The structure and the local proton mobility of poly(vinyl phosphonic acid) were studied by solid-state NMR under fast magic-angle spinning. At elevated temperatures, the signature of the hydrogen-bonded P-OH protons is observed in 1H magic-angle spinning (MAS) NMR as a single resonance at 10.5 ppm. Both 1H double-quantum NMR and variable-temperature experiments demonstrate that P-OH protons are mobile and thus able to contribute to proton conductivity. Below room temperature, two different types of hydrogen-bonded P-OH resonances are observed at 10.5 and 15 ppm, and 1H double-quantum NMR demonstrates that these protons are immobile on the NMR time scale. By means of first-principles calculations of a model polymer, we have assigned the additional hydrogen-bonded species at lower temperatures to phosphonic acid anhydride and charged anhydride. Also, in the 31P MAS NMR spectrum, two distinct resonances appear, arising from "normal" phosphonic acid and phosphonic acid anhydride. 31P double-quantum NMR experiments reveal that there is no phase segregation between normal and phosphonic acid anhydride and the condensation reaction occurs randomly throughout the system. The formation of acid anhydride leads to a decrease in proton conductivity through two mechanisms, (1) decrease in the number of charge carriers and (2) blockage of charge transport pathways through immobilization of charge carriers together with a hindered reorientation of the anhydride group. Our results provide strong evidence for these mechanisms by demonstrating that the conductivity is greatly influenced by the presence of phosphonic acid anhydride.
A combined experimental and ab initio study is presented of the 1H NMR chemical shift distribution of aqueous hydrogen chloride solution as a function of acid concentration, based on Car-Parrinello molecular dynamics simulations and fully periodic NMR chemical-shift calculations. The agreement of computed and experimental spectra is very good. From first-principles calculations, we can show that the individual contributions of Eigen and Zundel ions, regular water molecules, and the chlorine solvation shell to the NMR line are very distinct and almost independent of the acid concentration. From the computed instantaneous NMR distributions, it is further possible to characterize the average variation in hydrogen-bond strength of the different complexes.
Transition metal surfaces play a crucial role for many reactions in heterogeneous catalysis. Their catalytic functionality can be affected by a variety of factors, such as the morphology of the surface, defects, or poisoning. The most prominent example of poisoning is the adsorption of CO on platinum and similar surfaces. [1][2][3] Another important issue is the co-adsorption of several species, which may have an important influence on dissociation processes. [3,4] More generally, the adsorption of small molecules from the environment can significantly modify the catalytic efficiency of such metal surfaces. One particular case, in this scenario, is the adsorption of water. There are many theoretical and experimental studies of the structure and properties of water layers on metal surfaces in the literature. [5][6][7][8][9][10] Normally, the presence of a full layer is assumed in these investigations. However, the process of wetting, which is initiated by the adsorption of a single water molecule or small water clusters, is still poorly understood. From the view of an adsorbing water molecule, the surface has to compete thermodynamically with larger water clusters or simply the gas phase. Both phases provide a significantly larger entropic contribution to the free energy, which has to be compensated by a corresponding energy difference. Therefore, the theoretical investigation of the structural and energetic properties of the initial adsorption process on realistically modeled surfaces deserves particular attention.In particular, the crucial role of surface defects on adsorption processes is not always considered, especially in theoretical studies. Recently, we showed that a simple step defect on the nickel surface has the potential to enhance the adsorption energy of the initial water molecule by as much as 40 %. [11] Also the incremental adsorption energy of an additional second water molecule is higher than at a defect-free adsorption site. In principle, these adsorption energies can be determined experimentally, but spectroscopic parameters are often easier to obtain. First-principle calculations of experimentally accessible spectra are very scarce because of the relatively high computational cost involved in realistic and accurate calculations.Herein we want to bridge the gap between experiment and theory by providing ab initio calculations of IR peaks as a function of adsorption sites and cluster sizes, enabling for the first time a direct comparison of measurements and calculations. The initial steps of water adsorption on different nickel surfaces by means of their harmonic frequencies are characterized. The modification of the vibrational modes and frequencies of water clusters (monomer, dimer and trimer) upon adsorption are illustrated and these new vibrational modes involving the nickel-oxygen bond are described. Recent theoretical and experimental studies have shown that both the antisymmetric and the symmetric stretch vibrations can promote catalytic processes such as the chemisorption of methane on n...
We present (1)H NMR chemical shift calculations of liquid water based on first principles molecular dynamics simulations under periodic boundary conditions. We focus on the impact of computational parameters on the structural and spectroscopic data, which is an important question for understanding how sensitive the computed (1)H NMR resonances are upon variation of the simulation setup. In particular, we discuss the influence of the exchange-correlation functional and the size of the basis set, the choice for the fictitious electronic mass and the use of pseudopotentials for the nuclear magnetic resonance (NMR) calculation on one hand and the underlying Car-Parrinello-type molecular dynamics simulations on the other hand. Our findings show that the direct effect of these parameters on (1)H shifts is not big, whereas the indirect dependence via the structural data is more important. The (1)H NMR chemical shifts clearly reflect the induced structural changes, illustrating once again the sensitivity of (1)H NMR observables on small changes in the local chemical structure of complex hydrogen-bonded liquids.
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