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The Microwave Spectrum and Dipole Moment of Monofluorobenzene
JOURNAL OF MOLECULAR SPECTROSCOPY ARTICLE NO.
176, 146–147 (1996)
0071
The Microwave Spectrum and Dipole Moment of Monofluorobenzene Masao Onda,1,2 Hideki Mukaida, and Ichiro Yamaguchi Department of Chemistry, Faculty of Science and Technology, Sophia University, Kioi-cho, Chiyoda-ku, Tokyo 102, Japan Received April 20, 1995; in revised form December 15, 1995
The microwave spectrum of monofluorobenzene in the frequency region between 7.5 and 14 GHz was studied using a molecular beam Fourier transform microwave spectrometer. The rotational and centrifugal distortion constants were found to be A Å 5663.7369(38) MHz, B Å 2570.6535(4) MHz, C Å 1767.9134(4) MHz, DJ Å 0.165(9) kHz, dJ Å 0.077(1) kHz. The dipole moment was determined to be ma Å 1.54(1) D. q 1996 Academic Press, Inc. 1. INTRODUCTION
The microwave spectrum of monofluorobenzene has been investigated by several authors; the complete rs structure of the molecule was reported by the Copenhagen group (1). The dipole moment of this molecule, 1.66 D with an average error of 0.03 D, was reported by Kowalewski et al. in 1959 (2). Spieckermann and Sutter examined the reported dipole moments of the series of fluorine substituted benzenes (3). They deduced a ‘‘standard moment’’ m(F–H) Å 1.46 D for the fluorobenzenes. The standard moment is an effective dipole moment of an FCrrrCH unit, where F and H are para to each other. They pointed out that the dipole moment of monofluorobenzene calculated using a standard moment of 1.46 D deviated from the experimental value by 15%. This suggests that the reported dipole moment should be reexamined experimentally. In this article we present a precise determination of the dipole moment of monofluorobenzene and discuss this result in terms of the standard moment model.
was calibrated against the J Å 1–0, M Å 0 transition of OCS m Å (0.71521D) (4). The Stark coefficients could be obtained with higher precision than those measured by conventional microwave spectroscopy because of the narrower linewidth of 20 kHz. 3. RESULTS AND DISCUSSION
The observed transition frequencies of monofluorobenzene and the rotational constants obtained are shown in Table 1. The Stark effect data are summarized in Table
TABLE 1 Observed Frequencies (MHz) and Rotational Constants of Monofluorobenzene
2. EXPERIMENTAL DETAILS
The rotational transitions of monofluorobenzene have been measured between 7.5-14 GHz using a molecular beam Fourier transform spectrometer (MB-FTMW). The sample of monofluorobenzene was purchased from Tokyo Kasei Co., and was used without further purification. The sample gas was a mixture of a few percent of monofluorobenzene in argon. The stagnation pressure was approximately 1 atm. The Stark electric field was chosen to be parallel to the electric field of the microwave radiation. The Stark field 1 2
To whom correspondence should be addressed. E-mail; [email protected].
146 0022-2852/96 $18.00 Copyright q 1996 by Academic Press, Inc. All rights of reproduction in any form reserved.
AID
JMS 6963
/
6t09$$$261
03-11-96 21:11:20
mspas
AP: Mol Spec
147
DIPOLE MOMENT OF FLUOROBENZENE
2. The new measured dipole moment of 1.54(1) D is smaller than the previous value by more than one standard deviation. The reported dipole moments for the series of polar fluorobenzenes are summarized in Table 3. Most of the values were obtained by MB-FTMW spectroscopy except for 1,2and 1,3-difluorobenzenes and 1,2,4-trifluorobenzene. We evaluated the standard moment of 1.48 D using all the data in Table III. The calculated dipole moments with this value are shown in column I of Table 3. The maximum deviation has decreased to 5%. This method for evaluating the ‘‘standard moment’’ obviously neglects any interaction between two C – F groups. The large interaction may be an ortho-effect or a paraeffect. As a trial test, we divide the fluorobenzene molecules into two groups, one having two fluorine atoms para to each other (group B) and the other having no such configuration (group A). The standard moments were evaluated separately for each group. The standard moments obtained are 1.51 D and 1.43 D for groups A and B, respectively. The dipole moments calculated with these standard moments are shown in column II of Table 3. The maximum deviation is now only 2%. The difference in standard moments between the two groups is expected from the simple bond moment theory of m(C – F). The ortho-effect on the dipole moment of the substituted benzenes seems minor. Further detailed discussion requires a model beyond the concept of the standard moment. We are now ap-
TABLE 3 Dipole Moments of Polar Fluorobenzenes (D)
a
Calculated using the ‘‘standard dipole moment,’’ m (F–H), after (3). This work. c After (2). d After (5). e After (6). f After (3); angle between C–F bond and m (Total) estimated to be 87. g After (7). h After (8). b
proaching this problem through a theoretical calculation (Gaussian 92). REFERENCES
TABLE 2 Observed Stark Coefficients and Dipole Moment of Monofluorobenzene
1. B. Bak, D. Christensen, L. H. Nygaard, and E. T. Annenbaum, J. Chem. Phys. 26, 134–137 (1957); L. Nygaard, I. Bojesen, T. Pedersen, and J. R. Andersen, J. Mol. Struct. 2, 209–215 (1968). 2. D. G. de Kowalewski, P. Ko¨keritz, and H. Sele´n, J. Chem. Phys. 31, 1438 (1959). 3. J. Spieckermann and D. H. Sutter, Z. Naturforsch. A. 40a, 864–865 (1985). 4. J. S. Muenter, J. Chem. Phys. 48, 4544–4547 (1968). 5. L. Nygaard, E. R. Hansen, R. L. Hansen, J. R. Andersen, and G. O. Sørensen, Spectrochim. Acta Part A 23, 2813–2819 (1967). 6. M. Onda, H. Mukaida, H. Akiba, M. Mori, H. Miyazaki, and I. Yamaguchi, J. Mol. Spectrosc. 169, 480–483 (1995). 7. M. Onda, H. Yamada, H. Miyazaki, M. Mori, and I. Yamaguchi, J. Mol. Struct. 166, 247–248 (1994). 8. M. Onda, H. Yamada, M. Mori, H. Miyazaki, and I. Yamaguchi, J. Mol. Struct. 319, 297–299 (1994).
Copyright q 1996 by Academic Press, Inc.
AID
JMS 6963
/
6t09$$$261
03-11-96 21:11:20
mspas
AP: Mol Spec
176, 146–147 (1996)
0071
The Microwave Spectrum and Dipole Moment of Monofluorobenzene Masao Onda,1,2 Hideki Mukaida, and Ichiro Yamaguchi Department of Chemistry, Faculty of Science and Technology, Sophia University, Kioi-cho, Chiyoda-ku, Tokyo 102, Japan Received April 20, 1995; in revised form December 15, 1995
The microwave spectrum of monofluorobenzene in the frequency region between 7.5 and 14 GHz was studied using a molecular beam Fourier transform microwave spectrometer. The rotational and centrifugal distortion constants were found to be A Å 5663.7369(38) MHz, B Å 2570.6535(4) MHz, C Å 1767.9134(4) MHz, DJ Å 0.165(9) kHz, dJ Å 0.077(1) kHz. The dipole moment was determined to be ma Å 1.54(1) D. q 1996 Academic Press, Inc. 1. INTRODUCTION
The microwave spectrum of monofluorobenzene has been investigated by several authors; the complete rs structure of the molecule was reported by the Copenhagen group (1). The dipole moment of this molecule, 1.66 D with an average error of 0.03 D, was reported by Kowalewski et al. in 1959 (2). Spieckermann and Sutter examined the reported dipole moments of the series of fluorine substituted benzenes (3). They deduced a ‘‘standard moment’’ m(F–H) Å 1.46 D for the fluorobenzenes. The standard moment is an effective dipole moment of an FCrrrCH unit, where F and H are para to each other. They pointed out that the dipole moment of monofluorobenzene calculated using a standard moment of 1.46 D deviated from the experimental value by 15%. This suggests that the reported dipole moment should be reexamined experimentally. In this article we present a precise determination of the dipole moment of monofluorobenzene and discuss this result in terms of the standard moment model.
was calibrated against the J Å 1–0, M Å 0 transition of OCS m Å (0.71521D) (4). The Stark coefficients could be obtained with higher precision than those measured by conventional microwave spectroscopy because of the narrower linewidth of 20 kHz. 3. RESULTS AND DISCUSSION
The observed transition frequencies of monofluorobenzene and the rotational constants obtained are shown in Table 1. The Stark effect data are summarized in Table
TABLE 1 Observed Frequencies (MHz) and Rotational Constants of Monofluorobenzene
2. EXPERIMENTAL DETAILS
The rotational transitions of monofluorobenzene have been measured between 7.5-14 GHz using a molecular beam Fourier transform spectrometer (MB-FTMW). The sample of monofluorobenzene was purchased from Tokyo Kasei Co., and was used without further purification. The sample gas was a mixture of a few percent of monofluorobenzene in argon. The stagnation pressure was approximately 1 atm. The Stark electric field was chosen to be parallel to the electric field of the microwave radiation. The Stark field 1 2
To whom correspondence should be addressed. E-mail; [email protected].
146 0022-2852/96 $18.00 Copyright q 1996 by Academic Press, Inc. All rights of reproduction in any form reserved.
AID
JMS 6963
/
6t09$$$261
03-11-96 21:11:20
mspas
AP: Mol Spec
147
DIPOLE MOMENT OF FLUOROBENZENE
2. The new measured dipole moment of 1.54(1) D is smaller than the previous value by more than one standard deviation. The reported dipole moments for the series of polar fluorobenzenes are summarized in Table 3. Most of the values were obtained by MB-FTMW spectroscopy except for 1,2and 1,3-difluorobenzenes and 1,2,4-trifluorobenzene. We evaluated the standard moment of 1.48 D using all the data in Table III. The calculated dipole moments with this value are shown in column I of Table 3. The maximum deviation has decreased to 5%. This method for evaluating the ‘‘standard moment’’ obviously neglects any interaction between two C – F groups. The large interaction may be an ortho-effect or a paraeffect. As a trial test, we divide the fluorobenzene molecules into two groups, one having two fluorine atoms para to each other (group B) and the other having no such configuration (group A). The standard moments were evaluated separately for each group. The standard moments obtained are 1.51 D and 1.43 D for groups A and B, respectively. The dipole moments calculated with these standard moments are shown in column II of Table 3. The maximum deviation is now only 2%. The difference in standard moments between the two groups is expected from the simple bond moment theory of m(C – F). The ortho-effect on the dipole moment of the substituted benzenes seems minor. Further detailed discussion requires a model beyond the concept of the standard moment. We are now ap-
TABLE 3 Dipole Moments of Polar Fluorobenzenes (D)
a
Calculated using the ‘‘standard dipole moment,’’ m (F–H), after (3). This work. c After (2). d After (5). e After (6). f After (3); angle between C–F bond and m (Total) estimated to be 87. g After (7). h After (8). b
proaching this problem through a theoretical calculation (Gaussian 92). REFERENCES
TABLE 2 Observed Stark Coefficients and Dipole Moment of Monofluorobenzene
1. B. Bak, D. Christensen, L. H. Nygaard, and E. T. Annenbaum, J. Chem. Phys. 26, 134–137 (1957); L. Nygaard, I. Bojesen, T. Pedersen, and J. R. Andersen, J. Mol. Struct. 2, 209–215 (1968). 2. D. G. de Kowalewski, P. Ko¨keritz, and H. Sele´n, J. Chem. Phys. 31, 1438 (1959). 3. J. Spieckermann and D. H. Sutter, Z. Naturforsch. A. 40a, 864–865 (1985). 4. J. S. Muenter, J. Chem. Phys. 48, 4544–4547 (1968). 5. L. Nygaard, E. R. Hansen, R. L. Hansen, J. R. Andersen, and G. O. Sørensen, Spectrochim. Acta Part A 23, 2813–2819 (1967). 6. M. Onda, H. Mukaida, H. Akiba, M. Mori, H. Miyazaki, and I. Yamaguchi, J. Mol. Spectrosc. 169, 480–483 (1995). 7. M. Onda, H. Yamada, H. Miyazaki, M. Mori, and I. Yamaguchi, J. Mol. Struct. 166, 247–248 (1994). 8. M. Onda, H. Yamada, M. Mori, H. Miyazaki, and I. Yamaguchi, J. Mol. Struct. 319, 297–299 (1994).
Copyright q 1996 by Academic Press, Inc.
AID
JMS 6963
/
6t09$$$261
03-11-96 21:11:20
mspas
AP: Mol Spec