CONFORMATIONAL PROPERTIES OF SIMPLE MONOCYCLIC CONJUGATED ENEDIYNES

An investigation employing the AM1 semi-empirical SCF MO method to cal culate structure optimization and conformational interconversion pathw ays for Z-cyclonona-3-ene-1,5-diyne (1), Z-cyclodeca-3-ene-1,5-diyne ( 2), and Z-cycloundeca-3-ene-1,5-diyne (3) has been undertaken. The pla ne-symmetrical half-chair conformation of 1 is calculated to be 23.8 k J mol(-1) more stable than the planar C-2v transition-state geometry. Compound 2 has two energy-minimum conformations with the twist-boat (C -2) form being 31.1 kJ mol(-1) more stable than the twist (C-1) geomet ry. The calculated energy barrier for interconversion of C-2 and C-1 c onformations is 37.4 kJ mol(-1). The unsymmetrical twist-boat form of 3 is calculated to be the most stable conformation. The twist-boat geo metry can undergo two degenerate processes to achieve a time-averaged symmetry of C-2v. (C) 1998 Elsevier Science B.V.