1596
J . Phys. Chem. 1989, 93, 1596-1604
TABLE V: Thermodynamic Properties' of the Dimerization of Liquid 1,3-Cyclopentadiene at 298.15 K
method
AH"
AS"
AGO
(eq 9)
gas-phase and -15.Ib -27.5b -6.9b vapor pressure data Benford and gas-phase and -14.6b -21.8b -8.1b WassermannC vapor pressure data (eq 10) Wassermann and paraffin solvent -16.6b (-23.8)' (-9.5)' co-workers" endo-dicyclo-19.3b -32.5b -9.6b pentadiene solvent Turnbull and endo-dicyclo-18.5g (-25.7)* (-10.8)h Hull' pentadiene solvent MOLBD3
"Standard states are pure liquid monomer and dimer under 1 bar pressure; AH" and AGO, kcal/mol; So, cal/(mol.K). b A H o = R p { d In KJdTJ; ASo = (AH" - A G " ) / T ; AGO = -RT In K,. 'References 4, 16, 17, andd 18. dReference 21. 'Reference 4. /Estimated; see text. g Determined calorimetrically; ref 4. Estimated; ref 4. dp/dT is -1.0 g/(L.K). These values resulted in AHo = -16.6 kcal/mol and AS" = -23.8 cal/(mol.K) for paraffin solvent and a traditional Raoult's law standard state. AHo, ASo,and AGO from eq 9 and 10 from Turnbull and Hull4 and from Wassermann and co-workers22 are given in Table V. The AH" and AS" properties calculated by MOLBD3 plus experimental vapor pressures lie between the extremes of the direct experimental values, which differ by as much as 4.7 kcal/mol in AHo values and 10.7 cal/(mol.K) in AS". Hence, the calculated (25) Weast, Robert C. Handbook of Physics and Chemistry; CRC Press:
Boca Raton, FL, 1986.
thermodynamic values for the cyclopentadiene dimerization in solution appear to be at least as reliable as those based on experiment. Conclusions The MOLBD3 force-field program of Boyd et aL2 modified with diene parameters suggested by Anet and Yavari26and parameters developed in this investigation6 provided a satisfyingly accurate alternative to laboratory methods for determining thermodynamic properties for the dimerization of 1,3-~yclopentadieneover a range of temperature in the gas phase. Condensed-phase thermodynamic properties were obtained by coupling the gas-phase calculations with vapor pressure data. General applicability of the force-field method for calculating thermodynamic properties of reactions depends on the availability of parameters suitable for reactants and products. At present, the MOLBD3 program has parameters suitable for many hydrocarbons; special features such as internal It is rotation or pseudorotation must also be treated the contention of the authors that applicability of force-field methods having the capability of calculating vibrational frequencies (and therefore total entropies) as well as standard enthalpies of formation and optimized geometries will continue to widen as further investigation of new systems leads to the introduction of new parameters.
Acknowledgment. This work was supported by the Chemical Sciences Division, Office of Basic Energy Sciences, Office of Energy Research, of the U S . Department of Energy under Grant DE-FG02-86ER13582. We thank Professor Richard H. Boyd for providing the MOLBD3 program employed in our studies. Registry No. 1,3-CycIopentadiene,542-92-7;endo-dicyclopentadiene, 1755-01-7;exo-dicyclopentadiene, 933-60-8. (26) (a) Anet, F. L.; Yavari, I. Tetrahedron 1978,34,2879. (b) Anet, F. L.; Yavari, 1. J . Am. Chem. SOC.1978, 100, 7814.
Energetics and Nlechanlsms in the Reaction of Si' with SiCI,. Thermochemistry of SiCI, SiCI', and SiCI,+ M. E. Weber Department of Chemistry, University of California, Berkeley, California 94720
and P. B. Armentrout*Vt Department of Chemistry, University of Utah, Salt Lake City, Utah 84112 (Received: October 14, 1988)
The title reaction is studied by guided ion beam mass spectrometry. Absolute reaction cross sections are measured as a function of kinetic energy from thermal to 20 eV. Reaction occurs on virtually every collision at low energies. Production of SiCP + SiCI, is exothermic and is the dominant process. SiC13++ Sic1 and SiC12++ SiC12production are both seen to be slightly endothermic. Also observed are the dissociative channels Sic]+ + C1+ SiC12and SKI2++ CI + SiCI. No Si2CI,+ species are observed. Isotopic labeling studies indicate that SiCl' and SiC13+are produced by direct, coupled mechanisms, while SiC12+is formed through an intimate collision involving a symmetric intermediate. These mechanisms are interpreted in terms of the molecular orbital correlations. The cross section behavior and proposed mechanisms are consistent with those of a previous study of the Si+ + SiF4 reaction. Reaction thresholds are analyzed to derive the following thermochemical = 217 4 7 kcal/mol, IP(SiCI) = 7.44& 0.40eV, and AHrB8(SiCI2+) values: AHHf.02ss(SiC1)= 44 j= 6 kcal/mol, AHHf.0298(SiCI+) = 188 f 3 and 190 f 6 kcal/mol. These values and those in the literature are compared and evaluated.
Introduction the fabrication of microelectronic devices, chlorosilanes are used extensively in chemical vapor deposition (CVD) and plasma-enhanced CVD systems to deposit silicon layers.'-4 I n addition, chlorine-based plasmas that form silicon chloride ions and NSF Presidential Young Investigator, 1984-1989; Alfred P. Sloan Fellow; Camille and Henry Dreyfus Teacher-Scholar, 1988-1993.
0022-3654/89/2093-1596$01 S O / O
radicals are often used to etch such The study of the reactor composition, the decomposition of the starting material, ( I ) Inspektor-Koren, A. Surf. Coat. Technol. 1987, 33, 31. (2) Sherman, A. Chemical Vapor Deposition for Microelectronics; Noyes Publications: Park Ridee. NJ. 987, (3) Ban, V. J . Electrochem: Soc. 1975, 122, 1389. (4) Grossman, E.; Avni, R.; Grill, A. Thin Solid Films 1982, 90, 237. (5) Mucha, J. A,; Hess, D. W. ACS Symp. Ser. 1983, No. 219, 215.
,
0 1989 American Chemical Society
The Journal of Physical Chemistry, Vol, 93, No. 4, 1989 1597
Thermochemistry of SiC1, SiCl', and SiC12+ and the deposition and etching processes has been the focus of recent work.*-lo A detailed understanding of the chemical mechanisms involved can provide insight into the most important physical parameters of a plasma or CVD reactor. Thus, the investigation of the reactivity,"-'4 structure,l5-I9 and thermo~ h e m i s t r y ~of* ~silicon ~ chloride species has been an active area of interest. The role of gas-phase ion-molecule reactions in the polymerization process has been investigated. One investigation indicates that, in an SiC14-basedplasma, polymerization occurs exclusively at the surface.24 Yet in another study, Si,Cly ( x = 2-5) species are observed in the gas phase.25 Here, Avni et al. suggest that the SiC14/Ar plasma polymerization proceeds primarily through gas-phase ion-molecule reactions. The proposed mechanism is initiated by reactions that produce SiC14+ and SiC13+. These include electron impact ionization of SiC14,reactions with Ar+, and Penning ionization with metastable states of Ar. Propagation and polymerization are through reactions of these ions with SiCl, neutrals, as in reaction 1. In the same study, Avni et al. show
SIC4
SiC13+
+
Si2C15+ C12
-SiC14
Si,Cly+
(1)
that the addition of H2 to the plasma increases the decomposition rate of the SiCl., by a factor of 40.25 They provide evidence that this decomposition proceeds via reactions with hydrogen radicals rather than ionization. An earlier investigation by Avni et al. shows that increasing the H2 concentration decreases the deposition rate.26 This implies that polymerization via ion-neutral reactions proceeds much faster than via neutral-neutral reactions. This also suggests that polymerization, rather than decomposition of SiC14, is the rate-limiting step in the deposition of silicon layers. Previously in our laboratories, the interactions of Si+ with SiFz7 and SiH428were detailed. These studies measured absolute reaction cross sections for all observed product channels, including SiF,+, SiH,+, and Si2H,+ production, and evaluated the mechanisms and competition between these channels. Thermochemistry for both neutral and ionic species was systematically derived and compared to literature values. Recommended values reduced the uncertainty from as much as 20 kcal/mol to generally 1-3 kcal/mol. The present work is an extension of these studies and measures the reaction cross sections of the title reaction from (6) Schwartz, G. C.; Schaible, P. M. J . Vac. Sci. Technol. 1979,16,410. (7) Smith, D. L.; Bruce, R. H. Proc.-Electrochem. SOC.1982, 82, 462 (Proc. Symp. Plasma Process., 3rd, 1981). (8) Wormhoudt, J.; Stanton, A. C.; Richards, A. D.; Sawin, H. H. J . Appl. Phys. 1987,151, 142. (9) Bloem, J.; Claassen, W. A. P.; Valkenburg, W. G. J. N . J . Crysf. Growth 1982, 57, 177. (10) van der Putte, P.; Giling, L. J.; Boem, J. Cryst. Res. Technol. 1982, 17, 1535. (1 1) Safarik, I.; Ruzsicska, B. P.; Jodhan, A.; Strausz, 0.P.; Bell, T. N . Chem. Phys. Letf. 1985, 113, 71. (12) Stanton, A. C.; Freedman, A.; Wormhoudt, J.; Gaspar, P. P. Chem. Phys. Lett. 1985, 122, 190. (1 3) Pabst, R. E.; Margrave, J. L.; Franklin, J. L. Int. J . Mass Spectrom. Ion Phys. 1977, 25, 361. (14) Olsen, A,; Sale, F. R. J . Less-Common Met. 1977, 53, 277. (15) Tsuji, M.; Mizuguchi, T.; Nishimura, Y. Can. J . Phys. 1981,59,985. (16) Miller, J. H.; Andrews, L. J . Mol. Struct. 1981, 77, 65. (17) Hudson, A,; Jackson, R. A,; Rhodes, C. J.; Del Vecchio, A. L. J . Organomet. Chem. 1985, 280, 173. (18) Gosavi, R. K.; Strausz, 0. P. Chem. Phys. Lett. 1986, 123, 65. (19) Moc, J.; Latajka, Z.; Ratajczak, H. Chem. Phys. Lett. 1987,136,122. (20) Walsh, R. J . Chem. SOC.,Faraday Trans. 1 1983, 79, 2233. (21) Ho, P.; Coltrin, M. E.; Binkley, J. S.; Melius, C. F. J . Phys. Chem. 1985,89,4647. (22) Steele, W. C.; Nichols, L. D.; Stone, F. G. A. J . Am. Chem. SOC. 1962, 84, 4441. (23) Ihle, H. R.; Wu, C. H.; Miletic, M.; Zmbov, K. F. Adu. Mass Spectrom. 1978, 7A. 670. (24) Bruno, G.; Capezzuto, P.; Cicala, G.; Cramarossa, F. Plasma Chem. Plasma Process. 1986, 6, 109. (25) Manory, R.; Grill, A.; Carmi, U.; Avni, R. Plasma Chem. Plasma Process. 1983, 3, 235. (26) Grossman, E.; Avni, R.; Grill, A. Thin Solid Films 1982, 90, 237. (27) Weber, M. E.; Armentrout, P. B. J . Chem. Phys. 1988, 88, 6898. (28) Boo, B. H.; Armentrout, P. B. J . Am. Chem. SOC.1987, 109, 3549.
thermal energy to 20 eV. The probability of forming polysilicon chloride species via gas-phase ion-molecule reactions is examined. Also, thermochemistry of SiCl,+ and SiCl, species is derived and compared to literature values.
Experiment General. The ion beam apparatus and experimental techniques used in this work are described in detail elsewhere.29 Silicon ions are produced as described below. 28Si+or 30Si+ions are mass analyzed and decelerated to the desired translational energy. The ion beam is injected into an rf octopole ion beam which passes through the reaction cell containing the SiC14 reactant gas. The pressure of SiC14, semiconductor grade, is maintained sufficiently low, 0.025-0.25 mTorr, so that multiple ion-molecule collisions are improbable. The unreacted Si+ and product ions drift out of the gas chamber to the end of the octopole, where they are extracted and analyzed in a quadrupole mass filter. Finally, ions are detected by a secondary electron scintillation ion counter using standard pulse counting techniques. Raw ion intensities are converted to absolute reaction cross sections as described previ0us1y.~~ Only Si35C1,+ products are explicitly detected, so that the absolute magnitudes are corrected for the 75.77% isotopic abundance of 35Cl. Ion Collection Efficiency. The octopole beam guide utilizes rf electric fields to create a potential well which traps ions in the radial direction without affecting their axial energy.29 One advantage of the beam guide is highly efficient product collection, and absolute cross sections as small as A2 are measured. In general, we estimate that the absolute uncertainty of the cross sections is f 2 0 % and the relative uncertainty is *5%.30 However, in this work, the silicon isotopes in the product ions could not be completely mass resolved due to the complication presented by the chlorine isotopes. Therefore, the absolute magnitudes may be low by up to 8% depending on the reaction mechanism.31 Thus, the absolute uncertainty here is 4~30%. Energy Scale. Laboratory ion energies (lab) are converted to energies in the center-of-mass frame (CM) by using the conversion E(CM) = E(lab)M/(m M), where m is the ion mass and M is the target molecule mass.29 Here, the conversion factors used are 0.857 and 0.849, corresponding to the ?Si+ Si35C14and 3%i+ SP5C14interactions. Unless stated otherwise, all energies quoted in this work correspond to the C M frame. The absolute energy scale and the corresponding full width at half-maximum (fwhm) of the ion kinetic energy distribution are determined by using the octopole beam guide as a retarding potential analyzer.29 An accurate determination is possible since the interaction region and energy analysis region are physically the same. In this work, the uncertainty in the absolute energy scale is f0.04 eV and typical fwhms are 0.54.6 eV (lab). At very low energies, the slower ions in the ion beam energy distribution are not transmitted through the octopole, which results in a narrowing of the ion energy distribution. We take advantage of this effect to access very low interaction energies as described p r e v i o ~ s l y .Energies ~~ in data plots are mean ion energies which take into account this truncation of the ion beam distribution. Ion Source. Silicon ions are produced by surface ionization. In this source, a rhenium filament is resistively heated to -2200 K and is exposed to silane. The silane decomposes on the filament and ionized silicon desorbs. If the Si+ ions equilibrate at the filament temperature, the electronic-state distribution of the ions is M a x ~ e l l i a n . Since ~ ~ the first excited state of Si+ is 5.46 eV above the ground state,33exclusively ground-state Si+(2P)should be produced. Previous studies have verified that no excited ions are produced by this s o ~ r c e Both . ~ ~the ~ ~2Pl/2 ~ (0.0 eV) and 2P3/2 (0.036 eV) spin-orbit states are present, presumably with a
+
+
+
Ervin, K. M.; Armentrout, P. B. J . Chem. Phys. 1985, 83, 166. Teloy, E.; Gerlich, D. Chem. Phys. 1974, 4 , 417. The isotopic abundance of 28Si is 92.23%, 29Siis 4.67%. and 'OSi is 3.10%. (32) Elkind, J. L.; Armentrout, P. B. J . Phys. Chem. 1987, 91, 2037. (33) Moore, C. E. Natl. Stand. ReJ Data Ser. ( U S . , Natl. Bur. Stand.) 1970, 34.
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The Journal of Physical Chemistry, Vol. 93, No. 4, 1989
Weber and Armentrout ENERtY (eV. Lab)
0.38:0.62 Maxwellian population. Thermochemical Analysis. The threshold regions of endothermic reactions are analyzed by using the empirical model in eq 2, where ET is the translational threshold energy, uo is an uT(E) = uO(E - ET)"/Em
101
100
(2)
energy-independent scaling factor, and n and m are variable parameters. This general form has been discussed p r e v i o ~ s l y . ~ ~ * ~ ~ As in earlier studies, we have chosen to restrict the form of eq 2 to m = 1, a form predicted for translationally driven reaction^.'^ Furthermore, with m = 1, eq 2 has been found to be quite useful in describing the shapes of endothermic reaction cross sections and in deriving accurate thermochemistry from the threshold energies for a wide range of systems. These systems include the related reactions with tetraflu~rosilane~~ and silaneB and reactions of atomic transition metals with H2, D2,32and hydrocarbon^.^^ A complication here involves the treatment of the J = 3/2 and J= spin-orbit states of the reactant ion. This is handled by an explicit sum over the contributions of the individual states, weighted by their populations gJ, as shown in eq 3. Here, E 3 / 2 gl/2'0(E - E T ) R + g3/2f10(E+ E 3 / 2 uT(E) = E
ENERGY (eV, W
- ET)n (3)
is the energy of the J = 3 / 2 excited state, 0.036 eV. A statistical population is assumed, so that g1,2= 0.38 and g3/2 = 0.62. Equal reactivity is assumed for each spin-orbit state since the same uo is used. The reaction cross section for an endothermic process may decline at higher energies due to dissociation of the product ion. For such systems, cross sections are analyzed by using a model previously outlined, which makes a simple statistical assumption within the constraints of angular momentum c~nservation.~~ This model yields eq 4, a modified form of eq 2, where PD is the
probability for dissociation of the product ion. PD is a function of both ED, the energy at which dissociation begins, and p , a quantity related to the number of internal modes in the transition state. Thus for E C ED, PD is zero, and for E > ED, PD asymptotically approaches one. Optimized values of ET, uo, and n are obtained by using nonlinear least-squares regression analysis to give the best fit to the data, after convolving over the known ion beam and neutral energy distributions. In most cases, the data are fit with eq 3 from below threshold up to energies where dissociation can begin, ED. Equation 4 is used for endothermic processes where ET is close to ED, since product dissociation can have a significant effect on the shape of the threshold region of the cross section and the optimum ET. Here, ED is iteratively optimized, and the parameter p is treated as a variable but is limited to integer values. In all product channels, the cross section reproducibly becomes constant at energies below -0.2 eV. This behavior occurs because the experimental ion kinetic energy distribution cannot be modeled well by a Gaussian distribution at very low laboratory energies.29 Consequently, energies below 0.2 eV cannot be accessed. For the endothermic cross sections, this low-energy behavior complicates unambiguous determinations of the threshold energies ET. Thus in each case, a plausible value for ET is derived by modeling the cross sections above 0.2 eV with eq 3. An upper limit to ET is also derived by subtracting the nonzero low-energy cross section from the overall cross section, followed by modeling with eq 3. For each threshold determination, three data sets taken at different times and under different experimental conditions are analyzed. The uncertainties in ET reported here are based on the range of (34) Ervin, K. M.; Armentrout, P. B. J . Chem. Phys. 1986, 84, 6738. (35) Aristov, N.; Armentrout, P. B. J . Am. Chem. SOC.1986, 108, 1806. Sunderlin, L.; Aristov, N.; Armentrout, P. B. Zbid. 1987, 109, 78. (36) Chesnavich, W. J.; Bowers, M. T. J . Phys. Chem. 1979, 83, 900. (37) Weber, M. E.; Elkind, J. L.; Armentrout, P. B. J . Chem. Phys. 1986, 84, 1521.
Figure 1. Variation of product cross sections with translational energy in the laboratory frame of reference (upper scale) and the center-of-mass frame (lower scale) for reaction of Si' with SiCI,. The solid line shows the total reaction cross section, and the dotted line shows the lower error limit, -30%, in its magnitude. The dashed line shows the collision cross section given by the LGS model, eq 5 , calculated with a(SiCI,) = 11.27
A'.
threshold energies which reasonably reproduce each data set and include the upper limit.
Results In the reaction of silicon ions with tetrachlorosilane, three product ions are observed: SiC13+,SiC12+,and SiCl+. The reaction cross sections u(E) are shown as a function of relative translational energy in Figure 1. Despite a careful search, the present work found no SiCI4+or Si2C1,+ ions at any energy. This means that AZ. Below the cross section for such species must be less than -0.6 eV, the total experimental cross section declines as (72 f 22)p.5M.1and is due almost exclusively to SiCl+ production. This behavior is in good agreement with the collision cross section predicted by the Langevin-Gioumousis-Stevenson (LGS) model for ion-molecule reactions,38 given in eq 5. Here, E is the inU ~ G S=
~e(2a/E)'/~
(5)
teraction or center-of-mass (CM) energy of the reactants, e is the electron charge, and a is the polarizability of the target molecule SiC1,. The value for a(SiC14) has been measured as 11.27 A339 and can be calculated as 12.0 A3 by using the empirical method of Miller and S a v ~ h i k . ~These ~ , ~ values ~ result in an LGS cross section of (58 f l)E-0.5,which lies within the 30% uncertainty in the experimental magnitudes. Thus, at low energies, Si+reacts extremely efficiently with SiC14. If unit reaction efficiency is assumed, the experimental cross section magnitude suggests a( S i c 4 ) = 18 (+13,-9) A3. SiCl'. The dominant product ion at nearly all energies studied here is SiCI'. The cross section u(SiCl+) increases monotonically with decreasing energy, which indicates an exothermic process. This is consistent with the process Si+ SiC14 SiCl+ + SiC1, (6)
+
-
which is expected to be exothermic, as discussed later. As given . 1 to 2.5 above, the cross section initially behaves as p . 5From eV, u(SiC1') declines more steeply, as E-1.6*o,1, and at high energies, > 9 eV, the cross section again falls off rapidly, 1 14E-',3. (38) Gioumousis, G.; Stevenson, D. P. J . Chem. Phys. 1958, 29, 294. (39) Rothe, E. W.; Bernstein, R. B. J . Chem. Phys. 1959, 31, 1619. (40) Miller, K . J.; Savchik, J . A. J . Am. Chem. SOC.1979, 101, 7206. (41) The calculation is based on the value a(SiH,) = 4.339 A', taken from: Batsonov, S . S. Refractometry and Chemical Structure (translated by P . P. Sutton); Consultants Bureau: New York, 1961.
,
,
The Journal of Physical Chemistry, Vol. 93, No. 4, 1989 1599
Thermochemistry of SiCI, SiCl+, and SiC12+
ao
ENERGY ( e t Lab)
,
2;O
,
4;O
5;O
6;O
,
Si0
,
ENERGY fe V, W Figure 2. Isotopically labeled cross sections for SiCP production as a
function of translational energy. Diamonds denote the experimental cross section for process 7b, from mass 63 product ions. Triangles denote those for process 7a. The upright triangles are derived from the mass 65 product ions, while the inverted triangles arise from the mass 67 product ions (see ref 42). The dashed line shows the exothermic model for production of SiCI+ and bonded *SiC13,6 . 9 I P 5 . The dotted line is the threshold model for formation of SiCP and dissociated *SiCI2+ CI, from eq 3 with n = 1.0, uo = 2.2, and ET = 2.9 eV. The solid line shows the sum of these models. Below -2.5 eV, the solid and dashed lines coincide. But at intermediate energies, from 3 to 9 eV, the cross section function flattens and behaves as 16,5?.4. Information on the mechanism of SiCl+ formation and the origin of the change in behavior at 3 eV is gleaned from cross sections for isotopically labeled reactions, processes 7a and 7b. *Si+
-
+ SiC14
+ [SiCl, + CI]
(7a)
+ [*SiC12 + Cl]
(7b)
*SiCl+ SiCl+
EMffiY (eV, W Figure 3. Comparison of the cross section models to the data (squares)
for SiCI2+production. Energies are given in the laboratory frame of refereme (upper scale) and the center-of-mass frame (lower scale). The dashed line shows the threshold and falloff models for process 9, from eq 3 and 4 with n = 0.8, uo = 4.3, ET = 0.4 e V , p = 2, and EI, = 2.4 eV. The dotted line shows the threshold model for process 10, using eq 3 with n = 1.0, uo = 3.1, and ET = 5.4 eV. The solid line shows the sum of these models. Below -5 eV, the solid and dashed lines coincide. feature is described well by the threshold model of eq 3 with n = 1, a. = 2.2 A2, and ET = 2.9 eV. Both these exothermic and endothermic models are shown in Figure 2. We quote a large uncertainty, 0.3 eV, in the ET value for process 8 due to possible mass overlap and the difficulty in unambiguously deconvoluting the two features. SiC12+. Formation of SiC12+is the least favored process at all energies. The behavior of u(SiC12+) is characteristic of an endothermic reaction, as it rises from an energy threshold. At approximately 2 eV, a(SiC12+)reaches a maximum value of 2.8 A2. This low-energy feature in cr(SiCl,+) is due to process 9.
Here, the brackets indicate either bonded or dissociated SiC12-CI. These cross sections, shown in Figure 2, are obtained by employing 30Si+for the reactant ion beam and collecting product masses 63 amu (28Si35C1+),65 amu (28Si37C1+and 3oSi35C1+),and 67 amu Si+ SiC14 SiC12+ SiC12 (9) (3"Si37C1'). Mass 63 ions are produced exclusively from reaction 7b. Mass 65 and 67 ions are produced from both processes 7a Modeling of the threshold region results in good fits to the data and 7b, and each provides an independent determination of ~ ( 7 a ) . ~ ~up to about 2 eV. Below 0.5 eV, the data exceed these models. The fact that u(7a) derived from the mass 65 product ions is The following optimized parameters of eq 3 are obtained: n = virtually identical with that obtained from the mass 67 product 1.0, uo= 4.0, ET = 0.34 f 0.05 eV and n = 0.8, uo = 4.0, ET = ions (Figure 2) supports the derived cross sections. The cross 0.41 (+0.03,-0.08) eV. To derive an upper limit on ET, we sections in Figure 2 indicate that retention of the charge on the subtract a constant 0.49-A2cross section. Now the data can be reactant ion, reaction 7a, is favored over transfer of the charge, modeled from 0 to 2.0 eV with the following parameters: n = reaction 7b, by nearly an order of magnitude. Thus, reaction 1, uo= 3.6 A,, ET = 0.40 f 0.04 or n = 0.8, uo = 3.5, ET = 0.45 occurs preferentially by simple CI atom transfer, rather than via (+0.02,4.07) eV. Although these treatments of the data are quite rearrangement id a symmetric intermediate. different, the derived threshold energies do not change appreciably. The shapes of u(7a) and a(7b) indicate that the change in slope Our final value for the threshold is the average value ET = 0.4 observed in the overall S i C P cross section at -3 eV is due to a f 0.1 eV. We quote a conservative uncertainty due to the difsecond independent process that becomes energetically accessible ficulty in the modeling below 0.5 eV. at this energy. Specifically, in u(7b), the magnitude of the The SiC12+cross section begins to decline rapidly beyond 2 eV. exothermic feature is small, so that this second feature becomes The falloff behavior is modeled reasonably well up to 5 eV by using pronounced. On the other hand, in u(7a), the exothermic feature eq 4 with p = 2 and ED = 2.4 f 0.1 eV. This value for ED,which is so dominant that this second feature appears only as a slight is dependent upon the threshold model used, compares reasonably change in slope. This second process is most likely reaction 8, well to the threshold energy for the second feature in u(SiCl+), 2.9 eV. Furthermore, the magnitude of the u(SiCl+) endothermic Si+ + SiCI4 SiCI+ + CI + SiC1, (8) feature concurs with the decrease in the u(SiC12+)magnitude, and dissociation of SiCI2+ into SiCI+ and C1. We can determine a the sum of these cross sections is a reasonably smooth function. cross section for process 8 by subtracting the exothermic feature These two facts suggest that process 8 is responsible for both the from u(7b). The accuracy of this derived endothermic cross section decline in u(SiC12+)and the second feature in a(SiC1'). Further, is limited by our ability to establish the true exothermic function. the only other possible dissociative channel, formation of Si+ + If we assume here the function 6.9E1,5,the resulting endothermic CI,, cannot occur until 5.1 eV. At -5 eV, a(SiC1,') again rises rapidly. This second feature is most likely due to process 10, dissociation of SiCI3+into SiCI2+ (42) Mass 63 corresponds to **Si3'C1+[69.9%of u(7b)l. Mass 65 includes
+
-
+
-
28Si'7C1+[22.3%of u(7b)] and 3?Si35Cl+[75.8%of u(7a) and 2.3%of u(7b)l. Mass 67 corresponds to oSi37CI+[24.2%of u(7a) and 0.8% of u(7b)l.
Si+ + SiC14
-
SiC12+ + CI
+ Sic1
(10)
Weber and Armentrout
1600 The Journal of Physical Chemistry, Vol. 93, No. 4 , 1989 ENERGY (eV. Lab)
\
l0OU
' ' ' '
I\
" I '
I
101
100 ENERGY (eK CW
Figure 4. Comparison of the threshold models of eq 3 to the data (circles) for SiC13+production. Energies are given in the laboratory frame of reference (upper scale) and the center-of-mass frame (lower scale). The dashed line shows the threshold fit at low energies, with n = 1.0, uo = 7.8, and ET = 0.13 eV. The fit from 0.6 to 2.4 eV, with n = 0.2, uo = 7.0, and ET = 0.44 eV, is given by the dot-dash line. The dotted line gives the threshold model for the proposed second cross section feature, using n = 1 .O, uo = 2.8, and E T = 2.5 eV. The solid line shows the sum of these latter two models. Below - 2 eV, the dot-dash line and solid line coin-
and E T = 0.44 f 0.1 eV. This latter model (with uo = 7.0 instead) also nicely describes the falloff region of the original data, from 0.6 to 2.4 eV (Figure 4). Although these two models result in quite different cross section functions, the derived threshold energies vary by only 0.3 eV. We quote the average value ET = 0.3 eV and recommend the conservative uncertainty of 0.2 eV to reflect the difficulties in modeling all energies of the threshold region. From 1 to 2.5 eV, the SiC13+cross section declines as 6 . 3 p . 7 . Beyond 3 eV, the u(SiC13+)magnitude is virtually constant at 3.2 A2. This persistence of u(SiC13+) indicates that, at high kinetic energies, only a small fraction of the excess energy is placed in internal modes of the SiC13+product, which would otherwise lead to dissociation. This may arise from the dynamics of process 12. Alternatively, a new process for SiC13+production may become available. A threshold energy for this proposed second process is estimated by subtracting either the threshold model with n = 0.2 and E T = 0.44 eV or 6.3E4,7 from the overall cross section. This leaves an apparent second endothermic feature, which is modeled well with n = 1, uo = 2.4-2.8 A2, and ET = 2.5-2.9 eV, as shown in Figure 4. Isotopic labeling studies" show transfer of the charge from *Si to Si, process 13b, is favored over charge retention, process 13a. The cross section for (1 3b) is greater than a( 13a) by 20-40%. No difference in cross section shape is observed for processes 13a and 13b. *Si+
+ SEI4
-
+
*SiC13+ Sic1
cide.
and C1. The cross section for this process alone is derived by subtracting the cross section model for process 9 given above from u(SiCl2+). The resulting endothermic cross section is then modeled from 4 to 7 eV by using eq 3, and an excellent fit is obtained by using n = 1.0, uo = 3.1 A2, and ET = 5.40 eV. The cross section models for both processes 9 and 10 are compared to the data in Figure 3. Here, too, we report a large uncertainty, 0.2 eV, due to the ambiguity in deconvolution of processes 9 and 10. Information of the mechanisms of SiC12+production is found from the isotopic labeling studies discussed above. Here masses 98 amu (28Si35C12), 100 amu (3%i3sC12and 28Si35C137C1), and 102 amu (30Si35C137C1 and 28Si37C12) were collected, and cross sections for processes 1 1a and 1 1b are derived by correcting for isotopic *Si+
+ SiCI,
-
+ [SiCI + Cl] SiCI2+ + [*Sic1 + Cl] *SiC12+
(1 l a ) (1 1b)
a b ~ n d a n c e s . ~Here, ~ the brackets indicate either bonded or dissociated SiCl-Cl. In u( 1 l a ) and u( 1 lb), the first features, corresponding to reaction 9, have equivalent magnitudes. For the second feature, u( 1 1b) is favored by a factor of 5, which implies transfer of the charge from *Si to Si is favored over retention of the charge in process 10. SiC13+. Production of Sic&+,reaction 12, is the final product
Si+ + SiCI,
-
SiC13+
+ Sic1
(12) channel observed, and it is slightly endothermic. The cross section reaches a maximum value of 6.6 A2 at -0.9 eV. The threshold region of u(SiC13+) is modeled well from 0.2 to 0.8 eV by using eq 3 with n = 1.0, uo = 7.8 A2,and E T = 0.13 f 0.04 eV. While this model accurately describes the data below 0.8 eV (Figure 4), it cannot describe the decline in the cross section at higher energies. Further, there are no dissociative processes that can begin at this energy. However, if a constant cross section of 3.0 A2 is subtracted (to obtain an upper limit of ET), the resulting data are then modeled well from -0 to 1.0 eV with n = 0.2 0.1, uo = 3.9, (43) Mass 98 corresponds to 28Si35C1,[53.0%of u(1 lb)]. Mass 100 includes 3"Si35C1z[57.4%of u( 1 la) and 1.8%of u( 1 1 b)] and 28Si3sC137CI [ 16.9% of u( 1 1 b)], Mass 102 includes 3oSi35C137CI [ 18.4% of u( I la) and 0.6% of ~ ( l l b ) and ] 2sSi37CIz[22.3%of ~ ( l l b ) ] .
Discussion A . Thermochemistry. Although the determination of thermochemistry for the SiCI, and SiCl,+ species has been the focus of much recent work, still there is much uncertainty in most values. Heats of formation for SiCl, radicals come from two recent reviews of the literature thermochemistry by WalshZoand in the JANAF tables.45 Both establish the values AHfo(SiC12) = -40.3 f 0.8 kcal/mol and = -158.4 f 1.3 kcal/mol, but other recommended values conflict by as much as 13 kcal/mol. Thermochemistry of the ions is also available in the literature. In several independent studies,14~22*23,46,47 appearance potentials (APs) for the SiC1,' ions from SiC1, and SiC12 have been measured by using electron impact. Yet they conflict by as much as 43 kcal/mol. Theoretical calculations also provide values for the ionization potentials From the theoretical and experimental values, ion heats of formation can be calculated, and these are given in Table I. Here, we use the convention that the electron is a monatomic gas, which is adopted in the JANAF Thus, values from the literature which use the "stationary electron" convention are increased by 1.48 kcal/mol at 298 K. Heats of formation can be derived from the endothermicities of reactions 8,9, 10, and 12 determined above. The calculation of these values assumes that no energy barriers exist in excess of the true endothermicity of the reaction. This assumption is often quite reasonable for ion-molecule reactions since the long-range ion-induced dipole attraction eliminates small energy barrier^.^' (44) Masses 133, 135, and 137 amu were collected. Mass 133 corresponds to *8Si35C13[40.1%of u( 13b)l. Mass 135 includes 28Si37C13sC12 [ 12.8% of u(13b)l and 3oSi35C13[1.3% of u(13b) and 43.5% of u(13a)l. Mass 137 includes z8Si35C137C1z [4.1%of u(13b)l and 3?Si37C13sC12 [4.3%of u(13b) and 13.9% of u(l3a)l. (45) Chase, M. W., Jr.; Davies, C. A,; Downey, J. R., Jr.; Frurip, D. J.; McDonald, R. A,; Syerud, A. N. JANAF Thermochemical Tables, 3rd ed.; J . Phys. Chem. Re/. Data 1985, 14 (Suppl. 1). (46) Vought, R. H. Phys. Rev. 1947, 71, 93. (47) Andianov, K. A,; Tikhomirov, M. V.; Golubtsov, S.A,; Zubkov, V. I.; Potapov, V. K.; Sorokin, V. V. Dokl. Akad. Nauk SSSR 1970, 194, 1077. (48) Dewar, M . S.; Jie, C. Organometallics 1987, 6, 1486. (49) Hastie, J. W.; Margrave, J. L. J . Phys. Chem. 1969, 73, 1105. (50) Bosser, G.; Bredohl, H.; Dubois, I. J . Mol. Specrrosc. 1984, 106, 72. (51) Talrose, V. L.; Vinogradov, P. S.;Larin, I. K. In Gas Phase Ion Chemistry; Bowers, M . T., Ed.; Academic: New York, 1979; p 305.
The Journal of Physical Chemistry, Vol. 93. No. 4, 1989 1601
Thermochemistry of SiCl, SiCl', and SiC12+ TABLE I: Calculated Literature Values for Ion Heats of Formation
species SiCI' SiCl,
-
process SiCI' + C1 + e
+ 3CI + e
SiCI4
SiCI'
Sic1
SiCI+ + e
SiC12+ SiC12
SICI,
SiCI2++ e
-
SiCI2++ 2C1 + e
-
SiCI,' t CI + e
SiC13+' SiCI4
HSiCI,
-
SiCI,' t H t e
AP or IP of process, eV 12.50 f 0.10' 11.8 f 0.2d 19.20 f 0.10' 19.4 f 0.2d 6.82' 7.36 7.538 11.05 f O.lOc*h 10.2 f 0.2d 10.58 f 0.10' 11.6 f 0.41 11.9 f 0.3k 8.61' 17.64 f 0.1OC 17.7 f 0.2d 19.0 f 0.Q 18.4 f 0.3k 11.50 f O.lOCsh 12.3 f 0.2d 12.48 f 0.02" 11.9 f 0.03bJ"
AHf0.298
Of
TABLE 11: Thermochemical Values Derived and Adopted in This Work
species$b kcal/mol
220.5 f 0.4 204 f 5 198.9 f 2.6 202 f 5 203 215 219 216.0 f 2.4 196 f 5 205.2 f 2.4 229 f 9 235 f 7 159.8 191.9 f 2.6 193 f 5 223 f 14 209 f 7 79.3 f 2.6 98 f 5 101.9 f 1.4 105 f 1.7'
" Ion heats of formation, calculated by using the convention that the electron is a monatomic gas. bCalculated by using the appearance or ionization potential, along with the AHfo298values in Table 11. cReference 23. dReference 14. CFrom a Rydberg analysis, ref 50. JTheoretial value, ref 49. 8Theoretical value, ref 48. "Two values are reported in ref 23. The value from the extrapolated voltage difference method is expected to be more accurate than that from the semilogarithmic plot, and thus it is quoted here. 'From photoionization, ref 47. j From electron impact, ref 47. kReference 46. 'Other appearance potentials for SiCI,' are given in the literature, from Si2CI, and CH,SiCI,. The heats of formation for these neutrals are not well-known, however. Reference 22. In the strictest sense, the heats of formation derived by using this assumption are upper limits to the true values. There is also some ambiguity concerning the temperature of the products at thre~hold.,~We assume here that, except for the kinetic energy of the reactant Si', all reactants and products are characterized by a temperature of 298 K, the nominal temperature of the reactant SiCI4. In the following sections, the thermochemical values derived in this work are compared to the literature values for the SiCI, and SiCl,' species. Table I1 contains all relevant heats of formation adopted and used in this work. SiC1,'. AHfo298(SiC13+)can be derived from four AP measurements from the literature, three from Sic&and one from HSiC1, (Table I). Although one value differs significantly, three are in good agreement. These range from 98 to 105 kcal/mol and average 102 f 3 k ~ a l / m o l . This ~ ~ average value is adopted here for AHf0298(SiC13+). sic/. If AHfo298(sic13+)= 102 f 3 kcal/mol is used in conjunction with the threshold energy for reaction 12, ET = 0.3 f 0.2 eV, AHfo298(SiCI)= 44 f 6 kcal/mol results. This lies well within the uncertainty in the value of 37 f 10 kcal/mol recommended by Walsh. Moreover, it compares well to the value of 47.4 f 1.6 kcal/mol calculated in the JANAF tables45from studies by Farber and SrivastavaS3and with the value 45.3 kcal/mol recommended by Dewar et It also lies within the combined error limits of a recent theoretical value, 37.9 f 3 kcal/mol. We recommend here AHfo298(SiCI)= 44 h 4 kcal/mol, the average of these three literature values and our present value.s2 SKI2+ and SiC12. The threshold energy for process 9, 0.4 f 0.1 eV, results in the value 148 f 2.6 kcal/mol for the sum of AHfo298(sic12+)and AHfo298(sic12). If Afffo298(SiC12) = -40.3 ( 5 2 ) The uncertainty here corresponds to the pooled statistical error. See: Box, G . E. P.; Hunter, W. G.;Hunter, J . S. Staristics for Experimenters; Wiley: New York, 1978; p 319. (53) Farber, M.; Srivastava, R. D.J . Chem. SOC.,Faraday Trans. 1 1977, 73, 1672.
Si SiCl SiCI, SiCI, SiCI, HSiCI, CI
AHfo298 of neutral, kcal/mol
AHfo298 of ion,
107.6 f 1.9b 44 f 4d (44 f 6)e -40.3 f 0.8'~~
297.1 f l.Ob*' 210 f 9d (217 f 7)c 191 f 4d (188 f 3)e (190 f 6)' 102 f 39 115.0 f 1.3'
-78 f 3 4 -158.4 f 1.3b3c3h 119 f 2b,c 28.99 f 0.002b
kcal/mol
IP of neutral, eV 8.15 f 0.09 7.13 f 0.43 (7.44 f 0.40) 9.97 f 0.18 7.74 f 0.18 11.79 f 0.01'
"Calculated by using the convention that the electron is a monatomic gas. bReference45. CReference20. dAverageof the value obtained from the present study and literature values. Uncertainties calculated according to ref 52. eThis work. /Reference 21. #Average of literature values from Table I. Uncertainty calculated according to ref 52. The value from ref 23 is not included. hThe JANAF tables quote an uncertainty of 0.3 kcal/mol, but the more conservative error limits of Walsh are adopted here. 'IP of SiCI4 taken from: Lias, S . G.; Bartmess, J. E.; Liebman, J. F.; Holmes, J. L.; Levin, R. D.; Gary Mallard, W. J. Phys. Chem. Ref. Data 1988, 17 (Suppl. 1). f 0.8 kcal/mol is assumed, then AHfo298(SiC12+)= 188 f 3 kcal/mol results. This coincides with a second value for AHfo298(SiC12+)that can be calculated from the present work. Specifically, the threshold energy for reaction 10, 5.4 f 0.2 eV, and the value AHfo298(SiC1)= 44 f 4 kcal/mol, as given above, yield AHfo298(SiC12+) = 190 f 6 kcal/mol. Derived heats of formation for SiC12+from measured ionization potentials of SiC1, range from 196 to 235 kcal/mol, while one calculated I P yields 160 kcal/mol. These values for AHfo298(Sicl2+) depend on ~ i H f ' ~ ~ ~ ( S i c 1Alternatively, 2). values for AHfoZs8(SiCl2+)which are independent of the neutral heat of formation can be derived from the AP of SiCI2+from SiCl+ These range from 192 to 223 kcal/mol. The lowest values, 17.64 f 0.10 eV23and 17.7 f 0.2 eV,I4 are likely to be most accurate, and these yield values for AHfo298(SiC12+) of 191.9 f 2.6 and 193 f 5 kcal/mol, respectively. The average of these values and our values = 191 f of 188 f 3 and 190 f 6 kcal/mol is Akffo298(sic12+) 4 k c a l / m ~ l .Using ~ ~ this value, along with the value AHHrOB8(SiC12) = -40.3 f 0.8 kcal/mol, yields a value for IP(SiCI2) of 9.97 f 0.18 eV. This is in excellent agreement with the value of Olsen and Sale14 (Table I). SiCP. The threshold energy for process 8, 2.9 f 0.3 eV, together with the literature value for AHfo298(SiC12)= -40.3 kcal/mol, yields AHfo298(SiCI+)= 217 f 7 kcal/mol. From this, the value IP(SiC1) = 7.44 f 0.40 eV is derived from the present work (using our value for AHfo298(SiCl)of 44 f 6 kcal/mol given above). AH;298(SiC1+) = 217 f 7 kcal/mol is in good agreement = 220.5 f 2.4 kcal/mol, derived from the with AHfo298(SiC1+) AP of SiCI' from SiC12measured by Ihle et aLZ3This also concurs with two values, 21549and 21948kcal/mol (Table I), calculated from theoretical values for IP(SiCl), assuming AHfo298(SiC1)= 44 f 4 kcal/mol. Four additional values are calculated from the literature, which center around 202 f 5 kcal/mol (Table I). Our value lies just outside the combined uncertainties. There is no apparent reason to discount these values, and in Table 11, we adopt the average of our present value and all those given in Table I, 210 f 9 kcal/mol. SiC13. For AHfo298(SiC13), JANAF recommends -93.3 f 4 kcal/m01.~~ Yet Walsh20 presents a thorough evaluation of the literature values and recommends -80 f 2 kcal/mol, and Ho, Binkley, and co-workers calculate -76.5 h 3 kcal/mol. No definitive information on AHfoz98(sic13)is available from the present experiments, since reaction 6 is exothermic. This observation merely sets an upper limit of AHfo298(sic13)5 -69 & 7 kcal/mol. B. Comparison to Si+ + SiF4. The cross section functions for the Si+ + SiCI4 reactions are very similar to those of the Si+ + SiF4 reaction^.^' In each system, the silicon monohalide Six+,
1602 The Journal of Physical Chemistry, Vol, 93, No. 4, 1989
dihalide Sixz+,and trihalide Six3+product ions are produced, while no Six4+or SizXX+ions are observed. Furthermore, the relative magnitudes of each cross section are comparable. Specifically, Six+production is dominant, formation of Sixz+is the least favored process, and u(SiX3+) lies intermediate between u(SiX+) and u(SiX2+). The reaction energetics are also similar. In both the C1 and F systems, Six2+and Six3+production have comparable endothermicities: 0.4 and 0.3 eV with SiCI4 and 2.4 and 2.5 eV with SiF4. Likewise, Six' production is much more energetically accessible in each case: exothermic with SiC14and nearly thermoneutral with SiF4. The general shapes of the SiC12+ and SiF2+cross sections are analogous, as are the SiC13+and SiF3+ cross sections. Specifically, two distinct features appear in each u(SiXz+), due to production of both a bonded Sixzand dissociated Six X neutral fragment. The Six3+cross sections each rise from threshold to a maximum value and then decline consistently ) an energy 7-8 times the threshold as a power law ( E Xuntil energy. Here the cross section becomes virtually constant with increasing energy. C. High-Energy Behavior. At high kinetic energies, the excess energy available to the products must go either into translation or into internal modes of the neutral or ionic species. If enough energy lies in the internal modes of the SiCl,+ product, then the product ions will dissociate into reaction channels such as (8) and (10). We can compare the thresholds for these dissociative channels to the observed behavior of the reaction cross sections. A given cross section will peak and begin to fall off at the dissociation threshold ED if a fraction of the products are formed with all of the excess energy in the internal modes of the ion. Whether this occurs depends on the reaction mechanism, and thus the locations of the peaks in the cross sections can provide mechanistic information. The cross section for process 9, SiC12++ SiClZ,begins to decline at 2.4 f 0.1 eV. Furthermore, the threshold for reaction 8 is 2.9 f 0.3 eV. Both are in good agreement with the expected value for ED of 2.5 f 0.3 eV calculated from Table 11. This behavior is evidence for a mechanism involving a long-lived intermediate and randomization of the excess energy. On the other hand, u(SiC13+) peaks at about 1 eV, which is well below the expected dissociation threshold, ED = 4.3 f 0.1. Also at -1 eV, u(SiC1') begins to fall off much more steeply than the E-0.5 behavior observed at lower energies and predicted by the LGS model. However, the threshold for dissociation of SiCl', process 14, is
Weber and Armentrout
+
Si+ + SiC1,
-
Si+ + C1
+ SiC13
(14)
much higher, 4.7 f 0.1 eV. In each channel, the falloff cannot be attributed to competition from the dissociative process, and therefore, this behavior must be due to a dynamic effect. The fact that the falloff energies coincide is direct evidence that the two mechanisms are intimately coupled. Finally, we need to address the persistence of u(SiC13+)at high energies. As mentioned above, the SiF3+cross section in the Si' + SiF4 cross section showed identical behavior.z7 One possible explanation for this behavior in both systems is dissociative charge transfer, as in reaction 15. For the Si+ + SiC14 system, the
Si+ + SiCI,
-
SiC13+ + C1
+ Si
(15)
expected threshold for process 15 is 4.3 f 0.1 eV (calculated from Table 11). However, this energy is nearly 2 eV above the derived threshold energy for the second cross section feature, ET = 2.5-2.9 eV. An alternate possibility is that the dynamics of the reaction are such that excess energy is placed preferentially into translational modes of the products, rather than dissociative modes of the product ion. A third explanation is that this behavior is due to formation of excited-state products, either SiC13+ or SiC1. D. Reaction Mechanisms. *SiCI+ + SiCl, production is highly favored over SiCP + *SiC13 production in the isotopically labeled reaction of *Si+ with SiCI4. This is strong evidence for a direct mechanism in SiCI' production. In such a direct mechanism, the Si+interacts primarily with only one C1 atom on SiC14. Likewise, SiCI3+ + *Sic1 is preferred over *SiC13+ + SiC1. This also corresponds to a direct mechanism but involves the removal of
U (b)
Figure 5. Two possible orientations of the 3p(Si) orbitals with the u(CI-SiCI,) orbital as Si+approaches SiCI4collinearly. Case a is a u-type interaction and involves severe electron-electron repulsion. Case b involves r interaction, and overlap of the orbitals is much less repulsive. In both cases, not shown is an empty 3p(Si) orbital and a full pr(C1) orbital lying perpendicular to the plane of the paper. A third possible orientation is with this third 3p(Si) orbital singly occupied. Such an orientation is equivalent to case b.
C1- rather than C1 by the *Si+. This is consistent with two highly coupled mechanisms for production of either SiCl' or SiC13+,as discussed above. The second feature in u(SiClz+) is presumed to be due to dissociation of SiCI3+into SiClZ++ C1, process 10. Thus, the fact that u for SiClZ++ C1+ *Sic1 is greater than that for *SiC12++ Cl Sic1 is congruous with a dominant SiC13+ *Sic1 cross section. No preference is observed for production of *SiC12+ SiC12 versus SiClZ+ *SiClZ. This is additional evidence that SiClZ+ is formed through a long-lived intermediate involving rearrangement. Two such mechanisms are possible, and have been SiF4.z7 discussed in detail for the production of SiFz+ from Si' One is initiated by insertion of Si' into the C1-SiC13 bond. This is followed by C1 transfer to form [ClZSi-SiClz]+,and finally Si-Si bond cleavage occurs. The other involves approach of the Si+ toward SiC14in C, symmetry, such that Si+ interacts with and simultaneously abstracts two C1 atoms of SiCl,. Both mechanisms involve rearrangement to form a symmetric intermediate, and thus both mechanisms are likely to be hindered. No disilicon species are observed, which is analogous to the Si+ SiF4 systemz7but contrary to the Si+ SiH4 system.z8 This may imply that insertion of Si+ into the C1-SiCl3 bond to ultimately form SiClZ+,as proposed above, is unlikely. On the other hand, Si-Si dissociation may be preferred over dissociation of Si-C1 bonds, so that once the [ClZSi-SiClz]+ intermediate is formed, SiC12++ SiClz is the sole product. In any event, the fact that the Si-Si bond breaks before the Si-Cl bonds suggests that polymerization of silicon in a SiCl.,-based plasma may not proceed through gas-phase ion-molecule reactions. Additional studies of the SiCl,+ ( x = 1, 2, 3) + SiC14reactions are necessary to definitively resolve this question. E . Molecular Orbital Correlations. An understanding of the reaction mechanisms and dynamics can be gleaned from a molecular orbital (MO) correlation diagram.3z~s4*5s Such a diagram was used to explain the observed behavior in the Si' SiF4 system.27 Here, the strong coupling between the SiCl+and SiC13+ channels, as well as the similarities between the SiC14 and SiF4 systems, can likewise be explained by using the same MO arguments. In this paper, these arguments are only briefly reviewed. Ground-state Si' reactant has a ( 3 ~ )3p)' ~ ( electron configuration. Figure 5 demonstrates that the least electron-electron repulsion
+
+
+
+
+
+
+
+
(54) Mahan, B. H. J . Chem. Phys. 1971.55, 1436; Acc. Chem. Res. 1975, 8, 55. (55) Georgiadis, R.; Armentrout, P. B. J . Am. Chem. Soe. 1986, 108, 2119. Aristov, N.; Armentrout, P. B. J . Phys Chem. 1987, 91, 6178.
Thermochemistry of SiCI, SiCI', and SiC12'
The Journal of Physical Chemistry, Vol. 93, No. 4, 1989 1603
TABLE 111: Molecular Orbital Energies
energy, orbital
reactants p~(C1) P(Si) a(ACI)
eV
source
-1 1.8
IP(SiC14)" IP(Si) u(AC1)-pn(CI) spacing analogous to that in Si FHjblC u(ACI)-a*(ACI) spacing analogous to that in SiFH,b
-8.2 -13.3
a*(ACI)
+ 12.7
products ?r*(SiCI) u*(SiCI)
-7.1
a(SiC1)
-1 1.5
-4.3
IP(SiC1) u*-T* spacing is energy of A2Zt state,d KT)~(U)~(U*)~I~
a(SiC1)
u-T*
spacing is energy of B'2A state,d
[(n)4(4i(T*)21e
-12.2
T-T*
spacing is energy of C2H state:
[(*)3(u)2(T*)21e
4A')
-7.7
IP(SiC1,)
'Lias, S . G.; Bartmess, J. E.; Liebman, J. F.; Holmes, J. L.; Levin, R. D.; Gary Mallard, W. J. Phys. Chem. ReJ Data 1988, 17 (Suppl. I). bHout, Jr., R. F.; Pietro, W. J.; Hehre, W. J. A Pictorial Ap-
proach t o Molecular Structure and Reactivity; Wiley: New York,
1984; p 88. c I n Td symmetry, these orbitals are degenerate, but they split as the A-CI bond begins to break. dReference45. 'Reference 57.
results when Si+approaches with an empty 3p(Si+) orbital directed collinearly along a CI-SiC1, bond, in C3, symmetry. This clearly explains the small cross section for SiCI2++ SiCI2 formation, a process which must involve a noncollinear approach. Furthermore, this explains the observed direct reaction mechanisms for SiC1' and SiCI3+ production, since the lowest energy interaction of Si+ with SiCI4 involves only one of the CI atoms. The ordering and spacings of the SiC14molecular orbitals have been shown to be similar to those of SiF4, but shifted up by -4 eVSs6 Thus, construction of the reactant Si+and SiCI, molecular orbitals is analogous to those of Si+ SiF4.27 For simplication, we treat the SiCI3-Cl reactant as a diatomic species AC1. We have shown previously that full treatment of the reaction symmetry yields the same qualitative results.27 The highest occupied orbitals of ACI include the two filled p ~ ( C 1 orbitals, ) as well as the orbital of the Si-CI bond cleaved during reaction, denoted as u(AC1). The lowest unoccupied orbital of ACI is u*(ACl). For the SiCl and SiCI+ products, the valence electron configurations are ( T ) ~ ( u ) ~ ( A *for ) ' the SiC1(211) ground states7 and ( T ) ~ ( U )for ~ ground-state SiCI+('Z+).58 The nature of the u, u * , A, and A* orbitals should be similar to that of the SiF orbitals, which have been discussed p r e v i o ~ s l y . ~Lastly, ~ * ~ ~ in the A and A+ products, the only M O of interest is that which remains of the cleaved u(AC1) orbital, denoted here by u(A'). All M O energies for reactants and products are summarized in Table 111. Any uncertainty in the exact position of these orbitals is unlikely to affect the qualitative conclusions of the resulting M O diagram. The final ordering of the MOs is shown in Figure 6, with reactants on the left and products on the right. Qualitatively, this SiF4 system. This ordering duplicates the diagram of the Si' explains the similar reactivity observed between the present system and the fluorine system. The correlations between the reactant and product orbitals have been discussed in detail previously,27 and these are shown in Figure 6. The p(Si) and a*(SiCl) correlation corresponds to case a in Figure 5. The correlation between the doubly degenerate p(Si) and n*(SiCI) orbitals corresponds to case b. Crossing I involves two u orbitals and thus will be avoided in all symmetries. Crossings I1 and I11 involve a u and a A orbital and thus do not interact in the collinear geometry. However, if the geometry relaxes from collinear to C, symmetry or lower in the exit channel, there will be an avoided crossing at
+
+
(56) Bassett, P. J.; Lloyd, D. R. J . Chem. Soc. A 1971, 644. (57) Verma, R. D. Can. J . Phys. 1964, 42, 2345. Singhal, S. R.; Verma, R.D.Ibid. 1971, 49, 407. ( 5 8 ) Nishimura, Y . ;Mzuguchi, T.; Tsuji, M. J . Chem. Phys. 1983, 78, 7260. (59) Garrison, B. J.; Gcddard, W. A. J . Chem. Phys. 1987, 87, 1307.
SI
[SICI + A]'
+ ACI
Figure 6. Qualitative molecular orbital correlation diagram for reactions 6 and 12, where A denotes the SiC13 fragment. The orbital energies (along the vertical axis) are indicated by horizontal lines and are given in Table 111. Electrons are denoted by the vertical lines; the location of the 3p(Si) reactant electron in the product orbitals is unspecified. Roman numerals indicate orbital crossings for reference in the text. At crossing I (which is avoided in all geometries), solid and dashed lines indicate
adiabatic and diabatic correlations, respectively. Circles indicate that the crossings become avoided in C, or C, symmetry. I1 and 111. Whether crossing I11 is avoided or not is inconsequential, since all the corresponding orbitals are filled. The critical electron is that in the p(Si) orbital. If crossing I1 is avoided (corresponding to adiabatic behavior), then this electron occupies the a(A') orbital, producing ground-state SiC1, SiCl', process 6. On the other hand, if the electron passes through crossing I1 (corresponding to diabatic behavior), r*(SiCI) is instead occupied and SiC1,' + SiC1, reaction 12, results. The observed coupling between processes 6 and 12 is explained as competition for the odd electron. More direct evidence of this competition, as observed in the Si+ SiF4 system:7 is presumably masked here by the strong preference for the exothermic channel, SiCI' production. Since SiC1' production is so dominant, the T-u orbitals must mix very efficiently, which indicates that the reaction symmetry relaxes from strict C3u.
+
+
Summary Guided ion beam mass spectrometry has been used to study the reaction of Si' with SiCI4 from thermal to 20 eV CM. Three product ions are observed, SiCI', SiCI2+,and SiCI3+. No Si2C1,' species are observed, which may suggest that gas-phase polymerization of silicon in an SiC14plasma is unlikely. SiCP production is exothermic, while SiC12+and SiC13+formations are both slightly endothermic processes with threshold energies of 0.4 f 0.1 and 0.3 f 0.2 eV, respectively. This energetic information is then used to derive heats of formation for SiC1, SiCl', and SiC12+. These values lie within the error limits of previous literature values. The behavior of the SiCI3+cross section suggests that an excited state of SiC13', which lies -3 eV above the ground state, may be produced. The results of the isotopically labeled reaction of *Si+ with SiC14 indicate direct mechanisms are involved in production of both SiCl' and SiCI3+. Furthermore, interpretation of the locations of the cross section maxima indicates that these two processes are intimately coupled. On the other hand, analogous examination of the SiCI2+cross sections indicates that SiCI2+formation proceeds through a long-lived intermediate and involves statistical distribution of the excess energy. These mechanisms are consistent
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J . Phys. Chem. 1989, 93, 1604-1611
with those observed for the reactions of Si+ with SiF,. Furthermore, in both the fluorine and chlorine systems, the relative ordering of the cross section magitudes is comparable, as is the general behavior of the reaction cross sections. In each system, and Six3+ channels are approximately the energetics of the Six2+ equal, and the Six+channel is much more energetically accessible. The similarities between the SiF4 and SiC14 reactions with Si+
and the reaction mechanisms are interpreted in terms of molecular orbital arguments. Acknowledgment. This work was funded by the Air Force Wright Pm"hxJ ~ ~ b o r a t o r i e s . Registry No. Si, 7440-21-3; SiCl,, 10026-04-7; SiCI, 13966-57-9; Sic]+,5131 1-81-0; SiCI,+, 67047-80-7.
Modifying Role of Alkali-Metal Cations in Borate Glass Networks E. I. Kamitsos Theoretical and Physical Chemistry Institute, National Hellenic Research Foundation, 48, Vassileos Constantinou Aue., Athens 1I6 35, Greece (Received: January 8, 1988)
The cation vibration frequencies and their compositional dependence, which were obtained from far-infrared measurements, have been utilized to elucidate the role of the alkali-metal cation on the borate glass structure. The dependence of the cation motion frequency on the symmetry and size of the anionic network site has been studied by using a simplified type of the Born-Mayer potential to describe the cation-network interactions. Thus, comparison of calculated and experimental far-infrared data revealed that the anionic site charge density shows a strong cation dependence; that is, it decreases systematically upon increasing alkali-metal cation size. This is a manifestation of the dependence of the borate glass structure not only on the alkali-metal oxide content but also on the nature of the alkali-metal cation. These results have been discussed in connection with earlier as well as recent structural studies of alkali-metal borate glasses.
1. Introduction
The structure of alkali-metal borate glasses has been studied by a variety of techniques, including infrared,',* NMR,3-5 and Raman6 spectroscopies. The emphasis of these studies was focused mainly on elucidating the ways by which the glass structure changes upon increasing the alkali-metal oxide content. Thus, the presence of various boron-oxygen arrangements has been proposed and used to account for the peculiar compositional dependence of most physical properties of alkali-metal borate glasses.' We have recently measured the far-infrared spectra of the alkali-metal borate glass systems xM20.( 1 - x)B203 (M = Li, Na, K, Rb, Cs), in an effort to gain knowledge about the cation-glass network interactions and their compositional dependence.* One of the key results of this study was the linear dependence of the squares of the frequencies of the cation motion bands on alkali-metal oxide content, x . In addition, these plots have revealed kinks at about x = 0.20 for all but for the cesium borate glasses. To understand this behavior, we used a simplified type of the Born-Mayer potential as suitable to describe the cation-network site interaction^.^^^ Assuming then that each alkali-metal cation is surrounded by six nearest oxygen neighbors, arranged in an octahedral type configuration, it was shown that the following expression holds for the square of the cation motion frequency:
where qc and qA are the charge of cation and anionic site respectively, is the reduced mass of vibration, and ro is the equilibrium cation-oxygen distance. The various constants in( I ) Krogh-Moe, J . Phys. Chem. Glasses 1962, 3, 101. (2) Krogh-Moe, J . Phys. Chem. Glasses 1965, 6, 46. (3) Bray, P. J.; O'Keefe, J . G. Phys. Chem. Glasses 1963, 4 , 37. (4) Jellison, G. E.; Bray, P. J. J . Non-Cryst. Solids 1978, 29, 187. (5) Bray, P. J. J . Non-Cryst. Solids 1985, 73, 19, and references therein. (6) Konijnendijk, W. L.; Stevels, J. M. J . Non-Cyst. Solids 1975, 18, 307. (7) For a review article on borate glass structure see: Griscom, D. L. In Borate Glass: Structure, Properties and Applications; Pye, L. D., Frechette, V. D., Kreidl, N. K., Eds.; Plenum: New York, 1978. (8) Kamitsos, E. 1.; Karakassides, M. A.; Chryssikos, G. D. J . Phys. Chem. 1987. 9 .1,. -581-17 . .- . , . -(9) Kamitsos, E. I.; Chryssikos, G. D.; Karakassides, M. A. J . Phys. Chem. 1987, 91, 1067.
0022-3654/89/2093-1604$01.50/0
volved are as follows: c is the speed of light, eo the permittivity of free space, a a pseudo-Madelung constant, and p the repulsion parameter, taken to be p = 0.333 While the assumption of six-coordination is a reasonable one to start with, it should be noted that each alkali-metal cation has a preferable coordination geometry, which is not necessarily of the octahedral type. Representative crystallographic data for various alkali-metal borate compounds have been collected and are presented in Table I. No data for rubidium borate compounds are included, since the two crystallographic studies that we are aware of, Le., of Rbz0.B20323and Rb20-5B203:4 give no details on the Rb coordination and the average rubidium-xygen distance. However, it is reasonable to assume that rubidium behaves in a way similar to that of Cs or K. It is evident from Table I that six-coordination, on the basis of which eq 1 was derived, holds mainly for Na glasses but not for the rest of the alkali-metal borates. In this work we first investigate the effect of the coordination geometry on cation vibration. Fourfold coordination of alkali-
(10) Morris, C. D. J . Phys. Chem. Solids 1958, 5, 264; Proc. R. SOC. London, 1957, 242, 116; Acta Crystallogr. 1958, 11, 163. (1 1) Krogh-Moe, J. Acta Crystallogr., Sect. B Srruct. Crystallogr. Cryst. Chem. 1968, B24, 179. (12) Zachariasen, W. H. Acta Crystallogr. 1964, 17, 749. (13) Krogh-Moe, J. Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. 1974.830. 578. (14) Krogh-M&, J. Acta Crystallogr., Sect. B: Strucr. Crystallogr. Cryst. Chem. 1974,830, 747. (15) Hyman, A.; Perloff, A,; Mauer, F.; Block, S. 1967, 22, 815. (16) Krogh-Moe, J. Acta Crystallogr., Sect. B Struct. Crystallogr. Cryst. Chem. 1972, B28, 1571. (1 7) Marezio, M.; Plettinger, H. A.; Zachariasen, W. H. Acta Crystallogr. 1963, 16, 594. ( I 8) Krogh-Moe, J. Acta Crystallogr., Sect. B Struct. Crysrallogr. Cryst. Chem. 1972, B28, 168. (19) Krogh-Moe, J. Acta Crystallogr., Sect. B Struct. Crystallogr. Cryst. Chem. 1974,830, 1827. (20) Krogh-Moe, J. Acta Crystallogr., Sect. B Struct. Crystallogr. Cryst. Chem. 1972, B28, 3089. (21) Krogh-Moe, J. Acta Crystallogr., Sect. B Struct. Crystallogr. Cryst. Chem. 1974, 830, 1178. (22) Krogh-Moe, J. Acta Crystallogr., Sect. B Struct. Cryslallogr. Crysr. Chem. 1967, 23,427. (23) Schneider, W.; Carpenter, G. B. Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. 1970, 826, 1189. (24) Krogh-Moe, J. Ark. Kemi 1959, 14, 439.
0 1989 American Chemical Society
