High resolution infrared spectra and structure of borine carbonyl

JOURNAL OF MOLECULAR

High

SPECTROSCOPY

Resolution

The Analysis

53, 120-127 (1974)

Infrared Spectra of Borine Carbonyl

and Structure

of the vz bands of “BD,CO

C. PJ?PIN, L. LAMBERT, AND

and 10BD3C0

A. CABANA

D&parlementde Chimie, Universitb de Sherbrooke, Sherbrooke, Qutbec, Canada The high resolution infrared spectra of iOBD&O and “BDKO were obtained in the region of the y2 fundamentals. The K = 0 subbands of the ~2 and YS+ vg - ~8bands were assigned. The K structure is unresolvable with a spectral slit width of 0.03 cm-r. A series, almost as strong as the main series, has been identified in the spectrum of the nB molecule but has not been assigned. It is shown that the four structural parameters of borine carbonyl cannot be accurately determined using only the Bo rotational constants of four isotopically substituted molecules. INTRODUCTION

Borine carbonyl is a symmetric top molecule with CsUsymmetry. Therefore the molecule has four totally symmetric vibrations (labelled ~1 to ~4) and four doubly degenerate vibrations (labelled ~5 to ~8). Low resolution infrared and Raman spectra have been recorded and assigned. (1-3). The microwave spectra of the iOB, llB, Ha, and Da isotopic modifications have been obtained by Gordy, Ring, and Burg (4) and used to determine the four structural parameters. We have recently completed the high resolution infrared studies of the v4 (B-C stretching) vibrations of llBHBCO (5) and of l”BH3C0 (6). In this paper we wish to report the high resolution infrared spectra of 1°BD3C0 and of nBD&O in the region of the v2 fundamentals and to discuss the difficulties encountered in the determination of the structure of borine carbonyl using only the B. rotational constants of the four isotopic species referred to above. EXPERIMENTAL

METHODS

Isotopically enriched “BzD,r and l”B2Ds molecules were prepared by reduction of 95% enriched boron trifluoride etherate solution with lithium aluminum deuteride according to a method described by Shapiro et aE. (7). Deuterated borine carbonyl was then obtained from deuterated diborane by mixing the latter molecule with a large excess of CO. Since the infrared absorption of carbon monoxide coincides almost exactly with the bands of interest and since it is not possible to eliminate completely the presence of CO in borine carbonyl the spectrum of the latter molecule will always be superimposed on that of CO. However since the rotational constant of carbon monoxide is more than five 120 Copyright @ 1974 by Academic Press, Inc. AII rights of reproduction in any form reserved.

THE vq BANDS OF ‘“BDSCO AND “BDKO

121

times larger than the rotational constant B of DBSCO only about 20% of the lines of interest are overlapped by those of CO. A 2.5-m Littrow spectrometer was used to obtain the spectra. The grating, a 30 lines/mm Bausch and Lomb echelle blazed at 63’ (size 100 X 200 mm), was used in the thirteenth order (8). A liquid nitrogen cooled PbSe detector, obtained from the Santa Barbara Research Center, was used. The source was a carbon rod dissipating about 2 to 2.5 kW. The full width at half height of isolated absorption lines varied from 0.027 to 0.035 cm-‘. Our sample was constained in a l-m absorption cell of the type described by White (9). The mirrors were set to give four traversals. The spectra were recorded at various pressures in the range 0.5-4 Torr. The ratio of borine carbonyl to carbon monoxide was also varied; spectra with higher concentration of CO provide the best calibration since the CO absorption lines are then not significantly shifted by overlapping with the absorption bands of borine carbonyl. One of the spectra of the “BDQCO molecule was obtained at dry ice temperature in order to reduce the intensity of the “hot” bands : this however did not improve our results. The wavenumbers used for the calculation are average wavenumbers calculated from two spectra for the l”B molecule and from three spectra for the llB molecule. The observed wavenumbers are not reported in this paper but may be obtained from the authors or the Editorial Office upon request. The wavenumbers (in cm-*) of the 1 +- 0 absorption lines of CO used for the calibration were taken from Rao et al. (10). These wavenumbers were fitted with a polynomiaI of degree three; the average residues were of the order of 0.0019 cm-l. ASSIGNMENT

OF THE OBSERVED SPECTRA

Figure 1 shows a spectrum of ‘OBD&O in the region of 4.6 pm. This trace corresponds to the highest sample pressure used (4 mm). The spectrum of llBD&O is very similar and has not been reproduced here. The general appearance of the vz bands of both species is that of a parallel band of a symmetric top molecule with an intense hot band superimposed. Two Q branches are clearly observed in the crowded region of the band center and there is evidence for a third Q branch at slightly lower frequencies [around R (5)of CO; see Fig. 11. The assignment of the K = 0 subbands of the v2 and v2 •l- ~8 - vs bands of both molecules presented no difficulties. These lines account for less than one half of the measured lines, and much effort was spent in trying to assign the remaining lines either to other hot bands or to K structure. Despite the fact that the Boltzmann factor for the level 2~8 is 12% of the ground state, we have not succeeded in assigning unambiguously any other “hot” band. This is certainly attributable to the overlapping of a great number of transitions of nearly equal intensity occurring in the same region. Several attempts were made to assign some of the weaker features to various subbands but none of these proved to be successful. It was concluded that the subbands with different values of K all fall on top of each other, i.e., with the resolution available the K structure is unresolvable. A band of similar intensity to the main band is easily identified in the spectrum of the “R molecule. This is clearly shown by the Loomis-Wood diagram of Fig. 2. Such a series is not found on the spectrum of the l”B molecule.

122

F’GPIN, LAMBERT

AND CABANA

Since all the strong Q branches have already been assigned to either v2 or its accompanying hot bands, it must be assumed that this other band has a Q branch coincident with that of ~2. Both these strong bands have similar parabola on the LoomisWood diagram and consequently they must also have similar rotation-vibration constants. These assumptions and the similar strengths of both these bands imply a Fermi resonance. Finally this Fermi resonance must be of a high order for no binary or

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FIG. la. The R branch of the ~2 band of ‘(‘BDaCO.

FIG. lb. The band center of the v2 band of l”BDXO.

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FIG. Ic. The P branch

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of the vq band of 10BD,CO.

ternary combinations are expected to fall in this spectral region. This implies fairly rigid conditions on the separation of the unperturbed band origins and on the Fermi interaction parameter. DETERMINATION

OF CONSTANTS

Once the assignment of individual vibration-rotation lines had been veritied, by the calculation of appropriate combination differences, constants for the four bands were obtained using a method similar to that of Brown and Overend (II), but modified for a parallel band. In this method the band origin and the rotational constants for the upper and lower vibrational states are simultaneously determined by applying at least-squares adjustment to all of the observed wavenumbers using the standard equations for a symmetric top (12). The results are collected in Tables I and II. The agreement between the ground state rotational constants B derived in this study and those previously obtained from microwave (4) is excellent. THE

STRUCTURE

OF BORINE

CARBONYL

The internuclear distances and bond angles of borine carbonyl have been previously calculated by Gordy, Ring, and Burg (4) from measurements of the microwave spectra of four isotopic species. There are four molecular parameters describing the molecular geometry and in principle the four rotational constants are sufficient to determine the structure completely. However, it may be seen from the equations given below (4) that the BH bond length and the HBH angle are very strongly correlated. The moment of inertia IB is: Je

=

3Mn (x’ + dnn2 sin2ff/2) + Mn (x - f&n cos a)2 + Mo(dnc

+ dco - X +

~BHCOS~)~

+

MC(~BC

-

X +

~BH

COSa)’

124

P&PIN,

LAMBERT

AND

CABANA

I.S.10.

,.*.sa. I.L..OI 1.1.301

-., -4s -..

-¶a

THE

~2 BANDS

OF “‘BD,CO

AND

125

“BDXO

where

-t’s (Y= 180” -

< HBC,


= 2 sin-’

-sin 2

< HBC >

and

M&BC + durs cosa) + MB~BHcosa + Mo(dec

+ tlco + dBH cos a)

L=

These expressions were used to recalculate the bond lengths and bond angle using the infrared Bo rotational constants reported in this and previous papers (5, 6). The agreement between the set of parameters obtained in this way and the one previously reported (4) is very poor. In order to better understand this disagreement we have recalculated the four structural parameters using the data of Gordy et al. first with six figure precision, as quoted, and then rounding off to five figure precision. The results of the second calculation agree with those reported (4) but differ significantly with those of the first calculation. The results of the first calculation are reported in the column labeled m.w. of Table III.

FIG. 2. The Loomis-Wood diagram for the region of the ~2 band of “BD&O. The circles are for the main band. The series running across is the main hot band. The series almost parallel to the main band is not identified.

126

PEPIN,

LAMBERT

AND

CABANA

Therefore, it must be concluded that the structure of borine carbonyl is stiI1 not well determined. An accurate determination of the molecular structure would be achieved through the A rotational constant. This requires the assignment of forbidden vibrationrotation transitions. Such a study has been undertaken in this laboratory in the vg absorption region of *OBH&O and r’BH&O where Fermi and Coriolis resonances take place.

%.H.

Bauer,

Ref.

E. diff.=

PARAMETER

J.

A”.

cllem.

SOC.

“,

1804

4

m.w.

(1937)

ACKNOWLEDGMENTS Financial support was provided by the “Minist&re de 1’Education de la Province de QuCbec” and by

THE

~2 BANDS

OF “‘BD,CO

AND *‘BD,CO

127

the National Research Council of Canada. The authors wish to express their thanks to Dr. John M. R. Stone for several helpful discussions concerning the work reported in this paper. RECEIVED:

March 25, 1974 REFERENCES

1. 2. 3. J. 5. 6. 7. 8. 9. 10. 11. 12.

R. D. COWAN,J. Ckem. Phys. 18, 1101 (1950). G. W. BETH~E ANDM. K. WILSON, J. Chex~ Plzys. 26, 1118 (1957). R. C. TAYLOR, J. Chem. Phys. 26, 1131 (1957). W. GORDY, H. RING, ANDA. B. BURG, Phys. Rev. 78, 512 (1950). 1~.LAMBERT,C. PI%PIN,ANDA. CABANA,J. Mol. Spectrosc. 44, 578 (1972). C. PBPIN, L. LAMBEKT,AND A. CABANA, &z. J. Spectrosc. 18, 61 (1973). I. SHAPIRO,H. G. WEISS, M. SCHMICH,S. SI