7 Vibration Spectra and Structural Problems of Some Aminoboranes H E R M A N N J . BECHER and HANS T. BAECHLE
Downloaded by CORNELL UNIV on June 9, 2017 | http://pubs.acs.org Publication Date: January 1, 1964 | doi: 10.1021/ba-1964-0042.ch007
Laboratorium für anorganische Stuttgart, Germany
Chemie
, Technische
Hochschule,
The range of the BN-stretching frequencies and their coupling with the vibrations of adjacent bonds are discussed for monoaminoboranes with varying substitution. The decrease of the BN frequency by the introduction of phenyl groups on the nitrogen atom must be explained by a decrease in the BN force constant. The different resonance structures causing this change in the bond order as well as their dependence on steric strain are discussed, combining Raman, infrared, and ultraviolet spectroscopic data. The available observations suggest that a cis-configuration of two phenyl groups on the BN bond is unfavorable. Τ he molecular symmetry of a compound can be determined with the aid of its vibration spectrum, which also may permit a calculation or estimate of some bond force constants. Since the complete assign ment of vibrational spectra with many frequencies is very difficult, the interpretation of spectra is often restricted to vibrations of selected bonds or bond angles. However, the coupling between dif ferent vibrations can change the value of a special group frequency. This depends on the modes and values of frequencies which belong to the same symmetry species. Therefore coupling influences on more or less characteristic vibrations must be studied, either by compari son of different spectra with varying substituents on the characteristic group, or by frequency calculation of the isolated group frequencies on the one side and of a complete set of coupled frequencies on the other. The present paper deals with the assignment of vibrations of the BN linkage and of adjacent bonds or bond angles in aminoboranes, and also draws some conclusions concerning the bond order of the BN linkage and its dependence on steric influences. For some aminoboranes of the type X BNR (X = CH , CI, or B r and R = Η or CH ) a coplanar configuration of trigonal sp bonds for both Β and Ν atoms was evidenced by the interpretation of vibration spectra (5, 9). When substituents X and R of these aminoboranes are single atoms or can be regarded as nearly rigid groups with respect to the other vibrations of the molecule, the grouping X B N R exhibits 2
2
3
2
3
2
2
71
Niedenzu; Boron-Nitrogen Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1964.
72
A D V A N C E S IN C H E M I S T R Y SERIES
Table I.
(CH ) BNH
2
(CH ) BND
2
3
2
3
2
(CH ) BN(CH )
s 2
2
s
6
NC
Symmetric Vibrations
318
710
?
687
255?
v NC
2
s
s
433
645
C1 BN(CH )
2
257
391
530
923
Br BN(CH )
2
167
316
480
867
3
2
3
2
3
2
3
2
2
990
A l l frequencies i n c m .
Downloaded by CORNELL UNIV on June 9, 2017 | http://pubs.acs.org Publication Date: January 1, 1964 | doi: 10.1021/ba-1964-0042.ch007
Table II. Raman and Infrared Raman
Infrared
220 m - s
Ô
S
Ô
C
B
C
2
1
305 m 410 w 448 m - s 540 m
6' 1 A
"S 2 ÔC ,B B C 1
620 m
6
695 s
1
? 6* 2 TCH, B C
B
770 m - w
765 s
800 m
790 m
i, N-C ,A
835 vw
yCH, A
905
yCH,B
840 w
2
m-w
920 vs
925 v s
950 ?
955 m
2
6
1
2
2
BC1, as 10
