Mass Spectrometry in Boron Chemistry
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ISADORESHAPIRO, C. O.WILSON, J. F.DIITER, and W. J. LEHMANN
Downloaded by MONASH UNIV on June 27, 2013 | http://pubs.acs.org Publication Date: June 1, 1961 | doi: 10.1021/ba-1961-0032.ch014
Research Laboratory,OlinMathieson Chemical Corp., Pasadena, Calif.
The application of mass spectrometry to boron chemistry is particularly useful from both structural and analytical viewpoints. The technique of isotopic variation of the two stable boron isotopes as well as the hydrogen isotopes permits determination of the number of boron and hydrogen atoms in a molecule without the need to isolate the compound from a mixture. It also simplifies assignment of the various
fragmentation species in the mass spectral pattern. The mass spectra of all known boron hydrides are presented, and from their monoisotopic spectra relationships in fragmentation patterns have been deduced.
M a s s spectrometry is a particularly valuable tool i n the chemistry of boron-containing compounds because i t allows ready identification of compounds based on the two isotopes, boron-11 and boron-10. When a molecule contains several boron atoms i n natural isotopic abundance, its mass spectrum has a characteristic pattern which is different from other compounds and is fairly easy to identify, a feature that is i m portant, for example, for analysis of mixtures of hydrocarbons and boranes. T h e naturally occurring ratio of B / B is about 4 / 1 , but boron can also be obtained enriched to give relatively pure boron of the lighter isotope. The use of the boron-10enriched isotope in a compound is an additional powerful tool, because from the comparison of its mass spectrum with that prepared from isotopically normal boron one can calculate the number of boron atoms i n the compound without having to isolate it. One further technique that is applicable to boron-hydrogen systems is the substitution of deuterium for hydrogen. This increases the potential use of mass spectrometry i n this field, because i t allows one to vary the isotopic composition of boron and hydrogen independently. n
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Mass Spectra of Boron Hydrides The gross differences i n mass spectra of a boron hydride and an organic compound are illustrated i n Figure 1, which compares pentaborane(9) (five borons) and pentane (five carbons). The spectrum of pentane follows the usual pattern for large organic molecules; i t has a low parent peak (not shown) at mass number 72 plus several predominant peaks at lower mass numbers. T h e large peak at m/e 43 corresponds to the C H + ion fragment and the peaks at m/e 42, 41, etc., represent loss of hydrogen atoms from this fragment, while the small peak at mass 44 is due to the carbon-13 isotope. 3
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Present address, Aircraft Division, Hughes Tool Co., Culver City, Calif. Present address, National Engineering Science Co., Pasadena, Calif. 127 In BORAX TO BORANES; Advances in Chemistry; American Chemical Society: Washington, DC, 1961.
ADVANCES IN CHEMISTRY SERIES
128
Pentaborane
52
_L 53
j _ L 54
55
56
57
58
59
60
61
62
63
64
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η-Pentane
. 1 . 1 1 1 . 37
38
39
40
41
42
43
44
Figure 1. Comparison of principal peaks in mass spectra of a boron hydride and a hydrocarbon The spectrum of pentaborane(9), on the other hand, is considerably different. Its parent group has a large spread that is caused by distribution of the various boron-11 and boron-10 species, such as B H + , B B H - h , and B 3 B H + , as well as by successive abstractions of hydrogens from these species. If we could resolve its spectrum so that only the parent peaks occurred—e.g., b y lowering the ionizing voltage sufficiently—six different isotopic species would be visible. Because hydrogen atoms are lost readily from the parent group under normal operating conditions, the various fragment ions overlap one another to give the spectrum its pronounced spread. Although the mathematics becomes more cumbersome because of the larger number of peaks, this spread becomes useful i n separating the spectrum of penta borane (9) from that of a mixture because one can usually select a peak that is unique for pentaborane(9). The spread that is observed i n the above spectrum is characteristic of the spectra of all the known boron hydrides, the amount varying with the number of borons and hydrogens i n the parent molecule. T o illustrate this condition the polyisotopic mass 5
Table I. in a Bo"
Bi«o
Β
ί 0 χ
n
9
4
1 1
1 0
1 0
ί 1
ν
B2
10
Ba
Bs'o I
B4'°
10
Be
16
Β Be B7 1 x i o - « 3 Χ ΙΟ"* 5 X 10~
20.0
4.0
0.8
0.16
0.03
Bi"
80.0
32.0
9.6
2.56
0.60
0.15
0.04
B " 2
64.0
38.4
15.4
5..
1.54
0.43
0.11
0.03
Bs"
51.2
41.0
20.5
8.2
2.9
0.92
0.28
0.08
0.55
1
B4"
41.0
41.0
24.5
11.5
4.6
1.7
B "
32.8
39.2
27.5
14.7
6.6
2.6
Be"
26.2
36.7
29.4
17.6
8.8
B "
21.0
33.6
30.2
20.1
B " 8
16.8
30.2
30.2
Be"
13.4
26.8
Bio"
10.7
BxioBy" -
( x
Ί f
)!
X
10-*
(B»o),(B")
10
10
10
—
7
2
Normal Statistical Distribution of Boron Isotopes ( 8 0 % Boron-11, 2 0 % Boron-10) Β System W h e r e χ + y Varies from 1 to 10
Bo"
6
n
9
β
8 Χ ΙΟ"» 2 X 10~« 4 X 7 Χ
ΙΟ"»
f
In BORAX TO BORANES; Advances in Chemistry; American Chemical Society: Washington, DC, 1961.
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10~*
9
SHAPIRO ET AL.
129
Mass Spectrometry in Boron Chemistry
DECABORANE
NONANRANE
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OCTANRANE
HEXAIORANE
PENTANRANE-11
KNTAIORANE-S
TETRABORATE
DIBORANE
MASS
Figure 2.
NUMBER
A
m
Mass spectra of boron hydrides
spectra of a l l known boron hydrides are shown i n Figure 2. Although some spectra have been published (1-4, IS, H, 18), all those shown were prepared i n this laboratory. T o show how the distribution of boron species is affected b y the number of boron atoms present, Table I gives the statistical distribution for an 80-20 boron system i n which the concentration of species for any compound is given along the diagonal, as indicated for pentaborane (9). In addition to the observed increase i n spread with an increase i n the number of boron atoms, another useful feature of boron hydrides i n dealing with mixtures is the occurrence of doubly charged ion peaks. These are not illustrated i n Figure 2 because. In BORAX TO BORANES; Advances in Chemistry; American Chemical Society: Washington, DC, 1961.
ADVANCES IN CHEMISTRY SERIES
130
of their low intensities, but all of the boron hydrides exhibit them. The mass n u m bers of the doubly charged species are always less than one half the mass of the principal ion, indicating that they contain fewer hydrogen atoms. F o r ordinary analytical purposes the polyisotopic spectrum is sufficient. H o w ever, if one is concerned with structure changes, such as occur i n exchange work and in kinetics, it is necessary to simplify the spectrum by reducing it to its monoisotopic form, which allows direct comparison of species. The monoisotopic spectrum is cal culated by simply stripping out all species that contain boron-10. This can be done without any previous knowledge of the relative concentrations of the two boron isotopes, because the correct value to use is that which gives the minimum stripping residues. The authors and a number of other workers (2-4, 18, 17) have found that the op timum value of the ratio of the boron isotopes to use in stripping boron hydrides is 4.0, which is equivalent to 8 0 % boron-11 and 2 0 % boron-10. There has been considerable controversy in the past over the correct B / B ratio, and the difference between 4.0 and the "generally accepted" value of 4.3 exceeds simple experimental error. The value 4.3 was obtained (7) from the mass spectrum of boron trinuoride, but subsequent work (12, 15) has indicated that factors such as selective desorption of B F , as well as i m purities, were neglected and could account for the discrepancy. Recent work (11) also indicated possible selective fragmentation, so the "accepted isotopic boron ratio," 4.3, appears to be i n error.
Downloaded by MONASH UNIV on June 27, 2013 | http://pubs.acs.org Publication Date: June 1, 1961 | doi: 10.1021/ba-1961-0032.ch014
n
1 0
n
1,1,
POLYISOTOPIC SPECTRUM
m/n
ai
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