Molecular and Dissociative Adsorption of Water on (TiO₂)_<i>n</i> Clusters, <i>n</i> = 1–4

The low energy structures of the (TiO₂)_<i>n</i>(H₂O)_<i>m</i> (<i>n</i> ≤ 4, <i>m</i> ≤ 2<i>n</i>) and (TiO₂)₈(H₂O)_<i>m</i> (<i>m</i> = 3, 7, 8) clusters were predicted using a global geometry optimization approach, with a number of new lowest energy isomers being found. Water can molecularly or dissociatively adsorb on pure and hydrated TiO₂ clusters. Dissociative adsorption is the dominant reaction for the first two H₂O adsorption reactions for <i>n</i> = 1, 2, and 4, for the first three H₂O adsorption reactions for <i>n</i> = 3, and for the first four H₂O adsorption reactions for <i>n</i> = 8. As more H₂O’s are added to the hydrated (TiO₂)_<i>n</i> cluster, dissociative adsorption becomes less exothermic as all the Ti centers become 4-coordinate. Two types of bonds can be formed between the molecularly adsorbed water and TiO₂ clusters: a Lewis acid–base Ti–O­(H₂) bond or an O···H hydrogen bond. The coupled cluster CCSD­(T) results show that at 0 K the H₂O adsorption energy at a 4-coordinate Ti center is ∼15 kcal/mol for the Lewis acid–base molecular adsorption and ∼7 kcal/mol for the H-bond molecular adsorption, in comparison to that of 8–10 kcal/mol for the dissociative adsorption. The cluster size and geometry independent dehydration reaction energy, <i>E</i>_D, for the general reaction 2­(−TiOH) → −TiOTi– + H₂O at 4-coordinate Ti centers was estimated from the aggregation reaction of <i>n</i>Ti­(OH)₄ to form the monocyclic ring cluster (TiO₃H₂)_<i>n</i> + <i>n</i>H₂O. <i>E</i>_D is estimated to be −8 kcal/mol, showing that intramolecular and intermolecular dehydration reactions are intrinsically thermodynamically allowed for the hydrated (TiO₂)_<i>n</i> clusters with all of the Ti centers 4-coordinate, which can be hindered by cluster geometry changes caused by such processes. Bending force constants for the TiOTi and OTiO bonds are determined to be 7.4 and 56.0 kcal/(mol·rad²). Infrared vibrational spectra were calculated using density functional theory, and the new bands appearing upon water adsorption were assigned

Comparison of Computational Strategies for the Calculation of the Electronic Coupling in Intermolecular Energy and Electron Transport Processes

Electronic couplings in intermolecular electron and energy transfer processes calculated by six different existing computational techniques are compared to nonorthogonal configuration interaction for fragments (NOCI-F) results. The paper addresses the calculation of the electronic coupling in diketopyrrolopyrol, tetracene, 5,5′-difluoroindigo, and benzene–Cl for hole and electron transport, as well as the local exciton and singlet fission coupling. NOCI-F provides a rigorous computational scheme to calculate these couplings, but its computational cost is rather elevated. The here-considered ab initio Frenkel–Davydov (AIFD), Dimer projection (DIPRO), transition dipole moment coupling, Michl–Smith, effective Hamiltonian, and Mulliken–Hush approaches are computationally less demanding, and the comparison with the NOCI-F results shows that the NOCI-F results in the couplings for hole and electron transport are rather accurately predicted by the more approximate schemes but that the NOCI-F exciton transfer and singlet fission couplings are more difficult to reproduce