Dynamic polarization and relaxation of protons in Cr(V) complexes in dimethylformamide

Cr(V) complexes were prepared by reduction of hexavalent Cr salt by pyrocatechol in a highly polarized solvent - dimethyl formamide. The experiments were conducted in a magnetic field 0.89 T at temperatures 4.2 and 1.4 K. The proton enhancement factor (E) for the Cr(V): DMFA complex is given at a temperature of 1.4 K in dependence on the amount of pyrocatechol as is the dependence of proton relaxation times on the amount of pyrocatechol. It was found that an increase in nuclear polarization at a temperature reduced from 4.2 to 1.4 K may indicate than any further temperature decrease might increase E. (J.P.)