Variable-pressure dynamic NMR studies: effects of paramagnetic metal ions on NMR parameters in nonexchanging systems

Abstract

The possible effects of paramagnetic relaxation on the apparent volumes of activation for exchange reactions in solution, as measured by NMR at high pressures, are considered. Two model paramagnetic systems that do not undergo ligand exchange on the NMR time scale were examined: tri(acetylacetonato)chromium(III) in various perdeuterated solvents, and tris(ethy1enedi-amine)nickel(II) ion in ethylenediamine solvent. No pressure dependence was discernible up to 200 MPa for the chemical shifts of 'H (exemplifying nuclei of spin 112) in the Cr(II1) complex, or of solvent 1 4~ (representing quadrupolar nuclei) in the Ni(I1)-ethylenediamine case. The line widths hv,,,, however, were significantly dependent on pressure. For I H in the Cr complex, the increase of Av,, with pressure was less than expected from the theory of scalar interactions, and was small enough to imply that any contribution from this source to the observed volume of activation in exchanging systems may be neglected. For 1 4~ in liquid ethylenediamine, the increase of Av,,, with pressure was significantly greater when a paramagnetic solute was present. Thus, before the observed AvI1, at a pressure P (in MPa) measured by the NMR of a quadrupolar nucleus can be used to obtain a chemical exchange rate for a paramagnetic solute, it should be reduced by an amount Av, , : Can. J. Chem. 72,2188Chem. 72, (1994.On considbre les effets possibles de la relaxation paramagnetique sur les volumes apparents d'activation des rCactions d'Cchange en solution, tels que mesurCs par la RMN i pressions ClevCes. On a examine deux systbmes paramagnktiques modbles qui ne subissent pas d'Cchange de coordinat a 1'Cchelle de temps de la RMN : le tri(acCtylacCtonato)chrome(III) dans divers sol-"ants perdeuttrts et l'ion tris(Cthylbnediamine)nickel(II) dans 1'Cthylbnediamine comme solvant. Pour les dCplacements chimiques du 'H (reprtsentant le spin 112) du complexe de chrome(II1) ou du (representant les noyaux quadripolaires) presents dans le cas du nickel(II)$thylbnediamine, on n'a pas pu dCcelC de relation avec la pression jusqu'k 200 MPa. Toutefois, les largueurs des raies, Av,,,, varient beaucoup avec la pression. Pour le 'H du complexe de chrome, l'augmentation du Av,,, avec la pression est moins importante que celle prCvue par la thCorie des interactions scalaires et elle est suffisamment faible pour impliqier que toute contribution de cette source au volume d'activation observC dans les systbmes subissant un Cchange peut Ctre ntgligCe. Pour le 1 4~ dans I'tthylbnediamine liquide, l'augmentation du Av,,, avec la pression est beaucoup plus importante lorsqu'un solute paramagnttique est present. Donc, un AvIl2 d'un noyau quadripolaire obtenu a une pression donnCe P (en MPa) par [Traduit par la redaction]

Mechanism of water exchange on five-coordinate copper(ii) complexes

“…In addition, it has been pointed out that the outer sphere line broadening caused by a paramagnetic ion may affect the value of ∆V # as a result of the effect of pressure on the viscous flow and the compressibility of the solvent. 34 Fortunately, these effects fall within the experimental uncertainty of the measurements in the case of water as solvent. 34 The ∆V # values reported for solvent exchange on [Cu(tren)- 3 confirm the associative nature of the exchange mechanism as characterized by a more compact transition state than either the reactant or product states.…”

“…34 Fortunately, these effects fall within the experimental uncertainty of the measurements in the case of water as solvent. 34 The ∆V # values reported for solvent exchange on [Cu(tren)- 3 confirm the associative nature of the exchange mechanism as characterized by a more compact transition state than either the reactant or product states. Although the magnitude of these values suggests the operation of an associative interchange (I a ) mechanism, as argued above, the possible operation of a limiting A mechanism is not ruled out.…”

NMR spectroscopy of some electrolyte solutions to 1.9 GPa

“…The early work was directed at detailing solvent motions and relaxation mechanisms (Lee and Jonas, 1971;Lee et al, 1974;Akai and Jonas, 1976;Jonas et al, 1976;Defries and Jonas, 1977;Jonas, 1980;Lang and Lüdemann, 1993;Ballard et al, 1998). High-pressure NMR was then extended to estimate the activation volumes of homoleptic reactions (Merbach and Vanni, 1977;Asano and Noble, 1978;Ducommun, Earl, et al, 1979;Ducommun, Newman, et al, 1979;Ducommun et al, 1980;Monnerat et al, 1981;Swaddle and Merbach, 1981;Meyer et al, 1982;Hugi-Cleary et al, 1985;Cossy et al, 1987;Hugi-Cleary et al, 1987;Minirale, 1989;Pittet et al, 1990;Takagi et al, 1994;Swaddle et al, 2005;Dees et al, 2007) with an intent of assigning mechanisms to ligandexchange reactions (Helm and Merbach, 2005). The work has been reviewed several times (Asano and Noble, 1978;Eldik et al, 1989;.…”