Relaxation of 13C and 15N nuclei by solvent-refined coal

Abstract

The effects of changing various spectroscopic parameters on the solid-state and solution 13C-nuclear magnetic resonance (n.m.r.) spectra of solvent-refined coal have been investigated. Solution spectra were obtained with the use of broad band decoupling, inverse gated decoupling and coupling techniques. Conventional relaxation reagent (Cr(acac) 3) was sometimes added to the solvent-refined coal. The effects of pulse delay on the total signal intensity, and on the intensity of the signal from aromatic carbon have been measured. The results show that inverse gated decoupling with pulse delays of 10 s is needed for complete relaxation of solvent-refined coal, but pulse delays of 6 s can give accurate estimations of aromaticity. However, it is recommended that conventional relaxation reagent such as Cr(acac) 3 be added to ensure relaxation if shorter pulse delays are used. The effect of solvent-refined coal on relaxation of some pure compounds in solution has also been studied. Solvent-refined coal acts as a relaxation reagent on 13C nuclei in benzene, toluene and ethylbenzene. It can also relax 15N nuclei in aniline, N, N-dimethylaniline and nitromethane. Spin-lattice relaxation times ( T 1's) of selected nuclei have been measured and the contribution of solvent-refined coal to relaxation ( T 1SRC) has been calculated. Solid-state 13C-n.m.r. spectra have been obtained using the cross-polarization (CP) technique with magic-angle sample spinning (MASS). A variety of cross-polarization times and recycle times have been used. The results show that no serious errors in measurement of aromaticity ( f a ) are caused by using a contact time as short as 1 ms and a recycle time of 0.3 s. There is good agreement between f a 's obtained by solution and solid-state n.m.r., and as solid-state spectra can be obtained in only a fraction of the time needed to obtain a solution spectrum (≈ 1 20th ), CP-n.m.r. is the method of choice for analysis of f a of solvent-refined coal. The results also show that CP-MASS n.m.r. can be used to estimate the fraction of aromatic carbon which is unprotonated in solvent-refined coal and, hence, indirectly, the fraction of hydrogen which is aromatic.