As used herein, {HEAD ~ a paramagnetic metal ion} is selected from {TAIL ~ a group} consisting of Gd, Mn, Mn, Fe, Cr, Cu, Co, Ni, Dy, Tb, Ho, Er, Eu, and Nd 
is selected from
As used herein , { HEAD ~ a paramagnetic metal ion } is selected from { TAIL ~ a group } consisting of Gd , Mn , Mn , Fe , Cr , Cu , Co , Ni , Dy , Tb , Ho , Er , Eu , and Nd
OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH