Examples of -coupling and Chemical Shift Evolution

The angle between the two vectors is 45° and the center position (chemical shift position of the doublet) has rotated 67.5° (360° x 225 Hz x 0.0008333 s) to find itself exactly between the two vectors. We will represent this position with a dotted line (Fig. 6.13, r = 1/(87)). Note that the H = a vector moves faster than the H = f vector because the NMR line that corresponds to it is farther from the center of the spectral window. 

1 Experimental Example 1H Observe with J-Coupling Evolution Only 

The effect of /-coupling evolution can be observed directly using a sample of methyl iodide (CH3I) in CDCI3, which is enriched in 13C to the level of 60% (Fig. 6.14). Now we are looking at the one-bond coupling from the point of view of the three equivalent 

The antiphase doublet (Fig. 6.14(c)) is dispersive because /-coupling evolution to the antiphase state moves the vectors by 90°, from the +x axis to the +/ and —/ axes. This dispersive antiphase doublet can be phase corrected by moving the reference axis from the +x axis to the +/ axis (90° zero-order phase correction). Now the C = a peak is positive absorptive and the C = ft peak is negative absorptive (Fig. 6.15) and the central 12CH3l peak is pure dispersive because the vector is on the -hx/ axis and the reference axis is now +y (90° phase error). 

We usually ignore relaxation during short delays (e.g., 1/(2J) is usually milliseconds or tens of milliseconds) and consider it only when relaxation is essential to the experiment (e.g., inversion-recovery or nuclear Overhauser effect (NOE) experiments, typically hundreds of milliseconds or seconds for small molecules). Pulses are very short (tens of microseconds), so we do not usually worry about either evolution or relaxation during pulses. Although pulses may look fat in pulse sequence diagrams, they are really much shorter than most delays and their duration is not important in terms of evolution. Pulses lead to rotation of the net magnetization vector around the B axis, always in the counterclockwise direction. 

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