Results from MNDO and CCFF/pI+MCA calculations predict the boat-shaped conformation of 9,lO-dihydroanthracene to be of lowest potential energy in both the gaseous phase and the solid state. Entropy contribution favors further the boat-shaped conformation over the planar form.
The conformational analysis of the 1,4-cyclohexadiene ring system has been the subject of a substantial number of theoretical and experimental investigations.
l-3 The conformational preference of the parent 1,4-cyclohexadiene 1 and 1,4_dihydronaphthalene 2 ( Planar vs.
boat/equilibrating boats) has caused considerable controversy although by 1981 these difficulties appeared to be resolved and the most stable conformation for 1 and 2 were established to be planar. 1,2 On the other hand 9,10-dihydroanthracene 3 has been shown by -X-ray diffraction to be nonplanar in the solid state4 and is presumed to undergo rapid boat to boat ring inversion on the NMR time scale at least at -60 oC.5
We have carried out theoretical calculations of crystal and single molecule geanetries of 2 with the QCFF/PI+McA6 and m7 methods in order to clarify the relationship existing between the preferred conformations of 3 in the two states of matter. For comparison, the structures of 1 and 2 were also calculated with the MNDO method. The calculations were -performed with optimization of all independent bond lengths and bond angles for the C2v ( nonplanar structures) and D 2h ( in the case of planar structures) symmetry restrictions. Recently a careful computational study of l-3 has been published. 2a --Using molecular mechanics, MMI, it has been shown by calculations that the optimum structure is planar for each molecule although the energy required for nonplanar distortion is small and decreases regularly with the degree of benzannulation. A thorough investigation of 1 by means of 1@l2 and molecular orbital methods, MINDD/3, UNDO, and ab initio, gave qualitatively the same results.
2 A note added in proof cited the limited results of MM2 and MNDD calculations for 3-2a Both methods afforded an optimum geometry which is nonplanar, while the mm/3 method predicts the planar conformation to be preferred. It was concluded that the nonplanar X-ray