¹³³Cs, ¹H two‐dimensional heteronuclear Overhauser effect spectroscopy (HOESY): Structural analysis of organic caesium compounds by detection of short Cs,H contacts

Transient heteronuclear Overhauser effects were observed between 133 Cs and 1 H by two‐dimensional heteronuclear Overhauser effect spectroscopy (HOESY). In caesium tetraphenylborate (CsBPh 4 ) dissolved in pyridine‐ d 5 , Cs + is tightly attached to the BPh 4 − anion, thus forming a contact ion pair (CIP). By contrast, after addition of one equivalent of the crown ether 18‐crown‐6, caesium is located intimately at the site of the crown ether whereas the tetraphenylborate anion is remote (solvent‐separated ion pair, SSIP). The results are in agreement with experiments described earlier by Mei et al. Similar observations were made for caesium 3‐methylpentyl 3‐oxide in tetrahydrofuran‐ d 8 solution. 133 Cs, 1 H HOESY indicates the presence of a contact ion pair. Steady‐state 133 Cs { 1 H} experiments on the latter compound reveal weak fractional enhancements of 0.2%.