  The selective adsorption of a corrosive gas, SO$_2$, into two microporous
pillared paddle-wheel frameworks M(bdc)(ted)0.5 [M = Ni, Zn; bdc =
1,4-benzenedicarboxylate; ted = triethylenediamine] is studied by volumetric
adsorption measurements and a combination of in-situ infrared spectroscopy and
ab initio density functional theory (DFT) calculations. The uptake of SO$_2$ in
M(bdc)(ted)0.5 at room temperature is quite significant, 9.97 mol/kg at 1.13
bar. The major adsorbed SO$_2$ molecules contributing to the isotherm
measurements are characterized by stretching bands at 1326 and 1144 cm$^{-1}$.
Theoretical calculations including van der Waals interactions (based on vdW-DF)
suggest that two adsorption configurations are possible for these SO$_2$
molecules. One geometry involves an SO$_2$ molecule bonded through its sulfur
atom to the oxygen atom of the paddle-wheel building unit and its two oxygen
atoms to the C-H groups of the organic linkers by formation of hydrogen bonds.
Such a configuration results in a distortion of the benzene rings, which is
consistent with the experimentally observed shift of the ring deformation mode.
In the other geometry, SO$_2$ establishes hydrogen bonding with -CH$_2$ group
of the ted linker through its two oxygen atoms simultaneously. The
vdW-DF-simulated frequency shifts of the SO$_2$ stretching bands in these two
configurations are similar and in good agreement with spectroscopically
measured values of physisorbed SO$_2$.In addition, the IR spectra reveal the
presence of another minor species, characterized by stretching modes at 1242
and 1105 cm$^{-1}$ and causing significant perturbations of MOFs vibrational
modes (CH$_x$ and carboxylate groups). This species is more strongly bound,
requiring a higher temperature ($\sim$150 $^\circ$C) to remove it than for the
main physisorbed species.
