1. Purpose

The paramagnetic relaxation reagent Cr(acac)₃ is used to increase the intensity of the signals of quaternary ¹³C nuclei, which suffer from long relaxation times T₁. The addition of increasing amounts of the reagent results in a shortening of the relaxation times T₁ and an increase in line-width. The chemical shift is usually unaffected. The relaxation reagent is important for quantitative ¹³C NMR investigations (see Exp. 8.19). Cr(dpm)₃ was reported to be more soluble, but is not commercially available.

2. Literature

M. L. Martin, J. J. Delpuech, G. J. Martin, Practical NMR Spectroscopy, Heyden, London, 1980, Ch. 10.3.
G. C. Levy, U. Edlund, J. Am. Chem. Soc. 1975, 97, 4482–4485.
J. Tian, Y. Yin, H. Sun, X. Luo, J. Magn. Reson. 2002, 159, 137–144.

3. Pulse Scheme and Phase Cycle

Carbon-13 pulse sequence with composite pulse proton decoupling

4. Acquisition

Time requirement: 15 min

Sample: 20% ethyl crotonate in CDCl₃.

Load standard ¹³C NMR acquisition parameters (see Exp. 3.2) and record a ¹³C NMR spectrum. Then add about 20 mg of chromium acetylacetonate, Cr(acac)₃. The solution should become slightly purple. A concentration of 0.1 M is ideal for quantitative work. The highest concentration that is reported in the literature to give reasonable results is