Orientational ${\mathrm{ortho}-\mathrm{H}}_2$ pair interactions in the microporous framework MOF-5

Infrared spectroscopy is used to observe the orientational fine structure arising from $\text{ortho}-{\mathrm{H}}_2$ adsorbed at the primary site of the microporous framework MOF-5. The $Q_1\left(1\right)$ vibrational transition shows at least two symmetrically spaced fine structure bands on either side of the main band. These grow in relative intensity with increasing ${\mathrm{H}}_2$ concentration indicative of interacting ${\mathrm{H}}_2$ pairs. This interpretation is strongly supported by ${\mathrm{D}}_2$ addition experiments, which cause a large increase in intensity of the fine structure bands with only minimal change in the main band. The spectra are analyzed in terms of ${\mathrm{H}}_2-{\mathrm{H}}_2$ electric quadrupole-quadrupole interactions. Consistent with this approach we observe no fine structure bands for the $Q_1\left(0\right)$ vibrational transition arising from $\text{para}-{\mathrm{H}}_2$, which does not possess a quadrupole moment.

Figure 1

${\mathrm{Zn}}_4\mathrm{O}{({\mathrm{O}}_2\mathrm{C}–)}_6$ cluster showing two of the four possible primary sites occupied by ${\mathrm{H}}_2$ molecules. The sites are separated by 6.2 $\AA$ and each has threefold symmetry about a $Z$ axis joining it to the central oxygen atom at a distance of 3.8 $\AA$.

Figure 2

Energy diagram for the $J=1$ level of ${\mathrm{H}}_2$ at the primary site in MOF-5. The first column shows an isolated $J=1$ molecule, the second a single $J=1$ molecule in MOF-5 with binding energy $E_B$, the third two noninteracting ${\mathrm{H}}_2$ molecules in MOF-5, and the fourth two $J=1$ molecules interacting via the EQQ potential in MOF-5.

Figure 3

Infrared absorption spectrum at 15 K showing the $Q_1$(1) transition for a concentration of two ${\mathrm{H}}_2$ per MOF cluster. The instrument resolution is 0.1 ${\mathrm{cm}}^{-1}$. The stick spectrum is based on an $m=\pm 1$ splitting of 0.8 ${\mathrm{cm}}^{-1}$ and a $\mathrm{\Gamma }$ value of 0.06 ${\mathrm{cm}}^{-1}$.

Figure 4

Infrared absorption spectra at 15 K showing the $Q_1$(1) transition for ${\mathrm{H}}_2$ concentrations of 0.7 (black line), 1.3 (red line), 2.0 (green line), 2.7 (purple line), and 4 (blue line) ${\mathrm{H}}_2$ per MOF cluster. The instrument resolution is 0.1 ${\mathrm{cm}}^{-1}$ for all spectra.

Figure 5

Infrared absorption spectra at 15 K for a concentration of 2.9 ${\mathrm{H}}_2$ per MOF cluster at times of 40 (blue line), 100 (purple line), 130 (green line), 190 (red line), and 270 (black line) min after ${\mathrm{H}}_2$ was first introduced into the sample chamber at 77 K. The spectra show ortho to para conversion with time.

Figure 6

Infrared absorption spectra at 15 K for a concentration of 0.7 ${\mathrm{H}}_2$ per MOF cluster (black line) and a mixture of 0.7 ${\mathrm{H}}_2$ and 2.7 ${\mathrm{D}}_2$ per MOF cluster (red line). The instrument resolution is 0.1 ${\mathrm{cm}}^{-1}$.