A dynamic nuclear magnetic resonance (NMR) study of the
polar liquids pyridine and pentafluoropyridine
(PFP) confined to porous silica sol−gel glasses is reported. The
13C NMR spin−lattice relaxation times,
T
1,
of ortho (C2), meta (C3), and para (C4) carbons were measured in
glasses with pore radii ranging from 30
to 94 Å, over a temperature range of −22 to +20 °C. The
experimental data were analyzed in terms of the
two-state, fast exchange model, and the surface-layer relaxation times,
T
1s, were calculated. On the basis
of
surface enhancement factors,
T
1b/T
1s, where
T
1b is the relaxation time of the bulk liquid,
we conclude that
fluorination considerably weakens the interaction of pyridine with the
glass surface. This is consistent with
pyridine serving as a hydrogen bond acceptor for the surface. By
analyzing the relaxation differences in C2,
C3, and C4, we find that the reorientation of confined pyridine is
highly anisotropic, due to directional hydrogen
bonding between the N atoms of pyridine and the OH groups on the glass
surface. In contrast, we see no
confinement effect on the rotational anisotropy of PFP.