Positron Annihilation Studies of Styrene-N-X-Phenylmaleimides

Results of angular correlation of annihilation radiation and positron annihilation lifetime measurements are presented for five styrene copolymers: poly(co-styrene-phenylmaleimide) and its three derivatives with chlorine as well as for one with the OH group substituted in the benzene ring. It occurs that the chlorine substituted in three different positions in the benzene ring poly(co-styrene-o (or -m, -p)-chlorophenylmaleimide) inhibits the formation of the positronium to different extent. The greatest effect is observed in case of the chlorine atoms substituted in the benzene ring at ortho-position towards the nitrogen atom. In addition, electric dipole moments were measured for N-X-phenylmaleimide units, to check whether their magnitude influences the positron annihilation parameters. On the basis of the present results no correlation between the positron annihilation parameters and the electric dipole moments of the units was noticed

Excited-state forms of 2-methylamino-6-methyl-4-nitropyridine N-oxide and 2-butylamino-6-methyl-4-nitropyridine N-oxide

Excited-state quantum chemical calculations of two 2-alkyloamino-6-methyl- 4-nitropyridine N-oxides are presented. Several different calculation methods and different basis sets were used, which all lead to similar results, although the precise values of excited-state energies and excited-state dipóle moments differ. All methods used predict that in the Si excited state four types of isomers occur. In three cases, these excited-state local energy minima correspond to ground-state isomers, and these all have a ππ* character. The fourth excited-state minimum, which we denote L*, does not have a corresponding ground-state isomer and has an nπ* character. This isomer is stable and plays an important role in understanding the photophysics of these molecules. In addition, we also calculated barriers between these excited-state minima, using predescribed reaction pathways. The theoretical results derived in this Article are confronted with experimental data from earlier papers. © 2009 American Chemical Society