关键词: Raman磁共振, 强单量子跃迁的压抑, 异核AX_n体系

Abstract: Raman magnetic resonance experiment is a useful tool for direct detection of multiple-quantum signals in one-dimensional mode. It has been shown that all possible order quantum transitions in a spin system may be detected by the method using excitation radiofrequency field with proper offset and strengths. Among these quantum transition signals the single-quantum transition signals are usually strongest. The strong single-quantum signals may cause problems in dynamic range of receiver, spectral baseline roll and phase artifacts. Thus, suppression of strong single-quantum signals is necessary to obtain a high quality Raman magnetic resonance spectrum. Here we report an RMR experiment to suppress the strong single-quantum signals by alternatively accumulating FID signals with positive and negative irradiation offsets. It follows from the analysis of the product operator formalism that the strong single-quantum signals are symmetrical about offset, that is, when irradiation offset changes from positive value to negative, the strong single-quantum signals also invert their intensities for a heteronuclear spin system. Therefore, the accumulation of FID with positive and negative irradiation offset can suppress strong single-quantum signals in RMR experiments. The RMR experiments with alternative positive and negative offset accumulation acquisition on heteronuclear AX_n(n=1,2,3) spin systems (CHCl₃,CH₂Cl₂,CH₃OH) are carried out on an ARX-500MHz spectrometer, where proton is irradiated and canbon-13 is detected. The experimental results show that the strong single-quantum signals are suppressed effectively and the multiple-quantum signals with intensity similar to those of the single-quantum signals are acquired. This is in a good agreement with the above theoretical prediction.

Key words: Raman magnetic resonance, Suppression of strong single-quantum signals, Heteronuclear AX_n spin systems