The differences in g-values of two radicals, or the g-anisotropy of individual centers, become better resolved in high-field/high-frequency EPR. This is also illustrated in Fig. 2, where a spectrum obtained by conventional 9 GHz EPR is compared to spectra obtained at 95 GHz and at 275 GHz. Fig. 2 EPR spectra taken at increasing magnetic field/frequency strengths showing the increased spectral resolution obtained by high-field/high-frequency EPR. Shown is the frozen solution spectrum of a nitroxide spin label at 9 GHz (X-band), 95 GHz (W-band, bottom scale), and 275 GHz (J-band, top scale). All spectra have the same relative B 0-field scale. The g-tensor components

g xx , g yy , and g zz become increasingly separated. The separation (A zz ) between the three lines at the high field side high

field of the spectra remains constant, owing to PND-1186 order the independence of the hyperfine splitting from the external magnetic field. Figure modified from Finiguerra et al. (2006) Orientation selection has MK-8931 clinical trial been used to determine the relative orientations of the paramagnetic centers in photosynthesis (van der Est 2009; Savitzky and Möbius 2009; Kothe and Thurnauer 2009). Chemical shifts The chemical shift of nuclear resonances in NMR derives from the shielding of the external magnetic field at the position of the nucleus, which is caused by the magnetic field induced by the MLN2238 solubility dmso circulation of electrons in the molecule (Carrington and McLachlan 1979). So the electron density in the vicinity of the observed nucleus is important, and electron donating and withdrawing groups have a well-established effect on the chemical shift of the magnetic nuclei in a molecule. Chemical shift differences in the order of 10 ppm are common for protons, very 200 ppm for 13C nuclei. In a

400 MHz NMR spectrometer (9.4 T) the proton chemical shift range corresponds to a spread in the frequency of the lines of only 4 kHz. The magnetic field of an unpaired electron overwhelms this effect by far, since hyperfine splittings can be in the order of ten to hundreds of MHz, and therefore nuclei in the vicinity of or coupled to such an unpaired electron are shifted so far in the field that they cannot be observed under the usual conditions. Dipolar spin–spin interactions The interactions of electron and nuclear spins are often dipolar. Generally, a dipolar interaction between two magnetic moments μ1 and μ2 is given by $$ \Updelta E = \frac\overrightarrow \mu_1 \overrightarrow \mu_2 r^3 – \frac\left( \overrightarrow \mu_1 \overrightarrow r \right)\left( \overrightarrow \mu_2 \overrightarrow r \right)r^5 . $$ (3)Here r is the vector joining the two magnetic moments. Working out the scalar vector products under the condition that μ1 and μ2 are parallel results in $$ \Updelta E = \frac\mu_1 \mu_2 (1 – \cos^2 \theta )r^3 , $$ (4)where θ is the angle between r and the magnetic field.