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Microwave spectrum, structure, dipole moment and barrier to internal rotation of methyltrifluorosilane
Journal of Molecular Structure Elscvier Publishing Company,
Amsterdam.
Printed
in the Ncthcrland:
MICROWAVE SPECTRUM, STRUCTURE, DIPOLE MOMENT AND BARRIER TO INTERNAL ROTATlON OF METHYLTRIFLUOROSILANE
Y. S. LI AND
1. R. DURIG, Department (Rcccivcd
of Chemistry, 26 June
C. C. TONG*
Unicersity
of South
Carolina,
Columbiu,
SC.
29208
(U.S.A.)
1972)
ARSTRACT
rotational spectrum mcthyltrifluorosilane in ground and first three states of torsional mode have been in the of 12.540.0 The rotational for the ’ 'C isotopic species have also been measured. The following structural parameters have been determined: r(CH) = 1.081+0.004 A, L HCSi = 111”1’~30’, r(CSi) = 1.812+0.014 A, r(SiF) = 1.574+0.007 A, L FSiC = 112”20’+ l”6’. The structural parameters are compared to the corresponding ones for similar molecules. The dipole moment was determined to be 2.3310.10 D. From relative intensity measurements, the barrier to internal rotation was found to bc 0.93+0.09 kcal mol-‘; this value is consistent with the values obtained for other methylfluorosilanes.
INTRODUCTION
The microwave spectrum of methyltrifluorosilanc has been studied by several investigators 1 -4. In these studies’* ’ the barrier to internal rotation was estimated from the relative
intensities
of the torsional
vibrational
satellites.
The determined
barrier value was 1200 cal mol - ’ and the torsional vibrational frequency was estimated as 141+ 30 cm- ‘. Collins and Nielsen’ cstimatcd the torsional frequency at 982 to bc 156 cm- 1 from assigned sum and difference tones with a fundamental cm-‘. This assignment gives a torsional barrier of 1440 cal mol-‘. More recently
Kirtman6analyzed the microwave data of Thomas ef ~1.~ in terms of the vibrationalhindered rotational analysis and obtained a value of 1388+ 100 cal mol-’ for - ..-+ Taken in part from the thesis of C. C. Tong which was submitted to the Department of Chemistry
in partial
J. Mol. Structure.
fulfillment
14 (1972)
of the Ph.D.
degree
(1972).
255
the internal rotational barrier. Since the Hewlett-Packard microwave instrument has been shown to be quite good for intensity mcasurcmenfs’, it was felt that a re-investigation of the relative intensities for the ground and excited torsional states would provide a more consistent value for the barrier to internal rotation. In addition the dipole moment of methyltrifluorosilane had not been determined nor had a complete structure been calculated. Sheridan and Gordy obtained a carbon-silicon bond distance of 1.88 A with assumptions of SiF, and CH, bond distances from other molecules [r(SiF) = 1.555 A, r(CH) = 1.10 A, all angles tctrahcdral]
but this distance appears long in comparison
to the corrc-
spending
distance in other methylfluorosilane molecules_ These results for this molecule are particularly important for comparing to the values obtained for the barrier and dipole moment in the isoelectronic molccuk, H3BPF3*.
The sample of methyltrifluorosilane, CH,SiF,, sular Chcmresearch, Inc. and used without further spectrum was investigated by using the HP Model square-wave modulation frequency of 33.3 kHz on were measured with the Stark cell covered with Dry
was obtained
from Penin-
purification. The microwave 8640A spectrometer with a the Stark cell. Frequencies Ice.
TABLE1 ODSERVED
. J”
J’
FREQUENCIES
---..
-_-_---_ ‘*Cff,“SiF, -----1’ -_ 0
AND
--
,’ z
ROTATIONAL
.---.._
I
CONSTANTS
FOR
--
2 -* 3
____.---14844.72 22 267.49
3 -* 4 4+5
29725.02 37156.13
29689.87 37112.17
29656.55 37069.92
B
3715.64
3711.18
3707.1 I
-
E I4 829.49 22244.38
‘*CHJ2*SiF3 - . _.---
AND
-.--..
“CH,2ASiF~ “Cfi,2aSiF,
_____~_.
--
14862.54 22293.86
l-c2
(MHz) .- -.--
----_.. ,’ T 2
A _.__.. 14 828.45 22 242.30
-
-_
I’ = 3 _-_----__-
E
14813.84 22 220.73
14810.56 22215.52
14493.4 2 I 740.2
29627.67 37034.58
29620.81 37025.49
28985.9 36232.2
___--...-
29659.06 37073.62
I'= 0
A
_---_-
3 703.46
--
3 623.22
The microwave spectrum of the isotopic species, ’ 'CH ,SiF,, was observed and measured with the natural occurring abundance. The results arc listed in Table 1. The frequency accuracy is estimated to be 0.1 MHz. For internal consistency for the relative intensitymeasurements,we have also measured the microwave spectrum of CH$iF, in the ground vibrational state as well as in the first three vibrational excited states of the torsional mode. The uncertainty is estimated to be less than 0.04 MHz for the ground state frequencies, whereas it is larger for the vibrationally excited states. The results are also listed in Table 1. From this table, it can bc seen that all of the cxcitcd state transitions arc located on the lower frequency side of the ground state lines. 256
J. Mol.
Structure.
14 (1972)
STRUCTURE
The combination of our present experimental rotational constants as listed in Table I with those given in ref. 4 makes it possible to calculate the coordinates of the hydrogen and the carbon atcms in the same principal coordinate system’ - “_ molecule, and its principal The molecule, CH ,SiF, , was taken as the parent coordinate system was used in expressing the coordinates of the atoms. By letting
the X2 plane pass through
one of the hydrogen
atoms,
the 2 coordinate
of the
carbon atom was calculated by Kraitchman’s equation’. The relations between the X and 2 coordinates of the hydrogen atom and the mass and moments of inertia for the mono-, di-, and trisubstituted molecules can be expressed by I cl
=
I b2
=
I b3
=
and
respectively,
where
ClN = NMAm/(M+ A??1 = ?U’-??l nl’
= mass
m
=
of the mass of the M= the molecular N = the number 1, = the moment I gfu = the moment g-axis.
NAnr) substituted atom atom to be substituted weight of the parent molecule of fold of the substitution of inertia of the parent molecule along the b-axis of inertia of the N-fold substituted molecule along
the
Since all the moments of inertia are available, the solution of these equations gives the position of the hydrogen atoms. Thus we were able to obtain the r, structure for the methyl group. The results are given in Table 2 under method 1. Since the methyl group is not close to the center of mass of the parent molecule, TABLE
2
STRUCWRAL -_.-_ -
_--_-..r(Cf4)
L HCSi r(CSi) r(SiF) L FSiC
PARAMETERS FOR MtTtiYL _-_-.--
Method I .__.-._1.081 -co.004
1I I”S’f30
--_
A
THIFLUOROSILANC
_.._.
Method 2 _.___ 1.081 +a004
-
-
A
I1 1”1’&30’ 1.812_CO.Ol4A
1.57450.007 A I IZ”20’~ I”6’
----.. J. Mol. Structure, 14 (1972)
257
the structure
obtained
by this method
for the methyl group should be quite good. The given errors represent the internal consistencies in solving the available coordinates from different combinations of the isotopic species. The method used to calculate the complete molecular structure was by fitting of the structural parameters to the rotational constants from ref. 4 along with those in Table 1. This combination gave eight well-determined rotational constants for the six isotopic species of methyltrifluorosilane, which was expected to be sufficient to give a unique solution for all of the five structural parameters. A computer program was used to adjust these parameters to the experimental rotational constants by the least squares method and the results are listed inTable under method 2. The same results were obtained irrespective of the parameters ;Ised as the starting structure. The methyl group structure obtained by this method was found to be identical with those from method 1, cvcn though the rotational constants were less sensitive to the methyl structural parameters in comparison with the other structural parameters. From this point of view, it seems to indicate
that the resulting parameters, r(SiC), R(SiF) and L FSiC, are at least as good as those for the methyl group. On the other hand, the closeness of the silicon atom to the center of mass of this molecule structural parameters.
DIPOLE
gives some limitation
to the accuracy of the
MOMENT
The dipole moment of mcthyltrifluorosilane was determined by measuring the quadratic Stark effect of the following components: [MI = I, K = 0 of 2 e- 1 and 3 + 2 transitions, and IM1 = 3, K = 0 of the 4 t 3 transition. The electric fields were calibrated using a moment of 0.71521 D for 0CS13. The results are of the listed in Table 3. The resulting value (II = 2.33 &-0.10 D) is the average dipole moments obtained from the different Stark component measurements. The error given is the standard deviation from thcsc three measurements. TABLE3 MEASURED
__
DIPOLE
MOMENT
~.._..__.___
Tmnsirion
COMPONENTS ..__.__
IMi
AcJE~ x IO-~
,.-___...
-__
....----.-
[MHz~(~o~~cIII)~] __-_..-....-
---.-.--------_----
clalclrlafed Ekperitrierrrc~l _.__..._-
3-•4 i-*2 2 -* 3
0.440 2.569 0.115
3 I I _-_----~,T= 2.33D&O.10 D
TORSIONAL
_-_..
BARRIER
Relative 258
0.430 2.293 0.132 ____--_-.
intensity
measurements
had
been
performed J. Mol.
by Sheridan Structure,
and
14 (I 972)
Gordy’ at 200 K.Thc energy separation of the 0 --t 1, 0 + 2 and O-+ 3 states were measured to 145 cm- ‘, 255 cm-’ and 5 415 cm-‘, respectively, giving a mean value of 140 cm- ’ for the torsional frequency. Minden and Dailey’ had also measured the torsional frequency of this molecule by the intensity method and found it to be 141 f30 cm-‘. In both of these measurements the error is large. We have made relative intensity measurements on the u = 0 and u = 1 vibrational states of the transition J = 2 t 3 by the microwave bridge method’. The average of several measurements gave an energy separation of 114+6 cm-’ between these two states. The uncertainty represents the internal consistency of five different measurements. This result gives a barrier of 0.93 + 0.09 kcal mol’ ’ which is much lower than those given by other authors’* ‘* 6.
DISCUsslON
molecular
has been determined described in the Section. This method gave the same methyl group as that obtained from method 1. The errors listed under method 2 for these parameters structural determination. molecule, consideration determined position of this atom. From the cxperimental rotational constants, the separation determined with an uncertainty of only a few thousandhts of an However, a longer SIC bond distance along with a shorter SiF bond length and a smaller CSiF angle may give the same moment of inertia as the case given by a shorter SIC bond distance combined with a longer SiF bond and a larger CSiF angle. For a constant moment of inertia, such a possible change in the CSi bond distance is larger than that for the SiF bond. Thus, the uncertainty in the r(CSi) distance is larger than that for the r(SiF) distance. Based on the experimental uncertainty in the rotational constants along with the amount of the deviation from the calculated values, it is possible to elucidate the uncertainty in the silicon atom positions or the uncertainty in the r(CSi) bond length to be 0.041 A. Consequently, corresponding deviations in the r(SiF) bond distance and L CSiF are calculated to be 0.007 A and l.l”, respectively. With this determination of the structural parameters for methyltrifluorosilane, it is possible to make a comparison with those of mcthylsilane’4 and its fluorine substitutions”* I6 _ These comparisons are given in Table 4. It is interesting to note that both the SIC and SiF bond distances are decreasing with further substitution of the fluorine atoms for the hydrogen atoms attached to the silicon. Similar comparisons for the dipole moment and the barrier to the internal rotation are also given in Table 4. The dipole moment is found to be increasing with fluorine J. Mol.
Suucfrtre.
14 (1972)
259
substitution which definitcfy indicates that the dipole moment is negatively directed toward the fluorine atom. TABLE
4
STKUCTURAL
PARAMETERS, VtPOLE
AND THE hlETHYLFLIJOR0 _-. --. .-_-----
-...-_-.
r(SiC) r(SiF) 14 (D) V, (cd
_---.-
.---
’ See ref. 14.
---_-___-. CH,SiH2Fb
I .8668 f0.0005
I .848 f0.005 I.600 &O.OOS I.71 IS59 l 30
----._---..--...-.
0.73’ 1700*100
---.-b See ref. IS.
_--.-...
-..
c See Ref.
ROTATION
-
CH,SiH,=
(A) (A) mol- ‘) -__--
hIOMENT AND BARRIERS TO INTERNAL
DERIVATIVES .-.
--_.-..
Cff,SiHFzc .-__-
-
.-.-_--.---. 16.
* Prcscnt
.-
---.--..
I .833 f0.002 I .583 *0.002 2. I I 3to.02 1255;t30
OF .UETHYLSILANE _-_-
CH,SiFBd -
work.
--
1.812~0.014 1.574~0.007 2.3350.10 93o:t_90
_--
c From
ref. 17.
ACKNOWLEDGEMENT
The authors gratefully acknowledge the financial support given to this study by the National Science Foundation by Grant GP-20723.
REFERENCES I 2 3 4 5 6 7 8 9 IO II 12 I3 14 I5 I6 I7
J. SHERIVAN AND W. GORVY, Phys. Rec., 77 (1950) 719; J. C/tent. Php.. I9 (1951) ii. T. IMINDEN,J. M. MAYS AND B. P. DAII.EY, Pitys. Reo., 78 (1950) 347A. H. T. MIWEN ANL)B. P. DAILEY, Phys. Reu.. 82 (1951) 338A. I_. F. TIIOMAS,J. S. HEEKSAND J. SI~ERIVAN,Z. Elekrrorherrr., 61 (1957) 935. R. L. COLLINS AND J. R. NIEUEN, 1. Cinmt. Phys., 23 (1955) 351. B. KIRTMAN. J. C/rem. Phys.. 37 (1962) 2516. H. W. HARRINGTON. /. Chcm. Pitys., 44 (1966) 3481. R. L. KUCZKOWS~I AND D. R. LIDE. JH.. /. Cherrr.Phys.. 46 (1967) 357. K. J. KRAITCHLIAN. Amer. J. Phys.. 21 (1953) 17. C. C. COSTAIN, J. Chem. Phys.. 29 (1958) 864. A. CIIUTZIAN, J. Mol. Spec/msc., I4 (1964) 361. Y. S. Lt. K. L. KIZER AND J. R. DURIG. J. Mol. Spc~rrosc., 42 (1972) 430. J. S. MUENTER, J. Chew. Phys., 48 (1968) 4544. R. W. KILB AND L. PIERCE. J. Chem. Pirys.. 27 (1957) 108. L. PIERCE. 1. Chenr. Phys.. 29 (1958) 383. D. R. SWALEN AND B. P. STOICHEFF, J. Chem. Phys., 28 (1958) 671. D. R. LIVE. JR. AND K. D. COLES, Phys. Reu., 80 (1950) 91 I.
260
J. Mol.
Structure,
965.
I4 (I 972)
Amsterdam.
Printed
in the Ncthcrland:
MICROWAVE SPECTRUM, STRUCTURE, DIPOLE MOMENT AND BARRIER TO INTERNAL ROTATlON OF METHYLTRIFLUOROSILANE
Y. S. LI AND
1. R. DURIG, Department (Rcccivcd
of Chemistry, 26 June
C. C. TONG*
Unicersity
of South
Carolina,
Columbiu,
SC.
29208
(U.S.A.)
1972)
ARSTRACT
rotational spectrum mcthyltrifluorosilane in ground and first three states of torsional mode have been in the of 12.540.0 The rotational for the ’ 'C isotopic species have also been measured. The following structural parameters have been determined: r(CH) = 1.081+0.004 A, L HCSi = 111”1’~30’, r(CSi) = 1.812+0.014 A, r(SiF) = 1.574+0.007 A, L FSiC = 112”20’+ l”6’. The structural parameters are compared to the corresponding ones for similar molecules. The dipole moment was determined to be 2.3310.10 D. From relative intensity measurements, the barrier to internal rotation was found to bc 0.93+0.09 kcal mol-‘; this value is consistent with the values obtained for other methylfluorosilanes.
INTRODUCTION
The microwave spectrum of methyltrifluorosilanc has been studied by several investigators 1 -4. In these studies’* ’ the barrier to internal rotation was estimated from the relative
intensities
of the torsional
vibrational
satellites.
The determined
barrier value was 1200 cal mol - ’ and the torsional vibrational frequency was estimated as 141+ 30 cm- ‘. Collins and Nielsen’ cstimatcd the torsional frequency at 982 to bc 156 cm- 1 from assigned sum and difference tones with a fundamental cm-‘. This assignment gives a torsional barrier of 1440 cal mol-‘. More recently
Kirtman6analyzed the microwave data of Thomas ef ~1.~ in terms of the vibrationalhindered rotational analysis and obtained a value of 1388+ 100 cal mol-’ for - ..-+ Taken in part from the thesis of C. C. Tong which was submitted to the Department of Chemistry
in partial
J. Mol. Structure.
fulfillment
14 (1972)
of the Ph.D.
degree
(1972).
255
the internal rotational barrier. Since the Hewlett-Packard microwave instrument has been shown to be quite good for intensity mcasurcmenfs’, it was felt that a re-investigation of the relative intensities for the ground and excited torsional states would provide a more consistent value for the barrier to internal rotation. In addition the dipole moment of methyltrifluorosilane had not been determined nor had a complete structure been calculated. Sheridan and Gordy obtained a carbon-silicon bond distance of 1.88 A with assumptions of SiF, and CH, bond distances from other molecules [r(SiF) = 1.555 A, r(CH) = 1.10 A, all angles tctrahcdral]
but this distance appears long in comparison
to the corrc-
spending
distance in other methylfluorosilane molecules_ These results for this molecule are particularly important for comparing to the values obtained for the barrier and dipole moment in the isoelectronic molccuk, H3BPF3*.
The sample of methyltrifluorosilane, CH,SiF,, sular Chcmresearch, Inc. and used without further spectrum was investigated by using the HP Model square-wave modulation frequency of 33.3 kHz on were measured with the Stark cell covered with Dry
was obtained
from Penin-
purification. The microwave 8640A spectrometer with a the Stark cell. Frequencies Ice.
TABLE1 ODSERVED
. J”
J’
FREQUENCIES
---..
-_-_---_ ‘*Cff,“SiF, -----1’ -_ 0
AND
--
,’ z
ROTATIONAL
.---.._
I
CONSTANTS
FOR
--
2 -* 3
____.---14844.72 22 267.49
3 -* 4 4+5
29725.02 37156.13
29689.87 37112.17
29656.55 37069.92
B
3715.64
3711.18
3707.1 I
-
E I4 829.49 22244.38
‘*CHJ2*SiF3 - . _.---
AND
-.--..
“CH,2ASiF~ “Cfi,2aSiF,
_____~_.
--
14862.54 22293.86
l-c2
(MHz) .- -.--
----_.. ,’ T 2
A _.__.. 14 828.45 22 242.30
-
-_
I’ = 3 _-_----__-
E
14813.84 22 220.73
14810.56 22215.52
14493.4 2 I 740.2
29627.67 37034.58
29620.81 37025.49
28985.9 36232.2
___--...-
29659.06 37073.62
I'= 0
A
_---_-
3 703.46
--
3 623.22
The microwave spectrum of the isotopic species, ’ 'CH ,SiF,, was observed and measured with the natural occurring abundance. The results arc listed in Table 1. The frequency accuracy is estimated to be 0.1 MHz. For internal consistency for the relative intensitymeasurements,we have also measured the microwave spectrum of CH$iF, in the ground vibrational state as well as in the first three vibrational excited states of the torsional mode. The uncertainty is estimated to be less than 0.04 MHz for the ground state frequencies, whereas it is larger for the vibrationally excited states. The results are also listed in Table 1. From this table, it can bc seen that all of the cxcitcd state transitions arc located on the lower frequency side of the ground state lines. 256
J. Mol.
Structure.
14 (1972)
STRUCTURE
The combination of our present experimental rotational constants as listed in Table I with those given in ref. 4 makes it possible to calculate the coordinates of the hydrogen and the carbon atcms in the same principal coordinate system’ - “_ molecule, and its principal The molecule, CH ,SiF, , was taken as the parent coordinate system was used in expressing the coordinates of the atoms. By letting
the X2 plane pass through
one of the hydrogen
atoms,
the 2 coordinate
of the
carbon atom was calculated by Kraitchman’s equation’. The relations between the X and 2 coordinates of the hydrogen atom and the mass and moments of inertia for the mono-, di-, and trisubstituted molecules can be expressed by I cl
=
I b2
=
I b3
=
and
respectively,
where
ClN = NMAm/(M+ A??1 = ?U’-??l nl’
= mass
m
=
of the mass of the M= the molecular N = the number 1, = the moment I gfu = the moment g-axis.
NAnr) substituted atom atom to be substituted weight of the parent molecule of fold of the substitution of inertia of the parent molecule along the b-axis of inertia of the N-fold substituted molecule along
the
Since all the moments of inertia are available, the solution of these equations gives the position of the hydrogen atoms. Thus we were able to obtain the r, structure for the methyl group. The results are given in Table 2 under method 1. Since the methyl group is not close to the center of mass of the parent molecule, TABLE
2
STRUCWRAL -_.-_ -
_--_-..r(Cf4)
L HCSi r(CSi) r(SiF) L FSiC
PARAMETERS FOR MtTtiYL _-_-.--
Method I .__.-._1.081 -co.004
1I I”S’f30
--_
A
THIFLUOROSILANC
_.._.
Method 2 _.___ 1.081 +a004
-
-
A
I1 1”1’&30’ 1.812_CO.Ol4A
1.57450.007 A I IZ”20’~ I”6’
----.. J. Mol. Structure, 14 (1972)
257
the structure
obtained
by this method
for the methyl group should be quite good. The given errors represent the internal consistencies in solving the available coordinates from different combinations of the isotopic species. The method used to calculate the complete molecular structure was by fitting of the structural parameters to the rotational constants from ref. 4 along with those in Table 1. This combination gave eight well-determined rotational constants for the six isotopic species of methyltrifluorosilane, which was expected to be sufficient to give a unique solution for all of the five structural parameters. A computer program was used to adjust these parameters to the experimental rotational constants by the least squares method and the results are listed inTable under method 2. The same results were obtained irrespective of the parameters ;Ised as the starting structure. The methyl group structure obtained by this method was found to be identical with those from method 1, cvcn though the rotational constants were less sensitive to the methyl structural parameters in comparison with the other structural parameters. From this point of view, it seems to indicate
that the resulting parameters, r(SiC), R(SiF) and L FSiC, are at least as good as those for the methyl group. On the other hand, the closeness of the silicon atom to the center of mass of this molecule structural parameters.
DIPOLE
gives some limitation
to the accuracy of the
MOMENT
The dipole moment of mcthyltrifluorosilane was determined by measuring the quadratic Stark effect of the following components: [MI = I, K = 0 of 2 e- 1 and 3 + 2 transitions, and IM1 = 3, K = 0 of the 4 t 3 transition. The electric fields were calibrated using a moment of 0.71521 D for 0CS13. The results are of the listed in Table 3. The resulting value (II = 2.33 &-0.10 D) is the average dipole moments obtained from the different Stark component measurements. The error given is the standard deviation from thcsc three measurements. TABLE3 MEASURED
__
DIPOLE
MOMENT
~.._..__.___
Tmnsirion
COMPONENTS ..__.__
IMi
AcJE~ x IO-~
,.-___...
-__
....----.-
[MHz~(~o~~cIII)~] __-_..-....-
---.-.--------_----
clalclrlafed Ekperitrierrrc~l _.__..._-
3-•4 i-*2 2 -* 3
0.440 2.569 0.115
3 I I _-_----~,T= 2.33D&O.10 D
TORSIONAL
_-_..
BARRIER
Relative 258
0.430 2.293 0.132 ____--_-.
intensity
measurements
had
been
performed J. Mol.
by Sheridan Structure,
and
14 (I 972)
Gordy’ at 200 K.Thc energy separation of the 0 --t 1, 0 + 2 and O-+ 3 states were measured to 145 cm- ‘, 255 cm-’ and 5 415 cm-‘, respectively, giving a mean value of 140 cm- ’ for the torsional frequency. Minden and Dailey’ had also measured the torsional frequency of this molecule by the intensity method and found it to be 141 f30 cm-‘. In both of these measurements the error is large. We have made relative intensity measurements on the u = 0 and u = 1 vibrational states of the transition J = 2 t 3 by the microwave bridge method’. The average of several measurements gave an energy separation of 114+6 cm-’ between these two states. The uncertainty represents the internal consistency of five different measurements. This result gives a barrier of 0.93 + 0.09 kcal mol’ ’ which is much lower than those given by other authors’* ‘* 6.
DISCUsslON
molecular
has been determined described in the Section. This method gave the same methyl group as that obtained from method 1. The errors listed under method 2 for these parameters structural determination. molecule, consideration determined position of this atom. From the cxperimental rotational constants, the separation determined with an uncertainty of only a few thousandhts of an However, a longer SIC bond distance along with a shorter SiF bond length and a smaller CSiF angle may give the same moment of inertia as the case given by a shorter SIC bond distance combined with a longer SiF bond and a larger CSiF angle. For a constant moment of inertia, such a possible change in the CSi bond distance is larger than that for the SiF bond. Thus, the uncertainty in the r(CSi) distance is larger than that for the r(SiF) distance. Based on the experimental uncertainty in the rotational constants along with the amount of the deviation from the calculated values, it is possible to elucidate the uncertainty in the silicon atom positions or the uncertainty in the r(CSi) bond length to be 0.041 A. Consequently, corresponding deviations in the r(SiF) bond distance and L CSiF are calculated to be 0.007 A and l.l”, respectively. With this determination of the structural parameters for methyltrifluorosilane, it is possible to make a comparison with those of mcthylsilane’4 and its fluorine substitutions”* I6 _ These comparisons are given in Table 4. It is interesting to note that both the SIC and SiF bond distances are decreasing with further substitution of the fluorine atoms for the hydrogen atoms attached to the silicon. Similar comparisons for the dipole moment and the barrier to the internal rotation are also given in Table 4. The dipole moment is found to be increasing with fluorine J. Mol.
Suucfrtre.
14 (1972)
259
substitution which definitcfy indicates that the dipole moment is negatively directed toward the fluorine atom. TABLE
4
STKUCTURAL
PARAMETERS, VtPOLE
AND THE hlETHYLFLIJOR0 _-. --. .-_-----
-...-_-.
r(SiC) r(SiF) 14 (D) V, (cd
_---.-
.---
’ See ref. 14.
---_-___-. CH,SiH2Fb
I .8668 f0.0005
I .848 f0.005 I.600 &O.OOS I.71 IS59 l 30
----._---..--...-.
0.73’ 1700*100
---.-b See ref. IS.
_--.-...
-..
c See Ref.
ROTATION
-
CH,SiH,=
(A) (A) mol- ‘) -__--
hIOMENT AND BARRIERS TO INTERNAL
DERIVATIVES .-.
--_.-..
Cff,SiHFzc .-__-
-
.-.-_--.---. 16.
* Prcscnt
.-
---.--..
I .833 f0.002 I .583 *0.002 2. I I 3to.02 1255;t30
OF .UETHYLSILANE _-_-
CH,SiFBd -
work.
--
1.812~0.014 1.574~0.007 2.3350.10 93o:t_90
_--
c From
ref. 17.
ACKNOWLEDGEMENT
The authors gratefully acknowledge the financial support given to this study by the National Science Foundation by Grant GP-20723.
REFERENCES I 2 3 4 5 6 7 8 9 IO II 12 I3 14 I5 I6 I7
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