Wei Guang Liu

Mechanism for degradation of nafion in PEM fuel cells from quantum mechanics calculations

We report results of quantum mechanics (QM) mechanistic studies of Nafion membrane degradation in a polymer electrolyte membrane (PEM) fuel cell. Experiments suggest that Nafion degradation is caused by generation of trace radical species (such as OH(●), H(●)) only when in the presence of H(2), O(2), and Pt. We use density functional theory (DFT) to construct the potential energy surfaces for various plausible reactions involving intermediates that might be formed when Nafion is exposed to H(2) (or H(+)) and O(2) in the presence of the Pt catalyst. We find a barrier of 0.53 eV for OH radical formation from HOOH chemisorbed on Pt(111) and of 0.76 eV from chemisorbed OOH(ad), suggesting that OH might be present during the ORR, particularly when the fuel cell is turned on and off. Based on the QM, we propose two chemical mechanisms for OH radical attack on the Nafion polymer: (1) OH attack on the S-C bond to form H(2)SO(4) plus a carbon radical (barrier: 0.96 eV) followed by decomposition of the carbon radical to form an epoxide (barrier: 1.40 eV). (2) OH attack on H(2) crossover gas to form hydrogen radical (barrier: 0.04 eV), which subsequently attacks a C-F bond to form HF plus carbon radicals (barrier as low as 1.00 eV). This carbon radical can then decompose to form a ketone plus a carbon radical with a barrier of 0.86 eV. The products (HF, OCF(2), SCF(2)) of these proposed mechanisms have all been observed by F NMR in the fuel cell exit gases along with the decrease in pH expected from our mechanism.

Energetically Demanding Transport in a Supramolecular Assembly

A challenge in contemporary chemistry is the realization of artificial molecular machines that can perform work in solution on their environments. Here, we report on the design and production of a supramolecular flashing energy ratchet capable of processing chemical fuel generated by redox changes to drive a ring in one direction relative to a dumbbell toward an energetically uphill state. The kinetics of the reaction pathway juxtapose a low energy [2]pseudorotaxane that forms under equilibrium conditions with a high energy, metastable [2]pseudorotaxane which resides away from equilibrium.

Explanation of the Colossal Detonation Sensitivity of Silicon Pentaerythritol Tetranitrate (Si-PETN) Explosive

DFT calculations have identified the novel rearrangement shown here for decomposition of the Si derivative of the PETN explosive [PentaErythritol TetraNitrate (PETN), C(CH(2)ONO(2))(4)] that explains the very dramatic increase in sensitivity observed experimentally. The critical difference is that Si-PETN allows a favorable five-coordinate transition state in which the new Si-O and C-O bonds form simultaneously, leading to a transition state barrier of 33 kcal/mol (it is 80 kcal/mol for PETN) and much lower than the normal O-NO(2) bond fission observed in other energetic materials (approximately 40 kcal/mol). In addition this new mechanism is very exothermic (45 kcal/mol) leading to a large net energy release at the very early stages of Si-PETN decomposition that facilitates a rapid temperature increase and expansion of the reaction zone.

First-principles study of the role of interconversion between NO2, N2O4, cis-ONO-NO2, and trans-ONO-NO2 in chemical processes.

Experimental results, such as NO2 hydrolysis and the hypergolicity of hydrazine/nitrogen tetroxide pair, have been interpreted in terms of NO2 dimers. Such interpretations are complicated by the possibility of several forms for the dimer: symmetric N2O4, cis-ONO-NO2, and trans-ONO-NO2. Quantum mechanical (QM) studies of these systems are complicated by the large resonance energy in NO2 which changes differently for each dimer and changes dramatically as bonds are formed and broken. As a result, none of the standard methods for QM are uniformly reliable. We report here studies of these systems using density functional theory (B3LYP) and several ab initio methods (MP2, CCSD(T), and GVB-RCI). At RCCSD(T)/CBS level, the enthalpic barrier to form cis-ONO-NO2 is 1.9 kcal/mol, whereas the enthalpic barrier to form trans-ONO-NO2 is 13.2 kcal/mol, in agreement with the GVB-RCI result. However, to form symmetric N2O4, RCCSD(T) gives an unphysical barrier due to the wrong asymptotic behavior of its reference function at the dissociation limit, whereas GVB-RCI shows no barrier for such a recombination. The difference of barrier heights in these three recombination reactions can be rationalized in terms of the amount of B2 excitation involved in the bond formation process. We find that the enthalpic barrier for N2O4 isomerizing to trans-ONO-NO2 is 43.9 kcal/mol, ruling out the possibility of such an isomerization playing a significant role in gas-phase hydrolysis of NO2. A much more favored path is to form cis-ONO-NO2 first then convert to trans-ONO-NO2 with a 2.4 kcal/mol enthalpic barrier. We also propose that the isotopic oxygen exchange in NO2 gas is possibly via the formation of trans-ONO-NO2 followed by ON(+) migration.

Mechanism of O2 Activation and Methanol Production by (Di(2-pyridyl)methanesulfonate)PtIIMe(OHn)(2-n)- Complex from Theory with Validation from Experiment

The mechanism of the (dpms)Pt(II)Me(OH(n))((2-n)-) oxidation in water to form (dpms)Pt(IV)Me(OH)2 and (dpms)Pt(IV)Me2(OH) complexes was analyzed using DFT calculations. At pH < 10, (dpms)Pt(II)Me(OH(n))((2-n)-) reacts with O2 to form a methyl Pt(IV)-OOH species with the methyl group trans to the pyridine nitrogen, which then reacts with (dpms)Pt(II)Me(OH(n))((2-n)-) to form 2 equiv of (dpms)Pt(IV)Me(OH)2, the major oxidation product. Both the O2 activation and the O-O bond cleavage are pH dependent. At higher pH, O-O cleavage is inhibited whereas the Pt-to-Pt methyl transfer is not slowed down, so making the latter reaction predominant at pH > 12. The pH-independent Pt-to-Pt methyl transfer involves the isomeric methyl Pt(IV)-OOH species with the methyl group trans to the sulfonate. This methyl Pt(IV)-OOH complex is more stable and more reactive in the Pt-to-Pt methyl-transfer reaction as compared to its isomer with the methyl group trans to the pyridine nitrogen. A similar structure-reactivity relationship is also observed for the S(N)2 functionalization to form methanol by two isomeric (dpms)Pt(IV)Me(OH)2 complexes, one featuring the methyl ligand trans to the sulfonate group and another with the methyl trans to the pyridine nitrogen. The barrier to functionalize the former isomer with the CH3 group trans to the sulfonate group is 2-9 kcal/mol lower. The possibility of the involvement of Pt(III) species in the reactions studied was found to correspond to high-barrier reactions and is hence not viable. It is concluded that the dpms ligand facilitates Pt(II) oxidation both enthalpically and entropically.

Mechanistic Study of the Oxidation of a Methyl Platinum(II) Complex with O2 in Water: PtIIMe-to-PtIVMe and PtIIMe-to-PtIVMe2 Reactivity

The mechanism of oxidation by O2 of (dpms)Pt(II)Me(OH2) (1) and (dpms)Pt(II)Me(OH)(-) (2) [dpms = di(2-pyridyl)methanesulfonate] in water in the pH range of 4-14 at 21 °C was explored using kinetic and isotopic labeling experiments. At pH ≤ 8, the reaction leads to a C1-symmetric monomethyl Pt(IV) complex (dpms)Pt(IV)Me(OH)2 (5) with high selectivity ≥97%; the reaction rate is first-order in [Pt(II)Me] and fastest at pH 8.0. This behavior was accounted for by assuming that (i) the O2 activation at the Pt(II) center to form a Pt(IV) hydroperoxo species 4 is the reaction rate-limiting step and (ii) the anionic complex 2 is more reactive toward O2 than neutral complex 1 (pKa = 8.15 ± 0.02). At pH ≥ 10, the oxidation is inhibited by OH(-) ions; the reaction order in [Pt(II)Me] changes to 2, consistent with a change of the rate-limiting step, which now involves oxidation of complex 2 by Pt(IV) hydroperoxide 4. At pH ≥ 12, formation of a C1-symmetric dimethyl complex 6, (dpms)Pt(IV)Me2(OH), along with [(dpms)Pt(II)(OH)2](-) (7) becomes the dominant reaction pathway (50-70% selectivity). This change in the product distribution is explained by the formation of a Cs-symmetric intermediate (dpms)Pt(IV)Me(OH)2 (8), a good methylating agent. The secondary deuterium kinetic isotope effect in the reaction leading to complex 6 is negligible; kH/kD = 0.98 ± 0.02. This observation and experiments with a radical scavenger TEMPO do not support a homolytic mechanism. A SN2 mechanism was proposed for the formation of complex 6 that involves complex 2 as a nucleophile and intermediate 8 as an electrophile.

Mechanical bonds and topological effects in radical dimer stabilization

While mechanical bonding stabilizes tetrathiafulvalene (TTF) radical dimers, the question arises: what role does topology play in catenanes containing TTF units? Here, we report how topology, together with mechanical bonding, in isomeric [3]- and doubly interlocked [2]catenanes controls the formation of TTF radical dimers within their structural frameworks, including a ring-in-ring complex (formed between an organoplatinum square and a {2+2} macrocyclic polyether containing two 1,5-dioxynaphthalene (DNP) and two TTF units) that is topologically isomeric with the doubly interlocked [2]catenane. The separate TTF units in the two {1+1} macrocycles (each containing also one DNP unit) of the isomeric [3]catenane exhibit slightly different redox properties compared with those in the {2+2} macrocycle present in the [2]catenane, while comparison with its topological isomer reveals substantially different redox behavior. Although the stabilities of the mixed-valence (TTF2)(•+) dimers are similar in the two catenanes, the radical cationic (TTF(•+))2 dimer in the [2]catenane occurs only fleetingly compared with its prominent existence in the [3]catenane, while both dimers are absent altogether in the ring-in-ring complex. The electrochemical behavior of these three radically configurable isomers demonstrates that a fundamental relationship exists between topology and redox properties.

Oligorotaxane Radicals under Orders

A strategy for creating foldameric oligorotaxanes composed of only positively charged components is reported. Threadlike components—namely oligoviologens—in which different numbers of 4,4′-bipyridinium (BIPY2+) subunits are linked by p-xylylene bridges, are shown to be capable of being threaded by cyclobis(paraquat-p-phenylene) (CBPQT4+) rings following the introduction of radical-pairing interactions under reducing conditions. UV/vis/NIR spectroscopic and electrochemical investigations suggest that the reduced oligopseudorotaxanes fold into highly ordered secondary structures as a result of the formation of BIPY•+ radical cation pairs. Furthermore, by installing bulky stoppers at each end of the oligopseudorotaxanes by means of Cu-free alkyne–azide cycloadditions, their analogous oligorotaxanes, which retain the same stoichiometries as their progenitors, can be prepared. Solution-state studies of the oligorotaxanes indicate that their mechanically interlocked structures lead to the enforced interactions between the dumbbell and ring components, allowing them to fold (contract) in their reduced states and unfold (expand) in their fully oxidized states as a result of Coulombic repulsions. This electrochemically controlled reversible folding and unfolding process, during which the oligorotaxanes experience length contractions and expansions, is reminiscent of the mechanisms of actuation associated with muscle fibers.

Redox Control of the Binding Modes of an Organic Receptor

The modulation of noncovalent bonding interactions by redox processes is a central theme in the fundamental understanding of biological systems as well as being ripe for exploitation in supramolecular science. In the context of host-guest systems, we demonstrate in this article how the formation of inclusion complexes can be controlled by manipulating the redox potential of a cyclophane. The four-electron reduction of cyclobis(paraquat-p-phenylene) to its neutral form results in altering its binding properties while heralding a significant change in its stereoelectronic behavior. Quantum mechanics calculations provide the energetics for the formation of the inclusion complexes between the cyclophane in its various redox states with a variety of guest molecules, ranging from electron-poor to electron-rich. The electron-donating properties displayed by the cyclophane were investigated by probing the interaction of this host with electron-poor guests, and the formation of inclusion complexes was confirmed by single-crystal X-ray diffraction analysis. The dramatic change in the binding mode depending on the redox state of the cyclophane leads to (i) aromatic donor-acceptor interactions in its fully oxidized form and (ii) van der Waals interactions when the cyclophane is fully reduced. These findings lay the foundation for the potential use of this class of cyclophane in various arenas, all the way from molecular electronics to catalysis, by virtue of its electronic properties. The extension of the concept presented herein into the realm of mechanically interlocked molecules will lead to the investigation of novel structures with redox control being expressed over the relative geometries of their components.

Solid-State Characterization and Photoinduced Intramolecular Electron Transfer in a Nanoconfined Octacationic Homo[2]Catenane

An octacationic homo[2]catenane comprised of two mechanically interlocked cyclobis(paraquat-p-phenylene) rings has been obtained from the oxidation of the septacationic monoradical with nitrosonium hexafluoroantimonate. The nanoconfinement of normally repulsive bipyridinium units results in the enforced π-overlap of eight positively charged pyridinium rings in a volume of <1.25 nm(3). In the solid state, the torsional angles around the C-C bonds between the four pairs of pyridinium rings range between 16 and 30°, while the π-π stacking distances between the bipyridinium units are extended for the inside pair and contracted for the pairs on the outside--a consequence of Coulombic repulsion between the inner bipyridinium subunits. In solution, irradiation of the [2]catenane at 275 nm results in electron transfer from one of the paraphenylene rings to the inner bipyridinium dimer, leading to the generation of a temporary mixed-valence state within the rigid and robust homo[2]catenane.