J. Becker, A. Comotti, R. Simonutti, Piero Sozzani,
Molecular Motion of Isolated Linear Alkanes in Nanochannels.
J. Phys. Chem. B 109, 23285-23294 (2005).
The mobility of a series of linear alkanes in their inclusion compound with
tris(o-phenylenedioxy)- spirotriphosphazene is studied by high-resolution
carbon-proton magic-angle spinning solid-state NMR spectroscopy. Two different
carbon-proton dipolar recoupling experiments are compared with respect to their
ability to yield precise site-specific, motion-averaged dipolar coupling
constants. The most accurate results are obtained by analysis of extrema
positions in Lee-Goldburg cross-polarization build-up curves. We present a
comprehensive collection of coupling constants, which evidence a rotational
motion of the all-trans chains around the channel axis, with some further
averaging due to additional fluctuations, as previously found for alkanes in
other host matrices such as urea. The order parameter increases toward the inner
parts of the chains, and is largely independent of chain length. Notably, chains
in a TPP host are not more ordered than in urea, even though the average TPP
channel diameter is reported to be smaller. Significantly decreased order is
found for highly filled short-alkane samples, which is interpreted in terms of
an increased rate of mutual collisions. From residual dipolar couplings as well
as carbon chemical shifts, we derive similar amounts of gauche conformers.
Translational motions along the channels are further studied by proton
double-quantum spectroscopy, which probes guest-host dipolar couplings. The
extent of local-scale lateral motion is again correlated with the sample
filling, and is a weak function of temperature, as expected from a case in which
highly restricted single-file diffusion should dominate the mobility.
Characteristic effects of sample aging are apparent in all our experiments.