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MNDO study of the cyclopentathiazenium cation S5N5+
Journal of Molecular Structure (Theochem), 109 (1984) 367-371 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
MNDO STUDY OF THE CYCLOPENTATHIAZENIUM
CATION L&N;
WING-KWONG IP and WAI-KEE LI* Department of Chemistry, (How Kong)
The Chinese University
of Hong Kong, Shatin, N.T.
(Received 23 February 1984)
ABSTRACT An MNDO study has been carried out for the cyclopentathiazenium cation S,N& TWO stable structures, one heart-shaped and the other azulene-like, have been found on the potential energy surface. These structures are of comparable stability and they have dimensions similar to those determined by X-ray diffraction. The energy barrier for the interconversion between the two structures is low, about 8 kcal mol-‘. This leads to the conclusion that packing forces are responsible for determining which structure is predominant in a given crystal. The transition state for the pathway is of Dsh symmetry and the pathway maintains C,, symmetry throughout. INTRODUCTION
For the cyclopentathiazenium cation, S,N:, Banister and co-workers [ 1, 21 obtained two stable geometries using the X-ray diffraction technique. Both of the structures are nearly planar, as expected for a 14 n-electron system. One of the structures is heart-shaped with a sulfur atom located at the tip of the heart, while the other one is azulene-like. Comparing the bond angles and bond lengths of these two structures with those in other nitrogensulfur compounds, it is found that the azulene-like structure is less strained [ 3, 41. Based on their extended Hiickel molecular orbital calculation results, Bartetzko and Gleiter [5] suggested that S5Ng does not have two isomeric structures. Rather, superposition of two azulene-like structures may lead to the heart-shaped structure [ 61, as shown in Fig. 1. In this work, the MNDO model is employed to determine whether there exist two stable structures for this cation. Furthermore, if two structures are indeed found, a reaction coordinate calculation is carried out to study the pathway connecting these two structures.
*Author for correspondence. 0166-1280/84/$03.00
0 1984 Elsevier Science Publishers B.V.
368
Fig. 1. Diagram indicating to a heart-shaped one.
how the superposition
of two azulene-like
structures may lead
METHOD AND RESULTS
As mentioned above, the molecular orbital method chosen for the present study is the MNDO procedure developed by Dewar and Thiel [7]. For both structures, CZV symmetry was assumed and all structural parameters were allowed to vary until optimized geometries were reached. The calculated molecular dimensions are summarized in Fig. 2; the experimental values for the bond lengths and valence angles are enclosed in parentheses for ready comparison. To study the reaction path linking the two structures, a reaction coordinate calculation was carried out. The reaction coordinate d of the rearrangement is defined by the separation between the nitrogen atom N, and the dummy atom Dz, as shown in Fig. 2. Throughout this reaction pathway C, symmetry was assumed, with the symmetry plane containing atoms N1, DZ and S3. The separation is taken to be positive when N1 and S3 are on the opposite sides and is negative when they are on the same side. Thus, with reference to Fig. 2, the reaction coordinate equals 0.54 A and -0.70 A for the azulene-like and heart-shaped structures, respectively. The energy profile of the transformation between the two structures are shown in Fig. 3. The change in electronic distribution throughout the pathway is summarized in Fig. 4. DISCUSSION
According to the MNDO results, two stable structures have been found for E&N:, in agreement with the results of Banister et al. In addition, the two isomers are of comparable stability: the heats of formation for the heartshaped and the azulene-like structures are 434.3 and 434.7 kcal mol-I, respectively. As shown in Fig. 2, the MNDO structural parameters are quite close to the X-ray data.
369
s3
Fig. 2. The MNDO and experimental (in square brackets) structures for the heart-shaped and azulene-like cations of S,N:. Bond lengths am in A.
Referring to the MNDO pathway connecting the two structures, it is found that CzV symmetry is preserved throughout, even though only C, symmetry was imposed in the calculation. In the region 0.08 R < d < 0.54 A (azulene-like structure), energy rises smoothly to reach the transition state with energy 443.0 kcal mol-‘, yielding an activation energy of about 8 kcal mol-‘. The transition state has DSh symmetry, with all S-N bond lengths being 1.51 A and S-N-S angles 174”. It is obvious that as d decreases, atoms N6 and N, move away from the center of the ring until a structure with DSh symmetry results. It is noted that, at the transition state, all the bonds may be regarded as “typical” S=N bonds [8]. However, it is difficult to come up with a reasonable valence structure for the transition state. Nevertheless, the high energy state for the Dsh structure may be due to the strain around the N-S-N bond angles [ 3, 41. The charge flow in this region of the pathway has a general trend in that considerable electronic charge passes from sulfurs to the nitrogens. In total, 0.94e is transferred to the nitrogen atoms. As a consequence, the shortening of the bonds may be explained by the increase in bond polarities rather than multiple bond formation.
370
Fig. 3. The energy profile for the rearrangement of S,N: connecting the heart-shaped and azulene-like structures. Fig. 4. The atomic charge distribution along the rearrangement pathway of S,Nl.
For the region between the transition state and the heart-shaped structure, energy drops steadily to a minimum of 434.3 kcal mol-’ for the product. Most of the bond lengths increase in this region except the bond between S3 and N1,,. The bond between N1 and S4 has the greatest increase. This may be due to the greatest decrease in bond strength which in turn is a result of the decrease in bond polarity, as suggested by their atomic charges, shown in Fig. 4. This is also applicable to the shortening of the S3-N10 bond since the bond polarity is enhanced. The charge flow compared to the region before the transition state is in the opposite direction. This may be a response to the release of bond strains formerly established. It is noted that the charges on NIO and S6 are rather abnormal compared to the others. The loss in electronic charge on S6 may be due to the two attached “sp hybridized” nitrogens, NIO and Nh. The fact that the Nlo and Nn centers retain their charges in this region may be due to their unchanged environments. As a conclusion, with the low energy barrier found for the transformation between the two structures, it is not surprising for them to coexist. Crystal forces may be responsible for determining which structure is predominant. Finally, it is noted that Gleiter and Bartetzko [9] have apparently mistaken the Dsh structure to be a local minimum, while it is actually only a stationary point.
371 REFERENCES 1 A. J. Banister and H. G. Clarke, J. Chem. Sot., Dalton Trans., (1972) 2661. 2 A. J. Banister, J. A. Durant, I. Rayment and H. M. M. Shearer, J. Chem. Sot., Dalton Trans., (1976) 928. 3 A. J. Banister and J. A. Durant, J. Chem. Res. (S), (1978) 150. 4 A. J. Banister and J. A. Durant, J. Chem. Res. (S), (1978) 152. 5 R. Bartetzko and R. Gleiter, Inorg. Chem., 17 (1978) 995. 6 R. Gleiter, Angew. Chem. Int. Ed. Engl., 20 (1981) 444. 7 M. J. S. Dewar and W. Thiel, J. Am. Chem. Sot., 99 (1977) 4899. 8 N. C. Nyburg, J. Cryst. Mol. Struct., 3 (1973) 331. 9 R. Gleiter and R. Bartetzko, Z. Naturforsch., Teii B, 36 (1981) 956.
MNDO STUDY OF THE CYCLOPENTATHIAZENIUM
CATION L&N;
WING-KWONG IP and WAI-KEE LI* Department of Chemistry, (How Kong)
The Chinese University
of Hong Kong, Shatin, N.T.
(Received 23 February 1984)
ABSTRACT An MNDO study has been carried out for the cyclopentathiazenium cation S,N& TWO stable structures, one heart-shaped and the other azulene-like, have been found on the potential energy surface. These structures are of comparable stability and they have dimensions similar to those determined by X-ray diffraction. The energy barrier for the interconversion between the two structures is low, about 8 kcal mol-‘. This leads to the conclusion that packing forces are responsible for determining which structure is predominant in a given crystal. The transition state for the pathway is of Dsh symmetry and the pathway maintains C,, symmetry throughout. INTRODUCTION
For the cyclopentathiazenium cation, S,N:, Banister and co-workers [ 1, 21 obtained two stable geometries using the X-ray diffraction technique. Both of the structures are nearly planar, as expected for a 14 n-electron system. One of the structures is heart-shaped with a sulfur atom located at the tip of the heart, while the other one is azulene-like. Comparing the bond angles and bond lengths of these two structures with those in other nitrogensulfur compounds, it is found that the azulene-like structure is less strained [ 3, 41. Based on their extended Hiickel molecular orbital calculation results, Bartetzko and Gleiter [5] suggested that S5Ng does not have two isomeric structures. Rather, superposition of two azulene-like structures may lead to the heart-shaped structure [ 61, as shown in Fig. 1. In this work, the MNDO model is employed to determine whether there exist two stable structures for this cation. Furthermore, if two structures are indeed found, a reaction coordinate calculation is carried out to study the pathway connecting these two structures.
*Author for correspondence. 0166-1280/84/$03.00
0 1984 Elsevier Science Publishers B.V.
368
Fig. 1. Diagram indicating to a heart-shaped one.
how the superposition
of two azulene-like
structures may lead
METHOD AND RESULTS
As mentioned above, the molecular orbital method chosen for the present study is the MNDO procedure developed by Dewar and Thiel [7]. For both structures, CZV symmetry was assumed and all structural parameters were allowed to vary until optimized geometries were reached. The calculated molecular dimensions are summarized in Fig. 2; the experimental values for the bond lengths and valence angles are enclosed in parentheses for ready comparison. To study the reaction path linking the two structures, a reaction coordinate calculation was carried out. The reaction coordinate d of the rearrangement is defined by the separation between the nitrogen atom N, and the dummy atom Dz, as shown in Fig. 2. Throughout this reaction pathway C, symmetry was assumed, with the symmetry plane containing atoms N1, DZ and S3. The separation is taken to be positive when N1 and S3 are on the opposite sides and is negative when they are on the same side. Thus, with reference to Fig. 2, the reaction coordinate equals 0.54 A and -0.70 A for the azulene-like and heart-shaped structures, respectively. The energy profile of the transformation between the two structures are shown in Fig. 3. The change in electronic distribution throughout the pathway is summarized in Fig. 4. DISCUSSION
According to the MNDO results, two stable structures have been found for E&N:, in agreement with the results of Banister et al. In addition, the two isomers are of comparable stability: the heats of formation for the heartshaped and the azulene-like structures are 434.3 and 434.7 kcal mol-I, respectively. As shown in Fig. 2, the MNDO structural parameters are quite close to the X-ray data.
369
s3
Fig. 2. The MNDO and experimental (in square brackets) structures for the heart-shaped and azulene-like cations of S,N:. Bond lengths am in A.
Referring to the MNDO pathway connecting the two structures, it is found that CzV symmetry is preserved throughout, even though only C, symmetry was imposed in the calculation. In the region 0.08 R < d < 0.54 A (azulene-like structure), energy rises smoothly to reach the transition state with energy 443.0 kcal mol-‘, yielding an activation energy of about 8 kcal mol-‘. The transition state has DSh symmetry, with all S-N bond lengths being 1.51 A and S-N-S angles 174”. It is obvious that as d decreases, atoms N6 and N, move away from the center of the ring until a structure with DSh symmetry results. It is noted that, at the transition state, all the bonds may be regarded as “typical” S=N bonds [8]. However, it is difficult to come up with a reasonable valence structure for the transition state. Nevertheless, the high energy state for the Dsh structure may be due to the strain around the N-S-N bond angles [ 3, 41. The charge flow in this region of the pathway has a general trend in that considerable electronic charge passes from sulfurs to the nitrogens. In total, 0.94e is transferred to the nitrogen atoms. As a consequence, the shortening of the bonds may be explained by the increase in bond polarities rather than multiple bond formation.
370
Fig. 3. The energy profile for the rearrangement of S,N: connecting the heart-shaped and azulene-like structures. Fig. 4. The atomic charge distribution along the rearrangement pathway of S,Nl.
For the region between the transition state and the heart-shaped structure, energy drops steadily to a minimum of 434.3 kcal mol-’ for the product. Most of the bond lengths increase in this region except the bond between S3 and N1,,. The bond between N1 and S4 has the greatest increase. This may be due to the greatest decrease in bond strength which in turn is a result of the decrease in bond polarity, as suggested by their atomic charges, shown in Fig. 4. This is also applicable to the shortening of the S3-N10 bond since the bond polarity is enhanced. The charge flow compared to the region before the transition state is in the opposite direction. This may be a response to the release of bond strains formerly established. It is noted that the charges on NIO and S6 are rather abnormal compared to the others. The loss in electronic charge on S6 may be due to the two attached “sp hybridized” nitrogens, NIO and Nh. The fact that the Nlo and Nn centers retain their charges in this region may be due to their unchanged environments. As a conclusion, with the low energy barrier found for the transformation between the two structures, it is not surprising for them to coexist. Crystal forces may be responsible for determining which structure is predominant. Finally, it is noted that Gleiter and Bartetzko [9] have apparently mistaken the Dsh structure to be a local minimum, while it is actually only a stationary point.
371 REFERENCES 1 A. J. Banister and H. G. Clarke, J. Chem. Sot., Dalton Trans., (1972) 2661. 2 A. J. Banister, J. A. Durant, I. Rayment and H. M. M. Shearer, J. Chem. Sot., Dalton Trans., (1976) 928. 3 A. J. Banister and J. A. Durant, J. Chem. Res. (S), (1978) 150. 4 A. J. Banister and J. A. Durant, J. Chem. Res. (S), (1978) 152. 5 R. Bartetzko and R. Gleiter, Inorg. Chem., 17 (1978) 995. 6 R. Gleiter, Angew. Chem. Int. Ed. Engl., 20 (1981) 444. 7 M. J. S. Dewar and W. Thiel, J. Am. Chem. Sot., 99 (1977) 4899. 8 N. C. Nyburg, J. Cryst. Mol. Struct., 3 (1973) 331. 9 R. Gleiter and R. Bartetzko, Z. Naturforsch., Teii B, 36 (1981) 956.