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Internal rotation of trifluoromethyl groups in hexafluoroacetylacetone
315
Journal of Molecular Structure, 268 (1992) 315-318 Elsevier Science Publishers B.V., Amsterdam
Short Communication
Internal rotation of trifluoromethyl groups in hexafluoroacetylacetone Kinya Iijima, Yukino Tanaka Department
and Shigeki Onuma
of Chemistry, Faculty of Science, Shizuoka University,
Oya, Shizuoka 422 (Japan)
(Received 30 October 1991)
The potential energy barrier of internal rotation of the trifluoromethyl group is somewhat higher than that of the methyl group: the barrier height is 6.0 kcal mol-’ in (CF3)20 [l], 5.8 kcal mol-l in ClSCF, [2] and 5.2 kcal mol-’ in CF,SCF, [ 21. However, there are some compounds with comparatively low barrier heights such as 1.67 kcal mol-l in CFsCOF [3] and 1.19 kcal mol-l in CF, COCHzBr [ 41. Andreassen et al. [ 51 have determined the molecular structure of gaseous hexafluoroacetylacetone from electron diffraction data on the assumption of C, symmetry. They treated the internal rotation of the CF, groups as a rigid rotor and obtained the OCCF dihedral angle of 47’) although they mentioned that this is an “average structure” value. The present study was undertaken to investigate the rotational barrier height of CF, groups in hexafluoroacetylacetone by gas-phase electron diffraction. The molecular symmetry was checked again because acetylacetone has been revealed not to be symmetric [ 6,7]. Long and short camera-distance photographs were taken at room temperature by the use of an r3-sector. The exposure times were 50 and 95 s for the long and short camera-distance photographs, respectively, with an electron beam current of 0.6 ,uA. The wavelength of the electron beam was estimated from the measured accelerating voltage of 40 kV [ 81. The procedures of data reduction and molecular-structure analysis were the same as those used previously for acetylacetone [ 71. The vibrational mean amplitudes and the shrinkage corrections used in the analysis were calculated from the force field
[91. Assuming an unsymmetric ring skeleton the analysis did not achieve convergence, as in the previous study [5]. The molecular skeleton was then assumed to be symmetric. The analysis assuming the rigid rotor to the CF, groups Correspondence to: Professor K. Iijima, Department of Chemistry, Faculty of Science, Shizuoka University, Oya, Shizuoka 422, Japan.
0022-2860/92/$05.00
0 1992 Elsevier Science Publishers B.V. All rights reserved.
316
5
10
15
20
25
30
s/A
Fig. 1. Molecular intensities for gaseous hexafluoroacetylacetone. The upper curve corresponds to the long camera-distance data and the lower one to the short camera-distance data. The dots represent the observed data and the solid curves the best-fit calculated data. The differences are shown in the lower part of the figure.
resulted in the agreement factor R = C w ( sMobs- sMcd,)2/CsMobs2 of 0.064. However, the R-factor decreased to 0.045 when the CFBgroups were treated as a rotor with low potential barrier height. The molecular intensities were calculated from $= 0 to 120” at 20” intervals and summed with the weights of the Boltzmann distribution. The mean amplitudes calculated by excluding the torsional force constant around the C-C, bond were used. The potential minimum was found at the OCCF dihedral angle of 60 O. Next the rotation of the CF3 group was treated as a precession motion [lo]. The tilt angle 8 of the C,, symmetry axis of CF, with respect to the C-C bond was assumed as 8= 19,+ (l/2 ) 8, (1 + cos 3$), where $ is the rotational angle of the CF, group. This analysis reduced the R-factor to 0.030. The final experimental and calculated molecular intensities and the corresponding radial distribution curves are shown in Figs. 1 and 2, respectively. The molecular parameters obtained in the least-squares calculations are listed in Table 1. The uncertainties of the molecular parameters have much improved but the parameter values are in good agreement with those obtained in the previous study by Andreassen et al. [5], except for the parameters for the rotation of the CF3 groups. The symmetry of the rotational barrier of CF3 was threefold and a V, component was negligible. The R-factor versus the barrier height is shown in Fig. 3. The best V, value of 0.75 kcal mol-’ is much lower than those of other compounds including a CF3 group [l-4]. The potential minimum is at the position where the two fluorine atoms are staggered to the oxygen atom ($ = 60’ ) . The repulsion from the hydrogen atom is large in this conformation, tilting the C& symmetry axis away from the hydrogen atom (8= 4’ ) . The CBV
317
Hcxsfluoroacetrlacstone
1
1
2
3
4
5
6
7
r/A
Fig. 2. Radial distribution curve for hexafluoroacetylacetone. The dots represent the experimental data and the solid curve the theoretical data, with the difference shown below. The vertical bars represent bond distances and the scattering powers. The molecular model is shown at the upper right. TABLE 1 Molecular parameters of hexafluoroacetylacetone (bond distances and mean amplitudes in A, angles in degrees)
r(C-0)
r(C-G) r(C-GJ dC,-F) dC,-H) r(O-*-0)’ L
cc,c c,co c,cc,
L L L CC,F
L OH0 $ 01’
UC,-F) l(F*..F)
1.261 1.422 1.541 1.341 1.101 2.606 114.9 127.3 118.8 110.9 180 4.0 -2.2 0.041 0.060
1caleC
Ref. 5d
0.004 0.006 0.003 0.001 assumed 0.013 0.5 0.5 0.4 0.1 8 0.6 1.5
0.039
1.261(18) 1.409(31) 1.548(8) 1.338(7) 1.085 assumed 2.556(33) 115.2(23) 126.4(13) 119.7(15) 110.7(8) 176(11)
0.002 0.001
0.046 0.057
0.047 0.045 0.046 0.076 0.116
“Parameters obtained in the least-squares analysis. Angle parameters are F, parameters. bLimits of error. “Mean amplitudes calculated from the force field [ 91. dParameters in ref. 5 are converted to rg parameters by rg= r,+ l’/r,. ‘Dependent parameter. Tilt angle 13is defined as 0, + (l/2) 8, (1 + cos3#), where $ is a rotational angle of a CF, group.
318
0.032 -
0.030
t I
I
0.50
0.75
1.00
V3/ kcal mol-’ Fig. 3. Correlation of the R-factor in the least-squares with the potential barrier height, V,, of the internal rotation of the CF,.
symmetry axis tilts 2 o away from the oxygen atom towards the hydrogen atom at the potential maximum position ($ = 0” ) . A systematic difference was found in the range of the long camera-distance data on examination of the intensity curves in detail. It was inferred that this discrepancy arises from the F. * - F distances between the two CF, groups. The two CF3 groups were assumed to rotate independently in the present work, but there may be some correlation between the motions of the two rotors. A further analysis, however, was not carried out, since the difference between the experimental and the calculated molecular intensities was very small. The computations were carried out on AK5100 in the Shizuoka University Information Processing Center and on a super computer of HITAC S-820 in the Computer Center of the University of Tokyo.
REFERENCES 1 2 3 4 5 6 7 8 9 10
A.H. Lowrey, C. George, P. D’Antonio and J. Karle, J. Mol. Struct., 63 (1980) 243. H. Oberhammer, W. Gombler and H. Willner, J. Mol. Struct., 70 (1981) 273. J.H.M. ter Brake, R.A.J. Driessen, F.C. MijIhoff, G.H. Renes and A.H. Lowrey, J. Mol. Struct., 81 (1982) 277. J.R. Durig, T.G. Sheehan and J.A. Hardin, J. Mol. Struct., 243 (1991) 275. A.L. Andreassen, D. Zebelman and S.H. Bauer, J. Am. Chem. Sot., 93 (1971) 1148. A. Camerman, D. Mastropaolo and N. Camerman, J. Am. Chem. Sot., 105 (1983) 1584. K. Iijima, A. Ohnogi and S. Shibata, J. Mol. Struct., 156 (1987) 111. K. Iijima, T. Misu, S. Ohnishi and S. Onuma, J. Mol. Struct., 213 (1989) 263. H. Ogoshi and K. Nakamoto, J. Chem. Phys., 45 (1966) 3113. K. Iijima, J. Ohkawa and S. Shibata, J. Mol. Struct., 158 (1987) 315.
Journal of Molecular Structure, 268 (1992) 315-318 Elsevier Science Publishers B.V., Amsterdam
Short Communication
Internal rotation of trifluoromethyl groups in hexafluoroacetylacetone Kinya Iijima, Yukino Tanaka Department
and Shigeki Onuma
of Chemistry, Faculty of Science, Shizuoka University,
Oya, Shizuoka 422 (Japan)
(Received 30 October 1991)
The potential energy barrier of internal rotation of the trifluoromethyl group is somewhat higher than that of the methyl group: the barrier height is 6.0 kcal mol-’ in (CF3)20 [l], 5.8 kcal mol-l in ClSCF, [2] and 5.2 kcal mol-’ in CF,SCF, [ 21. However, there are some compounds with comparatively low barrier heights such as 1.67 kcal mol-l in CFsCOF [3] and 1.19 kcal mol-l in CF, COCHzBr [ 41. Andreassen et al. [ 51 have determined the molecular structure of gaseous hexafluoroacetylacetone from electron diffraction data on the assumption of C, symmetry. They treated the internal rotation of the CF, groups as a rigid rotor and obtained the OCCF dihedral angle of 47’) although they mentioned that this is an “average structure” value. The present study was undertaken to investigate the rotational barrier height of CF, groups in hexafluoroacetylacetone by gas-phase electron diffraction. The molecular symmetry was checked again because acetylacetone has been revealed not to be symmetric [ 6,7]. Long and short camera-distance photographs were taken at room temperature by the use of an r3-sector. The exposure times were 50 and 95 s for the long and short camera-distance photographs, respectively, with an electron beam current of 0.6 ,uA. The wavelength of the electron beam was estimated from the measured accelerating voltage of 40 kV [ 81. The procedures of data reduction and molecular-structure analysis were the same as those used previously for acetylacetone [ 71. The vibrational mean amplitudes and the shrinkage corrections used in the analysis were calculated from the force field
[91. Assuming an unsymmetric ring skeleton the analysis did not achieve convergence, as in the previous study [5]. The molecular skeleton was then assumed to be symmetric. The analysis assuming the rigid rotor to the CF, groups Correspondence to: Professor K. Iijima, Department of Chemistry, Faculty of Science, Shizuoka University, Oya, Shizuoka 422, Japan.
0022-2860/92/$05.00
0 1992 Elsevier Science Publishers B.V. All rights reserved.
316
5
10
15
20
25
30
s/A
Fig. 1. Molecular intensities for gaseous hexafluoroacetylacetone. The upper curve corresponds to the long camera-distance data and the lower one to the short camera-distance data. The dots represent the observed data and the solid curves the best-fit calculated data. The differences are shown in the lower part of the figure.
resulted in the agreement factor R = C w ( sMobs- sMcd,)2/CsMobs2 of 0.064. However, the R-factor decreased to 0.045 when the CFBgroups were treated as a rotor with low potential barrier height. The molecular intensities were calculated from $= 0 to 120” at 20” intervals and summed with the weights of the Boltzmann distribution. The mean amplitudes calculated by excluding the torsional force constant around the C-C, bond were used. The potential minimum was found at the OCCF dihedral angle of 60 O. Next the rotation of the CF3 group was treated as a precession motion [lo]. The tilt angle 8 of the C,, symmetry axis of CF, with respect to the C-C bond was assumed as 8= 19,+ (l/2 ) 8, (1 + cos 3$), where $ is the rotational angle of the CF, group. This analysis reduced the R-factor to 0.030. The final experimental and calculated molecular intensities and the corresponding radial distribution curves are shown in Figs. 1 and 2, respectively. The molecular parameters obtained in the least-squares calculations are listed in Table 1. The uncertainties of the molecular parameters have much improved but the parameter values are in good agreement with those obtained in the previous study by Andreassen et al. [5], except for the parameters for the rotation of the CF3 groups. The symmetry of the rotational barrier of CF3 was threefold and a V, component was negligible. The R-factor versus the barrier height is shown in Fig. 3. The best V, value of 0.75 kcal mol-’ is much lower than those of other compounds including a CF3 group [l-4]. The potential minimum is at the position where the two fluorine atoms are staggered to the oxygen atom ($ = 60’ ) . The repulsion from the hydrogen atom is large in this conformation, tilting the C& symmetry axis away from the hydrogen atom (8= 4’ ) . The CBV
317
Hcxsfluoroacetrlacstone
1
1
2
3
4
5
6
7
r/A
Fig. 2. Radial distribution curve for hexafluoroacetylacetone. The dots represent the experimental data and the solid curve the theoretical data, with the difference shown below. The vertical bars represent bond distances and the scattering powers. The molecular model is shown at the upper right. TABLE 1 Molecular parameters of hexafluoroacetylacetone (bond distances and mean amplitudes in A, angles in degrees)
r(C-0)
r(C-G) r(C-GJ dC,-F) dC,-H) r(O-*-0)’ L
cc,c c,co c,cc,
L L L CC,F
L OH0 $ 01’
UC,-F) l(F*..F)
1.261 1.422 1.541 1.341 1.101 2.606 114.9 127.3 118.8 110.9 180 4.0 -2.2 0.041 0.060
1caleC
Ref. 5d
0.004 0.006 0.003 0.001 assumed 0.013 0.5 0.5 0.4 0.1 8 0.6 1.5
0.039
1.261(18) 1.409(31) 1.548(8) 1.338(7) 1.085 assumed 2.556(33) 115.2(23) 126.4(13) 119.7(15) 110.7(8) 176(11)
0.002 0.001
0.046 0.057
0.047 0.045 0.046 0.076 0.116
“Parameters obtained in the least-squares analysis. Angle parameters are F, parameters. bLimits of error. “Mean amplitudes calculated from the force field [ 91. dParameters in ref. 5 are converted to rg parameters by rg= r,+ l’/r,. ‘Dependent parameter. Tilt angle 13is defined as 0, + (l/2) 8, (1 + cos3#), where $ is a rotational angle of a CF, group.
318
0.032 -
0.030
t I
I
0.50
0.75
1.00
V3/ kcal mol-’ Fig. 3. Correlation of the R-factor in the least-squares with the potential barrier height, V,, of the internal rotation of the CF,.
symmetry axis tilts 2 o away from the oxygen atom towards the hydrogen atom at the potential maximum position ($ = 0” ) . A systematic difference was found in the range of the long camera-distance data on examination of the intensity curves in detail. It was inferred that this discrepancy arises from the F. * - F distances between the two CF, groups. The two CF3 groups were assumed to rotate independently in the present work, but there may be some correlation between the motions of the two rotors. A further analysis, however, was not carried out, since the difference between the experimental and the calculated molecular intensities was very small. The computations were carried out on AK5100 in the Shizuoka University Information Processing Center and on a super computer of HITAC S-820 in the Computer Center of the University of Tokyo.
REFERENCES 1 2 3 4 5 6 7 8 9 10
A.H. Lowrey, C. George, P. D’Antonio and J. Karle, J. Mol. Struct., 63 (1980) 243. H. Oberhammer, W. Gombler and H. Willner, J. Mol. Struct., 70 (1981) 273. J.H.M. ter Brake, R.A.J. Driessen, F.C. MijIhoff, G.H. Renes and A.H. Lowrey, J. Mol. Struct., 81 (1982) 277. J.R. Durig, T.G. Sheehan and J.A. Hardin, J. Mol. Struct., 243 (1991) 275. A.L. Andreassen, D. Zebelman and S.H. Bauer, J. Am. Chem. Sot., 93 (1971) 1148. A. Camerman, D. Mastropaolo and N. Camerman, J. Am. Chem. Sot., 105 (1983) 1584. K. Iijima, A. Ohnogi and S. Shibata, J. Mol. Struct., 156 (1987) 111. K. Iijima, T. Misu, S. Ohnishi and S. Onuma, J. Mol. Struct., 213 (1989) 263. H. Ogoshi and K. Nakamoto, J. Chem. Phys., 45 (1966) 3113. K. Iijima, J. Ohkawa and S. Shibata, J. Mol. Struct., 158 (1987) 315.