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Quantum chemical calculations for the determination of the molecular structure of conjugated compounds
Journal of Molecular Structure, 51 (1979) 99-105 o Elsevier Scientific Publishing Company, Amsterdam -Printed
in The Netherlands
QUANTUM CHEMICAL CALCULATIONS FOR THE DETERMINATION OF TI-IE MOLECULAR STRUCTURE OF CONJUGATED COMPOUNDS* Part XIV. On the conformational structure of CX-diimineligands
ROLAND BENEDIX, PETER BIRNER, FRIEDER BIRNSTOCK and HORST HENNIG Sektion
Chemie,
HANS-JtjRG Sektion
Karl-Marx-University,
Leipzig
(G.D.R.)
HOFMANN**
Biowissenschaften,
Karl-Marx-University,
Leipzig
(G.D.R.)
(Received 5 May 1978)
ABSTRACT The conformational structure of some typical a-diimine ligands (2,2’-bipyridyl, glyoxaldiimine, glyoxal-N,N’-dimethyldiimine) is examined using the NDDO method. An analysis of the potential energy curves indicates the influence of specific lone-pair effects on the molecular structure. Based on a continuum model, changes of the molecular arrangement caused by solvent effects are estimated. INTRODUCTION
2,2’-bipyridyl, 1, is one of the most important representatives of a-diimine ligands in transition metal compounds. Its excellent ability to form complexes has been well known for many years and therefore it has been the subject of numerous investigations [l] . The simplest representatives of this group of ligands, glyoxaldiimine, 2, and its derivatives, have only comparatively recently been examined in detail [2-41.
c&-f3
H_Nd-H 1
2
*Part XIII. H.-J. Hofmann and F. Birnstock, J. Mol. Struct., 44 (1978) 231. **To whom correspondence should be addressed.
100
Using X-ray diffraction [5] and various indirect methods [6-111, respectively, a planar or approximately planar s-truns orientation was determined for 2,2’-bipyridyl both in the solid state and in solution. Unfortunately, the electron diffraction data for this compound are not very conclusive [12,13]. Apart from a maximum at the planar s-cis conformation, the rather flat potential curve does not show any well defined minimum but indicates a free rotation within a large range around the central single bond. NMR examinations on glyoxal-N,N’dimethyldiimine, 3, also suggest the transoid orientation of the nitrogen atoms for the simpler a-diimine ligands [ 141. Whereas the transoid configuration of the nitrogen atoms seems to be a characteristic feature of the isolated ligands, a planar or nearly planar cisoid arrangement exists in all complex compounds [ 15,161. For a better understanding of the complex formation process, it may therefore be important to get a more detailed knowledge of the conformational structure of these a-diimine ligands. Above all, exact information about the rotational barrier which must be overcome to reproduce the cisoid arrangement and the energy difference between the s-tram and s-cis forms are desirable. Furthermore, the molecular structure of 2,2’-bipyridyl may be compared with the orientation of the two rings in the iso-electronic compound biphenyl. It is known that the planar arrangement of biphenyl is distorted. The repulsion between the ortho hydrogen atoms enforces a torsion around the essential single bond by about 42” in the gas phase 1121 and 20” in solution [17]. A comparison between biphenyl and 2,2’-bipyridyl could be very useful in clarifying the influence of the destabilizing interactions between the ortho hydrogens and the nitrogen lone-pairs, respectively, on the molecular structure. The inclusion of 4,4’-bipyridyl, 4, may additionally illustrate the relations between these compounds. Torsional angles of about 37” [ 121 and 30” [ 181 have been determined for 4,4’-bipyridyl by means of electron diffraction and NMR techniques, respectively. These values are closely related to the rotational angles of biphenyl measured experimentally. Various quantum chemical methods have been used to gain information on the structure of 2,2’-bipyridyl. The results of n-electron methods connected with the calculation of the energy of the non-bonded interactions confirm the experimental data [ 19,201. EHT calculations show the planar s-tram conformation as the most stable form [21] . A second minimum was determined at 0 = 145”. The EHT torsional angle of about 60” estimated for 4,4’-bipyridyl seems to be somewhat too large and may be caused by an overemphasis of steric repulsions which is well known within this formalism. By contrast the CNDO/B method completely fails to predict the favoured conformations [ 22,231. Furthermore, numerous results support the conclusion that this method is not suited to describe the molecular structure of conjugated compounds correctly and should no longer be used to examine such problems [ 24-261. Moreover, due to the approximations inherent in this formalism,~no correct description of lone-pairs is possible [27-291. Recently, we have shown that the NDDO method is well suited for
101
determination of the conformational structure of conjugated compounds [ 25,30-331. Additionally, this formalism permits an adequate consideration of lone-pairs [ 27,291. Therefore, the NDDO method seems to be predestined to get precise results for this type of compound. To estimate the changes of the molecular structure under the influence of a solvent, we have combined the NDDO energy values with solvation energies calculated by means of a recently suggested continuum model [33,34] . DETAILS OF CALCULATIONS
The bond lengths and angles for 1 and 3 come from X-ray data [5,35]. All CH bond lengths amount to 1.08 A with the exception of the methyl group of 3 (Rcn = 1.09 A). One hydrogen atom of each methyl group in 3 has been assumed to be in an ecliptic position to the CN double bond. The pyridine geometry experimentally determined was used for compound 4 [ 361. The two rings are connected at a distance of 1.50 A in the 4-position. The bond lengths of 3 were also used for compound 2, which is as yet only known as constituent of complex compounds. The NH bond lengths are 1.03 A. The parametrization of the NDDO method and the basis of the continuum model used are described in refs. 29,33 and 34. RESULTS AND DISCUSSION
Figure 1 shows the NDDO potential energy curve for 2,2’ -bipyridyl. In accordance with the experimental results already mentioned, the planar
Fig. 1. NDDO potential energy curves without (-) solvent CCl, for 2,2’-bipyridyl.
and with (- - - -) inclusion of the
102
s-truns arrangement is the most stable form. In comparison with biphenyl, the loss of the steric interactions between the ortho hydrogen atoms in the tram configuration of 1 now enables the planar structure. A second minimum exists at a torsion angle of f3 = 130”. The energy difference and the barrier between the two minima amount to 19.2 and 19.7 kJ mol-‘, respectively. The position of the second minimum may be compared directly with the minimum conformation of biphenyl (NDDO: 8 = 45” = 135” [30]). Obviously, the nitrogen lone pair interaction in the planar cis conformation of 1 causes a similar destabilization to the corresponding interaction between the ortho hydrogen atoms in the planar form of biphenyl. According to the NDDO method, the energy difference between the maximum conformation at about 105” and the second minimum form is very small. Thus, a final valuation of the real importance of the second minimum for the molecular structure would only be possible after performing a complete geometry optimization of all conformations (for a discussion of this problem within semiempirical methods cf. ref. 32). In agreement with the electron diffraction data [ 12,131, the planar s-cis form represents the absolute maximum of the potential curve. The energy difference of 26.8 kJ mol-’ between the planar forms is relatively large. This strong destabilization of the s-cis conformation could be caused by the interaction of the nitrogen lone pairs and the steric hindrance between the ortho hydrogen atoms. Before we perform an analysis of these effects, it may be useful to compare the results obtained for 2,2’-bipyridyl with the potential energy curves of glyoxaldiimine and glyoxal-N,N’dimethyldiimine calculated by the NDDO method (Fig. 2). As in the case of 2,2’-bipyridyl and in accordance with the NMR data, the s-tram form is favoured. However due to the absence of a steric hindrance between hydrogen atoms, the second minimum exists nearby (0 = 165” ) or
Fig. 2. NDDO potential energy curves for glyoxaldiimine (-) diimine (- - - -).
and glyoxal-N,N’-dimethyl-
103
corresponds to the planar cis conformation, which no longer represents the absolute maximum of the potential energy curve. The energy differences between the minimum conformations amount to 12.5 kJ mol-’ for 2 and 10.5 kJ mol-’ for the derivative 3. The rotational barriers are 21.7 and 25.5 kJ mol-‘, respectively. The conformational structure of 2 and 3 can easily be compared with the situation in compounds such as glyoxal and acroleine [ 25,311. Based on an IRDO (Intermediate Retention of Differential Overlap) analysis [27,29], it is possible to gain further insight into the origin of the partially different potential curves of 2,2’-bipyridyl and the simpler adiimines. Within this analysis, we can selectively consider the interaction between various atoms of a molecule according to the NDDO or INDO and CND0/2 formalism, respectively. Firstly, we have performed an IRDO analysis on 2,2’-bipyridyl considering only the interaction of the nitrogen atoms according to the INDO approximations, simultaneously maintaining the NDDO formalism for the interaction between all other atoms. Thus, the directed character of the lone pairs is lost. It can be seen in Table 1 that the planar forms now have approximately the same stability. The value of the rotational barrier is only somewhat decreased. This fact demonstrates the importance of the directed interaction of the lone pairs for the energy difference between the planar cis and truns arrangements. Moreover, this result underlines the known fact that methods such as CNDO/B and INDO should not be used if lone pair effects play a dominant role. Considering the absence of strongly repulsive hydrogen interactions and the similar lone pair effect in the cisoid form of the simpler cudiimine systems, this conformation should be even more stable than the trans arrangement on performing the same IRDO treatment. Indeed, after the loss of the directed character of the nitrogen lone pairs, the planar cis form of glyoxaldiimine is favoured by about 8.1 kJ mol-‘. The shallow minimum at 8 = 45” estimated by the CNDO/B method for 2,2’-bipyridyl is not confirmed by our NDDO results. To explain the CNDO/B result, Borgen et al. [23] have postulated a repulsive effect between the TABLE 1 NDDO and IRDO values for the rotational barrier and the energy difference between the planar forms of 2,2’-bipyridyl (kJ mol-I)
a
*ENDDO
gob
9o" 180”
A$R.l10
0
0
0
18P 26.8
14.6 -9.4
19.2 27.2
a
*NN, interaction between the nitrogen atoms according to INDO; NH, interaction between the nitrogen atoms and the ortho hydrogens according to INDO. %-trcnS configuration. ‘Exact potential energy maximum at about 8 = 105”.
104
nitrogen lone pairs and the ortho hydrogen atoms in the planar s-Puns form. By means of the IRDO technique, it is possible to examine the existence of a specific interaction between these atoms. In a second IRDO treatment, we have therefore considered the interaction between the nitrogen and ortho hydrogen atoms within the INDO formalism maintaining the NDDO conventions for all other interactions. The results in Table 1 indicate that no change of the potential energy curve exists in comparison to the full NDDO treatment. Thus, the conclusion can be drawn that there should be no specific interaction between the nitrogen lone pairs and the ortho hydrogen atoms in the planar s-tram conformation. The dipole moments calculated for the conformations of 2,2’-bipyridyl are in the range of cr = O-3.42 D going from the s-tram to the s-cis arrangement. Therefore, in comparison to the other conformations a stronger stabilization of the s-cis form can be expected in solution. Figure 1 illustrates the potential energy curve estimated by means of our continuum model (solvent CCL,). Up to an angle of 0 = 90”) the increase of the cavity energy approximately compensates the stabilization caused by the electrostatic energy. Within the range of 0 = 90-MO”, the expected effect of the stabilization of the s-&s form appears. However, in complete agreement with the experimental data the planar s-tram form is also the most stable conformation in solution. Figure 3 shows the potential energy curves for 4,4’-bipyridyl calculated by the NDDO method with and without inclusion of the solvent CC14.The torsion angle of 8 = 40” estimated is in fair agreement with the gas phase data. Obviously, there is similarity between the molecular structure of 4,4’-bipyridyl and biphenyl. The rotational angle is somewhat decreased in solution (0 = 37” ). This result corresponds to the tendency determined experimentally [ 181.
Fig. 3. NDDO potential energy solvent CCI, for 4,4’-bipyridyl.
curves without
(-)
and with (-
- - -) inclusion of the
CONCLUSIONS
All NDDO results are qualitatively in good agreement with the experimental data and indicate the superiority of this method over the CNDO/B formalism for the description of the conformative properties of conjugated compounds and lone pair effects, respectively. The IRDO analysis represents a powerful tool to gain information about the influence of lone pairs on the molecular structure. The continuum model used is well suited to estimate changes of the conformative structure caused by solvents. Based on the NDDO calculations on typical cwdiimine ligands, it can be concluded that barriers of about 20-28 kJ mol- ’ must be overcome to produce the cis arrangement which is realized in the transition metal compounds. REFERENCES 1 W. R. McWhinnie and J. D. Miller, Adv. Inorg. Chem. Radiochem., 12 (1969) 135. 2 P. Krumholz, Struct. Bonding (Berlin), 9 (1971) 139. 3 H. tom Dieck, K. D. Franz and F. Hohmann, Chem. Ber., 108 (1975) 163. 4 H. tom Dieck and J. W. Renk, Chem. Ber., 104 (1971) 92,110. 5 L. L. Merritt and E. D. Schriider, Acta Crystallogr., 9 (1956) 801. 6 P. H. Cureton, C. G. Le Fevre and R. J. W. Le Fevre, J. Chem. Sot., (1963) 1736. 7 C. W. N. Cumper, R. F. A. Ginmann and A. I. Vogel, J. Chem. Sot., (1962) 1188. 8 D. Kranbuehl, D. Klug and W. Vaughan, N.A.S.N.R.C. Publ., 22 (1969). 9 K. Nakamoto, J. Phys. Chem., 64 (1960) 1420. 10 S. Castellano, H. Gunther and S. Ebersole, J. Phys. Chem., 69 (1965) 4166. 11 T. McL. Spotswood and C. I. Tanzer, Aust. J. Chem., 20 (1965) 1227. 12 A. Almenningen and 0. Bastiansen, K. Nor. Vidensk. Selsk. Skr., 4 (1958). 13 0. Bastiansen and M. Traetteberg, Tetrahedron, 17 (1962) 147. 14 J. M. Kliegmann and R. K. Barnes, Tetrahedron Lett., (1969) 1953. 15 H. Nakai, Bull. Chem. Sot. Jpn., 44 (1971) 2412. 16 M. Hinsmoto, S. Ovi and H. Kuroya, Bull. Chem. Sot. Jpn., 44 (1971) 586. 17 H. Suzuki, Electronic Absorption Spectra and Geometry of Organic Molecules, Academic Press, New York, 1967, p. 261. 18 J. W. Emsley, D. S. Stephenson, J. C. Lindon, L. Lunazzi and S. Pulga, J. Chem. Sot. Perkin Trans. 2, (1975) 1541. 19 G. Favini, Gazz. Chim. Ital., 94 (1964) 1287. 20 H.J. Hofmann, Z. Chem., 15 (1975) 76. 21 V. Galasso, G. De Alti and A. Bigotto, Tetrahedron, 27 (1971) 991. 22 M. Bossa, G. Ramunni and D. F. Franchini, Theor. Chim. Acta, 17 (1970) 327. 23 0. Borgen, B. Mestvedt and I. Skauvik, Acta Chem. Stand. Ser. A, 30 (1976) 43. 24 0. Gropen and H. M. Seip, Chem. Phys. Lett., ll(l971) 445. 25 H.-J. Hofmann, H.-J. Kbhler, K. Thieroff and P. Uhlmann, J. Prakt. Chem., 316 (1974) 659. 26 C. Sieiro, P. Gonzalez-Diaz and Y. G. Smeyers, J. Mol. Struct., 24 (1976) 345. 27 F. P. Chen and R. J. Jesaitis, J. Chem. Sot. Chem. Commun., (1970) 1533. 28 P. Birner, H.-J. KGhler and C. Weiss, Chem. Phys. Lett., 27 (1974) 347. 29 P. Birner, H.-J. Kijhler and C. Weiss, Int. J. Quantum Chem., 9 (1975) 917. 30 H.-J. Hofmann and P. Birner, Z. Chem., 15 (1975) 23. 31 H.-J. Hofmann and P. Birner, Chem. Phys. Lett., 37 (1976) 608. 32 H.-J. Hofmamr and P. Bimer, J. Mol. Struct., 39 (1977) 145. 33 H.-J. Hofmann and F. Bimstock, J. Mol. Struct.,,44 (1978) 231. 34 F. Bimstock, H.-J. Hofmann and H.-J. Klihler, Theor. Chim. Acta, 42 (1976) 311. 35 H. G. v. Schnering, K. Peters and F.-M. Peters, Chem. Ber., 109 (1976) 1665. 36 L. Nygaard, B. Bak and J. R. Andersen, J. Mol. Spectrosc., 2 (1958) 361.
in The Netherlands
QUANTUM CHEMICAL CALCULATIONS FOR THE DETERMINATION OF TI-IE MOLECULAR STRUCTURE OF CONJUGATED COMPOUNDS* Part XIV. On the conformational structure of CX-diimineligands
ROLAND BENEDIX, PETER BIRNER, FRIEDER BIRNSTOCK and HORST HENNIG Sektion
Chemie,
HANS-JtjRG Sektion
Karl-Marx-University,
Leipzig
(G.D.R.)
HOFMANN**
Biowissenschaften,
Karl-Marx-University,
Leipzig
(G.D.R.)
(Received 5 May 1978)
ABSTRACT The conformational structure of some typical a-diimine ligands (2,2’-bipyridyl, glyoxaldiimine, glyoxal-N,N’-dimethyldiimine) is examined using the NDDO method. An analysis of the potential energy curves indicates the influence of specific lone-pair effects on the molecular structure. Based on a continuum model, changes of the molecular arrangement caused by solvent effects are estimated. INTRODUCTION
2,2’-bipyridyl, 1, is one of the most important representatives of a-diimine ligands in transition metal compounds. Its excellent ability to form complexes has been well known for many years and therefore it has been the subject of numerous investigations [l] . The simplest representatives of this group of ligands, glyoxaldiimine, 2, and its derivatives, have only comparatively recently been examined in detail [2-41.
c&-f3
H_Nd-H 1
2
*Part XIII. H.-J. Hofmann and F. Birnstock, J. Mol. Struct., 44 (1978) 231. **To whom correspondence should be addressed.
100
Using X-ray diffraction [5] and various indirect methods [6-111, respectively, a planar or approximately planar s-truns orientation was determined for 2,2’-bipyridyl both in the solid state and in solution. Unfortunately, the electron diffraction data for this compound are not very conclusive [12,13]. Apart from a maximum at the planar s-cis conformation, the rather flat potential curve does not show any well defined minimum but indicates a free rotation within a large range around the central single bond. NMR examinations on glyoxal-N,N’dimethyldiimine, 3, also suggest the transoid orientation of the nitrogen atoms for the simpler a-diimine ligands [ 141. Whereas the transoid configuration of the nitrogen atoms seems to be a characteristic feature of the isolated ligands, a planar or nearly planar cisoid arrangement exists in all complex compounds [ 15,161. For a better understanding of the complex formation process, it may therefore be important to get a more detailed knowledge of the conformational structure of these a-diimine ligands. Above all, exact information about the rotational barrier which must be overcome to reproduce the cisoid arrangement and the energy difference between the s-tram and s-cis forms are desirable. Furthermore, the molecular structure of 2,2’-bipyridyl may be compared with the orientation of the two rings in the iso-electronic compound biphenyl. It is known that the planar arrangement of biphenyl is distorted. The repulsion between the ortho hydrogen atoms enforces a torsion around the essential single bond by about 42” in the gas phase 1121 and 20” in solution [17]. A comparison between biphenyl and 2,2’-bipyridyl could be very useful in clarifying the influence of the destabilizing interactions between the ortho hydrogens and the nitrogen lone-pairs, respectively, on the molecular structure. The inclusion of 4,4’-bipyridyl, 4, may additionally illustrate the relations between these compounds. Torsional angles of about 37” [ 121 and 30” [ 181 have been determined for 4,4’-bipyridyl by means of electron diffraction and NMR techniques, respectively. These values are closely related to the rotational angles of biphenyl measured experimentally. Various quantum chemical methods have been used to gain information on the structure of 2,2’-bipyridyl. The results of n-electron methods connected with the calculation of the energy of the non-bonded interactions confirm the experimental data [ 19,201. EHT calculations show the planar s-tram conformation as the most stable form [21] . A second minimum was determined at 0 = 145”. The EHT torsional angle of about 60” estimated for 4,4’-bipyridyl seems to be somewhat too large and may be caused by an overemphasis of steric repulsions which is well known within this formalism. By contrast the CNDO/B method completely fails to predict the favoured conformations [ 22,231. Furthermore, numerous results support the conclusion that this method is not suited to describe the molecular structure of conjugated compounds correctly and should no longer be used to examine such problems [ 24-261. Moreover, due to the approximations inherent in this formalism,~no correct description of lone-pairs is possible [27-291. Recently, we have shown that the NDDO method is well suited for
101
determination of the conformational structure of conjugated compounds [ 25,30-331. Additionally, this formalism permits an adequate consideration of lone-pairs [ 27,291. Therefore, the NDDO method seems to be predestined to get precise results for this type of compound. To estimate the changes of the molecular structure under the influence of a solvent, we have combined the NDDO energy values with solvation energies calculated by means of a recently suggested continuum model [33,34] . DETAILS OF CALCULATIONS
The bond lengths and angles for 1 and 3 come from X-ray data [5,35]. All CH bond lengths amount to 1.08 A with the exception of the methyl group of 3 (Rcn = 1.09 A). One hydrogen atom of each methyl group in 3 has been assumed to be in an ecliptic position to the CN double bond. The pyridine geometry experimentally determined was used for compound 4 [ 361. The two rings are connected at a distance of 1.50 A in the 4-position. The bond lengths of 3 were also used for compound 2, which is as yet only known as constituent of complex compounds. The NH bond lengths are 1.03 A. The parametrization of the NDDO method and the basis of the continuum model used are described in refs. 29,33 and 34. RESULTS AND DISCUSSION
Figure 1 shows the NDDO potential energy curve for 2,2’ -bipyridyl. In accordance with the experimental results already mentioned, the planar
Fig. 1. NDDO potential energy curves without (-) solvent CCl, for 2,2’-bipyridyl.
and with (- - - -) inclusion of the
102
s-truns arrangement is the most stable form. In comparison with biphenyl, the loss of the steric interactions between the ortho hydrogen atoms in the tram configuration of 1 now enables the planar structure. A second minimum exists at a torsion angle of f3 = 130”. The energy difference and the barrier between the two minima amount to 19.2 and 19.7 kJ mol-‘, respectively. The position of the second minimum may be compared directly with the minimum conformation of biphenyl (NDDO: 8 = 45” = 135” [30]). Obviously, the nitrogen lone pair interaction in the planar cis conformation of 1 causes a similar destabilization to the corresponding interaction between the ortho hydrogen atoms in the planar form of biphenyl. According to the NDDO method, the energy difference between the maximum conformation at about 105” and the second minimum form is very small. Thus, a final valuation of the real importance of the second minimum for the molecular structure would only be possible after performing a complete geometry optimization of all conformations (for a discussion of this problem within semiempirical methods cf. ref. 32). In agreement with the electron diffraction data [ 12,131, the planar s-cis form represents the absolute maximum of the potential curve. The energy difference of 26.8 kJ mol-’ between the planar forms is relatively large. This strong destabilization of the s-cis conformation could be caused by the interaction of the nitrogen lone pairs and the steric hindrance between the ortho hydrogen atoms. Before we perform an analysis of these effects, it may be useful to compare the results obtained for 2,2’-bipyridyl with the potential energy curves of glyoxaldiimine and glyoxal-N,N’dimethyldiimine calculated by the NDDO method (Fig. 2). As in the case of 2,2’-bipyridyl and in accordance with the NMR data, the s-tram form is favoured. However due to the absence of a steric hindrance between hydrogen atoms, the second minimum exists nearby (0 = 165” ) or
Fig. 2. NDDO potential energy curves for glyoxaldiimine (-) diimine (- - - -).
and glyoxal-N,N’-dimethyl-
103
corresponds to the planar cis conformation, which no longer represents the absolute maximum of the potential energy curve. The energy differences between the minimum conformations amount to 12.5 kJ mol-’ for 2 and 10.5 kJ mol-’ for the derivative 3. The rotational barriers are 21.7 and 25.5 kJ mol-‘, respectively. The conformational structure of 2 and 3 can easily be compared with the situation in compounds such as glyoxal and acroleine [ 25,311. Based on an IRDO (Intermediate Retention of Differential Overlap) analysis [27,29], it is possible to gain further insight into the origin of the partially different potential curves of 2,2’-bipyridyl and the simpler adiimines. Within this analysis, we can selectively consider the interaction between various atoms of a molecule according to the NDDO or INDO and CND0/2 formalism, respectively. Firstly, we have performed an IRDO analysis on 2,2’-bipyridyl considering only the interaction of the nitrogen atoms according to the INDO approximations, simultaneously maintaining the NDDO formalism for the interaction between all other atoms. Thus, the directed character of the lone pairs is lost. It can be seen in Table 1 that the planar forms now have approximately the same stability. The value of the rotational barrier is only somewhat decreased. This fact demonstrates the importance of the directed interaction of the lone pairs for the energy difference between the planar cis and truns arrangements. Moreover, this result underlines the known fact that methods such as CNDO/B and INDO should not be used if lone pair effects play a dominant role. Considering the absence of strongly repulsive hydrogen interactions and the similar lone pair effect in the cisoid form of the simpler cudiimine systems, this conformation should be even more stable than the trans arrangement on performing the same IRDO treatment. Indeed, after the loss of the directed character of the nitrogen lone pairs, the planar cis form of glyoxaldiimine is favoured by about 8.1 kJ mol-‘. The shallow minimum at 8 = 45” estimated by the CNDO/B method for 2,2’-bipyridyl is not confirmed by our NDDO results. To explain the CNDO/B result, Borgen et al. [23] have postulated a repulsive effect between the TABLE 1 NDDO and IRDO values for the rotational barrier and the energy difference between the planar forms of 2,2’-bipyridyl (kJ mol-I)
a
*ENDDO
gob
9o" 180”
A$R.l10
0
0
0
18P 26.8
14.6 -9.4
19.2 27.2
a
*NN, interaction between the nitrogen atoms according to INDO; NH, interaction between the nitrogen atoms and the ortho hydrogens according to INDO. %-trcnS configuration. ‘Exact potential energy maximum at about 8 = 105”.
104
nitrogen lone pairs and the ortho hydrogen atoms in the planar s-Puns form. By means of the IRDO technique, it is possible to examine the existence of a specific interaction between these atoms. In a second IRDO treatment, we have therefore considered the interaction between the nitrogen and ortho hydrogen atoms within the INDO formalism maintaining the NDDO conventions for all other interactions. The results in Table 1 indicate that no change of the potential energy curve exists in comparison to the full NDDO treatment. Thus, the conclusion can be drawn that there should be no specific interaction between the nitrogen lone pairs and the ortho hydrogen atoms in the planar s-tram conformation. The dipole moments calculated for the conformations of 2,2’-bipyridyl are in the range of cr = O-3.42 D going from the s-tram to the s-cis arrangement. Therefore, in comparison to the other conformations a stronger stabilization of the s-cis form can be expected in solution. Figure 1 illustrates the potential energy curve estimated by means of our continuum model (solvent CCL,). Up to an angle of 0 = 90”) the increase of the cavity energy approximately compensates the stabilization caused by the electrostatic energy. Within the range of 0 = 90-MO”, the expected effect of the stabilization of the s-&s form appears. However, in complete agreement with the experimental data the planar s-tram form is also the most stable conformation in solution. Figure 3 shows the potential energy curves for 4,4’-bipyridyl calculated by the NDDO method with and without inclusion of the solvent CC14.The torsion angle of 8 = 40” estimated is in fair agreement with the gas phase data. Obviously, there is similarity between the molecular structure of 4,4’-bipyridyl and biphenyl. The rotational angle is somewhat decreased in solution (0 = 37” ). This result corresponds to the tendency determined experimentally [ 181.
Fig. 3. NDDO potential energy solvent CCI, for 4,4’-bipyridyl.
curves without
(-)
and with (-
- - -) inclusion of the
CONCLUSIONS
All NDDO results are qualitatively in good agreement with the experimental data and indicate the superiority of this method over the CNDO/B formalism for the description of the conformative properties of conjugated compounds and lone pair effects, respectively. The IRDO analysis represents a powerful tool to gain information about the influence of lone pairs on the molecular structure. The continuum model used is well suited to estimate changes of the conformative structure caused by solvents. Based on the NDDO calculations on typical cwdiimine ligands, it can be concluded that barriers of about 20-28 kJ mol- ’ must be overcome to produce the cis arrangement which is realized in the transition metal compounds. REFERENCES 1 W. R. McWhinnie and J. D. Miller, Adv. Inorg. Chem. Radiochem., 12 (1969) 135. 2 P. Krumholz, Struct. Bonding (Berlin), 9 (1971) 139. 3 H. tom Dieck, K. D. Franz and F. Hohmann, Chem. Ber., 108 (1975) 163. 4 H. tom Dieck and J. W. Renk, Chem. Ber., 104 (1971) 92,110. 5 L. L. Merritt and E. D. Schriider, Acta Crystallogr., 9 (1956) 801. 6 P. H. Cureton, C. G. Le Fevre and R. J. W. Le Fevre, J. Chem. Sot., (1963) 1736. 7 C. W. N. Cumper, R. F. A. Ginmann and A. I. Vogel, J. Chem. Sot., (1962) 1188. 8 D. Kranbuehl, D. Klug and W. Vaughan, N.A.S.N.R.C. Publ., 22 (1969). 9 K. Nakamoto, J. Phys. Chem., 64 (1960) 1420. 10 S. Castellano, H. Gunther and S. Ebersole, J. Phys. Chem., 69 (1965) 4166. 11 T. McL. Spotswood and C. I. Tanzer, Aust. J. Chem., 20 (1965) 1227. 12 A. Almenningen and 0. Bastiansen, K. Nor. Vidensk. Selsk. Skr., 4 (1958). 13 0. Bastiansen and M. Traetteberg, Tetrahedron, 17 (1962) 147. 14 J. M. Kliegmann and R. K. Barnes, Tetrahedron Lett., (1969) 1953. 15 H. Nakai, Bull. Chem. Sot. Jpn., 44 (1971) 2412. 16 M. Hinsmoto, S. Ovi and H. Kuroya, Bull. Chem. Sot. Jpn., 44 (1971) 586. 17 H. Suzuki, Electronic Absorption Spectra and Geometry of Organic Molecules, Academic Press, New York, 1967, p. 261. 18 J. W. Emsley, D. S. Stephenson, J. C. Lindon, L. Lunazzi and S. Pulga, J. Chem. Sot. Perkin Trans. 2, (1975) 1541. 19 G. Favini, Gazz. Chim. Ital., 94 (1964) 1287. 20 H.J. Hofmann, Z. Chem., 15 (1975) 76. 21 V. Galasso, G. De Alti and A. Bigotto, Tetrahedron, 27 (1971) 991. 22 M. Bossa, G. Ramunni and D. F. Franchini, Theor. Chim. Acta, 17 (1970) 327. 23 0. Borgen, B. Mestvedt and I. Skauvik, Acta Chem. Stand. Ser. A, 30 (1976) 43. 24 0. Gropen and H. M. Seip, Chem. Phys. Lett., ll(l971) 445. 25 H.-J. Hofmann, H.-J. Kbhler, K. Thieroff and P. Uhlmann, J. Prakt. Chem., 316 (1974) 659. 26 C. Sieiro, P. Gonzalez-Diaz and Y. G. Smeyers, J. Mol. Struct., 24 (1976) 345. 27 F. P. Chen and R. J. Jesaitis, J. Chem. Sot. Chem. Commun., (1970) 1533. 28 P. Birner, H.-J. KGhler and C. Weiss, Chem. Phys. Lett., 27 (1974) 347. 29 P. Birner, H.-J. Kijhler and C. Weiss, Int. J. Quantum Chem., 9 (1975) 917. 30 H.-J. Hofmann and P. Birner, Z. Chem., 15 (1975) 23. 31 H.-J. Hofmann and P. Birner, Chem. Phys. Lett., 37 (1976) 608. 32 H.-J. Hofmamr and P. Bimer, J. Mol. Struct., 39 (1977) 145. 33 H.-J. Hofmann and F. Bimstock, J. Mol. Struct.,,44 (1978) 231. 34 F. Bimstock, H.-J. Hofmann and H.-J. Klihler, Theor. Chim. Acta, 42 (1976) 311. 35 H. G. v. Schnering, K. Peters and F.-M. Peters, Chem. Ber., 109 (1976) 1665. 36 L. Nygaard, B. Bak and J. R. Andersen, J. Mol. Spectrosc., 2 (1958) 361.