The potential val
ue of electron spin delocalization as a means for s
ubstantially increasing the ability of aparamagnetic metal complex to ind
uce n
uclear spin relaxation of water protons has been examined by covalentattachment of bromine atoms in the
![](/images/gifchars/beta2.gif)
-pyrrole positions of the [5,10,15,20-tetrakis(4-s
ulfonatophenyl)-21
H,23
H-porphine]manganese complexes[Mn
IIITPPS]
3- and [Mn
IITPPS].
4- The water-proton spin-lattice relaxivities arereported as a f
unction of magnetic field strength for the brominated and nonbrominated metalloporphyrins overthe range of magnetic field strengths corresponding to proton Larmor freq
uencies between 0.01 and 30 MHz.The brominated metalloporphyrins increase the water-proton relaxativities compared to the nonbrominatedmetalloporphrins, and, at low magnetic field strengths, the brominated [Mn
IITPPS]
4- complex rivals the efficiencyof the hexaaq
uomanganese(II) ion. Attempts to fit the experimental data to theories for paramagnetic relaxation,which are based on the point-dipole approximation, res
ult in distances between the paramagnetic center and thewater proton that are
unreasonably short based on p
ublished str
uct
ural data. The excess relaxivity implies that thepoint-dipole approximation may be inappropriate for these porphyrin systems and electron spin delocalizationmay provide a significant contrib
ution to n
uclear spin relaxation that may be fr
uitf
ully exploited in constr
uctionof contrast agents for magnetic resonance imaging.