Appearance Potentials, Ionization Potentials and Heats of Formation

18 Appearance Potentials, Ionization Potentials and Heats of Formation for Perfluorosilanes and Perfluoroborosilanes J. D . M c D O N A L D , C. H. WILLIAMS, J. C. T H O M P S O N , Downloaded by CORNELL UNIV on May 17, 2017 | http://pubs.acs.org Publication Date: June 1, 1968 | doi: 10.1021/ba-1968-0072.ch018

and J. L. MARGRAVE Rice University, Houston, Texas

A Bendix measure

time-of-flight

mass spectrometer

appearance

species containing

potentials silicon,

has been used to

and ionization

boron, and

potentials

fluorine.

From

data, a mutually

consistent set of average bond energies

thermodynamic

properties

has been

of these and

derived.

Τ η contrast to the l a r g e a m o u n t of d a t a r e p o r t e d o n the heats of f o r m a *

t i o n a n d b o n d strengths of h y d r o c a r b o n s a n d

fluorocarbons,

m u c h less

is k n o w n a b o u t t h e c o r r e s p o n d i n g s i l i c o n a n d s i l i c o n - b o r o n c o m p o u n d s . F o r t h e perfluorosilanes a n d b o r o n c o m p o u n d s , heats of f o r m a t i o n are reported for S i F B F (3). Si H 2

6

4

(12),

SiF

2

and S i F (I, 13), B F

3

(II), B F

2

(5)

and

T h e o n l y v a l u e f o r the s i l i c o n - s i l i c o n b o n d s t r e n g t h is b a s e d o n

d a t a (7),

a n d n o measurements at a l l h a v e b e e n m a d e o n t h e

s i l i c o n - b o r o n b o n d . A v e r a g e b o n d energies are u s e d to d e r i v e the o n l y p u b l i s h e d values of the heat of formation of S i F

3

I n this p a p e r ,

(8).

a p p e a r a n c e p o t e n t i a l studies, w h i c h l e a d to t h e i o n i z a t i o n potentials a n d heats of f o r m a t i o n of S i F , S i F , S i B F , B F , a n d S i F , are r e p o r t e d . 2

6

3

6

2

7

2

3

T h e s e are u s e d to c a l c u l a t e b o n d d i s s o c i a t i o n energies w h i c h m a y b e u s e d to c a l c u l a t e t h e r m o d y n a m i c p r o p e r t i e s of other

fluorosilanes.

Experimental T h e a p p e a r a n c e potentials g i v e n i n this w o r k w e r e m e a s u r e d o n a B e n d i x m o d e l 1 4 - 1 0 7 t i m e - o f - f l i g h t mass spectrometer. T h e i o n source electronics w e r e m o d i f i e d to p r o v i d e o p e r a t i o n a c c o r d i n g to the R e t a r d i n g P o t e n t i a l D i f f e r e n c e ( R P D ) m e t h o d (2, 6). W i t h t h e e l e c t r o n source filament at t h e e l e c t r o n e n e r g y p o t e n t i a l b e l o w g r o u n d , t h e five e l e c t r o n g r i d s of the s t a n d a r d m o d e l 14 i o n source w e r e c o n n e c t e d as f o l l o w s : 261

Margrave; Mass Spectrometry in Inorganic Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1968.

262

M A S S S P E C T R O M E T R Y IN ORGANIC C H E M I S T R Y

g r i d N o . 1, b i a s e d at — 5 volts d . c , p u l s e d to + 5 volts w i t h respect to the filament; grids N o . 2 a n d 4, + 1 v o l t d.c. w i t h respect to the filament; g r i d N o . 3, —1.0 or —1.1 v o l t d.c. to the filament to p r o v i d e a 0.1 v o l t R P D step; a n d g r i d N o . 5, at g r o u n d p o t e n t i a l . T h i s a p p a r a t u s , w h e n tested o n 0 , N , a n d the n o b l e gases, gave results s i m i l a r to those r e p o r t e d b y M e l t o n a n d H a m i l l ( 6 ) , b u t w h e n the s i l i c o n fluorides w e r e i n t r o d u c e d , some difficulty w a s e n c o u n t e r e d . T h e e l e c t r o n t r a p c u r r e n t , a n d h e n c e the i o n c u r r e n t , decreased w i t h t i m e , a n d the m e a s u r e d a p p e a r a n c e p o t e n t i a l of the s t a n d a r d ( N ) d r o p p e d several tenths of a n e.v. d u r i n g the experiment. B o t h effects w e r e p r o b a b l y c a u s e d b y the t h e r m a l d e c o m p o s i t i o n of the samples, d e p o s i t i n g a s i l i c o n m e t a l r e s i d u e o n the e l e c t r o n g r i d s . T h e effect w a s m i n i m i z e d b y o p e r a t i n g at l o w s a m p l e pressures ( 1 - 2 X 10" t o r r ) a n d c l e a n i n g t h e i o n source f r e q u e n t l y . T h e l o w e r signal-to-noise r a t i o c a u s e d b y l o w s a m p l e pressure r e s u l t e d i n r e d u c i n g the a c c u r a c y of measurement. T h e a p p e a r a n c e potentials ( A P ) w e r e c a l i b r a t e d against N / N , A P = 15.57 volts a n d H e / H e , AP — 24.58 volts ( 4 ) . 2

2

2

Downloaded by CORNELL UNIV on May 17, 2017 | http://pubs.acs.org Publication Date: June 1, 1968 | doi: 10.1021/ba-1968-0072.ch018

6

2

+

2

+

T h e S i F gas w a s p u r c h a s e d f r o m M a t h e s o n a n d w a s f r e e d of S 0 b y p a s s i n g i t o v e r i r o n at 8 0 0 ° C . S i F was p r o d u c e d b y flowing the S 0 - f r e e S i F over S i m e t a l at 1 1 7 5 ° C ; the gaseous p r o d u c t s w e r e l e a k e d i n t o the i o n source of the mass spectrometer b y a line-of-sight p a t h . A t t e m p t s to detect S i F i n the gaseous p r o d u c t s w e r e unsuccessful. T h e S i F , S13F8, a n d S i B F w e r e f r a c t i o n a l l y d i s t i l l e d f r o m the v o l a t i l e p r o d ucts f o r m e d b y h e a t i n g a S i — F or S i — F — Β p o l y m e r ( 9 , 1 0 ) ; the s a m p l e fractions w e r e a b o u t 9 7 % p u r e . 4

2

2

2

4

3

2

f i

2

7

Calculations T h e a p p e a r a n c e potentials m e a s u r e d for the p r o d u c t i o n of the v a r i o u s p a r e n t a n d f r a g m e n t ions are l i s t e d i n T a b l e I ; a l l h a v e a n e x p e r i m e n t a l r e p r o d u c i b i l i t y of ± 0 . 1 volt. T h e process of f o r m a t i o n of a g i v e n i o n m u s t b e chosen to p r o d u c e the most consistent set of results. I n the c a l c u l a t i o n s w h i c h f o l l o w the i n t e r n a l e x c i t a t i o n energy a n d excess k i n e t i c e n e r g y of t h e f r a g m e n t i o n are neglected. F o r this reason, o n l y those f r a g m e n t a t i o n reactions l i k e l y to i n v o l v e a m i n i m u m a m o u n t of i n t e r n a l e x c i t a t i o n energy a n d excess k i n e t i c energy w e r e u s e d i n the c a l c u l a t i o n s . A s a n e x a m p l e of the m e t h o d of c o m p u t a t i o n , c o n s i d e r Process 6: Si F = SiF 2

6

2

+

+ SiF

4

O n e has for the change i n e n t h a l p y , Δ Η , for this f r a g m e n t a t i o n r e a c t i o n Δ Η = ΔΗ,° ( S i F ) + I P ( S i F ) + Δ Η / ( S i F ) - Δ Η / ( S i F ) = 13.02 e.v. Γ

2

2

4

2

6

U s i n g the values for the heats of f o r m a t i o n of S i F a n d S i F g i v e n i n T a b l e I I a n d the appearance p o t e n t i a l of S i F f r o m S i F i n T a b l e I., this e q u a t i o n is s o l v e d for Δ Η / ° ( 8 ΐ Ρ ) = —565 k c a l . / m o l e . S u b t r a c t i n g 2

2

2

+

4

2

6


18.

MCDONALD ET AL.

Perfluowsilanes

263

and Perfluoroborosilanes

Process 6 f r o m Process 10 gives Si F 3

+ SiF =

8

2Si F

4

2

6

where AH

= 2AH/(Si F ) -

r

2

AH, (Si F ) e

6

e

B y i n s e r t i n g t h e v a l u e for A H ° ( S i F ) /

2

A H / ( S i F ) = 0.43 e.v.

e

4

calculated above a n d that for

6

A H ° ( S i F ) f r o m T a b l e I I , t h e h e a t of f o r m a t i o n of S i F /

4

3

=

aH,°(SiJFs)


Table I. Ion/Compound Si F Si F Si F Si F SiF SiF SiF SiF Si BF SiBF BF 3

8

8

7

2

5

2

4

4

3

2

Appearance

SiF

SiF^

2

Si F 2

6

12.89

+

15.71 16.20 27.35 28.75

+

+

11.29

+

c

Si F

Si BF

10.84 15.62 11.77 11.43

12.97 11.32

3

+

4

a

in Volts"

+

+

2

Potentials

+

2

is d e t e r m i n e d ,

8

- 7 5 4 kcal./mole.

14.30 (15.8) 13.02 14.16

8

2

15.51 13.45

15.42 13.21 11.56 15.36 11.95 14.67

+

+

+

Experimental reproducibility =

±

0.1 e.v.; Absolute accuracy =

±

7

0.3 e.v.

It is n o w p o s s i b l e to set u p t w o s i m u l t a n e o u s e q u a t i o n s i n v o l v i n g D(Si-F)

a v g

. andD(Si-Si)

a v g

.:

A H / ( S i F ) = 6 A H / ( F ) + 2ΔΗ/(Si,gas) 2

6

Atf/(Si F ) =8AH 3

8

/

(F)

e

+ 3ΔΗ/(Si,gas) -

B y s o l v i n g these, one finds D ( S i - F )

a v g

. =

141 a n d 142 k c a l . r e s p e c t i v e l y .

8D(SiF)

a v g

a v g

. - D(Si-Si)

. -

2

and S i F

4

.

a v g

2D(Si-Si)

138 k c a l . a n d D ( S i - S i )

58 k c a l . F r o m s i m i l a r c a l c u l a t i o n s o n S i F =

6D(SiF)

a v g

.

one gets D ( S i F )

A s a "best" value D ( S i - F )

a v g

.

w

— a v g

.

=

139 z t 3 is chosen.

W i t h t h i s v a l u e i n the a b o v e e q u a t i o n s one o b t a i n s

D(Si-Si)

10 k c a l .

. =

a v g

55±

T h e r e is a b r e a k i n the i o n i z a t i o n efficiency c u r v e for S i F Si F . 2

6

and

3

+

from

B e l o w the b r e a k the i o n i z a t i o n is a t t r i b u t e d to i o n p a i r p r o d u c t i o n

a b o v e the b r e a k to m o r e c o n v e n t i o n a l d i s s o c i a t i v e i o n i z a t i o n . W i t h

this

l a t t e r i n t e r p r e t a t i o n , Process

AH °(SiF ) f

8

=

-253

combined

with

k c a l . / m o l e as c o m p a r e d

5

with

Process —254

1

gives

kcal./mole

c a l c u l a t e d o n the basis of t h e average S i - F b o n d d i s s o c i a t i o n energy. using A H ° ( S i F ) f

IP(SiF ) 3

=

3

=

13.3 volts.

—253

kcal./mole

i n Process

1, one

By

calculates

Recent quantum mechanical calculations, h o w

ever, s t r o n g l y s u p p o r t I P ( S i F ) = 3

8.5 ±

1 volts [ H a s t i e , J . W . M a r g r a v e ,


264

M A S S S P E C T R O M E T R Y IN INORGANIC C H E M I S T R Y

J . L . ( u n p u b l i s h e d w o r k , 1967-1968)] a n d y i e l d t h r o u g h Process 1 the value A H ° ( S i F ) = f

-235

3

v a l u e for S i F

3

±

20 k c a l . / m o l e .

give A H ° ( S i F ) = f

2

r i

i n Process 4 y i e l d s I P ( S i F , ) = 2

r

—358 ±

7.5 ±

Process 9 a n d this l a t t e r

20 k c a l . / m o l e a n d this heat

1 volts. A c o m b i n a t i o n of a p p e a r -

ance potentials for Processes 8 a n d 12 y i e l d s A H ° ( S i B F ) = £

2

—637

7

±

25 k c a l . / m o l e . W i t h this v a l u e a n d Process 14, one calculates A H ° ( SiBF >) f

=

r

—454 ± 25 k c a l . / m o l e a n d f r o m Process 11 one derives A H ° ( B F ) f

-150 ±

20 k c a l . / m o l e . Process 15 y i e l d s I P ( B F ) — 2

9 ±

process s i m i l a r to that u s e d for s i l i c o n , one computes D ( B - F ) * 3 kcal./mole and D ( B - S i )

a v g

. =

55 ±

=

2

1 volts. B y a v f ?

.=

152

±

15 k c a l . / m o l e .

T h e results of these c a l c u l a t i o n s are s u m m a r i z e d i n T a b l e I I I .

The


uncertainties l i s t e d are o b t a i n e d b y a s s i g n i n g a n e r r o r of 0.1 v o l t or 2.3 k c a l . to e a c h a p p e a r a n c e p o t e n t i a l u s e d i n c a l c u l a t i n g a g i v e n q u a n t i t y . T h e final d a t a h a v e b e e n u s e d to c a l c u l a t e a n a p p e a r a n c e p o t e n t i a l f o r e a c h of t h e reactions i n T a b l e I I I . A l t h o u g h i n some processes t h e m e a s u r e d a n d c a l c u l a t e d values agree to w i t h i n 0.1 e.v., i n others—e.g., the p r o d u c t i o n of S i F

2

+

f r o m S i F , i n t e r n a l excitation energy a n d excess 4

k i n e t i c energy considerations are i m p o r t a n t , a n d the difference

between

t h e m e a s u r e d a p p e a r a n c e p o t e n t i a l a n d that c a l c u l a t e d for the "best fit" process is l a r g e r . Processes i n v o l v i n g e x c i t e d fragments w e r e not u s e d i n the c a l c u l a t i o n s of t h e r m o d y n a m i c properties g i v e n i n T a b l e I I . Table II.

AH °

Species SiF SiF SiF SiF Si F Si F Si F BF SiBF Si BF

f

3

4

5

2

6

3

8

2

5

2

7

D(Si-F) D(Si-Si) D(B-F) D(B-Si) . a v g

a v g

a v g

a v g

= 139 ± =55 ± =152 ± = 55 ±

1

7.3 ± 0 . 2 11.29 ± 0.1 8.5 ± I 15.7 ± 0.1 7.5 ± 1 (10.6 ± 1.0) 10.84 ± 0.1 9 ± I (11 ± 1.0)· (10.6 ± l ) c

5 20 ± 0.2 25 5 10 20 20 25

e

e

a

3kcal./mole 10kcal./mole 3kcal./mole 15 k c a l . / m o l e

Parent ion not observed. * Other quantities used in calculations:

a

AH °(F) àH °(F~) Atf,°[Si(g)] AH °[B(g)] AH °(BF ) r

f

r

r

c

3

= = = = =

6

IP (volts)

(kcal./mole- )

-2 ± 3 -136 ± -235 ± -386.0 -358 ± -565 ± -754 ± -150 ± -454 ± -637 ±

2

2

Heats of Formation and Ionization Potentials

18.8 ± 1 kcal./mole - 6 4 . 8 ± 1 (4) 105 ± (4) 133 ± 3 (4) - 2 7 1 ± 2 (11)

(4)

Hastie, J . W . , Margrave, J. L . (unpublished work, 1967).


a

18.

MCDONALD E TAL.

Perfluorosilanes

265

and Perfluoroborosttanes

Table III. Probable Fragmentation Reactions and Calculated Appearance Potentials Calc. A P , volts a


Process 1.

SiF = SiF

3

2.

SiF = SiF

2

4

4

Meas. A P , volts

+

+ F

16.2 ± 0.5

16.20

+

+ 2F

23.6 ± 0.5

27.35

3.

SiF = SiF + 3F

28.4 ± 0.5

28.75

4.

Si F = Si F

12.9 ± 1.0

12.89

5.

Si F = SiF

6.

Si F = SiF

7.

Si F = Si F

8.

Si F = Si F

9.

Si F = SiF

10.

Si F = SiF

11.

Si BF = Si F

5

12.

Si BF = Si F

4

13.

Si BF = SiF

14.

Si BF = SiF

15.

Si BF = BF

α

+

4

2

0

2

2

6

2

6

3

8

3

8

2

7

2

7

2

+ SiF

+

+ SiF

3

2

7

4

13.02

14 ± 1

11.77

—

11.43

+ SiF

4

+ Si F

5

+ Si F

+

2

2

2

7

2

15.8

13.0 ± 0.5

2

7

2

12 ± 1

3

+

4

+

3

+ SiF

+

5

2

8

3

2

+

2

8

3

3

+ F-

+

5

2

2

15.51

13.5 ± 0.5

13.45

+

+ BF

2

13 ± 1

12.97

+

+ BF

3

—

11.32

+

3

6

15.5 ± 0.5

+

+

+ SiBF

4

15.7 ± 1.0

15.42

+ SiBF

5

13 ± 1

13.21

15 ± 1

14.67

+ Si F 2

5

6

Based on Table II data.

(Meas.-Calc. Values) > experimental error; fragment may possess kinetic energy, or may be in excited electronic or vibrational states, or the reaction products may be different than assumed. 6

Conclusions F r o m these d a t a , o n e c a n n o w p r e d i c t t h e heats o f f o r m a t i o n o f Si-B-F

compounds

b a s e d o n t h e f o l l o w i n g average b o n d

dissociation

energies D(Si-F) D(B-F)

.

=

139 ± 3 k c a L / m o l e

.

=

152 ± 3 k c a l . / m o l e

a v g

. =

5 5 ± 10 k c a l / m o l e

.

55 ± 15 k c a l . / m o l e

a v g

a v g

D(Si-Si) D(Si-B)

a v g

=

F o r t h e m o n o s i l i c o n species o n e c a n c a l c u l a t e i n d i v i d u a l b o n d disso c i a t i o n energies as f o l l o w s : D(SiF -F)

=

170 ± 10 k c a l . / m o l e

D(SiF -F)

=

118 ± 10 k c a l . / m o l e

D(SiF-F)

=

153 ± 5 k c a l . / m o l e

D(SiF)

=

130 ± 3 k c a L / m o l e

3

2


266

MASS SPECTROMETRY I N INORGANIC

CHEMISTRY

Acknowledgments T h i s w o r k has b e e n s u p p o r t e d w i t h f u n d s f r o m t h e A d v a n c e d R e s e a r c h Projects A g e n c y , t h r o u g h t h e A r m y R e s e a r c h Office i n D u r h a m , and b y the Robert A . W e l c h Foundation.


Literature

Cited

(1) Ehlert, T.C.,Margrave, J. L.,J.Chem. Phys. 41, 1066 (1966). (2) Fox, R. E., Hickam, W. M., Grove, D. J., Kjeldass, T., Jr., Rev. Sci. Instr. 26, 1101 (1955). (3) Hildenbrand, D. L., Murad, E., J. Chem. Phys. 43, 1400 (1965). (4) Kiser, R. W., "Tables of Ionization Potentials," U. S. At. Energy Comm. TID-6142 (June, 1960). (5) Margrave, J. L.,J.Phys. Chem. 66, 1209 (1962). (6) Melton, C. E., Hamill, W. H., J. Chem. Phys. 41, 546 (1964). (7) O'Neal, H. E., Ring, Μ. Α., Inorg. Chem. 5, 435 (1966). (8) Stull, D. R., editor, "JANAF Thermochemical Tables," No. PB-168-370, Clearinghouse for Federal Scientific and Technical Information, Spring field, Va., August, 1960. (9) Timms, P. L., Kent, R. Α., Ehlert, T.C.,Margrave, J. L., J. Am. Chem. Soc. 87, 2824 (1965). (10) Ibid., 87, 3819 (1965). (11) Wise, S. S., Feder, Η. M., Hubbard, W. N., Margrave, J. L.,J.Phys. Chem. 67, 2157 (1961). (12) Ibid., 67, 815 (1963). (13) Zmbov, K. F., Hastie, J. W., Uy, M., Margrave, J. L. (unpublished, 1967). RECEIVED October 11, 1966.


Appearance Potentials, Ionization Potentials and Heats of Formation

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