Mass Spectrometric Study of Intermediates in the Photochemical

7 Mass Spectrometric Study of Intermediates in the Photochemical Oxidation of Diborane R I C H A R D F . P O R T E R and F . A . G R I M M

Downloaded by CORNELL UNIV on May 17, 2017 | http://pubs.acs.org Publication Date: June 1, 1968 | doi: 10.1021/ba-1968-0072.ch007

Cornell University, Ithaca, Ν. Y.

A mass spectrometric photolysis Total

of

low

sample

technique pressure

pressures

varied

with Β Η /O

ratio between

characterized

by two distinct

2

6

2

sive reaction slower

leading

initial

accelerates

presence

of B H 2

of H B O (g) 3

3

6

3

(a) an

(boroxine)

3

2

2

or (b) a

Reaction

3

time.

Analysis

of

of

HBO 2

2

and absence of O results in the 2

for the formation

of

of H B O . 2

HBO 2

2

3

2

is

A

3

b rate

mech

2

irradiation

is

explo

of O suggests that a chain

Continued

Hg

The reaction

to H B O (g).

and disappearance

3

mechanism

5 and 44 mm.

HBO 3

the

diborane-oxygen.

processes—either

irradiation

data for the disappearance anism is involved.

between

leading

with

of

5/1 and 1/5.

to gaseous

reaction

rapidly

has been used to study

mixtures

in

3

the

formation plausible

discussed.

* p \ i b o r a n e - o x y g e n reactions h a v e b e e n the subject of n u m e r o u s i n v e s t i gâtions.

T h e explosive r e a c t i o n , w h i c h c a n b e i n i t i a t e d t h e r m a l l y ,

has b e e n s t u d i e d i n some d e t a i l . I n a recent series of articles ( 1 0 ) , e a r l y w o r k o n this system is r e v i e w e d a n d m e c h a n i s m s for the explosive o x i d a t i o n are c o n s i d e r e d . G o l d s t e i n , et al. (5).

A pre-explosive r e a c t i o n has b e e n o b s e r v e d

by

A n a l t e r n a t i v e a p p r o a c h , as d e s c r i b e d i n this p a p e r ,

is to s t u d y the B H e - 0 r e a c t i o n p h o t o c h e m i c a l l y . I n these experiments 2

2

a p h o t o c h e m i c a l t e c h n i q u e is u s e d to s t u d y l o w pressure B H - 0 2

6

2

mix-

tures u n d e r c o n d i t i o n s that d o not n o r m a l l y l e a d to explosions at r o o m temperature.

A mass spectrometer w a s u s e d to a n a l y z e gases

issuing

f r o m a p i n h o l e i n a w a l l of the r e a c t i o n vessel. T h i s t e c h n i q u e has b e e n u s e d before o n flow systems to detect free r a d i c a l s (2, 3, 9). G o l d f i n g e r et al. (4)

I n 1961,

p r e s e n t e d a p a p e r o n the a d a p t a t i o n of a mass

spectrometer to s t u d y h i g h pressure static systems.

I n this p a p e r

we

discuss the use of a mass spectrometer to o b t a i n i n f o r m a t i o n o n t h e p h o t o c h e m i c a l r e a c t i o n of B H - 0 2

6

2

m i x t u r e s i n a static system. 94

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

7.

PORTER AND G R I M M

Apparatus

and

Photochemical

Oxidation

95

of Diborane

Experimental

T h e experiments w e r e c o n d u c t e d i n a s p e c i a l l y d e s i g n e d c e l l fitted w i t h a q u a r t z i m m e r s i o n w e l l ( F i g u r e 1). J o i n e d to the c e l l was a p i e c e of b o r o s i l i c a t e t u b i n g t e r m i n a t i n g i n a p i n h o l e . T h e p i n h o l e w a s m a d e b y the t e c h n i q u e of L o s s i n g a n d T i c k n e r ( 9 ) u s i n g a T e s l a c o i l d i s c h a r g e . T h e size of the p i n h o l e w a s a d j u s t e d u n t i l e n o u g h gas i s s u e d f r o m the h o l e to g i v e g o o d s e n s i t i v i t y o n t h e mass spectrometer. T h e pressure i n t h e mass spectrometer n e v e r e x c e e d e d 1 X 10 t o r r e v e n w h e n the pressure i n the r e a c t i o n vessel w a s a t m o s p h e r i c . P i n h o l e s p r e p a r e d i n this w a y w e r e f o u n d to b e s m a l l e n o u g h so that the pressure d r o p d u r i n g the course of the r a c t i o n w a s less t h a n 2 % . T h e mass spectrometer u s e d w a s a 1 0 " , 6 0 ° , d i r e c t i o n - f o c u s i n g i n s t r u m e n t . T h e m e d i u m pressure m e r c u r y l a m p a n d the q u a r t z i m m e r s i o n w e l l w e r e c a p a b l e of t r a n s m i t t i n g the c o m p l e t e s p e c t r u m f r o m the 1849-A. l i n e to the i n f r a r e d . D i b o r a n e absorbs i n the w a v e l e n g t h r e g i o n b e l o w a b o u t 2200 A . ( I , 8 ) a n d c a n also b e p h o t o s e n s i t i z e d b y m e r c u r y ( 6 ) .


5

Vacuum Stopcock To Vacuum Line To Mass Spectrometer

^Glass Disk with "pinhole" •Quartz Immersion Well

Figure 1.

Reaction cell

T h e r e a c t i o n c e l l w a s m a i n t a i n e d at a p p r o x i m a t e l y r o o m t e m p e r a t u r e b y flowing p r e - c o o l e d N a r o u n d the l a m p a n d m a i n t a i n i n g a v a c u u m of less t h a n 1 X 10" t o r r b e t w e e n the d o u b l e w a l l s of the i m m e r s i o n w e l l . A p i e c e of a l u m i n u m f o i l w a s p l a c e d a r o u n d the l a m p to p r e v e n t l i g h t f r o m r e a c h i n g the r e a c t i o n c e l l u n t i l the l a m p h a d s t a b i l i z e d . T h e e x p e r i m e n t w a s i n i t i a t e d w h e n the f o i l w a s r e m o v e d . E x p e r i m e n t s w e r e p e r f o r m e d b y a d d i n g first 0 a n d t h e n B H to the r e a c t i o n c e l l g i v i n g a m i x t u r e of k n o w n c o m p o s i t i o n . Since B H - 0 m i x t u r e s are e x t r e m e l y u n s t a b l e , a n d h i g h pressure m i x t u r e s are k n o w n to e x p l o d e , c a u t i o n m u s t b e exercised i n p r e p a r i n g a n d w o r k i n g w i t h these gas m i x t u r e s . 2

4

2

2

6

2

6

2

T h e d i b o r a n e w a s p r e p a r e d b y the m e t h o d of Jeffers ( 7 ) . grade oxygen was used without further purification.


Reagent

96

MASS SPECTROMETRY I N INORGANIC

CHEMISTRY

90 80 |3.0 70 2.6 60 |2.2 f

501 1.8

— »


40

χ

30

-1.4

20

-1.0

d0 dt \ o

S

10

-0.6

"0

20

40

60

80

100

120

140

160

180

0.2 220

200

Time (sec) Figure 2.

Plot of ion intensity for 0 (m/e = 32) and first derivative vs. time. Initial B and 0 equal 11.3 and 9.1mm. respectively 2

2

p

2

9

Results T h e course of the r e a c t i o n w a s f o l l o w e d b y c o n t i n u o u s l y m o n i t o r i n g t h e i o n i n t e n s i t y for one of t h e species as the m i x t u r e w a s i r r a d i a t e d . F o r 0

2

and B H 2

respectively.

6

i o n intensities w e r e o b s e r v e d for mass peaks 32 a n d 26, S h o w n i n F i g u r e 2 is a t y p i c a l p l o t of i o n i n t e n s i t y for

0

2

w i t h t i m e a l o n g w i t h a p l o t of the first d e r i v a t i v e . F i g u r e 3 illustrates the a p p e a r a n c e a n d d i s a p p e a r a n c e of H B 0 » a n d the d e l a y e d 2

appear

2

a n c e of H3B3O3. B y o b t a i n i n g the i o n intensities for 0

and B H

2

2

6

at the

b e g i n n i n g a n d e n d of the r e a c t i o n , it w a s possible to c a l c u l a t e t h e stoi c h i o m e t r y ( T a b l e I ) f r o m a c a l i b r a t i o n c u r v e of i o n i n t e n s i t y vs. pressure. It is i m p o r t a n t to note that a c a l i b r a t i o n c u r v e is necessary i f one is to relate i o n i n t e n s i t y to pressure. G o l d f i n g e r et al. (4)

f o u n d that i n t h e i r

a p p a r a t u s i o n i n t e n s i t y vs. pressure for a r g o n s h o w e d a b r e a k i n t h e p l o t at a b o u t 40 m m . pressure i n the r e a c t i o n c e l l . T h e p l o t w a s l i n e a r to 40 m m . a n d t h e n c h a n g e d slope a n d w a s l i n e a r to 400 m m . I n o u r case a l l pressures for a single species w e r e b e l o w 40 m m . a n d w e o b t a i n e d a l i n e a r p l o t w i t h no d i s c o n t i n u i t i e s . It w a s n o t e d that the s e n s i t i v i t y of t h e i n s t r u m e n t to B H « d e p e n d e d o n the r a t i o of B H / O 2

2

c e l l . It w a s f o u n d that for a g i v e n pressure of B H 2

0

0

2

i n the r e a c t i o n

the i o n i n t e n s i t y of


7.

PORTER A N D G R I M M

the m/e

=

Photochemical

Oxidation

of

97

Diborane

26 p e a k d e c r e a s e d u p o n a d d i t i o n of 0 . 2

The

H2B2O3-O2

system s h o w e d a m o r e p r o n o u n c e d effect i n t h e o p p o s i t e d i r e c t i o n w h i c h m a d e i t difficult to o b t a i n a n a c c u r a t e v a l u e for t h e a m o u n t of H2B2O3 produced.

T h i s effect w a s not o b s e r v e d for B H - H 2

i n t e n s i t y of t h e m/e

=

6

2

mixtures; the ion

26 p e a k d i d n o t c h a n g e u p o n a d d i n g H

to a

1

50.

ό /


2

100

200

300

V V

400

O

/

^

H

500

2

B

600

2°3

700

800

900

1000

Time (sec) Figure 3. Plot of ion intensity for H B 0 (m/e = 71) vs. irradiation time for a mixture of 0 and B H of 12.8 and 10.8mm. pressure respec tively, and ion intensity for H.B 0 (m/e = 83) vs. irradiation time for a mixture of 0 and B H of 13.3 and 10.0mm. pressure, respectively 2

2

2

:i

2

2

2

3

6

3

6

Table I. Experimental Determination of the Number of Molecules of 0 Reacting per Molecule of Β Η in Nonexplosive Reaction 2

2

β

Molecules 0 Reacting per molecule Β Η 2

Initial O, p

12.8 12.8 11.9 11.3 8.2 16.1 2.8 5.7 11.3 22.7 12.7 5.4 5.7 5.5 7.3

(mm.)

Initial Β Η ρ

α

6

(mm.)

2

11.0 11.0 9.3 9.1 16.2 7.9 2.5 5.1 10.3 20.3 9.3 5.0 6.9 6.5 5.8

2.60 2.54 2.04 2.18 2.45 2.51 2.60 2.96 2.51 2.13 2.86 2.84 2.30 2.30 2.10 2.46

e

e

α

Av.

e

e e e

e

e

e

e

" Indicates cell had initial coating of B2O3.


β

98


CHEMISTRY

g i v e n pressure of B H « . T h e presence of a s m a l l q u a n t i t y of 0 2

i n the

2

mass spectrometer f r o m the i n l e t leak m a y affect the s e n s i t i v i t y of the e l e c t r o n m u l t i p l i e r a n d / o r the emission characteristics of the i o n i z a t i o n filament. I n experiments u s i n g u n s c r a m b l e d

0 -

1 6

2

1 8

0

2

i n p l a c e of o r d i n a r y 0 , 2

i t w a s possible to observe the f o r m a t i o n of w a t e r , ( H

2

1 8

0 at m/e

w h i c h avoids the u s u a l l y h i g h b a c k g r o u n d of the m/e

=

=

18

20

peak),

d u r i n g the r e a c t i o n a n d to o b t a i n isotopic d i s t r i b u t i o n s i n the p r o d u c t s . T h i s d i s t r i b u t i o n for H B 0 2

2

3

is i l l u s t r a t e d i n F i g u r e 4.


Discussion T w o types of c h e m i c a l b e h a v i o r are o b s e r v e d w h e n l o w B H«-0 2

2

pressure

m i x t u r e s are p h o t o l y z e d w i t h short w a v e l e n g t h r a d i a t i o n f r o m

a m e d i u m pressure m e r c u r y l a m p .

T h e s e processes are either ( a )

an

explosive r e a c t i o n p r o d u c i n g H3B3O3, B O a , or a n u n i d e n t i f i e d b o r o n 2

hydride

(solid)

a n d H , or

a slower initial reaction,

(b)

2

producing

H B 0 , H , H 0 , a n d B C>3 f o l l o w e d b y the c o n v e r s i o n of H B C>3 to 2

2

3

2

2

2

2

2

H3B3O3.

O n l y R e a c t i o n b has b e e n s t u d i e d i n a n y d e t a i l , a n d w e w i l l confine o u r d i s c u s s i o n to this phase of the study. that a p p r o x i m a t e l y 2.5 molecules of 0 B H 2

6

H B 0 2

2

(Table I).

T h e stoichiometry indicated

are c o n s u m e d p e r m o l e c u l e

2

T h e rate of d i s a p p e a r a n c e of 0

a n d appearance

2

of of

are accelerated w i t h i r r a d i a t i o n times, a n d i n some cases a n

3

e x p l o s i o n is i n i t i a t e d . T h e i n t e r m e d i a t e p e r o x i d e , H B 0 , o b t a i n e d w h e n 2

Β Η({~( 2

1 Γ

2

3

Ό - 0 ) m i x t u r e s are i r r a d i a t e d , c o n t a i n o x y g e n atoms i n a n o n 2

1 8

2

statistical d i s t r i b u t i o n . T h i s i n d i c a t e d that m o l e c u l a r 0 i n t o the m o l e c u l e w i t h o u t r u p t u r e of the Ο — Ο

bond.

is i n t r o d u c e d

2

The following

m e c h a n i s m is consistent w i t h o u r observations. B H 2

+ h -»B H, + Η

e

v

(1)

2

or Hg + h - » Hg* v

Hg* + B H 2

( {

-> Β , Η , + Η + H g

+ M B H 2

r >

+ 0

->

2

OH + B H 2

(2) B H 0 + OH 2

-> B H

6

2

(3)

4

+ HoO

5

(4)

+ M B H 0 + 0 2

4

2

-»

2H-»H Net E q . : B H 2

f i

+ 20

2

hv -»

H B 0 2

2

3

4- 2 H

s

+ H 0 + H

(5)

2

H B O 2

2

2

2


PORTER AND G R I M M

Photochemical

Oxidation

of

Diborane


A

69 70 71 72 73 74 75 76 77 78

Figure

4.

Comparison of calculated 0- 0 tion in H B 0 with experimental 16

2

2

18

3

isotopic results

distribu

0.459 A.

Calculated from 0- 0 16

18

ratio =

B. Calculated from initial composition of = 0.424:0.0696:0.506

0>: 0 0: 0j

16

16

18

18

C. Calculated from the final composition of 0>: 0 0: 0> = 0.333:0.272.395 determined at the time the distribu tion in H>B;0.i was obtained. Note: This ratio is different from Β since isotopic exchange of 0> occurs photochemically 16

D. Ion intensity at m / e =

16

18

18

78 was corrected for background


100


CHEMISTRY

T h e inconsistency of the o b s e r v e d s t o i c h i o m e t r y w i t h the e x p e c t e d v a l u e b y this m e c h a n i s m is a t t r i b u t e d to f u r t h e r o x i d a t i o n of the p r o d u c t s . T h e t h i r d b o d y ( M ) i n E q u a t i o n s 2 a n d 4 w o u l d l i k e l y be the surface since w e k n o w that the rate of r e a c t i o n d e p e n d s u p o n the c o n d i t i o n of t h e surface. W e cannot neglect the p o s s i b i l i t y that some r a d i c a l s i n E q u a t i o n 1 m a y be p r o d u c e d manometers

by mercury photosensitization

w e r e u s e d to measure pressure.

w h i c h o i l manometers

(6)

2

2


step (6,

5

mercury

w e r e used, the rate of r e a c t i o n a p p e a r e d to

slower. It is possible to suggest r a d i c a l s other t h a n Β Η the c h a i n , b u t B H

since

I n several experiments i n

δ

be

for p r o p a g a t i n g

is a l i k e l y p r o d u c t for the p r i m a r y

photochemical

8).

The

mass

spectrometric

technique

as u s e d i n this s t u d y of

the

p h o t o c h e m i c a l o x i d a t i o n of B H

Mass Spectrometric Study of Intermediates in the Photochemical

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