Orientation selection in high-field RIDME and PELDOR experiments involving low-spin CoII ions

Abstract

Orientation selective (OS) RIDME and PELDOR were conducted on a low-spin Co^II complex coordinated by two nitroxide (NO) labelled 2,2′:6′,2′′-terpyridine ligands. Co–NO RIDME at W- and Q-band gave insight into the relative orientation between the Co–NO interspin vector (r_Co–NO) and the NO moiety. This was further supported by W-band Co–NO PELDOR that also allowed elucidating the relative orientation of the Co^II and NO g-tensors. Differences to earlier predictions were confirmed by DFT calculations. Finally, NO–NO PELDOR allowed retrieving the mutual orientations between the NO–NO interspin vector (r_NO–NO) and the NO moieties. The results demonstrate that OS-RIDME and -PELDOR can provide geometric structure information on a system containing a Co^II ion and two nitroxides. Especially, the high sensitivity and ease of interpretation of RIDME at W-band opens avenues for new applications of Co^II as orthogonal spin label.