Communication An in situ electrochemical cell for Q- and W-band EPR spectroscopy Paul R. Murray a , David Collison b , Simon Daff a , Nicola Austin a , Ruth Edge b , Brian W. Flynn c , Lorna Jack a , Fanny Leroux a , Eric J.L. McInnes b,⇑ , Alan F. Murray c , Daniel Sells b , Tom Stevenson c,1 , Joanna Wolowska b , Lesley J. Yellowlees a,⇑ a School of Chemistry, University of Edinburgh, Joseph Black Building, King’s Buildings, West Mains Road, Edinburgh EH9 3JJ, UK b EPSRC National UK EPR Facility, School of Chemistry and Photon Science Institute, The University of Manchester, Oxford Road, Manchester M13 9PL, UK c School of Engineering, The University of Edinburgh, Mayfield Road, Edinburgh, UK article info Article history: Received 25 July 2011 Revised 16 September 2011 Available online 22 September 2011 Keywords: Spectroelectrochemistry EPR ESR High frequency EPR Bipyridines Flavins abstract A simple design for an in situ, three-electrode spectroelectrochemical cell is reported that can be used in commercial Q- and W-band (ca. 34 and 94 GHz, respectively) electron paramagnetic resonance (EPR) spectrometers, using standard sample tubing (1.0 and 0.5 mm inner diameter, respectively) and within variable temperature cryostat systems. The use of the cell is demonstrated by the in situ generation of organic free radicals (quinones and diimines) in fluid and frozen media, transition metal ion radical anions, and on the enzyme nitric oxide synthase reductase domain (NOSrd), in which a pair of flavin rad- icals are generated. Ó 2011 Elsevier Inc. All rights reserved. 1. Introduction EPR spectroscopy is a powerful technique for the study of elec- tron transfer reactions because one-electron transfer must involve paramagnetic species. Redox generation of materials for EPR can be performed ex situ by chemical or electrochemical means followed by transfer to the spectroscopic cell. However, in situ electrochem- ical generation is preferable, giving access to less stable radicals. A comprehensive review of cell designs for in situ electrochemical EPR has been published by Compton and co-workers [1]. The vast majority of such cells are for X-band (ca. 9 GHz) spectroscopy, since this is the most common microwave frequency employed in continuous wave EPR, and have been developed for both static and flow (giving access to kinetic information) experiments and different resonator designs. However, in our electrochemical EPR work we have often found poorly resolved spectra from frozen solutions of electrochemically generated paramagnets at X-band due to the limited field resolution of the g-matrix. This motivated us to develop an in situ electrochemical cell for higher frequency EPR. In addition to enhanced field and orientation resolution of g-values, higher frequencies also offer advantages in, for example, detection of S > 1/2 species with large zero-field splitting, accessing faster motional dynamics, and higher sensitivity for small samples. X-band resonators have ample space for design of three-elec- trode cells and hence proper control of potential and segregation of electrodes. Higher microwave frequencies present severe space limitations when using resonant cavities, scaling with the wave- length. Simple in situ electrochemical experiments have been de- scribed at Q-band using crude two electrode cells [2], but this gives no control over potential and is only really of use for simple single redox event species. Here we detail a simple three-electrode in situ electrochemical cell which can be used in commercial Q- and W-band spectrometers (ca. 34 and 94 GHz, respectively). The cell is suitable for variable temperature use, allowing chilling and/or freezing of the samples (during or post-electrolysis) and hence access to the anisotropic g-values. We illustrate the use of the cell with coordination chemistry, free radical and biological examples. 2. Results and discussion Most EPR spectroelectrochemical cells (for X-band spectros- copy) are based on grid, coil or cylinder working electrodes, ensur- ing a high surface area and efficient electrolysis [1]. This is not possible for Q- and W-band (on commercial spectrometers with resonant cavities) because of the space restrictions: the inner diameter of the sample tubing is 1.0 and 0.5 mm for Q- and 1090-7807/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.jmr.2011.09.041 ⇑ Corresponding authors. Fax: +44 (0)161 275 4616 (E.J.L. McInnes), fax: +44 (0)131 650 6453 (L.J. Yellowlees). E-mail addresses: eric.mcinnes@manchester.ac.uk (E.J.L. McInnes), t.stevenson @ed.ac.uk(T. Stevenson), l.j.yellowlees@ed.ac.uk (L.J. Yellowlees). 1 Fax: +44 (0)131 650 6554. Journal of Magnetic Resonance 213 (2011) 206–209 Contents lists available at SciVerse ScienceDirect Journal of Magnetic Resonance journal homepage: www.elsevier.com/locate/jmr