Synthesis and characterization of a macrocyclic Schiff base Gd-III complex as a relaxation agent for a faster acquisition of H-2 NMR spectra of ethanol

Quant. 2H NMR anal. at the natural abundance represents a well-recognized and efficient method for the identification of the origin of EtOH from different sources. An intrinsic limitation of the protocol used is the long time required, .apprx.8 h, because of the long T1 values of the 2H resonances. The authors propose the use of a paramagnetic relaxation agent that significantly catalyzes the relaxation times and reduces the total time of the anal. This agent is the newly prepd. macrocyclic Schiff base complex [Gd(H2L)(H2O)3(EtOH)](Cl)3×2EtOH (H2L is the [1 + 1] macrocycle derived from the condensation of 3,3'-(3-oxapentane-1,5-diyldioxy)bis(2-hydroxybenzaldehyde) with 1,5-diamino-3-azamethylpentane), which is highly stable and sol. in the alc. soln. used. Elemental anal., IR and mass spectrometry have characterized this complex. The homogeneity of the complex and the correct Gd:Cl = 1:3 ratio was established by SEM-EDS measurements. Further characterization of the paramagnetic complex was achieved by measuring the magnetic field dependence of the 1H longitudinal nuclear magnetic relaxation time of a 1 mM soln. in CH3OD with a field-cycling relaxometer. The GdIII ion accommodates up to four MeOH mols. in its inner coordination sphere, whose rapid exchange with the bulk provides an efficient relaxation mechanism. The addn. of .apprx.37 mg of the complex to a soln. of EtOH (3.0 g) and tetramethylurea (TMU) (1.5 g) results in the redn. of the exptl. time of >50% with a S/N ratio compatible with that required for this application.