Dynamics of interspecies hydrogen bonding of partially fluorinated compounds and a stable nitroxide radical studied by1H and19F relaxation and lowhyphen;field DNP measurements

摘要

The coupling of ndash;OH protons of (CF3)3COH and C6F5OH interacting with 4hyphen;oxohyphen;2,2,6,6hyphen;tetramethylpiperidinohyphen;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 7times;10minus;11s in both cases and interaction distances of 2.5 and 2.3 Aring;, respectively. The dipolar correlation times are longer and the interaction distances are shorter than for the ndash;OH protons in the CF3CH2OH:TMPO interaction. These observations are consistent with the complex formation constants, as determined by ESR measurements, of 4.1plusmn;0.4, 4.4plusmn;0.4, and 1.9plusmn;0.6 mminus;1for 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.