  We consider the leading one-chain term in a skeleton expansion for QCD
observables and show that for energies Q^2>\Lambda^2, where Q^2=\Lambda^2 is
the Landau pole in the coupling, the skeleton expansion result is equivalent to
the standard Borel integral representation, with ambiguities related to
infrared (IR) renormalons. For Q^2<\Lambda^2 the skeleton expansion result is
equivalent to a previously proposed modified Borel representation where the
ambiguities are connected with ultraviolet (UV) renormalons. We investigate the
Q^2-dependence of the perturbative corrections to the Adler D function, the GLS
sum rule, and the polarized and unpolarized Bjorken sum rules. In all these
cases the one-chain result changes sign in the vicinity of Q^2=\Lambda^2, and
then exhibits freezing behaviour, vanishing at Q^2=0. Finiteness at
Q^2=\Lambda^2 implies specific relations between the residues of IR and UV
renormalons in the Borel plane. These relations, only one of which has
previously been noted (though it remained unexplained) are shown to follow from
the continuity of the characteristic function in the skeleton expansion. By
considering the compensation of non-perturbative and perturbative ambiguities
we are led to a result for the Q^2 dependence of these observables at all Q^2,
in which there is a single undetermined non-perturbative parameter, and which
involves the skeleton expansion characteristic function. The observables freeze
to zero in the infrared. We briefly consider the freezing behaviour of the
Minkowskian R_{e+e-} ratio.
