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Molecular-mechanics calculations for (X3C)2O and (X3C)2S molecules with X F, Cl and Br
Journal of Molecular Structure, 147 (1986) 179-184 Elsevier Science Publishers B.V., Amsterdam -Printed
in The Netherlands
MOLECULAR-MECHANICS CALCULATIONS FOR (X&O (X3C&S MOLECULES WITH X = F, Cl and Br
REIDAR ST@LEVIK Depart=nent
AND
and PIRKKO BAKKEN
of Chemistry,
AVH,
University
of Trondheim,
7000 Trondheim
(Norway)
(Received 26 March 1986)
ABSTRACT In the compounds (X,C),O and (X,C),S it has been predicted that for X = Cl, Br the molecules have C, structures while the fluoro compounds have C,, structures. Torsional force constants, rotational barrier heights and structural parameters have been obtained. For F,CSCF, the calculated values have been compared with experimental gas-phase data. A new value of the experimental rotational barrier height has been proposed for F,CSCF,. INTRODUCTION
Possible conformers and transition forms of those compounds are shown in Fig. 1. From gas-phase electron diffraction [l] it was concluded that F3CSCF3 exists in the CZVform. The gas-phase IR and liquid-phase Raman spectra of F3COCF3 were analyzed for a bent CZVmolecule [ 21. It is not known from experiment what are the stable forms for the chloro and bromo compounds. In the CZV form there are two pairs of parallel C-X bonds leading to strong non-bonded X1*** X3 interactions. If CZ is the stable conformer, then CZVcorresponds to the maximum in a typical double-minimum potential, as illustrated in Fig. 2. A previous paper [3] predicted the presence of such double minima in H&(CX,), and H$i(CX& molecules with X = Cl and Br. CALCULATIONS
AND RESULTS
For the molecules XJCOCXa and X,CSCX3 it is only necessary to include non-bonded interactions of the types X** *X and X* **C in the energy calculations. These interaction potentials are recorded in ref. 4. Force constants and reference values are found in Table 1 and Table 2. Excess atomic charges in Table 3 were obtained by a modified version of the procedure in ref. 5. Energy minima were found by adjusting all structural parameters simultaneously, but one (Cs*) or two (C,,*) torsional angles were held constant when finding the energies of the transition forms. The intrinsic torsional 0022-2860/86/$03.50
0 1986 Elsevier Science Publishers B.V.
* &+ *
s
Fig. 1. Possible conformers and transition forms (*) for X,C-O-CX, and X,C-S-CX, molecules. The symmetries (point groups) have been used as names. The numbering is Cl-02-C3 and Cl-S2-C3. The torsion angles are oIz = L(X*--C~-2-C3) and oz3 = /-(Cl-2-C3-X*). The C, form corresponds to o,* = **a, while the Cs forms have 9 11 = +9,,.
-60"
c2-
0"
c2+
+60"
Fig. 2. Potential energy function (E) for the molecules X,C-O-CX, and X,C-S-CX, where X is Cl or Br. The curve C2 corresponds to E(o). The curve C2 corresponds to E(o) when the torsion angles (@) are restricted as o12 = +o and ols = -o. The curve CS corresponds to E(o) when the torsion angles are restricted as o,l = tiz3 = @. Values of the parameters describing the potential functions are found in Table 4 and Table 5. The two forms C2- and C2’ are enantiomers.
181 TABLE 1 Force constants for X,COCX, X=F Bond stretch S-C O-C c-x
(mdyn A”) 3.30 5.05 6.03
Bond bend (mdyn A @ad)-‘) csc 0.95 cot 1.50 sex 0.83 ocx 0.83 xcx 0.70
and X,CSCX,
molecules
x = Cl
X = Br
3.30 5.05 3.32
3.30 5.05 2.57
0.95 1.50 1.21 1.21 1.10
0.95 1.50 0.96 0.96 1.10
TABLE 2 Reference values for X,COCX, X=F Bond lengths (A) S-C O-C c-x Bond angles (“) csc cot sex ocx xcx
and X,CSCX,
molecules
x = Cl
1.810 1.390 1.330
1.810 1.390 1.760
X = Br
1.810 1.390 1.925
97.3
97.3
97.3
111.5 109.5 109.5 109.5
111.5 109.5 109.5 109.5
111.5 109.5 109.5 109.5
TABLE 3 Excess atomic charges (q) for X,COCX, units X=F
and X,CSCX,
x = Cl
molecules.
X = Br
Charges in X,COCX, Q(X) Q(C) a(C)”
molecules -0.057 +0.172 -0.003
-0.034 +0.137 -0.070
-0.019 +0.121 -9.128
Charges in X,CSCX, Q(X) 9(C) 9w
molecules -0.070 +0.156 +O.llO
-0.050 +0.119 +0.065
-0.039 + 0.098 + 0.034
aThese charges do not enter calculation of energies.
Charges in electron
182
potential function was V = O.EiV!j (1 - cos(3@)) with Vi equal to 2.30 and 2.70 kcal mol-’ for sulfur and oxygen molecules respectively. RESULTS
FOR X,C-S-CX,
MOLECULES
Results are presented in Table 4. Our calculations agree with the gas-phase electron-diffraction finding [l] that F3C!SCF3 exists in the CzV form. Torsional force constants were derived from the vibrational amplitudes [ 11. Our diagonal value is in agreement with the observed value 0.16 f 0.06 mdyn A (rad)-‘. From this value a barrier height equal to 5.2 + 2.0 kcal mol-’ was calculated assuming a cos(3$)-potential function. Thus the assumed ratio between the barrier height B in kcal mol-’ and the experimental force constant F in mdyn A (rad)-* was 32.0. However, our calculations show that the ratio B/F is equal to 24.4, and using this value the experimental barrier height is reduced to 4.0 f. 1.5 kcal mol-‘. For C1$SCCIJ and Br3CSCBr3 the stable form is C2 with a barrier height equal to about 1 kcal mol-’ at CzV as illustrated in Fig. 2. The dihedral angles are about 18” in both cases. The diagonal torsional force constants TABLE
4
Results for X,C-S-CX,
molecules X=F
Torsion angles (“) @12 $13 Energies in (kcal mol-‘) C 1” C* CS* CW*
0 0
-
Oa
4.0 11.0
Torsional force constants (mdyn A (rad)“) PE/a$? 0.164 aZEla@a@’ +0.066 Bond lengths (A) S-C o-x c-x*
Bond angles (“) sex* sex csc xcx xcx*
1.806 1.328 1.326 109.1 109.5 96.5 109.2 109.7
aEnergies relative to this minimum.
x = Cl
X = Br
+17.5 -17.5
+18.3 -18.3
1.2 Oa 5.0 19.0
1.3 Oa 5.2 17.2
0.453 +0.027
0.430 -0.040
1.827 1.760 1.753
1.829 1.927 1.915
106.8 112.4 109.5 110.2 107.4
105.2 113.6 111.2 110.2 106.9
183
are nearly three times greater than in F3CSCF3. However, the Cs* barriers are only increased from 4 to 5 kcal mol -l. The non-diagonal torsional force constants are small in all three molecules. Large variations in bond-angle values have been obtained. Thus CSC varies from 97” to 111.2”, and the values of the SCX* angles are in the range 105~-log”, while SCX values range over 109.5-113.6”. The S-C bond length varies between 1.806 A and 1.829 A. RESULT FOR X,C-O-CX,
MOLECULES
Results are presented in Table 5. Again, the fluoro compound exists in the CzV form, while the chloro and bromo compounds have Cz structures with twist angles at about 21”, and a barrier height of 2.6 kcal mol-’ at CzV, as illustrated in Fig. 2. The Cs* barrier heights are in the range 4.08.3 kcal mol-I, and CzV* barrier heights are in the range lo-16 kcal mol-‘. Some of the bond angles show great variation in their values. Thus COC varies from 113.8” in FJCOCFs to about 123” in the chloro and bromo compounds. The difference between values of the OCX and OCX* angles TABLE 5 Results for X,C-O-CX,
molecules X=F
Torsion angles (“) @J12 @23
0 0
x = Cl
+20.8 -20.8
X = Br
t21.3 -21.3
Energies (kcal mol-‘) C,” C
- Oa
O8 2.6
Oa 2.6
cs* CW*
4.0 10.9
7.0 18.3
8.3 15.8
Torsional force constants (mdyn A (rad)“) a2ElaoZ 0.120 PEla@a@’ +0.066 Bond lengths (A) O-C c-x c-x* Bond angles (“) ocx* ocx cot xcx xcx*
1.395 1.329 1.325 108.1 110.6 113.8 109.5 109.0
aEnergies relative to this minimum.
0.484 -0.152
0.480 -0.106
1.414 1.767 1.751
1.415 1.935 1.914
105.1 114.1 123.0 110.3 106.2
103.2 115.4 123.5 110.1 105.7
184
are noticeable. In Br3COCBr3 the values are 103.2” and 115.4” for OCBr* and OCBr respectively. CONCLUDING REMARKS
Molecular-mechanics calculations, using non-bonded atomsatom interaction potentials derived from halogenated alkanes [4] , were able to reproduce [6] the correct conformation for the compounds FO-CFa, FS-CFJ and C1S-CC13. Conformational energies, rotational barrier heights and torsional force constants for the six molecules XO-CX3 and XS--CX3 with X = F, Cl and Br were also given in ref. 6. Reasonable values for the barrier heights as well as the torsional force constants were calculated. Our calculations also agree with the observation from the gas-phase study [l] that F&!SCF3 has the Czv form, and the correct value of the torsional force constant was obtained. The results for the chloro and bromo compounds of X&OCX3 and XBCSCXs, as well as the fluoro compound F,COCF,, ought to be equally reliable. Finally it is interesting to note that the fluoro compounds (F3C)2CHz and (F3C&SiH2 also have the Czv form [3], while the chloro and bromo compounds of (X$)&H2 and (X3C)$iH2 have the C2 form according to molecular-mechanics calculations [ 31, using the same non-bonded potential functions [ 41 as were used in this study. ACKNOWLEDGEMENT
Financial support from NAVF, is acknowledged.
Norges
Almenvitenskapelige
Forskningstid,
REFERENCES 1 2 3 4 5 6
H. H. R. R. R. R.
Oberhammer, W. Gombler and H. WiIlner, J. Mol. Struct., 70 (1981) 273. Burger and G. PaweIke, Spectrochim. Acta, Part A, 31(1975) 1965. St4levik and P. Bakken, J. Mol. Struct., 145 (1986) 287. Stdlevik, J. Mol. Struct., (Theochem), 109 (1984) 397. T. Sanderson, Chemical Bonds and Bond Energy, Academic Press, New York, 1976. St4levik and P. Bakken, J. Mol. Struct., 145 (1986) 143.
in The Netherlands
MOLECULAR-MECHANICS CALCULATIONS FOR (X&O (X3C&S MOLECULES WITH X = F, Cl and Br
REIDAR ST@LEVIK Depart=nent
AND
and PIRKKO BAKKEN
of Chemistry,
AVH,
University
of Trondheim,
7000 Trondheim
(Norway)
(Received 26 March 1986)
ABSTRACT In the compounds (X,C),O and (X,C),S it has been predicted that for X = Cl, Br the molecules have C, structures while the fluoro compounds have C,, structures. Torsional force constants, rotational barrier heights and structural parameters have been obtained. For F,CSCF, the calculated values have been compared with experimental gas-phase data. A new value of the experimental rotational barrier height has been proposed for F,CSCF,. INTRODUCTION
Possible conformers and transition forms of those compounds are shown in Fig. 1. From gas-phase electron diffraction [l] it was concluded that F3CSCF3 exists in the CZVform. The gas-phase IR and liquid-phase Raman spectra of F3COCF3 were analyzed for a bent CZVmolecule [ 21. It is not known from experiment what are the stable forms for the chloro and bromo compounds. In the CZV form there are two pairs of parallel C-X bonds leading to strong non-bonded X1*** X3 interactions. If CZ is the stable conformer, then CZVcorresponds to the maximum in a typical double-minimum potential, as illustrated in Fig. 2. A previous paper [3] predicted the presence of such double minima in H&(CX,), and H$i(CX& molecules with X = Cl and Br. CALCULATIONS
AND RESULTS
For the molecules XJCOCXa and X,CSCX3 it is only necessary to include non-bonded interactions of the types X** *X and X* **C in the energy calculations. These interaction potentials are recorded in ref. 4. Force constants and reference values are found in Table 1 and Table 2. Excess atomic charges in Table 3 were obtained by a modified version of the procedure in ref. 5. Energy minima were found by adjusting all structural parameters simultaneously, but one (Cs*) or two (C,,*) torsional angles were held constant when finding the energies of the transition forms. The intrinsic torsional 0022-2860/86/$03.50
0 1986 Elsevier Science Publishers B.V.
* &+ *
s
Fig. 1. Possible conformers and transition forms (*) for X,C-O-CX, and X,C-S-CX, molecules. The symmetries (point groups) have been used as names. The numbering is Cl-02-C3 and Cl-S2-C3. The torsion angles are oIz = L(X*--C~-2-C3) and oz3 = /-(Cl-2-C3-X*). The C, form corresponds to o,* = **a, while the Cs forms have 9 11 = +9,,.
-60"
c2-
0"
c2+
+60"
Fig. 2. Potential energy function (E) for the molecules X,C-O-CX, and X,C-S-CX, where X is Cl or Br. The curve C2 corresponds to E(o). The curve C2 corresponds to E(o) when the torsion angles (@) are restricted as o12 = +o and ols = -o. The curve CS corresponds to E(o) when the torsion angles are restricted as o,l = tiz3 = @. Values of the parameters describing the potential functions are found in Table 4 and Table 5. The two forms C2- and C2’ are enantiomers.
181 TABLE 1 Force constants for X,COCX, X=F Bond stretch S-C O-C c-x
(mdyn A”) 3.30 5.05 6.03
Bond bend (mdyn A @ad)-‘) csc 0.95 cot 1.50 sex 0.83 ocx 0.83 xcx 0.70
and X,CSCX,
molecules
x = Cl
X = Br
3.30 5.05 3.32
3.30 5.05 2.57
0.95 1.50 1.21 1.21 1.10
0.95 1.50 0.96 0.96 1.10
TABLE 2 Reference values for X,COCX, X=F Bond lengths (A) S-C O-C c-x Bond angles (“) csc cot sex ocx xcx
and X,CSCX,
molecules
x = Cl
1.810 1.390 1.330
1.810 1.390 1.760
X = Br
1.810 1.390 1.925
97.3
97.3
97.3
111.5 109.5 109.5 109.5
111.5 109.5 109.5 109.5
111.5 109.5 109.5 109.5
TABLE 3 Excess atomic charges (q) for X,COCX, units X=F
and X,CSCX,
x = Cl
molecules.
X = Br
Charges in X,COCX, Q(X) Q(C) a(C)”
molecules -0.057 +0.172 -0.003
-0.034 +0.137 -0.070
-0.019 +0.121 -9.128
Charges in X,CSCX, Q(X) 9(C) 9w
molecules -0.070 +0.156 +O.llO
-0.050 +0.119 +0.065
-0.039 + 0.098 + 0.034
aThese charges do not enter calculation of energies.
Charges in electron
182
potential function was V = O.EiV!j (1 - cos(3@)) with Vi equal to 2.30 and 2.70 kcal mol-’ for sulfur and oxygen molecules respectively. RESULTS
FOR X,C-S-CX,
MOLECULES
Results are presented in Table 4. Our calculations agree with the gas-phase electron-diffraction finding [l] that F3C!SCF3 exists in the CzV form. Torsional force constants were derived from the vibrational amplitudes [ 11. Our diagonal value is in agreement with the observed value 0.16 f 0.06 mdyn A (rad)-‘. From this value a barrier height equal to 5.2 + 2.0 kcal mol-’ was calculated assuming a cos(3$)-potential function. Thus the assumed ratio between the barrier height B in kcal mol-’ and the experimental force constant F in mdyn A (rad)-* was 32.0. However, our calculations show that the ratio B/F is equal to 24.4, and using this value the experimental barrier height is reduced to 4.0 f. 1.5 kcal mol-‘. For C1$SCCIJ and Br3CSCBr3 the stable form is C2 with a barrier height equal to about 1 kcal mol-’ at CzV as illustrated in Fig. 2. The dihedral angles are about 18” in both cases. The diagonal torsional force constants TABLE
4
Results for X,C-S-CX,
molecules X=F
Torsion angles (“) @12 $13 Energies in (kcal mol-‘) C 1” C* CS* CW*
0 0
-
Oa
4.0 11.0
Torsional force constants (mdyn A (rad)“) PE/a$? 0.164 aZEla@a@’ +0.066 Bond lengths (A) S-C o-x c-x*
Bond angles (“) sex* sex csc xcx xcx*
1.806 1.328 1.326 109.1 109.5 96.5 109.2 109.7
aEnergies relative to this minimum.
x = Cl
X = Br
+17.5 -17.5
+18.3 -18.3
1.2 Oa 5.0 19.0
1.3 Oa 5.2 17.2
0.453 +0.027
0.430 -0.040
1.827 1.760 1.753
1.829 1.927 1.915
106.8 112.4 109.5 110.2 107.4
105.2 113.6 111.2 110.2 106.9
183
are nearly three times greater than in F3CSCF3. However, the Cs* barriers are only increased from 4 to 5 kcal mol -l. The non-diagonal torsional force constants are small in all three molecules. Large variations in bond-angle values have been obtained. Thus CSC varies from 97” to 111.2”, and the values of the SCX* angles are in the range 105~-log”, while SCX values range over 109.5-113.6”. The S-C bond length varies between 1.806 A and 1.829 A. RESULT FOR X,C-O-CX,
MOLECULES
Results are presented in Table 5. Again, the fluoro compound exists in the CzV form, while the chloro and bromo compounds have Cz structures with twist angles at about 21”, and a barrier height of 2.6 kcal mol-’ at CzV, as illustrated in Fig. 2. The Cs* barrier heights are in the range 4.08.3 kcal mol-I, and CzV* barrier heights are in the range lo-16 kcal mol-‘. Some of the bond angles show great variation in their values. Thus COC varies from 113.8” in FJCOCFs to about 123” in the chloro and bromo compounds. The difference between values of the OCX and OCX* angles TABLE 5 Results for X,C-O-CX,
molecules X=F
Torsion angles (“) @J12 @23
0 0
x = Cl
+20.8 -20.8
X = Br
t21.3 -21.3
Energies (kcal mol-‘) C,” C
- Oa
O8 2.6
Oa 2.6
cs* CW*
4.0 10.9
7.0 18.3
8.3 15.8
Torsional force constants (mdyn A (rad)“) a2ElaoZ 0.120 PEla@a@’ +0.066 Bond lengths (A) O-C c-x c-x* Bond angles (“) ocx* ocx cot xcx xcx*
1.395 1.329 1.325 108.1 110.6 113.8 109.5 109.0
aEnergies relative to this minimum.
0.484 -0.152
0.480 -0.106
1.414 1.767 1.751
1.415 1.935 1.914
105.1 114.1 123.0 110.3 106.2
103.2 115.4 123.5 110.1 105.7
184
are noticeable. In Br3COCBr3 the values are 103.2” and 115.4” for OCBr* and OCBr respectively. CONCLUDING REMARKS
Molecular-mechanics calculations, using non-bonded atomsatom interaction potentials derived from halogenated alkanes [4] , were able to reproduce [6] the correct conformation for the compounds FO-CFa, FS-CFJ and C1S-CC13. Conformational energies, rotational barrier heights and torsional force constants for the six molecules XO-CX3 and XS--CX3 with X = F, Cl and Br were also given in ref. 6. Reasonable values for the barrier heights as well as the torsional force constants were calculated. Our calculations also agree with the observation from the gas-phase study [l] that F&!SCF3 has the Czv form, and the correct value of the torsional force constant was obtained. The results for the chloro and bromo compounds of X&OCX3 and XBCSCXs, as well as the fluoro compound F,COCF,, ought to be equally reliable. Finally it is interesting to note that the fluoro compounds (F3C)2CHz and (F3C&SiH2 also have the Czv form [3], while the chloro and bromo compounds of (X$)&H2 and (X3C)$iH2 have the C2 form according to molecular-mechanics calculations [ 31, using the same non-bonded potential functions [ 41 as were used in this study. ACKNOWLEDGEMENT
Financial support from NAVF, is acknowledged.
Norges
Almenvitenskapelige
Forskningstid,
REFERENCES 1 2 3 4 5 6
H. H. R. R. R. R.
Oberhammer, W. Gombler and H. WiIlner, J. Mol. Struct., 70 (1981) 273. Burger and G. PaweIke, Spectrochim. Acta, Part A, 31(1975) 1965. St4levik and P. Bakken, J. Mol. Struct., 145 (1986) 287. Stdlevik, J. Mol. Struct., (Theochem), 109 (1984) 397. T. Sanderson, Chemical Bonds and Bond Energy, Academic Press, New York, 1976. St4levik and P. Bakken, J. Mol. Struct., 145 (1986) 143.