High frequency EPR of a copper(II) trimer: experiment time scale effects in EPR spectroscopy Joris van Slageren a , Anife Ahmedova b , Dante Gatteschi a, *, Carlo Andrea Massa c , Luca A. Pardi c a Dipartimento di Chimica, Universita ´ degli Studi di Firenze and INSTM UdR Firenze, Polo Scientifico, Via della Lastruccia 3, I-50019 Sesto Fiorentino, FI, Italy b Department of Chemistry, University of Sofia, 1, J. Bourchier Avenue, 1126 Sofia, Bulgaria c Istituto per i Processi Chimico-Fisici, Consiglio Nazionale delle Ricerche, Via G. Moruzzi 1, I-56124 Pisa, Italy Received 9 October 2002; accepted 22 January 2003 Abstract High-frequency (190 GHz) EPR spectra are reported, that were recorded on an exchange coupled, trimeric copper cluster, octachlorodiadeniniumtricopper(II). The lines in the rhombic spectra shift as a function of temperature. This can be traced to the presence of two close lying spin doublet states, with different g tensors, that interconvert quite rapidly. Therefore, the lines observed in the spectrum have the Boltzmann weighted average of the two g values. The interesting observation is that signals due to the excited spin doublet are observed. This means that the interconversion rate is of the same order of magnitude as the Electron Paramagnetic Resonance experiment. Previous experiments at X-band frequency (9 GHz) could only ascertain that the interconversion rate was faster than that frequency. # 2003 Elsevier Science B.V. All rights reserved. Keywords: High-field high-frequency EPR; Exchange coupled clusters; Copper(II) compounds; Dynamic phenomena 1. Introduction Electron paramagnetic resonance (EPR) spectroscopy is one of the most powerful techniques to study the properties of paramagnetic exchange coupled cluster molecules. Properties such as zero-field splitting and g values are often readily extracted from EPR spectra. If low-lying spin excited states are present, the EPR spectra can present a Boltzmann weighted superposition of the spectra due to the individual spin states. Relevant examples can be found among dimeric [1,2], and octa- nuclear complexes of Cr III [3], and in Fe III compounds [4]. The observation of signals due to separate species implies that these species do not interconvert on a time scale comparable to or greater than the frequency of the EPR experiment at a certain temperature. In mononuclear complexes, spin-lattice relaxation can occur either directly or in a two-photon process. In exchange coupled species, the spin-lattice interaction causes modulation of the isotropic exchange interaction. This modulation can give rise to a fast interconversion between states of equal spin, as an additional relaxation pathway. Instead, for states differing in spin by DS /1, the anisotropic or antisymmetric parts of the dynamic exchange interaction are needed [5]. One example of a cluster where the interconversion between different spin states is too fast to allow EPR spectra due to them to be observed separately is octachlorodiadeniniumtricopper(II) (Fig. 1) [6]. In this complex, the three copper(II) ions are linked together in a linear fashion by the bridging chloride ions. The exchange interaction between these ions can be de- scribed by the spin Hamiltonian H J (S 1 × S 2 S 2 × S 3 ) J ?(S 1 × S 3 ) (1) If the resulting spin states are described as jS 1 S 2 S 3 S 13 S T /, using an intermediate spin operator * Corresponding author. Address: Universita di Firenze, Dipartimento di Chimica Generale, Via Maragliano 75/77, Firenze 50144, Italy. Tel.:  /39-055-354 841; fax:  /39-055-354 845. E-mail address: dante.gatteschi@unifi.it (D. Gatteschi). Inorganica Chimica Acta 351 (2003) 59  /62 www.elsevier.com/locate/ica 0020-1693/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0020-1693(03)00191-9