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Conformational identification and dipole moment of 4-fluoro-1-butene by microwave spectroscopy
Journal of Molecular Structure, 162 (1987) 305-311 Elsevier Science Publishers B.V., Amsterdam - Printed
CONFORMATIONAL 4-FLUORO-l-BUTENE
IDENTIFICATION BY MICROWAVE
YING-SING
LI and BIH-YING
Department
of Chemistry,
GAMIL
in The Netherlands
AND DIPOLE MOMENT OF SPECTROSCOPY
LIU
Memphis
State
University,
Memphis,
TN 38152
(U.S.A.)
A. GUIRGIS
Dyes and Pigments (Received
Division,
26 May 1987;
Mobay
Corporation,
in final form
16 July
Charleston,
SC 29411
(U.S.A.)
1987)
ABSTRACT The microwave spectrum of 4-fluoro-l-butene has been recorded in the frequency region 26.5-40.0 GHz. A-type rotational transitions in the ground vibrational state have been assigned. The effective rotational constants and centrifugal distortion constants were determined to be A = 19 196 k 323, B = 2132.48 ? 0.01, C = 2112.52 + 0.01 MHz, DJ= 0.70 + 0.03 and Da = -26.16 * 0.05 kHz. Analysis of the measured Stark effects gave a dipole moment of 1.80 D with components of ju,l = 1.62 f 0.01, lubj = 0.68 + 0.05 and (ucI = 0.39 + 0.14 D. From the experimentalrotational constants and dipole moments, it is suggested that the assigned spectra have resulted from the skew-trans conformer.
INTRODUCTION
Rotational isomerisms of haloalkanes and haloalkenes in the gas phase have been the topics of numerous studies. For example, microwave, infrared and Raman investigations of 1-halopropanes have identified both the gauche and tram conformers, with the gauche being predominant in the lower energy state [l-7]. The existence of both the cis and skew forms in ally1 fluoride has also been confirmed [8-121. In the latter studies, it was found that the cis form is more stable than the skew form. However, the microwave and electron diffraction studies of 1-butene have concluded that the corresponding skew form was more stable than the cis form [13, 141. Only one conformation (skew-tram) was detected in 4-bromo-l-butene by the low resolution microwave spectroscopy [ 151. As in 4-bromo-l-butene, there are five non-equivalent conformations in 4-fluoro-1-butene. Shown in Fig. 1 are the Newman projections of these conformers along the Cz-C3 and the CJ--C4 bonds. To the best of our knowledge, there has been neither any conformational nor any spectroscopic work done on this molecule. As a continuation of our conformational analyses in gases, we have investigated the microwave spectrum of 4-fluoro-1-butene
0022-2860/87/$03.50
o 1987
Elsevier
Science
Publishers
B.V.
306
FHF a-c2 H
H
H
H
C IS TRANS H
H @
H
FH,CCH,
C IS GAUCHE H -8.
H
H
SKEW TRANS H
FHE
Hy=CH,
HCH, H
F
H
H
SKEW GAUCHE H
FH,C @
H
HC=CH, I F
H
H
H
SKEW GAUCHE’ FH,C
H
H Fig. 1. Newman
projections
of 4-fluoro-l-butene
in different
conformations.
with emphasis on the identification of the conformers presented in this molecule. The results are reported herein. EXPERIMENTAL
The sample of 4-fluoro-1-butene used in the present study was prepared by the reaction of freshly distilled 4-bromo-1-butene (Aldrich Chemicals) with potassium fluoride in ethylene glycol at 200°C. The potassium fluoride was dried at 350°C for three hours before using. Additionally the reaction flask was fitted with a condenser cooled to 0°C by circulating ethanol sludge. The sample was purified by trap-to-trap distillation, and was checked for
307
purity by IR, CC and mass spectrometry. The spectrum was recorded in the frequency region 26.5-40.0 GHz using a conventional microwave spectrometer [16] with a Stark modulation frequency of 100 kH.z. A microwave signal source was generated from a Hewlett-Packard 8457A microwave synthesizer. The microwave spectrum was recorded while the Stark cell was being cooled with dry ice. The accuracy of frequency measurement was estimated to be 0.12 MHz. RESULTS
Spectrum The low resolution microwave spectrum of 4-fluoro-1-butene is predominated by three major bands observed in the R-band (26.5-40.0 GHz) frequency region. From their frequencies and separations, these bands were assigned as J = 7 +- 6, 8 + 7, and 9 +- 8 R-branch parallel transitions. To obtain some information about the characteristics of the rotational transitions for different conformers, rotational constants were calculated based on the structural parameters shown in Fig. 2. These assumed parameters were essentially obtained from those of 1-butene [13] and 1-fluoropropane [1]. All the bond lengths and bond angles were assumed to be the same for all the conformers except the dihedral angles involved in the internal rotations about the CZ-C3 and the C3-C4 bonds. The calculated rotational constants for the five possible conformers are listed in Table 1. As indicated in Fig. 1, all of the conformers have no elements of symmetry except the cis-trans form which has a symmetry plane.
Fig. 2. Structural
parameters
of 4-fluoro-1-butene.
308 TABLE 1 Calculated rotational constants (MHz) for five possible conformers of 4-fluoro-l-butene Conformers
A
B
c
Cis-tram &s-gauche Skew-trans Skew-gauche(I)a Skew-gauche(II)a
15 140.1892 8216.0732 20 516.8579 9884.7001 12 710.3864
2499.8969 3882.9181 2102.3244 3060.9991 2654.8538
2206.2230 2944.0842 2071.9408 2614.5426 2415.9091
Vhe fluorine atom is closer to the CC double bond in the skew-gauche(I) skew-gauche(I1) form.
K -0.954859 -0.643841 -0.996705 -0.877181 -0.953578 than in the
Rotational assignments including both J and K were made based on the low-resolution microwave spectrum, predicted rotational transitions, expected relative intensities, and qualitative Stark effects. A rigid rotor model was initially used for confirming the accuracy of the rotational assignments. An improvement of fitting the calculated frequencies to the observed ones resulted from the inclusion of centrifugal distortion constants DJ and DJK. Attempts to assign b-type and c-type transitions were unsuccessful, presumably because of small u,, and U, dipole moment components. Listed in Table 2 are the rotational transition frequencies, and Table 3 gives the effective rotational constants and centrifugal distortion constants of 4-fluoro-l-butene in its ground vibrational state. The accuracy of constant A is relatively poor because the molecule is a nearly prolate rotor and only u-type transitions are assigned. Attempts to assign the rotational transitions of another conformer failed in this present study. Dipole moment The electric dipole moments of 4-fluoro-1-butene were determined from IMI = 1, 2, 3,4, 5,6 and 7 components of the S18+ 717 transition, and IMI = 1, 2 and 3 of the 71, + 616 transition. All these components were shown to have a quadratic Stark effect. By varying the squares of the dipole moment components, the measured Stark components at different voltages were leastsquares fitted to the calculated frequencies from the second-order perturbation Stark coefficients [17]. The electric field was calibrated using the IMI = 2 component of the 3 + 2 transition of the OCS (carbonyl sulphide) in its ground vibrational state and its dipole moment of 0.7152 D [ 181. The results are listed in Table 4 with standard deviations. Conformation The calculated rotational constants shown in Table 1 for the five different conformers indicated that the rotational constants of 4-fluoro-1-butene are
309
TABLE 2 Rotational transition frequencies vibrational state
(MHz) of skew-tram
” (Observed)
4-fluoro-l-butene
in the ground
AV
(Obs. - talc.)
7,,---6‘6 7,,-61, 725-62, 7x-6,, 7,,--63, 7,,--64, 7,,-65, 7, z--6,,
29713.11 29644.40 29783.99 29715.39 29716.26 29717.51 29719.99 29723.26 29727.30
0.05 0.09 -0.05 0.02 -0.09 0.03 0.01 0.00 0.03
8*,--70, 8,,--71, 8,,-71, 8,,-72, 8,,---72, 8,,---73, 8,,--74, 8,,--75, 8,,--76, 8w--77,
33957.16 33878.81 34038.52 33960.05 33961.38 33962.52 33965.35 33969.11 33973.10 33979.11
0.06 0.02 0.04 0.02 -0.12 -0.02 -0.04 -0.01 -0.00 -0.03
9,,-81, 9x,--81, 9,,-82, 9,,--82, 9,,--8% 9,,---84, 9,,--85, 9w--86, 9,,--8,x
38113.11 38292.86 38204.57 38206.50 38207.46 38210.64 38214.83 38220.01 38226.10
0.01 0.11 0.04 -0.13 -0.03 -0.02 -0.02 0.01 -0.01
70,~-60,
TABLE 3 Rotational constants (MHz) and centrifugal distortion 4-fluoro-l-butene in the ground vibrational state A = 19 196 + 323.0 B = 2132.48 + 0.01 C = 2112.52 + 0.01 DJ= 0.70 t 0.03 D JK= -26.16 + 0.05 oa = 0.058 %I is the standard deviation of the frequency fit in MHz.
constants (kHz) of skew-tram
310 TABLE
4
Dipole moments pa P,, pc /+ era =
1.62 0.68 0.39 1.80 0.054
?: k f f
(Debye)
of 4-fluoro-1-butene
in the skew-tram
form
0.01 0.05 0.14 0.05
% is the standard
deviation
in MHz.
very sensitive to the conformation of the molecule. Such character has made it possible to identify the conformation of 4-fluoro-l-butene. A comparison of these calculated rotational constants with those experimental values (see Table 3) has left little question that the molecule predominantly exists in the skew-Pans form. The Stark effect study gave non-zero dipole moment components along the three principal axes. This indicates that the assigned rotational spectrum must arise from the conformer without any symmetry element except C1. If the total electric dipole moment of the molecule is made up essentially of contributions from the CC, double bond and the C-F bond moments, the dipole moment may be assumed as the vector sum of the dipoles of propene [19] and fluoromethane [20]. The choice of the orientation of the dipole vector in the propenyl part was found to be rather important. In our present calculation, we chose the orientation to be the same as that in propene [19]. The results of the calculations for the five different rotamers are listed in Table 5. A comparison of the calculated dipole moments with the experimental results apparently suggests that the molecule exists in the skew-truns form. CONCLUSION
Both the experimental rotational constants and the dipole moments obtained in the present microwave study on 4-fluoro-l-butene conclude that the molecule predominantly exists in the skew-truns form. No relative TABLE
5
Calculated
dipole moments
(Debye)
of 4-fluoro-l-butene
Cis-trans C&--gauche Skew-trans Skew-gauche Skew-gauche’
1.50 0.17 1.52 0.05 0.53
0.26 1.80 0.85 1.61 1.70
Experimental
1.62
0.68
in different
conformations
0.00
1.52
1.18 0.19
2.19 1.81
0.22 0.49
1.62 1.84
0.39
1.80
311
conformational stabilities could be derived without the identification of other forms. However, from the relative intensities of the unassigned lines in the spectrum, it is believed that the skew-truns form is the predominant conformation of 4-fluoro-l-butene. The predominance of the skew-truns form in 4-fluoro-1-butene is consistent with the results in 4-chloro-l-butene [21] and 4-bromo-l-butene [15, 211. A qualitative agreement between the experimental dipole moments and those calculated from the vector model suggests that the charge induction due to the fluorine atom in 4-fluoro-l-butene is insignificant. Such a suggestion is apparently supported by the results of our CNDO calculation [22]. REFERENCES 1 E. Hirota, J. Chem. Phys., 37 (1962) 283. 2 T. N. Sarachman, J. Chem. Phys., 39 (1963) 469. 3 C. Komaki, I. Ichishima, K. Kuratani, T. Miyazawa, T. Shimanouchi and S. Mizushima, Bull. Chem. Sot. Jpn., 28 (1955) 330. 4 K. Radcliffe and J. L. Wood, Trans. Faraday Sot., 62 (1966) 1678. 5 G. A. Crowder and H. K. Mao, J. Mol. Struct., 18 (1973) 33. 6Y. Ogawa, S. Imazeki, H. Yamaguchi, H. Matsunra, I. Harada and T. Shimanouchi, Bull. Chem. Sot. Jpn., 51 (1978) 748. 7 J. R. Durig, S. E. Godbey and J. F. Sullivan, J. Chem. Phys., 80 (1984) 5983. 8E. Hirota, J. Chem. Phys., 42 (1965) 2071. 9 P. Meakin, D. 0. Harris and E. Hirota, J. Chem. Phys., 51 (1969) 3775. 10 R. D. McLachlan and R. A. Nyquist, Spectrochim. Acta, Part A, 24 (1968) 103. 11 J. R. Durig, M. Zhen and T. S. Little, J. Chem. Phys., 81 (1984) 4259. 12 J. R. Durig, M. Zhen, H. L. Heusel, P. J. Joseph, P. Groner and T. S. Little, J. Chem. Phys., 89 (1985) 2877. 13 S. Kondo, E. Hirota and Y. Morino, J. Mol. Spectrosc., 28 (1968) 471. 14 D. Van Hemelrijk, L. Van den Enden, H. J. Geise, H. L. Sellers and L. Schafer, J. Am. Chem. Sot., 102 (1980) 2189. 15 W. E. Steinmetz, J. Hollenberg, F. Hickernell, C. E. Orr and L. H. Scharpen, J. Phys. Chem., 82 (1978) 940. 16 K. M. McAfee, R. H. Hughes and E. B. Wilson Jr., Rev. Sci. Instrum., 20 (1949) 821. 17 S. Golden and E. B. Wilson Jr., J. Chem. Phys., 16 (1948) 669. 18 J. S. Muenter, J. Chem. Phys., 48 (1968) 4544. 19 E. Hirota and Y. Marino, J. Chem. Phys., 45 (1966) 2326. 20 D. M. Larkin and W. Gordy, J. Chem. Phys., 38 (1963) 2329. 21 S. H. Schei, J. Mol. Struct., 128 (1985) 151. 22 Molecular Orbital program no. 141 from the Quantum Chemistry Program Exchange, University of Indiana. This program has been modified for second row elements as recommended by D. P. Santry, J. Am. Chem. Sot., 90 (1986) 3309 and J. R. Sabin, personal communication (1973).
CONFORMATIONAL 4-FLUORO-l-BUTENE
IDENTIFICATION BY MICROWAVE
YING-SING
LI and BIH-YING
Department
of Chemistry,
GAMIL
in The Netherlands
AND DIPOLE MOMENT OF SPECTROSCOPY
LIU
Memphis
State
University,
Memphis,
TN 38152
(U.S.A.)
A. GUIRGIS
Dyes and Pigments (Received
Division,
26 May 1987;
Mobay
Corporation,
in final form
16 July
Charleston,
SC 29411
(U.S.A.)
1987)
ABSTRACT The microwave spectrum of 4-fluoro-l-butene has been recorded in the frequency region 26.5-40.0 GHz. A-type rotational transitions in the ground vibrational state have been assigned. The effective rotational constants and centrifugal distortion constants were determined to be A = 19 196 k 323, B = 2132.48 ? 0.01, C = 2112.52 + 0.01 MHz, DJ= 0.70 + 0.03 and Da = -26.16 * 0.05 kHz. Analysis of the measured Stark effects gave a dipole moment of 1.80 D with components of ju,l = 1.62 f 0.01, lubj = 0.68 + 0.05 and (ucI = 0.39 + 0.14 D. From the experimentalrotational constants and dipole moments, it is suggested that the assigned spectra have resulted from the skew-trans conformer.
INTRODUCTION
Rotational isomerisms of haloalkanes and haloalkenes in the gas phase have been the topics of numerous studies. For example, microwave, infrared and Raman investigations of 1-halopropanes have identified both the gauche and tram conformers, with the gauche being predominant in the lower energy state [l-7]. The existence of both the cis and skew forms in ally1 fluoride has also been confirmed [8-121. In the latter studies, it was found that the cis form is more stable than the skew form. However, the microwave and electron diffraction studies of 1-butene have concluded that the corresponding skew form was more stable than the cis form [13, 141. Only one conformation (skew-tram) was detected in 4-bromo-l-butene by the low resolution microwave spectroscopy [ 151. As in 4-bromo-l-butene, there are five non-equivalent conformations in 4-fluoro-1-butene. Shown in Fig. 1 are the Newman projections of these conformers along the Cz-C3 and the CJ--C4 bonds. To the best of our knowledge, there has been neither any conformational nor any spectroscopic work done on this molecule. As a continuation of our conformational analyses in gases, we have investigated the microwave spectrum of 4-fluoro-1-butene
0022-2860/87/$03.50
o 1987
Elsevier
Science
Publishers
B.V.
306
FHF a-c2 H
H
H
H
C IS TRANS H
H @
H
FH,CCH,
C IS GAUCHE H -8.
H
H
SKEW TRANS H
FHE
Hy=CH,
HCH, H
F
H
H
SKEW GAUCHE H
FH,C @
H
HC=CH, I F
H
H
H
SKEW GAUCHE’ FH,C
H
H Fig. 1. Newman
projections
of 4-fluoro-l-butene
in different
conformations.
with emphasis on the identification of the conformers presented in this molecule. The results are reported herein. EXPERIMENTAL
The sample of 4-fluoro-1-butene used in the present study was prepared by the reaction of freshly distilled 4-bromo-1-butene (Aldrich Chemicals) with potassium fluoride in ethylene glycol at 200°C. The potassium fluoride was dried at 350°C for three hours before using. Additionally the reaction flask was fitted with a condenser cooled to 0°C by circulating ethanol sludge. The sample was purified by trap-to-trap distillation, and was checked for
307
purity by IR, CC and mass spectrometry. The spectrum was recorded in the frequency region 26.5-40.0 GHz using a conventional microwave spectrometer [16] with a Stark modulation frequency of 100 kH.z. A microwave signal source was generated from a Hewlett-Packard 8457A microwave synthesizer. The microwave spectrum was recorded while the Stark cell was being cooled with dry ice. The accuracy of frequency measurement was estimated to be 0.12 MHz. RESULTS
Spectrum The low resolution microwave spectrum of 4-fluoro-1-butene is predominated by three major bands observed in the R-band (26.5-40.0 GHz) frequency region. From their frequencies and separations, these bands were assigned as J = 7 +- 6, 8 + 7, and 9 +- 8 R-branch parallel transitions. To obtain some information about the characteristics of the rotational transitions for different conformers, rotational constants were calculated based on the structural parameters shown in Fig. 2. These assumed parameters were essentially obtained from those of 1-butene [13] and 1-fluoropropane [1]. All the bond lengths and bond angles were assumed to be the same for all the conformers except the dihedral angles involved in the internal rotations about the CZ-C3 and the C3-C4 bonds. The calculated rotational constants for the five possible conformers are listed in Table 1. As indicated in Fig. 1, all of the conformers have no elements of symmetry except the cis-trans form which has a symmetry plane.
Fig. 2. Structural
parameters
of 4-fluoro-1-butene.
308 TABLE 1 Calculated rotational constants (MHz) for five possible conformers of 4-fluoro-l-butene Conformers
A
B
c
Cis-tram &s-gauche Skew-trans Skew-gauche(I)a Skew-gauche(II)a
15 140.1892 8216.0732 20 516.8579 9884.7001 12 710.3864
2499.8969 3882.9181 2102.3244 3060.9991 2654.8538
2206.2230 2944.0842 2071.9408 2614.5426 2415.9091
Vhe fluorine atom is closer to the CC double bond in the skew-gauche(I) skew-gauche(I1) form.
K -0.954859 -0.643841 -0.996705 -0.877181 -0.953578 than in the
Rotational assignments including both J and K were made based on the low-resolution microwave spectrum, predicted rotational transitions, expected relative intensities, and qualitative Stark effects. A rigid rotor model was initially used for confirming the accuracy of the rotational assignments. An improvement of fitting the calculated frequencies to the observed ones resulted from the inclusion of centrifugal distortion constants DJ and DJK. Attempts to assign b-type and c-type transitions were unsuccessful, presumably because of small u,, and U, dipole moment components. Listed in Table 2 are the rotational transition frequencies, and Table 3 gives the effective rotational constants and centrifugal distortion constants of 4-fluoro-l-butene in its ground vibrational state. The accuracy of constant A is relatively poor because the molecule is a nearly prolate rotor and only u-type transitions are assigned. Attempts to assign the rotational transitions of another conformer failed in this present study. Dipole moment The electric dipole moments of 4-fluoro-1-butene were determined from IMI = 1, 2, 3,4, 5,6 and 7 components of the S18+ 717 transition, and IMI = 1, 2 and 3 of the 71, + 616 transition. All these components were shown to have a quadratic Stark effect. By varying the squares of the dipole moment components, the measured Stark components at different voltages were leastsquares fitted to the calculated frequencies from the second-order perturbation Stark coefficients [17]. The electric field was calibrated using the IMI = 2 component of the 3 + 2 transition of the OCS (carbonyl sulphide) in its ground vibrational state and its dipole moment of 0.7152 D [ 181. The results are listed in Table 4 with standard deviations. Conformation The calculated rotational constants shown in Table 1 for the five different conformers indicated that the rotational constants of 4-fluoro-1-butene are
309
TABLE 2 Rotational transition frequencies vibrational state
(MHz) of skew-tram
” (Observed)
4-fluoro-l-butene
in the ground
AV
(Obs. - talc.)
7,,---6‘6 7,,-61, 725-62, 7x-6,, 7,,--63, 7,,--64, 7,,-65, 7, z--6,,
29713.11 29644.40 29783.99 29715.39 29716.26 29717.51 29719.99 29723.26 29727.30
0.05 0.09 -0.05 0.02 -0.09 0.03 0.01 0.00 0.03
8*,--70, 8,,--71, 8,,-71, 8,,-72, 8,,---72, 8,,---73, 8,,--74, 8,,--75, 8,,--76, 8w--77,
33957.16 33878.81 34038.52 33960.05 33961.38 33962.52 33965.35 33969.11 33973.10 33979.11
0.06 0.02 0.04 0.02 -0.12 -0.02 -0.04 -0.01 -0.00 -0.03
9,,-81, 9x,--81, 9,,-82, 9,,--82, 9,,--8% 9,,---84, 9,,--85, 9w--86, 9,,--8,x
38113.11 38292.86 38204.57 38206.50 38207.46 38210.64 38214.83 38220.01 38226.10
0.01 0.11 0.04 -0.13 -0.03 -0.02 -0.02 0.01 -0.01
70,~-60,
TABLE 3 Rotational constants (MHz) and centrifugal distortion 4-fluoro-l-butene in the ground vibrational state A = 19 196 + 323.0 B = 2132.48 + 0.01 C = 2112.52 + 0.01 DJ= 0.70 t 0.03 D JK= -26.16 + 0.05 oa = 0.058 %I is the standard deviation of the frequency fit in MHz.
constants (kHz) of skew-tram
310 TABLE
4
Dipole moments pa P,, pc /+ era =
1.62 0.68 0.39 1.80 0.054
?: k f f
(Debye)
of 4-fluoro-1-butene
in the skew-tram
form
0.01 0.05 0.14 0.05
% is the standard
deviation
in MHz.
very sensitive to the conformation of the molecule. Such character has made it possible to identify the conformation of 4-fluoro-l-butene. A comparison of these calculated rotational constants with those experimental values (see Table 3) has left little question that the molecule predominantly exists in the skew-Pans form. The Stark effect study gave non-zero dipole moment components along the three principal axes. This indicates that the assigned rotational spectrum must arise from the conformer without any symmetry element except C1. If the total electric dipole moment of the molecule is made up essentially of contributions from the CC, double bond and the C-F bond moments, the dipole moment may be assumed as the vector sum of the dipoles of propene [19] and fluoromethane [20]. The choice of the orientation of the dipole vector in the propenyl part was found to be rather important. In our present calculation, we chose the orientation to be the same as that in propene [19]. The results of the calculations for the five different rotamers are listed in Table 5. A comparison of the calculated dipole moments with the experimental results apparently suggests that the molecule exists in the skew-truns form. CONCLUSION
Both the experimental rotational constants and the dipole moments obtained in the present microwave study on 4-fluoro-l-butene conclude that the molecule predominantly exists in the skew-truns form. No relative TABLE
5
Calculated
dipole moments
(Debye)
of 4-fluoro-l-butene
Cis-trans C&--gauche Skew-trans Skew-gauche Skew-gauche’
1.50 0.17 1.52 0.05 0.53
0.26 1.80 0.85 1.61 1.70
Experimental
1.62
0.68
in different
conformations
0.00
1.52
1.18 0.19
2.19 1.81
0.22 0.49
1.62 1.84
0.39
1.80
311
conformational stabilities could be derived without the identification of other forms. However, from the relative intensities of the unassigned lines in the spectrum, it is believed that the skew-truns form is the predominant conformation of 4-fluoro-l-butene. The predominance of the skew-truns form in 4-fluoro-1-butene is consistent with the results in 4-chloro-l-butene [21] and 4-bromo-l-butene [15, 211. A qualitative agreement between the experimental dipole moments and those calculated from the vector model suggests that the charge induction due to the fluorine atom in 4-fluoro-l-butene is insignificant. Such a suggestion is apparently supported by the results of our CNDO calculation [22]. REFERENCES 1 E. Hirota, J. Chem. Phys., 37 (1962) 283. 2 T. N. Sarachman, J. Chem. Phys., 39 (1963) 469. 3 C. Komaki, I. Ichishima, K. Kuratani, T. Miyazawa, T. Shimanouchi and S. Mizushima, Bull. Chem. Sot. Jpn., 28 (1955) 330. 4 K. Radcliffe and J. L. Wood, Trans. Faraday Sot., 62 (1966) 1678. 5 G. A. Crowder and H. K. Mao, J. Mol. Struct., 18 (1973) 33. 6Y. Ogawa, S. Imazeki, H. Yamaguchi, H. Matsunra, I. Harada and T. Shimanouchi, Bull. Chem. Sot. Jpn., 51 (1978) 748. 7 J. R. Durig, S. E. Godbey and J. F. Sullivan, J. Chem. Phys., 80 (1984) 5983. 8E. Hirota, J. Chem. Phys., 42 (1965) 2071. 9 P. Meakin, D. 0. Harris and E. Hirota, J. Chem. Phys., 51 (1969) 3775. 10 R. D. McLachlan and R. A. Nyquist, Spectrochim. Acta, Part A, 24 (1968) 103. 11 J. R. Durig, M. Zhen and T. S. Little, J. Chem. Phys., 81 (1984) 4259. 12 J. R. Durig, M. Zhen, H. L. Heusel, P. J. Joseph, P. Groner and T. S. Little, J. Chem. Phys., 89 (1985) 2877. 13 S. Kondo, E. Hirota and Y. Morino, J. Mol. Spectrosc., 28 (1968) 471. 14 D. Van Hemelrijk, L. Van den Enden, H. J. Geise, H. L. Sellers and L. Schafer, J. Am. Chem. Sot., 102 (1980) 2189. 15 W. E. Steinmetz, J. Hollenberg, F. Hickernell, C. E. Orr and L. H. Scharpen, J. Phys. Chem., 82 (1978) 940. 16 K. M. McAfee, R. H. Hughes and E. B. Wilson Jr., Rev. Sci. Instrum., 20 (1949) 821. 17 S. Golden and E. B. Wilson Jr., J. Chem. Phys., 16 (1948) 669. 18 J. S. Muenter, J. Chem. Phys., 48 (1968) 4544. 19 E. Hirota and Y. Marino, J. Chem. Phys., 45 (1966) 2326. 20 D. M. Larkin and W. Gordy, J. Chem. Phys., 38 (1963) 2329. 21 S. H. Schei, J. Mol. Struct., 128 (1985) 151. 22 Molecular Orbital program no. 141 from the Quantum Chemistry Program Exchange, University of Indiana. This program has been modified for second row elements as recommended by D. P. Santry, J. Am. Chem. Sot., 90 (1986) 3309 and J. R. Sabin, personal communication (1973).