X-band (9.5 GHz) time-resolved electron paramagnetic resonance
(TREPR) spectra of a 1,9-acyl−alkyl biradical
were obtained at room temperature in benzene and in liquid (950 psi)
carbon dioxide (CO2) solutions. The
spin exchange interaction (J) in this biradical is negative
and larger in magnitude than the hyperfine interaction
(q). This leads to the observation, in both solvents,
of spin-correlated radical pair (SCRP) spectra which are
net emissive. Spectra obtained at later delay times (>1.5 μs)
in CO2 exhibit alternating intensities of
their
SCRP transitions due to spin relaxation but do not show any significant
change in line width. The same
effect is observed in benzene, but on a slower time scale. Q-band
(35 GHz) experiments in benzene showed
that the phenomenon was found to be both field and temperature
dependent. It is also chain-length dependent,
being much stronger in short biradicals (
The photodegradation of two alternating R-CO copolymers (R = p-tert-butylstyrenyl and exo-norbornyl) was studied using time-resolved electron paramagnetic resonance (TREPR) spectroscopy at two different frequencies, X-band (9.5 GHz) and Q-band (35.1 GHz), and by product analysis. For the tert-butylstyrenyl-CO copolymer the EPR spectra displayed strong chemically induced dynamic spin polarization (CIDEP) expected for triplet-born radical pairs from the Norrish I a-cleavage reaction. Simulation of the spectra allowed unambiguous assignment of the radicals present as early as 200 ns after the laser flash. The 13C-substituted (13CO) tert-butylstyrenyl copolymer showed unusual magnetic field effects on its spin polarization patterns and an additional long-range 13C hyperfine interaction in the benzylic fragment. The exo-sorbornyl40 copolymer unexpectedly showed weak EPR signals of endo-substituted 2-norbornyl radicals from Norrish I a-cleavage, indicating that the polymer is not completely stereoregular. Product analysis suggests that this polymer also undergoes Norrish type I1 degradation processes, producing l,4-biradicals, which are unobservable by TREPR. A discussion of the mechanisms of photodegradation for both polymers and a spectroscopic analysis of the samples before and after photolysis are presented.
X-band time-resolved electron paramagnetic resonance (TREPR) spectra of three flexible biradicals of varying chain length and structure were obtained in liquid and supercritical carbon dioxide (CO2) solutions and compared to conventional solvents. For C16 acyl-alkyl biradical 1a, an average spin exchange interaction between the radical centers, J(avg), was obtained by spectral simulation using a simple model for spin-correlated radical pairs (SCRPs) and a small amount of T2 relaxation from a previously established J modulation mechanism. A large solvent effect on J(avg) was observed for the first time, varying by almost 1 order of magnitude from CO2 (J(avg) = -120 +/- 10 MHz) to heavy mineral oil (-11 +/- 3 MHz) for 1a. For C15 bis(alkyl) biradical 1b, spectra obtained under supercritical conditions are only slightly different from those detected in liquid CO2 but differ from spectra taken in benzene. For C10 acyl-alkyl biradical 2a, more emissive spin polarization due to S-T- mixing is observed in CO2 than in benzene. These results are discussed in terms of solvent properties such as dielectric constant, viscosity, and specific interactions. Both chain dynamics and changes to the equilibrium distribution of end-to-end distances can alter J(avg) and the observed ratio of S-T0 to S-T- mixing; however, faster chain dynamics is concluded to be the most likely cause of the observed effects in these systems.
An apparatus for the electron paramagnetic resonance (EPR) detection of stable and transient free radicals in pressurized liquids and supercritical fluids is described. The construction of a quartz EPR sample cell capable of withstanding pressures up to 3000 psi (204 atm) and the components of a high pressure flow system are explained and illustrated. In order to accommodate the thick-walled, high pressure sample tube, two X-band (9.5 GHz) microwave cavities were fabricated. One is a brass, cylindrical TE011 cavity for time-resolved (direct detection) EPR experiments where field modulation was not necessary, and the other is a silver wire-wound cylindrical TE011 cavity, mounted in a quartz support tube. The wire-wound cavity allows for simultaneous light excitation and field modulation for steady-state EPR. Both cavities are fitted with tunable end plates to facilitate tuning at X band in the face of large frequency shifts when the sample tube is introduced to the resonator and to allow for operation with smaller sample tubes at ambient pressure. Microwave coupling, flow optimization, and UV-light access for both cavities are described, and sample spectra in both time-resolved and steady-state modes are reported.
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