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The infrared spectrum of SiF4
Speetmchimica Acta,1964,Vol. 20,pp. 296to 298. Pergmcm Press Ltd. printed in NorthernIreland
The infrared spectrum
of SiF,*
JULIANHEICIUENand VESTERKNIGHT AerospaceCorporation, El Segundo, California (Receiud 26 July 1963) Abstract-The infrared spectra of normal (Si*8) and isotopically enriched (Si*g and Sia’J)SiF, were observed from 309 to 2100 cm-l on a grating in&ument operating in a vacuum. A band not previously reported for Sia8F, was found at 1803.0 cm-l and assigned as yg + 2~~ The assignments of the S?*F, band at 777.5 and 1063.1 cm-l were corrected from yg - yg and vp + 2v4 to 2v4 and v1 f vg, respectively. The anharmonicitiesassociated with the vibrations are small; the quadratic potential constant were calculated wily, and are listed in the report. The Urey-Bradley force constants also were computed and were found to be theoretically unsuitable.
THE Raman spectrum of liquids SiF, has been reported previously [ 1, 21 as has the infrared spectrum of gaseous SiF, [3-b]. We have’ re-examined the infrared spectrum in a vacuum (to eliminate water and carbon dioxide interference) at higher resolution. In addition to the SissF, spectrum, SiBoF and SiSOF,spectra have been obtained to determine unambiguously the vibrational assignments and quadratic potential energy constants. The infrared spectra from 300 to 2100 cm-l were measured on a modified Perkin-Elmer 13 U spectrometer. The instrument, which has a Nernst glower source, was operated single beam with CsI optics. The Littrow mirror was replaced by an appropriate grating, and a CsI foreprism was used. The gratings were blazed at 6, 12 and 226 p and had 150, 76 rend 40 grooves per mm, respectively. Sharp-cut filters removed unwanted higher-order rsdiation. A CsI windowed thermocouple was used as a detector. The entire spectrometer was placed in a large, air-tight box which then was evacuated to remove atmospheric absorption caused by water and carbon dioxide. The box also contained a cold finger filled with liquid nitrogen. By this procedure, the background absorption could be completely eliminated. Matheson tank SiF, was used for the naturally occurring isotopic mixture; its composition was 92.3 per cent Sia8,4.7 per cent Sisg,and 3-Oper cent Siso. The two isotopically enriched compounds were prepared by the Isotopes Speoialty Company (Burbank, California) and contained about 46 per cent SP and 30 per cent Siso, respectively. The samples were contained in a 10 cm cell with CsI windows while the spectra were being measured. * Presented at the 146th meeting of the American Chemical Society, New York. September S-13, 1963. [I] D. M. YOST, E. N. LASSETTRE, and 8. T. GBOSS,J. Chem. P&u. 4,326 (1936). [2] Unpublished results of BEST and TRAMPE ~EIquoted by D. M. YOST, Proc. Indian Acud. Soi. 8& 333 (1938). [3] C. R. BAILEY, J. B. HALE, andJ. W. THOYPSON, PTDC.Roy. Sot., London, A 167,666 (1938). [4] P. J. H. WOLTZ, E. A. JONES and A. H. NIELSEN, Phyu. Rew. 79,416 (1960). [a] E. A. JONES, J. 8. KIRRY&~ITH, P. J: H. WOLTZ end A. H. NEILSEN, J. Chem. Phy8. 19, 242 (1961). 296
J. HEICELEN and V. KNIom
296
The observed frequencies, relative intensities, and our assignments are listed in Table 1. The two intense bands at about 390 and 1030 cm-l must be the triply Product and sum rule calculations confirm degenerate fundamental vibrations. these assignments. Table 1. Frequencies of SiF, in cm1
SPF, 389.35 777.8 LO31*8 1063.1 1165.9 1190-4 1294.9 1803.0 1825.7 2060.0
* 0.15 f 0.2 f 0.3 f 0.3 f 0.4 rf: 0.2 f 0.2 f 0.3 k 0.3 f 1.0
387.8 f 775.0 f 1022.9 f -
0.6 0.3 0.3
1160.7 1188.4 1285.7 1791.3 1816.9 2041.6
0.4 O-2 0.2 0.3 0.3 1.0
f f f f f f
Relative intensity
SPF,
Siz9F4
386.35 771.6 1014.4 1063.1 1166.7 1186.8 1277.6 1780.5 1808.1 2024.5
f f f f f & f f f f
0.10 0.3 0.3 0.3 0.6 O-2 0.2 0.3 0.3 1.0
Strong Weak Strong Weak Weak Medium Weak Weak M0diulI-l Weak
Assignment v4 2114 VS v1 +v2
3114 v1 +v4 vs +va vg + 2v4 v1 +v, 2%
The va + 2vq band is very weak and has not been previously observed, probably because of the interference of background water in earlier studies. That this very minor peak was indeed real was validated by the shifts with isotopic substitution. A number of peaks were observed by JONES et al. [6] at very high pressures which we did not look for. The fundamental vibrations v1 and va are symmetry forbidden in the infrared but were observed in the Raman spectrum at 800 and 268 cm-‘, respectively [5]. These bands should not be altered by isotopic substitution as the appropriate U matrix elements depend only on the mass of the fluorine atom. From the observed combination bands, it appears that values of 801 and 264 cm-l fit the data more accurately. Consequently, these latter values were used for all calculations. The overtones and combinations can be assigned both by comparison with values calculated from the fundamentals and by the isotope shifts. Such has been done for all of the observed bands; it is readily apparent that the anharmonicities are quite small and positive in all cases. The calculated and observed isotope shifts are in excellent agreement. Two of the previous assignments have been corrected. The band at 778 was formerly assigned to vs - v2. There are two good reasons that this assignment must be erroneous. First, difference bands must be exact. This assignment could then only be correct if v2 were 254 cm-l, considerably lower than observed. Second, and of more salience is the fact that such an assignment requires that va be shifted significantly by isotopic substitution in direct conflict to theory. The assignment of 2v4 to this band is much more satisfactory. Agreement with the calculated frequency and of the isotope shifts is excellent. The assignment of the 1063 band has been changed from va + 2v, to vl + vg. The assignment of va + 2vq gives a calculated value of 1042 cm-l, much too low to be correct. Furthermore, this assignment predicts an isotope shift of 6.2 cm-l from the 29 to 30 isotope of Si. Shifts between 1 and 20 cm-l could have been observed easily. In fact, no isotope shift occurred at all, clearly identifying this
The ix&wed spectrum of SiF,
297
band as vl + Ye. The band y1 + va is an E-type band and is forbidden by symmetry considerations. However, since this band is near the very intense ys band, it appears because of the Coriolis interaction. Classical examples of this phenomenon are the appearance of the E-type r2 bands of CH,, SiH,, and GeH, because of the interaction with the intense vq bands [6]. The symmetry force constants can be computed directly by the F-G matrix method, and they are given in Table 2. There is nothing unusual about these constants, and they have values that might have been anticipated. Table 2. Force constants, millidynes per angstrom For Symmetry Coordinates P, = Fs = P ?,, = F “,O = F ,,a =
7.18 0.260 6.36 0.439 -0,269
Urey-Bradley Force Constants This Work
force constants
K=
F +,c
F=
(l/4)(&
F’
= Kc,
H= K/To2
-
can be computed
WCWLz
from the relationships
[7)
2Fr.a + 2J’,,cz -
+ W4W%,,
(1/4P’,,, =
5.4 0.43 -0.31 0.02 0.30
6.90 0.07 -0.47 - 0.035 0.127
K F F’ H KIT,”
The Urey-Bradley
From Ref. [8]
-
+ 3F”, -
&,A FA) + 2J’v.a + Il’7.r -
lia,
FE)
where K is the stretching constant; H is the bending constant; F’ and F are the first- and second-order nonbonded interaction constants, respectively; and K is the intramolceular tension. The values computed from the symmetry force constants, as well as those given by Shimanouchi, are shown in Table 2. The two sets are in fair agreement except for F and possibly K. However, these force For one thing, it is expected that F’ constants are really quite unsatisfactory. [6] For a discussion, see G. HERZBERU, Infrared and Raman Spectra of Polyatomic Molecuke pp. 463-458, D. Van Nostrand Co., Princeton, N.J. (1945). 173 T. SHI~~~NOTJCHI, J. Chem. Phys. 17, 848 (1949).
298
J. HEIBKLEN end V. I~I~IIT
be about (-l/10).8’. This is clearly not the case. Also, K/T$ should equal -0~96P [8]. In fact, Shimanouchi used this relationship to estimate the Urey-Bradley foroe constants. However, the values obtained from the symmetryforceconstants do not satisfy the relationship. It can only be concluded that the Urey-Bradley force field, although quite clever in conception, simply does not agree with the facts for SiF, and should be discarded. [S] T. SEIMANO~CEI,J. Chem. Php.
17, 246 (1949).
The infrared spectrum
of SiF,*
JULIANHEICIUENand VESTERKNIGHT AerospaceCorporation, El Segundo, California (Receiud 26 July 1963) Abstract-The infrared spectra of normal (Si*8) and isotopically enriched (Si*g and Sia’J)SiF, were observed from 309 to 2100 cm-l on a grating in&ument operating in a vacuum. A band not previously reported for Sia8F, was found at 1803.0 cm-l and assigned as yg + 2~~ The assignments of the S?*F, band at 777.5 and 1063.1 cm-l were corrected from yg - yg and vp + 2v4 to 2v4 and v1 f vg, respectively. The anharmonicitiesassociated with the vibrations are small; the quadratic potential constant were calculated wily, and are listed in the report. The Urey-Bradley force constants also were computed and were found to be theoretically unsuitable.
THE Raman spectrum of liquids SiF, has been reported previously [ 1, 21 as has the infrared spectrum of gaseous SiF, [3-b]. We have’ re-examined the infrared spectrum in a vacuum (to eliminate water and carbon dioxide interference) at higher resolution. In addition to the SissF, spectrum, SiBoF and SiSOF,spectra have been obtained to determine unambiguously the vibrational assignments and quadratic potential energy constants. The infrared spectra from 300 to 2100 cm-l were measured on a modified Perkin-Elmer 13 U spectrometer. The instrument, which has a Nernst glower source, was operated single beam with CsI optics. The Littrow mirror was replaced by an appropriate grating, and a CsI foreprism was used. The gratings were blazed at 6, 12 and 226 p and had 150, 76 rend 40 grooves per mm, respectively. Sharp-cut filters removed unwanted higher-order rsdiation. A CsI windowed thermocouple was used as a detector. The entire spectrometer was placed in a large, air-tight box which then was evacuated to remove atmospheric absorption caused by water and carbon dioxide. The box also contained a cold finger filled with liquid nitrogen. By this procedure, the background absorption could be completely eliminated. Matheson tank SiF, was used for the naturally occurring isotopic mixture; its composition was 92.3 per cent Sia8,4.7 per cent Sisg,and 3-Oper cent Siso. The two isotopically enriched compounds were prepared by the Isotopes Speoialty Company (Burbank, California) and contained about 46 per cent SP and 30 per cent Siso, respectively. The samples were contained in a 10 cm cell with CsI windows while the spectra were being measured. * Presented at the 146th meeting of the American Chemical Society, New York. September S-13, 1963. [I] D. M. YOST, E. N. LASSETTRE, and 8. T. GBOSS,J. Chem. P&u. 4,326 (1936). [2] Unpublished results of BEST and TRAMPE ~EIquoted by D. M. YOST, Proc. Indian Acud. Soi. 8& 333 (1938). [3] C. R. BAILEY, J. B. HALE, andJ. W. THOYPSON, PTDC.Roy. Sot., London, A 167,666 (1938). [4] P. J. H. WOLTZ, E. A. JONES and A. H. NIELSEN, Phyu. Rew. 79,416 (1960). [a] E. A. JONES, J. 8. KIRRY&~ITH, P. J: H. WOLTZ end A. H. NEILSEN, J. Chem. Phy8. 19, 242 (1961). 296
J. HEICELEN and V. KNIom
296
The observed frequencies, relative intensities, and our assignments are listed in Table 1. The two intense bands at about 390 and 1030 cm-l must be the triply Product and sum rule calculations confirm degenerate fundamental vibrations. these assignments. Table 1. Frequencies of SiF, in cm1
SPF, 389.35 777.8 LO31*8 1063.1 1165.9 1190-4 1294.9 1803.0 1825.7 2060.0
* 0.15 f 0.2 f 0.3 f 0.3 f 0.4 rf: 0.2 f 0.2 f 0.3 k 0.3 f 1.0
387.8 f 775.0 f 1022.9 f -
0.6 0.3 0.3
1160.7 1188.4 1285.7 1791.3 1816.9 2041.6
0.4 O-2 0.2 0.3 0.3 1.0
f f f f f f
Relative intensity
SPF,
Siz9F4
386.35 771.6 1014.4 1063.1 1166.7 1186.8 1277.6 1780.5 1808.1 2024.5
f f f f f & f f f f
0.10 0.3 0.3 0.3 0.6 O-2 0.2 0.3 0.3 1.0
Strong Weak Strong Weak Weak Medium Weak Weak M0diulI-l Weak
Assignment v4 2114 VS v1 +v2
3114 v1 +v4 vs +va vg + 2v4 v1 +v, 2%
The va + 2vq band is very weak and has not been previously observed, probably because of the interference of background water in earlier studies. That this very minor peak was indeed real was validated by the shifts with isotopic substitution. A number of peaks were observed by JONES et al. [6] at very high pressures which we did not look for. The fundamental vibrations v1 and va are symmetry forbidden in the infrared but were observed in the Raman spectrum at 800 and 268 cm-‘, respectively [5]. These bands should not be altered by isotopic substitution as the appropriate U matrix elements depend only on the mass of the fluorine atom. From the observed combination bands, it appears that values of 801 and 264 cm-l fit the data more accurately. Consequently, these latter values were used for all calculations. The overtones and combinations can be assigned both by comparison with values calculated from the fundamentals and by the isotope shifts. Such has been done for all of the observed bands; it is readily apparent that the anharmonicities are quite small and positive in all cases. The calculated and observed isotope shifts are in excellent agreement. Two of the previous assignments have been corrected. The band at 778 was formerly assigned to vs - v2. There are two good reasons that this assignment must be erroneous. First, difference bands must be exact. This assignment could then only be correct if v2 were 254 cm-l, considerably lower than observed. Second, and of more salience is the fact that such an assignment requires that va be shifted significantly by isotopic substitution in direct conflict to theory. The assignment of 2v4 to this band is much more satisfactory. Agreement with the calculated frequency and of the isotope shifts is excellent. The assignment of the 1063 band has been changed from va + 2v, to vl + vg. The assignment of va + 2vq gives a calculated value of 1042 cm-l, much too low to be correct. Furthermore, this assignment predicts an isotope shift of 6.2 cm-l from the 29 to 30 isotope of Si. Shifts between 1 and 20 cm-l could have been observed easily. In fact, no isotope shift occurred at all, clearly identifying this
The ix&wed spectrum of SiF,
297
band as vl + Ye. The band y1 + va is an E-type band and is forbidden by symmetry considerations. However, since this band is near the very intense ys band, it appears because of the Coriolis interaction. Classical examples of this phenomenon are the appearance of the E-type r2 bands of CH,, SiH,, and GeH, because of the interaction with the intense vq bands [6]. The symmetry force constants can be computed directly by the F-G matrix method, and they are given in Table 2. There is nothing unusual about these constants, and they have values that might have been anticipated. Table 2. Force constants, millidynes per angstrom For Symmetry Coordinates P, = Fs = P ?,, = F “,O = F ,,a =
7.18 0.260 6.36 0.439 -0,269
Urey-Bradley Force Constants This Work
force constants
K=
F +,c
F=
(l/4)(&
F’
= Kc,
H= K/To2
-
can be computed
WCWLz
from the relationships
[7)
2Fr.a + 2J’,,cz -
+ W4W%,,
(1/4P’,,, =
5.4 0.43 -0.31 0.02 0.30
6.90 0.07 -0.47 - 0.035 0.127
K F F’ H KIT,”
The Urey-Bradley
From Ref. [8]
-
+ 3F”, -
&,A FA) + 2J’v.a + Il’7.r -
lia,
FE)
where K is the stretching constant; H is the bending constant; F’ and F are the first- and second-order nonbonded interaction constants, respectively; and K is the intramolceular tension. The values computed from the symmetry force constants, as well as those given by Shimanouchi, are shown in Table 2. The two sets are in fair agreement except for F and possibly K. However, these force For one thing, it is expected that F’ constants are really quite unsatisfactory. [6] For a discussion, see G. HERZBERU, Infrared and Raman Spectra of Polyatomic Molecuke pp. 463-458, D. Van Nostrand Co., Princeton, N.J. (1945). 173 T. SHI~~~NOTJCHI, J. Chem. Phys. 17, 848 (1949).
298
J. HEIBKLEN end V. I~I~IIT
be about (-l/10).8’. This is clearly not the case. Also, K/T$ should equal -0~96P [8]. In fact, Shimanouchi used this relationship to estimate the Urey-Bradley foroe constants. However, the values obtained from the symmetryforceconstants do not satisfy the relationship. It can only be concluded that the Urey-Bradley force field, although quite clever in conception, simply does not agree with the facts for SiF, and should be discarded. [S] T. SEIMANO~CEI,J. Chem. Php.
17, 246 (1949).