Chemistry Reference
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Fig. 2 Estimate of R X T X value at the magnetic field strength at X MHz to provide the uncertainty of
the relaxation rate ( R X ) equivalent to that obtained at 600 MHz. The graph was calculated using ( 3 ),
in which ( I 0 / DI ) is proportional to ( B 0 ) 3/2 and R at 600 MHz is determined by a two-point
exponential fitting at T ¼ 0 and T ¼ 1/ R . For example, at X ¼ 900 MHz ( horizontal scale ),
R X T X can be reduced to 0.32 ( vertical scale ) than the optimal T X ¼ 1/ R X to obtain equivalent R X
uncertainty at 600 MHz
a Freeman-Hill phase cycle in which measured magnetization decays from an initial
value of I (0) to zero of magnetization starting from I (0) to z [ 49 ]. Since this phase
cycle avoids recovery from
I (0) to I (0), there is no need to record the magnetiza-
tion recovery till equilibrium is attained. Otherwise, the total experimental time is
enormously long [ 50 ]. In addition, because it is known that the I ( t ) approaches zero
at infinite time t , offset is not required as an unknown parameter in the exponential
fitting. Typically, only two unknown parameters ( R 1 and I 0 ) are optimized in the
exponential fitting.
The pulse scheme to suppress DD/CSA cross-correlation interference is similar to
that used to measure R 2 in which 1 H 180 pulses are applied during every 5-10 ms (at a
rate greater than the decay rate of the faster-relaxing components of the 15 N- 1 H
J-coupled two components) [ 37 ]. As the magnetic field strength increases, the magni-
tude of the
2
2
t R ð
1
þ t
R o
Þ
term decreases (Fig. 1 ). However, in the slow molecular
tumbling limit (
t R o N >>
1), the R 1 contribution by the CSA term is field independent
2
2
because
term. Thus,
although the relative contribution of DD/CSA increases, the absolute cross-correlation
effect on R 1 is not necessarily increased with increase in the magnetic field strength.
The application rate of the 1 H180 pulses may depend more on the apparent relaxation
rates of the two components, i.e., the 1 H spin-flip rate, but not necessarily on the
magnetic field strength.
To suppress DD/CSA cross-correlation in 15 N R 1 experiment, application of
accurate 1 H 180 inversion pulses is important. In principle, it is best to invert only
s CSA B 0 increases cancels by the reduction of the
t R ð
1
þ t
R o
Þ
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