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Microwave spectrum of cis-thiopropionic acid
Journal
of Molecular
Structure,
74
(1981) 43-47
Elsevier Scientific Publishing Company,
MICROWAVE
YOSHIKO Department
(Received
SPECTRUM
TAKAHASHI of Chemistry,
10 November
Printed in The Netherlands
OF C1.!%THIOPROPIONIC
and ICHIRO Sophia
Amsterdam -
ACID
YAMAGUCHI
University,
Tokyo
IO2
(Japan)
1980)
ABSTRACT The microwave spectrum of ck-thiopropionic acid in the ground vibrational state has been observed in the frequency range between 10 and 35 GHz. The rotational and centrifugal distortion constants, and the dipole moments have been determined. The conformation of this molecule is discussed on the basis of the observed results. INTRODUCTION
Microwave spectra of thioformic acid and thioacetic acid were studied extensively by Hocking and Winnewisser [ 1] and Naito et al. [Z] , respectively. Some interesting results on the structure and conformation of these molecules were reported. The thiol-form of thiopropionic acid is one of the monosubstituted propionyl compounds, CH&H2COX. The conformations of the compounds with X = H [3], F [4], Cl [ 5,6] and OH 1’71 have been much studied through their various molecular spectra; the conformers of cis and skew forms, as shown in Fig. 1, have been confirmed, the former being much more stable than the latter. The present study is a successor of the study on thioacetic acid [ 21. The observed results of the microwave spectrum of the cis form of thiopropionic acid in the ground vibrational state is presented. The rotational and centrifugal distortion constants and the dipole moments have been determined. The conformation of the molecule is also discussed. EXPERIMENTAL
The sample of thiopropionic acid was synthesized from propionic acid and phosphorus pentasulfide according to the method of Kitamura [IO] . The crude sample was purified by distillation in a vacuum system. The boiling point was 38°C at 41 torr. The sample obtained was identified through its IR and NMR spectra and confirmed to be the thiol form, CH,CH,C(=O)SH. The microwave spectrum in the range lo-35 GHz was obtained with a conventional spectrometer with 100 kHz Stark modulation. The frequency 0022-2860/81/000~000/$02.50
0 1981 Elsevier Scientific Publishing Company
44
(4 Fig.
1. Coaformation
of
monosubstituted
propionyl
compound.
(a)
Conformation
of
CH,CH,COX (X = H, OH, F, Cl). (b) ck-Thiopropionicacid. of the transitions was measured with a Hewlett-Packard 5245L Electronic Frequency Counter. The absorption cell was an X-band wave guide 1.5 m long. The sample gas was let flow in the cell at room temperature and the pressure was maintained around 0.04 torr. SPECTRUM
ANALYSIS
The model structure of the thiol-form was obtained from the molecular parameters of propionic acid [ 71 and thioformic acid [ 11. The molecular frame consisting of heavy atoms, was assumed to be in a plane for the cis form. For the skew form model, the C,-C, bond of the cis model was twisted by 120”. In these models, the S-H bond was assumed to be in a position trans to the CI-C2 bond. The dipole moments were estimated from the bond moment data [S] . The calculated rotational constants and the dipole moments are given in Table 3. The strong b-type Q-branch lines for the cis form were expected to appear. A series of J2,J-2 + J,qJ_l (2 < J < 10) was searched and the frequencies of the plausible lines were taken for the Q-branch plot and for the ASSIGNQ diagram developed in our laboratory [9]. The parameters found through these procedures are (A - C)/Z = 3403.3 MHz and K = -0.8628, which correspond to the cis form. If the molecule takes the cis conformation, only four hydrogen atoms in the ethyl group are out of the plane of the molecular frame. For such a planar l%nne molecule, the assumed structural parameters give a value amu A2. With this and the values of (A - C)/2 AI=& - I, -I,, = -6.259 and K obtained above, 3(A + C) = 31910 MHz was calculated, corresponding to the transition frequency of 3,. 2 -+ 21, 1_A search around 31910 MHz revealed three lines which may be assigned to the transition. If the one at 31954.77 MHz was taken for the transition, other eight R-branch transitions could be assigned consistently. This consistency can be shown on a diagram called “ASSIGNR”, which is a modification of the ASSIGNQ diagram to the R-branch transitions. On an ASSIGNR diagram, the correctly assigned transitions locate on a smooth curve just as in the case of ASSIGNQ diagram. Other transitions were assigned consecutively and a total of 32 transitions were identified (1 < J G 20, twenty four Q- and eight R-branch transitions). No evidence was observed for the splitting in the ground state lines due to the internal rotation of the methyl group.
45 TABLE
1
Rotational
A B C
&
dm dK dwJx dmx
10’ 10’
aP
and centrifugal
distortion
constants
of thiopropionic
acid= MHz MHz MHz kHz kHz
8729.128 f 0.030 2389.545 f 0.008 1922.584 f 0.009 -0.93 + 0.15 8.3 2 6.4 48 +- 20 0.58 * 0.06 -5.2 2 3.1 -6.53 + 0.16
aErrors are 2.5 times the standard deviation. baZ = I, -la - Zb. Conversion factor: 505376
kHZ
amuA2
amu A2 MHz.
The assigned 32 transitions were fitted to the Hamiltonian of Watson [ 111.
A
list of the observed transition frequencies and the centrifugal distortion
corrections is available from B.L.L.D. as Supplementary Publication No. S.U.P. 26192 (3 pages). The rotational and centrifugal distortion constants are given in Table 1. The Stark effect on the three rotational transitions was measured. The obtained Stark coefficients and the dipole moments are given in Table 2. DISCUSSION
Thio-alkyl acid molecule may have tautomer. One is thiol-form and the other thion-form. 7” R-C=0
YH R-C=S
thio1-form
thion-form
The thion-form, however, has not been identified so far from various spectroscopic observations. The study on thioformic acid by microwave spectroscopy [l] was only made for the thiol-form. For thioacetic acid the existence
of the thion-form was not confirmed in IR, Raman, microwave, or NMR studies, and only a couple of percent is suggested to be present [2,12, 131. The CNDO/2 calculation on this molecule showed that the thiol-form is the more stable [ 141. In this study on thiopropionic acid, the transition lines arising from the thion-form could not be identified, as expected from the IR and NMR spectra of the sample used. Table 3 compares the observed rotational constants and dipole moments with those calculated from the model structures and the bond moments. From the comparison, it is clear that the observed values are closer to those of the cis rather than the skew model. It is evident that the assigned spectrum was of the cis form of thiopropionic acid with the S-H bond bans to the
46 TABLE
2
Stark coefficients and dipole moments of thiopropionic Stark coefficienta
(Hz/(V/cm)‘)
Transition
IMI
5 1.3- 5**4 2 - 11.‘ 2.0 2 2.1 +- 1, . 0
5 1
Dipole moments (D) Obs_-Calc.
Ohs. -24.4 237.3 -215.6 9.5
1
0
acid
+ 2.1 + 9.6 + 8.3 f 2.3
0.3 9.3 7.1 1.3
I.ra J% J% ptotal
0.40 1.56 0.00 1.61
r 0.02 5 0.19 (assumed) + 0.20
aErrors are 2.5 times the standard deviation.
Cl---C2 bond (the cis form with the S-H bond cis to the C1-C2 bond has rather Wferent calculated values of the dipole moment), The fact that the transitions of the eis conformer are those which have been observed strongly and clearly suggests that this is the stable form at the observed temperature. The other conformer not observed in this study is the skezu form analogous to the other propionyl ~ompo~ds. The ground state of the skew form of thiopropionic acid is expected to lie about 400 cm-* above that of the cis form. This energy difference was estimated by analogy to CH&H&OF [4] and CH,CH,CHO [S] , where the energy difference was obtained as 449 and 315 cm-‘, respectively. If the energy difference for the thiopropionic acid molecule is of this ma~tude, the intensity of the Linesfrom the skew form may be several percent of the C&G form lines, i.e. very weak, The intensity of the ground state line of the skew form may be comparable with that of the fourth vibrationally excited state of the Cl-C2 bond torsional mode in the cis form, the tines for which have not yet been identified in this study. TAELE
3
Comparison between observed and calculated values of rotational constants (MHz), (amu A’), and Dipole Moments (D) Obs.
A B
c AI
C&-L
8729.128 2389.545 1922.584 -6.53 0.40
1.56 0.00 1.61
cis
skew
8732 2445 1957 -6.26
5152 3407 2252 -22.0
O-13 1.56 0.00 1.57
1.-38 0.52 0.22 1.49
AI
47
The orientation of the S-H bond with respect to the C-H bond has been reported for thioformic acid [ 11. In this molecule, the S-H bond takes both ck and Pans orientations with respect to the C-H bond. For thiopropionic acid, the dipole moment data strongly suggest that the S-H bond in the molecule points away from the alkyl group, i.e. tram to the CI-Cz bond. TXs should, however, be reconfirmed by observing the deuterated species. REFERENCES 1 W. H. Hocking and G. Winnewisser, Z. Naturforsch., Teil A, 31 (1976) 422,438,995, Teil A, 32 (1977) 1108. 2 T. Naito, 0. Ohashi and I. Yamaguchi, J. Mol. Spectrosc., 68 (1977) 32; T. Naito, Dissertation, Sophia University, 1979. 3 S. S. Butcher and E. B. Wilson, J. Chem. Phys., 40 (1964) 1671; H. M. Pickett and D. G. Scroggin, J. Chem. Phys., 61 (1974) 3954. 4 0. L. Stiefvater and E. B. Wilson, J. Chem. Phys., 50 (1969) 5385. 5 H. Karisson, J. Mol. Struct.. 33 (1976) 227. 6 J. E. Katon and W. R. Feairheller, Jr., J. Chem. Phys., 44 (1966) 144. 7 0. L. Stiefvater, J. Chem. Phys., 62 (1975) 233, 244. 8 W. Gordy and R. L. Cook, Microwave Molecular Spectra, Wiley, New York, 1970. 9 A. Yamazaki, K. Mogi, M. Koyama and I. Yamaguchi, J. Mol. Sttuct., 55 (1979) 185. 10 R. Kitamura, Yakugaku Zasshi, 57 (1937) 31. 11 J. K. G. Watson, J. Chem. Phys., 45 (1966) 1360. 12 H. S. Randhawa, W. Walter and C. 0. Meese, J. Mol. Struct., 37 (1977) 187. 13 N. Sheppard, Trans. Faraday Sot., 45 (1949) 693. 14 H. S. Randhawa and C. N. R. Rao, J. Mol. Struct., 21 (1974) 123.
of Molecular
Structure,
74
(1981) 43-47
Elsevier Scientific Publishing Company,
MICROWAVE
YOSHIKO Department
(Received
SPECTRUM
TAKAHASHI of Chemistry,
10 November
Printed in The Netherlands
OF C1.!%THIOPROPIONIC
and ICHIRO Sophia
Amsterdam -
ACID
YAMAGUCHI
University,
Tokyo
IO2
(Japan)
1980)
ABSTRACT The microwave spectrum of ck-thiopropionic acid in the ground vibrational state has been observed in the frequency range between 10 and 35 GHz. The rotational and centrifugal distortion constants, and the dipole moments have been determined. The conformation of this molecule is discussed on the basis of the observed results. INTRODUCTION
Microwave spectra of thioformic acid and thioacetic acid were studied extensively by Hocking and Winnewisser [ 1] and Naito et al. [Z] , respectively. Some interesting results on the structure and conformation of these molecules were reported. The thiol-form of thiopropionic acid is one of the monosubstituted propionyl compounds, CH&H2COX. The conformations of the compounds with X = H [3], F [4], Cl [ 5,6] and OH 1’71 have been much studied through their various molecular spectra; the conformers of cis and skew forms, as shown in Fig. 1, have been confirmed, the former being much more stable than the latter. The present study is a successor of the study on thioacetic acid [ 21. The observed results of the microwave spectrum of the cis form of thiopropionic acid in the ground vibrational state is presented. The rotational and centrifugal distortion constants and the dipole moments have been determined. The conformation of the molecule is also discussed. EXPERIMENTAL
The sample of thiopropionic acid was synthesized from propionic acid and phosphorus pentasulfide according to the method of Kitamura [IO] . The crude sample was purified by distillation in a vacuum system. The boiling point was 38°C at 41 torr. The sample obtained was identified through its IR and NMR spectra and confirmed to be the thiol form, CH,CH,C(=O)SH. The microwave spectrum in the range lo-35 GHz was obtained with a conventional spectrometer with 100 kHz Stark modulation. The frequency 0022-2860/81/000~000/$02.50
0 1981 Elsevier Scientific Publishing Company
44
(4 Fig.
1. Coaformation
of
monosubstituted
propionyl
compound.
(a)
Conformation
of
CH,CH,COX (X = H, OH, F, Cl). (b) ck-Thiopropionicacid. of the transitions was measured with a Hewlett-Packard 5245L Electronic Frequency Counter. The absorption cell was an X-band wave guide 1.5 m long. The sample gas was let flow in the cell at room temperature and the pressure was maintained around 0.04 torr. SPECTRUM
ANALYSIS
The model structure of the thiol-form was obtained from the molecular parameters of propionic acid [ 71 and thioformic acid [ 11. The molecular frame consisting of heavy atoms, was assumed to be in a plane for the cis form. For the skew form model, the C,-C, bond of the cis model was twisted by 120”. In these models, the S-H bond was assumed to be in a position trans to the CI-C2 bond. The dipole moments were estimated from the bond moment data [S] . The calculated rotational constants and the dipole moments are given in Table 3. The strong b-type Q-branch lines for the cis form were expected to appear. A series of J2,J-2 + J,qJ_l (2 < J < 10) was searched and the frequencies of the plausible lines were taken for the Q-branch plot and for the ASSIGNQ diagram developed in our laboratory [9]. The parameters found through these procedures are (A - C)/Z = 3403.3 MHz and K = -0.8628, which correspond to the cis form. If the molecule takes the cis conformation, only four hydrogen atoms in the ethyl group are out of the plane of the molecular frame. For such a planar l%nne molecule, the assumed structural parameters give a value amu A2. With this and the values of (A - C)/2 AI=& - I, -I,, = -6.259 and K obtained above, 3(A + C) = 31910 MHz was calculated, corresponding to the transition frequency of 3,. 2 -+ 21, 1_A search around 31910 MHz revealed three lines which may be assigned to the transition. If the one at 31954.77 MHz was taken for the transition, other eight R-branch transitions could be assigned consistently. This consistency can be shown on a diagram called “ASSIGNR”, which is a modification of the ASSIGNQ diagram to the R-branch transitions. On an ASSIGNR diagram, the correctly assigned transitions locate on a smooth curve just as in the case of ASSIGNQ diagram. Other transitions were assigned consecutively and a total of 32 transitions were identified (1 < J G 20, twenty four Q- and eight R-branch transitions). No evidence was observed for the splitting in the ground state lines due to the internal rotation of the methyl group.
45 TABLE
1
Rotational
A B C
&
dm dK dwJx dmx
10’ 10’
aP
and centrifugal
distortion
constants
of thiopropionic
acid= MHz MHz MHz kHz kHz
8729.128 f 0.030 2389.545 f 0.008 1922.584 f 0.009 -0.93 + 0.15 8.3 2 6.4 48 +- 20 0.58 * 0.06 -5.2 2 3.1 -6.53 + 0.16
aErrors are 2.5 times the standard deviation. baZ = I, -la - Zb. Conversion factor: 505376
kHZ
amuA2
amu A2 MHz.
The assigned 32 transitions were fitted to the Hamiltonian of Watson [ 111.
A
list of the observed transition frequencies and the centrifugal distortion
corrections is available from B.L.L.D. as Supplementary Publication No. S.U.P. 26192 (3 pages). The rotational and centrifugal distortion constants are given in Table 1. The Stark effect on the three rotational transitions was measured. The obtained Stark coefficients and the dipole moments are given in Table 2. DISCUSSION
Thio-alkyl acid molecule may have tautomer. One is thiol-form and the other thion-form. 7” R-C=0
YH R-C=S
thio1-form
thion-form
The thion-form, however, has not been identified so far from various spectroscopic observations. The study on thioformic acid by microwave spectroscopy [l] was only made for the thiol-form. For thioacetic acid the existence
of the thion-form was not confirmed in IR, Raman, microwave, or NMR studies, and only a couple of percent is suggested to be present [2,12, 131. The CNDO/2 calculation on this molecule showed that the thiol-form is the more stable [ 141. In this study on thiopropionic acid, the transition lines arising from the thion-form could not be identified, as expected from the IR and NMR spectra of the sample used. Table 3 compares the observed rotational constants and dipole moments with those calculated from the model structures and the bond moments. From the comparison, it is clear that the observed values are closer to those of the cis rather than the skew model. It is evident that the assigned spectrum was of the cis form of thiopropionic acid with the S-H bond bans to the
46 TABLE
2
Stark coefficients and dipole moments of thiopropionic Stark coefficienta
(Hz/(V/cm)‘)
Transition
IMI
5 1.3- 5**4 2 - 11.‘ 2.0 2 2.1 +- 1, . 0
5 1
Dipole moments (D) Obs_-Calc.
Ohs. -24.4 237.3 -215.6 9.5
1
0
acid
+ 2.1 + 9.6 + 8.3 f 2.3
0.3 9.3 7.1 1.3
I.ra J% J% ptotal
0.40 1.56 0.00 1.61
r 0.02 5 0.19 (assumed) + 0.20
aErrors are 2.5 times the standard deviation.
Cl---C2 bond (the cis form with the S-H bond cis to the C1-C2 bond has rather Wferent calculated values of the dipole moment), The fact that the transitions of the eis conformer are those which have been observed strongly and clearly suggests that this is the stable form at the observed temperature. The other conformer not observed in this study is the skezu form analogous to the other propionyl ~ompo~ds. The ground state of the skew form of thiopropionic acid is expected to lie about 400 cm-* above that of the cis form. This energy difference was estimated by analogy to CH&H&OF [4] and CH,CH,CHO [S] , where the energy difference was obtained as 449 and 315 cm-‘, respectively. If the energy difference for the thiopropionic acid molecule is of this ma~tude, the intensity of the Linesfrom the skew form may be several percent of the C&G form lines, i.e. very weak, The intensity of the ground state line of the skew form may be comparable with that of the fourth vibrationally excited state of the Cl-C2 bond torsional mode in the cis form, the tines for which have not yet been identified in this study. TAELE
3
Comparison between observed and calculated values of rotational constants (MHz), (amu A’), and Dipole Moments (D) Obs.
A B
c AI
C&-L
8729.128 2389.545 1922.584 -6.53 0.40
1.56 0.00 1.61
cis
skew
8732 2445 1957 -6.26
5152 3407 2252 -22.0
O-13 1.56 0.00 1.57
1.-38 0.52 0.22 1.49
AI
47
The orientation of the S-H bond with respect to the C-H bond has been reported for thioformic acid [ 11. In this molecule, the S-H bond takes both ck and Pans orientations with respect to the C-H bond. For thiopropionic acid, the dipole moment data strongly suggest that the S-H bond in the molecule points away from the alkyl group, i.e. tram to the CI-Cz bond. TXs should, however, be reconfirmed by observing the deuterated species. REFERENCES 1 W. H. Hocking and G. Winnewisser, Z. Naturforsch., Teil A, 31 (1976) 422,438,995, Teil A, 32 (1977) 1108. 2 T. Naito, 0. Ohashi and I. Yamaguchi, J. Mol. Spectrosc., 68 (1977) 32; T. Naito, Dissertation, Sophia University, 1979. 3 S. S. Butcher and E. B. Wilson, J. Chem. Phys., 40 (1964) 1671; H. M. Pickett and D. G. Scroggin, J. Chem. Phys., 61 (1974) 3954. 4 0. L. Stiefvater and E. B. Wilson, J. Chem. Phys., 50 (1969) 5385. 5 H. Karisson, J. Mol. Struct.. 33 (1976) 227. 6 J. E. Katon and W. R. Feairheller, Jr., J. Chem. Phys., 44 (1966) 144. 7 0. L. Stiefvater, J. Chem. Phys., 62 (1975) 233, 244. 8 W. Gordy and R. L. Cook, Microwave Molecular Spectra, Wiley, New York, 1970. 9 A. Yamazaki, K. Mogi, M. Koyama and I. Yamaguchi, J. Mol. Sttuct., 55 (1979) 185. 10 R. Kitamura, Yakugaku Zasshi, 57 (1937) 31. 11 J. K. G. Watson, J. Chem. Phys., 45 (1966) 1360. 12 H. S. Randhawa, W. Walter and C. 0. Meese, J. Mol. Struct., 37 (1977) 187. 13 N. Sheppard, Trans. Faraday Sot., 45 (1949) 693. 14 H. S. Randhawa and C. N. R. Rao, J. Mol. Struct., 21 (1974) 123.