Challenges in preparing, preserving and detecting para-water in bulk: overcoming proton exchange and other hurdles

Para-water is an analogue of para-hydrogen, where the two proton spins are in a quantum state that is antisymmetric under permutation, also known as singlet state. The populations of the nuclear spin states in para-water are believed to have long lifetimes just like other Long-Lived States (LLSs). This hypothesis can be verified by measuring the relaxation of an excess or a deficiency of para-water, also known as a "Triplet-Singlet Imbalance'' (TSI), i.e., a difference between the average population of the three triplet states T (that are symmetric under permutation) and the population of the singlet state S. In analogy with our recent findings on ethanol and fumarate, we propose to adapt the procedure for Dissolution Dynamic Nuclear Polarization (D-DNP) to prepare such a TSI in frozen water at very low temperatures in the vicinity of 1.2 K. After rapid heating and dissolution using an aprotic solvent, the TSI should be largely preserved. To assess this hypothesis, we studied the lifetime of water as a molecular entity when diluted in various solvents. In neat liquid H2O, proton exchange rates have been characterized by spin-echo experiments on oxygen-17 in natural abundance, with and without proton decoupling. One-dimensional exchange spectroscopy (EXSY) has been used to study proton exchange rates in H2O, HDO and D2O mixtures diluted in various aprotic solvents. In the case of 50 mM H2O in dioxane-d(8), the proton exchange lifetime is about 20 s. After dissolving, one can observe this TSI by monitoring intensities in oxygen-17 spectra of H2O (if necessary using isotopically enriched samples) where the AX(2) system comprising a "spy'' oxygen A and two protons X-2 gives rise to binomial multiplets only if the TSI vanishes. Alternatively, fast chemical addition to a suitable substrate (such as an activated aldehyde or ketone) can provide AX2 systems where a carbon-13 acts as a spy nucleus. Proton signals that relax to equilibrium with two distinct time constants can be considered as a hallmark of a TSI. We optimized several experimental procedures designed to preserve and reveal dilute para-water in bulk.

Study of fast exchanging processes by NMR Spectroscopy

Estel Canet Martinez

Nuclear magnetic resonance (NMR) spectroscopy is one of the most powerful and informative methods to probe molecular dynamics. Specifically, chemical exchange is an important phenomenon and one of the earliest and most vigorously investigated in NMR spectroscopy. Its effects can be observed in many NMR spectra; lines in a spectrum are broadened or averaged because the nuclei are exchanging magnetic environments. A wide variety of experimental methods exists to determine exchange rates of protons that hop between amino acids and water in the slow or intermediate exchange regime. However, so far there is a lack of methods to investigate hydrogen exchange in the fast exchange regime. In this thesis, we focus on the determination of very fast chemical exchange rates from the effects of scalar relaxation on a coupled nucleus. We quantify the effects of scalar relaxation caused by exchanging hydrogens, by detecting the decay of a 15N coherence under a multiple-refocusing Carr-Purcell-Meiboom-Gill (CPMG) pulse train in the presence or absence of hydrogen decoupling. In particular, we have adapted to the case of deuterium a method that was originally designed to study the exchange rate of the indole proton of tryptophan in water as a function of pH and temperature. We will present the exchange rates of the indole deuterium in tryptophan with solvent heavy water. After that, the proton exchanges rates have been compared with the deuterium exchange rates in order to describe the kinetic isotope effect. In a similar manner, we have quantified the proton exchange rates in pure water by using multiple refocusing of transverse 17O magnetization as a function of pH and temperature. Such measurements are widely used to obtain valuable information about molecular dynamics and structure of proteins, protein complexes and nucleic acids. The knowledge of the kinetic isotope effect may contribute to the characterization of reaction mechanisms and other dynamic parameters that can give insight into the stability of hydrogen bonded secondary structures in biomolecules. This information could form a foundation for a more thorough theoretical or modeling development of a number of long and outstanding fundamental questions. In the second part of the thesis, we combine fast exchange with dissolution dynamic nuclear polarization (D-DNP) to study intrinsically disordered proteins (IDPs). IDPs constitute a very large and functionally important class of proteins that lack stable secondary and tertiary structures. As a consequence of the broad dynamics and flexibility of IDPs, and specially under physiological conditions, the NMR spectra of IDPs typically show a strong signal overlap and broadening due to proton exchange. We present a method to overcome these limitations by means of hyperpolarized HDO produced by D-DNP. We could study the effects of conformational adaptations in Osteopontin (OPN), a metastasis- associated intrinsically disordered protein (IDP) by chemical exchange. The magnetization from hyperpolarized water is transferred to the rapidly exchanging protons of the solvent-exposed residues of the protein. The exchange with hyperpolarized water boosts the signals of fast exchanging residues above the detection limits. This allows one to follow individual resonances and draw conclusions regarding conformational adaptations due to ligand binding under physiological conditions. Thus, with our approach, exchange with the solvent is turned into an advantage

EPFL2017

Dark curing kinetics of UV printing inks by real-time ATR-FTIR spectroscopy

Eric Jacques Nouzille

In UV-curable printing technology, an ink or a varnish is exposed to the UV-light source within a fraction of a second. Subsequent polymerization therefore proceeds in the dark, referred to as dark curing. Because dark curing can be critical and contributes to a large part of the overall conversion and to the final properties of the printed ink, the aim of this work was to get a better understanding of the dark curing process. Because an ink is a complex mixture of components, simpler systems representative of free radical and cationic chemistry were studied. Special attention was given to free radical polymerization with a detailed kinetic modeling. For cationic systems, the study was more application oriented. Attenuated-Total-Reflection Fourier-Transform Infrared (ATR-FTIR) spectroscopy was found to be the method of choice to follow in real-time the progress of the reaction under UV irradiation and in the dark. This technique allowed the study of curing conditions close to those encountered in industry. Dark curing of free radical was examined in systems consisting of a monomer (multifunctional acrylate) and a photoinitiator. The reactivities of different monomers were investigated and it was found that the chemistry of the spacer between the acrylate functional groups had an influence on the extent of dark curing. For example, TPGDA (tripropylene glycol diacrylate) led to higher overall conversion than HDDA (hexanediol diacrylate) although both are diacrylate monomers. Several explanations were put forward, such as the effect of dissolved oxygen, primary cyclization and initiation efficiency. The efficiency of photoinitiators (PIs) was examined and different behaviours were highlighted. For example, it was observed that DMPA (dimethoxyphenyl acetophenone) has a better quantum yield for α-cleavage compared to HCPK (1-hydroxycyclohexylphenyl ketone), whereas HCPK is more efficient in initiating the reaction, resulting in a better overall conversion in the dark. It is suggested that the two primary radicals resulting from the cleavage of the PI behave differently for DMPA and HCPK. One radical effectively initiates the reaction and the second is mainly involved in the termination process. These two different behaviours were observed under specific experimental conditions: (i) by covering the sample with a quartz plate and taking into account dissolved molecular oxygen and (ii) by curing under inert conditions with nitrogen. Photo-crosslinking polymerization reactions are strongly influenced by diffusion effects due to the formation of an infinite network. Therefore, it was proposed that a redistribution of reactivity within the system occurs, such as the diffusion of unreacted photoinitiators towards concentrations of unreacted monomers. This diffusion process has two positive effects: (i) for the same UV dose, but different intensities, a higher extent of conversion was reached with longer irradiation times under the condition investigated; (ii) multiple exposures led to a higher overall conversion than for a single exposure, for a fixed overall UV dose. Other parameters were explored, such as temperature and diffusing oxygen from air. With temperature, different trends were observed depending on the photoinitiator and the start of dark curing, either starting before the maximum rate of polymerization (Rp,max), or after Rp,max. The development of a kinetic model to describe dark curing of free radical polymerization was proposed. Existing models based on physical parameters, such as those incorporating diffusion-controlled phenomena and free-volume are relatively complex and require extensive information on the system studied. Therefore, a semi empirical approach was chosen in order to study the variation of fitting parameters when changing curing parameters, such as the photoinitiator concentration and type, the irradiation time, the UV light intensity and the temperature. The model was derived from kinetic data and it was assumed that rate coefficients are time-dependent and that termination, which was considered to be a first-order process, is propagation dependent. The kinetic equation has the form of a stretched exponential function and incorporates four fitting parameters. The model was evaluated for a large number of experimental data and very good fits were obtained with a random distribution of residuals. The exponent β was found to be sensitive to the termination process and reflected different reactivities in the dark as in the case of DMPA and HCPK. Dark curing of cationic systems continues until complete monomer conversion but at a slower rate. Thus systems with high reaction rates, both during UV irradiation and during the first seconds of the dark period, are of most importance for UV-curable printing technology. The most widely used diepoxide monomer in cationic ink formulation is 3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexane carboxylate (DECC). In order to improve its reactivity during the first seconds of dark curing, the influence of several parameters was examined, such as the addition of an oxetane co-monomer (TMPO), the influence of relative humidity, UV dose, light intensity, temperature, and film thickness. The enhanced reactivity of DECC with TMPO was confirmed. During the cationic curing process, two reaction mechanisms compete; an activated chain end (ACE) mechanism and an activated monomer (AM) mechanism. It was observed that these two processes depend on the presence of hydroxyl containing compounds such as TMPO monomer, polyol compounds, and, most important, water. When the concentration of these components is increased the AM mechanism is enhanced, leading to higher conversion but at a slower curing rate, and a degradation of the mechanical properties. The presence of water is unavoidable and its concentration in the ink varies in presence of hydrophilic compounds, such as polyol. Furthermore, the water content increases with increasing relative humidity in air. It was shown that the reactivity of the system decreased as the percent of relative humidity increased. Moreover, although a higher overall conversion is achieved, the crosslink density of the film was severely reduced. This lower crosslink density was the result of the predominance of chain transfer reactions, leading to more pendant and short chains which were hydroxyl terminated. Moreover, the extent of conversion was thickness dependent since less water diffuses into the deeper layers occurred in thicker films.

EPFL2007

Study of N2O decomposition over Fe-ZSM-5 with transient methods

Petra Prechtl

Nitrous oxide is the third most important gas contributing to global warming and its concentration in the atmosphere is still on the rise. Nitric acid production represents the largest source of N2O in the chemical industry, with a total annual emission of about 400 kt N2O. An efficient way to reduce the N2O is the catalytic decomposition into N2 and O2. ZSM-5 zeolites containing iron are known to be effective in this reaction involving Fe(II)-extraframework species as active sites. But the nature of these sites and the reaction network is still under debate. Often the N2O is in the exhaust together with other gases like NOx, COx, oxygen, and water vapor, which may influence the catalyst efficiency towards N2O decomposition. The effect of these gases on the reaction mechanism is still not understood yet. The aim of this work is to study the decomposition mechanism of N2O over isomorphously substituted Fe-ZSM-5 (0.02-0.55% Fe) using transient response methods. Kinetic studies under steady-state conditions give overall information about the reaction, whereas transient experiments lead to information on the individual steps and the possibility to suggest a reaction model. Transient response methods are applied in vacuum using a Temporal Analysis of Products (TAP) setup and at ambient pressure using a Micromeritics AutoChem 2910 analyzer. The influence of different gases such as NO, H2O and NO2 on the decomposition of N2O over Fe-ZSM-5 is investigated to gain insight into their effect on this reaction. The TAP multifunctional reactor system is used to perform transient pulse experiments for micro-kinetic analysis of complex heterogeneous catalytic reactions. This setup allows to dose precisely reactants and to monitor product formation with submillisecond time resolution implying the absence of external mass or heat transfer. A reproducible low hydroxylation level of the catalyst can be attained in vacuum. The results from the transient responses with the Micromeritics analyzer are obtained under conditions more relevant to "operando". The mechanism of N2O decomposition over Fe containing ZSM-5 was studied by transient response methods at ambient pressure with the Micromeritics analyzer and under vacuum with the TAP setup. Both approaches showed that the mechanism strongly depends on the reaction temperature, but in all cases involves atomic oxygen loading from N2O with N2 evolution as the first reaction step. The surface oxygen, (O)Fe, formed from N2O possesses very high reactivity and oxidizes CO to CO2 already at 373 K. A complete saturation of the active sites can be reached by the catalyst exposure to N2O at low temperatures (T < 600 K in vacuum, T < 523 K at ambient pressure). Under these conditions oxygen is stored and does not desorb. On that basis, the concentration of sites active for the surface atomic oxygen loading from N2O is determined by pulse experiments using the TAP setup. The concentration of active sites is found to be the same as in a flow experiment with the Micromeritics analyzer. The mechanism of surface atomic oxygen recombination and desorption as O2 depends on the temperature. At temperatures higher than 773 K, the mechanism probably involves direct (O)Fe recombination. The investigation of the response profiles upon N2O pulses at 803 K evidences the O2 formation is the rate-determining step since the oxygen desorption is found to be slow compared to N2O and N2. Furthermore, a fit of the oxygen desorption curve corresponded to a first order dependence on the oxygen surface coverage. In the range of 523 to 773 K, the O2 formation is facilitated by NO, which is formed on the catalyst surface from N2O. The addition of NO to the gas phase confirmed the accelerating effect of NO on the oxygen desorption. NO and (O)Fe are adsorbed on adjacent, but different sites and are not competing for the same ones. Adsorbed NO accommodates oxygen from N2O forming higher oxidized nitrogen oxide species NO2/NO3 which decompose back to NOads and molecular oxygen. Thus, the recombination of surface oxygen loaded from N2O is accelerated by a NO/NOx redox cycle whereas adsorbed NOx acts as co-catalyst. Adsorbed NO is oxidized by N2O easily as shown also by temperature-programmed desorption (TPD) experiments. Therefore, the interaction of gaseous NO2 with Fe-ZSM-5 at 523 K in vacuum and at ambient pressure is investigated and demonstrates the formation of two different NOx surface species accompanied by the evolution of gaseous NO. These surface species block the sites for oxygen loading from N2O whereas the amount of reversible adsorbed N2O stays constant compared to the standard pre-treated catalyst. The formation of reactive surface oxygen from NO2 could neither be evidenced in TPD nor with CO oxidation experiments. Adsorbed water strongly influences the N2O decomposition inhibiting it completely, at least at temperatures ≤ 673 K. Water pulses at 523 and 593 K result in partial desorption of the loaded atomic surface oxygen as O2, indicating the competition of water for the same adsorption sites. The residual adsorbed oxygen desorbs during TPD experiments at 849 K instead of 715 K for the dry catalyst and it can not oxidize CO to CO2. It means that the active sites are transformed into an inactive form in the presence of water. The oxygen which stays on the catalyst seems to participate in the Fe(II) oxidation to the hydroxylated Fe(III) species. These Fe(III) species are inactive in N2O decomposition and could be activated again by high temperature treatment in He or in vacuum. During TAP experiments only small amounts of water are dosed and the water desorption is facilitated due to vacuum. Therefore, the effect of water could be shown to be reversible at temperatures > 700 K. At 803 K oxygen desorbs significantly faster with increasing water amount suggesting a water assisted O2 desorption. Simultaneously, the catalyst activity decreases to about a half of the initial activity of the dry zeolite. To conclude, from the TAP reactor data obtained under vacuum conditions and the results from the transient flow experiments at ambient pressure it follows that the mechanism of the N2O decomposition over Fe-containing ZSM-5 depends strongly on the reaction temperature and on the presence of NO, H2O and NO2. In general, below 523 K only surface atomic oxygen loading from N2O takes place with simultaneous evolution of N2 in the gas phase. The O2 formation proceeds via direct (O)Fe recombination at temperatures above 773 K. In the range of 523 to 773 K, the recombination of the atomic oxygen involves NOx adsorbed on the catalyst surface.

EPFL2007

Dark curing kinetics of UV printing inks by real-time ATR-FTIR spectroscopy

Eric Jacques Nouzille

EPFL2007

Study of N2O decomposition over Fe-ZSM-5 with transient methods

Petra Prechtl

EPFL2007

Study of fast exchanging processes by NMR Spectroscopy

Estel Canet Martinez

EPFL2017