J. Phys. Chem. 1993,97, 6806-6810
6806
Kinetic Studies of the Reaction of Chlorine Atoms with Tetramethylsilane Yannis G. Lazarou and Panos Papagiannakopoulos' Department of Chemistry, and Institute of Electronic Structure and Laser, University of Crete, and F.O.R.T.H., 71409 Heraklion, Crete, Greece Received: October 2, 1992; In Final Form: April 5, 1993
+
-
+
The reaction C1 (CH@i HC1 (CH3)3SiCHz has been studied with the very low pressure reactor (VLPR)technique in the temperature range 273-363 K. The rate constant for the forward reaction is given by the expression k = (3.58 f 0.76) X exp[(-490 f 240)/RT] cm3 molecule-' s-1 (R is expressed in cal mol-' K-l). The conventional transition-state (TS) theory indicates that the TS is bent with a C1- .H- .C angle ca. 160° and the C1 atom on the H- C- -Si plane and inside the methyl group cone. The (CH3)sSiCHz radical was observed as a reaction product, and it appears to be a rather stable radical due to a d-p electron delocalization.
.
-
Introduction
The gas-phase reactionsof chlorineatoms with organometallic compounds play a significant role in the synthesis of novel amorphous and polycrystalline thin films through the chemical vapor deposition (CVD) technique.'" The reaction of chlorine atoms with silane has been studied extensively,G while the reactions with alkylsilanes have not been reported in the past. However, the reaction of C1 atoms with the analogous hydrocarbon, neopentane C(CH3)4, has been studied previously, and the reported Arrhenius parameters are A = lO-9.69cm3 molecule-' s-1, E = 0.7 kcal/mol,' and A = 10-9.53 cm3 molecule-1 s-1, E = 0.9 kcal/mol.* Furthermore, the hydrogen abstraction reactions of H and CH3 radicals with methylsilanes have been investigated by several workers.e11 In this work we study the kinetics of the reaction
C1+ (CH,),Si
-
HCl + (CH,),SiCH,
(1) over a relatively wide temperature range using the very low pressure reactor (VLPR) technique, which has been discussed in great detail by Bensonet al.lz This technique has produced reliable rate constant values and Arrhenius parameters for a number of chlorine atom reactions with hydrocarbons. The technique is suitable for the study of extremely rapid reactions in the gas phase (by adjusting the residence times in the reactor), with very limited interferences from secondary reactions (by working in the milliTorr pressure regime). Experimental Section
Our experiments were performed with a very low pressure reactor (VLPR) apparatus,which has been described previously.l 3 The main features of the technique are as follows: The gas-phase reaction occurs in a Knudsen cell at a total steady-state pressure less than 5 mTorr. The reactants are introduced into the reactor through two or three separate capillary inlets and are allowed to react for a short period of time, since they are also discharged through a variable aperture into the first stage of a differentially pumped system. Thus, a continuous molecular flow is maintained, leading to a collimated molecular beam that is sampled with a quadrupole mass spectrometer, which is mounted in the secondstage vacuum chamber. The molecular beam is modulated with a tuning fork chopper at the entrance of the second vacuum chamber, in order to achieve amplification of the mass spectrometric signal. Chlorine atoms were generated by flowing 5% Clz in helium (ultrahigh purity) through a quartz tube coated with a dried slush of boric and phosphoric acid mixture and enclosed in a 2.45-GHz microwave cavity operating at 30 W. The complete 0022-3654/93/2097-6806S04.00/0
dissociation of Cl2 was checked by mass spectrometry. Our sensitivity to Clz was less than 1Olo molecule ~ m so- that ~ the ratio [ a 2 ]/ [Cl] after discharge was zero. Tetramethylsilane (TMS) was of NMR purity (Aldrich) and was degassed several times prior to use. The flow of tetramethylsilane was more uniform by diluting it in helium (5% TMS/He mixtures). Flow rates of all gases were determined by following the pressure drop (measured on Validyne model DP 15-30, and DP 15-28 transducers) in a known volume (700 cm3)as the gases flowed through a 1 mm X 20 cm capillary. The chlorine atom and TMS concentrationswere determined from their relative mass spectrum peaks, ICI(35 m / e ) and ITMS (88 m/e),and from accurate calibration curves (IMversus [MI). The uncertainty in IMmeasurementswas *5% and therefore the ratios RI = [Cll~/[Cl]= ICIO/ICI and R2 = [TMS]o/[TMS] = I T ~ / Iweredetermined T ~ with an accuracyof f7%. The TMS concentrationswere also determined from flow and escape rates. The reaction cell was mounted on a stainless-steel flange containing a 5-mm aperture. The interior surfaces of the cell ( V = 168 cm3) were coated with halocarbon wax in order to inhibit wall recombination. The escape constantof the cell was measured by following the first-order decay curve (monitored by the mass spectrometer) for various gases after a fast halt of the flow. k , was found to be 1.86(T/M)l12 s-l, where T is the absolute temperature and M is the molecular weight. The temperature of the reactor was held constant by circulating a thermostated liquid (water or ethanol) through an outer jacket surrounding the reactor. The temperature was controlled and monitored by a refrigeratedbath circulator (Haake Model F3) with an accuracy f l K. The molecular beam was chopped at 200 Hz and monitored by a quadrupole mass spectrometer (Balzers QMG 51 1) with a cross-beam analyzer. The signal was further amplified with a lock-in amplifier (NF Model 570). The electron energy of the ionizer was kept low at 19 eV, where the fragmentation of HCl ( m / e 36) to C1+ ( m / e 35) was less than 1%. Therefore, the formation of HC1 reaction product did not interfere with the monitoring of C1 atom concentration (mass peak m / e 35). Our VLPR system was frequently tested by measuring the rate constant of the well-known reaction of C1 with CH4, and the obtained rate constants at 303 K were in excellent agreement with the accepted value.14 Rt?SdtS
The mass spectrometricanalysis of the reaction products reveals the apperance of HCl (mass peak m / e = 36) and (CH3)3SiCH2 (mass peaks m / e = 87 and 72). Therefore,the chemical reaction 0 1993 American Chemical Society
The Journal of Physical Chemistry, Vol. 97, No. 26, 1993 6807
Reaction of Chlorine Atoms with Tetramethylsilane 300
_.
'r
-
I
m
El
.
PO0
G Y
\
N N
U
U
=;'
-
,
,
:loo[/ U
++
I
U
v
0
1so
100
30
0
(molecules/cm"
x
[TMSI
Figure 1. Plot of (R1 - 1 ) R 2 k d 1versus [TMS]o at 303 K. Symbol size reflects the propagated errors (2u).
under study is
-
HC1+ (CH,),SiCH,
The likely secondary reactions are
-ka
C1+ (CH,),SiCH,
-+
(CH3),SiCH2C1* (CH,),Si
(1)
/ [ClI
I
1
3
4
D
cm3
molecule-1 s-1 1.47i 0.07 1.56 i 0.08
10-10 om3
T, K
molecule-1 s-1
333 363
1.77 f 0.10 1.80 0.11
CH,C1 (2a) (2b) (24
k3
(CH,),SiCH,CH,Si(CH,),
(3)
which could not be detected in our experiments, since no traces of (CH&SiCHzCl, (CH&SiCl, CH3C1, CzHFl, C2H6, and ((CH3)3SiCH2)2 species were present in the mass spectrum analysis. The reverse reaction k-1 also could not be detected, since no increase in chlorine atoms and TMS concentrations were observedupon the addition of substantial amount of HCl through a third inlet of the reactor ([HCl]/[TMS] = 50). The steady-state concentration of chlorine atoms is given by the expression
where [Cl], is the initial chlorine atoms concentration in the absence of tetramethylsilane. Furthermore, the steady-state concentration of tetramethylsilane molecules is given by the expression
where koroTmis the escape constant of tetramethylsilane and [TMS]o is the initial concentration of tetramethylsilane in the absence of chlorine atoms. Expressions I and I1 can be written in the form
= k, [TMSI,
ki ( * 2 ~ ) ,
ki ( * 2 ~ ) ,
T,K 273 303
+ CH,CH,Cl CH,SiCH,Cl + CZH6
(R, - 1)R,k,l
D
Figure 2. Plot of p = ( R I - 1)R2/(R2 - 1)Rl versus [TMS]o/[Cl]o at 303 K. Symbol size reflects the propagated errors (24.
10-10
(CH,),Si
2(CH,),SiCH,
2
[TMSI
TABLE I: Measured Values of Rate Constant 4 at Various Temperatures
ki
C1+ (CH,),Si
I
I
I
(111)
within a factor of 20%. Hence, the [CI]o concentration can be estimated by substituting the rate constant kl in expression IV, and the obtained values were the same (within a factor of 20%) with those obtained by the calibration of C1. The ratio of expressions I11 and IV yields the expression
and a plot of p versus [TMS]o/[Cl]o should yield a straight line with zero intercept. Least-squares fits of the data at different temperatures yield straight lines with almost zero intercepts, and a typical plot at 303 K is shown in Figure 2. The slope of the lineswas 0.73 f 0.07, which is in good agreement with the expected value of k = ~ ~ ~ / =k0.63. d , Therefore, the proposed reaction kinetics is correct, and the secondary reactions do not affect our experimental results. Experiments were performed at four different temperatures, 273,303,333, and 363 K, and the rate constants kl obtained are presented in Table I. The precision of the kl rate constant measurements was ca. 10% (2a). Typical experimental data, flow rates of He carrier gas, concentrations of reacting species, R1, Rz, and p ratios at various temperatures are listed in Table 11. An Arrhenius plot for kl is presented in Figure 3. Linear least-squares analysis of the kl temperature-dependence data yields the activationenergy and the Arrhenius A factor for reaction 1:
E, = 490 f 240 (2a) cal/mol
A = (3.58 f 0.76) X IO-" (20.) cm3 molecule-' s-l (IV) (R, - 1)R,kwTMs = ki[C110 Therefore, a plot of (R1- 1 ) R & d 1 versus [TMS]o should yield Discussion a straight line (expression 111) with slope equal to kl and zero intercept. Least-squares fits of the data yield straight lines with The thermochemical kinetics version of the conventional zero intercepts, and a typical plot at 303 K is shown in Figure theory has been applied to reaction 1, assuming 1. In addition, the left side of expression IV,(Rz - l ) R ~ k o o ~ ~ u ~ stransition-state , a transition-state geometry.15 The entropy change AS# for was estimated at various tetramethylsilane concentrations and forming a mole of transition-state complex from the two reactants constant chiorineatomconcentration and was found tobe constant
Lazarou and Papagiannakopoulos
6808 The Journal of Physical Chemistry, Vol. 97, No. 26, 1993
TABLE II: TvDical Carrier Gas Flow Rates. Concentrations of Reacting Specie&RI, RL and p at Various Temperatures
Temperature 273 K 3.43 3.63 2.44 2.48 2.75
0.15 0.25 0.28 0.43 1.27
23.17 14.40 8.68 5.78 2.17
4.39 4.38 3.21 3.88 3.63
0.20 0.33 0.26 0.73 1.71
21.78 13.14 12.14 5.33 2.12
2.89 2.77 2.18 2.09 2.00
0.37 0.33 0.37 0.33 1.17
7.80 8.48 5.86 6.39 1.70
1.87 1.94 1.84 1.65 1.74
0.48 0.65 0.83 1.23 1.39
3.93 3.01 2.22 1.34 1.25
12.2 8.58 6.19 4.62 2.37
7.94 4.63 3.04 1.70 0.34
1.53 1.85 2.04 2.72 7.03
2.76 2.02 1.74 1.31 0.63
3.55 2.36 2.53 1.86 0.86
7.36 4.05 3.25 1.69 0.38
1.76 2.41 2.32 3.50 6.59
2.21 1.58 1.61 1.14 0.62
2.94 2.22 1.53 0.69
2.64 2.61 1.63 1.66 0.21
3.80 3.77 4.65 4.50 8.30
1.18 1.20 1.06 1.08 0.47
3.47 3.55 3.48 3.57 0.85
1.03 0.70 0.44 0.13 0.08
7.06 8.21 9.09 9.16 6.8 1
0.87 0.76 0.62 0.28 0.23
3.90 2.97 2.16 0.69 0.32
Temperature 303 K 12.9 9.75 7.53 5.92 2.5 1
2.35
Temperature 333 K 10.0 9.84 7.60 7.48 1.70
Temperature 363 - ~ ... .K ~~
-22.2
7.30 5.75 3.97 1.15 0.55
-
a
I
-22.4
-
3 E
a
7.
4
Y
h I
Y
C
0
H
d
-22.6
-
5.
X
*
-
u 3.
H
m H
I
I
-22.0 2.5
I
3.0
1000/T
3.8 (K-' )
I 4.0
Figure 3. Arrhenius plot of In kl versus 1/ T. Symbol size reflects the propagated errors (2u).
4
.
H 1.
C.
so.
c1
AS# = S'(comp1ex) - S'(TMS) - S"(C1) = AP(difference) - S'(C1) where MO(difference) is the difference in entropies between transition state and reactant TMS and includes changes in translation, vibration, external rotation, internal rotation, electronic, symmetry, and optical isomerism entropies. It is reasonable to assume that the reaction proceeds through a tight transition state in which the C- - -H and H- - -C1 bond distances are elongated by 0.3 relative to the normal covalent lengths.16 TheC1- -H- .C angle may extend at different intervals, depending on the approach of C1 atom toward a methyl group of tetramethylsilane. Those angle intervals are determined by the van der Waals radii of the adjacent hydrogen atoms and are found to be between 180' and 135'. The rotational entropy of the transition state depends on the product of the principal moments of inertia, which furthermore depends strongly on the C1. .Hs. .Cz angle and the dihedral angle a between the C1.s a6H.. -zC and 6H.s .zC.. .Si planes. (ZAZ&)# becomes maximum at a dihedral angle of ca. 180' and a C1. -6H. .zC angle of ca. 160°, as shown in Figure 4. Thus, the average
360.
a
Figure 4. Moment of inertia product of transition state as a function of dihedral angle and angle C1. .H. -C. (I
is given by
270.
180.
D i h r a r a l angle
-
.
H
\
-
Figure 5. Bent model of transition state for C1 + (CHI)&.
transition-state geometry is shown in Figure 5, and the detailed entropy calculations are presented in Table 111. The normal bond angles and bond distancesof TMS were taken from theliterature.16
Reaction of Chlorine Atoms with Tetramethylsilane
The Journal of Physical Chemistry, Vol. 97, No. 26, 1993 6809
TABLE IIk Estimation of the AP(298) for Reaction (CHo)&i
+ c1
degrces of freedom translational Mtr = 3/2R In(M*/M) rotational = 1/2R h ( ( l ~ I d c ) * / ( I J d ~ ) ) electronic A&= R ln(2S + 1) = R In(u/a*) = R ln(12) symmetry u = 12 X 3'+u* = 3 X 33 internal rotations about the C t & bond, S,Qr = 21.6 amu AZ) about the Si-CZ bond, Sf= 1/2R ln(I:/Zr) about the Si-C. bond, (n = 3-5) vibrational u(C-H), 3000 cm-1 u(C. .H), 2100 cm-1 Vb(H-c*i), 1030 Cm-' Vb(H' .csi),720 Cm-l
---
+
(2) Vb(H-c-H), 1450 Cm-'
bent 1 .o 2.73 1.40 4.97
AP(diff), cal mol-1 K-1 linear 2.58
7.67 3.48 0.04
r.c.
e
--
(2) Ub(H' .c-H), Cm-l (1) U b ( c . SH-CI). 600 cm-l (1 j Y;(c.. .H-c&600 cm-1
0.3 0.2 0.5
0.5
22.3
0.5 14.97
~~
AP(298) = 22.3 - 39.5 = -17.2 cal mol-' K-l bent -24.53 cal mol-' K-l linear
Am(cm3
s-l)
5
10-5.72(T/298)2exp(M*/R) = 10-9.47 (bent) 10-11.07
The main contributions to the entropy difference are due to changes in the external rotation and the one-dimensionalinternal rotation about the zC. -6Hbond. All internal rotations are treated as free rotations and are calculated by using the expression
-
S, = 4.6
+ R 1n(Zr1"/tr) + R / 2 ln(T/298)
where I, are the reduced moment of inertia of the rotor and u = 1 is the symmetry number. Finally, the A factor can be calculated by using the expression
A = 10-5.72(T/298)2exp(AS#/R) where the AS#(303) value is calculated in Table 111. The calculated values for A are lO-I1.O7cm3 molecule-1 s-l for the linear TS and 10-9.47 cm3 molecule-' s-I for the bent TS. The experimental value of A obtained by our Arrhenius plot (Figure 3) is 10-9.4SMJcm3 molecule-1 s-1, which is in good agreement with the conventional transition-state theory (bent model). Our experiments suggest that the most efficient approach of C1 atom toward TMS for the abstraction of a H atom is along a direction that comes with an angle to the loose C. .H bond and is located within the methyl group and close to the Ha. .C. .Si plane. This nonlinear (bent) transition state is expected, since the incoming C1 atom is initially sensing the attraction by all three H atoms of a given methyl group and subsequently has to form a covalent bond with one H atom and compensate for the attraction by the other two. The obtained Arrhenius parameters can be compared with those of a similar reaction of C1 with (CH&C, where the central Si atom has been replaced by a C atom and is expected to have little effect on the reactivity of the attached CH3 groups. The reported A factor values and 10-9.53cm3 molecule-' s-1 are in reasonable agreementwith our value, while the activation energy values 0.7 and 0.9 kcal/mol are slightly higher than our value.7~8 The higher activation energy in the neopentane reaction is an indication of a slightly weaker C- -H bond in tetramethylsilane, which is in accordancewith the reported bond dissociationenergies D[(CH3)3SiCHrH] = 99.2 f 2 kcal/mol17andD[(CH3)3CCH2H] = 101.2 f 2 kcal/mol.l* Finally, the rate constant at 303 K for the reaction of C1 with (CH&C has been reported as 1.1 f 0.3 X 10-lo cm3 molecule-l s-l,19 which is in good agreement with the k~ value. A second comparison can be made with the reaction of C1 and SiH4. The A factor for this reaction has been reported by Ding et al.4 as (1.56 f 0.1 1) X 10-10 cm3 molecule-1 s-1, which differs by a factor of 2 from our value. This difference is reasonable
-
-
(linear) consideringthe larger size of (CH3)dSi relative to SiH4. Finally, an equally high A factor of 1 0-9.25M.2 cm3 molecule-1 s-1 has been found for the reaction of CH3 with (CH3)4Si.10 The rate constant k-1 for the reverse reaction can be estimated by using the forward rate constant kl and the equilibrium relation K, = k,/k-, = exp(ASo/R) exp(-AHo/RT) or k-l = k, exp(-AS"/R) exp(AHo/RT) where ASo and PHO are the entropy and enthalpy changes for the forward reaction. At room temperature ASo = 7 cal/(K mol), since Sro((CH3)3SiCH2) = 86.9 cal/(K mol) and Sro((CH3)4Si) = 85 cal/(K m ~ l ) . ~Similarly, J~ AHo = -4 f 2 kcal/mol by assuming that the strength of the C- -H bond in tetramethylsilane is 99.2 f 2 kcal/mol.17 Thus, the rate constant k-1 at 300 K will be approximately 10-14.2*1.4 cm3 molecule-1 s-1, which is at least 2 orders of magnitude lower than k l . Therefore, the reverse reaction cannot compete with the foward reaction, which is in agreement with our experimental results. Finally, the chemical reactivity of the (CH3)3SiCH2 radical is expected to be small due to a possible delocalization of the unpaired electron into the 3d orbital of silicon, which leads to stabilization of the radical.20*21A comparison with the hydrocarbon analogue, the neopentyl radical, shows that the reactivity of neopentylradical is less than that of other primary alkyl radicals, and this is due either to a stabilization by hyperconjugation or to a steric hindrance of the 8-methyl groups.22 The unimolecular decomposition of (CH3)3SiCH2 radical by eliminating a methyl group is not likely, since the bond energy D[(CH3)2(CH2)Si- -CH3] is ca. 29.6 kcal/mol.23 However, the dimerization reaction 3 has been studied in the past in the liquid phase,24 and the extrapolated gas-phase rate constant k3 at room temperature has been reported as 1.7 X 10-11 cm3 molecule-' s-1, which is an order of magnitude smaller than that of the primary reaction. Therefore, under our experimental conditions, both secondary reactions 2 and 3 could not compete, and the (CH3)3SiCH2radical was observed in the reaction products.
-
-
References and Notes (1) Jasinski, J. M.;Meycrson, B.S.;ScoJt, B.A. Annu. Reu. Phys. Chem. 1987, 38, 109. (2) Jensen, K. F.; Mountziaris, T. J.; Fotiadis, D. I. In 111-V HeterostructuresforElectronfc/Pho!onicDeuices;Tu, C . W., Mattere, V. D., Gcmard, A. C., Eds.; Proc. Mater. Res. Soc. 1989, 145, 107. (3) Jasinski, J. M.; Gates, S.M. Acc. Chem. Res. 1991, 24, 9.
6810 The Journal of Physical Chemistry, Vol. 97, No. 26, 1993 (4) Ding, L.; Marshall, P. J. Phys. Chem. 1992, 96, 2197. (5) Nib, H.; Maker, P. D.; Savage, C. M.; Breitenbach, L. P. J. Phys. Chem. 1985,89, 1752. (6) Krasnoperov, L. N.; Chesnokov, E.N.; Panfilov, V. N. Chem. Phys. 1984, 89, 297. (7) Knox, J. H.; Nelson, R. L. Trans. Faraday Soc. 1959,55, 937. (8) Pritchard, H. 0.; Pyke,J. B.;Trotman-Dickenson,A. F. J.Am. Chem. Soc. 1955, 77, 2629. (9) Arthur, N. L.; Potzinger, P.; Reimann, B.; Steenbergen, H. P. J . Chem. Soc., Faraday Trans. 1990,86, 1407. (10) Austin, E.R.; Lampe, F. W. J . Phys. Chem. 1977,81, 1134. 111) Moms. E.R.: Thvnne. J. C. J. J. Phvs. Chem. 1969. 73. 3294. (12) Golden, D. M.';SGkes,'G. N.; Benson,-S. W. Angew. Chei.,Inr. Ed. Engl. 1973, 12, 534. (13) Lazarou. Y.; Michael, C.; Papagiannakopoulos, P. J. Phys. Chem. 1992, 96, 1705. (14) DeMore, W. B.; Molina, M. J.; Watson, R. T.; Golden, D. M.; Hamwon. R. F.: Kurvlo. M. J.: Howard. C. J.: Ravishankara. A. R. Chemical Kineiics a'nd Photochemical Data for Use in-Stratospheric Modelling, J. P. L. Publication 83-62;Jet Propulsion Laboratory, California Institute of Technology: Pasadena, CA.
Lazarou and Papagiannakopoulos (i5) Benson, S. W. Thermochemical Kinetics, 2nd ed.; Wiley-Interscience: New York, 1976. (16) Bcagley, B.;Monaghan, J. J.; Hewitt, T. G. J. Mol. Srruct. 1971,8, 401. (17) Walsh, R. Acc. Chem. Res. 1981, 14, 246. (18) Cohen, N.;Benson, S. W. In The Chemistry of Alkanes and Cycloalkanes; Patai, S., Rapporport, Z.,Eds.; 1992; p 215. (19) Atkinson, R.; Aschmann, S. M. Int. J . Chem. Kfnet. 1985, 17, 33. (20) Wilt, J. W.; Kolewe, 0.;Kraemer, J. F. J . Am. Chem. Soc. 1%9,91, 2624. (21) Doncaster, A. M.; Walsh, R. J . Chem. Soc. Faraday Tram. I1976, 72, 2908. (22) Wu, D.; Bayes, K. D. Int. J. Chem. Kinet. 1986,18, 541. (23) Using available thermochemical data one can calculate D[(CH3)2(CH2)C- -CHI] = 22.2kcal/mol. In molecules, Si- -C bonds are stronger than the analogous C- - -C bonds (seeref 17)andD[(CH3)2(CH2)Si- -CH!] - D[(CH&(CH*)C- -CH,] = 7.4 kcal/mol. Therefore, assuming thls differenceholds in the radicals, then D[(CH3)2(CH2)Si- -CH3] is estimated to be 22.2 + 7.4 = 29.6 kcal/mol. (24) Watts, G. B.; Ingold, K. U. J. Am. Chem. Soc. 1972, 94,491.
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