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The molecular structure of antimony pentafluoride
SpectroehimicaActa, 1957, Vol. 10, pp 57 to 60. Pergamon Press Ltd., London
The molecular structure of antimony pentafluoride J. GAUNT and J. B. AINSCOUOI~ United Kingdom Atomic Energy Authority, Atomic Energy Research Establishment Harwell, Didcot, Berks, and Research and Development Branch United Kingdom Atomic Energy Authority, Industrial Group Springfields Works, Salwick, Preston, Lancs.
(Received 6 June 1957)
Abstract~The infra-red and Raman spectra of antimony pentafluoride have been investigated. Five Raman lines were observed and have been assigned as fundamentals. An interpretation" of the infra-red spectrum has been made in a manner consistent with the rigid selection rules for the trigonal bipyramid structure (D3h). The thermodynamic properties have been calculated on the simple harmonic oscillator approximation.
Introduction I~ T~E previous paper [1] the results of an examination of the infra-red and R a m a n spectra of niobium pentaehloride have been reported. During the course of t h a t work it was decided that, to provide a comparison and in order to facilitate interpretation of the niobium pentachloride spectra, the infra-red and R a m a n spectra of a n t i m o n y pentafluoride should be investigated. This, to the best of our knowledge, had not previously been examined and it seemed probable t h a t niobium pentach]oride and a n t i m o n y pentafluoride might have the same structure.
Experimental The a n t i m o n y pentafluoride was supplied by Dr. P. L. ROBI~SOX of King's College, Newcastle. I t was a colourless, viscous liquid which fumed violently in moist air, and so was handled in an all-glass vacuum system. Since a n t i m o n y pentafluoride attacks hydrocarbon tap grease, all taps and joints which were likely to come into contact with the vapour were lubricated with a fluorocarbon grease. The vapour pressure of the pentafluoride is quite low, about 4 m m at 20°C [2], but was sufficient to give a satisfactory infra-red'sprectrum in a 35 cm gas ceil. The vapour attacked the windows of this cell forming a greenish-yellow deposit on them. The infra-red spectrum of the vapour was observed on a Hilger H 800 recording, double beam, prism speetrophotometer over the range 5000-310 cm -1, using the same prisms and filter as in the work on niobium pentachloride. The absorption cell was of glass with windows of both potassium bromide and caesium bromide attached by fluorocarbon grease. The infra-red spectrum of liquid antimony pentafluoride was also observed using a 0.1 mm cell fitted with caesium bromide windows. The apparatus used to excite the R a m a n spectrum was fairly conventional and has been described elsewhere [3]. The liquid sample was contained in a tube surrounded by three concentric vessels containing cooling water, a 0.5 cm layer 57
J . Gx~-~T a n d J . B. AINSCOUGH
of saturated sodium nitrite solution, and a 0.5 em layer of saturated rhodamine G solution to isolate the 4358 A mercury line which was used for excitation. The spectra were photograhed on two spectrographs, a Hilger F/4 and a Hilger medium, using glass optics; exposure times ranged from a few hours to a week. Spectra were calibrated by superimposing an iron are on the R a m a n lines. The plates were examined under a travelling microscope and on a recording microphotometer.
Discussion So far as we are aware no work has previously been carried out to elucidate the molecular structure of a n t i m o n y pentafluoride. However, like niobium pentachloride, the molecule is probably either a plane pentagon, a square pyramid, or a trigonal bipyramid. The observation of five R a m a n lines immediately excludes the first of these but is not sufficient evidence to distinguish between the square pyramid and the trigonal bipyramid as the former has seven and the latter six R a m a n active frequencies [4]. The more probable structure seems to be a trigonal bipyramid and an a t t e m p t has been made to interpret the spectra on this assumption. The strong, sharp line observed in the R a m a n spectrum at 667 cm -1 can probably be assigned as ~1, the symmetrical stretching frequency; and the infra-red absorption at 710 cm -1, which is also observed in the R a m a n spectrum at 716 cm -1, can probably be assigned as v5. An a t t e m p t has been made to assign the other observed R a m a n frequencies as fundamentals and then to interpret the observed infra-red absorption bands in a way consistent with the rigid selection rules for a molecule of D3~ symmetry. I n order to make a complete assignment of the fundamental bands, a R a m a n line at 498 cm -1, which was not in fact observed, has been postulated, though an adsorption band at 491 cm -1 was observed in the infra-red spectrum. The assignment also requires an infra-red fundamental at 294 cm -1. No definite peak could be observed here as this is outside the present instrumental range but the spectrophotometer trace shows t h a t a very strong absorption occurs below 315 cm -~ and this is strong evidence for a fundamental absorption band in this region. Weak absorption bands at 1195, 1130 and 1076 cm -~ were also observed but as these were found in the spectrum of an e m p t y cell after the a n t i m o n y pentafluoride had been pumped out, t h e y were attributed to absorption by the deposit formed on the cell windows. An absorption at 1030 cm -1 was attributed to silicon tetrafluoride. Some supporting evidence for the assumption of the D3h structure, in the case of the niobium compound, has been cited previously [1] and as there are quite marked similarities in the order of the assignments of the fundamental frequencies of the two molecules we feel t h a t this provides strong evidence for assuming t h a t a n t i m o n y pentafluoride molecule is also a trigonal bipyramid. The fundamental frequencies with their respective symmetries, degeneracies and activities are given in Table 1, while the interpretation of the infra-red spectrum is given in Table 2. In order to calculate the thermodynamic properties of a n t i m o n y pentafluoride it has been necessary to estimate the S b - - F bond length. The A s - - F distance in arsenic trifluoride is 1.71 A while the As--C1 and Sb--C1 distances in the trichlorides are 2.16 A and 2.37 A respectively. Assuming t h a t the difference between 58
The molecular structure of antimony pentafluoride
the bond lengths in the two fluorides is the same as in the chlorides a S b - - F bond length of 1.92 A is obtained for antimony trifluoride. There is probably some increase in bond length in going from the trifluoride to the pentafluoride as there T a b l e 1. F u n d a m e n t a l v i b r a t i o n s of a n t i m o n y p e n t a f l u o r i d e Activity
F r e q u e n c y (cm -1) calculated
A 1' A 1" A2 ~
R R IR IR
667 264 294 212
~'5
E'
R and IR
713
~'6
E I E' E~
R and IR R and IR R
498 107 228
Assignment
Symmetry
7'1
?)2 ?)3 ?"4
Y7 ~'8
Degeneracy
F r e q u e n c y (cm -1)
667 s. 264 m. (diffuse) Not observed Not observed 710 (IR) s. 716 (R) s. (diffuse) 491 ( I R only)m. ca. 90(R only)v.w. 228 m. (diffuse)
T a b l e 2. I n t e r p r e t a t i o n of t h e i n f r a - r e d s p r e c t u m of a n t i m o n y p e n t a f l u o r i d e Observed frequency
(era-l)
1419 m.* 1140 w. t 760 s. 727 s. 710 s. 684 s. 517 s. 491 m. 478 w. 439 w. 335 s. 326 s. c a 300 s.
Calculated frequency ( c m -1 )
1426 1139 762 726 713 684 522 498 476 440 335 321 294
Assignment
2v 5 2~ 4 + v 5 ~e + vs
Y5 3~ 8
~e ~2 -~- ~4 ~'4 "~ ~8 ~7 -+- v8
3v 7
* Observed only in the spectrum of the liquid. Observed in b o t h the liquid and vapour spectra.
is, for example, in the phosphorus fluorides where the P - - F distance is 1-52 A in the trifluoride and 1.57 A in the pentafluoride and so 2.00 A has been taken as the S b - - F distance in antimony pentafluoride. The sum of the covalent radii is 2.05 A. An error of 0.01 A in the bond length leads to a error of about 0.03 eal/deg.mole in the values of S O and of --(G- Eo)/T. On the basis of this bond length and using the fundamental frequencies given in Table 1, the thermodynamic properties have been calculated on the simple harmonic oscillator approximation and are given in Table 3. 59
J. GAVNT and J. B. AINSCOVGH
Table 3. Calculated thermodynamic properties of antimony pentafluoride gas T(°K)
100 150 200 250 273.1 298.1 350 400 500
So
--(G--Eo)/T
(cal/deg. mole)
(cal/deg. mole)
63.03 69.01 74.92 80.07 82.24 84.46 88"69 92.25 98.74
15.36 19-14 21.98 24.15 24.96 25.71 26.97 27.88 29.10
(cal/deg. mole)
52.25 56.04 60.04 63.54 65.03 66.57 69.54 72.17 76.89
A c k n o w l e d g e m e n t s - - T h e a u t h o r s w i s h t o t h a n k D r . P . L . ROBINSON f o r t h e p r e paration of the sample of antimony assistance throughout this work.
p e n t a f l u o r i d e a n d Mr. A . M. DEANE i b r h i s
References
[1] [2] [3] [4]
GAunT J. and AII~SCOUGHJ . B . Spectrochim. Acta 1957 50 52--56. SC}tAIRR. C. and SCHURm W . F . Industr. Engng. Chem. 1951 43 1624. GAUNT J. Trans..Faraday Soc. 1954 50 209. WILSON E . B . J. Chem. Phys. 1934 2 432.
60
The molecular structure of antimony pentafluoride J. GAUNT and J. B. AINSCOUOI~ United Kingdom Atomic Energy Authority, Atomic Energy Research Establishment Harwell, Didcot, Berks, and Research and Development Branch United Kingdom Atomic Energy Authority, Industrial Group Springfields Works, Salwick, Preston, Lancs.
(Received 6 June 1957)
Abstract~The infra-red and Raman spectra of antimony pentafluoride have been investigated. Five Raman lines were observed and have been assigned as fundamentals. An interpretation" of the infra-red spectrum has been made in a manner consistent with the rigid selection rules for the trigonal bipyramid structure (D3h). The thermodynamic properties have been calculated on the simple harmonic oscillator approximation.
Introduction I~ T~E previous paper [1] the results of an examination of the infra-red and R a m a n spectra of niobium pentaehloride have been reported. During the course of t h a t work it was decided that, to provide a comparison and in order to facilitate interpretation of the niobium pentachloride spectra, the infra-red and R a m a n spectra of a n t i m o n y pentafluoride should be investigated. This, to the best of our knowledge, had not previously been examined and it seemed probable t h a t niobium pentach]oride and a n t i m o n y pentafluoride might have the same structure.
Experimental The a n t i m o n y pentafluoride was supplied by Dr. P. L. ROBI~SOX of King's College, Newcastle. I t was a colourless, viscous liquid which fumed violently in moist air, and so was handled in an all-glass vacuum system. Since a n t i m o n y pentafluoride attacks hydrocarbon tap grease, all taps and joints which were likely to come into contact with the vapour were lubricated with a fluorocarbon grease. The vapour pressure of the pentafluoride is quite low, about 4 m m at 20°C [2], but was sufficient to give a satisfactory infra-red'sprectrum in a 35 cm gas ceil. The vapour attacked the windows of this cell forming a greenish-yellow deposit on them. The infra-red spectrum of the vapour was observed on a Hilger H 800 recording, double beam, prism speetrophotometer over the range 5000-310 cm -1, using the same prisms and filter as in the work on niobium pentachloride. The absorption cell was of glass with windows of both potassium bromide and caesium bromide attached by fluorocarbon grease. The infra-red spectrum of liquid antimony pentafluoride was also observed using a 0.1 mm cell fitted with caesium bromide windows. The apparatus used to excite the R a m a n spectrum was fairly conventional and has been described elsewhere [3]. The liquid sample was contained in a tube surrounded by three concentric vessels containing cooling water, a 0.5 cm layer 57
J . Gx~-~T a n d J . B. AINSCOUGH
of saturated sodium nitrite solution, and a 0.5 em layer of saturated rhodamine G solution to isolate the 4358 A mercury line which was used for excitation. The spectra were photograhed on two spectrographs, a Hilger F/4 and a Hilger medium, using glass optics; exposure times ranged from a few hours to a week. Spectra were calibrated by superimposing an iron are on the R a m a n lines. The plates were examined under a travelling microscope and on a recording microphotometer.
Discussion So far as we are aware no work has previously been carried out to elucidate the molecular structure of a n t i m o n y pentafluoride. However, like niobium pentachloride, the molecule is probably either a plane pentagon, a square pyramid, or a trigonal bipyramid. The observation of five R a m a n lines immediately excludes the first of these but is not sufficient evidence to distinguish between the square pyramid and the trigonal bipyramid as the former has seven and the latter six R a m a n active frequencies [4]. The more probable structure seems to be a trigonal bipyramid and an a t t e m p t has been made to interpret the spectra on this assumption. The strong, sharp line observed in the R a m a n spectrum at 667 cm -1 can probably be assigned as ~1, the symmetrical stretching frequency; and the infra-red absorption at 710 cm -1, which is also observed in the R a m a n spectrum at 716 cm -1, can probably be assigned as v5. An a t t e m p t has been made to assign the other observed R a m a n frequencies as fundamentals and then to interpret the observed infra-red absorption bands in a way consistent with the rigid selection rules for a molecule of D3~ symmetry. I n order to make a complete assignment of the fundamental bands, a R a m a n line at 498 cm -1, which was not in fact observed, has been postulated, though an adsorption band at 491 cm -1 was observed in the infra-red spectrum. The assignment also requires an infra-red fundamental at 294 cm -1. No definite peak could be observed here as this is outside the present instrumental range but the spectrophotometer trace shows t h a t a very strong absorption occurs below 315 cm -~ and this is strong evidence for a fundamental absorption band in this region. Weak absorption bands at 1195, 1130 and 1076 cm -~ were also observed but as these were found in the spectrum of an e m p t y cell after the a n t i m o n y pentafluoride had been pumped out, t h e y were attributed to absorption by the deposit formed on the cell windows. An absorption at 1030 cm -1 was attributed to silicon tetrafluoride. Some supporting evidence for the assumption of the D3h structure, in the case of the niobium compound, has been cited previously [1] and as there are quite marked similarities in the order of the assignments of the fundamental frequencies of the two molecules we feel t h a t this provides strong evidence for assuming t h a t a n t i m o n y pentafluoride molecule is also a trigonal bipyramid. The fundamental frequencies with their respective symmetries, degeneracies and activities are given in Table 1, while the interpretation of the infra-red spectrum is given in Table 2. In order to calculate the thermodynamic properties of a n t i m o n y pentafluoride it has been necessary to estimate the S b - - F bond length. The A s - - F distance in arsenic trifluoride is 1.71 A while the As--C1 and Sb--C1 distances in the trichlorides are 2.16 A and 2.37 A respectively. Assuming t h a t the difference between 58
The molecular structure of antimony pentafluoride
the bond lengths in the two fluorides is the same as in the chlorides a S b - - F bond length of 1.92 A is obtained for antimony trifluoride. There is probably some increase in bond length in going from the trifluoride to the pentafluoride as there T a b l e 1. F u n d a m e n t a l v i b r a t i o n s of a n t i m o n y p e n t a f l u o r i d e Activity
F r e q u e n c y (cm -1) calculated
A 1' A 1" A2 ~
R R IR IR
667 264 294 212
~'5
E'
R and IR
713
~'6
E I E' E~
R and IR R and IR R
498 107 228
Assignment
Symmetry
7'1
?)2 ?)3 ?"4
Y7 ~'8
Degeneracy
F r e q u e n c y (cm -1)
667 s. 264 m. (diffuse) Not observed Not observed 710 (IR) s. 716 (R) s. (diffuse) 491 ( I R only)m. ca. 90(R only)v.w. 228 m. (diffuse)
T a b l e 2. I n t e r p r e t a t i o n of t h e i n f r a - r e d s p r e c t u m of a n t i m o n y p e n t a f l u o r i d e Observed frequency
(era-l)
1419 m.* 1140 w. t 760 s. 727 s. 710 s. 684 s. 517 s. 491 m. 478 w. 439 w. 335 s. 326 s. c a 300 s.
Calculated frequency ( c m -1 )
1426 1139 762 726 713 684 522 498 476 440 335 321 294
Assignment
2v 5 2~ 4 + v 5 ~e + vs
Y5 3~ 8
~e ~2 -~- ~4 ~'4 "~ ~8 ~7 -+- v8
3v 7
* Observed only in the spectrum of the liquid. Observed in b o t h the liquid and vapour spectra.
is, for example, in the phosphorus fluorides where the P - - F distance is 1-52 A in the trifluoride and 1.57 A in the pentafluoride and so 2.00 A has been taken as the S b - - F distance in antimony pentafluoride. The sum of the covalent radii is 2.05 A. An error of 0.01 A in the bond length leads to a error of about 0.03 eal/deg.mole in the values of S O and of --(G- Eo)/T. On the basis of this bond length and using the fundamental frequencies given in Table 1, the thermodynamic properties have been calculated on the simple harmonic oscillator approximation and are given in Table 3. 59
J. GAVNT and J. B. AINSCOVGH
Table 3. Calculated thermodynamic properties of antimony pentafluoride gas T(°K)
100 150 200 250 273.1 298.1 350 400 500
So
--(G--Eo)/T
(cal/deg. mole)
(cal/deg. mole)
63.03 69.01 74.92 80.07 82.24 84.46 88"69 92.25 98.74
15.36 19-14 21.98 24.15 24.96 25.71 26.97 27.88 29.10
(cal/deg. mole)
52.25 56.04 60.04 63.54 65.03 66.57 69.54 72.17 76.89
A c k n o w l e d g e m e n t s - - T h e a u t h o r s w i s h t o t h a n k D r . P . L . ROBINSON f o r t h e p r e paration of the sample of antimony assistance throughout this work.
p e n t a f l u o r i d e a n d Mr. A . M. DEANE i b r h i s
References
[1] [2] [3] [4]
GAunT J. and AII~SCOUGHJ . B . Spectrochim. Acta 1957 50 52--56. SC}tAIRR. C. and SCHURm W . F . Industr. Engng. Chem. 1951 43 1624. GAUNT J. Trans..Faraday Soc. 1954 50 209. WILSON E . B . J. Chem. Phys. 1934 2 432.
60