Determination of End-to-End Distances in Oligomers by Pulsed EPR

Abstract

Conformations of end-labeled alkanes in a rigid polymer matrix were investigated using the pulsed double electron-electron resonance (DEER) method. To this end, dipolar spectra of a series of aliphatic dicarbonic acids with 8−20 methylene units and chain ends bifunctionalized with nitroxide radicals were recorded in a polystyrene matrix. The biradicals are derived from hindered amine light stabilizers (HALS), used as polymer additives. Analysis of the dipolar spectra allows the determination of the end-to-end distances of the oligomer biradicals. The experiments show that the “flexible” alkane chains are almost completely stretched. This observation is consistent with the results of force field calculations.

Dead-Time Free Measurement of Dipole–Dipole Interactions between Electron Spins

“…The correspondingly lower Q L value (about 100) leads to a loss of signal. However, handling a single-mode mw resonator proved to be more convenient and reliable compared to a bimodal resonator used before (18). In contrast to previous works, the position of irradiating the A spins is now located in the left shoulder of the nitroxide spectrum; see Fig.…”

“…We use the formalism of Pfannebecker et al (18) for understanding the new pulse sequence. For simplicity, we restrict our consideration to an isolated electron-electron two-spin system S A ϭ 1 2 and S B ϭ 1 2 .…”

“…Biradical 1 was used as a model compound because of its known end-to-end distance (16,18). This sample was also used for determining the optimum experimental conditions.…”

Direct Conversion of EPR Dipolar Time Evolution Data to Distance Distributions

“…Distortions due to orientational selection are not expected for this flexible biradical, as the orientations of the molecular frames of the two nitroxide groups are not strongly correlated. At least part of the deviation between the two curves is due to the fact that the end-to-end distance of these biradicals is not expected to exhibit a Gaussian distribution in a G(r ) plot (40,41). The access to experimental distance distributions, rather than only mean distances, should allow for more detailed checks of theoretical models for chain conformation statistics.…”

Determination of the Nanostructure of Polymer Materials by Electron Paramagnetic Resonance Spectroscopy

“…More detailed information can be obtained by measurements of intergroup distances between defined points on the chain, for example, between the chain ends. [117] The application of EPR distance measurements to statistically labeled chains can also be of interest, since such measurements require much less expensive facilities than neutron scattering. [29] A CW EPR study on spin-labeled poly(vinylpyridine) made use of the fact that the expected spectral lineshapes, computed from the pair correlation function, are different for Gaussian chains, self-avoiding chains, and collapsed chains.…”