14 Analysis of Boranes and Carboranes by Mass Spectrometry
Mass Spectrometry in Inorganic Chemistry Downloaded from pubs.acs.org by MIDWESTERN UNIV on 01/23/19. For personal use only.
J . F . D I T T E R , F . J A M E S G E R H A R T , and R O B E R T E .
WILLIAMS
Space-General Corporation, Center for Research and Education, Los Angeles, Calif.
Boranes mass
and carboranes
spectral
fragment
each have their own
patterns.
rather
Boranes
severely,
the
more so than the "stable," as their alkylated spectral
profiles.
resistant
Conversely,
have sharp
cut-off
and carbon C-containing
peak,
somewhat
they, as
broad, rounded
the stable
class of
points
at their
high
compounds,
ion that appears for
above
CΒΗ , 4
6
12
the
quite profiles
mass numbers.
involves
of the mass spectral
as illustrated
are
the exact number
atoms in carboranes
of
careful normal
dimethyl
A boron
measure
peak owing the
well mass
and hence their spectral
for determining
ment of the intensity 13
impact
ones
(and their alkyl derivatives),
to fragmentation,
useful technique
"unstable" display
characteristic
electron
and consequently
derivatives,
the closo-carboranes
under
to the cut-off
derivative
of C Β Η . 2
Various
6
8
i o n g r o u p s i n the mass s p e c t r a l patterns of b o r o n h y d r i d e s
( n a t u r a l a b u n d a n c e of
n
B/
1
0
B =
4.0) h a v e c h a r a c t e r i s t i c a l l y b r o a d ,
r o u n d e d profiles that g e n e r a l l y a l l o w easy i d e n t i f i c a t i o n i n c h e m i c a l m i x tures (24).
I n e a c h i o n g r o u p the b o r o n isotopes are essentially d i s
t r i b u t e d i n a statistical m a n n e r , a n d , as a consequence, the greater the n u m b e r of borons i n a p a r t i c u l a r i o n g r o u p , the f a r t h e r r e m o v e d w i l l b e the p e a k of m a x i m u m intensity f r o m the t o p mass n u m b e r . A n e v e n m o r e i n f l u e n t i a l factor i n d e t e r m i n i n g the p o i n t of m a x i m u m i n t e n s i t y , h o w ever, is the ease of a b s t r a c t i o n of h y d r o g e n atoms, a n d i n boranes this a b s t r a c t i o n occurs r e l a t i v e l y easily. W i t h o u t e x c e p t i o n , at n o r m a l i o n i z i n g voltages, the i o n of h i g h e s t i n t e n s i t y has f e w e r h y d r o g e n s t h a n the p a r e n t molecule.
F u r t h e r m o r e , these same s p e c t r a l characteristics are c a r r i e d
over to a l k y l d e r i v a t i v e s of t h e boranes. 191
192
MASS S P E C T R O M E T R Y IN INORGANIC C H E M I S T R Y
Figure
1.
Molecular
configurations
of typical
boron
hydrides
O n the other h a n d , for the stable closed-cage cZoso-carboranes
the
f r a g m e n t a t i o n patterns are representative of m i n i m a l h y d r o g e n abstract i o n . ( T h e terms closo- a n d nido-, p e r t a i n i n g to carboranes, w e r e a d o p t e d at the latest m e e t i n g of the B o r o n N o m e n c l a t u r e C o m m i t t e e , A m e r i c a n C h e m i c a l Society M e e t i n g , N e w Y o r k C i t y , S e p t e m b e r 12-16, 1966. )
The
s p e c t r a l patterns d i s p l a y s h a r p cut-oflEs at the h i g h e s t mass n u m b e r s , a n d the h i g h e s t i n t e n s i t y p e a k almost i n v a r i a b l y is that of the p a r e n t i o n ; i n a f e w cases the i o n w i t h one or t w o h y d r o g e n s a b s t r a c t e d is of s l i g h t l y h i g h e r i n t e n s i t y t h a n the p a r e n t i o n . T h e second, t h i r d , etc. generations of ions ( those w i t h one, t w o , etc. b o r o n a n d / o r c a r b o n atoms a b s t r a c t e d ) h a v e intensities that are o n l y s m a l l percentages
of t h e i r p a r e n t
group
14.
DiTTER E T A L .
intensities.
193
Boraties and Carboranes
A s w i t h boranes, these characteristics are reflected i n the
r e l a t e d a l k y l a t e d cZoso-carboranes, a n d once a g a i n the o v e r - a l l profile serves as a r e a d y " f i n g e r p r i n t " for q u i c k i d e n t i f i c a t i o n .
Figure
2.
Molecular
configurations
of typical
B e c a u s e of the
closo-carboranes
194
MASS S P E C T R O M E T R Y IN INORGANIC C H E M I S T R Y
resistance
to h y d r o g e n
a b s t r a c t i o n , cZoso-carborane
spectra
resemble,
s o m e w h a t , the s p e c t r u m of e l e m e n t a l b o r o n . N i d o - c a r b o r a n e s , w h i c h h a v e o p e n m o l e c u l a r structures a n d w h i c h g e n e r a l l y c o n t a i n h y d r o g e n b r i d g e b o n d i n g , resemble boranes m o r e t h a n t h e y r e s e m b l e cZoso-carboranes
i n t h e i r f r a g m e n t a t i o n patterns. A t least
this is the case w i t h the f e w nido-carboranes that h a v e b e e n s y n t h e s i z e d a n d a n a l y z e d mass s p e c t r a l l y . I n the f o l l o w i n g discussion the s t r u c t u r a l configurations of
boranes
w i l l b e c o m p a r e d w i t h the t w o classes of carboranes, a n d s t r u c t u r a l d i f ferences w i l l t h e n b e r e l a t e d i n a g e n e r a l w a y to mass s p e c t r a l f r a g m e n tation patterns. T h e mass spectra of a l l cZoso-carboranes a n d the s p e c t r u m of the m e t h y l d e r i v a t i v e of the
and derivatives
rado-carborane,
CB >H , r
w e r e o b t a i n e d w i t h a P e r k i n - E l m e r H i t a c h i R M U - 6 D spectrometer
9
(80
volts i o n i z i n g p o t e n t i a l ) l o c a t e d at W e s t C o a s t T e c h n i c a l S e r v i c e , S a n G a b r i e l , C a l i f . T h e r e m a i n i n g spectra w e r e o b t a i n e d w i t h a C o n s o l i d a t e d M o d e l 21-620 spectrometer. f r a g m e n t a t i o n patterns w i t h
A l t h o u g h there are some d i s s i m i l a r i t i e s i n different
instruments a n d w i t h
different
voltages, the o v e r - a l l mass s p e c t r a l p r o f i l e s — w h i c h are o u r m a i n interest h e r e — d o n o t c h a n g e a p p r e c i a b l y . I n several instances, as i n d i c a t e d , the exact i s o m e r i c structures of s o m e of the cZoso-carboranes w e r e u n k n o w n , b u t i t is h i g h l y d o u b t f u l w h e t h e r i s o m e r i c differences
c o u l d effect a n y
gross changes i n s p e c t r a l patterns.
Borane
Structures
M o l e c u l a r structures of some t y p i c a l b o r o n h y d r i d e s are s h o w n i n F i g u r e 1.
T h o s e h a v i n g the m o r e c o n d e n s e d
structures ( B , H , r
9
B Hi , 6
0
a n d B10H14) h a v e one t e r m i n a l h y d r o g e n for e a c h b o r o n a t o m , w h i l e those w i t h o p e n structures ( B H i 4
a n d B H n ) also h a v e B H
0
5
2
groups.
T h e f o r m e r are the s o - c a l l e d " s t a b l e " boranes, w h i l e the latter, e x c e p t i n g d i b o r a n e , are the " u n s t a b l e " boranes, d e n o t e d as s u c h i n a c c o r d a n c e w i t h t h e i r r e l a t i v e t h e r m a l stabilities ( 2 5 ) .
A better d e s i g n a t i o n is the f o r m u l a
( B H ) ( B H ) a . , w h e r e η has a n i n t e g r a l v a l u e , a n d χ = w
3
boranes a n d 3 for unstable boranes
(6).
2 for
of a l l boranes is the presence of three-center Β — Η — Β ( h y d r o g e n bonding (12).
stable
O n e significant c h a r a c t e r i s t i c bridge)
O n electron i m p a c t there is a p r o n o u n c e d t e n d e n c y
for
the h y d r o g e n atoms to b e a b s t r a c t e d i n p a i r s , a n d some mass s p e c t r a l evidence
with bridge-deuterated
decaborane
(23)
and labeled tetra-
b o r a n e ( 7 ) i n d i c a t e d t h a t the first p a i r of h y d r o g e n s a b s t r a c t e d consists of a t e r m i n a l h y d r o g e n a n d its adjacent h y d r o g e n b r i d g e a t o m . w o r k w i t h zero source contact mass s p e c t r o m e t r y (14),
Later
h o w e v e r , suggests
that, for tetraborane at least, this m e c h a n i s m m a y b e incorrect.
14.
DiTTER E T A L .
Boranes and
195
Carboranes
Figure 3. Structural similarities of hexaborane-10 and three of the nido-carboranes. The structure of C B He> which is analogous to the others, is not shown /t
Closo-Carborane As
2
Structures
mentioned previously, two
k n o w n : c l o s e d cage (closo-)
g e n e r a l classes of
a n d o p e n (nido-)
carboranes
compounds.
The
are closo-
c o m p o u n d s as a class are s i g n i f i c a n t l y m o r e stable u n d e r e l e c t r o n i m p a c t , are less r e a c t i v e c h e m i c a l l y , a n d are t h e r m a l l y m o r e stable t h a n e i t h e r the boranes or nicZo-carboranes. A d d i t i o n a l l y , e a c h b o r o n a n d e a c h c a r b o n in a
rfoso-carborane
has a s i n g l e t e r m i n a l h y d r o g e n , a n d g e n e r a l l y there
are n o h y d r o g e n b r i d g e s .
W i t h one e x c e p t i o n , C B > H r
7
(15),
the
closo-
carboranes r e p o r t e d to d a t e c o n t a i n t w o c a r b o n atoms a n d h a v e t h e g e n e r a l f o r m u l a C B „ H „ . T h e structures of representative c o m p o u n d s 2
+ 2
of this t y p e are s h o w n i n F i g u r e 2. The
s i m p l e s t m e m b e r of the series a n d the one first d i s c o v e r e d is
C2B3H5, cZoso-l,5-dicarbapentaborane ( 5 , 10, 13, 19, 2 1 ) , a t r i g o n a l b i p y r a m i d w i t h a c a r b o n i n e a c h of the t w o apex positions. Grimes (9)
Recently
has p r e s e n t e d e v i d e n c e for a n a l k y l d e r i v a t i v e of the 1,2-
i s o m e r i n w h i c h one skeletal c a r b o n is i n a n apex p o s i t i o n a n d the other is e q u a t o r i a l . T h e s e c o n d c a r b o r a n e i n the closo -series is C B 4 r I e , a 2
196
MASS S P E C T R O M E T R Y
I N INORGANIC C H E M I S T R Y
t e t r a g o n a l b i p y r a m i d , of w h i c h t w o isomers are k n o w n : t h e 1,2- a n d t h e 1,6-dicarba-compounds
17, 19, 22). S i m i l a r l y , g o i n g u p t h e scale
(10,13,
b y successive a d d i t i o n o f BH units, there a r e C B H 2
21 ), C B H (13,28,31), 2
6
2
(8, 13, 26), a n d C B H i 2
r )
C B H (13, 28), C B H
8
1 0
7
9
2
8
(1,10,11,13,17,19,
7
1 0
(13,20, 28), C B H 2
9
13L
(5, 12, 13, 19). I n e a c h case there are k n o w n
2
o r p o t e n t i a l isomers w i t h carbons a r r a n g e d i n v a r i o u s positions i n t h e m o l e c u l a r skeletons.
I n t h e case o f C B i H i , t h e s t r u c t u r e is a c l o s e d 2
0
2
i c o s a h e d r o n , a n d a l l atoms o c c u p y e q u i v a l e n t p o s i t i o n s ; t h e n u m b e r i n g system i s s u c h t h a t o n e a r b i t r a r i l y selects o n e o f t h e carbons as t h e apex ( N o . 1 ) , a n d t h e e q u a t o r i a l positions a r e t h e n n u m b e r e d c l o c k w i s e i n each plane. Monocarbahexaborane
( 7 ) , t h e o n l y r e p o r t e d cZoso-carborane
one c a r b o n a t o m ( I S ) , is isoelectronic w i t h C B H . 2
4
6
with
I t has a b r i d g e
h y d r o g e n a n d is t h e o n l y cZoso-carborane t o date w i t h this feature. Nido-Carborane
Structures
T h e n i d o - c a r b o r a n e s that h a v e b e e n d i s c o v e r e d a n d c h a r a c t e r i z e d
to date i n c l u d e four, CB H 5
C B H 4
2
BH
2
6
9
(16), C B H 2
4
8
(18, 19), C B H 3
3
7
(4), a n d
( 3 ) , t h a t arise f r o m systematic s u b s t i t u t i o n o f CH groups f o r
groups w i t h i n B H i w i t h w h i c h t h e y are isoelectronic. A l l o f these 6
0
species, i n c l u d i n g B H i , h a v e p e n t a g o n a l p y r a m i d structures. B H i has 6
a
10 11
B2
20
22
0
6
B4
B3
30 32
40 42 44
B8
*7
Id ft 74 76 78 80 82 84 86
B
+
5
50 52 54
+
60 62 64 66
B|0
B9
88 90 92 «
*
0
*
100 102T o n k 1Ô8 110
m/e
Figure
4.
Synthetic
polyisotopic mass spectrum occurring abundance
of boron of
naturally
14.
DiTTER E T A L .
Boranes and
197
Carboranes
f o u r b r i d g e h y d r o g e n s , w h i l e the n i d o - c a r b o r a n e s h a v e three, t w o , one, a n d zero b r i d g e h y d r o g e n s , r e s p e c t i v e l y ; e x c e p t i n g C B H 4
2
the structures
6
of these c o m p o u n d s are s h o w n i n F i g u r e 3. I n the case of C B H , o n l y 5
9
m e t h y l derivatives h a v e b e e n r e p o r t e d , w h i l e the p a r e n t species itself r e m a i n s u n d e t e c t e d . S i m i l a r l y , e v i d e n c e for the p a r e n t c o m p o u n d
C B H 3
3
7
is s t i l l tentative, b u t three C - m e t h y l d e r i v a t i v e s h a v e b e e n i s o l a t e d a n d characterized.
A fifth n i d o - c a r b o r a n e , C B H i 2
9
(19, 2 9 ) ,
3
presumably
containing two bridge hydrogens a n d h a v i n g an open icosahedral structure, also has b e e n p r e p a r e d . Tebbe, Garrett, a n d H a w t h o r n e (27) have reported an example of a t h i r d t y p e of c a r b o r a n e , C B H i , w h i c h t e n t a t i v e l y has b e e n d e t e r m i n e d 2
7
3
to c o n t a i n t w o m e t h y l e n e g r o u p s a n d t w o h y d r o g e n b r i d g e s . It does not f a l l i n t o either the closo- or nidoGeneral Characteristics
categories.
of Boron Spectra
If e l e m e n t a l b o r o n c o n s i s t i n g o n l y of
10
B - i s o t o p e w e r e s u b j e c t e d to
e l e c t r o n i m p a c t a n d t h e n a n a l y z e d mass s p e c t r o m e t r i c a l l y , one observe o n l y m o n o i s o t o p i c species at m/e to
B , B ,
1 0
+
1 0
2
+
1 0
would
10, 20, 30, etc., c o r r e s p o n d i n g
B , etc. S i m i l a r l y , the B - i s o t o p e i n p u r e e l e m e n t a l f o r m 3
+
n
w o u l d generate peaks at m/e 11, 22, 33, 44, etc. ( B , . . . ) . I n b o r o n of n
naturally occurring composition
( B/ n
1 0
+
B == 4.0), h o w e v e r , the result
w o u l d b e a p o l y i s o t o p i c mass s p e c t r u m of essentially s t a t i s t i c a l l y d i s tributed
l l
B and
1 0
B , as d e p i c t e d i n F i g u r e 4. F o r i o n groups c o n t a i n i n g
s m a l l n u m b e r s of borons
(say B i to B ) the s p e c t r a l profile is statis+
4
+
t i c a l l y w e i g h t e d i n f a v o r of the h i g h e r mass n u m b e r s of the
group,
because of the h i g h c o n c e n t r a t i o n ( 8 0 % ) of B ; — i . e . , i n the B - g r o u p , n
n
B
is the most a b u n d a n t i o n , w h i l e i n the B - g r o u p , B
+
3
3
n
4
4
+
and
1 1
B
3
1 0
B
1
are i n e q u a l a b u n d a n c e . A s the n u m b e r of borons increases b e y o n d f o u r , h o w e v e r , the m a x i m u m i n t e n s i t y p e a k ( o r p e a k s ) begins to shift a w a y f r o m the cut-off peak.
I n the B , - g r o u p , B r
n
4
1 0
Bi
is the most a b u n d a n t
+
i o n , w h i l e i n the B i o - g r o u p , it is " B s ^ B / , w h i c h is 2.81 times as a b u n d a n t as
n
B
1 0
+
.
T h e a b u n d a n c e ratios of a l l the i s o t o p i c species i n a p a r t i c u l a r i o n g r o u p are r e a d i l y c a l c u l a t e d b y s i m p l e statistics. I f the b o r o n occurs i n n a t u r a l a b u n d a n c e , the c o n c e n t r a t i o n of a n y i o n , B n
x
1 0
B , is g i v e n b y y
[ H B / o B J = W(.80)*(.20)*. I n this expression, W is the statistical w e i g h t — t h e n u m b e r of different ways
n
B and
1 0
B atoms c a n b e a r r a n g e d i n the a v a i l a b l e skeletal positions
— n u m e r i c a l l y e q u a l to (x
+
y)\/x\y\. T h e values of W
i n m a t h e m a t i c a l h a n d b o o k s as " b i n o m i a l coefficients."
are t a b u l a t e d
198
MASS S P E C T R O M E T R Y
MONOISOTOPIC
POLYISOTOPIC
5 9
B
52
54
56
58
60
IN INORGANIC C H E M I S T R Y
H
8
62 64
6
4
2
0
H'S ABSTRACTED
m/e
B9H|5 .1
102 104 106 108 110 112
Figure 5.
14
12 10
8
6
4
Parent group mass spectra of pentaborane-9 nonaborane-15
2
and
A s a n e x a m p l e , c o n s i d e r a c o m p o u n d w i t h five b o r o n atoms. B a s e d o n t h e a b o v e e q u a t i o n the concentrations a n d i s o t o p i c a b u n d a n c e ratios, r e l a t i v e to
1 1
B , are as f o l l o w s : 5
Abundance
Concentration
Isotopic Species
1(.80) 5(.80) (.20) 10(.80)3(.20)2 10(.80) (.20) 5(.80)(.20) 1(.20) 5
ιΐΒ ιοβ
"Bg^B* "Bo^Ba B^°B B 11
1 0
5/4 10/4 10/4 5/4 1/4
4
4
2
3
4
4
δ
5
2 3 4 δ
Ratio
1.0 =1.25 = 0.625 = 0.156 = 0.0195 = 0.00098
S i m i l a r c a l c u l a t i o n s c a n b e c a r r i e d o u t f o r a l l other i o n g r o u p s ( a l r e a d y t a b u l a t e d i n T a b l e I of R e f . 2 4 ) , a n d the e l e m e n t a l b o r o n s p e c t r u m i n F i g u r e 4 is b a s e d o n these d a t a . T h e u t i l i t y of s u c h d a t a lies i n c a l c u l a t i n g m o n o i s o t o p i c s p e c t r a f r o m p o l y i s o t o p i c d a t a since the concentrations of a l l i s o t o p i c species c a n b e d e t e r m i n e d b y m e a s u r i n g o n l y those ions c o n taining
1 1
B-atoms.
F o r e x a m p l e , the m o n o i s o t o p i c s p e c t r u m of B > H is o b t a i n e d b y first r
measuring
n
B H 5
9
+
(m./e. 64), then subtracting B n
9
4
1 0
BH
9
+
,
l l
B
8
, o
B H 2
0
+
,
14.
DiTTER E T A L .
Boranes
and
199
Carboranes
etc. ( r e l a t i v e values 1.0, 1.25, 0.625, etc.) f r o m m/e 63 d o w n to m/e 5 9 (
1 0
B H 5
9
+
).
T h e r e s i d u a l o f m/e 6 3 t h e n is B H n
intensities of B H 5
8
+
, B H 5
7
+
1 1
B
4
1 0
BH
, . . . B
5
8 +
+
,
n
B
3
1 0
5
8
, a n d f r o m this t h e
+
B H \ etc., a n d u l t i m a t e l y a l l ions, B , H , 2
8
r
are d e t e r m i n e d . N e x t i n l i n e is t h e B
4
9
+
H / group,
t h e n Β Η / , etc. 3
I n e a c h g r o u p , i f t h e correct n u m b e r of b o r o n atoms is a s s u m e d , t h e d a t a s h o u l d r e d u c e to a neat m o n o i s o t o p i c s p e c t r u m . I f t h e n u m b e r o f b o r o n atoms a s s u m e d is s m a l l e r t h a n the a c t u a l n u m b e r present, t h e d a t a also m a y r e d u c e to n e g l i g i b l y s m a l l r e s i d u a l s , b u t i f t h e n u m b e r a s s u m e d is greater t h a n t h e a c t u a l n u m b e r present, i n o r d i n a t e l y large n e g a t i v e residuals w i l l o c c u r d u r i n g t h e r e d u c t i o n process. T h e c r i t e r i o n , therefore, is the largest n u m b e r of b o r o n atoms that w i l l s u i t a b l y r e d u c e t h e d a t a . Mass Spectra of Boranes If, to t h e b o r o n i o n species i n F i g u r e 4 w e c o u l d a d d h y d r o g e n s , t h e cut-off p e a k w o u l d shift u p w a r d i n p r o p o r t i o n to t h e n u m b e r of h y d r o g e n s a d d e d , a n d the profiles w o u l d b e t h e same as f o r e l e m e n t a l b o r o n . F o r
METHYLPENTABORANE
I
66 68 70 72 74
7T
10
8
6
4
2
0
H'S ABSTRACTED
m/e
ETHYLPENTABORANE
12
Figure
6.
10
8
6
4
2
0
Parent group mass spectra of two alkyl derivatives pentaborane-9
of
200
MASS S P E C T R O M E T R Y
example, B H 5
+
9
IN INORGANIC C H E M I S T R Y
w o u l d o c c u p y the r e g i m e f r o m m/e 59 to m/e 64, c o r r e
s p o n d i n g to t h e mass r a n g e o f m/e 50 to m/e 55 of the B
5
m/e 63 because the species
1 0
B H 3
+
9
to
1 1
B H 5
9
+
r a t h e r t h a n the r a n g e
species; the highest i n t e n s i t y p e a k w o u l d b e
+
n
B4
1 0
BH
9
+
w o u l d b e statistically m o s t p r o b
able. H o w e v e r , since e l e c t r o n i m p a c t also k n o c k s h y d r o g e n s off some of the p e n t a b o r a n e m o l e c u l e s , the entire s p e c t r u m of ions f r o m m/e (
1 0
B
5
+
) to m/e 64 ( B , H 1 1
r
+
9
) shows u p . F u r t h e r m o r e , B H 5
h i g h e s t i n t e n s i t y , a n d v a r i e d y i e l d s of the v a r i o u s Β Η „ δ
+
5
50
is the i o n of
ions c o m b i n e d
+
w i t h t h e statistical d i s t r i b u t i o n o f b o r o n isotopes causes the profile to p e a k at m/e 59 to m/e 60, as s h o w n i n F i g u r e 5. T h e effect is e v e n m o r e p r o n o u n c e d for the B H - c o n t a i n i n g " u n s t a b l e " b o r a n e , B H i , w h i c h has 2
9
5
its m a x i m u m i n t e n s i t y p e a k n i n e mass n u m b e r s b e l o w its m o l e c u l a r w e i g h t of 114.
A l s o , the t w o ions B H i 9
+ 5
and B H 9
X 4
+
are of n e g l i g i b l y s m a l l
intensities, a c h a r a c t e r i s t i c w h i c h h o l d s t r u e for a l l " u n s t a b l e " boranes. I n essence, t h e n , the b o r o n isotope d i s t r i b u t i o n a n d the ease of a b s t r a c t i o n o f h y d r o g e n s g i v e boranes t h e i r b r o a d , r o u n d e d profiles, a n d this c o n tributes to easy i d e n t i f i c a t i o n .
B H 5
9
• 1
52
1 I
54
I
57
1 I
58
I
I I
60
62
1 1 1
64
m/e
CBH 2
3
8
6
4
0
5
ι I I 1 1 I I I 56 58 60 62
Figure
2
H'S ABSTRACTED
4
2
7. Comparison of mass spectra of 1,5-C B H 2,4-C B H closo-carboranes with that of B H 2
2
5
7
3
and
5
5
0
9
14.
DiTTER E T A L .
Boranes
and
201
Carboranes
B|0H|4 110 112 114 116 118 120 122 124
14 12 10
8
6
4
2
H'S ABSTRACTED
m/e
C2BgH,0 , . M i l l
10
108 110 112 114 116 118 120 122
8
6
4
2
0
C 2 B, 0 H | 2
4
130 132 134 136 138 140 142 T44
Figure
I?
8. Comparison of mass spectra of C B H CB H closo-carboranes with that of B 2
2
10
12
8
and 1,2-
10
H
10
u
T h e same g e n e r a l f r a g m e n t a t i o n patterns h o l d t r u e f o r t h e r e l a t e d a l k y l - b o r a n e s , a n d t w o t y p i c a l examples a r e s h o w n i n F i g u r e 6. A d d i n g m e t h y l a n d e t h y l groups m e r e l y shift t h e cut-offs u p w a r d 14 a n d 28 mass n u m b e r s , r e s p e c t i v e l y , reflecting t h e a d d i t i o n of C H a n d C H 2
2
4
to t h e
m o l e c u l a r w e i g h t . T h e fact that t h e p a r e n t g r o u p profile is c h a n g e d o n l y s l i g h t l y suggests
that the hydrogen
a b s t r a c t i o n u p o n electron
impact
occurs p r e f e r e n t i a l l y o n t h e b o r a n e skeleton, n o t o n t h e a l k y l s i d e c h a i n s , a n d this has b e e n v e r i f i e d b y mass s p e c t r a l analysis of c o m p o u n d s
with
d e u t e r a t e d a l k y l groups ( 3 0 ) .
Mass Spectral Profiles of CJoso-Carboranes. cZoso-carboranes, 5
2
3
5
I n F i g u r e 7 t h e p a r e n t g r o u p spectra of t w o
1,5-C B H 2
spectrum of B H . while C B H
Carboranes
9
3
(C B H 2
5
7
5
a n d 2 , 4 - C B H , are c o m p a r e d 2
5
7
w i t h the
has t h e same n u m b e r of borons as B H ,
has t h e same n u m b e r of s k e l e t a l atoms.)
5
9
T h e spectral
202
MASS S P E C T R O M E T R Y
IN INORGANIC C H E M I S T R Y
differences are q u i t e p r o n o u n c e d . T h e s m a l l degree of h y d r o g e n abstract i o n i n the cZoso-carboranes
c a n b e seen i n their m o n o i s o t o p i c
spectra;
i n e a c h case the p a r e n t i o n is of c o n s i d e r a b l y greater i n t e n s i t y t h a n a n y of its offspring.
T h e same g e n e r a l profile is e v i d e n t for the
C B H i o , shown i n comparison with B i H i 2
8
0
i n F i g u r e 8.
4
carborane,
C B H i o was 2
8
p r e p a r e d i n y i e l d s of less t h a n 1 % , a n d w e d o not k n o w w h i c h isomer w a s f o r m e d . T h e s p e c t r u m of l , 2 - C B 2
1 0
H i , also s h o w n i n F i g u r e 8, has 2
a f a i r l y intense p a r e n t i o n , b u t its highest i n t e n s i t y p e a k for some reason is t w o mass n u m b e r s b e l o w that of its p a r e n t . T h e reason f o r this a p p a r ent d i s c r e p a n c y is n o t k n o w n . I n F i g u r e 9 are spectra of the 1,2- a n d 1,6isomers of c Z o s o - C B H , a n d here w e see t h a t the C B H , 2
4
6
2
4
r
i o n is the
+
one of highest intensity. I n g e n e r a l , h o w e v e r , the o v e r - a l l s p e c t r a l c h a r acteristics c l o s e l y resemble other
cZoso-carboranes.
A s w i t h boranes, a l k y l d e r i v a t i v e s of carboranes f r a g m e n t n e a r l y t h e same as d o the n o n a l k y l a t e d parents. T h e h y d r o g e n s f r o m the a l k y l side chains a n d those f r o m the c a r b o r a n e skeleton a p p e a r to h a v e a b o u t the same g e n e r a l resistance to a b s t r a c t i o n . F i g u r e 10 c o m p a r e s the s p e c t r u m of 2 , 4 - C B H 2
5
7
w i t h that of a C - m e t h y l d e r i v a t i v e , C H C B H 6 , w h i l e 3
2
5
F i g u r e 11 shows spectra of the d i m e t h y l d e r i v a t i v e s of C B H i , C B H , 2
8
0
SYM-
H'S ABSTRACTED
m/e
UNSYM
64
66
68
Figure 9.
70
72
6
74
4
2
0
Parent group mass spectra of the 1,6- and 1,2isomers of CBH 2
A
6
2
7
9
14.
DiTTER E T A L .
C
Boranes
and
203
Carboranes
2 5 7 B
74
H
76 78 80 82
6
84 86
m/e
CH -C2B H 3
5
. . . .
88
Figure
C B H 2
6
8
4
2
0
H'S ABSTRACTED
6
ι ι ι I
90 92 94 96 98 100
10.
Similarities
in the mass spectra C-methyl derivative
of 2,4-C B H 2
(exact i s o m e r i c structures as y e t u n k n o w n ) .
5
and a
7
T h e spectra are
self-explanatory. 2V/do-Carboranes. A s m e n t i o n e d p r e v i o u s l y t h e n i d o - c a r b o r a n e s c o n t a i n h y d r o g e n b r i d g e s , a n d t h e y are n o t as t h e r m a l l y or c h e m i c a l l y stable as t h e i r cZoso-counterparts.
F i g u r e 12 shows t h e spectra o f C B H 2
4
8
and
the m e t h y l d e r i v a t i v e o f C B - , H , a n d i t is a p p a r e n t that t h e i r f r a g m e n t a 9
t i o n patterns are m o r e closely a l l i e d w i t h boranes t h a n w i t h t h e closocarboranes. T h i s is also d e m o n s t r a t e d r a t h e r d r a m a t i c a l l y b y c o m p a r i s o n w i t h the spectra of the t w o cZoso-carborane isomers of C B H 2
Over-all
Comparison
4
6
i n F i g u r e 9.
of Boranes and Carboranes
T o s h o w i n a g e n e r a l w a y h o w t h e p o l y i s o t o p i c mass s p e c t r a of t h e b o r a n e a n d c a r b o r a n e series c h a n g e as t h e n u m b e r o f b o r o n atoms i n creases, w e h a v e p l o t t e d i n F i g u r e 13 t h e r e l a t i v e positions of t h e m a x i mum
i n t e n s i t y peaks f o r a l l t h e c o m p o u n d s f o r w h i c h w e h a v e s p e c t r a .
F o r e x a m p l e , f o r t h e l o w e r m o l e c u l a r w e i g h t cîoso-carboranes t h e m a x i mum
i n t e n s i t y p e a k occurs o n e mass n u m b e r less t h a n t h e m o l e c u l a r
w e i g h t o f t h e p a r e n t m o l e c u l e , w h i l e f o r B i H i , w h i c h is a " B H " - t y p e 0
4
b o r a n e , t h e r e l a t i v e p o s i t i o n of this peak is seven mass n u m b e r s b e l o w the mass n u m b e r o f t h e p a r e n t .
I n s e v e r a l instances—e.g., B , H , t h e r
9
204
MASS S P E C T R O M E T R Y
IN INORGANIC C H E M I S T R Y
spectra s h o w t w o peaks of essentially e q u a l " m a x i m u m " i n t e n s i t y , a n d w e chose to use average ( n o n u n i t ) values i n the p l o t . I n the p l o t w e note a g e n e r a l t r e n d u p w a r d for a l l b o r o n
compounds
o w i n g to the w a y i n w h i c h b o r o n i s o t o p i c species are d i s t r i b u t e d ; h o w ever, t h e p r e d o m i n a n t f a c t o r g o v e r n i n g the p o s i t i o n of the m a x i m u m i n t e n s i t y p e a k arises f r o m the ease of a b s t r a c t i o n of h y d r o g e n s f r o m p a r e n t ions. 4
B H - c o n t a i n i n g boranes
lose h y d r o g e n s
2
most readily, w h i l e the
s t a b l e " B H - b o r a n e s a n d the n i d o - c a r b o r a n e s are s o m e w h a t m o r e resistant
to h y d r o g e n a b s t r a c t i o n . T h e cZoso-carboranes most stable of these species. s u c h r e l a t e d species
are w i t h o u t q u e s t i o n the
O n e w o u l d p r e d i c t that the mass s p e c t r a of
as B i H ( C O ) 0
8
2
w o u l d also r e s e m b l e
the
closo-
carboranes. Determination
of Boron and Carbon
by ^C-lsotope
Analysis
W i t h a n y c a r b o n - c o n t a i n i n g c o m p o u n d of n a t u r a l i s o t o p i c a b u n d a n c e , there w i l l b e mass s p e c t r a l c o n t r i b u t i o n s f r o m i o n species c o n t a i n i n g t h e
(0Η3) 0 Β Η 2
2
8
8
140 142 144 146 148 150
10
JL_L 8
6
Mill 4
2
0
H'S ABSTRACTED
m/e
(CH^JgCgByHy
128 130 132 134 136 138
(CH ) C BgH 3
2
2
II.
ι • ».
8
6
I I » I I
4
2
0
6
116 118 120 122 124 126
Figure
10
1I ι 1I I I 1 _
10
Parent group mass spectra of three closo-carboranes
8
6
4
2
0
dimethyhted
14.
DiTTER E T A L .
B
Boranes
and
205
Carboranes
6 I0 H
64 66 68 70 72 74 76
10
m/e
C H
3
C B
6
I I t
4
2
0
H'S ABSTRACTED
5 8 H
Mill
10
78 80 82 84 86 88 90
8
6
4 2
8
6
4
0
2 4 8
C
B
H
66 68 70 72 74 76
Figure
13
8
C-isotope.
12.
Comparison of the spectra of two with that of hexaborane-10
2
nido-carboranes
T h e r e also w i l l b e s m a l l c o n t r i b u t i o n s f r o m d e u t e r i u m -
c o n t a i n i n g species, b u t i n c a l c u l a t i n g a b u n d a n c e ratios t h e d e u t e r i u m c a n b e c o n v e n i e n t l y l u m p e d together w i t h t h e C - s p e c i e s . 13
I n some cases,
w h e r e the n u m b e r o f h y d r o g e n atoms is s m a l l , i t c a n b e n e g l e c t e d e n t i r e l y . F o r a c o m p o u n d w i t h a n u n k n o w n c o m b i n a t i o n of c a r b o n , b o r o n , a n d h y d r o g e n ( o r other elements, f o r that m a t t e r ) , mass s p e c t r a l analysis g e n e r a l l y c a n b e u s e d to d e t e r m i n e t h e exact n u m b e r of b o r o n a n d c a r b o n atoms i n t h e p a r e n t c o m p o u n d .
T h e r e q u i r e m e n t s a r e : ( 1 ) a mass spec-
trometer t h a t gives g o o d p e a k d e f i n i t i o n , a n d ( 2 ) n e g l i g i b l e c o n t r i b u t i o n s f r o m i m p u r i t i e s i n t h e v i c i n i t y of t h e p a r e n t i o n mass n u m b e r .
Sharp
cut-offs at t h e p a r e n t i o n m/e are also d e s i r a b l e , a n d . i n this respect t h e cZoso-carboranes are almost i d e a l l y s u i t e d . T h e m e t h o d c a n b e i l l u s t r a t e d with C B H i 2 , the dimethyl derivative of C B H . 4
6
2
6
8
First Approximation. T h e c o m p o u n d i n q u e s t i o n h a s a b o r a n e profile a n d , e x c e p t i n g a s m a l l
13
closo-car-
C - i s o t o p e p e a k at m/e 127 ( r e l a -
t i v e i n t e n s i t y 1.0), t h e h i g h mass cut-off occurs at m/e 126 ( i n t e n s i t y
206
MASS S P E C T R O M E T R Y I N I N O R G A N I C C H E M I S T R Y
2 3 . 9 ) . N o w the cut-off p e a k w o u l d h a v e to b e a t t r i b u t e d to B - c o n t a i n i n g 1 1
ions f r o m species s u c h as C B H i , C B o H i , o r C B H n , etc. 3
quently, the C-isotope 1 3
C
C
1 2
1
B H
1 1
2
7
,
+
1 3
1 3
C
1
1 2
C
3
7
3
peak
13
B H
1 1
6
4
2
5
5
Conse-
a t m/e 127 w o u l d t h e n h a v e t o b e
1 2
+
,or
1 3
C
1
1 2
C
4
n
B H 5
1 1
+
, etc. A s w i t h b o r o n
isotopes, i t is a f a i r l y s i m p l e m a t t e r t o c a l c u l a t e r e l a t i v e a b u n d a n c e s o f ions c o n t a i n i n g
1 3
C and
1 2
C ; b a s e d o n n a t u r a l a b u n d a n c e o f the t w o i s o -
topes, the c a l c u l a t i o n s are as f o l l o w s : General formula: ForC B H 3
7
For C B H 4
6
1 3
2
2
[
2
1 2
2
C l
[
[
1 2
4 (.98931) 3 (.01069) 1(.98931)
4
4
C l
=
0.03242
=
0.04322
=
0.05402
1 (.98931)3
c ]
[l C 13 ]
CRBKIL
2
3
C l
y
3(.98931) (.01069)
c ]
3
1 2
x
C y
[l C 13 ]
:
2
For
x
[l C 13 ]
:
1 2
[ l C 1 3 ] = W(.98931) (.01069)
4
5 ( . 9 8 9 3 1 ) (.01069) 4
1 (.98931)
c ] 5
5
T h e o b s e r v e d r a t i o o f m/e 127 t o m/e 126 i s 1/23.9, o r 0.0418, p o i n t i n g to
C B Hi 4
We 1 2
C
6
as t h e c o r r e c t
2
could 4
1 1
B
6
1
H
n
2
formula for the compound
have
Hi ,
t o m/e 127, a l t h o u g h this is unnecessary w h e n t h e
+
included
t h e d e u t e r i u m isotope
i n question.
also
contribution,
Lu
4 5 6 NO. OF BORONS Figure
7 8 9 IN MOLECULE
1 3 . Comparison of spectral profile boranes and carboranes
characteristics
of
14.
DiTTER E T A L .
Figure
14.
Boranes and
Distribution
207
Carboranes
of ion species in the mass spectrum "second approximation'
of
B C, H — 6
t
]2
(Legend: Similar cross-hatched areas represent related boron isotopic species; ion formulas in boxes represent related carbon isotopic species of interest)
c a l c u l a t i o n is o n l y a first a p p r o x i m a t i o n .
Just for the sake of c o m p l e t e
ness, h o w e v e r , the c o n t r i b u t i o n of this i o n w o u l d b e P E W ] [Ή
1 2
12(.99984)«(.0001β)
]~ ~
(Λ8984)~ΰ
A d d e d to the p r e v i o u s r e s u l t for
1 H
~
°'
00192
C w e get a t o t a l c a l c u l a t e d c o n t r i b u t i o n
of .04322 - f .00192 — .04514, c o m p a r e d w i t h the o b s e r v e d i n t e n s i t y r a t i o of 0.0418.
I t is not necessary to d e r i v e these values for e a c h s i t u a t i o n , h o w e v e r , since tables of i s o t o p i c a b u n d a n c e (2)
ratios are a l r e a d y a v a i l a b l e .
Beynon
for e x a m p l e , has t a b u l a t e d t h e m for v a r i o u s c o m b i n a t i o n s of c a r b o n ,
h y d r o g e n , n i t r o g e n , a n d o x y g e n ( no b o r o n , u n f o r t u n a t e l y ) u p to mass 250. I n some cases ( i n c l u d i n g the C H 4
1 2
p o r t i o n of C B « H 4
1 2
) , it is necessary
to extrapolate ( l i n e a r l y ) the d a t a i n his t a b l e . Second Approximation.
I n the first a p p r o x i m a t i o n w e a s s u m e d that
o n l y one i o n w a s c o n t r i b u t i n g to m/e
126, n a m e l y
1 2
C
4
l l
B {Hi (
2
+
.
T h i s is
208
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
Figure 15. Distribution of ion species in the mass spectrum of B C H —"third approximation' (Legend: same as in Figure 14) 6
not strictly true, however, terium neglected 1 1
B
5
1 0
Jt
because two
4
1 1
B i - c o u n t e r p a r t of m/e
C - c o n t a i n i n g isotopes
(deu-
One
is the
127 a n d , f r o m statistical c a l c u l a t i o n s , i t is 127; the other i o n is the
B H n — t h e p r i n c i p a l i o n of m/e
13
C - c o u n t e r p a r t of
125. A s s h o w n i n F i g u r e 14, s u b -
+
6
1 3
a g a i n for s i m p l i c i t y ) also are present.
1.5 times the i n t e n s i t y of m/e C
12
t r a c t i o n of 1.5 f r o m the i n t e n s i t y of m/e 126 leaves a r e s i d u a l v a l u e of 22.4, w h i c h , as a s e c o n d a p p r o x i m a t i o n , w e c a n a t t r i b u t e to the i o n . T h e o b s e r v e d ratio of
1 3
C
1 2
C / 3
1 2
C
4
1 2
C
4
n
B Hi 6
2
+
n o w is 1.0/22.4, or 0.0446. T h i s
is closer to o u r c a l c u l a t e d v a l u e ( d e u t e r i u m i n c l u d e d ) of 0.04514 t h a n w a s o u r first a p p r o x i m a t i o n ( . 0 4 1 8 ) . S i m i l a r l y , i f it w e r e necessary to p e r f o r m these same for C B H i 3
7
3
computations
a n d C B H n , the a m o u n t s u b t r a c t e d f r o m m/e 5
5
b e 1.75 a n d 1.25, r e s p e c t i v e l y , since these are the counterparts of m/e
127. T h e residuals of m/e
a n d 22.6, r e s p e c t i v e l y , a n d the ratios of m/e b e 0.0452 a n d 0.0442, r e s p e c t i v e l y ; these
n
B6
l 0
126 w o u l d
B i and
n
B
4
1 0
Bi
126 t h e n w o u l d b e 22.1
127 to these residuals w o u l d
figures
deviate b y about
25%
f r o m the r e q u i r e d a b u n d a n c e ratios of 0.032 a n d 0.054 ( d e r i v e d b y m e t h ods g i v e n i n the "first a p p r o x i m a t i o n " section ).
14.
DiTTER E T A L .
Boranes and
209
Carboranes
Further Refinement o f Spectral Data. A s m e n t i o n e d i n t h e p r e v i o u s section a n d as s h o w n i n F i g u r e 15, t h e d a t a c a n b e f u r t h e r r e f i n e d b y t a k i n g i n t o a c c o u n t t h e C - a n a l o g of C 1 3
4
B H i i , the principal contributor
1 1
6
+
to m/e 125. M o r e o v e r , several i t e r a t i v e processes c a n b e u t i l i z e d to "zero i n " o n t h e exact c o n t r i b u t i o n of this i o n , f r o m w h i c h one c a n c a l c u l a t e p r e c i s e l y t h e i n t e n s i t y of
1 3
Ci
1 2
C
3
1 1
B Hi e
1
+
i n m/e 126. W e p e r f o r m e d this
task w i t h t h e d a t a at h a n d a n d o b t a i n e d n e a r l y exact checks w i t h t h e statistically c a l c u l a t e d v a l u e 1 3
C
1
1 2
C
3
1 1
B H e
1 2
+
of 0.04514 f o r t h e a b u n d a n c e
r a t i o of
.
T h e p o i n t of this w h o l e p r o c e d u r e is t h a t g i v e n g o o d spectra, o n e c a n use t h e m to d e t e r m i n e t h e exact n u m b e r s o f borons, carbons, h y d r o gens, etc. ( b a r r i n g t h e absence of p a r e n t i o n s ) i n a c o m p o u n d .
In many
instances this t e c h n i q u e c a n p r e c l u d e t h e necessity f o r e l e m e n t a l c h e m i c a l analyses. I f t h e m a t e r i a l i n q u e s t i o n is i m p u r e , t h e mass s p e c t r a l analyses m a y n o t g i v e correct a n s w e r s — b u t , t h e n , n e i t h e r w i l l e l e m e n t a l analyses. F u r t h e r m o r e , w i t h a s m a l l a m o u n t of f r a c t i o n a t i o n , i t is g e n e r a l l y easy t o t e l l b y shifts of s p e c t r a l peaks w h e t h e r o r n o t i t is a r e l a t i v e l y p u r e c o m pound.
T h e t e c h n i q u e c a n b e u t i l i z e d w i t h a n y i s o t o p i c elements o f
k n o w n abundance. Acknowledgment T h i s i n v e s t i g a t i o n w a s s p o n s o r e d b y t h e Office o f N a v a l R e s e a r c h .
Literature
Cited
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210
MASS SPECTROMETRY
I N INORGANIC
CHEMISTRY
(16) Onak, T. P., Dunks, G. B., Spielman, J. R., Gerhart, F. J., Williams, Robert E., J. Am. Chem. Soc. 88, 2061 (1966). (17) Onak, T. P., Gerhart, F. J., Williams, Robert E., J. Am. Chem. Soc. 85, 3378 (1963). (18) Onak, T. P., Williams, Robert E., Weiss, H. G., J. Am. Chem. Soc. 84, 2830 (1962). (19) Onak, T. P., "Advances in Organometallic Chemistry," Vol. 3, p. 263, F. G. A. Stone, R. West, eds., Academic Press, New York, 1965. (20) Potenza, J. Α., Lipscomb, W. N., Inorg. Chem. 5, 1301 (1966). (21) Shapiro, I., Good, C. D., Williams, Robert E., J. Am. Chem. Soc. 84, 3837 (1962). (22) Shapiro, I., Keilin, B., Williams, Robert E., Good, C. D., J. Am. Chem. Soc. 85, 3167 (1963). (23) Shapiro, I., Lustig, M., Williams, Robert E., J. Am. Chem. Soc. 81, 838 (1959). (24) Shapiro, I., Wilson, C. O., Ditter, J. F., Lehmann, W. J., ADVAN. CHEM. SER. 32, 127 (1961). (25) Stock, Alfred, "Hydrides of Boron and Silicon," Cornell University Press, Ithaca, New York, 1933. (26) Tebbe, F. N., Garrett, P. M., Hawthorne, M. F., J. Am. Chem. Soc. 86, 4222 (1964). (27) Tebbe, F. N., Garrett, P. M., Hawthorne, M . F., J. Am. Chem. Soc. 88, 607 (1966). (28) Tebbe, F. N., Garrett, P. M., Young, D. C., Hawthorne, M . F., J. Am. Chem. Soc. 88, 609 (1966). (29) Wiesboeck, R. Α., Hawthorne, M . F., J. Am. Chem. Soc. 86, 1642 (1964). (30) Williams, Robert E., unpublished data. (31) Williams, Robert E., Gerhart, F. J., J. Am. Chem. Soc. 87, 3513 (1965). RECEIVED
October 24, 1966.
