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