. (7):

One selects reference compounds M2 that are structurally similar to M, to ensure that the

’ AeO = - AEA because the electron affinity is defined as the negative of the energy of the following reaction (OK): X + e- - X-.

R,A. Seburg, R.R. Squires~International Journal of Mass Spectrometry and Ion Processes 167/168 (1997) 541-557

546

M2+eEA(M2)]~

M1 +eI EA(M1)

above for Aeo, and using Eq. (10), Eq. (9) becomes - ]'~ (~2)+(s-1)( AEA j In ItM2]~J ~ l n

,~Eo -AEAI =

(11)

Fig. 1. Potential energy diagram for dissociation of negative ion

Again, a plot of ln(I[M1]/I[M2]) vs AEA is linear, but the slope has a slightly different interpretation. The reciprocal of the slope is the excess internal energy per degree of freedom in the activated complex. The unimolecular rate coefficient from RRKM theory is Eq. (12) [46]:

clusters of M ~and M2, illustrating the relationship of the difference in electron affinities (AEA) and the difference in activation or critical energies (Ae0).

k-

MI-+M2

~

= [MI"-..M?_] =:

= [M'I..-M2-]

.

" MI+M2-

ratio of the partition functions for the two transition states is unity. If Eq. (6) is Used, then it is not crucial that Q*I/Q~= 1 (or vl/v2 = 1), just that it is constant over the series of compounds used. The kinetic method can also be rationalized using the RRK unimolecular rate theory [49]. The classical RRK rate coefficient is given as Eq. (8) [46]: k= v

( )Sl

(8)

where v is the frequency factor for the reaction, s is the number of vibrational degrees of freedom of the reactant, E0 is the critical energy for the reaction (corresponding to e0 in Fig. 1), and E is the total energy of the reactant. For two competing reaction pathways, Eq. (9) is obtained: ln(~)

Vl

=1n(72)+(s-

E-E1

I)In(E_-C-E-7)

(9)

G*(E-Eo) hN where G*(E - Eo) is

the sum of states of the active degrees of freedom in the transition state, h is Planck's constant, and N is the density of states of the active degrees of freedom in the energized reactant. For two competing processes,

kl- G*I(E-E1) k2

(E-E')

In ~

E2-E'

(lO)

~ E-E1

Equating E~ - E2 with the negative of the difference in electron affinities ( - AEA) as described

(13)

G;(E-E2)

Because of the RRKM assumption of rapid intramolecular energy transfer, the densities of states of the reactant for the two pathways are equivalent and cancel each other out. The ensuing equation is obtained by following the prescription of Forst [50] as set forth by Lee and Beauchamp [51], using a semiclassical expression for G*(E) given in terms of the quantum numbers and degeneracies of the degrees of freedom, where m, at and a2 are constants and P is a sum of constants. l f/[M1]" ~

n L/[--~J

If the difference in the critical energies, IEl - E 2 I, is small relative to the excess internal energy, E - E t, then

(12)

~

m.ln(a2)+p( AEA \al,/

k,~J

(14)

The subscripts designate the modes involved in the bond cleavage for the two dissociation channels. To achieve Eq. (14), the following assumptions were made: the two transition states differ significantly only in the modes involved in the bond cleavage, and these modes are of the same general type (such as hydrogen bonds in

R.A. Seburg, R.R. Squires~International Journal of Mass Spectrometry and Ion Processes 167/168 (1997) 541-557

the case of proton-bound dimers). The first assumption is also made when absolute rate theory is used for the derivation of the kinetic method working equation (vide supra), but not in the RRK derivation. Eq. (14) was obtained, however, without making any assumption regarding the specific nature of the active modes of the reactant or the transition states. If one assumes harmonic oscillators for vibrational modes and free rotors for internal rotational modes, then Eq. (14) reduces to Eq. (15), the same as that derived from RRK theory [49]: (l[Mi-]'~

(~2)

(AEA'~

lnk,#[-[-~,} ~ln ~ +(s-Ilk, E-El) (15) Thus, all three theories of unimolecular reactions predict a linear correlation between the logarithm of the yield ratio of ionic products and the difference in electron affinities.

3.2. Experimental design

ions contain alkyl substituents for which the electron affinities of the corresponding alkyl radicals were both accurately and precisely known. The EAs for the selected radicals were each determined by laser photoelectron spectroscopy and have relatively small experimental uncertainties [53]. The calibration series comprises 2-phenylethanethioate (3; R = PhCH2, benzyl), propenethioate (4; R = CH2 = CH, vinyl), 3-butenethioate (5; R = CH2=CHCH2, allyl), and 2-oxopropanethioate [6; R = C H 3 C ( O ) , acetyl]. The EAs of the corresponding radicals are listed in Table 1. A precisely known value for the EA of OCS is not crucial because OCS is common to all reactant ions and thus its EA enters the calibration plot as a constant in the intercept. To illustrate, Eqs. (6) and (15) are rewritten in the following forms. The calibration plots are constructed using the equations in these forms.

[ I[R-] "~_RTln(Ul~ EA(R-)=RTln t I[OCS-'],,] \ u2J +EA(OCS-)

For this investigation we employed the single reference kinetic method, in which one dissociating fragment is constant for all calibration compounds and the unknown. The single reference was OCS, and the calibration series was the thiocarboxylate class of ions, RCOS- (Scheme 2). The thiocarboxylates were selected in part because of their synthetic accessibility. Additionally, the electron affinity of OCS (0.46 ___0.20 eV [52] 3) is similar to that predicted for cyclopropyl radical (0.36 eV [28]). Thus, the CID of cyclopropane thiocarboxylate (2) is expected to yield both cyclopropyl anion and OCS- in similar yields. In order to construct a reliable calibration plot, it was necessary that the thiocarboxylate

3 Because of the large uncertainty range of the experimental EA, we computed the G2 EA of OCS using the isodesmic reaction below and the experimental EA of CS2, 0.51 - 0.10 eV [59]. The EA for OCS according to G2 theory is 0.30 eV, 0.16 eV lower than the experimental value. OCS + CS~---, OCS + CS2

547

EA(R-)=

(16)

E-E(R-), i/ I[R-] "~

~

In t l[O---C-ffz.j)

E-E(R-)ln ( Pl~ + EA(OCS-) (17) s- 1 \ u2]

3.3. Ion synthesis Two synthetic routes to the thiocarboxylate ions were employed. Reaction of OCS with Table 1 Electron affinities of relevant species Compound

Electron affinity (eV)

Reference

PhCH2 (benzyl) CH2=CH2 "(vinyl) CHz=CHCH2 (allyl) CH~C(Oj (acetyl) OCS OCS (G2, isodesmic)

0.912 _+ 0.006 0.667 ~ 0.024 0.481 -- 0.008 0.423 _+ 0.037 0.46 _+ 0.20 0.30

[53a] [53b] [53c] [53d] [52] This work

548

R.A. Seburg, R.R. Squires/International Journal of Mass Spectrometry and Ion Processes 167/168 (1997) 541-557

benzyl and allyl anions afforded thiocarboxylates 3 and 5 [Eqs. (18) and (19)]. PhCH 2 + OCS ---*PhCHzCOS-

(18)

CH2=CHCH~ - + OCS ---*CH2=CHCH2COS(19) Adduct formation is efficient, providing very high ion intensities of 3 and 5. Products from competing reactions are negligible for the benzyl system. In the allyl system, the sulfur abstraction product CHz=CHCH2S - (m/z 73) is the major side product. These observations reproduce those reported by DePuy and Bierbaum for the same systems [54]. The same route was not feasible for ions 2, 4 and 6 because cyclopropyl and vinyl anions are not easily accessible in a flowing afterglow, and acetyl anion is known to react with OCS by sulfur atom transfer only [55]. Electron impact ionization has been used to generate thiocarboxylates from thiocarboxylic acids and anhydrides [56], but no other ion-molecule routes have been reported. We developed a novel thiocarboxylate synthesis using addition/elimination chemistry at an acyl group. Reaction of acyl chlorides with SH- in the flow reactor generates the desired anions 2, 4 and 6, according to Eqs. (20)-(22). (20) -

+

CH3COC1 + SH- ~ CH3C(O)SH + C1b

CH3C(O)S- + HC1

(23)

3.4. CID products from thiocarboxylates

c-C3HsCOC1 + SH- ---*c-C3HsCOS- + HC1

CH2=CHCOC1 + SH- ---*CH2=CHCOS

through a tetrahedral transition state to the product ion-dipole complex, [RC(O)SH-C1-]. An alternative mechanism involving a tetrahedral intermediate (rather than a transition state) is also possible [58]. The product complex then dissociates to C1- and RC(O)SH, or the C1- deprotonates the nascent RC(O)SH in the ion-dipole complex and escapes as HC1, leaving RCOS- behind. Supporting this mechanism is the large ammount of C1- in the product mass spectra. In the synthesis of 2 and 6, the ratio of C1-/RCOS- is approximately 2, whereas for 4 the ratio is much greater, about 15 (employing neutral flow rates that maximize RCOS- production). This indicates that the majority of C1- generated in the ion-molecule complex escapes before proton transfer occurs. The overall enthalpy change for the model reaction shown in Eq. (23) is - 2 kcal mol-~; the first step (a) is exothermic by 19 kcal mol -~, while the second step (b) is endothermic by 17 kcal mol -l [59]:

HC1 (21)

CH3C(O)COC1 + SH- ---*CH3C(O)COS- + HC1 (22) Reaction of SH- with acyl chlorides in an ICR has been reported by Asubiojo and Brauman [57]. No thiocarboxylate products were recorded; the major product was C1-. Reaction Eqs. (20)(22) are believed to proceed by the stepwise mechanism suggested by Asubiojo and Brauman [57]. That is, SH- forms an ion-dipole complex with RCOC1, and the reaction coordinate passes

For applications of the kinetic method, dissociation of the precursor ions via two channels is desirable. The formation of side-products can reduce the intensity of the ions of interest either by direct competition or by sequential fragmentation of the primary ions. Most kinetic method investigations employ noncovalent, ionbound clusters that dissociate in a well-behaved fashion by only two pathways. In each of the previous uses of the kinetic method for EA determination, additional fragmentation pathways were not observed [32-35]. Collision-induced dissociation of thiocarboxylate anions under the multiple-collision conditions employed in this study produces several different

R.A. Seburg, R.R. Squires~International Journal of Mass Spectrometry and Ion Processes 167/168 (1997) 541-557

549

Table 2 Products from collision-induced dissociation of thiocarboxylate ions, RCOS -a RCOS-

CID products

Relative abundance

Apparent onset (eV) b

2 c-C3HsCOS-

C2H4OS + HCC- (m/z 25) c-C3HsCO + S-(m/z 32) C4H40 + SH- (m/z 33) OCS + c ~ 3 H ~- (m/z 41) CS + c-C3H50- (mlz 57) c-C3H5"+ OCS- (m/z 60) CO + c-C3HsS- (m/z 73) PhCH 2' + OCS- (m/z 60) OCS + PhCH2(m/z 91) H2S + PhCCO- (m/z 117) CO + PhCH2S- (rn/z 123) OCS + CH2=CH - (m/z 27) CH2=CHCO + S- (m/z 32) CH2CCO + SH- (m/z 33) CH2=S + HCCO- (mlz 41) CH2=CI:t + OCS " (mlz 60) CH2=CHCH2CO + S- (m/z 32) OCS + CH2=CHCH/ (mlz 41) CH2=CHCH2"+ OCS- (mlz 60) CO + CH2=CHCH2S - (mlz 73) CH3C(O)CO + S- (m/z 32) CH2C(O)CO + SH- (m/z 33) OCS + CHACO- (ndz 43) CH3CO + OCS- (m/z 60) CO + CH3COS- (m/z 75)

11 34 44 100 55 31 36 1 100 < 1 < 1 100 43 33 65 8 18 100 18 16 42 100 76 53 17

3.0 7.2 2.9 3.9 2.6 4.2 3.0 5.1 1.6

3 PhCH2COS-

4 CH2CHCOS-

5 CH2CHCH2COS-

6 CH3C(O)COS-

3.4 7.4 2.2 2.6 4.9 6.4 2.0 3.5 1.9 6.5 2.5 3.1 3.8 2.0

a Collisions with argon in Q2 at a pressure of 2 x 10 -4 Tort. Less than 20% diminution of parent ion beam. b Because of the multiple-collision conditions employed for CID, the apparent onsets are not necessarily equivalent to the true appearance energies of the product ions.

ionic products. 4 CID spectra were obtained at collision energies of 10, 11 and 12 eV (CM) for each ion 2-6, employing 0.200 mTorr of argon as target gas in the second quadrupole. Table 2 presents the CID products for 2 - 6 at 10 eV (CM) collision energy. Ion appearance curves were obtained for each product ion in order to estimate the energy onsets. These energies are also given in Table 2. In each system, the number and identities of the fragment ions are constant at the three collision energies, although the relative intensities vary somewhat. The CID of 2-phenylethanethioate (3) produces predominantly PhCH/ (m/z 91, R-) and a relatively small 4In a previous study [56], other thiocarboxylate anions fragmented only to R- and OCS, or did not fragment at all, in the field-free region of a sector instrument under single-collision conditions, using N2 as a collision gas.

amount of OCS- (m/z 60) by direct cleavage. Two other trace products are PhCHaS(m/z 123) and PhCCO- (m/z 117). Propenethioate (4) yields five CD products: H2C=CH - (m/z 27,

R-), S- (m/z 32), SH- (m/z 33), HCCO- (m/z 41), and OCS- (m/z 60). The CID fragments from 3butenethioate (5) are S- (m/z 32), CH2=CHCH~ (m/z 41, R-), OCS(m/z 60), and CH2=CHCH2S - (m/z 73). 2-Oxopropanethioate (6) generates five CID products: S- (m/z 32), SH- (m/z 33), CH3C(O )- (m/z43, R-), OCS(m/z60), and CH3C(O)S- (m/z75). Finally, upon CID cyclopropane thiocarboxylate (2) fragments to HCC- (re~z25), S- (m/z32), SH(m/z 33), C3H 5 (m/z 41, R-), C3H50- (m/z 57), OCS-' (m/z 60), and C3H5S- (m/z 73). Only one fragmentation pathway is common to all ions 2-6: the desired C - C bond cleavage to produce

550

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R- and OCS-. Ions 2 and 4 - 6 produce S-, and the CO-loss channel (RS-) occurs in ions 2, 3, 5 and 6. Ions 2, 4 and 6 exhibit a rearrangement channel producing m/z33, SH-. H2CS loss occurs from 4 (m/z 41, HCCO-). Although the multitude of ionic fragments seems daunting, only those ions resulting from secondary fragmentation of R- and O C S (where R denotes the entire alkyl moiety of the thiocarboxylate, R - C O S - ) are of concern vis-hvis the electron affinity determination. All other ionic products resulting from primary fragmentation channels should have no effect on the R - / O C S - yield ratio and can be ignored. In order for an ion to be a secondary fragment, it must have a lower m/z value than its precursor, it must have a chemical formula contained in that of its precursor, and it must have a higher appearance energy than its precursor. With respect to the R- channel, only one of the CID product ions in Table 2 meets the first two criteria, HCC- (m/z 25) from the CID of 2. The apparent onset for the appearance of HCC-, however, is 0 . 9 e V lower than that of c - C 3 H 5 (rn/z41), 3.0 eV vs 3.9 eV. Thus, HCC- must result either from a primary process or from some other primary fragment; it cannot originate entirely from c - C 3 H 5. Because neither R- nor O C S contains both S and H atoms, any ion possessing these two atoms can be eliminated from consideration as a relevant secondary fragment. Likewise, any product ion containing both O and H atoms can be disregarded. Thus, on this basis, every other CID product for systems 2 - 6 in Table 2 can be ignored except S-. Incorporating the S- data for construction of the calibration plots is not as straightforward as might appear. If S- arises only through secondary fragmentation of OCS-, then simply adding the measured ion intensity of S- to that of O C S - is the appropriate course of action. In the thiocarboxylate systems, however, direct C - S bond cleavage can generate S- (and an acyl radical). The known thermochemistry for a related model system aids in evaluating the relative importance

of each process [60]. The C - S bond energy in HCOS- is 87 kcal mo1-1 (3.8 eV), while in O C S it is 37kcalmo1-1 (1.6eV). The C - H bond energy in HCOS- is 67 kcal mo1-1 (2.9 eV). H C O S - ---, HCO + S -

AH298= 87 kcal mol- l (24)

H C O S - ~ IT+ O C S -

AH298= 67 kcal mol- 1 (25)

O C S - ~ CO + S -

At/-/298= 37 kcal mol- 1 (26)

Thus, formation of S- from HCOS- via secondary fragmentation of O C S - requires at least 104 kcal mo1-1 (4.5 eV), which is 17 kcal mol -l (0.74 eV) greater than the energy necessary for primary C - S cleavage. To the extent that the HCOS- model can be extended to the ions 2 - 6 , the thermochemistry suggests that most of the Syield originates from primary C - S bond cleavage in RCOS-. The apparent onsets for S- formation upon CD of ions 2 - 6 are all about 6 - 7 eV. This is consistent with both primary and secondary pathways for the production of S-. Without knowledge of the relative contribution of each pathway, adding the measured intensities of Sto those for O C S - to obtain the requisite yield ratios is tenuous. Indeed, the EA vs yield ratio data with S- intensities included gives a calibration plot with a much poorer linear correlation than that constructed from only the O C S - intensities. Therefore, the S- data is excluded from further analysis.

3.5. Collision energies An important experimental parameter is the collision energy employed for CID of the reactant ions. Figs. 2 and 3 present ion appearance curves for formation of R- (benzyl and cyclopropyl) and O C S - from CID of thiocarboxylates 3 and 2, respectively. The corresponding

R.A. Seburg, R.R. Squires/International Journal of Mass Spectrometry. and Ion Processes 167/168 (1997) 541-557

appearance curves for R- and O C S - from ions 4 - 6 are qualitatively similar. In each case, I[R-] increases rapidly after the onset then slowly decreases as E(CM) increases, while I[OCS-] rises slowly from its onset and remains relatively constant at energies greater than about 10 eV (CM). The onsets of R- and O C S - should differ simply by the difference in the electron affinities of 1~ and OCS. However, the shifts in the apparent onsets for O C S - (see Table 2) significantly exceed AEA. The shift is most pronounced for 3, where the R- and O C S - onsets are 1.6 and 5.1 eV, respectively. The reason for the shift is unclear, although we note that the absolute yields of O C S - are consistently less than those for R-. Because the electron affinity of OCS is rather low (see Ref. [52] and Footnote 3), autodetachment might be suppressing the measured cross-section of OCS-. Inasmuch as the same behavior is evident in the energy-resolved CID spectrum of C6H5SO 2 [35], where EA(SO2) = 1.107 eV [60] and EA(C6Hs) = 1.097 eV [54](a), this explanation seems unlikely. The appearance curves aid in choosing appropriate collision energies for the quantitative analysis of the fragment yield ratios. Unlike the electrostatically bonded clusters ions that are usually examined with the kinetic method, the thiocarboxylate ions require much higher collision energies for dissociation because of the stronger covalent bond that must be broken (see Figs. 2 and 3 and Table 2). The collision energy must be sufficiently high to ensure that the excess internal energy of the reactant is much greater than the difference in electron affinities (AEA) of R and OCS. This is equivalent to the condition that IE~ - E21 be much less than E - E~ (or E - E2) in the RRK and RRKM justifications of the kinetic method given earlier. For our calibration series, the largest AEA is 0.45 eV (benzyl vs OCS). In addition, the collision energy must be sufficiently high to avoid the region of the O C S onset shift described above. Finally, the yield ratio I[R-]/I[OCS-] should be invariant in the region of the selected collision energy, in order

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to minimize error in measuring the yield ratio. This is especially important for the less abundant fragment, O C S - These criteria necessitate collision energies of about 10 eV or greater. Gas-phase acidities measured by the kinetic method have been shown to be insensitive to changes in collision energy within a small range (near 2.5 eV, CM) [61]. Therefore, experiments were performed at three different collision energies to ascertain whether the same is true for 0.20

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R.A. Seburg, R.R. Squires~International Journal of Mass Spectrometry and Ion Processes 167/168 (1997) 541-557

552

Table 3 Experimental values of ln(l[R ]//[OCS-]) for the product ion yields from CID of thiocarboxylate ions, RCOS -a RCOS-

2 3 4 5 6

c-C3HsCOS PhCHzCOSCH2=CRCOS CH2=CHCH2COSCH3C(O)COS-

In (I[R-]//[OCS 10eV (CM)

-])h

0.931 _+ 0.053 4.36 _+ 0.024 2.50 +_ 0.050 1.54 _+ 0.046 0.468 _+ 0.041

II eV (CM)

12eV (CM)

0,754 _+ 0.042 4,26 _+ 0.019 2.26 _+ 0.055 1,43 _+ 0.039 0,326 _+ 0.038

0.673 _+ 0.060 4.25 _+ 0.061 2.26 + 0.055 1.35 _+ 0.057 0.159 _+ 0.029

"Collisions with argon in Q2 at a pressure of 2 x 10 -4 Ton'. b Each yield ratio is a weighted average of three independent measurements for ion 3 and four for ions 2 and 4-6,

measurements of electron affinities from CID of covalently bound anions.

EA = (0.144 _ 0.026)ln(l[R - ] / I [ O C S - ] ) + (0.299 _+ 0.066) eV

3.6. Electron affinity of cyclopropyl radical Table 3 presents the logarithms of the measured yield ratios of R- and OCS-, In (I[R-]//[OCS-]), from CID of thiocarboxylate ions 2 - 6 . These were measured at collision energies of 10, 11 and 12 eV (CM). The data from ions 2 and 4 - 6 represent weighted averages of four measurements made over two different days. For 3, three measurements were made. The measured yield ratios range from 1.17 (6, at 12 eV) to 78.3 (3, at 10 eV), all well within the dynamic range of our instrument (ca. 100), though the latter requires a very intense reactant ion beam to measure I[OCS-] reliably. Plots of EA(R) against the experimental ln(I[R-]//[OCS-]) values are presented in Fig. 4. The experimental uncertainties in the EAs are represented by the vertical error bars. A weighted linear least-squares analysis of the data gives the following calibration relations: Eq. (27) for data acquired at 10 eV (CM) collision energy, Eq. (28) for collision energy of 11 eV (CM), and Eq. (29) for 12 eV (CM) collision energy.

(29)

The dashed lines indicate where the data for cyclopropyl radical (1) fall. From the calibration plots, the measured EA of 1 is 0.402 + 0.068, 0.393 -+ 0.070 and 0.396 -+ 0.069 eV at collision energies of 10, 11 and 12 eV (CM), respectively. The best-fit lines lie closer to the reference EAs

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(28)

In (I[R-] / I[OCS'-]) Fig. 4. Calibration plots of the electron affinities of R vs the natural logarithm of the yield ratios, In (I[R ]/I[OCS-]), obtained upon collision-induced dissociation of thiocarhoxylate ions at collision energies of (a) lO eV (CM), (h) 1 l eV (CM), and (c) 12 eV (CM).

R.A. Seburg, R.R. Squires~International Journal of Mass Spectrometry and Ion Processes 167/168 (1997) 541-557

that have the smallest experimental uncertainties, because the fitting procedure is a weighted leastsquares analysis. The uncertainties in the calibration lines are calculated as previously reported [62], based on the analysis of Bevington [41], including both the uncertainties in the EAs and in the measured yield ratios. If one fits the data using a simple unweighted least-squares analysis, assuming that the reference EAs all have the same uncertainties (or none at all), then the calibration lines split the data more evenly, and the EA for cyclopropyl increases to about 0.45 eV. This is, of course, inappropriate since the uncertainties in the EAs are known. The EAs obtained at the three different collision energies are quite consistent, and display no systematic variations with collision energy. Averaging the three values gives EA(1) = 0.397 - 0.069 eV (Table 4).

3.7. Comparison of EA(1) with theory and other thermochemistry The electron affinity for cyclopropyl radical measured by the kinetic method is in good agreement with theoretical predictions and with estimates from other experiments. For comparison, Table 4 presents the experimental electron affinity measured by the kinetic method, other experimental estimates from thermochemical cycles, and G2 values. Calculated G2 energies for relevant species are presented in Table 5, which includes some entries taken from the literature [63,64]. The energy we calculate for Table 4 Experimental and calculated (G2) electron affinities for cyclopropyl radical Method

Electron affinity

Reference

(eV) Kinetic method Thermochemical cycle a Thermochemical cycle" G2 theory, isodesmic G2 theory, direct

0.397 +_ 0.069 0.36 0.51 0.503 0.464

This work [28] [66] This work This work

Measured AHacid of cyclopropane and obtained EA using the thermochemical cycle presented in the text.

553

Table 5 Total energies, E (0 K), and enthalpies, H (298 K), of relevant species calculated by G2 theory a Species

E (OK)

H (298K)

Cyclopropyl radical Cyclopropyl anion Cyclopropane b CH,~ CH 3'' CH 3'

- 116.95773 - 116.97480 - 117.63118 -40.41088 -39.74648 -39.74509

- 116.95334 - 116.97043 - 117.62681 -40.40706 -39.74267 -39.74084

a Energies and enthalpies in hartrees, b Ref. [63]. ¢ Ref. [64].

cyclopropyl anion is 2.89 millihartrees lower than that reported by Glukhovtsev et al. [65], even though the MP2(full)/6-31G* geometries are equivalent. We will use the G2 energy of cyclopropyl anion determined in the present study for further analyses. Previous experimental investigations have provided estimates of EA(1) via the following thermochemical cycle: z~r"/acid (C -- C 3H 6) = DH298(c-C3Hs-H) - EA(1) + IE(H). The homolytic bond enthalpy, DH298(c-C3Hs-H), is 106.3 _ 0.25 kcal mo1-1, as determined by radical kinetics measurements in a low pressure reactor [6]. With this value and the well-known IE(H), two groups estimated EA(1) by measuring AHacid(C-C3H6) using the silane cleavage method. DePuy et al. measured z~t--/acid(C-C3H6) = 411.5 _ 1 kcalmol -~ with a FA-SIFT instrument and derived EA(1) = 0.36 eV [28]. Peerboom et al. obtained ~ J - - / a c i d ( C - C 3 H 6 ) = 408.2 _ 1 kcalmol -~ with a FT-ICR and derived EA(1) = 0.51 eV [66]. An estimate for Z ~ t - / a c i d ( c - f 3 H 6 ) of 408 _-_ 5kcalmo1-1 was obtained in our laboratory from energy-resolved CID of cyclopropanecarboxylate [22]. In computing the electron affinity from the acidity and bond enthalpy, DePuy et al. and Peerboom et al. made the commonly used assumption that the EA, defined as a 0 K energy, is equivalent to the corresponding 298 K reaction enthalpy, Z~taIEA,298(1). This is in fact a good assumption for cyclopropyl radical. Using the 0 - 2 9 8 K integrated heat capacities for cyclopropyl radical and cyclopropyl anion in the G2 output, the

554

R.A. Seburg, R.R. Squires/International Journal of Mass Spectrometry and Ion Processes 167/168 (1997) 541-557

difference between EA(1) and AHEA,298(1) is found to be insignificant, 0.0004 eV (using the 'ion convention', the electron has no integrated heat capacity [29]). Thus, our experimental EA for cyclopropyl radical falls between those predicted from the two different gas-phase acidity measurements, but closer to the EA(1) = 0.36 eV of DePuy et al. The G2 energies and enthalpies can be used to calculate the thermochemical quantities of interest. From Table 5, the direct G2 predictions are DH298(c-CaHs-H) -- 110.3 kcal mo1-1, ~nacid(C-C3n6) -- 413.4 kcal mol -l, and EA(1) = 10.7 kcal mol -l (0.464 eV). A more reliable method derives these values from the G2 energies and enthalpies of the isodesmic reaction Eqs. (30)-(32) and the following well-established experimental quantities: DH298(CHa-H) = 104.9 +_ 0.1 kcal mol -j, AHacid(CH4) = 416.7 +_ 0.7 kcal tool -l, and EA(CH3) = 1.8 _+ 0.7 kcal tool -l [67]. Using the isodesmic reaction approach, c---Call 6 + CH3"---* c - C 3 H 5 ' - I - C H 4

AH298(G2) = 4.52 kcal mol- l

(30)

c--C3H6 + C H 3 ---' c - C 3 H 5 + C H 4 Z~-/298(G2 ) = - 5 . 0 5

kcal mol-l

(31)

c-C3H5'+ CH 3 ---. c_C3H ~- + CH 3" AE0(G2) = - 9 . 8 4 kcal mol-1

(32)

the calculated G2 values are DHz98(c-C3Hs-H) = 109.4 kcal mo1-1, AHacid(C-C3H6) = 411.6 kcalmol -I, and EA(1) = l l . 6 k c a l m o l -j (0.503 eV). Of the experimental thermochemical values, the AHacid(C-C3H6) = 411.5 kcal mol -j of DePuy et al. [28] agrees extremely well with the G2 acidity. The other two experimental acidities are 3.6 kcal mol -~ lower than the G2 acidity. The experimental value for EA(1) determined in the present study, 9.2 kcal mol -t (0.397 eV), differs by 2.4 kcal mol -l from the G2 prediction. The experimental value for DHz98(c-C3Hs-H),

106.3kcalmo1-1, is 3.1 kcalmol -l below the G2 bond dissociation enthalpy. 5 Thus, the G2 thermochemical results involving cyclopropyl radical fall just outside the nominal _+ 2 kcal mol-1 accuracy claimed for this method [43].

3.8. Evaluation of the method To derive the working equations for the kinetic method, some simplifying assumptions are necessary (vide supra). First, the two dissociation pathways proceed from the same reactant. In loosely bound cluster ions, the rate of ion or electron transfer between the two components must be rapid to satisfy this assumption. The thiocarboxylates employed in this work are discrete covalently bonded species, thereby obviating the need for this assumption. The excess internal energy of the reacting species must be much greater than the difference in activation energies. For the thiocarboxylates, these quantities differ by a factor of -> 10. Next, no secondary fragmentation of product ions occurs. Our thermochemical analysis (vide supra) indicates that secondary fragmentation of O C S - is minor. The good linearity of the calibration plots is evidence that secondary fragmentation of O C S - to S- is negligible. Another assumption is that the dissociation channels do not have reverse activation barriers. The dissociations of R C O S - to R-/OCS and R / O C S - are direct cleavages and therefore probably meet this criterion. The former channel is a heterolytic bond cleavage. Theory suggests that nucleophilic addition of strongly basic anions to CO2 and CS2 is a barrierless process [68]; by analogy the same should apply to OCS. The latter channel is a homolytic bond cleavage. 5Montgomery et al. report a value for D0(c-C3Hs-H) of 107.9kcal mol -~ based on CBS-QC1/APNO calculations [J. A. Montgomery, Jr, J. W. Ochterski, G. A. Petersson, Journal of Chemistry and Physics 101 (1994) 5900]. This corresponds to DH298(c-C3H~-H) = 109.4kcal mol -I, assuming the same heat capacity corrections computed by G2 theory.

R.A. Seburg, R.R. Squires~International Journal of Mass Spectrometry and Ion Processes 167/168 (1997) 541-557

This process is spin- and symmetry-allowed and should also occur without a barrier. The presence of a barrier in the homolytic dissociation channel might explain the higher apparent onsets for OCS- (vide supra). If so, the barrier varies systematically with the EA of 1~, and the end effect is minor, for the quality of the calibration lines in Fig. 4 is good. Finally, entropy effects must be dealt with in some manner. Analysis of the calibration Eqs. (27)-(29) using Eq. (16) affords estimates for the effective temperatures of the dissociating species and for the entropy effect. In terms of absolute rate theory the slope is RT, providing a measure of the effective temperature, Teff, of the activated species. At 10, 11 and 12 eV collision energies, the effective temperatures are essentially the same, 1729 ± 280, 1717 _ 301 and 1671 ± 302 K, respectively. For the intercepts, ln(Q * [R-]/Q * [OCS-]) is substituted for ln0,1h,2) in Eq. (16) (where [R-] and [OCS-] denote the dissociation channels to R- and OCS-, respectively). This more general expression for the entropy effect is used because OCS is linear whereas OCS- is bent (or ~ 113 ° [69]); therefore, their vibrational frequencies differ significantly. This expression also accounts for any significant differences in the vibrational frequencies of 1~ and R-. The intercept of the calibration plots is then EA(OCS) - RTeff ln(Q * [R-]/Q * [OCS-]). Using Teff from the slope and the known EA of OCS (0.46 ± 0.20 eV), the ratios of partition functions at the three collision energies are Q * [R-]/Q * [OCS-] = 3.8 _ 2.5, 3.3 --- 2.2 and 3.1 ___2.1. In many applications of the kinetic method (typically for proton-bound dimers) this ratio is assumed to be unity [31]. The fact that these derived ratios for dissociation of the RCOS- ions are greater than 1 means that the R-/OCS channel is entropically favored over the R/OCS-channel [48,69]. Implicit in this analysis is the assumption that the relative contributions of R- and 1~ to the partition functions Q * [R-] and Q * [OCS-], respectively, remain constant over the series of

555

R groups. To satisfy this condition, the alkyl groups R should be structurally as similar as possible. Because the pool of available reference candidates having precisely known electron affinities is relatively small, the selection of a calibration series of structurally similar compounds is limited. Of the five radicals in this study, the molecular weights range from 27 amu (vinyl) to 91 amu (benzyl). Additionally, the radicals are formally of two types, a (cyclopropyl, acetyl, and vinyl) and 7r (allyl and benzyl), which might have different entropy effects in their respective dissociation channels. It may be noteworthy that the two a-radicals in the calibration plots appear to define a slightly different line than the two r-radicals. However, no reliable conclusion can be drawn without more reference compounds of both types. Nevertheless, the good overall linearity of the calibration plots in Fig. 4 indicates that the effects due to these structural differences in the reference radicals are minor. Thus, the kinetic method proves to be a robust means for measuring electron affinities, even when the competing dissociations involve fragments of different structural types.

4. Conclusions The electron affinity of cyclopropyl radical is accurately determined to be 0.397 ___0.069 eV by measuring yield ratios of R- to OCS- upon collision-induced dissociation of thiocarboxylate ions (RCOS-). This experimental value is in good agreement with previous estimates made by measuring the gas-phase acidity of cyclopropane and applying a thermochemical cycle, and is also in relatively good agreement with the prediction from G2 theory. Potentially detrimental effects due to multiple fragmentation channels, dissimilarities in the structures of the competing fragments, and structural variations within the series of thiocarboxylates are shown to be minor. This study demonstrates the successful

556

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use of the kinetic method to determine electron affinities of radicals from systems of covalently bonded precursor ions. Wider application of this procedure is expected. Acknowledgements This work was supported by the National Science Foundation and by the Department of Energy, Office of Basic Energy Science. R.A.S. thanks the National Science Foundation for a Postdoctoral Research Fellowship.

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