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Microwave spectrum and molecular structure of arsenic tribromide1
Journal of Molecular Structure, 35 (1976) 81-84 oEIsevier Scientific Publishing Company, Amsterdam -
MICROWAVE SPECTRUM AND MOLECULAR ARSENIC TRIBROMIDE”
Printed in The Netherlands
STRUCTURE OF
A G. ROBIETIE Department of Chemistry, The University,
Reading,
Berkshire RG6
2AD
(Great Britain)
(Received 4 May 1976)
ABSTRACT
The microwave spectrum of arsenic ttibromide has been recorded in the frequency range 26.5-40.0 GHz. From the rotational constants for the symmetric top species “AzBr, and *‘AsBra the following structural parameters have been deduced: r,(AsBr) = 2.324 * 0.003 A, e,(BrAsBr) = 99.8 f 0.2 “_ These results are in excellent agreement with the parameters obtained by Samdal et al., in a concurrent electron diffraction study. INTRODUCTION
In recent years there have been several structural studies of the phosphorus and arsenic trihalides by electron diffraction and microwave spectroscopy [l-6]. Samdal et al. [7] have investigated arsenic tribromide, AsBrs, by electron diffraction in order to determine the structural parameters and vibrational amplitudes: they have used the amplitudes to characterise the quadratic force field by the methods of Iwasaki and Hedberg Cl]. Since the microwave spectrum of arsenic tribromide has apparently never been reported, the present work was undertaken. EXPERIMENTAL
The sample, of commercial origin, was provided by Dr. S. Samdal. It was sublimed in a vacuum line before use. No microwave lines attributable to impurities were observed. Microwave spectra were recorded on a Hewlett Packard 84608 spectrometer in the frequency range 26.5-40.0 GHz. Sample pressures were - 20-30 mtorr and the spectra were recorded at room temperature. All features in the spectra were rather broad since they contained many unresolved components of different K, and consequently the accuracy of measurement is believed to he only +-0.1 MHz.
*For details of a concurrent electron diffraction study of gaseous AsBr, by SamdaI, Bamhart and Hedberg, see pages 67-80 of this issue.
82
OBSERVED SPECTRUM
The characteristic spectra of the symmetric top isotopic species As7’Br, and As8*Br3 were readily identified. Clumps of lines due to the asymmetric top species As7’BrZg1Br and As7’Brg1Br2 were also observed, but these spectra were not sufficiently well resolved for an analysis to be possible. Complex patterns of vibrational satellites were observed for all isotopic species; unfortunately these overlapped to such an extent that no convincing assignment could be made even for the symmetric top species. Thus, only the spectra of the two symmetric top species in their vibrational ground states are reported here. The (J+ 1) + J transitions of the symmetric top species give rise to features which are noticeably asymmetric, degrading to high frequency. This is caused by the unresolved K structure. Computer simulations showed that the asymmetry of the envelope is compatible with D,, 2: -0.2 kHz. The measured peak positions were corrected to give estimated K = 0 transition origins using this value of DJK, and the resulting frequencies are given in Table 1, The frequencies were fitted to the equation Y = 28,(J+
1) -40&J+
1)3
for each isotopic species. The molecular constants obtained are also given in Table 1, together with values of D, calculated for both species from the general harmonic force field of Samdal et al. [7]. HARMONIC FORCE FIELD
The agreement of observed and calculated DJ constants shown in Table 1 is reasonable, bearing in mind that the force field was derived from vibrational frequencies not corrected for anharmonicity, and that the observed D, constants have not been extrapolated to equilibrium values. The force field also predicts D,, to be -0.25 kHz in satisfactory agreement with the rough estimate of -0.2 kHz from the observed asymmetry of the microwave peaks. The force field of Samdal et al. [ 73 was therefore adopted without change from the molecular structure calculations described below. It is possible to adjust the harmonic force field to give slightly better agreement with the observed distortion constants at the expense of slightly worse agreement with the observed vibrational amplitudes, but such changes in the force field are quite insignificant from the point of view of the structure calculations. MOLECULAR STRUCTURE
The two experimental B. constants may be used to derive the traditional “rO structure”, which is in fact a mixed ro/rS structure. It is, however, preferable to calculate the zero-point average or rZ structure [S] by the methods of Kuchitsu and co-workers 19, lo] since the rz structure has a
83 TABLE
1
Observed microwave transitions, frequency fit, and rotational and centrifugal distortion constants for arsenic tribromide (J+l)cJ
As79Br3
As*‘Br 3
%IXM*
A MHza
“oh MHz
26795.74
0.00 -0.03 0.01 0.01 Q.03 0.04 0.04 -0.05
-
15 + 14 16+ 15 17 + 16 18 + 17 19 + 18 206 19 21620 22-+ 21
28581.75 30367.76 32153.67 33939.46 35725.28 37510.94 39269.44
B, MHzb DJ kHz (obs)b 0, kHz (calc)C
893.269 A 0.005 0.172 +- 0.006 0.156
27939.22
29685.08 31430.84 33176.62 34922.29 36667.80 38413.22
A MHza
-0.02 -0.01 -0.03 0.04 0.07 0.01 -0.05
873.183 ZFZ 0.006 0.159 * 0.007 0.150
aA =v - vcalcbUnce%&nties quoted are 30. CCalculated from force field of Samdal et al., (see text).
clearly defined physical meaning and is convenient for comparison with the electron diffraction results. These calculations are summarised in Table 2. The isotopic change in the rz bond length, defined as 6 r, = rz (As”Br) - rz (As’lBr), was calculated using published formulae [9, lo] to be (1.7 f 0.5) X lo-’ A, assuming the Morse anharmonicity constant a for the bonds to be 1.8 + 0.5 A-‘. The isotope effect on LBrAsBr, subsequently denoted 8, was assumed to be zero. Since for this molecule both 6r, and the rotational constant corrections (Bo -B, ) are small, the rz structure differs little from the r. structure. The preferred results are those including the effect of 6 rz, and the final structure is taken to be: r,(AsBr) = 2.324 If:0.003 A, 8, = 99.8 h 0.2 O. The errors quoted are subjective estimates of the possible systematic error in the procedure used to derive the rz structure, which is likely to be much more important than the purely experimental errors in the rotational constants. From the spectroscopic rz sticture it ia possible to predict the electron diffraction structural parameters [9, lo]. For a temperature of 300 K the predicted param etersare:r, (AsBr) = 2.326 f 0.003 II, 0, = 995 It O-2 “_ These compare very satisfactorily with those determined experimentally by Samdal et al. [73 at 373 K, r,(AsBr) = 2.3244 + 0.0024 A, 8, = 99.64 f 0.13 O.
84 TABLE
2
r, and r, structures
for arsenic tribromide r( AsBr)
r, structure rz structure (6rz = 0) i-z structure (6’; = 1.7 X lO_ Estimated uncertainty in ‘; structure
A)
2.323, 2.324, 2.323, 0.003
LBrAsBr A A A A
99.7; 99.7,O 99.8, 0.2 o
In the rz structure calculations, for AsT9Br3, 0.76, MHz.
the calculated values of ($3, - EL ) used were 0.79, MHz MHz for As8’Brn. The conversion factor used was I X B = 505379 u A2
ACKNOWLEDGEMENT
I am indebted to Professor Hedberg and Dr. Samdal for drawing this problem and for interesting discussions of the electron diffraction results. to my attention
REFERENCES 1 M. Iwasaki 2 Y. Morino, 3 Y. Merino, T. Shibata, 4 S. Konaka
and K. Hedberg, J. Chem. Phys., 36 (1962) 594. T. Ukaji and T. Ito, Bull. Chem. Sot. Jpn., 39 (1966) 71. K. Kuchitsu and T. Moritani, Inorg. Chem., 8 (1969) 867; K. Kuchitsu, A. Yokozeki and C!. Matsumura, Inorg. Chem., 10 (1971) 2584. and M. Kimura, Bull. Chem. Sot. Jpn., 43 (1970) 1693; S. Konaka, Bull.
Chem. Sot. Jpn., 43 (1970) 3107_ 5 F. B. Clippard and L. S. Bartell, Inorg. Chem., 9 (1970) 805. 6 G. Cazzoli, J. Mol. Spectrosc., 53 (1974) 37. 7 S. Samdal, D. M. Bamhart and K. Hedberg, J. Mol. Struct., 35 (1976) paper). 8 T. Oka, J. Phys. Sot. Jpn., 15 (1960) 2274. 9 K. Kuchitsu, 10 K. Kuchitsu,
J. Chem. Phys., 49 (1968) 4456. T. Fukuyama and Y. Merino, J. Mol. Struct., 1 (1968)
T. Fukuyama and Y. Morino, J. Mol. Struct., 4 (1969)
41.
67, (preceding
463;
K. Kuchitsu,
MICROWAVE SPECTRUM AND MOLECULAR ARSENIC TRIBROMIDE”
Printed in The Netherlands
STRUCTURE OF
A G. ROBIETIE Department of Chemistry, The University,
Reading,
Berkshire RG6
2AD
(Great Britain)
(Received 4 May 1976)
ABSTRACT
The microwave spectrum of arsenic ttibromide has been recorded in the frequency range 26.5-40.0 GHz. From the rotational constants for the symmetric top species “AzBr, and *‘AsBra the following structural parameters have been deduced: r,(AsBr) = 2.324 * 0.003 A, e,(BrAsBr) = 99.8 f 0.2 “_ These results are in excellent agreement with the parameters obtained by Samdal et al., in a concurrent electron diffraction study. INTRODUCTION
In recent years there have been several structural studies of the phosphorus and arsenic trihalides by electron diffraction and microwave spectroscopy [l-6]. Samdal et al. [7] have investigated arsenic tribromide, AsBrs, by electron diffraction in order to determine the structural parameters and vibrational amplitudes: they have used the amplitudes to characterise the quadratic force field by the methods of Iwasaki and Hedberg Cl]. Since the microwave spectrum of arsenic tribromide has apparently never been reported, the present work was undertaken. EXPERIMENTAL
The sample, of commercial origin, was provided by Dr. S. Samdal. It was sublimed in a vacuum line before use. No microwave lines attributable to impurities were observed. Microwave spectra were recorded on a Hewlett Packard 84608 spectrometer in the frequency range 26.5-40.0 GHz. Sample pressures were - 20-30 mtorr and the spectra were recorded at room temperature. All features in the spectra were rather broad since they contained many unresolved components of different K, and consequently the accuracy of measurement is believed to he only +-0.1 MHz.
*For details of a concurrent electron diffraction study of gaseous AsBr, by SamdaI, Bamhart and Hedberg, see pages 67-80 of this issue.
82
OBSERVED SPECTRUM
The characteristic spectra of the symmetric top isotopic species As7’Br, and As8*Br3 were readily identified. Clumps of lines due to the asymmetric top species As7’BrZg1Br and As7’Brg1Br2 were also observed, but these spectra were not sufficiently well resolved for an analysis to be possible. Complex patterns of vibrational satellites were observed for all isotopic species; unfortunately these overlapped to such an extent that no convincing assignment could be made even for the symmetric top species. Thus, only the spectra of the two symmetric top species in their vibrational ground states are reported here. The (J+ 1) + J transitions of the symmetric top species give rise to features which are noticeably asymmetric, degrading to high frequency. This is caused by the unresolved K structure. Computer simulations showed that the asymmetry of the envelope is compatible with D,, 2: -0.2 kHz. The measured peak positions were corrected to give estimated K = 0 transition origins using this value of DJK, and the resulting frequencies are given in Table 1, The frequencies were fitted to the equation Y = 28,(J+
1) -40&J+
1)3
for each isotopic species. The molecular constants obtained are also given in Table 1, together with values of D, calculated for both species from the general harmonic force field of Samdal et al. [7]. HARMONIC FORCE FIELD
The agreement of observed and calculated DJ constants shown in Table 1 is reasonable, bearing in mind that the force field was derived from vibrational frequencies not corrected for anharmonicity, and that the observed D, constants have not been extrapolated to equilibrium values. The force field also predicts D,, to be -0.25 kHz in satisfactory agreement with the rough estimate of -0.2 kHz from the observed asymmetry of the microwave peaks. The force field of Samdal et al. [ 73 was therefore adopted without change from the molecular structure calculations described below. It is possible to adjust the harmonic force field to give slightly better agreement with the observed distortion constants at the expense of slightly worse agreement with the observed vibrational amplitudes, but such changes in the force field are quite insignificant from the point of view of the structure calculations. MOLECULAR STRUCTURE
The two experimental B. constants may be used to derive the traditional “rO structure”, which is in fact a mixed ro/rS structure. It is, however, preferable to calculate the zero-point average or rZ structure [S] by the methods of Kuchitsu and co-workers 19, lo] since the rz structure has a
83 TABLE
1
Observed microwave transitions, frequency fit, and rotational and centrifugal distortion constants for arsenic tribromide (J+l)cJ
As79Br3
As*‘Br 3
%IXM*
A MHza
“oh MHz
26795.74
0.00 -0.03 0.01 0.01 Q.03 0.04 0.04 -0.05
-
15 + 14 16+ 15 17 + 16 18 + 17 19 + 18 206 19 21620 22-+ 21
28581.75 30367.76 32153.67 33939.46 35725.28 37510.94 39269.44
B, MHzb DJ kHz (obs)b 0, kHz (calc)C
893.269 A 0.005 0.172 +- 0.006 0.156
27939.22
29685.08 31430.84 33176.62 34922.29 36667.80 38413.22
A MHza
-0.02 -0.01 -0.03 0.04 0.07 0.01 -0.05
873.183 ZFZ 0.006 0.159 * 0.007 0.150
aA =v - vcalcbUnce%&nties quoted are 30. CCalculated from force field of Samdal et al., (see text).
clearly defined physical meaning and is convenient for comparison with the electron diffraction results. These calculations are summarised in Table 2. The isotopic change in the rz bond length, defined as 6 r, = rz (As”Br) - rz (As’lBr), was calculated using published formulae [9, lo] to be (1.7 f 0.5) X lo-’ A, assuming the Morse anharmonicity constant a for the bonds to be 1.8 + 0.5 A-‘. The isotope effect on LBrAsBr, subsequently denoted 8, was assumed to be zero. Since for this molecule both 6r, and the rotational constant corrections (Bo -B, ) are small, the rz structure differs little from the r. structure. The preferred results are those including the effect of 6 rz, and the final structure is taken to be: r,(AsBr) = 2.324 If:0.003 A, 8, = 99.8 h 0.2 O. The errors quoted are subjective estimates of the possible systematic error in the procedure used to derive the rz structure, which is likely to be much more important than the purely experimental errors in the rotational constants. From the spectroscopic rz sticture it ia possible to predict the electron diffraction structural parameters [9, lo]. For a temperature of 300 K the predicted param etersare:r, (AsBr) = 2.326 f 0.003 II, 0, = 995 It O-2 “_ These compare very satisfactorily with those determined experimentally by Samdal et al. [73 at 373 K, r,(AsBr) = 2.3244 + 0.0024 A, 8, = 99.64 f 0.13 O.
84 TABLE
2
r, and r, structures
for arsenic tribromide r( AsBr)
r, structure rz structure (6rz = 0) i-z structure (6’; = 1.7 X lO_ Estimated uncertainty in ‘; structure
A)
2.323, 2.324, 2.323, 0.003
LBrAsBr A A A A
99.7; 99.7,O 99.8, 0.2 o
In the rz structure calculations, for AsT9Br3, 0.76, MHz.
the calculated values of ($3, - EL ) used were 0.79, MHz MHz for As8’Brn. The conversion factor used was I X B = 505379 u A2
ACKNOWLEDGEMENT
I am indebted to Professor Hedberg and Dr. Samdal for drawing this problem and for interesting discussions of the electron diffraction results. to my attention
REFERENCES 1 M. Iwasaki 2 Y. Morino, 3 Y. Merino, T. Shibata, 4 S. Konaka
and K. Hedberg, J. Chem. Phys., 36 (1962) 594. T. Ukaji and T. Ito, Bull. Chem. Sot. Jpn., 39 (1966) 71. K. Kuchitsu and T. Moritani, Inorg. Chem., 8 (1969) 867; K. Kuchitsu, A. Yokozeki and C!. Matsumura, Inorg. Chem., 10 (1971) 2584. and M. Kimura, Bull. Chem. Sot. Jpn., 43 (1970) 1693; S. Konaka, Bull.
Chem. Sot. Jpn., 43 (1970) 3107_ 5 F. B. Clippard and L. S. Bartell, Inorg. Chem., 9 (1970) 805. 6 G. Cazzoli, J. Mol. Spectrosc., 53 (1974) 37. 7 S. Samdal, D. M. Bamhart and K. Hedberg, J. Mol. Struct., 35 (1976) paper). 8 T. Oka, J. Phys. Sot. Jpn., 15 (1960) 2274. 9 K. Kuchitsu, 10 K. Kuchitsu,
J. Chem. Phys., 49 (1968) 4456. T. Fukuyama and Y. Merino, J. Mol. Struct., 1 (1968)
T. Fukuyama and Y. Morino, J. Mol. Struct., 4 (1969)
41.
67, (preceding
463;
K. Kuchitsu,