•Thermal degradation of fluoropolymers can be probed through Reactive Force Field (ReaxFF) molecular dynamics (MD) simulations.•ReaxFF MD simulation accurately reproduced the bulk density and glass ...transition temperature of polytetrafluoroethylene (PTFE).•Pyrolysis of PTFE takes place via random CC bond cleavage followed by C2F4 unzipping through β–scission.•Major products from pyrolysis of PTFE are C2F4, C2F6, C3F6.
The formation of products of incomplete destruction (PIDs) from fluoropolymer incineration is poorly understood and it is imperative to environmental impact studies. The lack of analytical standards limits the experimental approaches targeting product analysis. To navigate this challenge, computational modeling of the thermal degradation of fluoropolymers provides simulated product distributions. However, it is essential to benchmark reactive forcefields to accurately simulate fluoropolymer pyrolysis. The present work describes a protocol to perform accurate simulations of the thermal degradation of fluoropolymers to probe the PIDs. The ReaxFF force field was applied to reproduce the experimental bulk density and glass transition temperature of polytetrafluoroethylene (PTFE). The benchmarked methodology developed has been extended to provide simulated product distributions and mechanistic insights which are in excellent agreement with primary literature. On the basis of our simulated data, we observe a degradation mechanism that proceeds through three primary steps: 1) initiation of random backbone cleavage, 2) C2F4 unzipping through β–scission (as opposed to CF2 unzipping), and 3) secondary product formation. An extension of the developed protocol has the potential to simulate the thermal degradation of non-polymeric per- and polyfluoroalkyl substances (PFASs) in addition to long-chain fluoropolymers.
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1,2‑H Atom Rearrangements in Benzyloxyl Radicals Van Hoomissen, Daniel J; Vyas, Shubham
The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory,
01/2019, Letnik:
123, Številka:
2
Journal Article
Recenzirano
The rate constants for solvent-assisted 1,2-H atom rearrangements in para-substituted benzyloxyl radicals were studied with density functional theory. The rate of the radical rearrangement, ...calculated through transition state theory with Eckhart tunneling corrections, was shown to be drastically impacted by the presence of both implicit and explicit solvent molecules, with a quantitative agreement with laser flash photolysis studies for a variety of electron-donating and -withdrawing substituents. The rate of rearrangement was found to be correlated to the distance between the rearranging hydrogen atom and the α-carbon in the transition state, which could be modified through the para substituent and the type of assisting solvent molecule (e.g., water, ethanol, methanol, acetic acid, or a mixture of the latter). Natural bond orbital analysis showed that the rearrangement does not proceed through a hydrogen radical but through a quasi-proton exchange and charge transfer between the benzyl carbon and the adjacent oxygen atom. Energetic and spin population results indicated that electron-withdrawing groups induce faster rearrangement kinetics. Understanding 1,2-H atom shifts in benzyloxyl radicals are essential for tuning the rate of superoxide production in aqueous systems, as the resonance-stabilized carbon radical produced from the rearrangement can bind oxygen and decompose to produce superoxide radical anion, an important reactive intermediate in environmental and biological systems.
In this article, an experimental comparison between open sun and solar drying is done. The thermal performance of evacuated tube based solar dryer is investigated with drying characteristics of ...Phyllanthus Emblica (Anvla), Aloe Vera, Aegle Marmelos (Bel) and leaves of Azadirachta Indica (Neem), Aegle Marmelos (Bel) and Psidium Guajava (Guava). In this setup, an evacuated tube collector, shell and tube heat exchanger and drying chamber are used. It was found that the maximum temperature difference between hot air and ambient air is 35.4°C and maximum efficiency of the setup is calculated as 55%. The average drying rate of Phyllanthus Emblica (Anvla), Aloe Vera and Aegle Marmelos (Bel) is measured as 0.46 g/min, 0.44 g/min, and 0.39 g/min respectively which are higher than that of Open Sun Drying. The leaves of Azadirachta Indica (Neem), Aegle Marmelos (Bel) and Psidium Guajava (Guava) also get dried with faster rates of 0.18 g/min, 0.17 g/min, and 0.14 g/min respectively.
The success of solar fuel technology relies on the development of efficient catalysts that can oxidize or reduce water. All molecular water-oxidation catalysts reported thus far are transition-metal ...complexes, however, here we report catalytic water oxidation to give oxygen by a fully organic compound, the N(5)-ethylflavinium ion, Et-Fl(+). Evolution of oxygen was detected during bulk electrolysis of aqueous Et-Fl(+) solutions at several potentials above +1.9 V versus normal hydrogen electrode. The catalysis was found to occur on glassy carbon and platinum working electrodes, but no catalysis was observed on fluoride-doped tin-oxide electrodes. Based on spectroelectrochemical results and preliminary calculations with density functional theory, one possible mechanistic route is proposed in which the oxygen evolution occurs from a peroxide intermediate formed between the oxidized flavin pseudobase and the oxidized carbon electrode. These findings offer an organic alternative to the traditional water-oxidation catalysts based on transition metals.
Electronic and structural properties of the room temperature ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulonyl)imide are studied using density functional theory (DFT) methods in ...addition to infrared and UV–vis spectroscopy. The DFT methods were conducted for both gas phase and solution phase using the integral equation formalism polarizable continuum model, while optical absorption experiments were conducted using neat and dilute methanol solutions. Three energetically similar conformers were obtained for each of the gas phase and solution phase DFT calculations. These multiple configurations were considered when analyzing the molecular interactions between the ion pair and for a molecular-level interpretation of the experimental IR and UV–vis spectroscopy data. Excitation energies of low-lying singlet excited states of the conformers were calculated with time-dependent DFT and experimentally with UV–vis absorption spectra. Difference density plots and excited-state calculations in the gas phase are found to be in good agreement with the experimental findings, while the implicit solvation model calculations adversely impacted the accuracy of the predicted spectra.
This work investigates stability and chemical bonding in possible per- and polyfluoroalkyl substances (PFAS) generated through the disposal of munitions in controlled detonations and open burns. ...Density functional theory (DFT) calculations were used to determine bond dissociation enthalpies (BDEs), activation energies, and other chemical properties. Calculated parameters were used to determine the functional groups most likely to be present based on the level of fluorination and the position of fluorines. In compounds that form C–O bonds, the presence of α-fluorines significantly strengthens the C–O bond by ∼4–18 kcal/mol. The results of this study indicate that fluoroalkyl alcohols are a very likely product of the disposal of munitions. This work was designed to expedite the analytical process of confirming that PFAS are created from current disposal methods of energetic devices by providing insight as to of what types of compounds should be expected. The PFAS generated in such reactions are expected to contain some functional groups (i.e., nitro and nitrite) that have not been known to exist as a result of the environmental degradation of industrially relevant PFAS, therefore, they may have been overlooked before. These initial results imply that PFAS with nitro functionalities may be formed in these conditions considering the abundance of NO2 radicals expected to be present as well as the strength of the C–N bond that can form (∼40–50 kcal/mol) whereas with nitroso functionalities are not expected to be found since the bonds formed are much weaker (∼25–35 kcal/mol), and nitrosoalkanes are known to decompose under mild conditions. Although these results are promising, analytical work is needed to assess the conclusions of this study in real systems.
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•Radical combination of products of fluoropolymer degradation and energetic compound decomposition can lead to new relatively unexplored PFAS.•Fluorination affects the chemical bonding and stability of per- and polyfluoroalkyl substances with NO2, NO, ONO, and OH functional groups.•The stability of PFAS with NO2, NO, ONO, and OH functional groups can be explained using the stereoelectronics.•The most stable isomer out of nitro and nitrite isomers depends on the degree of fluorination of the alkyl unit.
This study explored the fundamental chemical intricacies behind the interactions between metal catalysts and carbon supports with graphitic nitrogen defects. These interactions were probed by ...examining metal adsorption, specifically, the location of adsorption and the electronic structure of metal catalysts as the basis for the metal–support interactions (MSIs). A computational framework was developed, and a series of 12 transition metals was systematically studied over various graphene models with graphitic nitrogen defect(s). Different modeling approaches served to provide insights into previous MSI computational discrepancies, reviewing both truncated and periodic graphene models. The computational treatment affected the magnitudes of adsorption energies between the metals and support; however, metals generally followed the same trends in their MSI. It was found that the addition of the nitrogen dopant improved the MSI by promoting electronic rearrangement from the metals’ d- to s-orbitals for greater orbital overlap with the carbon support, shown with increased favorable adsorption. Furthermore, the study observed periodic trends that were adept descriptors of the MSI fundamental chemistries.
We have determined the identity of the complexes extracted into the ALSEP process solvent from solutions of nitric acid. The ALSEP process is a new solvent extraction separation designed to separate ...americium and curium from trivalent lanthanides in irradiated nuclear fuel. ALSEP employs a mixture of two extractants, 2-ethylhexyl phosphonic acid mono-2-ethylhexyl ester (HEHEHP) and N,N,N′,N′-tetra(2-ethylhexyl)diglycolamide (TEHDGA) in n-dodecane, which makes it difficult to ascertain the nature of the extracted metal complexes. It is often asserted that the weak acid extractant HEHEHP does not participate in the extracted complex under ALSEP extraction conditions (2–4 M HNO3). However, the analysis of the Am extraction equilibria, Nd absorption spectra, and Eu fluorescence emission spectra of metal-loaded organic phases argues for the participation of HEHEHP in the extracted complex despite the high acidity of the aqueous phases. The extracted complex was determined to contain fully protonated molecules of HEHEHP with an overall stoichiometry of M(TEHDGA)2(HEHEHP)2·3NO3. Computations also demonstrate that replacing one TEHDGA molecule with one (HEHEHP)2 dimer is likely energetically favorable compared to Eu(TEHDGA)3·3NO3, whether the HEHEHP dimer is monodentate or bidentate.
The photochemistry of 2-naphthylsulfonyl azide (2-NpSO2N3) was studied by femtosecond time-resolved infrared (TR-IR) spectroscopy and with quantum chemical calculations. Photolysis of 2-NpSO2N3 with ...330 nm light promotes 2-NpSO2N3 to its S1 state. The S1 excited state has a prominent azide vibrational band. This is the first direct observation of the S1 state of a sulfonyl azide, and this vibrational feature allows a mechanistic study of its decay processes. The S1 state decays to produce the singlet nitrene. Evidence for the formation of the pseudo-Curtius rearrangement product (2-NpNSO2) was inconclusive. The singlet sulfonylnitrene 1(2-NpSO2N) is a short-lived species (τ ≈ 700 ± 300 ps in CCl4) that decays to the lower-energy and longer-lived triplet nitrene 3(2-NpSO2N). Internal conversion of the S1 excited state to the ground state S0 is an efficient deactivation process. Intersystem crossing of the S1 excited state to the azide triplet state contributes only modestly to deactivation of the S1 state of 2-NpSO2N3.
The photochemistry of 2-naphthoyl azide was studied in various solvents by femtosecond time-resolved transient absorption spectroscopy with IR and UV–vis detection. The experimental findings were ...interpreted with the aid of computational studies. Using polar and nonpolar solvents, the formation and decay of the first singlet excited state (S1) was observed by both time-resolved techniques. Three processes are involved in the decay of the S1 excited state of 2-naphthoyl azide: intersystem crossing, singlet nitrene formation, and isocyanate formation. The lifetime of the S1 state decreases significantly as the solvent polarity increases. In all solvents studied, isocyanate formation correlates with the decay of the azide S1 state. Nitrene formation correlates with the decay of the relaxed S1 state only upon 350 nm excitation (S0 → S1 excitation). When S n (n ≥ 2) states are populated upon excitation (λex = 270 nm), most nitrene formation takes place within a few picoseconds through the hot S1 and higher singlet excited states (S n ) of 2-naphthoyl azide. The data correlate with the results of electron density difference calculations that predict nitrene formation from the higher-energy singlet excited states, in addition to the S1 state. For all of these experiments, no recovery of the ground state was observed up to 3 ns after photolysis, which indicates that both internal conversion and fluorescence have very low efficiencies.
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