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Understanding NMR Spectroscopy phần 9

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chiều quang phổ. Đây được xem là chi tiết hơn trong phần 9.5.6. 9.5.5.2 xung Phân nhóm với nhau, ngược lại thứ tự hiển thị có thể được sử dụng để tạo ra nhiều lượng tử gắn kết từ từ hóa cân bằng, trong tiếng vọng spin từ hóa chống-giai đoạn phát triển và xung cuối cùng chuyển thành nhiều sự gắn kết lượng tử. | dimensional spectra. This is considered in more detail in section 9.5.6. 9.5.5.2 Grouping pulses together The sequence shown opposite can be used to generate multiple quantum coherence from equilibrium magnetization during the spin echo anti-phase magnetization develops and the final pulse transfers this into multiple quantum coherence. Let us suppose that we wish to generate double quantum with p 2 as show by the CTP opposite. As has already been noted the first pulse can only generate p 1 and the 180 pulse only causes a change in the sign of the coherence order. The only pulse we need to be concerned with is the final one which we want to generate only double quantum. We could try to devise a phase cycle for the last pulse alone or we could simply group all three pulses together and imagine that as a group they achieve the transformation p 0 to p 2 i.e. Áp 2. The phase cycle would simply be for the three pulses together to go 0 90 180 270 with the receiver going 0 180 0 180 . It has to be recognised that by cycling a group of pulses together we are only selecting an overall transformation the coherence orders present within the group of pulses are not being selected. It is up to the designer of the experiment to decide whether or not this degree of selection is sufficient. The four step cycle mentioned above also selects Áp 6 again we would have to decide whether or not such high orders of coherence were likely to be present in the spin system. Finally we note that the Áp values for the final pulse are 1 3 it would not be possible to devise a four step cycle which selects all of these pathways. Pulse sequence for generating double-quantum coherence. Note that the 180 pulse simply causes a change in the sign of the coherence order. 9.5.5.3 The last pulse We noted above that only coherence order -1 is observable. So although the final pulse of a sequence may cause transfer to many different orders of coherence only transfers to p -1 will result in observable .