Raman磁共振谱中强单量子信号的压抑

通常单量子信号的强度在Raman磁共振谱中要比其它多量子信号的强度大得多,因而易引起接收机饱和,使信号发生变形,不利于多量子信号的检测,因此压抑强的单量子峰是必要的.积算符理论分析表明,异核体系Raman磁共振谱的单,多量子峰的共振频率及其信号强度关于频偏满足对称关系:当频偏由正变为负时,单量子信号的强度亦随之改变了符号.因此,对正负频偏(数值相等)的两次Raman磁共振实验的FID信号进行累加,就可压抑强的单量子峰,而对于多量子峰则在大的频偏条件下不会相互抵消.因此,这样的实验累加可以保持多量子信号的强度而压抑掉强的单量子峰.作为实验验证,我们给出了异核AX_n体系(CHCl₃,CH₂Cl₂,CH₃OH)的按上述正负频偏循环累加采样方法得到Raman磁共振¹³C实验谱.实验结果同理论预言完全一致.

Suppression of Strong Single-Quantum Signals in Raman Magnetic Resonance Spectroscopy in Heteronuclear Spin Systems

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 (CHCl-3,CH-2Cl-2,CH-3OH) 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. ;