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Title: Ab initio and force field investigations of physical hydrogen adsorption in Zeolitic Imidazole Frameworks
Author: Alqahtani, Huda
ISNI:       0000 0004 7651 5748
Awarding Body: Cardiff University
Current Institution: Cardiff University
Date of Award: 2018
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Recent theoretical calculations and experiments have considered that metal-organic frameworks are promising for storing molecular hydrogen (H2). Optimizing the geometry and the interaction energy of storing for enormous H2 storage is of great current interest. In this work, we used specific category of MOFs, Zeolitic Imidazole Frameworks (ZIFs). We carried out calculations through high-accuracy electronic structure calculations (MP2, CCSD and CCSD(T)) levels of theory, with controlled errors. Also we established and calibrated a computational protocol for accurately predicting the binding energy and structure of weakly bound complexes. Then, we applied the protocol to a number of models for metal-organic frameworks. For example, we have built many systems of noncovalently bound complexes [H2...benzene, H2....imidazole, CO.... imidazole, N2... imidazole, NH3...imidazole and H2O ...imidazole] and we have optimized geometries of these systems through calculating numerical gradients at MP2/CP level and LMP2 level of theory and extrapolated from aug-cc-PVTZ and aug-cc-PVQZ basis set to evaluate the binding energy by using Hobza's scheme to obtain correct interaction energies. We found that NH3 and H2O with imidazole prefer to form hydrogen bonds rather than physical adsorption (London dispersion force). Also, the perpendicular position of hydrogen has the lowest potential energy surface, while the parallel hydrogen position has the highest potential energy surface. We have confirmed that by using a high level of basis set at MP2 such as ccpVXZ (x= Q, 5, 6) and aug-cc-pVXZ (x=D, T, Q, 5, 6), and by using the same basis sets at CCSD and CCSD(T) as the high level of theory. Also, it is clear from these results that the binding energies are sensitive to improvement of the size of basis sets. In terms of applying Hobza's scheme to obtain correct interaction energies, we found that this scheme CCSD(T)/ [34] = MP2/ [34] + (CCSD(T)/ [23] – MP2 [23]) achieved the highest accurate of interaction energy for CO...imidazole. On the other hand, this scheme CCSD(T)/ [34] = MP2/ [34] + [CCSD(T)/AVDZ– MP2/AVDZ] produced the highest accurate of interaction energy for H2...imi, N2...imi and H2...Benzene. Regarding to Basis Set Superposition Error (BSSE) and counterpoise examination (CP), Ab initio and Force field investigations of physical hydrogen adsorption in Zeolitic Imidazole Frameworks we found that the MP2/CP and LMP2 methods yield very similar results at the basis set limit and the convergence of MP2 and LMP2 with increasing size of basis sets is different since the BSSE in LMP2 is reduced. Furthermore, we found that the extrapolation to the CBS limit cannot offer an alternative to the counterpoise correction where the differences in the values of bending energies are large so we need to use both techniques together to overcome the BSSE problem. Then to confirm our result regard to the potential energy surface, we calculated corresponding potential energy surfaces using several popular force fields potential, and compare critically with best ab initio results, where we focused on the adsorption of H2 on imidazole as the organic linker in ZIFs. We carried out ab initio calculations at the MP2/CCSD(T) levels with different basis sets, basis set extrapolation and Lennard-Jones potential for the three directions X, Y and Z for 294 positions of H2. Also, we have fitted ab initio binding energy at the MP2/CCSD(T) levels with different basis set and basis set extrapolation to Lennard-Jones (12-6 LJ) binding energy by applying the nonlinear least squares method. Then we estimated the fitted binding energy using Hobza’s schemes to reduce the errors. We found that the 12-6 LJ formula produced unreasonable fit for ab initio calculated potential energy surface PES, for both the equilibrium and attractive regions, to improve this fitting, we found the good fit is only achieved by the exponential formula of repulsion region. It is hoped that this study could facilitate the search for a “good” application to store the H2 molecule conveniently and safely.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QD Chemistry