  [Abridged] Barium stars are moderately rare, chemically peculiar objects,
which are believed to be the result of the pollution of an otherwise normal
star by material from an evolved companion on the asymptotic giant branch
(AGB). We aim to derive carbon, nitrogen, oxygen, and fluorine abundances for
the first time from the infrared spectra of the barium red giant star HD 123396
to quantitatively test AGB nucleosynthesis models for producing barium stars
via mass accretion. High-resolution and high S/N infrared spectra were obtained
using the Phoenix spectrograph mounted at the Gemini South telescope. The
abundances were obtained through spectrum synthesis of individual atomic and
molecular lines, using the MOOG stellar line analysis program, together with
Kurucz's stellar atmosphere models. The analysis was classical, using 1D
stellar models and spectral synthesis under the assumption of local
thermodynamic equilibrium. We confirm that HD 123396 is a metal-deficient
barium star ([Fe/H] = -1.05), with A(C) = 7.88, A(N) = 6.65, A(O) = 7.93, and
A(Na) = 5.28 on a logarithmic scale where A(H) = 12, leading to [(C+N)/Fe] ~
0.5. The A(CNO) group, as well as the A(Na) abundances, is in excellent
agreement with those previously derived for this star using high resolution
optical data. We also found A(F) = 4.16, which implies [F/O] = 0.39, a value
that is substantially higher than the F abundances measured in globular
clusters of a similar metallicity, noting that there are no F measurements in
field stars of comparable metallicity. The observed abundance pattern of the
light elements (CNO, F and Na) recovered here as well as the heavy elements
(s-process) studied elsewhere suggest that the surface composition of HD 123396
is well fitted by the predicted abundance pattern of a 1.5Msun AGB model star
with Z = 0.001.
