100
W. Morris et al. / Journal of Molecular Structure 1004 (2011) 94–101
may be associated with libration of the ring, we analyzed whether
its pre-exponential factor would match the pre-exponential factor
for the twofold flipping process [18,50], recognizing that they
should have the same attempt frequency [51]. From a correlation
the 99.6% natural abundance quadrupolar 14N in the amino group.
Dipolar coupling of the 14N to adjacent spin-½ nuclei (both 1H and
13C) occurs not only in the static sample but also in the MAS exper-
iments at the magnetic field of 7 T used in this study. As a result,
the spin dynamics and the cross-polarization dynamics are af-
fected. In the MAS experiments, isotopic replacement of the natu-
ral abundance 14N with 15N significantly improves resolution in the
1H, 13C and 15N spectra.
time of 2.4 ꢀ 10ꢁ6 s derived from the 1H T1 data at 260 K, one
q
can determine a pre-exponential factor of ca. 1.3 ꢀ 107 sꢁ1 for
the low barrier process. A similar analysis for the ring flipping mo-
tion reveals that a correlation time of 7.9 ꢀ 10ꢁ7 s at 365 K is asso-
ciated with a pre-exponential factor of 1.19 ꢀ 109 sꢁ1 [52]. The
difference of two orders of magnitude between the two processes
indicates that they do not originate from elementary motions
within the same potential energy well.
Acknowledgments
With the results from the 1H T1 data as a guideline, we ana-
This work was supported by National Science Foundation under
Grant CHE-0956006 (CD), Grants DMR0605688 and CHE0551938
and creativity extension DMR0937243 (MAGG).
q
lyzed the 1H T1 data in Fig. 4. We note that thermal limitations of
the NMR probe used in this study prevented determination of
the temperature at which the maximal rate for 1H spin–lattice
relaxation occurs. However, the simulation of the data in Fig. 4 also
yields an activation energy of 5.0( 0.2) kcal/mol. The similarity of
this value suggests that the motion is the same one responsible
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q
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