The solvent effect on di-t e r t-butyl nitroxide. A dipole–dipole model for polar solutes in polar solvents

Abstract

Articles you may be interested inMolecular near-field antenna effect in resonance hyper-Raman scattering: Intermolecular vibronic intensity borrowing of solvent from solute through dipole-dipole and dipole-quadrupole interactions Electron spin relaxation of ditertiarybutyl nitroxide in supercooled water New measurements of the solvent effect on the nitrogen hyperfine coupling constant of di-tert-butyl nitroxide are reported. These, together with literature data, are used to test various models for the solvent effect. At the Huckel level of approximation, aN is a linear function of the applied electric field. Thus various reaction field theories may be considered. The widely used Onsager reaction field does not account for the effects of the more polar solvents or for the differences between polar and nonpolar solvents. The Wertheim and Block-Walker reaction fields are better, especially for very polar solvents. However none of these continuum reaction fields is entirely satisfactory theoretically or experimentally. We propose a dipole-dipole model for polar solvents which is superior to the continuum models. From the dipole-dipole model, we suggest that the quantity IJp/M is a convenient linear parameter for polar solvent effects, the factors being solvent dipole moment, density, and molecular weight. The dipole-dipole model should apply to a wide range of polar solutes. Some special situations are not explained by the model, including hydrogen-bonding solvents, halogenated aromatiCS, and solvents with more than one conformation. The temperature dependence of the solvent effect is also considered.

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“…The expression given by Eq. [8] holds for Kirkwood's theory (10) only in so far as the correlation parameter, g h , remains relatively constant, independent of concentration [PH]. Departures of g h from values for the neat hydrogen-bonding fluid are expected most at high dilution, for which one anticipates that g h ≈ 1.…”

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