Rigid Orthogonal Bis-TEMPO Biradicals with Improved Solubility for Dynamic Nuclear Polarization

Abstract

The synthesis and characterization of oxidized bis-thioketal-trispiro dinitroxide biradicals that orient the nitroxides in a rigid, approximately orthogonal geometry is reported. The biradicals show better performance as polarizing agents in dynamic nuclear polarization (DNP) NMR experiments as compared to biradicals lacking the constrained geometry. In addition, the biradicals display improved solubility in aqueous media due to the presence of polar sulfoxides. The results suggest that the orientation of the radicals is not dramatically affected by the oxidation state of the sulfur atoms in the biradical, and we conclude that a biradical polarizing agent containing a mixture of oxidation states can be used for improved solubility without a loss in performance.

Observation of strongly forbidden solid effect dynamic nuclear polarization transitions via electron-electron double resonance detected NMR

“…Also, note that the relative intensities of the various SE transitions remain similar in the three experiments in Figure 2. This is consistent with (5), where all SE transition amplitudes are linear to the microwave field strength (ω 1S ). One expects the degree of electron saturation for the SE to be proportional to the square of the transition amplitude when the relaxation rate constant is significantly larger than the transition amplitude, and the electron does not become fully saturated.…”

“…In our experiments, we observe higher order transitions-transitions involving two or more nuclei-and see that the intensity of these transitions is much higher than predicted by (5). Therefore, the nuclear-nuclear couplings must contribute to the mixing that leads to the DNP enhancement, since the electron-nuclear coupling cannot account for the transition intensity alone.…”

“…Observation of transitions for n > 1 is not entirely unexpected: Higher order terms in (5) suggest that higher order transitions should exist. Also, the n = 2 transition was observed via 1 H NMR by de Boer, although the relative intensity was far lower than observed here.…”

“…1,2 In recent years, the state of the art of DNP experiments has improved dramatically with the introduction of many new radicals [3][4][5][6][7][8][9][10][11] and improved technique and instrumentation. [12][13][14][15][16] Because NMR is an inherently insensitive technique, DNP allows prohibitively long experiments to become feasible.…”

Hydroalkoxylation of alkynes by a nitroxyl containing alcohol, 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl: synthesis of spin-labeled enol ethers

“…[1][2][3] Among such species, 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) and its congeners are of special interest. They have found wide applications as reagents and catalysts in organic synthesis, 3 antioxidants, 4 materials for radical batteries, 5,6 magneto-active materials, 7 agents for DNP-NMR, 8,9 as well as radiation protective agents 10 and polymerization catalysts. 11 Furthermore, 4-hydroxy-TEMPO exhibits an antihypertensive effect.…”