Chemistry Reference
In-Depth Information
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