ftil, /l It{

Reprinted from THE JOURNAL OF CHEMICAL PHYSICS, Vol. 47. No. 12, 5031-5037, 15 December 1967 Printed ID U. S. A.

Infrared Spectrum, Force Constants, and Thermodynamic Functions if SiFt

V. M. KHANNA,- R. HAUGE, R. F. CURL, JR., AND J. L. MARGRAVE

Department 0/ Chemistry, Rice University, HOfuton, Texas

(Receivfd 10 July 1967)

nCT KHAN - VM 67 -111 9

The infrared spectrum of gaseous SiFI has been recorded from 1050 to 400 em-I. Two bands centered at 872 and 855 cm-I are, respectively, assigned to the antisymmetric stretching (II.) and symmetric stretching ("1) frequencies of the molecule. The symmetric bending frequency ("2) as determined from the earlier ultraviolet absorption studies lies at 345 cm-I and could not be observed because this was well below the low-frequency limit of the spectrometer used. The observed vibrational frequencies are combined with the available microwave data to obtain the following values of the quadratic force constants:/.= 5.019 mdyn/ A, / •• =0.310 mdynj A, /ajrl =0.440 mdyn/ A, /ar/r=0.123 mdyn/ A. Two cubic potential constants have been evaluated, K2'!3= -9.5 cm-I and KII2=9.9 cm-I • The average structure of SiFI in its ground state is computed and found to be (r)= 1.5947 A, (a)= 100°53', where r is the Si-F bond distance and a the F-Si-F angle. A table of thermodynamic functions of SiFI is presented.

INTRODUCTION

The silicon difluoride molecule, SiF2, was first de­tected and investigated spectroscopically by Johns, Chantry, and Barrowl who photographed its ultra­violet emission spectrum following the decomposition of silicon tetrafluoride vapors in an electrical discharge. Two vibrational intervals observed in the spectrum at about 937 and 427 cm-l were assigned to the symmetric stretching and bending modes of the ground state of the nonlinear molecule. Later Rao and Venkateswarlu2

made an extended study of the emission bands and interpreted their data in terms of the vibrational fre­quencies, 111=778 cm-I, 112=427 cm-l , and 113=947 cm-l

for the ground state. The recent discovery by Timms et al.3 that gaseous

monomeric silicon difluoride, produced by the action of silicon tetrafluoride on silicon at high temperatures and low pressures, exists as a relatively long-lived species, made absorption spectroscopic studies of this molecule possible. The principal features of the absorp­tion spectrum4•5 recorded in the ultraviolet region were the presence of a long progression in the bending fre­quency of the upper electronic state and a few "hot" bands which yielded a value of about 345 cm-1 for the bending frequency of the ground state. Despite the fact that the absorption spectrum extended farther on the high-frequency side than the earlier emission studies, all of the observed bands could be explained purely in terms of the upper and lower state bending modes and no evidence for the excitation of stretching mode vibrations was obtained. These observations cast

- Present address : Department of Chemistry, University of New Dehli, New Dehli 7, India.

I J. W. C. Johns, G. W. Chantry, and R. F. Barrow, Trans. Faraday Soc. 54, 1589 (1958).

2 D. R. Rao and P. Venkateswarlu, J. Mol. Spectry. 1, 287 (1961) .

a P. L. Timms, R. A. Kent, T. C. Ehlert, and J. L. Margrave, J. Am. Chem. Soc. 87, 2842 (1965); Nature 207, 187 (1965).

• V. M. Khanna, R. Hauge, G. Besenbruch, and J. L. Margrave, Proc. Intern . Symp. Mol. Struct. Spectry. Columbus, Ohio, 1966.

6 V. M. Khanna, G. Besenbruch, and J. L. Margrave, J. Chern. Phys. 46, 2310 (1967) .

serious doubt on the vibrational numberings given by Johns et al.l and also by Rao and Venkateswarlu.2

Rao, Curl, et ai.6 obtained the microwave spectrum of SiF2, deduced the bond distance and bond angle, and reported a value of 345 cm-I, in agreement with the ultraviolet absorption studies, for the bending fre­quency. In addition these workers attempted to calcu­late the force constants and therefrom the vibrational frequencies for the molecule, utilizing the centrifugal­distortion data obtained from the microwave studies.8b

The large uncertainties in their estimated values of the vibrational frequencies were indicative of the fact that the force field in this molecule is very poorly de­termined by the centrifugal-distortion effects alone.

Bassler, Timms, and Margrave7 studied the infrared spectrum of SiF2 by the matrix-isolation technique, but there was extensive splitting and/or matrix shifts so that the results were difficult to interpret in terms of fundamental vibrational frequencies.

Because of the failure of past workers to fix the char­acteristic frequencies of SiF2, an infrared study of the gas-phase species was undertaken with a view to ob­taining reliable values of the vibrational frequencies. In addition, although the force constants may be poorly determined from the centrifugal-distortion data alone, it is well known8.9 that these can be very accurately determined by combining the observed vibrational frequency data with' that of the centrifugal-distortion effects. The remarkable stability of SiF2 and in par­ticular its long lifetime (~2 min) as comparee. to its carbon analog CF2 (~1 sec) has aroused CO!l­

siderable interest in the nature of chemical bonding in this molecule. The determination of accurate values for the force constants and consequently reliable data on the force field should give further insight into the electronic structure of the molecule.

• (a) V. M. Rao, R. F. Curl, P. L. Timms, and J. L. Margrave, J. Chem. Phys. 43, 2557 (1965). (b) V. M. Rao and R. F. Curl, ibid. 45, 2032 (1966).

7 J. M. Bassler, P. L. Timms, and J. L. Margrave, Inorg. ChelL. 5,729 (1966).

• D. Kivelson, J. Chern. Phys. 22, 904 (1954). I L. Pierce, N. DiCianni, and R. H. Jackson, 1. Chem. Phys.

38,730 (1963). . . .

5031

--,

, ,

9 196B

5032 KHANNA, HAUGE, CURL, AND MARGRAVE

VACUUM VACUUM

BECKMAN

IR - 9

t t 24 "

LIGHT SOURCE

12 " r-, : I

1 I

He

KBr FIG. 1. Schematic diagram of the WINOOW apparatus used for recordin!:? the in­

frared spectrum of gaseous SIFt.

---QUARTZ TUBE CONTAINING SILICON

_SIF. L-__________ ~,~

EXPERIMENTAL DETAILS

Figure 1 represents the apparatus used for recording the infrared spectra of SiF2• Silicon difluoride was pre­pared by passing commercial silicon tetrafluoride first over steel wool heated to SOO°C to free it from impurities such as sulfur dioxide and 'oxygen and then over small pieces of silicon contained in a quartz tube heated to about 1150°C. The emerging gases (a mixture of SiF4

and SiF2) were then pun'lped 'through the absorption cell during the measurements. The absorption cell was made of Pyrex with KEr windows attached.

Initially, the infrared measurements were made with an optical cell smaller in size than shown in Fig. 1 and :a Perkin-Elmer spectrophotometer (Model 12-C) , The main difficulty encountered here was the tendency of SiF, to condense on the KBr windows to form a thin film of polymer. The condensate showed strong broad absorption bands centered at about 840, 930,

,. ; '1 I

OJ z ~ 2 ....

a. 15 III ID C

L~~~~~'~~~'-7-~ ~ ~

WAVENUMBER (em-I)

FIG. 2. The SiF, polymer bands recorded I'~th (Curve A) and without (Curve B) SiF4 flowing through the system and also at two different concentrations.

' .

NEEDLE VALVE

and 960 cm-1 as can be seen from a trace of the spectrum presented in Fig. 2. These bands were also. seen in earlier infrared investigations and were asSigned to an SiF2 polymer.

Because both the silicon difluoride polymer and the gas-phase species absorbed in about the region S00-1000 cm-t, the intense polymer. spectru~ tended to obscure the bands due to weakly absorbmg gaseous species. Therefore, it was necessar~ to preve~t the condensation of SiF2 on the KBr wmdows. ThiS was achieved by first lengthening the absorption cell; second, circulating a non absorbing gas, heli~, thro~gh the cell (as shown in Fig. 1). Although the clr~ulatlOn of helium minimized the condensation on the wmdows, it did not completely stop it. However, by cool~ng the cell near the ends, no deposition of SiF2 on the wI~dows was found as evidenced by the absence of (SIF2)n­polymer absorption bands in the spectrum.

Because of the lower resolution of the Perkin-Elmer instrument the final measurements were recorded witlt the IR-9 Beckmann spectrophotometer. The instr:u­ment was operated in its single-beam m.ode and, Its optical system was modified to adapt I~ for, hIgh­temperature molecule studies. T~e alteratl?ns m the optical system reduced the resolution of the lDstrument

F

FIG. 3. Molecular structure of SiF3•

\-

.,1

"

INFRARED SPECTRUM OFi SiFt 5033.

and this was of the order of 2 em-I for the region scanned (1050-800 em-I).

The pressure of silicon tetrafluoride and hence that of SiF2 through the absorption cell V:as controlled by means of a needle valve. Thus, spectra were measured as a function of concentration.

It was found during the course of measurements that for better conversion the SiF2 molecules once formed in the furnace, should be quickly pumped out of the hot zone. Accordingly, two vacuum pumps were used to ensure fast pumping of the reaction products out of the furnace.

INFRARED SPECTRUM AND VIBRATIONAL ASSIGNMENT

Silicon diflu~ride is a bent molecule possessing C2v

symmetry6a Wlth bond angle, LF-Si-F=lOl° and bond length, r(Si-F), = 1.591 A. The rotational con­stants are A = 1.021, B=0.294, and C=0.228 em-I, and from these the value of the asymmetry parameter, K[K=(2B-A-C)/(A-C)], can be computed to be -0.833. The molecule can, therefore be considered as being close to a prolate symmetric' top (for a true prolate symmetric top K = -1) and the vibration­rotation spectra of SiF2 should resemble those of a prolate symmetric top.

Symmetric nonlinear SiF2 has three fundamental vibrations all of which are infrared active. Two vibra­tions (Ill and 112), belonging to the symmetry species al are totally symmetric whereas the other (lIa) , belongin~ to the symmetry species bl , is antisymmetric. Since during the symmetric vibrations the dipole moment changes along the Ib axis (a model of SiF2 structure is given in Fig. 3) which is perpendicular to the unique axis of the top (in this case the axis of least moment of inertia, i.e., LJ), these vibrations will exhibit a per­pendicular-type band (Type B) with a doublet contour. On the other hand the antisymmetric strntch (lIa) will display a parallel-type band (Type A) " with PQR branches because the dipole-moment change occurs a~ong .the axis of least moment of inertia during this VIbratlOn. Thus, the fundamentals of SiF2 can be reliably assigned on the basis of their band contours provided the resolution of the instrument is sufficien~ to record the contours.

Figure 4 depicts the infrared spectrum of SiF2• The spectrum clearly shows two overlapping bands. One band reveals a PQR contour with a sharp Q branch at about 872 cm-I and broad PR branches. Although the noise level of any trace was such ~o make the PQR features (particularly the Q branch) somewhat doubt­ful, the above statement about the features is based upon the fact that they reproduce in a number of traces. The second band centered at about 855 cm-I exhibits a doublet structure. In the light of the discussion in the preceding paragraph these two bands at 872 and 855 em-I should, respectively, be assim,ed to the ar:\~-

z Q l­n. 0:: o <J)

CD ~

WAVENUMBER (em-I)

FIG. 4. Bands of SiFt.

symmetric stretching frequency (lIa) and symmetric stretching frequency (Ill) of the molecule.

The symmetric bending frequency (112) as deter­min~d f~om the mic::rowave and ultraviolet absorption studies hes at about 345 cm-I and could not be observed in the present study because this was well below the low-frequency limit of the instrument.

It is clear from the figure that the P branch of the 113 stretch overlaps rather strongly with the R side of the III stretch. Therefore, the P-R separation in the antisymmetric stretch could not be accurately esti­mated but a rough estimate gave a value of about 22 cm-I, which is in reasonable agreement with the theoretical valuelo of 25.1 cm-I. Also it must be pointed out that although the band at 855 cm-I with a doublet contour is, more likely than not, due to the symmetric stretch, for its unequivocal identification its rotational fine structure must be resolved. If this band arises from the symmetric stretching vibration, as suggested in the present analysis, then the fine structure lines must show a spacing of 2[A -HB+C) ] = 1.5 cm-I. U n­fortu~ately, the resolution obtained with the present expenmental setup was not sufficient to resolve the fine structure of the rotational-vibrational bands.

The absorption spectrum was scanned from 1200 to 400 em-I (the lower-frequency limit of the spectrom­eter) and no bands other than those discussed above plus.one due to the antisymmetric stretching frequency of SIF. at 1030 cm-I were observed in the gas phase. The intensities of the SiF2 bands varied with the tem­perature of the Si tube, as expected. Also mass-spectro­metric analysis of the reaction products revealed that SiF2 and SiFt account for at least 99.5% of the gaseous species present in the absorption cell. These observa­tions are definite indications that the bands at 872 ~nd 8~5. cm-l are due to the SiF2 molecule and not any Impuntles. Furthennore, the present gas-phase assign­ments of SiFt receive corroborative support from the

I.S. L. Gerhard and D. M. Dennison, Ph),! · Rev 43, 197 (1933). . .

__ ------------~---~---'..:." III ----------~~~--~------~----~----.. I

. -.. ,.

5034 KHANNA, HAUGE, CURL, AND MARGRAVE

.~ c >.

"'" E

3

2

0.1735

o

800 816

:-;---2"89---:'28~4~ 260

-1L--~--------~2--------~3---------47-------~5~~----~6~-----

FII (mdyn/A)

FIG. 5. Plot of Fa /,. versus Fu for SiF,. The number without parentheses adjacent to each point represents the rms deviation (in megacycles/second) of the observed and calculated rotational frequencies when the set of force constants corresponding to that value of Fu is used to compute the centrifugal-distortion effects. The number within parentheses is the value of the inertial defect, AOIll_ Arm (in atomic mass units per square angstrom) obtained from the corresponding force constant. The observed value is 0.3784 amu·As.

about 17 and 12 cm-1, respectively. These are not un­usually large matrix shifts.

Table I summarizes all the frequencies observed both in gas-phase and matrix studies.

QUADRATIC FORCE CONSTANTS

A suitable quadratic potential function for a sym­metrical-bent triatomic molecule may be given by

2V =1r (6.'1'+ 6.'22) +21rrMIM2+1a6.a2

+ 21,a (Ml + 6.'2) 6.a,

infrared studies made by the matrix-isolation tech­nique. The matrix spectral studies showed, in the region 800-850 cm-I , three sharp peaks at 855, 843, and 811 cm-I in order of decreasing intensity. No band at 811 cm-I was observed in the gas-phase study and also this band was found to disappear much more rapidly than others during warmup experiments in the matrix study. This band cannot be ascribed to normal SiF2 molecule and must be due to some more reactive species as suggested by Bassler el al.7 The bands at 855 and 843 cm-I should accordingly belong to the anti­symmetric stretching and symmetric stretching fre­quencies of SiFt. This represents downward shifts of

TABLE I. Spectral data on SiFs and SiFt polymer.

where 6.'1, 6.1'2, and 6.a are the displacements of the two (Si-F) bonds and the apex angle from the equilib­

" rium configuration, fr, fa, frr, and fTa are the bond-

Frequency (em-I)

Matrix (Bassler,

Gas phase Timms,and (this work) Margrave) Band type Assignment

730- SiFt polymer 830- 840 SiF, polymer 855 843 Doublet "1

(B type) 872 855 PQR '" (A type)

SiFI polymer 930- 930 96()a 970 SiFl polymer

- These band. were observed only when SiF. condensed on KBr wiodo_ Gueoua ailicon dilluorlde mowed DO banda due to polymer. ..... .

TABLE II. Observed values of moments of inertia, inertial defects. and Coriolis coupling constants.

Ground state Excited state (lit =0,111=0,11.=0) (111 =O,IIz= 1. 11.=0)

I. (amu·!') 16.5145 16 (amu.!I) 57.2765

I. (amu·1I) 73 .9900 A (amu·!t) (1.-1.-16) 0 . 1990

AOIll-Arm (amu·!I) =0.3784

Coriolis coupling constants

1)"2I(0IJ'=0.8164 I)"u(')] =0. 1836

16.2740 57.3789 74.2303

0.5774

/

INFRARED SPECTRUM OF SiFt 5035

stretching constant, the angle-deformation constant, the bond-bond interaction constant, and the angle-bond interaction constant, respectively. In order to utilize the observed fundamentals for the evaluation of force constants, it is necessary to assume that the observed fundamental frequencies 1'1 and 1'3 are the same as the harmonic frequencies WI and Wa since the anharmonicity constants Xij are unknown. This has been done. Since there are only three fundamental frequencies available, these data are not sufficient to evaluate four force con­stants. Additional information for this molecule is, however, available from microwave studies which furnished data on the centrifugal-distortion effects and the Coriolis coupling constants. The infrared measure­ments can thus be combined with either of these ob­served quantities to determine uniquely the four force constants. In the present study the force constants were obtained from both. First the quadratic force con­stants were evaluated utilizing the measured infrared frequencies and the centrifugal-distortion constants and these values were compared with those obtained from using the Coriolis coupling constants and vibrational frequencies in order to check internal consistency.

Centrifugal-Distortion Effects

To evaluate the force constants from the observed fundamental frequencies and the centrifugal-distortion constants, . the procedure followed was essentially the same as used by Pierce et al.9 for OF2• For a series of assumed values for Fll , the remaining force constants (F22, Fu, Faa) were calculated from the following

TABLE Ill. Rotational constants, force constants, and vibrational frequencies of SiFt.

Inertial defect

(Coriolis Centrifugal- couplin~

distortion constant Microwave constants and and infrared

data infrared data data

A (Me/sec) 30602.08 30601.33 B (Mc/sec) 8823.47 8823.26 C (Me/sec) 6830.35 6830.15

T ... a~ (Mc/see) -1.9807 -1.9734 Tbbbl> (Me/sec) -0.0715 -0.07213 T.obb (Me/sec) 0.2613 0.2659 T.bub (Me/sec) -0.0359 -0.0433

f. (mdyn/A) 5.42 5.019 5.01 fry (mdyn/ A) -0.25 0.310 0.299 fal.s (mdyn/ A) 0.444 0.440 0.441 far}. (mdyn/ A) 0.132 0.123 0.112

WI (cm-I) 841 855 855 WI (em-I) 347 345 345 ·Wi (em-I) 957 872 872

~.'

TABLE IV. Vib-rot interaction constants of SiFt.

a2 (Mc/sec) (J2 (Mc/sec) 'Ys (Mc/sec)

K2"!2 (em-I) K122 (cm-I)

-452 . 23 15.75 22 . 11

Cubic potential constants

SiFs SOa· OF2b

-9.5 9.9

-6.6 9.3

-16 -9.8

• Y. Morino, Y. Kiklchl. S •. Saito. and E. Hirota, J. Mot. Spectry. ll. 95 (1964).

by. Morino and S. SaIto. J. Mot. Spectry. 19, 435 (1966).

equations:

Fu (Gn) = -Gu FlI

[Gn Gn'lAi·"'2 JI12 ± Fll (>-1+>-2) - (GllGn-Gu2) - F1l2(GuGn-Gu2) ,

F33G33 = >-3,

( 1)

(2)

(3)

where >-;= (27rCWi) 2 and F and G are the matrices with respect to the symmetry coordinates.lO,l!

Because Eq. (1) is quadratic, for each value of Fll ,

there are two values of Fu [and hence two corre­sponding values for F'O. from Eq. (2)]; Fa3 is fixed uniquely by Eq. (3). Thus two sets of quadratic po­tential constants are obtained corresponding to each assumed value for Fll• A graphical plot of the solutions for various values of Fll is shown in Fig. 5 where the bottom half of the curve represents one possible solu­tion (for Fu) and the top half represents the other one. The various sets of potential constants thus obtained were used to compute centrifugal-distortion constantsl2

and these in turn were used to account for the cen- · trifugal-distortion effects in the least-squares fit of the microwave data with rotational constants. By choosing a set of force constants which is completely consistent with the measured fundamental frequencies and also such that it yields values of T that give mini­mum deviations between the ' calculated and observed rotational frequencies, one can make a very sensitive determination of the potential constants. Deviations for a number of sets of force constants are given in Fig. 5. The best set of force constants which gave

11 The sy=etry coordinates are SI=v2'-I(Arl+dr2), S2=Aa, and S3=v2'-I(drl-Ars).

11 Explicit formulas for the evaluation of T'S in terms of the four quadratic potential constants are given by D. Kiveison, J. Chem. Phys. 22, 908 (1954).

, : ! , • r

.f'

•

5036 KHANNA, HAUGE, CURL, AND MARGRAVE

TABLE V. Ground-state moments of inertia and structure of SiFt.

Moments of inertia (amu· At)

IA Is Ie Ie-lA-Is

Effective 16.5145 57.2765 73.9900 0.1990

Average 16.6177 57.4370. 74.0547 0.000

Structure

Effective , (Si-F) 1. 5913 A L F-Si-F 100° 59'

Average r(Si-F) 1.5947 A L F-Si-F 100° 53'

a standard deviation of 0.7 Mc/sec was found to be

Fu=5.3292 mdyn/A,

F12/r=0.1736mdyn/ A,

F22/r2=0.4400 mdyn/A,

F3a=4.709 mdyn/A.

Coriolis Coupling Constant

The inertial defect is related to the Coriolis constant by the following relation13 ;

!lOIO_t:.rI:~= (h/re) [wNW02(waL W022) J(r23(c»', (4)

(r23(C»2+ (rI3(C»2= 1, (5)

where !l0IO and !lOOO are the inertial defects in the excited (Til" = 0, 'll2" = 1, V3" = 0) and ground (VI" = 0, t12" = 0, va" =0) state, respectively.

From the observed values of the inertial defects and the fundamental frequencies, the value of (rI3(c»)2 could be readily determined and was found to be 0.1836 (Table II).

The Coriolis coupling constant (r13(c»2 is related to the force constant Fll by the equation13

(6)

where ~1 is the inverse of the G matrix. By using estimated values of (rI8(c»2, the constant

Fu was determined from the above relation and then all the remaining force constants were determined with the aid of Eqs. (1)-(3).

Table III summarizes all the data on the rotational constants, force constants, and vibrational frequencies. It is obvious from the table that the values of the force constants derived from utilizing centrifugal-distortion constants agree very well with those obtained using Coriolis coupling constants.

In order to clarify the question as to which of the

11 Y. Morino, Y. Kikichi, S. Saito, and E. Hirota, J. Mol. Spectry. 13, 95 (1964).

two methods gives more sensitive determination of the potential functions, the inertial defects (!l0IO_!lOOO) corresponding to a few selected sets of potential con­stants were computed and are shown in Fig. S. It is evident that whereas both methods yield fairly accurate values of the potential constants these are much more sensitive to centrifugal-distortion constants than to the inertial defects (or Coriolis coupling coefficients).

CUBIC POTENTIAL CONSTANTS

Silicon difluoride is one of the few polyatomic mole­cules for which it is possible to obtain some information on the anharmonicity of its force field. From the changes in the rotational constants that occur when the molecule is vibrationally excited, the vibration­rotation interaction constants which are a function of cubic potential constants can be determined. Since in the microwave investigations on SiF2 only the t12 = 1 state was studied, only three constants, (X2( = AOlO-Aooo), (32( =BOIO-Booo), and 'Y2( =COIO-COOO )

could be estimated. The observed values are given in Table IV. From these values and the information on the harmonic potential constants, two cubic constants, Km and K 122, were determined, using the expressions given by Morino et al.13 These values are given in Table IV where a comparison is made with the corre­sponding values obtained for S02 and OF2.

AVERAGE GROUND-STATE STRUCTURE

The average structure of the molecule in its ground state can be determined from the ground-state rota­tional constants if the harmonic force constants are known. By means of the relations given by Laurie and Herschbach,14 the average structure parameters' for SiF2 were computed and are given in Table V. The slight increase in bond distance but decrease in bond angle in going from the effective structure to average structure is typical of such molecules. It may be noted that the requirement of planarity (Le., I.=I",+Ib) is

TABLE VI. Thermodynamic functions for SiF •.

Cal mole-I·deg-I kcal mole-I

T(OK) COp SOT -(POT-HOm)/T HOT-H291

100 8.298 51.187 69.947 -1.876 200 9.476 57.294 62.235 -0.988 298 10 .638 61.299 61.299 0 .000 500 12.214 67 . 221 62 .569 2.326

1000 13.396 76.163 67 .354 8.809 1500 13 . 672 81.658 71.265 15.589 2000 13.775 85.607 74 .380 22.455 2500 13.823 88.686 76.944 29 .355 3000 13.849 91.209 79 . 118 36.274 3500 13.865 93 .345 81.002 43.203 4000 13.876 95.198 82 .663 50. 139

It V. W. Laurie and D. R. Herschbach, J. Chern. Phys. 37,1687 (1962).

•

• r

INFRARED SPECTRUM OF SiFt 5037

nicely satisfied for the average structure values of the moments of inertia, and hence any pair among the three moments of inertia leads to identical molecular structural parameters.

THERMODYNAMIC FUNCTIONS

Thermodynamic functions for SiF2 reported earlier in JANAF tables have been recalculated with this new data.15 Table VI lists a few values at selected

U H. Prophet and D. R. Stull (private communication), 1967.

)

temperatures obtained using the following values for the vibrational frequencies and molecular dimensions:

w1=855, "'2=345, w3=872; LF- Si-F=lOlo,

and r(Si-F) = 1.591 A. ACKNOWLEDGMENTS

This work has been supported by the U. S. Atomic Energy Commission, the U. S. Army Research Office, Durham, the Robert A. Welch Foundation, and the National Science Foundation.

• . -i"

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