Orientation selection in high-field RIDME and PELDOR experiments involving low-spin CoII ions†
Orientation selective (OS) RIDME and PELDOR were conducted on a low-spin CoII 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 (rCo–NO) and the NO moiety. This was further supported by W-band Co–NO PELDOR that also allowed elucidating the relative orientation of the CoII 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 (rNO–NO) and the NO moieties. The results demonstrate that OS-RIDME and -PELDOR can provide geometric structure information on a system containing a CoII ion and two nitroxides. Especially, the high sensitivity and ease of interpretation of RIDME at W-band opens avenues for new applications of CoII as orthogonal spin label.