210 J. CHEM. RESEARCH (S), 1997 J. Chem. Research (S), 1997, 210–211† Conformational Properties of Cyclododeca-1,5,9-triyne† Issa Yavari,*a Avat (Arman) Taherpourb and Arash Jabbarib aChemistry Department, Tarbiat Modarres University, P.O. Box 14155-4838, Tehran, Iran bChemistry Department, Islamic Azad University, P.O. Box 19395-1775, Tehran, Iran Semi-empirical AM1 SCF-MO calculations are used to calculate the structure optimization and conformational interconversion pathways in cyclododeca-1,5,9-triyne; two symmetrical energy-minimum conformations, viz.chair (D3) and twist-boat (C2), with similar strain energies are found which are separated by a fairly low (9.6 kJ molµ1) energy barrier. Cyclododeca-1,5,9-triyne (1), with three acetylenic chromophores alternatively inserted in carbon–carbon bonds of cyclohexane, could experience six-electron cyclic interactions of both the in-plane and out-of-plane p bonds of the three acetylene moieties, and might exhibit homoaromaticity.1 This ‘exploded cyclohexane’ is expected to manifest special conformational properties, since the torsional strain and transannular van der Waals repulsions, which play such a crucial rule in determining the relative energies of the various conformations of cyclohexane, will be greatly reduced.2–4 Ab initio calculations with the STO-3G minimal basis set have been reported1 for the chair-like (D3) and planar D3h geometries of 1.According to these calculations the D3 conformation is 67 kJ molµ1 more stable than D3h.The photoelectron spectrum of 1 has been interpreted in terms of a chair-like conformation of D3 symmetry.1 We present the results of Austin Model 1 (AM1) semiempirical SCF-MO calculations5 on 1 that allow interesting conclusions to be drawn about the conformational properties of this molecule. Two symmetrical energy-minimum conformations, viz. chair (D3) and twist-boat (C2), with similar strain energies6 are found which are separated by a fairly low energy barrier.The planar D3h geometry of 1 was found to be about 26.9 kJ molµ1 less stable than the D3 conformation. Semi-empirical calculations were carried out using the AM1 method with the MOPAC 6.0 program7,8 implemented on a VAX 4000-300 computer. Energy-minimum geometries were located by minimizing energy with respect to all geometrical coordinates, and without imposing any symmetry constraints. The structures of the transition-state geometries were obtained using the optimized geometries of the equilibrium structures according to the procedure of Dewar et al.9 (using keyword SADDLE).All structures were characterized as stationary points, and true local energy-minima and energy-maxima on the potential energy surface were found using the keyword FORCE. All energy-minima and energymaxima geometries obtained in this work are calculated to have 3Nµ6 and 3Nµ7 real vibrational frequencies, respectively. 10 Results and Discussion The results of semi-empirical AM1 calculations for various molecular geometries of cyclododeca-1,5,9-triyne (1) are shown in Table 1 and Fig. 1. The conformational possibilities available to 1 parallel those of cyclohexane.11 Thus, chair and twist-boat conformations should be accessible. The chair conformation, which has D3 symmetry, is calculated to have the lowest heat of formation (DHf°). Since the twist-boat form has a higher (by ca. 2.3 kJ molµ1) DHf than the chair form, it is expected to be significantly populated at room temperature.The chair conformation is the same as the structure determined by photoelectron spectroscopy.1 The energy surface for the interconversion of the energyminimum conformations of 1 was investigated in detail by changing different torsional angles. The results are shown in Fig. 1. There are two distinct, different transition states (not counting mirror images) which are required to describe the conformational dynamics in cyclododeca-1,5,9-triyne. The internal and torsional angles of these transition states are given in Table 1.The simplest conformational process, and the one with the lowest barrier, is the degenerate interconversion of the chair conformation with its mirror image via the twist-boat intermediate. If this process is fast the time-averaged symmetry of the chair conformation becomes D3h, which is the maximum symmetry allowed by the chemical structure of cyclododeca- 1,5,9-triyne.A second, and higher-energy, process undergone by the chair conformation is also degenerate, and involves the planar transition state, which has D3h symmetry. The calculated heat of formation for planar D3h geometry is 26.9 kJ molµ1, which is much higher than those for twist and halfchair geometries (see Table 1). Two significant differences can be anticipated between the conformational features of cyclododeca-1,5,9-triyne (1) and cyclohexane. The first derives from the fact that torsional effects and transannular van der Waals repulsion should diminish in 1, since the dimensions of the ring will be magni- fied while the CH2CH2 groups will remain unchanged in size.Consequently, conformations such as chair and twist-boat should hardly differ in energy from one another. By comparison, cyclohexane exists mainly in the chair conformation (a99%) at room temperature.11 The other conformational feature of 1 concerns its flexibility. The ease with which the C·C�C bond angles can be deformed from linearity and the large number of sp carbon atoms over which angle strain can be spread will practically reduce the barriers to conformational interconversions in 1.*To receive any correspondence. †This is a Short Paper as defined in the Instructions for Authors, Section 5.0 [see J. Chem. Research (S), 1997, Issue 1]; there is therefore no corresponding material in J. Chem. Research (M). Fig. 1 Calculated strain energy (kJ molµ1) profile for the degenerate interconversion of the chair conformation of cyclododeca-1,5,9-triyne with its mirror image geometry via twist-boat intermediatesJ.CHEM. RESEARCH (S), 1997 211 Thus, the barrier separating the chair from the twist-boat conformation should be only a small fraction of the 45 kJ molµ1 required for the same conformational change in cyclohexane.11 In conclusion, AM1 calculations provide a picture of the conformations of cyclododeca-1,5,9-triyne (1) from both the structural and energetic points of view.Compound 1 is predicted to exist as a mixture of chair (D3) and twist-boat (C2) conformations. There is good agreement between the AM1 structure of the chair form and the photoelectron results.1 Received, 31st January 1997; Accepted, 27th February 1997 Paper E/7/00723J References 1 K. N. Houk, R. W. Strozier, C. Santiago, R. W. Gandour and K. P. C. Vollhardt, J. Am. Chem. Soc., 1979, 101, 5183. 2 R.Gleiter, W. Schafer and H. Sakurai, J. Am. Chem. Soc., 1985, 107, 3046. 3 M. J. S. Dewar and M. K. Holloway, J. Chem. Soc., Chem. Commun., 1984, 1188. 4 L. T. Scott, G. J. DeCicco, J. L. Hyun and G. Reinhardt, J. Am. Chem. Soc., 1985, 107, 6546. 5 M. J. S. Dewar, E. G. Zeobisch, E. F. Healy and J. J. P. Stewart, J. Am. Chem. Soc., 1985, 107, 3902; M. J. S. Dewar and K. M. Dieter, J. Am. Chem. Soc., 1986, 108, 8075. 6 E. M. Arnett, J. C. Sanda, J. M. Bollinger and M. Barber, J.Am. Chem. Soc., 1967, 89, 5389. 7 J. J. P. Stewart, J. Comput. Aided Mol. Des., 1990, 4, 1. 8 J. J. P. Stewart, QCPE 581, Department of Chemistry, Indiana University, Bloomington, IN, USA. 9 M. J. S. Dewar, E. F. Healy and J. J. P. Stewart, J. Chem. Soc., Faraday Trans., 1984, 80, 227. 10 J. W. McIver, Acc. Chem. Res., 1974, 7, 72; O. Ermer, Tetrahedron, 1975, 31, 1849. 11 F. A. L. Anet and A. J. R. Bourn, J. Am. Chem. Soc., 1967, 89, 760; M. E. Squillactotte, R. S. Sheridan, O. L. Chapman and F. A. L. Anet, J. Am. Chem. Soc., 1975, 97, 3244. Table 1 Heats of formation, bond angles and dihedral angles for cyclododeca-1,5,9- triyne Chair Twist-boat Twist Half-chair Planar Feature D3 C2 C2 CS D3h DHf°/kJ molµ1 DDHf ° /kJ molµ1 494.5 0.0 496.8 2.3 504.1 9.6 502.6 8.1 521.4 26.9 Bond angle (y/°) y123 y234 y345 y456 y567 y678 y789 y8,9,109,10,11 y10,11,12 y11,12,1 y12,1,2 178 112 112 178 178 112 112 178 178 112 112 178 177 112 113 177 178 113 113 178 177 113 112 177 176 112 113 178 176 116 116 176 178 113 112 176 177 112 113 178 178 115 115 178 178 113 112 177 176 116 116 176 176 116 116 176 176 116 116 176 Dihedral angle (f/°) f2345 f3478 f6789 f7,8,11,12 f10,11,12,1 f11,12,3,4 51 µ69 51 µ69 51 µ69 46 11 µ51 11 46 µ83 49 µ27 0 µ27 49 µ76 50 µ51 0 51 µ50 0 000000 aThe standard strain energy in each geometry of a molecule is defined as the difference between the standard heats of formation (DHf°) for that geometry and the most stable conformation of the molecule.6