J . Phys. Chem. 1991,95,4765-4772
TABLE MI:
ROtOartiOn MiCmwmts It 298
histidylglycine log kN log kIm 108 krn log kk
7.77 7.27 6.85 6.35
histidine 9.58 7.40 8.65 6.47
K in H20 histamine 10.32 7.64 9.30 6.62
9.58, log K2 = 6.47; histamine, log KI= 10.32, log K2 = 6.62. The microconstantscalculated for H 2 0 at 298 K are reported in Table VII. Arrhenius plots of the microscopic rate constants k++ and ko+ provided the microscopic activation energies E++and Eo+ for C(2) hydrogen-deuterium substitution in the N+,Im+ and N,Im+ protonation states, respectively. The values obtained for E++and Eo+, respectively, are 84 and 86 kJ/mol for histidylglycine, 83 and 90 W/mol for histidine, and 82 and 90 kJ/mol for histamine.
Conclusions The interactivity parameter is approximately 0.9 log k units for histidine and histidylglycine. It is slightly larger and more temperature dependent for histamine, apparently due to the lack of a second, bulkier substituent (e.g., carboxylate or peptide group) that could also interact with the imidazole. When one of the histamine a-hydrogens is replaced by the -COO- or -CONHCH2-C00- groups, the adjacent amino basicity decreases by approximately 0.7 and 2.5 log k units, respectively, whereas the basicity of the more remote imidazole groups is diminished by only 0.2 and 0.4 units. Consequently, the amino/imidazole basicity ratio increases dramatically in the order histidylglycine C histidine C histamine. The method used in this study provides overall basicity data for the imidazole ring. However, all the imidazole constants are a composite of the N I and N3 basicities. Tanokura has determined relative values, for the N3 and N I basicities by studying N3 and N I methyl derivatives."J2 For histidine, he obtained a N,/N3 basicity ratio of 4 at 37 'C and I = 0.1. However, this data is valid only when the a-amino group is protonated. When the a-amino is blocked by acetylation, the Nl/N3 ratio is significantly lower (1.6-2.5 in different derivatives), and there are no data when the amino group is in its unmodified, neutral form. Consequently, no individual interactivity parameters can be determined for the
4765
amino-imidazole N I and the amino-imidazole No interaction without further experiments. The difference between the log k++ and log kD+ microscopic rate constants measures the effect of the amino protonation on the C(2) hydrogen-deuterium substitution. This C(2) amino interactivity parameter is approximately 0.6 in all cases but has an opposite sign from the imidazole amino interactivity. Thus, when the amino group is protonated, the imidazole N basicity is decreased, however, the C(2)deprotonation rate increased. Both phenomena are apparently a consequence of the decrease in electron density on the imidazole ring upon protonation of the amino group. For simplicity, we investigated relatively small molecules in the present study. Nevertheless, these data can also be of practical use in larger biopolymers. N-Acetylhistidine is a model for Cterminal histidine residues in peptides, while the histidyl moiety in histidylglycine is an N-terminal histidine. By taking into account the basicity differences between the corresponding microforms of histidylglycine and histidine and combining this value with the log K value for N-acetylhistidine, we estimate log k 7.2 in H 2 0 at 298 K and I = 2.0 M dm-3 for the imidazole of a histidyl residue in a peptide chain. By an analogous procedure, the rate constants for C(2)-hydrogen substitution for histidyl residues in peptides is log k 6.2, where k has units of dm3mol-' h-' in D20 at 313 K and I = 2.0 M dm-). These values may be modified in most peptides and proteins by intra- and intermolecular interactions. The microscopic activation energy values have an estimated uncertainty of 2-4 kJ mol-'. However, these parameters show unambiguously that the rupture of the C(2)-proton bond requires a few kJ mol-' less activation energy when both the imidazole and the amino groups are protonated. Acknowledgment. This research was supported by National Institutes of Health Grant AI24216. B.N. gratefully acknowledges a fellowship from the Soros Foundation. The NMR instrumentation was supported in part by BRSG 2 SO7 RR 07010-20 awarded by Biomedical Research Resources, National Institutes of Health, and B.P. America. Rdstry No. H-His-Gly-OH, 2578-58-7; H-His-OH, 71-00-1; AcHis-OH, 2497-02-1; 4-methylimidazole, 822-36-6; histamine dihydrochloride, 56-92-8; imidazole-4-acetic acid, 645-65-8; L-j3-imidazolelactic acid, 14403-45-3.
=
Reactions of Os+, Ar+, Ne+, and He+ with SICI,: Thermochemistry of SiCi,+ ( x = 1-3) Ellen R. Fisher and P.B. Armentrout**t Department of Chemistry, University of Utah, Salt Lake City, Utah 84112 (Received: December 12, 1990)
Reactions of 02+, Ar+, Ne+, and He+ with SiC14 are studied by using guided ion beam mass spectrometry. All reactions are found to be fairly efficient with thermal energy rate constants that exceed 66% of the collision rate. The major products and Ar+ reactions are SiC14+and SiC13+,while for the Ne+ and He+ reactions, the major product is observed in the 02+ SiCI'. Thermochemistry derived from these SiCI, systems includes the determination of AfH(SiC13+)= 99.8 f 1.6 kcal/mol system, and ArH(SiCI2+)= 184.9 f 2.6 kcal/mol and AfH(SiCI+)= 203.9 and AfH(OSiC13+)< 97 kcal/mol from the 02+ f 2.5 kcal/mol from the Ar+ system. The ionization energies IE(SiC13) = 7.65 f 0.15 eV, IE(SiCI2) = 9.81 f 0.10 eV, and IE(SiC1) = 6.79 f 0.24 eV are also derived after consideration of other literature thermochemistry.
Introduction In the fabrication of microelectronic devices, chlorosilanes are used extensively in chemical vapor deposition (CVD) and plasma-enhanced CVD systems to deposit silicon layers.' Chlorine-based plasmas that form silicon chloride ions and radicals are also often used to etch such layers.lc.2 A detailed understanding of the chemical mechanisms involved in these plasmas 'Camille and Henry Dreyfus Teacher-Scholar. 1987-1992.
0022-3654/9l/2095-4765$02.50/0
can provide insight into the most important physical parameters of a plasma or CVD reactor. Thus the investigation of the reactivity,) structure: and thermochemistry>* of silicon chloride (1) (a) Inspektor-Koren, A. Sur/. Coat. Technol. 1987,33,3I . (b) Ban, V. J . Electrochem. SOC.lWS,122,1389. (c) Much, J. A.; Ha,D. W. ACS Symp. Ser. 1983, 219, 215. (2) (a) Schwartz, G. C.; Schaible, P. M. J . Voc. Scl. Technol. 1979, 16, 410. (b) Wormhoudt, J.; Stanton, A. C.; Richards, A. D.; Swain, H. H. J. Appl. Phys. 1981,61, 142.
63 1991 American Chemical Society
4166 The Journal of Physical Chemistry, Vol. 95, No. 12, 1991
species has been an active area of research. Many additives such as rare gases (Rg) and oxygen are used as diluents in these plasma systems? The amount and nature of the diluent has been found to have profound effects on deposition rates and film properties.lo*" Rare gases are used extensively in the plasma starting materials, often comprising up to 90% of the system. One study of a SiCI4/Ar plasma suggests that polymerization of silicon-containing species proceeds primarily through gas-phase ion-molecule reactions.I2 The proposed mechanism is initiated by reactions that produce SiCI4+and SiCI3+. These include electron impact ionization of SiC14,reactions with Ar+, and Penning ionization with metastable states of Ar. In our efforts to explore ion-molecule reactions related to plasma systems, we have previously examined the interactions of rare-gas ions with SiFq,13CF4,14and SiH4,ISthe reactions of Si+ with SiCI4I6and SiF4,I7and the reactions of O+ and 02+ with CF, and C2F6,18 and SiF419 These studies measured absolute reaction cross sections for all observed product channels, and found that these reactions produce primarily dissociative charge-transfer products. Little thermochemistry was derived in the Rg+ + SiH4, Rg+ + CF4, O+ and 02+ + CF4 and C2F6 systems because these reactions are controlled primarily by vertical transitions to electronic states of SiH4+,CF4+and C2F6+. The present study exAr+, Ne+, and He+ with SiC14. amines the reactions of 02+, Similar to the CF4 and c2F6 systems, the high energy dissociative charge-transferreactions are also controlled by verticil transitions to the SiC14+electronic states. At lower energies, however, we are able to derive thermochemical information for SiCI3+,SiC12+, and SiCP and for silicon oxychloride ions produced in the 02+ system. Thermal energy rate constants are derived for the four reaction systems studied here and are found to be close to the collision limit at all energies examined.
Experimental Section General. The ion beam apparatus and experimental techniques used in this work are described in detail elsewhere." The rare-gas and molecular oxygen ions are produced as described below. The ion beams are mass analyzed, decelerated to the desired translational energy, and injected into an rf octopole ion beam guide,21
(3) (a) Safarik, I.; Ruwicska, P. B.; Jodhan, A.; Strausz, 0. P.; Bell, T. N . Chem. Phys. Leii. 1985, 113, 71. (b) Stanton, A. C.; Freedman, A,; Wormhoudt, J.; Gaspar, P. P. Chem. Phys. Len. 1985, 122, 190. (c) Pabst, R. E.; Margrave, J. L.; Franklin, J. L. Ini. J . Mass Specirom. Ion Phys. 1977, 25, 361. (4) (a) Tsuji, M.; Mizuguchi, T.; Nishimura, Y. Can. J . Phys. 1981, 59, 985. (b) Miller, J. H.; Andrews, L. J . Mol. Srrucr. 1981, 77, 65. (c) Gosavi, R. K.; Strausz, 0. P. Chem. Phys. Leii. 1986, 123, 65. (5) Walsh, R. J . Chem. Soc., Faraday Trans. 11983, 79, 2233. (6) Steele, W. C.; Nichols, L. D.; Stone, F. G. A. J . Am. Chem. Soc. 1%2, 84, 4441, (7) Ho, P.; Coltrin, M. E.; Binkley, J. S.; Melius, C. F. J . Phys. Chem. 1985, 89, 4647. ( 8 ) (a) Ho, P.; Melius, C. F. J . Phys. Chem. 1990, 94, 5120. (b) Dixon, D. A. J . Phys. Chem. 1988, 92, 86. (9) Boyd, H.; Tank, M. S. Solid State Technol. 1979. 22, 133. (IO) Chu, T. L.; Chu, S. S.; Ang, S.T.; Duong, A,; Han, Y . X.; Liu, Y. H. J . Appl. Phys. 1986, 60. 4268. ( I I ) Flamm, D. L.; Donnelly, V. M. Plasma Chem. Plasma Process. 1981, I , 317. Morgan, R. A. In Plasma Erching in Semiconducior Fabrication; Elsevier: New York, 1985. (12) Manory, R.; Grill, A.; Carmi, U.; Avni, R. Plasma Chem. Plasma Process. 1983, 3, 235. (13) Weber, M. E.; Armentrout, P. B. J . Chem. Phys. 1989, 90, 2213. (14) Fisher, E. R.; Weber, M. E.; Armentrout, P. B. J . Chem. Phys. 1990, 92, 2296. (15) Fisher, E. R.; Armentrout, P. 8. J . Chem. Phys. 1990, 93, 4858. (16) Weber, M. E.; Armentrout, P. B. J . Phys. Chem. 1989, 93, 1596. (17) Weber, M. E.; Armentrout, P. B. J . Chem. Phys. 1988, 88, 6898. (18) Fisher, E. R.; Armentrout, P. B. J . Phys. Chem., in press. (19) Fisher, E. R.; Armentrout, P. Chem. Phys. Len., in press. (20) Ervin, K . M.; Armentrout, P. B. J . Chem. Phys. 1985. 83, 166. (21) Teloy, E.; Gerlich, D. Chem. Phys. 1974, 4, 417.
Fisher and Armentrout which passes through the reaction cell containing the neutral reactant gas. The neutral pressure is sufficiently low, 0.01-0.07 mTorr, that multiple ion-molecule collisions are improbable. The transmitted reactant 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. 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 previously." In all four systems, mass peaks corresponding to different C1 isotopes were observed, with relative intensities consistent with the natural abundances of the 35Cland 37Clisotopes. For convenience, only Si35CI,+ products were explicitly detected and then corrected for the 75.77% isotopic abundance of 3sCI to yield absolute magnitudes for all cross sections. The absolute zero and the 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.20 The uncertainty in the absolute energy scale is f0.05 eV (lab). The distribution of ion energies has an average fwhm of 0.3 eV (lab). Laboratory ion energies (lab) are converted to energies in the center-of-mass frame (CM) by using the conversion ECM = Ela,,M/(m+ M), where m is the ion mass and A4 is the neutral molecule mass. Unless stated otherwise, all energies quoted in this work correspond to the CM frame. Ion Source. The Rg+ ions are produced by electron impact of argon, neon, or helium gas which have ionization energies (I&) of 15.755,21.559, and 24.580 eV, respectively.22 The first excited electronic states of the ions are 29.3,48.5, and 65.4 eV, respectively, above the neutral ground states.22 To prevent formation of ionic excited states, the nominal electron energy used is thus 20 eV for Ar+, 40 eV for Ne+, and 60 eV for He+. Only the ground state of He+ is formed, but both the 'P3/2 and 2P!/2 spin-orbit states of Ar+ and Ne+ are produced, presumably with a 2:1 statistical population ratio. The 2Pl/2states of Ar+ and Ne+ lie 0.178 eV and 0.097 eV, respectively, above the 2P312ground state.22 The 02+ ions are formed in their ground electronic and vibrational states by using a recently constructed flow tube (FT) ion source.23 This flow tube (100 cm in length) operates at a pressure of 0.4-0.7 Torr with a helium flow rate of 3500-7000 standard cm3/min. In this source, He+ and He* are formed by a microwave discharge. Further downstream, these species interact ions (IE(02) = 12.071 f 0.001 with the reagent gas to form 02+ eV24925)through charge transfer and possibly Penning ionization. is obtained by self-quenchingcollisions Primary cooling for 02+ with O2molecules. It is known that electronically excited metastable ions of 02+ are rapidly deexcited in collisions with O2(& = 3.1 X 1O-Io cm3 and that vibrationally excited 02+ ions are known to be rapidly quenched by O2( k = 3 X cm3s-I).~~ The internal energy content of the ions is evaluated in several ways that have been discussed in detail previously.28 Results of these tests show that the 02+ ion beams used here are >99.9% ground u = 0). state 02+(*ng, Ion Collection EfJciency. For charge transfer (CT) and dissociative CT reactions, products may be formed through a long-range electron jump such that little or no forward momentum is transferred to the ionic products.29 In such instances, it is (22) Moore, C. E. Natl. Srand. ReJ Data Ser. (US., Nail. Bur. Srand.) 1970, 34. (23) Schultz, R. H.; Armentrout, P. B. I n i . J . Mass Specirom. Ion Processes,
in press.
(24) Chase, M. W., Jr.; Davies, C. W.; Downey, J. R., Jr.; Frurip, D. J.; McDonald, R. A.; Syverud, A. N. J. Phys. Chem. Ref.Data 1985, 14, SUPPI. .. 1 (JANAF Tables):
(25) Lias, S. G.; Bartmess, J. E.; Liebman, J. F.; Holmes, J. L.; Levin, R. 1. D.: Mallard. W. G. J . Phvs. Chem. Ref. Data 1988. 17. SUDDI. (26) Lindinger, W.; Aibritton, D. L.-; McFarland, M.; Feh;enfeld, F. C.; Schmeltekopf, A. L.; Ferguson, E. E. J . Chem. Phys. 1975,62,4101. Glosik, J.; Rakshit, A. B.; Twiddy, N . D.; Adams, N. G.;Smith, D. L. J . Phys. B 1978. 1 1 . 3365. (27) khringer, H.; DurupFerguson, M.; Fahey, D. W.; Fehsenfeld, F. C.; Ferguson, E. E. J . Chem. Phys. 1983, 79, 4201. (28) Fisher, E. R.; Armentrout, P. B. J . Chem. Phys. 1991, 94, 1150.
The Journal of Physical Chemistry, Vol. 95, No. 12, 1991 4767
Reactions of 02+, Ar+, Ne+, and He+ with SiCI4 possible that up to 50%of these ions may have no forward velocity in the laboratory and will not drift out of the octopole to the detector. Such slow product ions that do traverse the octopole may be inefficiently transmitted through the quadrupole mass filter.20 Exothermic or nearly thermoneutral reaction channels, in particular, can be subject to these effects. In the present results, cross section features and magnitudes were found to be sensitive to the extraction and focusing conditions following the octopole. Nevertheless, results reported here were reproduced on several occasions and the cross section magnitudes shown are averaged results from all data sets. Based on reproducibility, conservative estimates on the uncertainty in the absolute cross sections are &30% in the Ne, Ar, and O2systems, while that for the He system is &50%. This is due to difficulties associated with collecting a low-mass ion such as He+, as discussed previou~ly.~~ Collision Cross Sections. The collision cross section ucol for ion-molecule reactions at low energies is predicted by the Langevin-Gioumousis-Stevenson (LGS) model,31eq 1, where e is the
= 7re(2a/E)'I2 (1) electron charge, a is the polarizability of the target molecule, and E is the relative kinetic energy of the reactants. Many exothermic reaction cross sections follow this type of energy dependence, although deviations are commonly seen.32 In this work, we use a(SiCl4) = 1 1.27 A3.33 At high kinetic energies, ucoIfor CT reactions may better be represented by the hard-sphere limit, uHS= 7rR2.We estimate that uHS(O2+/SiCl4)= 54 A2,34uHs(Ar+/SiC14) = 60 A2,35 uHs(Ne+/SiCl.,) = 47 A2,Mand uM(He+/SiCl4)= 40 A2?' Here, we take ucoIto be the maximum of ULGS and uHS. Thermochemical Analysis. Cross sections for endothermic reactions of species having a distribution of electronic states, denoted by i, can be analyzed by using eq 2, which involves an
ENERGY lev. Lab)
102
E a =? 101
38
100
ULOS
u(E)
CgiuiJE - Eo + E,)"/E
ENERGY (eV. CM)
Figure 1. Variation of product cross section with translational energy in laboratory frame of reference (upper scale) and center-of-mass frame (lower scale) for reaction of Ozt with SiCI, at low energies. The solid line shows the total reaction cross section. The dashed line shows the collision cross section, given by the maximum of am or uLOs The dotted line is for the model discussed in the text for the SiCI3+cross scction. The solid line running through the data points is for this model convoluted with the experimental kinetic energy distributions. ENERGY l e t Lab) 0.0
40.0
I
I
(2)
i
explicit sum of the contributions of individual states weighted by their populations, g,. Here, Eo is the threshold for reaction of the lowest electronic level of the ion, E, is the electronic excitation of each particular electronic level, ofois an energy-independent scaling factor, and n is an adjustable parameter. For the J = 3/2 and J = 1/2 spin-orbit states of the Ar+ reactant ions, a 2:l statistical population ratio and equal reactivity are assumed. Equation 3 has only a single term for 02+, since the 211 , u = 0 state is the only electronic, vibrational state of the ion formed. Equation 2 has been derived as a model for translationally driven reactions38and has been found to be quite useful in describing the shapes of endothermic reaction cross sections and in
E It: 0 c
0 A 0
1
(29) Hierl, P. M.; Pacak, V.; Herman, 2.J . Chem. fhys. 1977,67,2678. (30) Ervin, K. M.;Armentrout, P. B. J . Chem. Phys. 1987, 86, 6240. (31) Gioumousis, G.; Stevenson, D. P. J . Chem. fhys. 1958, 29, 292. (32) Armentrout, P. B. In Structure/Reactiviry and Thermochemistry of Ions; Awla#i, P., Lias, S.G.,Eds.; D. Reidel: Dordrecht, The Netherlands, 1987; p 97. (33) Rothe, E. W.; Bernstein, R. 8. J . Chem. fhys. 1959, 31, 1619. (34) For the Ozt + SiC14 system, R is approximated as re(Ot-C1) re(Cl$i-Cl) re(OZt)/2 = 4.1 5 A. The value re(OtC1) = 1.57 A is based on the equilibrium bond length of OCI from OHare, P. A. G.; Wahl, A. C. J. Chem. fhys. 1971,54,3770. The value for re(C13Si-Cl)= 2.02 A is taken from Tossell, J. A.; Davenport, J. W. J. Chem. fhys. 1984,80,813. The value for r,(O,+) = 1.12 A is taken from Hubcr, K.P.; Herzberg, G. In Molecular Spectra and Molecular Structure Constants of Diatomic Molecules; Van Nostrand Reinhold: New York, 1979. (35) In the Art SiC14 systems, R is estimated by re(ArCIt) + re(CI~SI-CI)= 4.38 A if re(ArCIt) = 2.36 A. This value is an estimate obtained by comparing re O W ) = 1.57 A to the ratio of re(RgF+) = 2.0 AI3 and re(OF+)= 1.33 The value for re(OF+) is based on the equilibrium bond distance for OF from OHare, P. A. G.; Wahl, A. C. J . Chem. Phys. 1970, 53, 2469. (36) For Net, R is estimated by re(NeCIt) + re(CI3SiCl) = 3.86 A if re(NcCI*) = 1.84 A. This value for re(NeCIt) is obtained by comparing r (OCP) * 1.57 A to the ratio of re(NeF+)= 1.56 ,&''and re(OF+) 1.33
+
+
+
h.
+
-
(37) For the Het systems, R is estimated by re(HeCIt) re(C13Si-Cl) = 3.59 A if re(HeCIt) = 1.57 A. This value is obtained by comparing re OCP) = 1.57 A to the ratio of re(HeF+) = 1.33 AI3 and r,(OF+) = 1.33 (38) Chesnavich, W. J.; Bowers, M. T. J . Phys. Chem. 1979, 83; 900.
h
t
L
1
A.
20.0
0.0
O2
8
-4 0 . I 10.0
t
SiC14
+
I
I
I
20.0
30.0
40.0
50.0
ENERGY leV, CM)
Figure 2. Variation of product cross section with translational energy in laboratory frame of reference (upper scale) and center-of-mass frame (lower scale) for reaction of Ozt with S i c 4 at elevated energies.
deriving accurate thermochemistry for a wide range of systems.I7J9 The parameters n, ulorand Eo are allowed to vary freely to best fit the data as determined by nonlinear least-squares analysis. Errors in threshold values are determined by the variation in Eo obtained from several data sets and the absolute uncertainty in the energy scale, -0.05 eV (lab). Results 02+ SiC.1,. In this system, we observe formation of Sic&+ ( x = 1-4) ions, processes 3-6, and OSiCI,+ ( x = 0-3) ions, processes 7-10. The cross section results for these reactions are shown in Figures 1 and 2, where for clarity the products are divided
+
into two parts. At all energies, the reaction efficiency is high, (39) (a) Aristov, N.; Armentrout, P. B. J . Am. Chem. Soc. 1986, 108, 1806; (b) Sunderlin, L. S.;Aristov, N.; Armentrout, P.B. Ibid. 1987, 109, 78.
Fisher and Armentrout
4768 The Journal of Physical Chemistry, Vol. 95, No. 12, 1991
02+ + sic4
Sic4'
+ 02
(3)
+ o2 + CI sic12+ + 02 + XI Sic$+
+ 0, + 3CI
SiCI'
osicb' + (0+ CI) osick' + (0+ ~ C I ) OSiCI+ + (0+ XI)
+
SiO'
(0 + 4CI)
(4)
(5) (6)
(7) (8)
TABLE I: Literature TbennocaaiStry 8t 298 C ArH(R). ArH(R+), R kcal/mol kcal/mol Si 107.6 f 1.9 297.1 f 1.0 CI 28.99 f 0.002 c102 25 f 1.5 CIO 24.2 0.5
CI,O
(10)
with ala -83% of uwiat the lowest kinetic energies (Figure 1). At intermediate kinetic energies, between 0.3 and 10 eV, the total cross section rises to nearly twice a,. This discrepancy suggests that our estimate of uHS is low. At the lowest energies, formation of SiC14+ dominates the observed reactivity, with 0SiCl3+the only other product. Both cross sections decrease as -EO*4 between 0.1 and 4 eV (Figure l), in reasonable agreement with the predicted behavior of eq 1. This exothermic behavior for the CT process is consistent with the relative ionization energies of O2and SiC14 (Table I). As the kinetic energy increases, u(SiC14+) continues to decline as formation of SiC13+,process 4, becomes thermodynamically accessible (Figure 1). From 1.5 eV to the highest kinetic energies, SiC13+dominates the reactivity. The sum of these two cross sections (essentially ulo,in Figure 1) is smooth, indicating competition between the two channels. The remaining products are all formed in strongly endothermic processes (Figure 2). The cross section for 0SiCl2+rises from an apparent threshold of -4 eV, about 1.0 eV lower than the apparent threshold for SiCI2+. There appears to be substantial competition between these various processes but it is difficult to quantify this because of the number of product ions. The smallest cross section in this system is that for S O + , formed in process 10, which is observed only at the highest kinetic energies, with an apparent threshold of 12 eV. Ar' SiC14. Results for the reactions of Ar+ with SiC14are shown in Figure 3. At all energies, the reaction efficiency is fairly high. The total cross section declines at the highest energies, but this behavior could be due to poor ion collection efficiency at these energies. In this system, only CT and dissociative CT reactions are observed, processes 1 1-1 5. ArCP is not observed at any energy.
-
SiClz
21.0 f 1.6 44 f 4c 46 f 5d -40.3 f 0.8
SiC13
-78 f 3'
SiCl
(9)
SiCI, Si0 OSiCl
OSiClz
*
-158.4 0.3 -24 f 2 -87 f 7' -168 f 7'
OSiC1,
210 f 9c 203.9 f 2Sd 191 f 4c 184.9 f 2.6d 187.3 f 2.2' 102 f 3c 99.8 f 1.6d 115.0 f 1.3f 245 f 5 194 f 4d 165 f 3d 20 eV), this branching ratio changes to 13:8:72:7. The increase in the Si+cross section, which begins near 1 eV (Figure 4), may be due to formation of 2C1 + C12rather than 2CI2. The threshold for this process is calculated to be 0.64 f 0.05 eV. The reaction efficiency in the He system is also very high (Figure 5 ) . All five possible reaction products, SiCI,+ (x = 0-4) are observed. The cross sections for these products all increase monotonically with decreasing energy, indicating exothermic processes. This is consistent with the Table I thermochemistry regardless of the neutral products. Below 0.8 eV, the total ex~ .good l. perimental cross section decreases as (61 f 3 0 ) ~ 8 P ~ in . Similar to the case of the Ne system, agreement with u u ; ~57E-O.'. formation of SiCP is significantly favored over the other products a t all energies. In the He system, the branching ratio for
*
(40) Olsen, A.; Sale, F. R. J . Less-Common Mer. 1977, 53, 277. (41) Ihle, H. R.; Wu, C. H.; Miletic, M.; Zmbov, K. F. Ado. Muss Spectrom. 1978, 7A, 670. (42) Recommended value from ref 16. (43) Vought, R. H. Phys. Reo. 1947, 71, 93.
4770 The Journal of Physical Chemistry, Vol. 95, No. 12, 1991
eV, although a value of 8.6 eV has also been reported." More recent measurements include the photoelectron spectroscopy of SiCI2 taken by Bock et al.45and Kruppa et ala4 The adiabatic IE of SiC12can be estimated from these data as 9.8 f 0.1 eV (see Figure 3 of ref 46). When combined with the generally accepted ~.~~ literature value for AfH(SiC12)of -40.3 f 0.8 k c a l / m ~ l ,this gives ArH(SiCI2+) = 187.2 f 2.4 kcal/mol, in good agreement with the value derived here from the Ar+ system. It might be noted that a recent theoretical study by Ho et al. finds a slightly higher neutral heat of formation, -37.6 k ~ a l / m o lwhich ,~ leads to AfH(SiCI2+) = 189.9 f 2.4 kcal/mol, somewhat higher than the value derived here. The final values available for AfH(SiCI2+)come from studies by WA of the endothermic reactions 16 and 17.16 For reaction
-
Si+ + SiCI4
+
Si+ SiC14
+
SiCI2+ SiClz
SiCI2++ Sic1
(16)
+ CI
16, the measured threshold can be combined with AfH(SiC12)= -40.3 f 0.8 kcal/mol to yield 4H(SiC12+) = 188.2 f 2.6 kcal/mol or with 4H(SiCI2) = -37.6 kcal/mol to yield 4H(SiCI2+)= 185.5 f 2.6 kcal/mol. For reaction 17, the measured threshold is combined with an updated value for &H(SiCI) = 46 f 5 kcal/mol derived below to obtain AfH(SiC12+)= 188 f 7 kcal/mol. All these values are in good agreement with that derived here from the Ar+ system. These various considerations lead us to recommend that ArH(SiCI2+) = 187.3 f 2.2 kcal/mol, which is an average of all values cited above excluding the AE measurements. This is within experimental error of the value recommended previously by WA, 191 f 4 kcal/mol, which was an average that relied heavily on the AE measurements. If we accept the 187.3 f 2.2 kcal/mol value for the ion, we can combine this with the threshold for reaction 16 to yield AfH(SiC12) = -39.4 f 3.4 kcal/mol. This heat of formation is within experimental error of the literature value and the theoretical value of Ho et ah7 When the recommended heat of formation for SiC12+is combined with the literature value for the neutral, we calculate IE(SiCI2) = 9.81 f 0.10 eV, in agreement with the IE measured via photoelectron spectroscopy .45.46 SiC12+ is also formed in an endothermic reaction in the O2 system. Analysis of the cross section for process 5 using eq 2 was difficult since this cross section rises very slowly from threshold. Adequate reproduction of the data could be obtained with threshold values ranging from about 6.0 to 6.6 eV. If it is assumed that the neutral products are O2 2 CI, then these thresholds can be converted to ArH(SiCI2+) = 202-216 kcal/mol, considerably higher than the value found above for the Ar+ system. Because formation of SiCI2+is very endothermic in this system, it competes with a variety of other processes such that its threshold could be shifted to higher energies. An alternate possibility is that the neutral products are CI20 0 such that AfH(SiCI2+) = 179-193 kcal/mol, within experimental error of the value derived for the Ar+ system. However, these neutral products are not consistent with any reasonable mechanism for this system.47 S i C r . Analysis of the low-energy feature in the cross section for reaction 14 from five independent data sets yields the eq 2 parameters n = 1.0 f 0.2, a, = 0.18 f 0.04, and Eo = 1.15 f 0.1 1 eV. This threshold yields A,H(SiCI+) = 203.9 f 2.5 kcal/mol, assuming that the neutrals formed in this reaction are Ar + CI + C12. This value is within experimental error of the value recommended previously from the reaction of Si+ SiCI4, AfH(SiCi+) = 210 f 9 kcal/mol. Good agreement is also found between the present value and four previously reported values
+
+
+
(44) Andianov, K. A.; Tikhomirov, M. V.; Golubtsov, S. A.; Zubkov, V. 1.; Potapov, V. K.; Sorokin, V. V. h k l . Akad. Nauk SSSR 1970, 194, 1077. (45) Bock, H.; Solouki, B.; Maier, G. Angew. Chem., Inr. Ed. EngL 1985, 24, 205. (46) Kruppa, G. H.; Shin, S. K.;Beauchamp, J. L. J . Phys. Chem. 1990, 94, 327. (47) If the 0 + OC12neutral products accompany the SiCI2+product, then the precursor to the ionic product must either be Si0Cl2+ or SiOCI,+. The latter is not observed and the former does not have a threshold which corresponds to formation with the 0C12 neutral. See text.
Fisher and Armentrout TABLE 11:
Bond Dissociation Energies (kcal/mol)'
BDE
BDE 14 (2) 116 (2) 46 (3) 122 (3j
bond SiC13-CI SiC12-CI SiCI-CI Si-CI
109 (3) 67 (3) 115 ( 5 ) 91 (5j
OSiCI-CI OSi-CI
1 IO (IO) OSiCI+CI 92 (7) OSi+-CI
bond SiC13+-CI
SiCI2+-CI SiCI+-CI
si+-CI
bond
BDE
OSiC12+-CI 297 (5) O-SiCI,+ 62 (5) 58 (5) 0-SiCl2+ 82 (4) 80 (5) O-SiCI+ 69 (5) +Si+ 112
"These values are based on the ion and neutral heats of formation derived here and listed in Table I. derived from AE and IE measurements that center around 202 f 5 kcal/m01.~~ Further support of the value derived in the Ar system comes system. Analysis of from the threshold for process 6 in the 02+ the SiCP cross section from three independent data sets for the 02+ system yields the eq 2 parameters n = 1.8 f 0.1, u, = 0.15 f 0.03, and Eo = 7.48 ;t 0.13 eV. Assuming loss of 3C1 and 02, this threshold then leads to AfH(SiC1+) = 207.0 f 3.0 kcal/mol, in good agreement with the value determined for the Ar system. Because of the difficulties associated with definitive assignment of the neutral products for the O2reaction system and the possibility discussed above that the thresholds are elevated due to competition, we take the value determined for the Ar system as our most reliable heat of formation. If the value derived above for AfH(SiC13+)is used in conjunction with the 0.3 f 0.2 eV threshold measured by WA for reaction 18, we find ArH(SiCl) = 46 f 5 kcal/mol. This value is somewhat Si+
-
+ SiC14
+
SiC13+ Sic1
(18)
higher than the value previously recommended by Walsh,' 37 f 10 kcal/mol,'but lies within the generous error bars. Our value compares well with the 47.4 f 1.6 kcal/mol value cited in the JANAF tablesz4 that is taken from studies by Farber and Sri~astava:~but it is higher than the 37.9 kcal/mol value calculated by Ho et al.7 If we combine our values for A,H(SiCl+) and P,H(SiCl), we derive IE(SiC1) = 6.79 f 0.24 eV. This compares very well with the lone experimental measurement derived from a Rydberg analysis, 6.82 eV.50 Comecutiue Bond Dissociation Energies. The thermochemistry derived above allows us to calculate the sequential bond dissociation energies (BDEs) for the SiCl,,+-CI and SiCI,I-Cl series of bonds (Table 11). The BDEs for the neutral bonds are nearly identical with those discussed previously by W a l ~ h . ~The ' consecutive BDEs for the neutral series follow an alternating strong-weak pattern. Walsh has attributed the very weak SiCI2-CI bond to a stabilizing effect in the SiC12species, which is associated with an enhancement in the low energy s orbital character of the lone-pair ~ r b i t a l . ~For ' the ionic series, a similar alternating pattern in consecutive BDEs is observed where the strongest bonds for both the ionic and neutral silicon chloride species are comparable. Now, the pattern is displaced by one ligand such that the weakest bond by far is SiCI3+-CI and the strongest is Si+-CI. This is reasonable since both SiC13+and SiCP are probably singlet ions which are isoelectronic at the silicon center with SiC14and SiCIz, respectively. B. Thermochemistry of Silicon Oxychloride Ions. Thermochemical information on the neutral and ionic silicon oxychlorides is sparse. Here we are able to derive tentative heats of formation for OSiCl,+ ( x = 1-3) from the 02+ system. Definitive thermochemistry is difficult to assign since the neutral products in reactions 7-10 cannot be ascertained unequivocally. Analysis of the trends in the consecutive OSiCl,,+-CI bond energies is used to choose among several possible values. The bond energy dis(48) These values are listed and discussed in ref 16. (49) Farber, M.; Srivastava, R. D. J . Chem. Soc., Faraday Trans. 1 1977, 73, 1672. (50) Bosser, G.; Bredohl, H.; Dubois, I. J. Mol. Specrrosc. 1984, 106, 72. (51) Walsh, R. Acc. Chem. Res. 1981. 14, 246.
Reactions of 02+, Ar+, Ne+, and He+ with SEI4 cussion implicitly assumes that the structures of the OSiCl,+ specits have no S i C I linkages but only S i 4 and Six1 bonds.S2 The results are necessarily somewhat speculative. OSiC13+. In the 02+ SiCI, system, OSiC13+ is formed at thermal energies. If we assume that the neutral product is OC1, this observation enables us to place an upper limit on AfH(OSiCI3+)of
