Chemical-ionization mass spectrometry of chlorosilanes

221 k&rna~ionat Jo&mudof iifm Spccrrome~ and Ion Phpics, 21 (1976) 221-230 @ Ekevkr Sckncific Publishing Company~ Amsterdam -Printed in l-he Ncth~ds

CHEMICAL-IONIZATION SILANES

J. R. KRAUSE,

P. PDTZlXGER

MASS

AND

SPECfRO%¶ETRY

OF

CHLORO-

B_ REIMAbW

Ins!itut fcir Straklenchenzie im 2&f--PI&-fditut &dir (FE cc-n-r)

fEE Kohle&omchmg,

D-4330 Miilheim ad

(Received 9 Dcce-m.bcr 197.5)

ABSIRACT

Chemical-ionimtion mass spectra for chlorosilane, trichiorosilane and tetrach!orosi!ane have been obtained using methane as the reactant gas_ Three types of ionic-transfer reactions were detected, Proton transfer to sample molecules was found for al! three chlorinated sil;on compounds and was the dominant chemicaEionization reaction at higher ion-source pressure. Hydrideion transfer took place if the sampie molecule contained at least one Si-H bond and chloride-ion transfer occurred in those systems where the sampIe molecule contained three or four S&C! bonds_ Reaction mechanisms are proposed which account for the formation of the product ions observed and the relative rate constants for the chemical-ionization processes have been determined_

In a recent publication we reported the rcsu!ts of our chemical-ionization (CI j studies of silane, disilane and the methy!si!anes in methane [I]_ Even tioua proton transfers from CHs’ and C,H,’ are thermochemica!!y feasible reactions, it was found that the only CI processes which took p!ace were hydride-ion transfers from sample molecules to reactant ions_ Evrimenta! evidence indicated that the relative degree of hydride-ion transfer increased with the number of methyl groups bound to silicon- This-W% att?iiuted to ad inductive effect of the methyl goups which leads to a charge distribution favouring hydride transferThe current investigation was undertaken to examine the effect of electronwithdrawing ligands such as chlorine. It was intended to compare the tyPes and rates of the reactions of Si&I,C!, SiHCI, and %I, with CH5‘+ and C2H5’.

EXPERIMENTAL

The mass spectrometer has been described previously [I]. In all experiments the repeIIer fieId strength, E(V cm-‘), inside the ion-source box was set at 20 S E I 25 because it was found that the collection efficiency for CI product ions formed at source pressures greater than 02 torr was a maximum at these settingsAt the same time it was noted that the observed intensity ratio of CH5 * to CtH5+ decreased with increasing repeller field. This effect has been found previously and results from the fact that the crosssections for formation of CHs’ and C2HsC depend somewhat dil%erentIyon ion-source repeller fieId strengths [3]_ In pure methane, using low repelIer fieId strengths, we have indeed observed IcH5+J I CzHs*G 50 : 40 staying constant up to 1 torr. The chiorinated silanes were obtained from commercial sources or prepared by iiterature procedure [2]_ The methane used was high-purity grade purchased from the Matheson Company- Mass-spectrometric analysis revealed no major impurities in any of the substances usedAll ionic intensities were corrected for contributions from the naturalIy occunin,o isotopeS_ RESULTS AND DISCUSSION

The relative abundances

of the ions in the CH.&CIH3

as a function of ion-source pressure in Fig_ 1.

4

f

*

I,

.I

I

I

O-2 O-3 O-4 O-5 O-6 O-7 bnSourceRtuuc(Toe~)

I

A

H30+

system are plotted

223 It is notabIe that the processes leading to chemical ionization are much Iess effective than in the cases we investigated earlier [I 3; this is also true for a11other chlorosilanes reported here_ The oniy product ions from the chemical ionization are SiCII&+ and SiCiHJ* and HsO*_ It was not possible to remove water completely from the system and its presence, even in trace amounts, is noted by the appearance in the mass spectrum of a peak at m/e 19 due to H30i_ However from the form of the’ H,O’ curve in Fig_ I itwas concluded that H,O’ is a product ion that, Iike C3HSt, does not undergo any further reactions_ The SiCI&* ion can only be formed by proton transfer_ Both major ions of our system, CH5+ and C,HS*, can readily transfer a proton to an acceptor which is a stronger base than methane or ethylene, * respcctiveIy_ Since CH, + is the major reactant ion in a11cases, a11processes are iliustrated in terms of this ion_ If SiCIH,’ and SiCIH,’ Fre formed by proton transfer followed by stabilization and dissociation respectively of an activated intermediate, viz eqns_ (l)-(3) CH, *f

SiCIH, + (SiCIH,*)*+CH,

(SICIH,“)*

--+ SiCIH,*

(SiCIH,i)*+CH,

(I)

+ Hz

(2)

--+ SiClH59 +CH,

(3)

it can easily be shown that the relative yield of product ions should be dependent on the methane concentration given by ‘SiCll?.z*

+lSiCU*=*

~SiClJfa+

=

k,[CH,]

+ k,

(4)

bCCH,f

However, when plotted as a function of ion-source pressure the ion current ratio does not approach the limiting high-pressure value of I predicted by eqn_ (4) but instead revels off at a value some 40 T< hi&er_ CkarIy a proton-transfer mechanism where the product ion is only partially stabilized does not account fulIy for the measured product-ion intensity mtio. One has to look for anothq reaction which also yields SiCIH,*.

Hydride

transfer may lead to this ion:

CH,‘+SiCIH,

+ SiCIH,++CH,+H,

(5)

It is we11known from previous mass-spectrometric studies that hydride-ion transfer to CHs * is a major reaction in the ion-molecule chemistry of silicon-hydridemethane mixtures [I, 4-7]_ Assuming a steady-state concentration of (SiCIH*‘)* it follows that

A plot of the product-ion ratio against l/[CHJ

should yieid a straight line with

224

I !SiCIH++ %iCI"+ 4 I

2

-

ECIH;

t_ 2

I-

t 1

I

I 2

I

1

I 6

Fig- 2 PIot of prbduct-ion intensityratio for SiClH, v_ reciprocal presume

-l-ABLE

I

-l-EVE

b flbkuiikc,-

RATE

COSSGXSSS

FOR

PROXOX

AXi

SECATLVE-X0X

77LSXSFER

REACTlOSS

CI&-SiCIH~

CH.&iCI~H

CH.,-SiCI*

95 -

10 5

l-5

intercept (k1fk5)/kI and slope equal to k#, (kl i-k&k,. The relative rate constantsso derivedfrom the plot of Fig_ 2 are given in Table 1,

The pressure-dependence of the relativeionic abundancesof the reactant - ad product ~OIE is graphedin Fig_ 3_ The pIot of ee relativeabundanceof &Of at m/e 19 has been negkcted _ becauieits maximum -valueis ~ca,3 % and its presexxe,as exphxinedearlier, does _-

225

O-0

O-1

O-2

O-3-

O-4

Ion soura

O-5

O-6

Pressure(Tocl)

Fig_ 3_ The pressure-dependence of ion abundances for rhe mixture C&-SiCiaHposition 200 : I_ Repeller field strength 20 V cm-‘_

Sample com-

not vitiate our treatment of the expzrimentaI msu&_ The product ions from the methane chemical ionization are SiCI,H=+, SiCl,+ and SIC&H*_ The presence of these ions suggests that proton transfer which may either from CH,+ to SiCI,H gives rise to an activated (SiCI,H2*)* dissociate by loss of HCI or Hz or be stabilized by collision_ The fact that a plot of product-ion intensities vs. pressure does not approach the value of 1 at the highpressure limit would seem to indicate that some of the trivalent ions are being formed by direct hydride- or chloride-ion transfers from SiC13H to reactant CH5 + ions_ It is reasonable to expect that hydride-ion transfer is taking place since the hydride affinity of CH,+ is 16 kcal mol- r larger than the hydride aI%nity of Sic1 3+ (AP(SiC13i) = 11.91 eV [S]). Hydride transfer alone cannot account for the experimental resu1t.ssince the same intercept is not seen in either Fig_ 4_ or Fig_ 5_ Chtoride-ion transfer, on the other hand, is thermochemicaIly feasibte if the appearance potential of SiCI,H+ 5 1290 eV_In that ease thechloride affinity [9] of SiCIIHt given by -M? of reaction (7) SiCIzH+ -ICI-

4 SiCI,H

(7)

would be less than the chloride aI3inityof CHs+ calculated from - AJY of reaction PI CH,+ -ICI-

--, CH,+HCI

(S)

226 From appearance-potential measurementsit is known [lo] that in the ion the SiCl bond is ca, 0.8 eV strongerthan the Si-H bond (AP(SiCI,H+) - 12.7 eV) and hence that chloride-ionabstractionby CH,’ is an exothermicprocess_ The folIowinSmechanismis proposed to account for the product-ion formationCH5* -I-SiCi,H + (SiC13Hzt)*+ CH, (SicI,H,*)*

(9)

--, SiC13++-Hz

(10)

(SiC13H2*)* --, SiCI,H* t HCI

(W

(SiC13H2*)*+CH,

(12)

CH,’

+ SiC13H2* +CH,

+SiCI,H --, SiCI,c +CH,+H,

CHs *tSiCI,H

03)

-P SiCIrHC+HCi+CH,

(14)

From a kineticana.I_ysis and assuminga steady-stateconcentrationof (SiC13Hzi)* one cbtains in anaIo_gy to eqn. [6) the expression ISiCI$f:+

+lSiCI,eilSiCI*H+ I SiCI3H=+

=-

1

_

(kl&k,

D-L1

b2

,) &+k,&;r5*J k9

From the same kineticanalysisit follows that

The interceptsof Figs_ 4 and 5 may be used to soIve for simuhaneousvaIuesof

k,, k, 3 and kl,_ The vahresso obtainedare summarizedin Table l-

The efEct of source pressureon the relativeintensitiesof ions in the CH4SiCI, systemis ilhstrated in Fig 6.ThereIativeabundanceof H30+ is again small and has been ne&ctecL The only product ions from the chemical ionization are SiCI,H’ and SiCI,*. The_SiCI, + ions could be formed by ebmination of HCi from the protonated intermediate(SiC14H*)* or by direct chloride-iontransfer from SiCILto CHs+ or to CzHs+ which seems to pIay a greaterrole here than in the other two cases e&m&e&- The chloride a5ity of CH,+ is 10 kcal mol-* greater $rau the ichtorideat&&y of SiCi3* (AP(SiCI,*) = 12.48 eV [S]). The folIowing me+anism-is proposed to a&u& for the pr&sure-dependenceof the _ _ _ _

-

_.

I

_

.-__

__-

227

I

1

I 1

1

I

I

2

5

,,,CH413U0,1:

-

Fig- 4_ Plot of product-ion intensityratio for SiCLH v- reciprocal pressure

I siC13H2-f+ I

sia;

%iCI_H:

I

2

3 &I

(TL?)

6

7

8

9

d Fig_ 5_ Piot of product-ion intensityratio for SiCllH v. reciprocal pressure-

0CH;

0

C2H;

D

SiC14H' +

m SKI3

iA

0.3

0.4

0.5

0.6

Ion !source Rtnwe

Fig-6_Thcplzssurc

-dcpatdma

positioiz 200 ; I- Rep&x

=3Hs

(brl)

of ion abundanas

for the misturc CI&-SK&-

Sampk com-

fkid stmnth 2OV an-‘-

product ions observed C&+

+SiCI,

+ (SiCI,H*)*

+ CH,

(171

(SiCI,Ht )* + SiCI 3+ +- HCI

(W

(SiCI,H ‘)* f CH,

(19)

Ci-I,‘-f-SiCI,

-P SiC14H+ + CH,

+ SiCl,* +CH,+HCI

P-9

Using the steady-state assumption for the activated intermediate the product ratio plotted in Fig 7 is given by

as

I SiCtaH+

kt8

flSiCts+

I Sic&H

*

=

&I19

(~t+~iO) k,,

-f

kT+bd

@0

47

The values of the relative rate constants cafculatcd are given in Table I. Proton and negatic-ion transfer Three types of ionic-transfer reactions were found in the current CI studto chIorosiIanes and hydride und chloride-

ies, namelyproton transferfnom CHs*

ion tknsfkrs from the chlorinated silanes to reactant CH,* ions. - Proton transfer from reactant ions to sample mokcuIes is a major reaction pathway _ -- of the ChIorosiIane systems, This is not a rezsuitthat one wouId -_. i&all three .- -

- _

Fig- 7, PIOLof product-ion

intensity ratio for SiCi, v. mciproul

prcssunz_

on the basis of previous mass-spectrometric studies <1,4-7) It is known for example [7] that onIy insignificant amounts, if any, of SiH,’ are formed by the proton-transfer process

anticipate

CHs*-t-SiH,

--+ SiHs++CH,

(22)

In the current studies however the rate constants for proton transfer to SiCIH, and SiCI,H (k&) are greater than the rate constants for hydride-ion transfer (kH- ) by factors of 2.5 and 10 respectiveIy, and are also greater than the rate constants for chloride-ion transfer (k,-)_ Chloride-ion transfer has not previously been observed for silicon compounds but halide-ion transfer processes have been found in studies of ion-molecule reactions of structurally similar hatogenated methanes [ll-131. As in the carbon system it was noted in this study that chIoride-ion transfer to hydrocarbon ions was a signiikant reaction when the reactant mokcule contained more than two chlorine atomsThe chlorosilanes therefore differ significantly from other silicon-hydride compounds. Replacement of only one hydrogen atom by chIorine in silane dramatical~y reduces the propensity for hydride transfer and permits a proton transfer to the apparently more nucleophilic siiicon atom, Attaching three chlorine atoms to sibcon increases the rate constant for proton transfer and also gives rise to chloride transfer_ Going from SiHCIa ?o SiCI, (Table 1) the rate constant for

-230 proton transfer is chauged very fittIe but chloride transfer is very much enhancedOur experimental i%ndingsmay be interpreted by the same simple reasoning used for explaining our results in the chemical ionization of silanes and methylsihnes_ While electron-donating ligands like methy groups increase the hydride character of the H atom in the Si-H bond leading exciusively to hydride-transfer reaction,

e?essron-withdrawing

groups

on the other hand decrease

the ekctron

density at the H atom and may even increase the electron d,xxsity at the Si atom by back-bonding- Proton transfer is therefore able to com;xte succcssfu!ly with hydride transfer-

We thank Dr_ A_ Ritter and Dr_ J- Leitich for valuable discussions.

1 J- R KKMJSC and P- Powiaga, IIU. J- IWESSSpecmm fan Ph>x, IS (1975) 303_ t k G- MacDkrmid, in W- L. Jolly (Ed.), Prcpararice lnerganic Rcactiorts, VoL 1. Inwscicncc, New York, 1964, pp- MS-20x 3 F- H- FieI& J- t FmnkIin and F- W_ Lampc. J_ &ner_ Chem_ Sot-, 79 (1957) z419_ 4 c- W- Stcwar~ J- M- S- H&s and P- P- Gaspar, I- Chem- Ph_rs_,52 (1972) 1990. 5 T- M- EL Ckng, -I-- Y- Yu stud F- W_ Lampe, J- Phj-s Chem, 77 (1973) 2587, 6 G- W- Stcsut, J- M- S- Hcnis and P- P- Gasp, J- Cfzem~P&s., 57 (192) 2247_ 7 M- D- Scfcik, J- M- S- Harir and P- P- Gaspars J- Ckenz- P&s-, 61 (1974) 43299. 8 W- C- S~cck, L D- Nicfrols and E G- A- Stone, J- Anwr- Chem Six_, 84 (1962) 444L 9 J- EL J- Dawson, W- G- Henderson. R M- o’hfaky and K- R- Jcnnisp InraJ_ Afuss Sjucrronzkm Phys, II (1973) 61_ 10 P- Potzinger. A- Rittcr and J. Krasc, 2. Nuturforsrfz~A, 30 (1975) 347. 1I N- A- McAskiU, Am_ I_ Chetm, 23 (1970) 893_ 12 A- G- Hanimn a118N- k hlkAskilJ, Anrr, L C/em_, 24 (1971) 1611_ 13 I- L- Buucixunp. D- Hok, S- D_ Woodgate and S_ L- Pact, J- Amer- t&m Sk., 94 (1972)