Richard D. Bates

Use of nitroxide spin labels in studies of solvent–solute interactions

Complexities imposed on the studies of dynamic polarization of solvent nuclei by the three‐line ESR spectrum of solute nitroxide radicals are examined. These include (1) the field dependence of proton polarizations, (2) the unequal magnitude of the polarizations at frequencies corresponding to the nitroxide radical peaks, and (3) the radical concentration dependence of the extrapolated enhancements. These features are analyzed with respect to existing models and with respect to a new mathematical model based on an exchange interaction that couples radicals in different 14N spin states. This model successfully accounts for the nonlinear extrapolation of enhancements as a function of reciprocal radical concentration. The model is tested experimentally using samples of 4‐hydroxy‐2,2,6,6‐tetramethyl piperid‐1‐yloxy free radical in benzene, with both DNP and relaxation data used in the analysis.

Temperature‐dependent relaxation of CO(v=1) by HD, D2, and He and of D2(v=1) by D2

Rate constants for the vibrational deactivation of CO in collisions with HD, D2, and He have been measured as a function of temperature using the laser excited vibrational fluorescence technique. Throughout the 109–630 °K range, CO–He and CO–HD samples exhibit a single exponential decay, dominated by V–T,R transfer, with rates increasing rapidly with temperature. Typical collision deactivation rate constants at 630 °K are 22.7 sec−1 torr−1 for He and 82.5 sec−1 torr−1 for HD, and, at 109 °K, 0.067 sec−1 torr−1 for He and 0.27 sec−1 torr−1 for HD. At low temperatures, diffusion and radiative decay become important contributions to the observed rates. In CO–D2 mixtures, double exponential decay of CO fluorescence at large D2 mole fractions is obtained, corresponding to rapid V–V transfer between the (v=1) vibrational levels of CO and D2, followed by coupled V–T,R deactivation. The V–V transfer rate (ΔE=−850 cm−1) increases from 0.26 sec−1 torr−1 at 202 °K to 69.4 sec−1 torr−1 at 633 °K. The V–T,R deactivati...

Relaxation of fluorine nuclei by collisions with free radicals

Intermolecular nuclear relaxation during liquid‐state collisions has been examined for 16 combinations of four free radicals and four fluorocarbons. The spectra of electron‐induced scalar relaxation transitions are resolved into high‐ and low‐frequency components. All samples are found to have a spectral component with a short correlation time of around 1×10−11 sec, corresponding to fast diffusion‐controlled collisions. Those samples which show stronger scalar hyperfine coupling show a second component with time up to 44×10−11 sec, corresponding to sluggish stereospecific collisions or transient complexation. Strong scalar coupling is found to occur with a proportionally strong dipolar coupling, which is augmented well beyond that expected from feasible molecular approach distances. Monopoly of spin‐rich sites by the fluorocarbon might account for part of the dipolar augmentation but does not satisfactorily account for the entire range of observations. An induced anisotropic hyperfine coupling, after the ...

Dynamic polarization of carbon-13 nuclei in free radical solutions

Abstract In contrast to previously studied nuclei, solvent carbon-13 NMR enhancements at 74 G for a series of free radicals show increased contribution of scalar coupling for well shielded, saturated carbons. Enriched 13 CCl 4 with BDPA gives an ultimate enhancement of +850.

Interspecies vibrational energy flow in CO2 or N2O mixtures with the series of deuterated methanes

Time‐resolved laser‐induced infrared fluorescence was used to study the rates of deactivation of vibrationally excited CO2 and N2O by the series of deuterated methanes CHnD4−n, where n=0–4. Rates varied from 4.7 msec−1 torr−1 for CO2–CH4 to 199 msec−1 torr−1 for N2O–CD4. Within experimental error, the deactivation probabilities for CO2 or N2O by the same collision partner are identical. The rates observed were determined to increase linearly with the number of deuterons per substituted methane, and thus with the number of C–D stretches nearly resonant with the 001 state of CO2 or N2O. The results are interpreted by examining the role that rotational state changes of the collision partners can play in reducing the vibrational energy defect, thus minimizing the amount of energy that must go into translations.

Dynamics of interspecies hydrogen bonding of a fluorinated alcohol and a nitroxide free radical

The combined use of (1) the magnetic field dependence of solvent nuclear relaxation times induced by paramagnetic solutes and (2) the dynamic nuclear polarization of the same solvent nuclei is reported in a study of the dynamics of interactions between trifluoroethanol and 4‐oxo‐2,2,6,6‐tetramethyl piperidino‐oxy free radical. The –OH proton behavior is dominated by dipolar coupling with the spin‐label unpaired electron that is characterized by rotational diffusion of the complex with a correlation time of 5×10−11 s and an interaction distance of 2.6 A. The interaction with the –CH2 protons and –CF3 fluorine nuclei is weaker, dominated by dipolar coupling with the free radical, and characterized by a shorter correlation time.

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

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 A, 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 spi...

Rapid interspecies energy flow between vibrationally excited SF6 and the asymmetric stretch of N2O

Rapid, selective collision-dependent excitation of N2O following pumping of SF6 with a CO2 laser is reported. The N2O fluorescence rise depends on the pressure of each component and is dominated by the SF6-dependent contribution of 2290 ms−1 Torr−1. The subsequent fall is governed by V→V processes among SF6 vibrational modes.

Rapid sensitization of vibrational energy levels of N2O in collisions with SF6 excited by a CO2 laser

Abstract Experimental investigations of mixtures containing predominantly N2O and small amounts of SF6 demonstrate that rapid interspecies pooling of vibrational energy can occur to produce a pulse of excess vibrational energy in the ν3 mode of N2O following excitation of SF6 by a Q-switch CO2 laser. This increased population in the ν3 mode of N2O can occur on a time scale shorter than that on which collision-induced VV processes redistribute vibrational energy among the modes of SF6. The equilibration takes place in three discernible stages: (1) a rapid pooling of energy between a limited number of levels of the SF6 and N2O, then (2) a slower collision-dependent VV process that equilibrates all the vibrational modes in the system, with (3) a subsequent VT,R process that returns the system to its initial state. Argon is shown to accelerate selectively process (2) with an efficiency consistent with the previously measured ability of argon to accelerate the VV process in pure SF6. The experimental evidence indicates that other modes in N2O do not become involved on the time scale on which direct crossing to ν3 occurs. Additionally, on the time scale preceding the SF6 VV equilibration, a fast collision-dependent process competes with the transfer of excitation to N2O. The production of a pulse of excitation in N2O is eliminated when isotopically substituted N2O (14N15NO) is used instead under the same conditions because the crossing rate to the ν3 mode of N2O is decreased sufficiently when 15N is substituted for 14N that it no longer can compete with the VV equilibration among the modes in SF6.

Deactivation of vibrationally excited CD3H using laser-induced fluorescence

Abstract Infrared fluorescence observed after exciting to ν 6 (ν=1) of CD 3 H with a Q -switched CO 2 laser yields the exponential deactivation rate constant of 0.84 ms −1 torr −1 . Rate constants for deactivation of CD 3 H by rare gases vary from 1.4 (for He) to 0.029 (for Xe) ms −1 torr −1 .