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AB initio study of the conformation of 1,4-dithiin
Jourrzal of %‘olecuh?-
Structure,
85 (1981)
159~-162
THEOCHEM Elsevier
Scientific
AB INITIO
Publishing
STUDY
Company,
Amsterdam
-
Printed
OF THE CONFORMATION
in The Netherlands
OF 1,4-DITHIIN
V. GALASSO Istituto
di Chimica,
(Received
8 October
Universitti
di Trieste,
I-34127
Trieste
(Italy
1980)
ABSTRACT A full geometry optimization at the ab initio STO--3G level shows that 1,4-dithiin, as an isolated molecule in the gaseous state, exists preferably in a fixed boat conformation, in agreement with the structural model inferred from X-ray diffraction analysis. The theoretical, relatively high energy barrier to planarity could not be reconciled with the hypothesis of an easy boat-to-boat inversion. INTRODUCTION
There is still uncertainty about the conformation of 1,4-dithiin despite several investigations using different techniques. According to an X-ray
diffraction study [ 11, in the crystal the 1,4-dithiin ring is non-planar with a dihedral angle 0 of ca. 137” between the planes containing the H,CSSCH, and H,CSSCH, atoms, but from determinations of NMR proton coupling constants in a nematic phase [2] it was not possible to choose between a boat, planar, or rapidly inverting structure. A recent UV-PES investigation [ 31 has provided no further elucidation of this problem and, unfortunately, far-IR and Raman spectra, which could give quantitative information about deformation of the ring, have not been obtained. Bearing in mind the chemical [ 41 and biological [ 51 importance of 1,4-dithiin and also the fact that ring inversion or ‘butterfly flapping’ in 1,4-dithiins is of current interest in the field of the stereochemistry of dicoordinated sulphur [ 61, there is an urgent need for an ab initio study of its conformation. A full geometry optimization of 1,4-dithiin was the aim of the present work. COMPUTATIONAL
DETAILS
The full geometry optimization of 1,4-dithiin was performed with ab initio SCF-MO computations of the restricted HF type at a minimum 0166-1280/81/0000-0000/$02.75
0 1981
Elsevier
Scientific
Publishing
Company
160
basis set level (STO-3G) using the GAUSSIAN-70 series of programs [ 71. The potential function governing the ‘butterfly flapping’ motion of the molecule was calculated by keeping all bond lengths and valence angles (apart from angles CSC) constant and equal to the optimized values for the equilibrium conformation and computing the total energy for various values of the dihedral angle f3. Taking then the potential energy as zero at the planar conformation, the potential function may be expressed as V(a) = A@ * + IN4 where @ = (n - 0 )/2 is the coordinate of the ‘butterfly flapping’ motion, and the coefficients A and B are obtained by a best-fit procedure of the theoretical data. RESULTS
AND
DISCUSSION
The ab initio full geometry optimization leads to the structural parameters reported in Table 1 together with the X-ray experimental values; taking into account the limited basis set (STO-3G) employed in the calculation and the experimental uncertainties, the overall agreement between theoretical and experimental values is good. For 1,4-dithiin, in the gaseous state, the energetically more favourable conformation predicted by the theory is, therefore, a non-planar boat-like conformation (E,,, = -938.24763 a.u.) with a dihedral angle of 131”, which is somewhat lower than the ca. 137” found in the crystal [l] . This finding confirms previous theoretical conclusions based on HMO [8] and EHT [9] investigations restricted to the folding motion only; incidentally, note that a MIND0/3 full geometry optimization performed in the present study lead instead to a flattened structure. The equilibrium structure of 1,4-dithiin, a compound not obeying the Hiickel (4n t- 2) rule, is clearly derived from a compromise between two opposing factors: the n-interaction between the two ethylenic moieties and the chalcogen atoms, which is a maximum in the planar arrangement, and the strains about the chalcogens which force the ring to deviate from planarity. According to the present results, for 1,4-dithiin the balance of the two competing steric and conjugative effects is reached at the boat rather than the TABLE
1
Experimental
and theoretical
structural
parameters
of 1,4-dithiin
Theory 1.781 1.319 1.084
c-x c=c C-H
ccs csc CCH Dihedral
angle
123.18” 99.29” 121.29” 131.14”
Experimental 1.78 t 0.05 1.29 k 0.05
A A A
124.5 100.2
_
137
t 2” ? 2”
! 2”
[ 11 A A
161
J
E $30yo
20 -
10.
O-
-10
-
I
Fig.
I
100
1. Potential
140
180
220
Dihedral energy
function
iygle
for the ‘butterfly
flapping’
motion
of 1,4-dithiin.
planar conformation; this behaviour may be ascribed to strong strains at the sulphur atoms caused by their diffuse lone-pairs. In this regard, it is worth mentioning the different conformational situation of 1,4-dithiin relative to its congener system 1,4-dioxin, whose planarity has been assessed both on experimental [lo] and theoretical [ 111 grounds and most likely derives from the relatively large angle COC (116”) which causes release of sufficient strain energy to balance the conjugative and steric factors in the planar arrangement. The potential energy function for the ‘butterfly flapping’ motion of 1,4-dithiin, depicted in Fig. 1, is calculated to be V(a)
= -154.64+’
+ 407.57Q4
where V is in kcal mol-’ and ct, in radians. The barrier height to ring inversion is 14.6 kcal mall’, remarkably higher than the value of 6.4 kcal mol-’ calculated by Kreevoy [8] with a simple HMO approach. However, while the conclusion of the most favourable non-planar conformation of 1,4dithiin appears to be quite reliable, the use of the minimal basis set STO-3G does not permit the estimate of the barrier to inversion to be regarded as a reliable quantitative prediction. Therefore, a definitive speculation about the possible boat-to-boat inversion demands further information both from experimental evidence and more sophisticated calculations, which are beyond the scope of this work. At any rate, the relatively high energy barrier found in the present calculations would seem to suggest that 1,4-dithiin is not a flexible molecule but exists in a fixed boat conformation, even in the gaseous state. The pattern of net atomic charges calculated for the theoretical equilibrium conformation of 1,4-dithiin, + 0.186 (S), -0.165 (C), and + 0.072 (II), is
consistent with its chemical reactivity [4], and the theoretical dipole moment of 0.93 D compares well with the experimental value of 1.16 D [ 121. ACKNOWLEDGEMENT
This work
was supported
by the C.N.R.
of Italy.
REFERENCES 1 P. A. Howell, R. M. Curtis and W. N. Lipscomb, Acta Crystallogr., 7 (1954) 489. 2 R. C. Long, Jr. and J. H. Goldstein, J. Mol. Spectrosc., 40 (1971) 632; J. Russell, Org. Magn. Reson., 4 (1972) 433. 3 F. P. Colonna, G. Distefano and V. Galasso, J. Electron Spectrosc. Relat. Phenom., 18 (1980) 75. 4 D. S. Breslow and H. Skolnik, Multi-sulfur and Sulfur and Oxygen Five- and Sixmembered Heterocycles, Part II, Interscience, New York, 1963, Ch. 12, and refs. therein. 5 L. L. Gershbein, Res. Commun. Chem. Pathol. Pharmacol., 11 (1975) 445. 6 P. H. Laur, in A. Senning (Ed.), Sulfur in Organic and Inorganic Chemistry, Vol. 3, Dekker, New York, 1972. 7 W. J. Hehre, W. A. Lathan, M. D. Newton and J. A. Pople, Program No. 236,Q.C.P.E., Indiana University, Bloomington, IN. 8 M. M. Kreevoy, J. Am. Chem. Sot., 80 (1958) 5543. 9 N. K. Ray and P. T. Narashimhan, J. Mol. Struct., 1 (1968) 489. 10 J. Y. Beach, J. Chem. Phys., 9 (1941) 54;J. E.Connett, J. A. Creighton, J. H.S. Green and W. Kynaston, Spectrochim. Acta, 22 (1966) 1859. 11 B. Tinland and C. Decoret, J. Mol. Struct., 9 (1971) 205. 12 D. S. Sappenfield, Ph.D. Thesis, University of Minnesota (1962), as quoted in D. S. Sappenfield and M. M. Kreevoy, Tetrahedron, 19, Suppl. 2 (1963) 157.
Structure,
85 (1981)
159~-162
THEOCHEM Elsevier
Scientific
AB INITIO
Publishing
STUDY
Company,
Amsterdam
-
Printed
OF THE CONFORMATION
in The Netherlands
OF 1,4-DITHIIN
V. GALASSO Istituto
di Chimica,
(Received
8 October
Universitti
di Trieste,
I-34127
Trieste
(Italy
1980)
ABSTRACT A full geometry optimization at the ab initio STO--3G level shows that 1,4-dithiin, as an isolated molecule in the gaseous state, exists preferably in a fixed boat conformation, in agreement with the structural model inferred from X-ray diffraction analysis. The theoretical, relatively high energy barrier to planarity could not be reconciled with the hypothesis of an easy boat-to-boat inversion. INTRODUCTION
There is still uncertainty about the conformation of 1,4-dithiin despite several investigations using different techniques. According to an X-ray
diffraction study [ 11, in the crystal the 1,4-dithiin ring is non-planar with a dihedral angle 0 of ca. 137” between the planes containing the H,CSSCH, and H,CSSCH, atoms, but from determinations of NMR proton coupling constants in a nematic phase [2] it was not possible to choose between a boat, planar, or rapidly inverting structure. A recent UV-PES investigation [ 31 has provided no further elucidation of this problem and, unfortunately, far-IR and Raman spectra, which could give quantitative information about deformation of the ring, have not been obtained. Bearing in mind the chemical [ 41 and biological [ 51 importance of 1,4-dithiin and also the fact that ring inversion or ‘butterfly flapping’ in 1,4-dithiins is of current interest in the field of the stereochemistry of dicoordinated sulphur [ 61, there is an urgent need for an ab initio study of its conformation. A full geometry optimization of 1,4-dithiin was the aim of the present work. COMPUTATIONAL
DETAILS
The full geometry optimization of 1,4-dithiin was performed with ab initio SCF-MO computations of the restricted HF type at a minimum 0166-1280/81/0000-0000/$02.75
0 1981
Elsevier
Scientific
Publishing
Company
160
basis set level (STO-3G) using the GAUSSIAN-70 series of programs [ 71. The potential function governing the ‘butterfly flapping’ motion of the molecule was calculated by keeping all bond lengths and valence angles (apart from angles CSC) constant and equal to the optimized values for the equilibrium conformation and computing the total energy for various values of the dihedral angle f3. Taking then the potential energy as zero at the planar conformation, the potential function may be expressed as V(a) = A@ * + IN4 where @ = (n - 0 )/2 is the coordinate of the ‘butterfly flapping’ motion, and the coefficients A and B are obtained by a best-fit procedure of the theoretical data. RESULTS
AND
DISCUSSION
The ab initio full geometry optimization leads to the structural parameters reported in Table 1 together with the X-ray experimental values; taking into account the limited basis set (STO-3G) employed in the calculation and the experimental uncertainties, the overall agreement between theoretical and experimental values is good. For 1,4-dithiin, in the gaseous state, the energetically more favourable conformation predicted by the theory is, therefore, a non-planar boat-like conformation (E,,, = -938.24763 a.u.) with a dihedral angle of 131”, which is somewhat lower than the ca. 137” found in the crystal [l] . This finding confirms previous theoretical conclusions based on HMO [8] and EHT [9] investigations restricted to the folding motion only; incidentally, note that a MIND0/3 full geometry optimization performed in the present study lead instead to a flattened structure. The equilibrium structure of 1,4-dithiin, a compound not obeying the Hiickel (4n t- 2) rule, is clearly derived from a compromise between two opposing factors: the n-interaction between the two ethylenic moieties and the chalcogen atoms, which is a maximum in the planar arrangement, and the strains about the chalcogens which force the ring to deviate from planarity. According to the present results, for 1,4-dithiin the balance of the two competing steric and conjugative effects is reached at the boat rather than the TABLE
1
Experimental
and theoretical
structural
parameters
of 1,4-dithiin
Theory 1.781 1.319 1.084
c-x c=c C-H
ccs csc CCH Dihedral
angle
123.18” 99.29” 121.29” 131.14”
Experimental 1.78 t 0.05 1.29 k 0.05
A A A
124.5 100.2
_
137
t 2” ? 2”
! 2”
[ 11 A A
161
J
E $30yo
20 -
10.
O-
-10
-
I
Fig.
I
100
1. Potential
140
180
220
Dihedral energy
function
iygle
for the ‘butterfly
flapping’
motion
of 1,4-dithiin.
planar conformation; this behaviour may be ascribed to strong strains at the sulphur atoms caused by their diffuse lone-pairs. In this regard, it is worth mentioning the different conformational situation of 1,4-dithiin relative to its congener system 1,4-dioxin, whose planarity has been assessed both on experimental [lo] and theoretical [ 111 grounds and most likely derives from the relatively large angle COC (116”) which causes release of sufficient strain energy to balance the conjugative and steric factors in the planar arrangement. The potential energy function for the ‘butterfly flapping’ motion of 1,4-dithiin, depicted in Fig. 1, is calculated to be V(a)
= -154.64+’
+ 407.57Q4
where V is in kcal mol-’ and ct, in radians. The barrier height to ring inversion is 14.6 kcal mall’, remarkably higher than the value of 6.4 kcal mol-’ calculated by Kreevoy [8] with a simple HMO approach. However, while the conclusion of the most favourable non-planar conformation of 1,4dithiin appears to be quite reliable, the use of the minimal basis set STO-3G does not permit the estimate of the barrier to inversion to be regarded as a reliable quantitative prediction. Therefore, a definitive speculation about the possible boat-to-boat inversion demands further information both from experimental evidence and more sophisticated calculations, which are beyond the scope of this work. At any rate, the relatively high energy barrier found in the present calculations would seem to suggest that 1,4-dithiin is not a flexible molecule but exists in a fixed boat conformation, even in the gaseous state. The pattern of net atomic charges calculated for the theoretical equilibrium conformation of 1,4-dithiin, + 0.186 (S), -0.165 (C), and + 0.072 (II), is
consistent with its chemical reactivity [4], and the theoretical dipole moment of 0.93 D compares well with the experimental value of 1.16 D [ 121. ACKNOWLEDGEMENT
This work
was supported
by the C.N.R.
of Italy.
REFERENCES 1 P. A. Howell, R. M. Curtis and W. N. Lipscomb, Acta Crystallogr., 7 (1954) 489. 2 R. C. Long, Jr. and J. H. Goldstein, J. Mol. Spectrosc., 40 (1971) 632; J. Russell, Org. Magn. Reson., 4 (1972) 433. 3 F. P. Colonna, G. Distefano and V. Galasso, J. Electron Spectrosc. Relat. Phenom., 18 (1980) 75. 4 D. S. Breslow and H. Skolnik, Multi-sulfur and Sulfur and Oxygen Five- and Sixmembered Heterocycles, Part II, Interscience, New York, 1963, Ch. 12, and refs. therein. 5 L. L. Gershbein, Res. Commun. Chem. Pathol. Pharmacol., 11 (1975) 445. 6 P. H. Laur, in A. Senning (Ed.), Sulfur in Organic and Inorganic Chemistry, Vol. 3, Dekker, New York, 1972. 7 W. J. Hehre, W. A. Lathan, M. D. Newton and J. A. Pople, Program No. 236,Q.C.P.E., Indiana University, Bloomington, IN. 8 M. M. Kreevoy, J. Am. Chem. Sot., 80 (1958) 5543. 9 N. K. Ray and P. T. Narashimhan, J. Mol. Struct., 1 (1968) 489. 10 J. Y. Beach, J. Chem. Phys., 9 (1941) 54;J. E.Connett, J. A. Creighton, J. H.S. Green and W. Kynaston, Spectrochim. Acta, 22 (1966) 1859. 11 B. Tinland and C. Decoret, J. Mol. Struct., 9 (1971) 205. 12 D. S. Sappenfield, Ph.D. Thesis, University of Minnesota (1962), as quoted in D. S. Sappenfield and M. M. Kreevoy, Tetrahedron, 19, Suppl. 2 (1963) 157.