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Conformation and structure of ethane-1,2-dithiol from ab initio HF and MP3 study
Journal of Molecular Structure (Theo&m), 257 (1992) 465-490 Elsevier Science Publishers B.V., Amsterdam
485
Conformation and structure of ethane-1,2-dithiol from ab initio HF and MP3 study Nino Russo, Emilia Sicilia and Marirosa Toscano Dipartimento di Chimica, Universita’della Calabria, I-87030 Arcavacata di Rende (Italy) (Received 25 September 1991)
Abstract The structure and the conformational equilibrium of ethane-1,2-dithiol (CH$HCHxSH) were determined by performing a geometry optimization at the HF/6-31G** level for both the anti and gauche conformers. The estimated geometries agree with the experimental values. Third-order Moller-Plesset (MP3) single-point calculations were done on the optimized geometries in order to obtain the final energies. At the Hartree-Fock (HF) level the gauche form is the most stable, while the inclusion of correlation effects reverses the stability order. Owing to the small energy difference it is possible that the two conformers coexist.
INTRODUCTION
The simple molecules that afford the possibility of intramolecular hydrogenbond formation (i.e. ethylene glycol, ethanedithiol, ethylenediamine, 2-mercaptoethanol, 2-fluoroethanol and 2-chloroethanol) are interesting from many points of view and have been subject to several theoretical and experimental investigations [l-lo]. Because these compounds exist as mixtures of different conformers (i.e. anti and gauche) due to rotation about a central single bond, the structural properties of these molecules are not easily obtainable experimentally. On the whole, however, the presence of different conformers does not affect non-empirical quantum-mechanical computations and these can be used to obtain quantitative results which complement the experimental evidence. In the present study we focused our attention on the structural properties of ethane-1,2dithiol. In recent years this molecule has been studied experimentally with microwave (MW) [ 111, electron diffraction (ED) [ 12-141 and IR [ 151 techniques and theoretically at the ab initio 4-31G level [ 161. Despite these investigations, some interesting aspects of this molecule, such as the Correspondence to: N. Russo, Dipartimento di Chimica, Universita della Calabria, I-87030 Arcavacata di Rende, Italy.
0166-1260/92/$05.00
0 1992 Elsevier Science Publishers B.V. All rights reserved.
488
relative stability, the dipole moment and other monoelectronic properties and the nature and position of the S. - H hydrogen bond remain to be clarified. In this paper we report the ab initio computation using a large basis set (6-31G**) of the structure, relative stability and electronic properties of both the anti and gauche conformers of ethane-1,2dithiol. The third-order Mraller-Plesset (MP3)treatment was used in determining the correlation energy. l
COMPUTATIONAL DETAILS
The structure of both the anti and the gauche conformer was completely optimized by using gradient methods at the 6-31G** level [ 171 employing the GAUSSIAN at3package [ 181 on the CRAY-YMP computer of CINECA. For the anti conformer Cab symmetry was imposed, while the gauche form was optimized without constraint. A single-point calculation Meller-Plesset treatment of the correlation energy [ 171 truncated at third order (MP3) was done on the Hartree-Fock (HF) optimized structures. RESULTS AND DISCUSSION
The two most stable conformers of ethane-1,2dithiol the geometrical parameters are shown below.
and the definitions of
H-1
Anti
Gauche
The 6-31G** optimized structures of the anti and gauche conformers are reported in Table 1 together with the most recent experimental and theoretical results. It can be seen from Table 1 that the results obtained in the present study are in generally good agreement with the results of ED [ 141 and MW [ 111 studies. The small discrepancy between our values for the bond lengths and valence angles and those obtained from MW and ED studies can be accounted for by the fact that in the experimental measurements many geometrical parameters (see Table 1) are assumed to be equivalent. Comparison of the present results with ab initio 4-31G results [ 161 reveals that the C* *S and S- * *H bond lengths calculated with the 4-31G basis set are overestimated and that the C* **C and Cm- lH values are underestimated. This is essentially due to the fact that no polarization functions were added in the latter computation. With l
487 TABLE 1 Optimized 6-31G** geometrical parameters of the gauche end anti (in parentheses) conformers of ethane-1,2-dithiol Parameter
Method ED”
6-31G** Bond length Cl-C2 Cl-S1 c2-s2 Sl-Hl S2-H2 Cl-H3 Cl-H4 C2-H5 C2-H6
MWb
4-31G”
(li,
Bond angle (deg.) CB-Cl-S1 Cl-c2-s2 Cl-Sl-Hl C2-S2-H2 C2-Cl-H3 C2-Cl-H4 Cl-C2-H5 Cl-C2-H6
1.527 1.828 1.821 1.326 1.326 1.083 1.081 1.083 1.083
(1.525) (1.827)
1.534
1.538
1.512
1.820
1.821
1.894
1.338
1.340
1.358
1.103
1.097
1.077
(1.327) (1.082)
110.9 (110.9) 115.6 97.9 (97.6) 98.4 108.9 (110.2) 110.1 108.6 110.9
113.1
112.5
110.0
94.0
96.5
97.6
111.1
110.7
108.9
69.0 - 141.0 -40.0
64.8 - 162.9 -43.4
103.0 - 177.2 - 60.0
Dihedral angk (deg.) ;
e dd (A)
74.9 (0.0) - 166.8 (180.0) -64.2 (180.0) 3.040
2.7
*From ref. 14. bFrom ref. 11. “From ref. 16. the Sl*--H2 hydrogen bond distance.
regard to the torsional angle ly, we found a value of 74.9” while the MW and ED values are 64.8” and 69.0”, respectively. The two CCSH dihedral angles # and 19are - 166.8” and - 64.2’ as measured as a counterclockwise rotation of the S***H bonds relative to an eclipsed arrangement of these bonds at a cy ( SCCS ) torsional angle of 0 ‘. Our value of @ is slightly higher than the ED value (# = - 141.0 ’ ), but is in good agreement with MW measurement (# = - 162.9’ ) . The present value of
488
angle 8 is about 20” higher than the corresponding MW and ED results, but is in agreement with the 4-31G value. The rotational constants, the dipole moment and relative energy between the two conformers are reported in Table 2. The 6-31G** values for the rotational constants and dipole moments are in agreement with the experimental values [ 11,121, note in particular that our computed dipole moment is 2.068 D while the experimental value [ 111 is 2.031 D. In the present study it was found (at the HF level) that the anti form is 0.36 kJ mol-’ more stable than the gauche one. With the introduction of correlation effects, the gauche conformer was found to be the most stable. In particular, at the MP2 level the energy difference is 2.27 kJ mol-’ and at the MP3 level this difference decreases to 1.71 kJ mol-l. Both conformers are observed experimentally and the relative stability of the conformers cannot be unambiguously resolved. The ED radial distribution function shows the presence of substantial amounts of both anti and gauche heavy atom conformations with the anti form being the dominant conformer (about 60%) [ 13,141. The anti conformer is also observed in the IR spectrum [ 151. However, MW experiments indicate that the gauche form is the most stable one although the anti conformer is present. Our theoretical values confirm the very small energy TABLE 2 Spectroscopic, electronic and energetic parameters of ethane-1,2-dithiol from 6-31G** and MP3 computations (the values in parentheses are for the anti conformer) Parameter
Method 6-31G**
Rotational constants (MHz) A 9826.6 (25573.3) B 2127.5 (1515.6) C 1873.8 (1456.5) Dipole moments (0) Px & k ,&4x
1.196 1.606 0.517 2.068
ED”
Mwb
9289.6 2233.9 1935.9
9292.5 2239.2 1935.6
0.650 1.650 0.660 1.892
Relative energy, AE (gauche-anti) (kJ molV1) HF 0.36 MP2 - 2.27 -1.71 MP3 Erp. 1.71 “From ref. 14. bFrom ref. 11.
0.774 1.749 0.683 2.031
difference between the two conformers. Different experimental conditions could favour one of the two conformers. The coexistence of two conformers has been also postulated in the case of ethylene glycol [9,10]. Finally, the present study confirms the presence of an effective S---H hydrogen bond in the gauche form, as indicated previously [ 141. In our structure the distance of H2 from the acceptor atom Sl (d) is 3.02 A, which is only 0.1 A less than the sum of the van der Waals radii [ 191. In this situation, the SH- H hydrogen bond is quite weak. However, a smaller distance d increases the repulsive interactions of non-bonding electrons which destabilizes the gauche form. This destabilization is not compensated for by the consequently increased hydrogen bond force. In the experimental ED structure [ 141 the angles $ and 8 are 20” lest than the corresponding theoretical values and the distance d is shorter (2.70 A). This means that in this structure the non-bonded interactions are higher than in the present case and the gauche structure is slightly destabilized relative to the anti one. l
l
CONCLUSIONS
On the basis of our study the following conclusions can be drawn. (i) The HF/6-31G** optimization gives geometrical parameters that are in agreement with the experimentally determined values. (ii) The presence of a weak hydrogen bond formation in the gauche isomer is confirmed. (iii) The stability of the anti and gauche conformers of ethane-1,2dithiol is due to the delicate balance between the hydrogen bond formation and the nonbonded repulsive interactions in the gauche conformer. Because of the small energy difference between the two conformers, different experimental conditions or levels of theoretical treatment can give rise to apparently contradictory results. In order to draw a more definite conclusion about the absolute minimum energy, a full optimization at the HF/MP level is required in future computations. ACKNOWLEDGMENTS
We gratefully acknowledge research support from CNR and MURST. One of us (E.S.) is indebted to the Costituendo Istituto “Membrane e Reattori Chimici” de1 CNR for a fellowship. REFERENCES 1 2
K.M. Marstokk and H.J. Mollendal, J. Mol. Struct., 49 (1978) 221. W. Caminati and G. Corbelli, J. Mol. Spectrosc., 90 (1981) 527.
490 3
8 9 10 11 12 13 14 15 16 17 18
19
C. Van AIsenoy, L. Van Den Enden and L. Schafer, J. Mol. Struct. (Theochem), 108 (1984) 121. G. Chidichimo, D. ImbardeIIi, M. Longeri and A. Saupe, Mol. Phys., 65 (1988) 1143. K. Hagen and K. Hedberg, J. Am. Chem. Sot., 95 (1973) 8263. E.M. Sung and M.D. Harmony, J. Am. Chem. Sot., 99 (1977) 5603. A. AIgemenningen, D. Bastiansen, L. Femholt and K. Hedberg, Acta Chem. Stand., 25 (1971) 1946. R.N. Nandi, M.F. BoIand and M.D. Harmony, J. Mol. Spectrosc., 92 (1982) 419. H. Frei, H.A. Tae-Kyu, R. Meyer and H.H. Gunthard, Chem. Phys., 25 (1977) 271. B.J. Costa CabraI, L.M.P.C. Alberquerque and F.M.S. SiIva Femandes, Theor. Chim. Acta, 78 (1991) 271. R.N. Nandi, CF. Su andM.D. Harmony, J. Chem. Phys., 81 (1984) 1051. G. Schultz and I. Hargittai, Acta Chim. Acad. Sci. Hung., 75 (1973) 381. I. Hargittai and G. Schultz, J. Chem. Phys., 84 (1986) 5220. S.L. Barkowski, L. Hedberg and K. Hedberg, J. Am. Chem. Sot., 108 (1986) 6898. M. Hayashi, Y. Shiro, T. Oshima and H. Mu&a. Bull. Chem. Sot. Jpn., 38 (1965) 1734. T. Shida and T. Momose, J. Mol. Struct., 126 (1985) 159. W.J. Hehre, L. Radom, P.v.R. Schleyer and J.A. Pople, Ab initio Molecular Orbital Theory, Wiley, New York, 1986. M.J. Frisch, M. Head-Gordon, B.B. SchIegel, K. Raghavachari, J.S. BinkIey, C. Gonzales, D.J. DeFrees, D.J. Fox, R.A. Whiteside, R. Seeger, C.F. Melius, J. Baker, R.L. Martin, L.R. Kahn, J.P. Stewart, E.M. Fhider, S. Topiol and J.A. Pople, GAUSSIANEXJ, Gaussian Inc., Pittsburg, PA, 1988. L. Pauling, The Nature of Chemical Bond, 3rd edn., Cornell University Press, Ithaca, NY, 1960.
485
Conformation and structure of ethane-1,2-dithiol from ab initio HF and MP3 study Nino Russo, Emilia Sicilia and Marirosa Toscano Dipartimento di Chimica, Universita’della Calabria, I-87030 Arcavacata di Rende (Italy) (Received 25 September 1991)
Abstract The structure and the conformational equilibrium of ethane-1,2-dithiol (CH$HCHxSH) were determined by performing a geometry optimization at the HF/6-31G** level for both the anti and gauche conformers. The estimated geometries agree with the experimental values. Third-order Moller-Plesset (MP3) single-point calculations were done on the optimized geometries in order to obtain the final energies. At the Hartree-Fock (HF) level the gauche form is the most stable, while the inclusion of correlation effects reverses the stability order. Owing to the small energy difference it is possible that the two conformers coexist.
INTRODUCTION
The simple molecules that afford the possibility of intramolecular hydrogenbond formation (i.e. ethylene glycol, ethanedithiol, ethylenediamine, 2-mercaptoethanol, 2-fluoroethanol and 2-chloroethanol) are interesting from many points of view and have been subject to several theoretical and experimental investigations [l-lo]. Because these compounds exist as mixtures of different conformers (i.e. anti and gauche) due to rotation about a central single bond, the structural properties of these molecules are not easily obtainable experimentally. On the whole, however, the presence of different conformers does not affect non-empirical quantum-mechanical computations and these can be used to obtain quantitative results which complement the experimental evidence. In the present study we focused our attention on the structural properties of ethane-1,2dithiol. In recent years this molecule has been studied experimentally with microwave (MW) [ 111, electron diffraction (ED) [ 12-141 and IR [ 151 techniques and theoretically at the ab initio 4-31G level [ 161. Despite these investigations, some interesting aspects of this molecule, such as the Correspondence to: N. Russo, Dipartimento di Chimica, Universita della Calabria, I-87030 Arcavacata di Rende, Italy.
0166-1260/92/$05.00
0 1992 Elsevier Science Publishers B.V. All rights reserved.
488
relative stability, the dipole moment and other monoelectronic properties and the nature and position of the S. - H hydrogen bond remain to be clarified. In this paper we report the ab initio computation using a large basis set (6-31G**) of the structure, relative stability and electronic properties of both the anti and gauche conformers of ethane-1,2dithiol. The third-order Mraller-Plesset (MP3)treatment was used in determining the correlation energy. l
COMPUTATIONAL DETAILS
The structure of both the anti and the gauche conformer was completely optimized by using gradient methods at the 6-31G** level [ 171 employing the GAUSSIAN at3package [ 181 on the CRAY-YMP computer of CINECA. For the anti conformer Cab symmetry was imposed, while the gauche form was optimized without constraint. A single-point calculation Meller-Plesset treatment of the correlation energy [ 171 truncated at third order (MP3) was done on the Hartree-Fock (HF) optimized structures. RESULTS AND DISCUSSION
The two most stable conformers of ethane-1,2dithiol the geometrical parameters are shown below.
and the definitions of
H-1
Anti
Gauche
The 6-31G** optimized structures of the anti and gauche conformers are reported in Table 1 together with the most recent experimental and theoretical results. It can be seen from Table 1 that the results obtained in the present study are in generally good agreement with the results of ED [ 141 and MW [ 111 studies. The small discrepancy between our values for the bond lengths and valence angles and those obtained from MW and ED studies can be accounted for by the fact that in the experimental measurements many geometrical parameters (see Table 1) are assumed to be equivalent. Comparison of the present results with ab initio 4-31G results [ 161 reveals that the C* *S and S- * *H bond lengths calculated with the 4-31G basis set are overestimated and that the C* **C and Cm- lH values are underestimated. This is essentially due to the fact that no polarization functions were added in the latter computation. With l
487 TABLE 1 Optimized 6-31G** geometrical parameters of the gauche end anti (in parentheses) conformers of ethane-1,2-dithiol Parameter
Method ED”
6-31G** Bond length Cl-C2 Cl-S1 c2-s2 Sl-Hl S2-H2 Cl-H3 Cl-H4 C2-H5 C2-H6
MWb
4-31G”
(li,
Bond angle (deg.) CB-Cl-S1 Cl-c2-s2 Cl-Sl-Hl C2-S2-H2 C2-Cl-H3 C2-Cl-H4 Cl-C2-H5 Cl-C2-H6
1.527 1.828 1.821 1.326 1.326 1.083 1.081 1.083 1.083
(1.525) (1.827)
1.534
1.538
1.512
1.820
1.821
1.894
1.338
1.340
1.358
1.103
1.097
1.077
(1.327) (1.082)
110.9 (110.9) 115.6 97.9 (97.6) 98.4 108.9 (110.2) 110.1 108.6 110.9
113.1
112.5
110.0
94.0
96.5
97.6
111.1
110.7
108.9
69.0 - 141.0 -40.0
64.8 - 162.9 -43.4
103.0 - 177.2 - 60.0
Dihedral angk (deg.) ;
e dd (A)
74.9 (0.0) - 166.8 (180.0) -64.2 (180.0) 3.040
2.7
*From ref. 14. bFrom ref. 11. “From ref. 16. the Sl*--H2 hydrogen bond distance.
regard to the torsional angle ly, we found a value of 74.9” while the MW and ED values are 64.8” and 69.0”, respectively. The two CCSH dihedral angles # and 19are - 166.8” and - 64.2’ as measured as a counterclockwise rotation of the S***H bonds relative to an eclipsed arrangement of these bonds at a cy ( SCCS ) torsional angle of 0 ‘. Our value of @ is slightly higher than the ED value (# = - 141.0 ’ ), but is in good agreement with MW measurement (# = - 162.9’ ) . The present value of
488
angle 8 is about 20” higher than the corresponding MW and ED results, but is in agreement with the 4-31G value. The rotational constants, the dipole moment and relative energy between the two conformers are reported in Table 2. The 6-31G** values for the rotational constants and dipole moments are in agreement with the experimental values [ 11,121, note in particular that our computed dipole moment is 2.068 D while the experimental value [ 111 is 2.031 D. In the present study it was found (at the HF level) that the anti form is 0.36 kJ mol-’ more stable than the gauche one. With the introduction of correlation effects, the gauche conformer was found to be the most stable. In particular, at the MP2 level the energy difference is 2.27 kJ mol-’ and at the MP3 level this difference decreases to 1.71 kJ mol-l. Both conformers are observed experimentally and the relative stability of the conformers cannot be unambiguously resolved. The ED radial distribution function shows the presence of substantial amounts of both anti and gauche heavy atom conformations with the anti form being the dominant conformer (about 60%) [ 13,141. The anti conformer is also observed in the IR spectrum [ 151. However, MW experiments indicate that the gauche form is the most stable one although the anti conformer is present. Our theoretical values confirm the very small energy TABLE 2 Spectroscopic, electronic and energetic parameters of ethane-1,2-dithiol from 6-31G** and MP3 computations (the values in parentheses are for the anti conformer) Parameter
Method 6-31G**
Rotational constants (MHz) A 9826.6 (25573.3) B 2127.5 (1515.6) C 1873.8 (1456.5) Dipole moments (0) Px & k ,&4x
1.196 1.606 0.517 2.068
ED”
Mwb
9289.6 2233.9 1935.9
9292.5 2239.2 1935.6
0.650 1.650 0.660 1.892
Relative energy, AE (gauche-anti) (kJ molV1) HF 0.36 MP2 - 2.27 -1.71 MP3 Erp. 1.71 “From ref. 14. bFrom ref. 11.
0.774 1.749 0.683 2.031
difference between the two conformers. Different experimental conditions could favour one of the two conformers. The coexistence of two conformers has been also postulated in the case of ethylene glycol [9,10]. Finally, the present study confirms the presence of an effective S---H hydrogen bond in the gauche form, as indicated previously [ 141. In our structure the distance of H2 from the acceptor atom Sl (d) is 3.02 A, which is only 0.1 A less than the sum of the van der Waals radii [ 191. In this situation, the SH- H hydrogen bond is quite weak. However, a smaller distance d increases the repulsive interactions of non-bonding electrons which destabilizes the gauche form. This destabilization is not compensated for by the consequently increased hydrogen bond force. In the experimental ED structure [ 141 the angles $ and 8 are 20” lest than the corresponding theoretical values and the distance d is shorter (2.70 A). This means that in this structure the non-bonded interactions are higher than in the present case and the gauche structure is slightly destabilized relative to the anti one. l
l
CONCLUSIONS
On the basis of our study the following conclusions can be drawn. (i) The HF/6-31G** optimization gives geometrical parameters that are in agreement with the experimentally determined values. (ii) The presence of a weak hydrogen bond formation in the gauche isomer is confirmed. (iii) The stability of the anti and gauche conformers of ethane-1,2dithiol is due to the delicate balance between the hydrogen bond formation and the nonbonded repulsive interactions in the gauche conformer. Because of the small energy difference between the two conformers, different experimental conditions or levels of theoretical treatment can give rise to apparently contradictory results. In order to draw a more definite conclusion about the absolute minimum energy, a full optimization at the HF/MP level is required in future computations. ACKNOWLEDGMENTS
We gratefully acknowledge research support from CNR and MURST. One of us (E.S.) is indebted to the Costituendo Istituto “Membrane e Reattori Chimici” de1 CNR for a fellowship. REFERENCES 1 2
K.M. Marstokk and H.J. Mollendal, J. Mol. Struct., 49 (1978) 221. W. Caminati and G. Corbelli, J. Mol. Spectrosc., 90 (1981) 527.
490 3
8 9 10 11 12 13 14 15 16 17 18
19
C. Van AIsenoy, L. Van Den Enden and L. Schafer, J. Mol. Struct. (Theochem), 108 (1984) 121. G. Chidichimo, D. ImbardeIIi, M. Longeri and A. Saupe, Mol. Phys., 65 (1988) 1143. K. Hagen and K. Hedberg, J. Am. Chem. Sot., 95 (1973) 8263. E.M. Sung and M.D. Harmony, J. Am. Chem. Sot., 99 (1977) 5603. A. AIgemenningen, D. Bastiansen, L. Femholt and K. Hedberg, Acta Chem. Stand., 25 (1971) 1946. R.N. Nandi, M.F. BoIand and M.D. Harmony, J. Mol. Spectrosc., 92 (1982) 419. H. Frei, H.A. Tae-Kyu, R. Meyer and H.H. Gunthard, Chem. Phys., 25 (1977) 271. B.J. Costa CabraI, L.M.P.C. Alberquerque and F.M.S. SiIva Femandes, Theor. Chim. Acta, 78 (1991) 271. R.N. Nandi, CF. Su andM.D. Harmony, J. Chem. Phys., 81 (1984) 1051. G. Schultz and I. Hargittai, Acta Chim. Acad. Sci. Hung., 75 (1973) 381. I. Hargittai and G. Schultz, J. Chem. Phys., 84 (1986) 5220. S.L. Barkowski, L. Hedberg and K. Hedberg, J. Am. Chem. Sot., 108 (1986) 6898. M. Hayashi, Y. Shiro, T. Oshima and H. Mu&a. Bull. Chem. Sot. Jpn., 38 (1965) 1734. T. Shida and T. Momose, J. Mol. Struct., 126 (1985) 159. W.J. Hehre, L. Radom, P.v.R. Schleyer and J.A. Pople, Ab initio Molecular Orbital Theory, Wiley, New York, 1986. M.J. Frisch, M. Head-Gordon, B.B. SchIegel, K. Raghavachari, J.S. BinkIey, C. Gonzales, D.J. DeFrees, D.J. Fox, R.A. Whiteside, R. Seeger, C.F. Melius, J. Baker, R.L. Martin, L.R. Kahn, J.P. Stewart, E.M. Fhider, S. Topiol and J.A. Pople, GAUSSIANEXJ, Gaussian Inc., Pittsburg, PA, 1988. L. Pauling, The Nature of Chemical Bond, 3rd edn., Cornell University Press, Ithaca, NY, 1960.