Crystallographic studies provide the time-averaged position of atoms, and whilst some insight into atom dynamics can be gained through analysis of the average structure and atomic displacement parameters, detailed information regarding the dynamics of the guest and host-guest system are better gained through other neu­tron-scattering methods. Inelastic neutron scattering (INS) combined with compu­tational calculations permit visualization of the atom dynamics and allow elucidation of the interaction between adsorbed guest molecules and the host.

Measurement of the interaction between CO2 molecules and the porous host is crucial to understanding the detailed binding mechanism and therefore the observed selectivity and guest-uptake properties of porous hosts. INS cannot directly detect the CO2 binding interaction within an adsorbent because the incoherent neutron­scattering cross section for these elements (for their naturally-abundant isotopes) is effectively zero, being 0.001 barns each. One approach to overcome this problem is to combine INS and DFT to visualize captured CO2 molecules within a porous host by investigating the change in the dynamics of the other atoms of the adsorbent structure. INS spectra can be calculated directly using DFT-based computations to obtain the force constants, and then making the harmonic approximation to obtain the eigenvectors and eigenvalues to determine the spectral intensities and fre­quencies, respectively [43]. INS and DFT-based calculations are a powerful com­bination in understanding the working mechanism of functionalized materials containing specific gas molecule binding-sites, probing directly the impact of functional groups, and other host features such as topology and pore shape and size on the orientation and type of binding of CO2 in the host.

An example of this is the application of INS and DFT to study CO2 in the material Al2(OH)2(bptc), where bptc = biphenyl-3, 3′,5, 5′-tetracarboxylate and also known as NOTT-300, where the neutron-scattering signal comes primarily from the H atoms in the Al2(OH)2(bptc) hydroxyl groups and benzene rings of the ligand, and the INS signal is perturbed by the binding of CO2 (Fig. 3.3) [44]. Al2(OH)2(bptc) is an Al-hydroxyl functionalized porous-solid exhibiting high chemical and thermal stability as well as high selectivity and uptake capacity for CO2 and SO2. The

material exhibits no apparent adsorption of H2 and N2, which is attributed to the slow diffusion of these gases through the narrow pore channels. In contrast, unusually high and selective CO2 and SO2 uptakes were observed, including at low-pressure. The INS spectra revealed two major increases in peak intensity upon adsorption of 1.0 CO2 into the formula unit: peak I at lower energy transfers (30 meV) and peak II at higher energy transfer (125 meV). Peaks in the range 100-160 meV were slightly shifted to higher energies in the CO2-adsorbed material, indicating a hardening of the motion of the Al2(OH)2(bptc) host upon CO2 adsorption.

The INS spectrum derived from DFT calculations show good agreement with the experimental spectrum and confirm that the adsorbed CO2 molecules are located end-on to the hydroxyl groups. The O-H distance between the CO2 molecule and the hydroxyl group is 2.335 A, indicating a moderate to weak hydrogen bond, with the optimized C-O bond distances in CO2 being 1.183 A at the hydrogen-bonded end and 1.178 A at the free end. The CO2 is linear with a O-C-O bond angle of 180°. Each adsorbed CO2 molecule is found to be surrounded by four aromatic C-H groups, forming weak cooperative supramolecular interactions between the O(5-) of the CO2 and the H(5+) of the — CH (where O-H = 3.029 and 3.190 A and each occurs twice). Peak I in the INS spectrum was assigned to the O-H group wag, occurring perpendicular to the Al-O-Al direction and attributed to the presence of the CO2. Peak II in the spectrum was assigned to the wag of the four aromatic C-H groups on four benzene rings adjacent to each CO2, in conjunction with the OH group wag. Hence, in this work the direct visualization of host-guest interactions through INS and DFT calculations was crucial in rationalizing the material’s high selectivity for CO2 and in understanding the detailed binding mechanism of CO2 in the material. The low H2 uptake of the material was rationalised in a similar manner, with the contribution from H2 in the material to the INS data, consistent with that expected for liquid H2, indicating a weak interaction with the material.