A study of weak exchange interactions mediated by resonance assisted hydrogen bonds (RAHB)

abstract Article history:Received 21 December 2011Received in revised form 10 April 2012Accepted 11 April 2012Available online 1 May 2012Keywords:EPR spectroscopyExchange interactionCopper complexesModel systemsBiomolecules EPR measurements have been performed on single crystals of [Cu(L‐tyrosine) 2 ] ∞ at 33.8 GHz and at roomtemperature. The EPR spectra display partially resolved EPR lines for most orientations of the magneticfield in the ab plane, and only one resonance for orientations close to the crystal axes, while only a singleline is observed along any direction in the ca and cb crystal planes. This behavior is a result of the selectivecollapse of the resonances corresponding to the four copper sites in the unit cell produced by the exchangeinteractions between the copper ions. The magnitudes of the exchange interactions between the copperions were evaluated from the angular variation of the line width and the collapse of the EPR lines. Thevalue |J AD /k B |=0.8 K between neighboring copper atoms at 4.942 A is assigned to a syn−anti equatorial−apical carboxylate bridge with a total bond length of 6.822 A, while the small value |J