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Force field, centrifugal distortion constants, coriolis constants and mean amplitudes for trifluoroacetonitrile
Journal of Molecular Structure Elsevier Publishing Company, Amsterdam. Printed in the Netherlands
FORCE
FIELD,
CONSTANTS TRILE J. A. FANIRAN*
Department S.
CENTRIFUGAL
DISTORTION
AND MEAN AMPLITUDES
AND
49
CONSTANTS,
CORIOLIS
FOR TRIFLUOROACETONI-
H. F. SHURVELL
of Chemistry,
Queen’s University,
Kingston
(Canada)
J. CYVIN
Institute of Theoretical
Chemistry,
Technical
University
of Norway,
TrondIzeim (Norway)
(Received March 29th, 1971)
ABSTRACT
CaIculations
of valence force fields for CF,CN
have been carried out.
Several sets of force constants have been found to reproduce the observed frequencies exactly. The best of these has been selected on the basis of the qualitative description of the normal modes given by the potential energy distribution and the agreement between experimental and calculated values of centrifugal distortion constants and Coriolis constants. Mean amplitudes of vibration and perpendicular amplitude correction coefficients have also been calculated for CF,CN.
INTRODUCTION
Force constants have been calculated by Edge11 and Potter1 and Galasso and Bigotto* for CF3CN. However, the principal force constants differ greatly between the two calculations. Since these constants are essential for the description of the normal modes of CF,CN and since no potential energy distributions are available from the previous calculations, we have undertaken a recalculation of the normal coordinates for this molecule. Several sets of force constants have been obtained that reproduce our observed frequencies3. A final force field was fixed on the basis of an examination of the potential energy distributions and the comparison between observed and calculated centrifugal distortion constants OJ, D JK and DK and the Coriolis 5 constants. * Present address: Department of Chemistry, University of Ibadan, Ibadan, Nigeria. J. Mol.
Structure,
10 (1971) 49-55
50
J. A. FANIRAN,
FORCE CONSTANT
H. F. SHURVELL,
S. J. CYVIN
CALCULATIONS
The Wilson FG matrix method4 was used to obtain force fields for CF,CN. The symmetry coordinates used were the same as those of Brunvoll and Cyvin”. The G matrix was calculated using the following structural parameters6: r = C-F = 1.33 A, R = C-C = 1.5 A, D = C=N = 1.15 A, cc = Fa fi =C!?
= 1 lO”42’ and 4 = C?N
= 180”. The calculations
= 108”30’,
were carried out:
(a) on an IBM 360/50 computer using a program written by Schachtschneider7 and modified by Brooks8; and (b) with the ALGOL program series of Cyvin et al. adapted to a UNIVAC 1108 computer. Several sets of force constants were examined. Different approaches were applied on the basis of valence coordinates on one hand and the symmetry coordinates on the other. Table 1 shows our final force field in terms of the symmetry coordinate?; it is among the sets which reproduce the observed frequencies3 exactly. During the evaluation of force constants it was found necessary to introduce several interaction terms. Particularly for the CF stretching and the relatively large CF/CF interaction constants we get f, = 5.80 mdyn/A and f,, = 1.30 mdyn/A, respectively. The CN and CC stretching constants from our final set arefD = 18.00 mdyn/A andf, = 5.60 mdyn/A, respectively. TABLE
1
SYMMETRY
FORCE
CONSTANTS
(IN
mdyn/W) FOR CFJCN
Species a, 8.405 0.677
5.597
0.565 0.295
0.332 -0.476
18.000 -0.287
0.690 0.012
0.415
0.018
0.083
Species
0.742
e
4.504 -0.227 -0.001 0.050
POTENTIAL
ENERGY
0.189
DISTRIBUTION
gives a qualitative picture of each The potential energy distribution7-g vibrational mode. PED’S were calculated for every set of force constants obtained in the present work. Apart from the a, fundamental at 2275 cm-l, the various sets J. Mol. Structure,
10 (1971) 49-55
FORCE TABLE
FIELD
FOR
TRIFLUOROACETONITRILE
51
2
VIBRATIONAL
FREQUENCIES
(IN
Cm-‘)
Species a1
k5.0
SI (C-F) &
0.003 0.132
0.886 0.003
Species e S, (C-F) WFCF) S3(CCF) S,(CCN)
VS
AND
POTENHAL
v2
ENERGY
DISTRIBUTION
FOR
CF&N
V3
v4
801.7
52z.o
0.591
0.436
0.005
0.409
0.370
0.079 0.325
0.165
0.033 0.117
0.012 0.642
V-I
V8
0.060 0.868 0.001 0.005
0.026 0.007 0.764 0.638
1227.2
Z214.3
FZ8.3
0.797 0.145 0.106 0.035
0.139 0.001 0.225 0.425
462.7
196.0
of force constants gave different descriptions of the normal modes. The force field of Table 1 is consistent with the PED as given in Table 2 along with the vibrationa frequencies. The description of normal modes according to the PED is compatible with approximate descriptions based on the group vibration concept. The PED indicates that the C=N stretching vibration is largely uncoupled. About 10 o? contribution from C-C stretching is as predicted theoretically” for C=N stretching modes in the nitriles. The potential energy distribution also shows that the C-F stretch coupled with the C-C stretch gives rise to both v2 and vs. v2 also invokes some FCF bending. The v, mode arises from FCF bending with some admixture of C-C stretching. In species e there is considerable mixing between v6 and vs, while it is natural to assign these frequencies to CFs deformation and CCN linear bending, respectively. This feature suggests that some improvements of the force field might possibly be achieved, but no further refinements were found to be necessary at the present stage for this reason alone. In moIecuIes of the CFsX and CXs-CY type, it has been observedll*‘Z that C-F stretching vibrations are found in the range 1400-1000 cm-‘, while C-C stretching frequencies lie between 1000 and 750 cm-‘. These modes are certainly mixed in these compounds.
CENTRIFUGAL
DISTORTION CONSTANTS
Expressions for .the rotational distortion constants DJ, DJK and DK of a symmetric top molecule as a function of the force constants have been given by Kivelson and WilsonL3. A computer program was written for calculating these .I. Mol. Slructure,10 (1971) 49-55
52 TABLE
J. A. FANIRAN,
H. F. SHURVELL,
8. J. CYVIN
3
CENTRIFUGALDISTORTIONCONSTANTS(IN
kc/set) FOR CF,CN
AND RELATED MOLECULES
Atdtors
DJ
D JK
DK
Reference
Present work
0.28
6.29
-5.08
-
Galasso
-4.50
0.31
5.60
Burrus and Gordy
and Bigotto
0.31
5.81
-
15
CF&CH
0.24
0.63
-4.80”
16
CF&CD
0.26
0.62
-4.60”
16
2
a These values were taken from ref. 2.
constants from sets of force constants that reproduced the observed frequencies. The elements (J&J, were evaluated by the methods of Cyvin and Hagen14 in terms of the equilibrium position vectors Xz, Y,’ and 2: of the atoms a of the molecule. The values of D,, DJK and D, obtained for the force constants of Table 1 are given in Table 3. This table also includes the observed centrifugal distortion constants of CF,CN and the related molecules CF,CCH and CF,CCD2V’5S’6. The present vahres are all seen to have the right orders of magnitude. A better quantitative agreement with observed values could be desired, but the results are reasonably satisfactory.
CORIOLIS COUPLING
CONSTANTS
Two experimental determinations of Coriolis constants for CF3CN have been made3#“_ in the present work the [= matrix, which contains the most important Coriolis constants for a symmetric top molecule, was evaluated from the relations of Meal and Polo l* . This method involves the evaluation of the C= and L matrices_ C= was obtained from the B matrix, which is made up from the Wilson s vectors4, and L was taken from the present normal coordinate analysis. The results are shown in Table 4, with values obtained by other methods. TABLE
4
CALCULATED
AND
EXPERIMENTAL
VALUES
OF
CORIOLIS
COUPLING
CONSTANTS
Source
5s
56
57
58
This work
0.87
0.78
-0.80
0.41
Ref. 3
0.76
0.69
-0.24
0.16
Ref. 17
0.84
0.68
-
0.42
L Mol.
Structure,
10 (1971)49-S
FOR
CF&N
FORCE
FIELD
FOR
53
TRIFLUOROACETONITRILE
The calculated results display a generally good agreement with observed values_ The [ values are known to be very sensitive to changes in the force fieId.
This feature was confirmed during the present analysis; the values of C6,C7 and CR even changed sign during our evaluation of the force field.
MEAN
AMPLITUDES
OF VIBRATION
AND
SHRINKAGE
EFFECT
The developed force field was used to calculate the mean amplitudes of vibrationlg and reiated quantities by standard methods with the aid of a wellestablished computer program. The results of calculated mean amplitudes (I) are given in Table 5 along with perpendicuIar amplitude correction coefficients (K). Also the linear shrinkage effect for the CCN chain was computed, and gave the results of 0.0057 A and 0.0082 A at 0 “K and 298 “K, respectiveiy. AI1 these quantitieslg are of great interest in modern gas electron diffraction studies. The molecules CF,CN and CH,CN have been investigated” by the visual method of electron diffraction_ Temperature factors, which are related to the mean amplitudes, have been used in that investigation but no reliable values of I can presumably be extracted from the material. Also the mean amplitudes from the more recent electron diffraction investigation of CCI,CNZ1 are very inaccurate. In that work a visual interpretation
of sectored photographs
was employed.
The magnitudes of all the mean amplitudes calculated here are reasonable. The I values for the bonded distances agree well with the data of Galasso and Bigotto2. These values (see TabIe 6) were deduced from the reported mean-square amplitude quantities ‘. Table 6 shows a comparison between mean amplitudes for some related distances in CF,CN and other molecules. The different types; of distances are found to have fairly characteristic mean amplitude values. This is true especially for the C&N mean amphtude TABLE MEAN (K)
in accord with previous experiencelg.
5 AMPLITUDES
FOR
CF,CN;
Distance
C-F C-C
_
OF IN
VIBRATION A
(0
AND
PERPENDICULAR
AMPLITUDE
CORRECTION
COEFFIECIENTS
UNITS
Equil. dist. G-0
1 0°K
298 “K
0°K
298 “K
(1.330)
0.0450
0.0455
0.0025
0.0037
0.0464
0.0020
0.0021
(1 SOO)
0.0459
K
CzN
(1.150)
0.0344
0.0344
0.0056
0.0098
C .. .N
(2.650)
0.0490
0.0497
0.0019
0.0037
C---F
(2.326)
0.0596
0.0705
0.0014
0.0018
N---F
(3.354)
0.0717
0.0977
0.0008
0.0015
F...F
(2.159)
0.0546
0.059 1
0.0018
0.0034
J. Mol. Siructure, 10 (1971) 49-55
54
J. A. FANIRAN,
TABLE
H. F. SHURVELL,
S. J. CYVIN
6 OF CALCULATED
COMPARISON DIFFERFHT
MEAN AMPLITUDES
(IN
A)
AT 298 “K
FOR
RELATED
DISTANCES
IN
MOLECULES
F...F
Ref.
0.0497
0.0591
present
0.0340
..
..
2
0.0343
0.0504
-
22
0.0345
0.0483
-
23
-
-
0.0547
19
Molecule
C-F
C-C
CEN
c,
CF&N
0.0455
CO464
0.0344
CF&N
0.0456
0.0486
CH&N
-
0.0476
CICCCN
-
0.0453
=CF,
0.0433
-
..N
CONCLUSIONS
A reasonable force field has been found for CFsCN. The potential energy distribution gives a plausible qualitative description of the normal modes and the calculated values of the centrifugal distortion constants and Coriolis constants are in satisfactory agreement with experimental values of these quantities. No claim is made for uniqueness of the force field reported here. However, we have found that although other force fields reproduce the observed frequencies exactly, they are inferior. Either the qualitative description that they give of the normal modes is less satisfactory, or the calculated values of the Coriolis constants are not in agreement with experimental values. On the other hand all the force fields obtained in the present work gave acceptable values for the centrifugal distortion constants.
ACKNOWLEDGMENTS
We gratefully Council of Canada.
acknowledge
the financial support
of the National
Research
REFERENCES 1 W.
F. EDCELL AND E. M. POTTER, J. Chem. Phys., 24 (1956) 80.
~V.GALASSO
AND A. BICOITO, Spectrochim. Actu, 21 (1965) 2085. 3 J. A. FANIRAN AND H. F. SHURVELL, Spectrochim. Actu, in press. 4 E. B. WILSON, J. C. DECNS AND P. C. CROSS, Molecular Wbrutions, McGraw-Hill,
New
1955. J. Mol. Structure, 6 (1970) 289. qiss. Abstr., 19 (1958) 50. 7 J. H. SCHACHTSCHNEIDER, Technical Report No. 57-65, Shill Development Co.;1965.
5 3. BRUNVOLL
AND S. J. CYVIN,
6 R. E. fbDERSON; 8. W.
V. F. B~~&+private
9 Y. M&&o
.i. Mol. Strut+
communication..
AND K~&rrsu,‘J. .’ -10,(i971):49-55
Chem. Phys., 20 (1952) X80&
York
FORCE
FIELD
FOR
TRIFLUOROACETONITRILE
55
10 S. BESNAINOU. B. THOMAS AND S. BRATOZ. J. Mol. Soectrux.. 21 (1966) 113. -Van Nostrand, New 11 G. HERZBERG~ Infrared and Raman Spectra of Polyhtomic h&x&es, York, 1945. 12 M. ST. C. FLETT, Characteristic Frequencies of CfiemicaZ Groups in the Infrared, Elsevier, Amsterdam, 1963. 13 D. KIVEL~~N AND E. B. WILSON, JR., J. Chem. Phys., 20 (1952) 1575; 21 (1953) 1229. 14 S. J. CW~N AND G. HAGEN, Chem. Phys. Left., 1 (1968) 645. 15 C. A. BIJRRUSAND W. GORDY, J. Chem. Phys., 26 (1957) 391. 16 W. E. ANDERSON, R. TRAMBARULO, J. SHERIDAN AND W. GORDY, Phys. Rev., 82 (19.51) 58. 17 R. W. VALENTINE, Ph.D. Thesis, Purdue University, 1957. 18 J. H. MEAL AND S. R. POLO, J. Chem. Phys., 24 (1956) 1119, 1126. 19 S. J. CYVIN, Molecular Vibrations and Mean Square Amplitudes, Universitetsforlaget, Oslo, and Elsevier, Amsterdam, 1968. 20 M. D. DANFORD AND R. L. LIVINGSTON, J. Amer. Chem. Sot., 77 (1955) 2944. 21 R. L. LIVINGSTON, W. L. PAGE AND C. N. RAMACHANDRA RAO, J. Amer. Chem. Sot., 82 (1960) 5048.
22 S. J. CYVIN AND V. DEVARAJAN,J. Mol. Structure, in press. 23 P. KLABOE, E. KLOSTER-JENSEN AND S. J. CYVIN, Spectrochim.
Acta, 23A
J. Mol.
(1967) 2733.
Structure,
10 (1971)
49-55
FORCE
FIELD,
CONSTANTS TRILE J. A. FANIRAN*
Department S.
CENTRIFUGAL
DISTORTION
AND MEAN AMPLITUDES
AND
49
CONSTANTS,
CORIOLIS
FOR TRIFLUOROACETONI-
H. F. SHURVELL
of Chemistry,
Queen’s University,
Kingston
(Canada)
J. CYVIN
Institute of Theoretical
Chemistry,
Technical
University
of Norway,
TrondIzeim (Norway)
(Received March 29th, 1971)
ABSTRACT
CaIculations
of valence force fields for CF,CN
have been carried out.
Several sets of force constants have been found to reproduce the observed frequencies exactly. The best of these has been selected on the basis of the qualitative description of the normal modes given by the potential energy distribution and the agreement between experimental and calculated values of centrifugal distortion constants and Coriolis constants. Mean amplitudes of vibration and perpendicular amplitude correction coefficients have also been calculated for CF,CN.
INTRODUCTION
Force constants have been calculated by Edge11 and Potter1 and Galasso and Bigotto* for CF3CN. However, the principal force constants differ greatly between the two calculations. Since these constants are essential for the description of the normal modes of CF,CN and since no potential energy distributions are available from the previous calculations, we have undertaken a recalculation of the normal coordinates for this molecule. Several sets of force constants have been obtained that reproduce our observed frequencies3. A final force field was fixed on the basis of an examination of the potential energy distributions and the comparison between observed and calculated centrifugal distortion constants OJ, D JK and DK and the Coriolis 5 constants. * Present address: Department of Chemistry, University of Ibadan, Ibadan, Nigeria. J. Mol.
Structure,
10 (1971) 49-55
50
J. A. FANIRAN,
FORCE CONSTANT
H. F. SHURVELL,
S. J. CYVIN
CALCULATIONS
The Wilson FG matrix method4 was used to obtain force fields for CF,CN. The symmetry coordinates used were the same as those of Brunvoll and Cyvin”. The G matrix was calculated using the following structural parameters6: r = C-F = 1.33 A, R = C-C = 1.5 A, D = C=N = 1.15 A, cc = Fa fi =C!?
= 1 lO”42’ and 4 = C?N
= 180”. The calculations
= 108”30’,
were carried out:
(a) on an IBM 360/50 computer using a program written by Schachtschneider7 and modified by Brooks8; and (b) with the ALGOL program series of Cyvin et al. adapted to a UNIVAC 1108 computer. Several sets of force constants were examined. Different approaches were applied on the basis of valence coordinates on one hand and the symmetry coordinates on the other. Table 1 shows our final force field in terms of the symmetry coordinate?; it is among the sets which reproduce the observed frequencies3 exactly. During the evaluation of force constants it was found necessary to introduce several interaction terms. Particularly for the CF stretching and the relatively large CF/CF interaction constants we get f, = 5.80 mdyn/A and f,, = 1.30 mdyn/A, respectively. The CN and CC stretching constants from our final set arefD = 18.00 mdyn/A andf, = 5.60 mdyn/A, respectively. TABLE
1
SYMMETRY
FORCE
CONSTANTS
(IN
mdyn/W) FOR CFJCN
Species a, 8.405 0.677
5.597
0.565 0.295
0.332 -0.476
18.000 -0.287
0.690 0.012
0.415
0.018
0.083
Species
0.742
e
4.504 -0.227 -0.001 0.050
POTENTIAL
ENERGY
0.189
DISTRIBUTION
gives a qualitative picture of each The potential energy distribution7-g vibrational mode. PED’S were calculated for every set of force constants obtained in the present work. Apart from the a, fundamental at 2275 cm-l, the various sets J. Mol. Structure,
10 (1971) 49-55
FORCE TABLE
FIELD
FOR
TRIFLUOROACETONITRILE
51
2
VIBRATIONAL
FREQUENCIES
(IN
Cm-‘)
Species a1
k5.0
SI (C-F) &
0.003 0.132
0.886 0.003
Species e S, (C-F) WFCF) S3(CCF) S,(CCN)
VS
AND
POTENHAL
v2
ENERGY
DISTRIBUTION
FOR
CF&N
V3
v4
801.7
52z.o
0.591
0.436
0.005
0.409
0.370
0.079 0.325
0.165
0.033 0.117
0.012 0.642
V-I
V8
0.060 0.868 0.001 0.005
0.026 0.007 0.764 0.638
1227.2
Z214.3
FZ8.3
0.797 0.145 0.106 0.035
0.139 0.001 0.225 0.425
462.7
196.0
of force constants gave different descriptions of the normal modes. The force field of Table 1 is consistent with the PED as given in Table 2 along with the vibrationa frequencies. The description of normal modes according to the PED is compatible with approximate descriptions based on the group vibration concept. The PED indicates that the C=N stretching vibration is largely uncoupled. About 10 o? contribution from C-C stretching is as predicted theoretically” for C=N stretching modes in the nitriles. The potential energy distribution also shows that the C-F stretch coupled with the C-C stretch gives rise to both v2 and vs. v2 also invokes some FCF bending. The v, mode arises from FCF bending with some admixture of C-C stretching. In species e there is considerable mixing between v6 and vs, while it is natural to assign these frequencies to CFs deformation and CCN linear bending, respectively. This feature suggests that some improvements of the force field might possibly be achieved, but no further refinements were found to be necessary at the present stage for this reason alone. In moIecuIes of the CFsX and CXs-CY type, it has been observedll*‘Z that C-F stretching vibrations are found in the range 1400-1000 cm-‘, while C-C stretching frequencies lie between 1000 and 750 cm-‘. These modes are certainly mixed in these compounds.
CENTRIFUGAL
DISTORTION CONSTANTS
Expressions for .the rotational distortion constants DJ, DJK and DK of a symmetric top molecule as a function of the force constants have been given by Kivelson and WilsonL3. A computer program was written for calculating these .I. Mol. Slructure,10 (1971) 49-55
52 TABLE
J. A. FANIRAN,
H. F. SHURVELL,
8. J. CYVIN
3
CENTRIFUGALDISTORTIONCONSTANTS(IN
kc/set) FOR CF,CN
AND RELATED MOLECULES
Atdtors
DJ
D JK
DK
Reference
Present work
0.28
6.29
-5.08
-
Galasso
-4.50
0.31
5.60
Burrus and Gordy
and Bigotto
0.31
5.81
-
15
CF&CH
0.24
0.63
-4.80”
16
CF&CD
0.26
0.62
-4.60”
16
2
a These values were taken from ref. 2.
constants from sets of force constants that reproduced the observed frequencies. The elements (J&J, were evaluated by the methods of Cyvin and Hagen14 in terms of the equilibrium position vectors Xz, Y,’ and 2: of the atoms a of the molecule. The values of D,, DJK and D, obtained for the force constants of Table 1 are given in Table 3. This table also includes the observed centrifugal distortion constants of CF,CN and the related molecules CF,CCH and CF,CCD2V’5S’6. The present vahres are all seen to have the right orders of magnitude. A better quantitative agreement with observed values could be desired, but the results are reasonably satisfactory.
CORIOLIS COUPLING
CONSTANTS
Two experimental determinations of Coriolis constants for CF3CN have been made3#“_ in the present work the [= matrix, which contains the most important Coriolis constants for a symmetric top molecule, was evaluated from the relations of Meal and Polo l* . This method involves the evaluation of the C= and L matrices_ C= was obtained from the B matrix, which is made up from the Wilson s vectors4, and L was taken from the present normal coordinate analysis. The results are shown in Table 4, with values obtained by other methods. TABLE
4
CALCULATED
AND
EXPERIMENTAL
VALUES
OF
CORIOLIS
COUPLING
CONSTANTS
Source
5s
56
57
58
This work
0.87
0.78
-0.80
0.41
Ref. 3
0.76
0.69
-0.24
0.16
Ref. 17
0.84
0.68
-
0.42
L Mol.
Structure,
10 (1971)49-S
FOR
CF&N
FORCE
FIELD
FOR
53
TRIFLUOROACETONITRILE
The calculated results display a generally good agreement with observed values_ The [ values are known to be very sensitive to changes in the force fieId.
This feature was confirmed during the present analysis; the values of C6,C7 and CR even changed sign during our evaluation of the force field.
MEAN
AMPLITUDES
OF VIBRATION
AND
SHRINKAGE
EFFECT
The developed force field was used to calculate the mean amplitudes of vibrationlg and reiated quantities by standard methods with the aid of a wellestablished computer program. The results of calculated mean amplitudes (I) are given in Table 5 along with perpendicuIar amplitude correction coefficients (K). Also the linear shrinkage effect for the CCN chain was computed, and gave the results of 0.0057 A and 0.0082 A at 0 “K and 298 “K, respectiveiy. AI1 these quantitieslg are of great interest in modern gas electron diffraction studies. The molecules CF,CN and CH,CN have been investigated” by the visual method of electron diffraction_ Temperature factors, which are related to the mean amplitudes, have been used in that investigation but no reliable values of I can presumably be extracted from the material. Also the mean amplitudes from the more recent electron diffraction investigation of CCI,CNZ1 are very inaccurate. In that work a visual interpretation
of sectored photographs
was employed.
The magnitudes of all the mean amplitudes calculated here are reasonable. The I values for the bonded distances agree well with the data of Galasso and Bigotto2. These values (see TabIe 6) were deduced from the reported mean-square amplitude quantities ‘. Table 6 shows a comparison between mean amplitudes for some related distances in CF,CN and other molecules. The different types; of distances are found to have fairly characteristic mean amplitude values. This is true especially for the C&N mean amphtude TABLE MEAN (K)
in accord with previous experiencelg.
5 AMPLITUDES
FOR
CF,CN;
Distance
C-F C-C
_
OF IN
VIBRATION A
(0
AND
PERPENDICULAR
AMPLITUDE
CORRECTION
COEFFIECIENTS
UNITS
Equil. dist. G-0
1 0°K
298 “K
0°K
298 “K
(1.330)
0.0450
0.0455
0.0025
0.0037
0.0464
0.0020
0.0021
(1 SOO)
0.0459
K
CzN
(1.150)
0.0344
0.0344
0.0056
0.0098
C .. .N
(2.650)
0.0490
0.0497
0.0019
0.0037
C---F
(2.326)
0.0596
0.0705
0.0014
0.0018
N---F
(3.354)
0.0717
0.0977
0.0008
0.0015
F...F
(2.159)
0.0546
0.059 1
0.0018
0.0034
J. Mol. Siructure, 10 (1971) 49-55
54
J. A. FANIRAN,
TABLE
H. F. SHURVELL,
S. J. CYVIN
6 OF CALCULATED
COMPARISON DIFFERFHT
MEAN AMPLITUDES
(IN
A)
AT 298 “K
FOR
RELATED
DISTANCES
IN
MOLECULES
F...F
Ref.
0.0497
0.0591
present
0.0340
..
..
2
0.0343
0.0504
-
22
0.0345
0.0483
-
23
-
-
0.0547
19
Molecule
C-F
C-C
CEN
c,
CF&N
0.0455
CO464
0.0344
CF&N
0.0456
0.0486
CH&N
-
0.0476
CICCCN
-
0.0453
=CF,
0.0433
-
..N
CONCLUSIONS
A reasonable force field has been found for CFsCN. The potential energy distribution gives a plausible qualitative description of the normal modes and the calculated values of the centrifugal distortion constants and Coriolis constants are in satisfactory agreement with experimental values of these quantities. No claim is made for uniqueness of the force field reported here. However, we have found that although other force fields reproduce the observed frequencies exactly, they are inferior. Either the qualitative description that they give of the normal modes is less satisfactory, or the calculated values of the Coriolis constants are not in agreement with experimental values. On the other hand all the force fields obtained in the present work gave acceptable values for the centrifugal distortion constants.
ACKNOWLEDGMENTS
We gratefully Council of Canada.
acknowledge
the financial support
of the National
Research
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