Probing the unusual kinetic behavior of Si2C+2 reactions with unsaturated and aromatic compounds

ELSEVIER

International Journal of Mass Spectrometry and Jon Processes 138 (1994) 307-324

Probing the unusual kinetic behavior of Si& reactions with unsaturated and aromatic compounds Denise C. Parent’ Chemistry Division/Code 6113, Naval Research Laboratory, Washington, DC 20375-5320,

7,lSA

Received 29 October 1993; accepted 28 March 1994

Abstract The unusual non-first-order form ion cyclotron resonance

kinetic behavior of Si& ions produced by direct laser vaporization in a Fourier transmass spectrometer was probed using a variety of experimental techniques to specify the energy content of these ions. The ions apparently are formed with at least 0.8 eV of excess energy in an excited electronic

state, possibly as isomers of the ground state. Reactions with a variety of substituted benzenes and unsaturated hydrocarbons were also investigated. The results of the substituted benzene reactions display a number of similarities to the reactions of Si+ with benezene or NOz, proceeding through adduct formation for the former and producing silicon oxides in the latter. Carbon-13 labeling of Si2Cz in the reactions with 2- and 3-carbon unsaturated neutrals reveals the detailed mechanism of these reactions. The C3H4 isomers (allene and propyne) both appear to react by insertion into single bonds, leading to different products in these two cases. This is in contrast to the reactions with ethylene and acetylene, where insertion into the multiple carbon-carbon bonds is important Keywords:

FT-ICR; Internal energy; Ion-molecule

reactions; Isomers; Kinetics; SizC:

1. Introduction

I/lo = exp(-kiVt)

Previous work in this laboratory on the reactions of silicon carbide cluster cations with acetylene had noted that the disilicon carbide (Si,C;, y > 1) ions exhibited unusual kinetic behavior [ 11.Specifically, the ion decay was non-linear on a first-order plot, as shown in Fig. 1. For comparison, curves for a firstorder decay, described by a single exponential (dotted line)

as well as a biexponential decay (broken line)

’ Present address: Laboratoire de Physico-Chimie des Rayonnements, Universitt de Paris Sud, Bltiment 350, 91405 Orsay Cedex, France.

I/l0 =fi exp(-klNt)

(1)

+&exp(--k2Nt)

(2)

that describes the kinetic behavior when two forms of the reactant ion are present, are also plotted on Fig. 1. (I/lo is the reactant ion fraction remaining at reaction time t, N is the neutral concentration, k, kl and k2 are rate constants, and fi and f2 are reactant ion fractions.) In Eq. (2) the two forms of the ion can be different isomers [2] or different energy states. For the biexponential decay to be observed, interconversion between the two

0168-l 176/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDZ 0168-1176(94)04060-5

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Solving the rate equations for this system (see Appendix) leads to the following expression for the total concentrations of Al plus AZ: [Al + A21 = IF2 -&I E

m-1.25 8 % -1.6 -

\

x (1 - L4210MR2

‘.

\

\

._

----.----._

-2 0.5

1

1.5

2

2.5

3

3.5

1 4

Reaction Time (s)

Fig. 1. Plots of the natural logarithm of the reactant ion fraction as a function of reaction time illustrating three different rate laws (the neutral density is 2 x lo9 cm3 for each plot): ., single exponential decay as described by Eq. (l), with k = 2.5 x lo-” cm3 s-‘. - - -, b&exponential decay as described by Eq. (2;, with k’ = 2.5 x 10e9 cm3 s-‘, k2 = 2.5 x lo-” cm3 SK’, fi = 0.75 and fi = 0.25; 0, evolution of the S&Cl signal in the presence of ‘3CsH2. The line is a fit of Eq. (4) to the data with [Aslo set to zero and k,, = 3.0 x 10-‘“cm3 s-‘, k,, = -3 x 10-l’ cm3 s-’ and kr2 = 2.9x lo-” cm3 s-l (see Appendix for a fuller discussion regarding this fit).

forms of the ion must be slow compared to the reaction of the more reactive ion. The decay curve [3] obtained for Si2C; represents a third possibility: initially only the less reactive form of the reactant ion is present; isomerization or relaxation of this ion produces the more reactive form. A possible mechanism, involving two forms of the species A, is the following: Al + B 3 Al+(M)-A2 A2 + B 5

4)

‘.

-----_-__

f

0

-4

‘.

Products kd Products

(3a) (3b) (3c)

where B is the reactant neutral and M can be a non-reactive collision partner as well as B. Unimolecular radiative emission as a relaxation process is also included in Eq. (3b).

x

exp( -R2t)

(4)

where R1 = krl[B], D = kd[M], R2 = kr2[B], K = D + R1,and [A210is the initial concentration of AZ. In Fig. 1 the solid line is a fit of Eq. (4) to the data points. In the reaction of Si,C,‘( y > 1) with acetylene, two ion products were observed. At short reaction times the only product was S&Cl + C2H2 + Si2Cy+ lHt + C

(5a)

while at longer reaction times adduct formation Si&$ + C2H2 --+ Si2Cy+2H 2+

(5b)

was observed to compete with the first channel. The ratio of the two products, [Eq. (Wl/[Eq. WI, increased from zero to a constant value with increasing reaction time. Though heats of formation for most of these silicon carbide species are unknown, and thus reaction enthalpies cannot be calculated, the formation of the CH2 addition product in Eq. (5a) is surely less energetically favorable than adduct formation in Eq. (5b). Assuming that the reaction in Eq. (5a) is endothermic, these results suggest that the less reactive form of the Si2Cl is an excited ion which relaxes to a more reactive ion. The possibility of an isomerization of Si2CT in conjunction with the relaxation process cannot be discounted. Carbon-13 labeling of the reactants in Eq. (5) revealed that the Si2C: undergo carbon exchange with the

D.C. Parent/International Journal of Mass Spectrometry and Ion Processes 138 (1994) 307-324

acetylene, for example, SiZ13Cz +12C2H2 + Si2 13cy_ 1 12c+ +12C13CH2

(6)

before any other reaction products are observed. This reaction may be intimately linked with the isomerization/relaxation process that the initial Si2C; population appears to undergo. In this paper the behavior of Si2Cz, as the representative Si2C; ion, will be explored in more detail. There are several questions, alluded to above, on which this paper focuses. Do the Si2Cl ions, as initially formed, contain excess energy? If they do, how much and in what form is this excess energy? Are there different isomers involved? What process occurs at short reaction times to convert one form of the ion into another? In addition, this paper presents the results of the reactions of Si2Cz with other linear unsaturated hydrocarbons and aromatic (substituted benzenes) compounds. The unsaturated hydrocarbons were studied for comparison with the acetylene reaction, in particular to determine if carbon exchange was also occurring in these reactions. Reactions with the aromatic compounds were used to bracket the ionization potential of the silicon carbide ion.

2. Experimental All of the experiments were performed with a Fourier transform ion cyclotron resonance (ICR) mass spectrometer (FTMS) that has been previously described [2]. The apparatus consists of a custom vacuum system with a 1 in x 1 in x 2 in cell, a 3 T superconducting magnet and a Nicolet FTMS/lOOO data system. The procedure for forming silicon carbide cluster ions was given in the previous paper

309

and will only be summarized here. The solid sample pellets were made of a mixture of silicon and graphite powders (Alfa Products) or amorphous 13C graphite (Isotec, 99.5% labeled) and mounted just external to one of the ICR cell trapping plates. Cluster ions are produced by direct laser vaporization (DLV) of the pellet with the focused 532 nm light from an Nd:YAG laser, and trapped in the ICR cell. Typical laser fluence was a few (<5)mJ per pulse at the pellet. In most of the experiments, the initial Si&$ ion population was isolated (mass selected) and allowed to react with a static pressure of a neutral gas. The charge transfer reactions were studied with both the initial Si2Ct ion population and with Si2Ct ions that were re-isolated after a variable delay time. Kinetic excitation of Si.&! in some experiments was achieved by applying a very low amplitude short (~200 ps) r.f. pulse at the resonance frequency of the ion. To determine absolute reaction rate constants, the pressures of the neutral gases measured by the ionization gauge must be calibrated. For acetylene, ethylene, propyne and benzene, the pressure of each gas was calibrated by studying known reactions of one or more ions in that neutral gas and comparing the measured rate constant(s) with the literature value(s). This procedure gives an experimental pressure correction factor (PCF,,) for each gas. The PCF is the ratio of two components, PCF = If/R,: the instrumental factor If is a constant, while R, is the response of the ionization gauge to the particular gas. Values of R, have been compiled [4] for a variety of gases and were used to calculate an average value for If (from PCF,,,) that was in agreement with our previous determinations of this constant. From this If and literature Rx values, calculated PCF values were determined for the other gases used in this study. For those gases where R, was unknown, the empirical

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formula R, = 0.36~~+ 0.3 (where o is the polarizability in A3) was used to estimate R,. Rate constants are quoted with la error limits. These limits are measured reproducibilities from 2-4 sets of data. Aside from the usual random fluctuations, there are two sources of error that can affect the values of k determined in these experiments. The first is the pressure calibration, which is a source of error in all of our FT-ICR kinetic studies and is discussed above. In this work, the variation either in If (calculated from all the calibration reactions) or in individual PCFs was less than 10%. The second error source is less common, and arises from the non-linear behavior of the kinetic plots. All determinations of k were made from the linear portion of the reactant ion decay. This provides a lower limit for the true rate constant of the reaction of the ground state ion. If the reactant ions are not completely relaxed, then the measured rate constant will be too low. The branching ratios vary with reaction time. This arises from two effects: (i) the chemistry of the two forms of the reactant ion are different, and (ii) secondary reactions are observed in a few cases (see Table 3 for example). Average values are listed in the tables and variation from this average is generally 25% or less. The non-conventional behavior of this system makes it necessary to report both initial (taken at the shortest reaction time or extrapolated to zero reaction time) and final (long reaction time, when the reaction is more than 75% complete) values for the branching ratios. These values should not be taken as definitive; rather they are meant to give an indication of the different chemistries of the two forms of the reactant ion.

3. Results and discussion It is difficult to determine if the non-firstorder kinetic behavior is caused by excess

and Ion Processes 138 (1994) 307-324

energy in an ion or by the presence of different isomers, since collisions are the agent for both relaxing and isomerizing the ions. Instead, it has been necessary to rely on less direct methods. A variety of experiments, aimed at elucidating the nature of the initially formed Si$z ion population, were performed and are discussed in the first part of this section. In the second part, the reactions with substituted benzenes and unsaturated hydrocarbons are discussed in turn. In the section that follows, various types of excitation of the S&Cl ion will be considered. These are translational or kinetic excitation, vibrational/rotational (VR) excitation and electronic excitation. A word of explanation about the latter: in this paper, “electronic” excitation is used in its broadest form to encompass any state that is not the lowest energy (ground) state (excluding kinetic or VR excitation of course). This includes isomers as well as purely electronic excitation of a particular isomer. 3.1. Probing the twoforms of the Si&

ion

3.1.1. Kinetic energy eflects in the reaction with fiuorobenzene

The reaction of S&Cl with fluorobenzene displayed the non-first-order kinetics (similar to that shown in Fig. 1) and varying product branching ratios typical for the reactions of this ion. In particular, the amount of charge transfer product decreased with increasing reaction time. Observation of the charge transfer product in itself is unusual, since the ionization potential (IP) of fluorobenzene is 9.20 eV while that of Si2C2 has been estimated [5] to be only 8.2eV. These two observations imply that S&C; may initially contain a significant amount of excess energy. Fig. 2 reproduces several spectra, taken under different conditions, for the reaction with fluorobenzene. The bottom trace shows the results for the normal (or control)

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(Adduct -

311

small neutrals)+

Fig. 2. Mass spectra taken during the reaction of Si2Cl with fluorobenzene: spectrum a, S&Cl was isolated and reacted for 1.5 s with fluorobenzene; spectrum b, after reacting for 1.5 s, as in spectrum a, the remaining S&C: was reisolated and again reacted for 1.5 s with fluorobenzene; spectra c-e, same as spectrum b except that the reisolated Si& ions were given additional kinetic energy before the second 1.5 s reaction period. The maximum amount of kinetic energy increases from 1.3 to 3.2 to 6.3 eV in spectra c, d and e respectively.

experiment, in which Si&t has been isolated and allowed to react with about 7 x lo-* Torr of fluorobenzene for 1.5 s. The charge transfer product at m/z96 is seen, as well as several other products between m/z 120 and m/z 180 that are formed by loss of small neutral fragments from the adduct. For the next trace, the experiment is the same as in the bottom trace, except that after the 1.5 s reaction time all products are ejected and the Si&,f is allowed to react for an additional 1.5 s. Now no charge transfer product is observed, owing to depletion of the excited Si2C2f, while the other products show increased intensity. This latter observation is due to the increase in reaction rate previously discussed. It is also clear that the behavior of the reactant ion, vis-a-vis charge transfer, varies with reaction time. The next three traces represent the same experiment as the second trace (reisolation) except that the Si& are kinetically excited

just before the second 1.5 s reaction time. The maximum laboratory kinetic energy of the ion is increased from 1.3 to 6.3 eV in this series of experiments. Again, no charge transfer product is formed, while the other products decrease in intensity as the kinetic energy is increased. Neither result is surprising. The products arising from adduct formation will be inhibited as the energy of the reaction system is increased and the lifetime of the ion/neutral complex is thus decreased. More importantly, kinetic energy does not drive the charge transfer reaction, ruling out excess kinetic energy in Si2Ci as the origin of its unusual behavior. This leaves VR or electronic excitation to consider. 3.1.2. Activation of the acetylene reaction by d@eren t gases

The kinetics of the reaction of Si&Jt with about 6.5 x lo-* Torr of 13C-labeled

D.C. Parent/International Journal of Mass Spectrometry and Ion Processes 138 (1994) 307-324

312

0

0.5

1

1.5 Reaction

2

2.5

3

3.5

4

Time (s)

Fig. 3. Plot of the decay of the Si2C: signal as a function reaction time with 6.5 x lo-* Torr of 13C-labeled acetylene: pure acetylene, A, about 1.5 x lo-‘Torr of SF6 added; about 3.2 x lo-‘Torr of SF,, added. The lines are fits Eq. (4) to the data (see Appendix for discussion).

of 0, n, of

acetylene [6] are shown in Fig. 3. The top curve, with pure acetylene, shows the nonfirst-order kinetics typically observed in this reaction. To determine the effect of nonreactive collisions on the behavior of this reaction, argon or SF6 was added in varying amounts to the ICR cell. The other two curves in Fig. 3 present some of the results with SF6, and we can see that the curvature of the plots becomes less pronounced as SF6 is added. Increasing the total pressure in the ICR cell has the effect of “speeding up” the reaction with acetylene or “activating” the Si2Ci towards reaction. Similar results were seen with argon and also when the pressure of acetylene was increased. To quantify the relative efficiencies of the different gases in activating Si$z towards reaction with acetylene, the following analysis was carried out. The onset of linearity in the kinetics plots’ was determined for each experiment, and the number of collisions of the reactant ion with acetylene and argon or SF6 during the non-linear portion was calculated [7]. In pure acetylene, an average of 5.7 f 0.2 collisions were required to reach the onset of

the linear decay. For the experiments with added argon or SF6, the contribution of the acetylene was subtracted out before calculating the efficiency of the argon or SF6 relative to acetylene in activating the reaction. With argon, the relative efficiency was 0.35 f 0.11 while with SF6 it was 0.31 ZIZ 0.03. Argon is as efficient as SF6 in activating the reaction of S&C: with acetylene, and both are only one-third as efficient as acetylene itself. SF6 is generally considered to be a good quencher of vibrational energy but a poor electronic quencher [8]. The vibrational frequencies [9] of SF, all lie below 1000 cm-‘, ranging from 344 to 769cm-‘; the vibrational frequencies of acetylene are mostly near or above 2000 cm-‘, varying from 612 to 3373 cm-‘. For comparison, the vibrational frequencies of neutral Si2C2 have been calculated [lo] for various geometries: all lie below 2000 cm-’ and most are below lOOOcm_‘. While there are no calculations of the rotational constants of Si2C2, it is expected to have a non-linear geometry. From these comparisons, one would expect SF6 to be a better quencher of vibrational and rotational energy than acetylene, yet it is less efficient than acetylene and no more so than argon, which has no internal modes. This strongly suggests that excess energy in the vibrational/rotational modes is not the factor behind the observed behavior of S&C,+. The enhanced efficiency of acetylene in activating Si2Cz is quite likely linked to the carbon exchange reaction (Eq. (6)) described above. This reaction, involving as it does the exchange of atoms between the two collision partners, provides an excellent mechanism for electronic state quenching [l I] and/or isomerization [12]. An example of the latter is carbon scrambling in the collisionallymediated isomerization of linear CsH$ to cyclic C3H$ using labeled acetylene as the collision partner.

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Spectrometry and Ion Processes 138 (1994) 307-324

3.1.3. Ionization potential bracketing Charge transfer reactions between an ion and a series of neutral reagents of known ionization potential (IP) can be used to bracket the unknown IP of the corresponding neutral [13]. This technique was applied to the Si&$ ions to determine the amount of excess energy in the initial population. The experimental methodology used to make these measurements and the possible pitfalls have been discussed in a number of papers [13]. The same technique was utilized in this work, except that there was no thermalizing buffer gas pulse since the excited state population was the one of interest. The neutral reagents were a series of ten substituted benzenes having IPs ranging from 7.72eV (aniline) to 9.91 eV (hexafluorobenzene). The initial Si& ion population charge transfers with all reagents having IPs less than or equal to 9.25eV (benzene) but not with those having IPs greater than or equal to 9.45eV (o-nitrotoluene). This places the IP of the neutral corresponding to the initially formed Si$Z$ at 9.35 f O.lOeV. Determining the ionization potential of the neutral corresponding to the relaxed Si&l was more problematic, since it involved reisolating the reactant ions after a sufficient delay, as described above for the reactions with fluorobenzene. The reisolated ions were observed to undergo charge transfer with reagents having IPs less than or equal to 8.04eV (durene) but generally not [14] with those having IPs greater than or equal to 8.44eV (p-xylene). This gives an IP for S&C2 of 8.24 f 0.20eV, which is in remarkably good agreement with the previous estimate [5] of 8.2 eV. These two IP values lead to an excess energy of at least 0.8eV in the initially formed Si& ions. As we have previously eliminated kinetic and VR excitation, this excess energy must be electronic, which can include different isomers. One caveat as regards these experiments must be mentioned. The lack of charge

313

transfer for the initial Si$$ ion population with reagents having IPs at or above 9.45eV is not an absolute guarantee that this is the upper limit on the ionization potential of the corresponding neutral silicon carbide (IP = 9.45 eV) species. With o-nitrotoluene and nitrobenzene (IP = 9.86 eV) efficient reactions into other product channels were observed. It is possible that the charge transfer reactions are exothermic but were not observed. With hexafluorobenzene (IP = 9.9 1 e V) S&C,+ was essentially non-reactive, leading to a good upper limit on the IP of the excited state of Si2C2. 3.1.4. Identity of the excited state of Si2Ci We have seen that the 0.8eV (about 19 kcal mol-‘) of excess energy in the initially produced S&C; ions is, by default, in the form of electronic excitation. Unfortunately, it is difficult to define this excited state further, including whether the excited state is in a different spin manifold than the ground state or if it is an isomeric form of the ion. The only calculations available to guide us are by Lammertsma and Giiner [10(b)] for neutral Si2C2. They have calculated a number of geometric configurations of S&C2 in both the singlet and triplet spin states. The lowest energy state is a singlet spin symmetric rhombus (to be referred to as the ground state). Within 1 eV of this ground state they found two excited states in the singlet manifold, a rhomboidal isomer and a linear isomer. The lowest triplet state has a linear configuration and is essentially isoenergetic with the singlet ground state. The first excited triplet state is a rhomboidal isomer. The rhomboidal isomers can be converted to the linear isomers by simple cleavage of the silicon-silicon bond. This bond is not present in the ground state singlet rhombus, necessitating a more complex rearrangement to obtain this isomer. We now assume that the ion has similar geometric isomers in the different spin state

314

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manifolds [15]. Under the conditions in which the silicon carbide ions are produced, it is quite reasonable for the ions to be initially formed in a linear configuration. If the relaxed Si& ions are also linear, then the difference in reactivity arises solely from spin effects [ 161.If this is the case, however, then the low reactivity of the relaxed ion when compared to that of Si2C+ is puzzling. As noted in the previous paper, S&C+ is presumed to have a bent Si-C-Si geometry [17], and the differences in reactivity between Si2C+ and the larger S&Cl was ascribed to a change in geometry, most likely cyclic for the larger ions in the series. A second alternative for the relaxed ion is a rhombus, similar to the ground state of the Table 1 Summary of the reactions of Si$z Neutral IP (eV) Benzene 9.25

Toluenee 8.82

k (lO-‘” cm3 s-I)~ % of krb 5.6 f 0.8 46

-6 50

p-Xylene 8.44

8.8 f 0.3 66

Durene 8.04

13 90

neutral. Isomerization between the linear and rhombic forms requires rearrangement of the Si-C skeleton, which is supported by the observations on the relative quenching efficiencies of SF6 and acetylene, as well as the carbon exchange process observed in the acetylene reaction. In both the linear and rhombic isomers, the silicon atoms are divalent, while the carbon atoms are tetravalent in the linear isomer but only threefold coordinated in the rhombus. Thus one might expect the rhombus to be somewhat more reactive than the linear form, but less so than the linear carbene geometries found for Sic; and C;. Without direct experimental evidence or detailed calculations on the

with benzene and methyl-substituted

benzenes

Major ion productsC

Branching ratiod

CT A A-H A - C2H2 A - SiCzH A-H2 A-CHs A-SiH CT CT-H CT - CH3 A-H A-Hz A-CHz A-CH3 A- SiH CT CT - CH3 A-CH A-CH3

40 + 5 9 -+ 59 9-45 24+ 18 11-+2 7 47 26 22 + 5 9-3 9-l 5 5 I 31+49 ll+ 15 22+ 10 7+2 I 64 -+ 14

aThe la error limits, where listed, are derived from the experimental reproducibilities. The pressure calibration can introduce an additional uncertainty of 10% in the absolute values. bFor durene, the Langevin rate constant /q was calculated using an estimated value of 19.26A3 for the polarizability. For benzene, toluene and p-xylene the literature values [26] of 10.64 A3, 12.5 A3 and 14.95 A3 respectively were used. ‘Only products representing 5% or more of the total abundance are listed. Product abbreviations are CT (charge transfer) or A (adduct formation) with possible neutral loss -X. When different ion products are possible for a given m/z, only the most feasible is noted (i.e. A - SiH and not A - CzHs). dBranching ratios are indicated as (short reaction time) + (long reaction time, reaction more than 75% completed) unless constant. Numbers may not add up to 100 since minor products are not listed. eCharge transfer was observed at short reaction times but was less than 5% of the total products.

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315

Table 2 Summary of the reactions of SizCT with other substituted benzenes Neutral IP (eV)

k (lo-” cm3 s-l)a % of kLb

Major ion produ&

Branching ratiod

Aniline 7.12

7.0 f 0.4 54

o-Nitrotoluene 9.45

5.5 41

Nitrobenzene 9.86 Fluorobenzene 9.20

4.2 f 1.1 33 2.5 22

1,2,4-Trichlorobenzene 9.04

2.2 17

CT A-H Si&N+ CT-O CT - OzH SiO+ SiOH: SizCsH + CT-O CT-O2 CT A A - SiF CT A - Cl A - SiC12 A - S&CC1 SiCl+ SiCrCl+ SiC< SiC&lH+ SizC3C12Ht A+CCl

85 + 47 ---t 23 12 23+ 14 7 38 + 24 0 * 30 11 53 25 53 --t 5 47 -+ 88 17 64 + 26 5 15 5 36 --t 6 12 12 6 6 11 (secondary product)

’ The la error limits, where listed, are derived from the experimental reproducibilities. The pressure calibration can introduce an additional uncertainty of 10% in the absolute values. b For aniline, o-nitrotoluene and 1,2,4-trichlorobenzene, the Langevin rate constant kr was calculated using the estimated values of 13A3, 16.6A3 and 16.5A3 respectively for the polarizability. For nitrobenzene and fluorobenzene the literature values [26] of 14.75 A3 and 10.23 A3 respectively were used. ’ Only products representing 5% or more of the total abundance are listed. Product abbreviations are CT (charge transfer) or A (adduct formatron) with possible neutral loss -X. When different ion products are possible for a given m/z, only the most feasible is noted (i.e. Si&N+ and not SizC3H:). d Branching ratios are indicated as (short reaction time) -+ (long reaction time, reaction more than 75% completed) unless constant. Numbers may not add up to 100 since minor products are not listed.

Si2C,f ion, however, these ideas remain purely speculative. 3.2. Reactions of Si2CT ions

data set is rather limited, particularly for those neutrals where no rate constant error limits are given. The results are summarized and briefly discussed in this section.

3.2.1. Reactions with substituted benzenes The primary reason for studying the reaction of Si2Cl with the aromatic compounds was to bracket the ionization potential, as described above. It was noted that these compounds also yielded a variety of other products when reacted with Si2Cl. The branching ratios and rate constants were measured; however, the

Benzene and methyl-substituted benzenes. The reactions with benzene, toluene, p-xylene and durene (1,2,4,5_tetramethylbenzene) were studied and the results are summarized in Table 1. Though all are fast reactions, the rate constant and efficiency increase as the number of methyl substituents increases. The dominant product in the reaction with

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benzene at short reaction times is the charge transfer product, which we have seen is due to the excited Si2C2f. As the reaction proceeds, the charge transfer product becomes negligible while adduct formation comes to dominate. For the methyl-substituted benzenes, the major product is loss of methyl from the adduct (A - CHs), especially at longer reaction times. Formation of the adducts in these reactions is reminiscent of the reactions of Si+ with benzene and naphthalene studied by Bohme and co-workers [18] in which they observed adduct formation to be the major product. In the case of benzene the charge remains on the silicon atom, while in the naphthalene adduct the charge is transferred to the naphthalene. With Si2C2f, the adduct ion is only observed with benzene while loss of methyl from the adduct is observed with the other neutrals. Elimination of methyl requires about 100 kcal mol-’ in the case of toluene, which must be provided primarily by the bonding between the ion and neutral. With p-xylene, the energy requirement is about 80 kcal mol-’ if no charge transfer occurs, but only about 56 kcal mol-’ if charge transfer to the aromatic radical occurs [19]. The heat of formation of the 1,2,4_trimethylbenzene radical is not known; thus we cannot say what the energy requirement would be, but presumably durene would follow the trend seen above. Other substituted benzenes. The reactions with aniline, o-nitrotoluene, nitrobenzene, fluorobenzene and 1,2,4-trichlorobenzene were studied and the results are given in Table 2. Again, all reactions are relatively fast, with the rate constant and efficiency decreasing from aniline to 1,2,4-trichlorobenzene. Aside from charge transfer, the major products generally involve the substituent group, with the formation of silicon oxides and halides driving the chemistry. For example, an important process in the reaction of the nitro-substituted compounds is the loss

This of o-, presumably to form Si&O. behavior is analogous to that seen [20] in the reactions with NOz of Si+ and Si,‘, which yield predominantly SiO and S&O species. The halogen-substituted benzenes form neutral or ionized silicon halide species. 3.2.2. Reactions with unsaturated hydrocarbons Acetylene. The detailed results for the reaction with acetylene have already been published [l] and are reviewed here for reference (also see Table 4). The reaction is slow, with a rate constant of 2.9 * 0.5 x lo-” cm3 s-l, which is less than 3% of the Langevin rate. The major products are S&&H,‘, which increases in relative abundance with increasing reaction time, and Si&Hl, which is the major product but decreases in relative abundance with reaction time. Si2C4Ht and Si2C3H+ are also formed in small amounts. Using labeled reactants, carbon exchange is observed to be the dominant process at short reaction times, leading to carbon scrambling in the Si2C3H$ product. This result is also proof that the carbon-carbon triple bond is cleaved in this reaction, which presumably accounts for its inefficiency. Ethylene. The reaction of unlabeled S&Cl with ethylene is at first glance quite simple [21], sequentially adding H2 and then C2H3, as indicated below: S&C; + C2H4 -+ Si&Hz

+ C2H2

Si2C2Hl + C2H4 --f Si2C4Hz + H

(7a) (7b)

The measured rate constant for the dehydrogenation of ethylene (Eq. (7a)) is 2.9 f 0.1 x lo-” cm3 s-l, which is 27% of the Langevin rate constant. Use of the labeled ion Sii3Cr (m/282) reveals a complexity that was hidden in Eq. (7). Fig. 4 presents a detailed view of this

D.C. Parent/International Journal of&lass Spectrometry and Ion Processes 138 (1994) 307-324

Si213C2+

2

Si2W2H2+

(82)

,’

w

,*’

16%

I. Si2WXC+

9

,’

: Si2WPCH2+ w

: [Si21QH2+] WI

1:4.1:[-]

,'

: Si212C2+ @a

w 3:l

I Si2W2f2C2H5+ (IW

: Si213CW3H5+ : Si2QC4Hs+ (110)

W)

1:3.6:1.6

Fig. 4. Scheme of the reactions of labeled S$Cl with ethylene, showing the ion masses and branching ratios for the primary and secondary reactions.

reaction. The first difference to note is that 16% of the reactant ions undergo carbon exchange, analogous to that seen in the acetylene reaction, to form the m/z 81 and 80 ions. The ratio of m/z 8 1:80 is 3: 1, compared to a statistical ratio of 4:l. Given the very low signal levels of m/z80 this is probably within the error limits. Reaction of m/z 81 to give m/z80 via carbon scrambling would also decrease the ratio. The H2 addition reaction (Eq. (7(a)) also shows carbon scrambling, forming the m/z84 and 83 ions in a ratio of 1:4.1, again quite close to the statistical ratio of 1:4. The amount of the m/z 82 product ion, which is presumably also formed, cannot be determined as it is masked by the reactant ion. However, its presence is revealed in the secondary reaction (Eq. (7b)), where there is a one-to-one correspondence between the m/284, 83 and 82 secondary ions and the m/z 111, 110 and 109 tertiary ions [22]. The ratio of these tertiary ions is 1:3.8:1.6. The m/z 109 product (and thus the m/z82 product) appears to be enhanced from the complete scrambling ratio of 1:4: 1. Part of this enhancement could undoubtedly be due to reaction of the m/z 8 1 and 80 ions, which we have neglected thus far in the analysis. The products from these ions can only add lo-20% to the expected value for the m/z82 product, however, giving a relative value of 1. l- 1.2 for m/z 109, which is still well below

317

the 1.6 which was measured. Thus we must search for another explanation. The m/z 82 and 84 products of the dehydrogenation reaction are complementary ones. They presumably arise from a symmetric intermediate in which each carbon has one hydrogen attached to it, such as Si2 (13C2Hz)(12C2H2)’ (this formula is not meant to imply anything about the bonding of the silicon atoms to each other or to the carbon atoms). Interestingly, the loss of 13C acetylene is preferred by about 60% over the loss of 12Cacetylene. This suggests a concerted process in which 13C2H2 leaves as the 12C2H2 group is integrated into the product, after the initial transfer of two hydrogen atoms to the silicon carbide ion. The most abundant product of the dehydrogenation reaction is m/283, in which 13C12Cacetylene is lost. This requires that the carbon atoms in the reactant ion bond to those in the ethylene. One possible reaction intermediate is shown below:

(The H atom on each C has been left off for clarity.) The ethylene comes in parallel to the C-C axis of a rhombic Si2Cg. (As indicated above, calculations on neutral Si2C2 find the ground state to be a symmetric rhombus.) If all the carbon atoms are equivalent, the loss of any two adjacent carbon atoms as acetylene, which is different from complete scrambling, would give a ratio of 1:2: 1 for the m/z 84, 83 and 82 ions (the measured ratio is 0.6:2.4: 1.O). Using this model, we see that the m/z84 product is the least favored, owing to the

D.C. Parent/International Journal of Mass Spectrometry and Ion Processes 138 (1994) 307-324

318

greater propensity for 13C acetylene loss noted above. Thus we see an isotope fractionation effect in the dehydrogenation reaction, with a preference for 13C in the neutral acetylene product. A similar effect was observed in the sequential reactions of Fe( 13CO)+ with Fe(12C0)5 to produce Fe,(13CO),(12CO),’ where there was a tendency to lose the labeled CO@= 0) from the adduct ions [23]. Another possible reaction intermediate arises from addition of acetylene across the C-C bond of the linear Si-C-C-S1 ion, with hydrogen transfer and bond formation between the acetylene carbon atoms and the

Table 3 Product branching Primary

products

ratios for the reaction

of Si$z

Allene BR(0)a

BR(t)b

BR(t)b

_ _ _

_ _ _

23

14

9

22

11

8

_

_

+ C3H3

12

S&C: + C2H4 S&H+ + C2H;

32 _

_

Si&H:

21

10

_

_

Si&H: Si&: + S&H+ S&&H: Si2C4H: Si,CSH:

+ CzH2 + C2Hd CH4 + CH3 + CH2 + C + H

Secondary products C6Hq and C,Hz Si&,H: S&H; S&C,H:

8e

with allene and propyne

BR(0)a

I 6

Si&H+

(Again, the H atom on each C has been left off for clarity.) However, loss of acetylene from this intermediate leads to formation of a (c-C2H2Si)Si structure, which was

Propyne

S&H Si2C2 + SiC2H2 + SiC2H

C3Ht + C3H$ + S1C3H: SiC3Ht

silicon atoms [24]. This leads to the symmetric trigonal prism shown below:

8e

0

9

24

6

0

11 _

_ 12

3

_

_ _

6

11

19

37

0

12 30 _

_

0 _ 0

2

0 _

Labeling

70% 30% 40% 60%

resultsC

Si”C3H$ Si13CJ2C2H: Sii2C2H+ Sii3C2H+

15% Sii2C3Ht 55% Si:‘CL2C2H: 30% Si:3C:2CHC 2

Si’3C:2C2H+ 2

Sii3C:‘C

3 H+3

9 _

aProduct branchmg ratio extrapolated to zero reaction time. bProduct branching ratio at the longest reaction time studied (reaction about 90% complete). ‘Labeling of the ion products in the reaction of Si13C: with propyne dThese products may be either primary products whose branching ratio increases with time or they may be secondary products. Labeling results could not be determined because of interference from the Si2C3Hz product which is more abundant at short reaction times. eTotal branching ratio for all four Si2C4H,f (n = O-2, 4) products.

D.C. Parent/International Journal of Mass Spectrometry and Ion Processes 138 (1994) 307-324

calculated to lie 74.3 kcalmol-’ above the ground state [10(b)] for the neutral species. Thus this reaction mechanism seems less likely than addition to the rhombus isomer of the ion, which we expect to be the relaxed ion. Allene and propyne. The reactions of Si2Ci with the C3H4 isomers allene and propyne were studied to compare the reactivity of the silicon carbide ion toward double and triple bonds in a larger system, vis-a-vis the twocarbon neutrals acetylene and ethylene, which are not themselves isomers. Table 3 presents a synopsis of the results for these two neutrals. It is obvious from the number of products that the reactions are quite a bit more complex than were the reactions with acetylene and ethylene. Both reactions are also quite fast, with rate constants of 6.5 f 0.1 x lo-” cm3 s-l and 5.9 f 0.7x lo-” cm3 s-’ for allene and propyne respectively. These values are 56% and 52% of the Langevin rate constants. In the reaction with allene, the two major product pairs are Si2Cc/C2H4 and Si2C3Ht/ C2H2, which together account for more than 50% of the reaction products. The balance is split among ten other products. Many of the products are similar, differing only in the number of hydrogen atoms they contain. Because of this and the low intensities of many of the products, the 13C-labeling reaction was not studied. With the exception of the Si2C4H,f series of ions, which are of very low intensity, all of the products contain either 2, 3 or 5 carbon atoms. This implies that the products can be formed simply by transfer of one or more hydrogen atoms from the allene to the silicon carbide ion, followed by fission of the reaction complex. None of the carbon-carbon double bonds in allene need be broken. This idea is further supported by the observation mentioned above, i.e. most ions belong to series that differ only in the number of hydrogen atoms. Further indirect

319

evidence for this mechanism can be found by comparing the rate constants of the reactions with the different unsaturated hydrocarbons. The rate constant for product formation increases by a factor of 10 in going from acetylene to ethylene, even though 60% of the ethylene reaction involves double bond cleavage (formation of the 12C13CH2 product). The reactions with allene and propyne are another factor of 2 faster. The results for propyne, discussed below, indicate that the reaction proceeds by single-bond cleavage, suggesting that this also holds for allene. However, a definitive answer would depend on the results of labeling experiments. Compared to the allene reaction, that with propyne is relatively simple. Only five products are initially formed, and they all differ by more than the number of hydrogen atoms, in sharp contrast to the allene reaction. Two other products, Si2C3H+ and Si2C3Ht, which increase in intensity with reaction time, can be either primary or secondary products [24]. The product distribution from the reaction of the 13C-labeled ions indicates that several ions are formed by more than one mechanism. Product ions containing zero or three 12C atoms can originate from attack at the acetylenic carbon-hydrogen bond, while those ions containing one or two 12C atoms can arise from attack at the carbon-carbon single bond [25]. These two sites of attack are equally favored, with each yielding about 50% of the products. However, the transfer of one or more hydrogen atoms must be assumed in the case of Si13C12C2Hz and the Si2C3Hl products. Up to 3% of the propyne reaction may be carbon exchange to form Sii3C12C+. It is difficult to determine the exact yield since the Sii2C2H+ product interferes with its detection. This places an upper limit of 1.8 x lo-” cm3 s-l on the rate of carbon exchange compared to (5-6) x lo-” cm3 s-l for acetylene and ethylene.

19% Si&H: 12% Si2C4Ht 24% Si,C,H: + 22% SisCsH 23% SiCsH: 5.9 &0.7(-lo),

60% SiCsHt 25% SiCaH+ 15% SiCH: 1.2(-9), SIFT [32]

C,H; C,H: CsH; C2H; CsH: 0.5(-g),

30% 30% 20% 10% 10% 1.9 f

H?C-CCH

Si4CzH: 1.2(-l l), SIDT [39]

Si&sHz Si&H: Si&H$ SirC: Si&H+ SiCsH: CsH: 0.1 (-lo),

SiaC2H: 2.9 f O.l(-lo),

FTMS,

FTMS,

FTMS,

FTMS

this work

this work

this work

[35]

The entries indicate the products and their branching ratios, with rate constants given as a(b) = a x 10mb cm3 s-‘. The order of multiple sets of branching ratios corresponds to that of the references. Experimental codes are as follows: FA, flowing afterglow; FTMS, Fourier transform mass spectrometry; ICR, ion cyclotron resonance mass spectrometry; SIDT, selected ion drift tube; SIFT, selected ion flow tube; TMS, tandem mass spectrometry. FTMS and ICR are low pressure techniques while the others are high pressure techniques.

SIFT [28]

SIFT [28]

7% 8% 21% 31% 12% 6% 13% 6.5 f

70% SiCsH: 20% SiCsH+ 10% SiCH: 1.2(-9), SIFT [32]

C4H: C,H: CsH; C2H; 0.3(-9),

[34]

40% 15% 25% 20% 1.4 f

S1aC4H: + Si2C4H Si&H: + Si,CsH &0.5(-ll),

H2C=C=CH2

39% CsH: 9% CsH+ 17% C4H; 8% CsH: 27% C2H4+ 1.4(-9), FTMS

16% 3% 74% 7% 2.9

0, 60% SiC*H: 100, 40% SiC*H: 7.4 f 2.0(-ll), TMS [38] 5.6(-lo), SIFT [32]

[31]

8&O, 0% C3H: 0, 30, 60% C3H: 0, 40, 20% C,H: 0, 0, 5% CsH+ 15, 15, 10% CaH: 0, 15, 5% C,H; 1.62(-g), SIFT [36] 1.3 f 0.3(-9), SIFT [28] 1.7(-9), SIFT [37]

(24

No reaction <2(-ll), FTMS

100, 100,76% C6H’ 0, 0, 24% C,H; 1.5(-g), ICR [29] 1.4(-9), SIFT [33] 1.7(-g), FTMS [34]

Si$l

0, 0, 30% SiQH: 100, 100, 70% SiCaH’ 1.8 f 0.7(-lo), TMS [30] 3 f 2(-lo), FTMS [31] 3.5(-lo), SIFT [32]

Si$

CsH+ 2.7(-9), FA [27] 2.2 f 0.7(-9), SIFT [28] 2.8(-9) ICR [29]

molecules

WI

neutral C4’

with various

st+

of Cc, Si+, C:, Si: and Si&

C+

of the reactions

Neutral

Table 4 Comparison

D.C. Parent/International Journal of Mass Spectrometry and Ion Processes 138 (1994) 307-324

TriJEuoropropyne. Trifluoropropyne was used as a reactant for possible insight into the mechanism of the propyne reaction. The reaction with trifluoropropyne is also about 50% efficient, with a rate constant of 4.3f 0.1 x lo-” cm3 s-l. However, the presence of the fluorine atoms completely changes the chemistry of the system. Formation of the silicon-fluorine species is the dominant motif in this reaction. Most of the product channels do not have analogs in the propyne reaction. Also each product is formed with only one isotopic carbon configuration in the labeling study, in contrast to the propyne reaction for which different mechanisms lead to the same product.

3.2.3. Comparison of the reactivity of Si2Ct and pure cluster and atomic ions

Reactions of S&Cc with hydrocarbons generally lead to incorporation of carbon and hydrogen into the parent ion (new bond formation), as has been seen in the reactions of C+, C,’ and Si’. Comparison of the reactions of the pure carbon and pure silicon species (see Table 4) shows that C+ and Cz react more rapidly than Si+ and Siz, as well as producing different products. For the atomic species in particular, C+ produces a greater variety of products, including elimination of HZ, C and CH, which do not occur in the reactions of Si+. With Si+, H and methyl elimination are preferred. The reactions of Si&l appear to be intermediate between those of C+ and Cc on the one hand and Si+ and Si$ on the other. Adduct formation and elimination of H and methyl are analogous to the reactions of the silicon species while the total rate constants (with acetylene) follow the order Cz > Sic; > S&C: > Siz. Thus the incorporation of carbon into the silicon cluster increases the reactivity, and the silicon exerts its influence in determining to a large extent the identity of the products formed.

321

4. Summary (1) A variety of experimental results suggest that Si2Ci is formed in an excited state at least 0.8eV above the ground state. This excited state is less reactive than the ground state and may be a different isomer than the ground state, i.e. a linear Si-C-C-S1 configuration versus a rhombus of alternating Si and C atoms. (2) The ionization potential of Si2C2 was found to be 8.24 k 0.20eV, in good agreement with the only other determination of this value. (3) The reactions of S&Cl with methyl-substituted benzenes proceed via adduct formation, with subsequent loss of methyl. This is similar to the reaction of Si+ with unsubstituted aromatic neutrals where adducts were observed, and indicates that strong adduct bonds are formed. (4) The reactions of Si& with nitro- and halide-substituted benzenes, as well as trifluoropropyne, indicate that formation of silicon oxides and silicon halides is the driving force behind these reactions. (5) The products of the reactions of Si& with unsaturated hydrocarbons depend on the degree of unsaturation. With acetylene, triple bond cleavage is observed but is very slow. Double-bond cleavage in ethylene is five times as efficient, while single-bond cleavage in allene or propyne is three times as efficient again. Labeling studies indicate that 60% of the products arise from double-bond cleavage in ethylene, while in propyne only single-bond cleavage is observed. Allene was not studied with labeled reactants, but the results are consistent with single-bond cleavage.

5. Note added in proof The excess energy in the initially formed S&C; ions relative to the relaxed S&Cl ions

322

D.C. Parent/International Journal of Mass Spectrometry and Ion Processes 138 (1994) 307-324

was found to be at least 0.8 eV. This assumes that the corresponding neutrals lie at the same energy (as is the case for the linear triplet and the singlet rhombus states). This is the most plausible scenario, as explained in the text. The excess energy can be larger if the state of the neutral corresponding to the initially formed ions is higher in energy (e.g. the linear singlet state of the neutral). The excess energy could also conceivably be less if the corresponding neutral state of the excited ion is lower in energy. This would be the case if the relaxed Si2Ci ions do not correlate to either the linear triplet or singlet rhombus states, but to an excited neutral state.

Acknowledgments This work was carried out at the Naval Research Laboratory with the support of the Office of Naval Research. The author would like to thank G. Parlant (Orsay) for his assistance with the derivation of Eq. (4) and SW. Buckley (University of Arizona) for useful discussions.

Appendix

To solve Eq. (A2), let [A,] be defined as the following arbitrary function of time: [A21 =

MO

+.Y)

W-W

W)

from which d[A2]/dt = {dx/dt - K(x(t) + y)} x exp(-Kt)

(A4)

Substituting Eqs. (A3) and (A4), along with the solution for [At], into Eq. (A2) gives dx/dt = (K - R2)x(t) + (@%]o -

ROY + KY)

W)

In order to be able to integrate this function the second term on the right-hand side is set to zero, such that y = D[AI10/(R2 - K). The integration of Eq. (A5) gives x(t) = x0 exp{ (K - R2)t} (with x0 being the value of x(t) at time zero). Substituting the solutions for x(t) and y into Eq. (A3) we obtain the following expression for [A21: L421 =

-@oexpKK - R2)tl + Db%10/(& - K)) ew(-Kt)

At

time

646)

zero

we obtain x0 = [A210and along with [Atlo = V%41Io/(R2 - K)h 1 - [A210,this leads to L421 = [A210 exp(-R20

Derivation of Eq. (4)

+ -Ml

From Eq. (3), the dependence of the concentrations of the At and A2 species can be written as d[A,]/dt = -(Ri + D)[A,] = -K[A,] (K=Ri

+D)

(Al)

and d[AJldt

-

R2L421

(AZ)

The solution of Eq. (Al) is trivial and gives hl = L%loexp(-W where [At], is the concentration of At at time zero.

L4210W2

-

QHew(-W

exp(-R2#

647)

The A species “concentration” terms are now actually fractions relative to the total initial concentration of A. Since the mass spectrometer does not differentiate between At and A2, the measured quantity is

L%l+ L421 = D]AJ -

-

=

(1 -

+

B210)

exp(-Kl)

[A210 ewW2t)

+ {W

-

MOM& - KU

x {exp(-Kt) - exp(-R2t)}(AS)

D.C. Parent/International Journal of Mass Spectrometry and Ion Processes 138 (1994) 307-324

Eq. (A8) can be rearranged to give Eq. (4) of the text. The four parameters are the fraction of the initial population of A that is in AZ, and the three (rate constants x neutral concentration) products R1 (included in K), D and R2.

323

includes the carbon-scrambling process, which is not observed with unlabeled reactants. It is not possible to provide.a value for k,, since this parameter was usually negative; however, it is necessarily much smaller than either kd or kr2.

Fits of experimental data with Eq. (4) The experimental data for the reaction of Si2Cz with 13C2H2, with or without an additional buffer gas, were fitted with Eq. (4), as shown in Figs. 1 and 3. However, the results are less than satisfactory since very large errors are associated with the values found for the parameters during the fit. In addition, nonphysical (i.e. negative) values were also obtained for the parameters [A& and/or D. This is due in part to the data itself, which are not extensive enough for these types of kinetics fits. Unfortunately, a single data set having a sufficient number of points at both short and long reaction times is unavailable, since the data were not taken with this type of analysis in mind. Another factor that is adversely affecting the ability to obtain meaningful fits is the term (R2 - R1 - II), which appears in the denominator of Eq. (4). For this particular system, this quantity is apparently very small and close to zero, and thus the expression [Ai + A21 is very sensitive to small changes in this quantity. The negative values for [A& were interpreted as meaning that the initial fraction of A2 is very small or zero. Thus all fits were actually obtained with this parameter set to zero. Speaking very generally, the global values for kd and kr2 are in the low lo-” cm3 s-l range. This value for kd is similar to that deduced [l] for the carbonscrambling process. However, since the data do not extend to long reaction times, the value for kr2 is rather uncertain. Note that this rate constant is not the same as that previously reported [I] for the reaction of Si&’ with acetylene, since this value for kr2

References and notes VI D.C. Parent, Int. J. Mass Spectrom. Ion Processes, 116 (1992) 257. DC. Parent and SW. McElvany, J. Am. Chem. Sot., 111 (1989) 2393. 131 Buckner and co-workers have observed similar kinetic plots from non-thennalized atomic metal ions formed in a mixture of electronic states: R.M. Pope, S.L. VanOrden, B.T. Cooper and S.W. Buckner, Organometallics, 11 (1992) 2001; personal communication, 1992. 141 (a) J.E. Bartmess and R.M. Georgiadis, Vacuum, 33 (1983) 149 and references cited therein. (b) F. Nakao, Vacuum, 25 (1975) 431. [51 J. Drowart, G. De Maria and M.G. Inghram, J. Chem. Phys., 29 (1958) 1015. 161 The use of labeled acetylene allows the reaction to be observed at a lower extent of reaction, owing to the carbon exchange channel. Since this reaction channel involves an intimate collision, it is expected to be very sensitive to the initial energy content of the Si&. The curvature in the kinetics plot of Fig. 3 supports this assumption. normal Langevin formulation, kL =2.34x [71 The 10e9 cm3 s-l (a/p) I’* where o is the neutral polarizability and p is the reduced mass, was used to calculate the collision frequency and thus the number of collisions experienced by the ion during a given time. t81 (a) B.M. Toselli, J.D. Brenner, M.L. Yerram, W.E. Chin, K.D. King and J.R. Barker, J. Chem. Phys., 95 (1991) 176. (b) I.J. Wysong, J.B. Jeffries and D.R. Crosley, J. Chem. Phys., 91 (1989) 5343. (c) H. Helvajian, J.S. Holloway and J.B. Koffend, J. Chem. Phys., 89 (1988) 4450. (d) R.A. Morris, A.A. Viggiano, F. Dale and J.F. Paulson, J. Chem. Phys., 88 (1988) 4772. (e) R.F. Heidner III, H. Helvajian, J.S. Holloway and J.B. Koffend, J. Phys. Chem., 93 (1989) 7813. (f) R.A. Copeland, M.L. Wise and D.R. Crosley, J. Phys. Chem., 92 (1988) 5710. 191 G. Herzberg, Molecular Spectra and Molecular Structure, Van Nostrand Reinhold, New York, 1966, Vol. III. HOI (a) G.B. Fitzgerald and R.J. Bartlett, Int. J. Quantum Chem., 38 (1990) 121. (b) K. Lammertsma and O.F. Gtiner, J. Am. Chem. Sot., 110 (1988) 5239. 1111 R.G. Shortridge and M.C. Lin, J. Chem. Phys., 64 (1976) 4076. This reference provides one example, the quenching

PI

324

D.C. Parent/International Journal of Mass Spectrometry and Ion Processes 138 (1994) 307-324

of O(‘Dz) by CO through complex formation as shown by isotopic labeling of the oxygen. [12] M. Moini, J. Am. Sot. Mass Spectrom., 3 (1992) 631. [13] (a) S.B.H. Bach and J.R. Eyler, J. Chem. Phys., 92 (1990) 368. (b) J.A. Zimmerman, J.R. Eyler, S.B.H. Bach and SW. McElvany, J. Chem. Phys., 94 (1991) 3556. (c) J.A. Zimmerman, S.B.H. Bach, C.H. Watson and J.R. Eyler, J. Phys. Chem., 95 (1991) 98. (d) S.B.H. Bach and S.W. McElvany, J. Phys. Chem., 95 (1991) 9091. (e) M.A. Cheeseman and J.R. Eyler, J. Phys. Chem., 96 (1992) 1082. [ 141 Conflicting results were occasionally obtained, presumably because insufficient time had been allowed to relax the SizC: ions completely. [15] Lammertsma and Giiner [10(b)] have presented the molecular orbitals for the ground state ‘As rhombus and the first excited singlet state, the ‘A’ rhomboid, of SizCz. For both states, the HOMO is essentially a non-bonding orbital; thus ionization would not greatly affect the geometry. [16] We are assuming here that both excited and relaxed linear configurations are Si-C-C-Si since calculations [10(b)] find linear structures with interior Si atoms to lie much higher in energy. [17] By analogy to the neutral S&C, whose geometry has been calculated by (a) R.S. Grev and H.F. Schaefer, J. Chem. Phys., 82 (1985) 4126. (b) G.H.F. Diercksen, N.E. Grtiner, J. Oddershede and J.R. Sabin, Chem. Phys. Lett., 117 (1985) 29. [18] (a) D.K. Bohme and S. Wlodek, Astrophys. J., 342 (1989) L91. (b) D.K. Bohme, Int. J. Mass Spectrom. Ion Processes, 100 (1990) 719. (c) D.K. Bohme, S. Wlodek and H. Wincel, J. Am. Chem. Sot., 113 (1991) 6396. [19] These values assume that the .(ChHd)CHr radical rearranges to the (CsHS)CH,. radical. The ionization potential of this radical is less than that of Si2C2 so that charge transfer stabilizes the adduct. [20] (a) M.L. Mandich, V.E. Bondybey and W.D. Reents, Jr., J. Chem. Phys., 86 (1987) 4245. (b) S. Wlodek and D.K. Bohme, J. Chem. Sot., Faraday Trans. 2, 85 (1989) 1643. The reaction of SiO+ with NO1 yields mostly NO+ and presumably SiOz. Higher oxides of Si+ (SiOi, n = 2-4) may be produced in the reaction of SiOf with NzO.

[21] The primary ion also adds C2H3 and C2H4, but these products are only l-2% of the total reaction and so are neglected. The secondary ion also adds C2H4 but again it is a negligible amount of the total reaction. [22] Carbon exchange of the Si&Hl products in competition with Eq. (7b) is probably not a factor. The rate constant of the reaction in Eq. (7b), though not measured, appears to be relatively fast. Also the cross-over (equal intensities) in the branching ratios of m/z 83 and 110 occurs at the same reaction time as the cross-over for m/z 84 and 111, which also supports the notion that there is a direct correspondence between the primary and secondary product branching ratios. [23] E.L. Kerley and D.H. Russell, J. Am. Chem. Sot., 112 (1990) 5959. [24] Since the goal of this work was not to map out all of the reaction pathways, the double resonance experiments were not performed. [25] Attack on the carbon-carbon triple bond could also yield these products, but is surely energetically much less favorable. [26] A.A. Marylott and F. Buckley, Nat. Bur. Stand. (U.S.) Circ., 537 (1953) 1. [27] HI. Schiff and D.K. Bohme, Astrophys. J., 232 (1979) 740. [28] D.K. Bohme, A.B. Rakshit and HI. Schiff, Chem. Phys. Lett., 93 (1982) 592. [29] V.G. Anicich, W.T. Huntress, Jr., and M.J. McEwan, J. Phys. Chem., 90 (1986) 2446. [30] T.M. Mayer and F.W. Lampe, J. Phys. Chem., 78 (1974) 2645. [31] W.R. Creasy, A. O’Keefe and J.R. McDonald, J. Phys. Chem., 91 (1987) 2848. [32] S. Wlodek, A. Fox and D.K. Bohme, J. Am. Chem. Sot., 113 (1991) 4461. [33] J.S. Knight, C.G. Freeman, M.J. McEwan, V.G. Anicich and W.T. Huntress, Jr., J. Phys. Chem., 91 (1987) 3898. [34] S.W. McElvany, J. Chem. Phys., 89 (1988) 2063. [35] D.C. Parent, Int. J. Mass Spectrom. Ion Processes, 116 (1992) 257. [36] A.B. Rakshit, Z. Naturforsch., Teil A, 35 (1980) 1218. [37] E. Herbst, N.G. Adams and D. Smith, Astrophys. J., 269 (1983) 329. [38] T.M. Mayer and F.W. Lampe, J. Phys. Chem., 78 (1974) 2433. [39] M.F. Jarrold, J.E. Bower and K. Creegan, J. Chem. Phys., 90 (1989) 3615.