title: Electron spin-lattice relaxation mechanisms of nitroxyl radicals in ionic liquids and conventional organic liquids: temperature dependence of a thermally activated process.
abstract: During the past two decades, several studies have established a significant role played by a thermally activated process in the electron spin relaxation of nitroxyl free radicals in liquid solutions. Its role has been used to explain the spin relaxation behavior of these radicals in a wide range of viscosities and microwave frequencies. However, no temperature dependence of this process has been reported. In this work, our main aim was to investigate the temperature dependence of this process in neat solvents. Electron spin-lattice relaxation times of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and 4-hydroxy-TEMPO (TEMPOL), in X-band microwave frequency, were measured by the pulse saturation recovery technique in three room-temperature ionic liquids ([bmim][BF4], [emim][BF4], and [bmim][PF6]), di-isononyl phthalate, and sec-butyl benzene. The ionic liquids provided a wide range of viscosity in a modest range of temperature. An auxiliary aim was to examine whether the dynamics of a probe molecule dissolved in ionic liquids was different from that in conventional molecular liquids, as claimed in several reports on fluorescence dynamics in ionic liquids. This was the reason for the inclusion of di-isononyl phthalate, whose viscosities are similar to that of the ionic liquids in similar temperatures, and sec-butyl benzene. Rotational correlation times of the nitroxyl radicals were determined from the hyperfine dependence of the electron paramagnetic resonance (EPR) line widths. Observation of highly well-resolved proton hyperfine lines, riding over the nitrogen hyperfine lines, in the low viscosity regime in all the solvents, gave more accurate values of the rotational correlation times than the values generally measured in the absence of these hyperfine lines and reported in the literature. The measured rotational correlation times obeyed a modified Stokes-Einstein-Debye relation of temperature dependence in all solvents. By separating the contributions of g-anisotropy, A-anisotropy and spin-rotation interactions from the observed electron spin-lattice relaxation rates, the contribution of the thermally activated process was obtained and compared with its expression for the temperature dependence. Consistent values of various fitted parameters, used in the expression of the thermal process, have been found, and the applicability of the expression of the thermally activated process to describe the temperature dependence in liquid solutions has been vindicated. Moderate solvent dependence of the thermally activated process has also been observed. The rotational correlation times and the spin-lattice relaxation processes of nitroxyls in ionic liquids and in conventional organic liquids are shown to be explicable on a similar footing, requiring no special treatment for ionic liquids.
authors.affiliation: †Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai 400005, India.
authors.forename: Krishnendu
authors.initials: K
authors.lastname: Kundu
authors.affiliation: #Physical and Theoretical Chemistry Laboratory, University of Oxford, South Parks Road, Oxford, OX1 3QZ, United Kingdom.
authors.forename: Daniel R
authors.initials: DR
authors.lastname: Kattnig
authors.affiliation: §Institute of Physical and Theoretical Chemistry, Graz University of Technology, Stremayrgasse 9/Z2, A-8010 Graz, Austria.
authors.forename: Boryana Y
authors.initials: BY
authors.lastname: Mladenova
authors.affiliation: §Institute of Physical and Theoretical Chemistry, Graz University of Technology, Stremayrgasse 9/Z2, A-8010 Graz, Austria.
authors.forename: Günter
authors.initials: G
authors.lastname: Grampp
authors.affiliation: †Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai 400005, India.
authors.forename: Ranjan
authors.initials: R
authors.lastname: Das
mesh_terms: D001555:Benzene Derivatives / Q000737:chemistry; D003497:Cyclic N-Oxides / Q000737:chemistry; D004578:Electron Spin Resonance Spectroscopy; D019811:Hydroxylamine / Q000737:chemistry; D052578:Ionic Liquids / Q000737:chemistry*; D009584:Nitrogen / Q000737:chemistry; D009589:Nitrogen Oxides / Q000737:chemistry*; D010795:Phthalic Acids / Q000737:chemistry; D011522:Protons; D012399:Rotation; D012997:Solvents / Q000737:chemistry; D013696:Temperature; D014783:Viscosity
chemical_list: D001555:Benzene Derivatives; D003497:Cyclic N-Oxides; D052578:Ionic Liquids; D009589:Nitrogen Oxides; D010795:Phthalic Acids; D011522:Protons; D012997:Solvents; D019811:Hydroxylamine; C012125:diisononyl phthalate; C063840:sec-butylbenzene; C039900:nitroxyl; D009584:Nitrogen; C505333:TEMPOL-H; C003959:TEMPO