The nuclear modulation effect in electron spin echoes for complexes of Cu2+ and imidazole with 14N and 15N

Abstract

We have studied the nuclear modulation effect in the electron spin echo decay envelope for the complexes Cu2+–ethylenediamine–imidazole and Cu2+– (imidazole)4 with specific attention to the prominent low frequency patterns due to coupling between Cu2+ and the remote nitrogen nucleus of bound imidazole. (Nitrogen nuclei directly coordinated with Cu2+ do not contribute to the effect.) Some experiments were also made with 15N substituted imidazole in order to distinguish between contributions due to the electron nuclear coupling and to the quadrupolar interaction of 14N. Complexes involving Cu2+ and imidazole were selected for study because of their common occurrence in copperproteins. It was found that the remote nitrogen of imidazole is coupled to Cu2+ by a pseudodipolar term which is several times larger than the classical dipolar coupling and which can be represented by assigning an effective radial distance r eff=2.9 Å to the nitrogen nucleus in place of the actual physical distance r=4.16 Å. There is also a contact term AI⋅S, where A=1.75 MHz for 14N (A=2.5 MHz for 15N). The magnitude of this contact term is such that, when the electron spin echo experiments are performed in the X‐band range (i.e., at fields?3 kG), it almost cancels the Zeeman term for one of the M S =±1/2 manifolds, thus permitting the observation of the zero field quadrupolar frequencies of 14N in the echo envelope. These frequencies are substantially the same as those reported by Hunt, Mackay, and Edmonds (Ref. 21) for the protonated nitrogen in imidazole, thus showing that the imidazole ligands are protonated and that the two complexes studied here carry a net positive charge of 2. Computer simulations of the two‐pulse echo envelope have been made in order to obtain an estimate for the pseudodipolar interaction, and in order to establish that the mixture of 14N (or 15N) superhyperfine states is such as to yield modulation patterns of approximately the observed depths.