An all-atom 5 nanosecond molecular dynamics simulation of a water-solvated micelle containing 60 sodium dodecyl sulfate monomers was performed. Structural properties such as the radius of gyration, eccentricity, micellar size, accessible surface area, dihedral angle distribution, carbon atom distribution, and the orientation of the monomers toward the micelle center of mass were evaluated. The results indicate a stable micellar system over the duration of the simulation. Evaluation of the structure and motion of the sodium counterions show (1) a long equilibration time (1 nanosecond) is required to achieve a stable distribution of counterions and (2) approximately 25% of the sodium ions are located in the first shell and 50% are located in the first two shells of the micelle during the course of the simulation. The structure of the micelle oxygen-sodium ion radial distribution function reveals two distinct peaks which divide the counterions into those close to the micelle (first shell) those far from the micelle (bulk) and those between (second shell). Finally, values of the diffusion coefficient for sodium ions followed a decreasing trend for ions in the bulk of the micellar system (D ) 1.9 × 10 -5 cm 2 /s), ions in the second shell of the micelle (D ) 1.4 × 10 -5 cm 2 /s), and those in the first shell of the micelle (D ) 1.0 × 10 -5 cm 2 /s).

The emitting metal-to-ligand charge transfer (MLCT) excited state of [Ru(bpy)2(bpz)]2+ (bpy is 2,2‘-bipyridine; bpz is 2,2‘-bipyrazine) is reductively quenched by hydroquinone (H2Q) by proton-coupled electron transfer (PCET), most likely by concerted electron−proton transfer (EPT). The identity of the transient products ([Ru(bpy)2(bpzH•)]2+ and HQ•) and the kinetics of their formation and disappearance have been established by steady-state emission and time-resolved emission, absorption, and EPR measurements. The protonated, reduced complex [Ru(bpy)2(bpzH•)]2+ functions as a H-atom reductant toward quinone or benzaldehyde with potential implications for net photochemistry and energy conversion.

Abstract— Steady‐state and time‐resolved electron paramagnetic resonance (TREPR) experiments are described. Comparison of the TREPR continuous wave method to other time domain EPR techniques such as Fourier transform EPR (FT‐EPR) is made, and the advantages and disadvantages of each are presented. The role played by several mechanisms of chemically induced dynamic electron spin polarization (CIDEP) in the appearance of the spectra is explained. The advantages of using higher frequency spectrometers than the standard X‐band (9.5 GHz) are presented and discussed. Examples are presented that are relevant to organic photochemistry and electron donor‐acceptor chemistry. The use of TREPR to study polymer photodegradation, polymer chain dynamics, free radical initiator chemistry and biradical spin exchange interactions is described. Emphasis is placed on magnetic field effects studied by multiple frequency TREPR in these systems. Finally, several future directions in the field are discussed in terms of new developments in microwave and magnetic field technology.

Spin and conformational dynamics in symmetric alkyne-bridged multi[copper(II) porphyrin] structures have been studied in toluene solution at variable temperature using steady-state electron paramagnetic resonance (EPR) spectroscopy. Comparison of the dimer EPR spectra to those of Cu porphyrin monomers shows evidence of an isotropic exchange interaction (J) in these biradicaloid structures, manifested by a significant line broadening in the dimer spectra. The extent line broadening depends on molecular structure and temperature, suggesting J is modulated by conformational dynamics that impact the torsional angle distribution between the porphyrin-porphyrin least-squares planes. Computational simulation of the experimental EPR spectra, using a developed algorithm for J modulation in flexible organic biradicals, supports this hypothesis. Comparison of ethyne and butadiyne alkyne bridges reveals remarkable sensitivity to orbital interactions between the spacer and the metal, reflected in measurements of J as a function of temperature. The results suggest orbital symmetry relationships may be more important than recognized in design of optimized molecular spintronic devices.

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Expressions for contributions to TI and T2 from modulation of the exchange interaction (J) in spin-correlated radical pairs (SCRPs) are derived and incorporated into SCRP theory for simulation of direct detection EPR spectra of alkane chain biradicals. The T2 term is consistent with that previously derived by Luckhurst to explain the temperature-dependent alternating line widths observed in EPR spectra of stable nitroxide biradicals.Alternating line-width patterns are observed for symmetric bis(alky1) biradicals at high temperatures, where the rapid modulation of J is caused by conformational jumping. However, in unsymmetrical systems such as acyl-alkyl biradicals, J modulation manifests itself at low temperatures as an increased broadening of the spectral lines with increasing distance of the transition from the center of the spectrum, with no alternation.Several experimental examples are presented, simulated, and discussed. Contrary to a suggestion by Maeda et al., J modulation is shown to have only a minimal effect on TI in SCRP theory for the case of biradicals undergoing rapid conformational interconversion. A connection is proposed between the magnitude of the relaxation matrix elements and the biradical conformational distribution and chain dynamics.

The time-resolved electron paramagnetic resonance spectrum of a 1,14-bis(alkyl)biradical linked via the para position of an aryl ether moiety has been studied. Previously the spectrum obtained at 50°C could not be simulated using spin-correlated radical pair (SCRP) theory. It is successfully simulated here by adding modulation of the exchange interaction (J) to the fitting routine as a T 2 relaxation mechanism. Inclusion of this line-broadening effect allowed an accurate value of the exchange interaction to be determined for this temperature. Comparison of this J value with that obtained in previous work for the meta isomer of the same biradical showed that the dominant mechanism of J coupling is through-solvent rather than through-bond. The magnitude of the J modulation matrix elements in the two isomers are discussed as are their temperature dependencies.In a recent paper 1 we presented the experimental and simulated time-resolved electron paramagnetic resonance (TREPR) spectra of biradical 1a (shown in Scheme 1) obtained in n-octane solution at two temperatures. The purpose of that work was to attempt to differentiate between two coupling mechanisms for the spin exchange interaction J: through-bond (TB) and throughsolvent (TS). It has been shown in another of our papers 2 that the TB and TS mechanisms have opposite temperature dependencies for flexible alkane chain biradicals. While the TS coupling becomes greater at higher temperatures, the TB mechanism dominates at the lower ones. This means that a temperature dependence of the TREPR spectrum provides an excellent way to test the dominant mechanism of J coupling in these structures. As demonstrated in additional published work from our laboratory, construction of biradical precursors with double bonds 3,4 or conjugated π-systems 1 can change both mechanisms simultaneously. Figure 1A shows the X-band TREPR spectrum of biradical 1a at 50°C, obtained by laser flash photolysis of precursor ketone 1. All experimental details regarding the collection of this data are given in ref 1. To get a better idea of which electronic coupling mechanism is dominant, our previous experiments were also performed on the meta isomer of 1a, and the temperature dependencies of the TREPR spectra were also studied for both isomers. 1 At higher temperatures (105°C ), good quality simulations of spectra from 1a were obtained using SCRP theory in its most basic form. By comparison to the meta isomer, it could be concluded that the dominant mechanism was TS at 105°C. However, at lower temperatures, a very poor quality fit was obtained, as shown in Figure 1B. It is more accurate to state that we were unable to simulate the spectrum of 1a at 50°C. At that temperature, however, we could stimulate the spectrum from the meta isomer very well.In this paper we demonstrate that the inclusion of J modulation (caused by conformational motion) as a relaxation mechanism into the SCRP simulation routine allows us to resolve this situation and reach excellent agreement between the experimental and sim...

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