Para-positronium Conversion Research Articles

Positronium formation in sol–gel-prepared silica-based glasses: temperature and positron-irradiation effect

Silica, titania-silica, and zirconia-silica samples were studied at room temperature (RT) and low temperature (LT ∼50 K) by means of positron annihilation lifetime spectroscopy. It was confirmed that the samples were composed of oxide particles with finer pores dispersed within them and larger pores between the particles. The effect of e +-irradiation during the measurements was checked. The o-Ps lifetime originating from the larger pores was quenched at LT for all of the samples. For the zirconia-silica sample, this effect was observed even at RT. Quenching is an indication of ortho– para positronium conversion on paramagnetic centers formed due to e +-irradiation. These centers appear to be the reason why the longest lifetime as a function of the temperature does not follow the Goworek–Gidley model at temperatures below RT. Above RT, the Goworek–Gidley model fits well with the experimental values for the longest lifetime.

Electron Delocalization Caused in 3d Complexes by Azide and Chloride Ligands Checked by Positronium Spin-Exchange Reactions

The rate constants kCR of orthointo para-positronium conversion reactions promoted by 3d complexes are linearly correlated with the electron delocalization of unpaired metal electrons caused by ligands, as measured by the ratio β between the interelectronic repulsion parameters in complexes and in the free gaseous ions (1 > β ≥ 0). The β values may be evaluated by means of the following empirical equation: β = 1 tMhL, where tM and hL are characteristic constants of 3d ions and ligands, respectively. By measuring kCR and applying the correlations mentioned above, hL constants may be derived providing that tM is known. The method was used to measure the hL value of N−3, not yet known, and to check that of Cl− ligand.

Positronium Spin-Conversion Reactions Promoted by 3d Complexes in Highand Low-Spin States

Continuing in our work on the correlations between the rate constants, kCR, of the ortho into para-positronium conversion reactions, CR, promoted by complexes of various 3d ions and the metal electron delocalization β caused by the ligands, the relationship between the parameters of the correlation lines pertinent to highand low-spin complexes of CrII, MnII, and CoII ions was ascertained. Moreover, it was experimentally verified, for the first time, that the statistical probability of the CR promoted by paramagnetic compounds with S = 1/2 is three times larger than that of the CR caused by compounds with S > 1/2.

Electron delocalization in CrII complexes checked by ortho- into para-positronium conversion reactions

Abstract The metal electron delocalization caused by ligands in 3 d complexes may be evaluated by means of the empirical equation β =1- t M h L where t M and h L are constants depending on metal ion and ligand types, respectively. t M and h L are till now derived by UV-VIS absorption spectroscopy. By applying a completely different approach, i.e. the correlation found between β and the rate constants of the ortho - into para -positronium spin conversion reactions, the t M value of Cr II was determined and that of Ni II checked.

Electron Delocalization in 3d Complexes Caused by Dimethylurea Checked by Positron Annihilation

The rate constants, kCR, of ortho- into para-positronium conversion reactions promoted by paramagnetic 3d complexes are linearly correlated with the electron delocalization, β, of unpaired metal electrons caused by ligands, β being the ratio between the inter-electronic repulsion parameters in complexes and in the free gaseous ions. By applying a procedure previously described the β values of MnII, CoII, NiII complexes with dimethylurea were deduced from the mentioned correlations and compared with those of complexes with urea obtained both by the method of Ps reactions here applied and that based on UV-Vis absorption spectroscopy.

Positronium Conversion Reactions for Testing the Electron Delocalization in 3d Complexes Caused by Ligands

The rate constants of ortho- into para-positronium conversion reactions promoted by paramagnetic 3d complexes were found to be linearly correlated with the delocalization, β of unpaired metal electrons caused by ligands. It is shown here that βs, usually obtained by UV/Vis absorption spectroscopy, may also be deduced from the correlations mentioned above.

On the Ortho- into Para-Positronium Conversion Reactions Promoted by V<sup>II</sup> Complexes

On the Ortho- into Para-Positronium Conversion Reactions Promoted by V<sup>II</sup> Complexes

Electron Delocalization in Cr<sup>II</sup> Complexes Probed by the Rate Constants of Ortho- in Para-Positronium Conversion Reactions

Electron Delocalization in Cr<sup>II</sup> Complexes Probed by the Rate Constants of Ortho- in Para-Positronium Conversion Reactions

Further results on electron delocalization in 3d metal complexes probed by positronium atoms

A. L. Fantola Lazzarini, E. Lazzarini and M. Mariani, J. Chem. Soc., Faraday Trans., 1997, 93, 231 DOI: 10.1039/A604860I

Effectiveness of Cr III , Mn II , Co II and Ni II paramagnetic complexes in promoting the ortho- into para-positronium conversion reactions

The conversion reactions of ortho- into para-positronium, Ps, atoms promoted by CrII complexes with water, urea and ethylenediaminetetraacetic acid, by the complexes of CrIII, CoII and NiII ions with ethylenediaminetetramethylene-phosphonic acid, nitrilotrismethylene-phosphonic acid, urea and by the complexes of MnII ions with urea, glycine and iminodiacetic acid have been investigated in order to ascertain the role of the metal ion in the reactions. All the complexes considered are octahedral and of high spin type. The reaction rate constants of the complexes are correlated to the corresponding electron delocalization caused by the ligands as described by the ratio B/B0=β between the inter-electronic repulsion parameters in complexed and free gaseous metal ions. The rate constant per fully delocalized unpaired electron of the metal ion was deduced and found to be (3.7 ± 0.6) dm3 mol–1n s–1 electron–1.

The Spin-Orbit Coupling and the Rate Constants of Ortho- into Para-Positronium Conversion Reactions Promoted by 3d Ions

The Spin-Orbit Coupling and the Rate Constants of Ortho- into Para-Positronium Conversion Reactions Promoted by 3d Ions

Further results on the relationship between the rate constants of o-Ps into p-Ps conversion and the electron delocalization in 3d complexes

The ortho into para-positronium conversion reactions promoted by the paramagnetic complexes formed with ethylenediaminetetraacetic acid (H 4 EDTA) by Mn(II), Fe(II), Co(II), Ni(II) ions were studied at various temperatures. The results are discussed together with those previously obtained for the Cr(III) and Cu(II) complexes with the same ligand. It was found that the rate constants, k SE , are larger for complexes: A) whose 3d inter-electronic repulsion parameter attains smaller values; and B) for which spin orbit coupling occurs. These findings give further support to our proposal that the o-Ps into p-Ps conversion reaction may be used to investigate the electron delocalization in paramagnetic 3d complexes

3d electron delocalization studied by ortho- to para-positronium conversion reactions

The ortho- to para-positronium conversion reactions promoted by the paramagnetic complexes formed by ethylenediaminetetraacetic acid (H4EDTA) with MnII, FeII CoII, NiII ions have been studied at various temperatures. The results are discussed together with those previously obtained for the CrIII and CuII complexes with H4EDTA. It was found that the rate constants, kCR, are larger for complexes in which spin–orbit coupling occurs and for which the 3d inter-electronic repulsion parameter attains smaller values. These findings give further support to our proposal that the o-Ps to p-Ps conversion reaction may be used to investigate electron delocalization in paramagnetic 3d complexes.

The nephelauxetic effect and ortho- to para-positronium conversion reactions promoted by Cr(III) complexes

The ortho- (o-Ps) to para-positronium (p-Ps) conversion reactions promoted by ten Cr(III) complexes were investigated at several temperatures. The results obtained are discussed together with those of three other Cr(III) complexes measured previously.