ba-1961-0032.ch015

Infrared Spectrometry in Boron Chemistry

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W. J.LEHMANN, C. O.WILSON, J. F.DIHER, and ISADORE SHAPIRO

Downloaded by IOWA STATE UNIV on February 18, 2015 | http://pubs.acs.org Publication Date: June 1, 1961 | doi: 10.1021/ba-1961-0032.ch015

Research Laboratory,OlinMathieson Chemical Corp., Pasadena, Calif.

Various techniques in the application of infrared spectrometry to structural problems in borane chemistry are discussed. These include compensation methods with double-beam instruments, isotopic substitution techniques, and the determination of trends in series of structurally related compounds. Vibrational effects of deuterium and alkyl substitutions on diborane are discussed.

T h e purpose of this discussion is to give an over-all picture of how the authors have applied infrared spectrometry i n their laboratory to structural problems i n borane chemistry. Rather than detailed data only the general techniques involved are considered, including topics such as compensation methods with double-beam instruments, isotopic substitution techniques, and trends i n series of structurally related compounds. The paper is further limited to examination of the vibrational effects of deuteration and alkyl substitutions i n diborane. The published literature contains relatively little information about vibrational spectra of boron hydrides and their derivatives. Aside from diborane, the vibrations of which have been thoroughly investigated (5, 13, 19, 22, SI, 84), vibrational spectra are reported for tetraborane (20), stable and unstable pentaborane (11, 20, 85), and decaborane (12). The borane derivatives that have been studied b y Raman and/or infrared spectroscopy are: borane carbonyl (1, 6, 7, 82), ether and amine borane complexes (including ammonia trimethylborane) (9, 23, 25), phosphorus trifluoride borane (38), a few trialkylboranes (8, 17, 18, 27, 80), some dialkoxyboranes (14, 16), and a number of boron halides. Dimethylaminodiborane has been studied (4, 21, 29), and a little has appeared on the spectra of alkyldiboranes (6, 24, 26, 28). Diborane In diborane the two boron atoms are linked b y bridges consisting of two hydrogen atoms, which, according to accepted theory (8), are attached to the boron atoms b y two three-centered bonds. I t is the only known compound containing a doublehydrogen bridge. I n addition, each boron has two terminal hydrogens, the four bonds around each boron being approximately tetrahedral. The two borons and the four terminal hydrogens lie in one plane, perpendicular to the plane formed b y the borons and the two bridge hydrogens. Perhaps from a chemical point of view this molecule may be considered simple; from a vibrational point of view, however, i t is complex. The eight atoms of diborane have 24 degrees of freedom, 18 of which are internal vibrations (1). These vibrations Present address, Aerospace Corp., 2400 East E l Segundo Blvd., E l Segundo, Calif. 'Present address, National Engineering Science Co., Pasadena, Calif. • Present address, Hughes Tool Co., Culver City, Calif. 139 1

In BORAX TO BORANES; Advances in Chemistry; American Chemical Society: Washington, DC, 1961.

ADVANCES IN CHEMISTRY SERIES

140


B-H

BH

asym./in phase

B-H' Figure 1.

rocking

2

ΗΒΗ

out of p h a s e

Four of the infrared active vibrations of B H 2

e

will cause scattering (the R a m a n effect) or absorption (the infrared spectrum). Whether a particular vibration appears i n one or the other spectrum (or both, or neither) is determined by selection rules (10), which are beyond thé scope of this discussion. One vibration is forbidden i n both spectra—i.e., "totally inactive"—while WAVE NUMBER

·· ·· ·· · 5000

3000

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1200

\ \ \\ B-H

\

V

4

x

1000

900

BH d*f. 2

B-H'

800

BH wae. 2

\

ό

8

WAVE LENGTH

Figure 2.

1

····· ····· · ·

2000

comb.

c m

10

12

MICRONS

Frequency shifts: B2H6 to BgDç


700

LEHMAN N ET AL.

141


all the others are either infrared active or Raman active. I n this case, because of the center of symmetry, the vibrations are active i n one or the other but not i n both. Figure 1 shows four of these vibrations. Each involves all or most of the atoms of the molecule, yet each can be described essentially as stretching or distortion of certain bonds—i.e., they are characteristic of a certain group. The upper left v i b r a tion involves stretchings of the Β—Η (terminal) linkage, with the two hydrogens oscillating asymmetrically about each boron (one stretches, while one contracts) and with the two ends of the molecule vibrating i n phase with each other. T h e upper right vibration represents an in-plane rocking of- the two B H parts of the molecule. The lower left vibration is one of the Β—IT (bridge) stretching modes, while the final one represents a deformation of the Η Β Η angle, the two ends vibrating out of phase with each other. A l l of these vibrations involve changes of dipole moment and hence are infrared active. I t is fortunate, especially for the analyst, that whenever certain atom groups or certain bonds appear i n a molecule, the characteristic frequencies associated with them usually appear i n the infrared and Raman spectra. Thus there are frequencies characteristic of C—Η bonds, Β—Η bonds, Β—C bonds, etc. I t is important to be able to recognize infrared bands as being characteristic of certain bonds, for this information can then be used to identify bonds and groups and to help determine structures of new compounds. This fundamental knowledge is best obtained through the study of the simpler molecules, of which diborane is an example. The infrared spectrum of diborane, with labels for the "characteristic" bands, is shown i n the upper part of Figure 2. These same bands appear (sometimes slightly shifted) i n other molecules containing the same atom groupings.


2

Deuterated Diboranes The lower half of Figure 2 illustrates the effect of replacing hydrogen atoms b y deuteriums. This generally causes a lowering of the frequencies—the heavier atoms

PERCENT DEUTERIUM

Figure

3.

Normal

distribution of Β Η _ Λ function of deuterium 2

β

species

as



142

can be pictured simply as moving more sluggishly—and for each B H band there is a corresponding one for B D at a lower frequency. Whenever B H and B D are mixed together, there is a rapid redistribution of hydrogen and deuterium, and an equilibrium is established among B H , B H D , B H D , B H D , B H D , B H D , and B D . Their statistically calculated concentrations as functions of the deuterium content are given i n Figure 3, where W represents the statistical weighting factors (binomial coefficients for six entities). I n order to obtain a spectrum of B H D a mixture of 9 5 % B H and 5 % B D was chosen. Figure 3 shows that this will result i n about 7 4 % of B H , 2 3 % of B H D , and a small amount of B H D — a rather unhandy mixture. B u t i n the Perkin-Elmer M o d e l 21 double-beam spectrophotometer, one may compensate for the 7 4 % of B H " i m p u r i t y " by using pure B H i n the compensating beam at the appropriate pressure. The small amount of B H D present can be ignored. 2

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5000 · ·

3000 · •

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Figure 4.

2000 ·

1500 •.»

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cm-'

1200 · ·

1000 900 · · · ·

·

8

10

WAVE LENGTH

MICRONS

a. Mixture of 9 5 % of B H and 5 % of B D B H D) b. Compensated B H D e

2

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e

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WAVE NUMBER

» ·

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2

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e

e

800 ·

700 ·

·

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6

·

14

( 7 4 % of B H + 2 3 % of 2

e

5

5

Figure 4 shows a spectrum of the uncompensated mixture and the compensated spectrum of B H D (the arrows indicate B H bands). A t this point an experimental difficulty should be mentioned. Whenever the absorption i n the compensating beam exceeds about 80 or 9 0 % , not enough energy is available to activate the pen. The pen response becomes sluggish and eventually ceases altogether. The curve traced out under this dead-pen condition has no relation to the absorption i n the sample beam and thus is meaningless. This condition is illustrated i n Figure 5. The center spectrum shows the 95/5 mixture when compensated b y B H . Whenever B H (in the compensating beam) absorbs strongly, the pen is dead (see arrows). I t is at these points that the principal curve i n Figure 4 is interrupted. T h e obvious remedy is to reduce sample pressures in both cells until the pen responds again (lower chart of Figure 5 ) . 2

5

2

2

e

6

2

6


143


LEHMANN ET A L

3 1«

t Ν


CO

id

>_ 41

"DEAD

" 21

B H 0 2

Figure 5. WAVE

5000 •

·

·

3000 ·

·

2000 ·...,

,.,·

Dead pen

NUMBER

1500 ·.

cm'

1200

• · . . . · - · .

ό

5

• • • f,

1

1000 900

800

8

WAVE

Figure 6.

LEN6TH

Contamination

a . B2D0 with some B2D3H b.

700

• m . a r r T m n ι π η τ π τ τ ^ΊΤΠΤΓΓΤΓΓΡΤΓIΤΓΒΜΤΙ 111 I I I I I I I I I I 1.1 Γ

C o m p e n s a t e d BsDe


ADVANCES IN CHEMISTRY SERIÇS

144

T o interpret the spectrum properly it must be realized that the formula " B H D " actually represents two compounds; the single substituent, deuterium, can be at the terminal or at the bridge position. Simple statistics would predict these two compounds to be present in a ratio of 2 to 1. A similar situation applies to B H D . T h e situation gets worse for B H D , where five compounds are possible—viz., an unsym2

2

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WAVE NUMBER

c m

1

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2000

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5000

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WAVE LENGTH

Figure 7.

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MICRONS

Diborane a n d its deutero derivatives


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LEHMANN ET AL.

145


metrically substituted di-terminal, a cis- and a trans-compound, a terminal-bridge, and a double-bridge substitution. Altogether the five molecular formulas for the partially deuterated diboranes represent 19 different species, each with its own spectrum (13). Because of this complexity the authors were unable to obtain good results for any but the B H D and B D H groups. Because deuterium normally is contaminated by a small amount of protium, an ordinary spectrum of deuterodiborane, B D , is contaminated b y B H D . Figure 6 presents B D contaminated with the usual amount (ca. 0.7%) of hydrogen, which produces ca. 4 % of B H D ; the lower chart has that contribution subtracted (indicated by arrows). B y use of compensation techniques the authors were able to de2

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2

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6 WAVE

Figure 8.

t 1EMGTH

10

12

MICRONS

Two methyldiboranes a n d parent compounds


14


146

termine the amount of B H D present to within about ± 0 . 2 % (out of 4%)—i.e., an accuracy of about 1 part i n 20. Thus they were able to determine the protium con centration to be 0.80 ± a few hundredths per cent. W i t h i n this error they were u n able to detect any deviations from the predicted statistical distribution, such as might be due to differences in bond strengths of Β—Η and Β—D. Figure 7 compares the spectra of B H , B H D , B H D , and B D and indicates some of the transitions. One interesting trend is that of the strong A - t y p e B H de formation band near 1200 c m . i n the upper chart. I n B H D this splits into two bands—a B H deformation still near 1200 c m . , and a B H D deformation near 1100 2

5

2

e

2

5

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e

2

- 1

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- 1


2

2


LEHMANN ET AL.

λ 47


c m . . B H D has a B H D band as well as a B D band, of a frequency (880 c m . ) similar to that i n B D . - 1

2

6

- 1

2

2

6

Alkyldiboranes


The substitution of alkyl groups (methyl and ethyl) on the parent diborane mole cule allows a study of the effects of terminal substitution on diborane and helps to resolve bridge vs. terminal assignments. One can substitute up to four a l k y l groups at the terminal positions but none at the bridges. Thus, even though the alkyl groups add complexity b y introducing vibrations internal to the alkyl groups, the picture as a whole is somewhat simplified by the elimination of bridge substitution. Although these alkyl compounds do disproportionate, the disproportionations are sufficiently slow to permit their spectra to be recorded.

Figure

10.

Schematic comparison of some monosubstituted diboranes

A s expected, the spectra of the alkyldiboranes have some of the characteristics of both diborane and trialkylborane, as shown i n Figure 8. A l l but B H have C—Η bands at 3000 c m . - , and a l l but B ( C H ) have Β—Η (terminal) bands at 2500 cm. and Β — H ' (bridge) bands at 1600 c m . - . Such comparisons aid considerably in the assignment of bands to group vibrations. Another technique that aids i n making assignments is to compare isotopic v a r i ants of a compound—for example, the monomethyldiboranes i n Figure 9. Bands caused by vibrations of the — B H part of the molecule will be common to the spectra of C H B H and C D B H , while — C H bands will be common to C H B H and C H B D . I n an analogous manner the — B D and — C D absorptions are de termined. Further checks on the — B H and — B D assignments can be obtained from examination of the spectra of B H D and B D H (as well as other mono-substi tuted diboranes). These relationships are schematically shown i n Figure 10. 2

1

3

- 1

1

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2

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h

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ή

V α

b Figure 11.

c

ci

e

B — H r m . band of some methyldiboranes t e


f

5


148 CLEAVAGE

α b c

unsym

d

OF DIBORANES

B

2

H

4

R B

2

H

5

- R

s y m - R

B

2

B

2

2

H H

2

" 1 * H ^

4

R

3

B

2

H

3

f

R

4

B

1

H

2


h

2R BH

3

3

2

2

2

2

*" 1 R B H 2

OF FRAGMENTS

R BH + BH X

2

3

R B + RBH* 3

R B

R B H

ba-1961-0032.ch015

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