EXTENDED CONJUGATION IN ENYNONES, DIENONES AND RELATED SPECIES - A THEORETICAL AND EXPERIMENTAL-STUDY - THE MOLECULAR-STRUCTURES OF 3-ETHYNYL-2-METHYLCYCLOPENT-2-ENONE, 3-ETHENYL-2-METHYLCYCLOPENT-2-ENONE AND 3-ETHYL-2-METHYLCYCLOPENT-2-ENONE, AS STUDIED BY GAS ELECTRON-DIFFRACTION

The molecular structures of three 3-R substituted conjugated enones ha ve been determined by the gas electron diffraction method. Depending o n the nature of the substituents the molecules represent examples of m olecules with no additional conjugation, relative to that of the enone system (R = CH3CH2-), or those with extended conjugation involving di fferent pi systems (R = CH2=H-; R = HC=C-). The molecular structures o f the three enones have also been calculated by ab initio MP2/6-31G o ptimizations. In order to better understand the conjugative effects of ethenyl and ethynyl substituents on a conjugated enone system, a larg e number of hydrogenation reactions and substituent exchange re;action s have been studied, based on the energies of the various molecules in volved in the reactions, as calculated by MP2/6-31G optimizations. Th e studies indicate that the conjugative effect of an ethenyl substitue nt on a conjugated enone system is approximately 4.2 kcal mol(-1), whi le that of an ethynyl substituent is considerably lower and more varia ble; on average ca. 2.3 kcal mol(-1).