Dimérisation des sels de l'ion nitrosodisulfonate à l'état solide

Abstract

La dim~risation de plusieurs sels du radical libre nitrosodisulfonate est ~tudi6e par R.S.E. Les spectres d'~chantillons polycristallins de deux formes cristallographiques des sels de sodium, potassium et rubidium, sont caract~ristiques d'un ~tat triplet accessible thermiquement. Ces spectres sont attribu~s ~ des paires de groupements NO radicalaires, coupl6s par ~change et sont d~crits par l'Hamiltonien de spin : ~s =/,~BgS + SDS + 89 + I2).Les param~tres de d~placement en champ nul, les composantes du tenseur g le long des directions principales X, Y, Z du tenseur fin et la composante du tenseur Ale long de l'axe Z (dans le cas oft la structure hyperfine est r6solue), ont Ot~ mesur~s et discut~s. A partir de ces mesures, on montre que l'axe Z est dirig~ suivant les orbitales 2p des atomes d'azote et d'oxyg~ne, et que l'axe X est dirig~ selon la liaison NO. Par suite, l'arrangement des deux groupements NO d'une paire est toujours sensiblement rectangulaire.Dans le cas des deux formes cristallines du potassium et du rubidium, l'6cart singulet triplet d6pend de la temperature, ainsi que les param&res de d~placement en champ nul, ces variations avec la temperature sont attribu6es l'expansion thermique du r~seau. La presence d'une structure hyperfine bien r~solue dans certains de ces sels, et la forte anisotropie des largeurs de raies dans les autres sels, sugg6rent que les excitations sont fortement localis6es, et que si elle sont mobiles leur fr~quence de saut est inf~rieure ~ 10 ~ Hz.The dimerization of several salts of the free radical nitrosodisulfonate ion is investigated by use of E.S.R. spectroscopy. The E.S.R. spectra of powdered samples of two crystalline modifications of the sodium, potassium and rubidium salts are characteristic of a thermally accessible triplet state. The observed spectra are attributed to pairs of radical fragments NO interacting through exchange and are described by the spin Hamiltonian : ~s = ~BBgS + SDS + 89 + I2).The zero-field splitting parameters, the components of the g tensor along the principal directions X, Y, Z of the zero-field splitting tensor, and the component of the A tensor along the Z axis (whenever the hyperfine structure is resolved), have been measured and are discussed. From these measurements it is shown that the Z axis is directed along the 2p orbitals of the nitrogen and oxygen atoms, and that the X axis is directed along the NO bond. Therefore, within a pair, the arrangement of the two neighbouring NO fragments is always nearly rectangular.For both crystalline modifications of the potassium and rubidium salts, the singlet triplet energy gap is shown to be temperature dependent, as well M.P. 5 T Downloaded by [Columbia University] at 16:23 08 December 2014 1672 M.-C. Schouler et al.as the zero-field splitting parameters. It is suggested that these variations with temperature are due to the thermal expansion of the crystal lattice. The presence of a well-resolved hyperfine structure in some of these salts, and in the other salts the strong anisotropy of the line...

A fulleropyrrolidine binitroxide: synthesis, EPR and electrochemical features

“…The negative temperature coefficient ([13 cm~1) was attributed to the thermal expansion of the crystal lattice. 19 In our case, the biradical conformation is not unique, owing to the possible rotation around the r bond connecting one of the TEMPO units to the pyrrolidine ring. A negative temperature coefficient should therefore be attributed to the change of the relative weight of di †erent conformers.…”

“…r 0 On the other hand, an approximately linear temperature dependence of the exchange interaction with a negative slope was observed for alkali metal salts of nitrosodisulfonate (Fremy salts), which are dimeric in the crystalline state. 19 In the dimers, the NO bonds are almost parallel and point in opposite directions, the nitrogen atom of one molecule being separated by 2.86 from the oxygen atom of the second. In A this conformation the exchange interaction is positive and of the order of 103 cm~1 (e.g., for the Rubidium salt J \ 1047 cm~1 at 270 K).…”

Spin polarization in fullerene derivatives containing a nitroxide group. Observation of the intermediate photoexcited quartet state in radical triplet pair interaction

“…It depends on the relative distance between the partners as well as on their relative orientation [42].…”