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qHNMR Factor 3: Relaxation Delay (d1). The delay, in
seconds, which precedes the pulsed qHNMR experiment is referred
to as the relaxation delay, denoted on most commercial NMR
spectrometers as d1. This delay is inserted to allow the excited
nuclei to re-establish their equilibrium z-magnetization after the
acquisition of the FID information and prior to the application of
the next pulse or pulse train. If the pulse excitation is a 90-deg
pulse, i.e., all equilibrium z-magnetization is converted into
transverse (x, y) magnetization, the relaxation delay is generally
set to 5 times the longest proton relaxation time (T1) in the sample
(determined by considering all proton resonances in the sample)
in order to avoid distortion of integrated signal intensity due to
relaxation effects. If the pulse excitation is a <90-deg pulse, then
a shorter relaxation delay can in principle be employed. However,
Figure 1. Inverse-gated decoupling scheme for eliminating the 13C
satellites from 1H NMR spectra, proposed as a routine experiment
for acquisition of qHNMR data sets. The main building blocks of
this experiment are the relaxation delay (a), the pulse width [pw]
(b), the acquisition time (c), and the composite pulse decoupling
GARP (d).
the relationship between the d1 delay, the proton relaxation times
of the sample, and the “flip angle” of the pulse used must also be
considered. A further aspect of setting of the relaxation delay that
requires comment relates to the application of composite pulse
decoupling, in this instance 13C GARP decoupling, and the heat
produced from the broadband decoupling during the acquisition
time. While the length of the relaxation delay can be reduced, thus
avoiding relaxation time effects and obtaining good quantitative
results, decoupling of the carbon frequency range occurs during
the acquisition time. Therefore, it is recommended to lengthen the
relaxation delay to maintain a reasonable duty cycle (relaxation
delay + acquisition time =pulse repetition rate) for the composite
pulse decoupling and to minimize unfavorable heating effects.
Minimizing heating effects serves to reduce excessive line broadening
and, if the sample is heat sensitive, degradation of the sample
during the course of the NMR data acquisition. In general, a duty
cycle of 10-20% is recommended. _0 c8 D+ b9 t( @+ B
# C% Q4 Y( Q' R8 q4 R1 H" _
前半部分说d1=5T1,后半部分看不懂,duty
cycle是什么意思 1 P4 A7 X2 A$ |* w) s/ _8 M3 N
[此贴子已经被作者于2011-2-18 9:07:58编辑过]
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发表于 2011-2-18 09:06:47
