A theoretical investigation of the π-polarization mechanism. The importance of localized and extended polarization

Abstract

Ab initio molecular orbital theory has been used to examine the π-polarization mechanism. ‘Isolated molecule’ calculations have been made for the pairs C₃HX/H₂CCH–HCO, with X being varied through a range of common substituents. The dipole of X is able to polarize the π electrons of the acrolein unit such that both polarization of the entire conjugated system and separate polarization of the individual CC and CO π units occurs. The relative importance of extended and localized polarization varies for different atomic positions within the acrolein probe molecule. At the terminal oxygen atom, extended polarization accounts for ca. 60% of the total polarization whilst for the carbonyl carbon it accounts for only 20%. Most of the polarization at this position arises from localized polarization of the carbonyl unit. These results are in agreement with ¹³C s.c.s. data in related systems.