Dynamics of interspecies hydrogen bonding of partially fluorinated compounds and a stable nitroxide radical studied by 1H and 19F relaxation and low-field DNP measurements

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The coupling of –OH protons of (CF3)3COH and C6F5OH interacting with 4-oxo-2,2,6,6-tetramethylpiperidino-oxy (TMPO) free radical is governed by dipolar coupling of the two spins. The magnetic field dependence of the proton relaxation indicates that the interaction is dominated by rotationally modulated diffusion of the interspecies complex with a correlation time of 7×10−11 s in both cases and interaction distances of 2.5 and 2.3 Å, respectively. The dipolar correlation times are longer and the interaction distances are shorter than for the –OH protons in the CF3CH2OH:TMPO interaction. These observations are consistent with the complex formation constants, as determined by ESR measurements, of 4.1±0.4, 4.4±0.4, and 1.9±0.6 m−1 for the (CF3)3COH:TMPO, C6F5OH:TMPO, and CF3CH2OH:TMPO complexes, respectively. Dynamic nuclear polarization (DNP) measurements also indicate that the fluorine atoms in the first two complexes show significant scalar interaction of the fluorines with the unpaired electron on the spin label.