固态交叉极化魔角旋转NMR中的极化转移

在普通有机固体中,直接键合的S和I自旋之间的偶极相互作用强于SI_n基团和其它质子之间的I-I耦合。因此,SI_n子系统中的相互作用支配了交叉极化过程中的自旋动力学,在这一新概念的指引下,样品魔角旋转时极化转移的理论和技术有了飞跃发展,特别是,各种新技术涌现出来,以提高高速魔角旋转时极化转移的效率。这些新技术的共同特点是仅调制射频辐射的位相或幅度,而保持高速度的魔角旋转不变。一系列谱编辑方法也已发展起来,它能依据相连质子的个数,产生碳的4个子谱,即C,CH,CH₂和CH₃子谱,这个技术已被用于一组煤样,在煤的CPMAS谱中,4种类型的碳谱是完全互相交迭在一起的,该技术的应用,已取得了令人鼓舞的成果。

POLARIZATION TRANSFER IN SOLIDSTATE CPMAS NMRHiaoling

or normal organic solids, the heteronuclear dipolar interaction between the directlybonded S and I spins is stronger than the I-I coupling between the SL_n groups and theother protons. As a result, the interactions in the SI_n subsystem dominates the spin dy-namics of the cross polarizatinn (CP) process. In the light of this new concept, theoretitalstudies and technical developments of polarization transfer under magic angle spinning(MAS)have leaped forward. Especially, a variety of new techniques have emerged to en-bance the polarization transfer efficiency at high MAS speeds. The common feature ofthese approacbes is to modulate the phases or amplitudes of the radio frequencyirradiations, while keeping the high MAS speeds unchanged. A series of spectralediting methods have been developed, which can generete four subspectra according to theproton multiplidties, i. e. the nonprotonated-C, methine, methylene, and methylsubspectra. This technique has been applied to a set of coal samples, where four types ofsignals orerlap completely in the normal CPMAS spectrum, and has yielded encouragingresults.