LOCAL DESCRIPTION OF A POLYENIC RADICAL-CATION

The various local electronic events occurring in a radical cation of a linear polyene with even number of centers are investigated by means of the calculation of the expectation values of second quantized densi ty operators, in the framework of the general poly-electron population analysis. Two series of calculations in two limit geometries (a stron g alternant and a polaron-like one) are performed by using as analyser s both natural AOs in ab initio correlated wave functions, as well as the model orthogonal AOs in PPP + full CI ones. The probabilities of f inding simultaneously the positive charge (+) and the radical center ( .) follows, in accord with basic chemical intuition, an oscillating (e ven-odd) law, even at distant AO positions. The probability of having a transmission of the (+) charge through the rr-bonds (when the () is located in one extremity of the polyene) is greater than this of the t ransmission of the (). Comparing the radical cation with the parent po lyene, it is shown that oxidation creates an important trend of single -double bond inversion even in strongly alternant geometry; this effec t is more pronounced in bonds of the middle. The examination of variou s CDW structures shows that some of them can have small or negligible contributions; this counterintuitive and cooperative effect is rationa lized by means of Moffitt's theorem. All the above effects are not the consequence of the polaron-like geometry, but are controlled from the topology of n-centers linearly disposed and involving (n-1) electrons .