[Show abstract][Hide abstract] ABSTRACT: We use ab initio calculations to investigate the energetics and kinetics associated with carbinolamine formation resulting from the addition of dimethylamine to formaldehyde catalyzed by a single water molecule. Further, we compare the energetics for this reaction with that for the analogous reactions involving respectively, methylamine and ammonia. We find that the reaction barrier for the addition of these nitrogen containing molecules onto formaldehyde decreases along the series: ammonia, methylamine, and dimethylamine. Hence, starting with ammonia, the reaction barrier can be "tuned" by the substitution of an alkyl group in place of a hydrogen atom. The reaction involving dimethylamine has the lowest barrier with the transition state (TS) being 5.4 kcal/mol below the (CH3)2NH + H2CO + H2O separated reactants. This activation energy is significantly lower than that for the bare reaction occurring without water, H2CO + (CH3)2NH, which has a barrier of 20.1 kcal/mol. The negative barrier associated with the single-water molecule catalyzed reaction of dimethylamine with H2CO to form the carbinolamine (CH3)2NCH2OH suggests that this reaction should be energetically feasible under atmospheric conditions. This is confirmed by rate calculations which suggest that the reaction will be facile even in the gas phase. As amines and oxidized organics containing carbonyl functional groups are common components of secondary organic aerosols (SOA), the present finding has important implications for understanding how larger, less volatile organic compounds, can be generated in the atmosphere by combining readily available smaller components as required for promoting aerosol growth.
The Journal of Physical Chemistry A 09/2015; DOI:10.1021/acs.jpca.5b04887 · 2.69 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Atmospheric Chemistry. In their Communication on page 11404 ff., H. Beckers et al. report the isolation of methylsulfonyloxyl radicals, key intermediates in the atmospheric oxidation of dimethyl sulfide.
[Show abstract][Hide abstract] ABSTRACT: Interkulturelle Kooperation ist, wenn sie funktioniert, synergistisch und bringt ein Verstehen von Partnern mit sich, das jeder für sich allein wohl kaum erreichen kann. Überall auf der Welt gibt es Menschen, die etwas wissen, doch niemand weiß alles. Internationale Kooperationen bringen Wissen und Ressourcen zusammen und ziehen Nutzen aus deren weltweiter Verbreitung und dem menschlichen Streben nach mehr Wissen …” Lesen Sie mehr im Editorial von Joseph S. Francisco.
[Show abstract][Hide abstract] ABSTRACT: Cross-cultural collaboration, when it works, is synergistic, and brings understanding between partners that neither is likely to be able to develop alone. There are people in the world that know something, but nobody knows everything. International collaborations in science bring together and capitalize on the dispersal of knowledge and resources across the globe, and the human desire to advance knowledge …” Read more in the Editorial by Joseph S. Francisco.
Angewandte Chemie International Edition 09/2015; DOI:10.1002/anie.201505267 · 11.26 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Recently reported synthetic organic nanopore (SONP) can mimic a key feature of natural ion channels, i.e., selective ion transport. However, the physical mechanism underlying the K(+)/Na(+) selectivity for the SONPs is dramatically different from that of natural ion channels. To achieve a better understanding of the selective ion transport in hydrophobic subnanometer channels in general and SONPs in particular, we perform a series of ab initio molecular dynamics simulations to investigate the diffusivity of aqua Na(+) and K(+) ions in two prototype hydrophobic nanochannels: (i) an SONP with radius of 3.2 Å, and (ii) single-walled carbon nanotubes (CNTs) with radii of 3-5 Å (these radii are comparable to those of the biological potassium K(+) channels). We find that the hydration shell of aqua Na(+) ion is smaller than that of aqua K(+) ion but notably more structured and less yielding. The aqua ions do not lower the diffusivity of water molecules in CNTs, but in SONP the diffusivity of aqua ions (Na(+) in particular) is strongly suppressed due to the rugged inner surface. Moreover, the aqua Na(+) ion requires higher formation energy than aqua K(+) ion in the hydrophobic nanochannels. As such, we find that the ion (K(+) vs. Na(+)) selectivity of the (8, 8) CNT is ∼20× higher than that of SONP. Hence, the (8, 8) CNT is likely the most efficient artificial K(+) channel due in part to its special interior environment in which Na(+) can be fully solvated, whereas K(+) cannot. This work provides deeper insights into the physical chemistry behind selective ion transport in nanochannels.
Proceedings of the National Academy of Sciences 09/2015; 112(35):10851-6. DOI:10.1073/pnas.1513718112 · 9.67 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We present a comprehensive computational study of NH2 (radical) solvation in a water nanodroplet. The ab initio Born-Oppenheimer molecular dynamics simulation shows that NH2 tends to accumulate at the air-water interface. The hydrogen bonding analysis shows that compared to the hydrogen bond of HNH··OH2, the hydrogen bond of HOH··NH2 is the dominant interaction between NH2 and water. Due to the loose hydrogen bonding network formed between NH2 and the droplet interface, the NH2 can easily move around on the droplet surface, which speeds up the dynamics of NH2 at the air-water interface. Moreover, the structural analysis indicates that the NH2 prefers an orientation such that both N atom and one of its H atoms interact with the water droplet while the other H atom is mostly ex-posed to the air. The exposed hydrogen becomes a more probable reactive site for reaction at the water interface. More importantly, the solvation of NH2 modifies the amplitude of vibration of the N-H bond, thereby affecting the Mulliken charges and electrophilicity of NH2. As such, reactive properties of the NH2 are altered by the droplet interface and this can either speed up reactions or allow other reactions processes to occur in the atmosphere. Hence, the solvation of NH2 on water droplets, in chemistry of the atmosphere, may not be negligible when considering the effects of clouds.
Journal of the American Chemical Society 09/2015; DOI:10.1021/jacs.5b07354 · 12.11 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The SNO and OSN radical isomers are likely to be of significance in atmospheric and astrochemistry, but very little is known about their gas phase spectroscopic properties. State-of-the-art ab initio composite quartic force fields are employed to analyze the rovibrational features for both systems. Comparison to condensed-phase experimental data for SNO has shown that the 1566.4 cm−1 ν1 N–O stretch is indeed exceptionally bright and likely located in this vicinity for subsequent gas phase experimental analysis. The OSN ν1 at 1209.4 cm−1 is better described as the antisymmetric stretch in this molecule and is also quite bright. The full vibrational, rotational, and rovibrational data are provided for SNO and OSN and their single 15N, 18O, and 34S isotopic substitutions in order to give a more complete picture as to the chemical physics of these molecules.
The Journal of Chemical Physics 08/2015; 143(8). DOI:10.1063/1.4929472 · 2.95 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Even though quartic force fields (QFFs) and highly accurate coupled cluster computations describe the OCHCO(+) cation at equilibrium as a complex between carbon monoxide and the formyl cation, two notable and typical interstellar and atmospheric molecules, the prediction from the present study is that the equilibrium C∞v structure is less relevant to observables than the saddle-point D∞h structure. This is the conclusion from diffusion Monte Carlo and vibrational self-consistent field/virtual state configuration interaction calculations utilizing a semi-global potential energy surface. These calculations demonstrate that the proton "rattle" motion (ν6) has centrosymmetric delocalization of the proton over the D∞h barrier lying only 393.6 cm(-1) above the double-well OCHCO(+) C∞v minima. As a result, this molecule will likely appear D∞h, and the rotational spectrum will be significantly dimmer than the computed equilibrium 2.975 D center-of-mass dipole moment indicates. However, the proton transfer fundamental, determined to be at roughly 300 cm(-1), has a very strong intensity. This prediction as well as those of other fundamentals should provide useful guides for laboratory detection of this cation. Finally, it is shown that the two highest energy QFF-determined modes are actually in good agreement with their vibrational configuration interaction counterparts. These high-level quantum chemical methods provide novel insights into this fascinating and potentially common interstellar molecule.
The Journal of Chemical Physics 08/2015; 143(071102). DOI:10.1063/1.4929345 · 2.95 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Thiophosphoryl nitrene, R2P(S)N, is a thiazirine-like intermediate that has been chemically inferred from trapping products in early solution studies. Herein, photolysis of the simplest thiophosphoryl azide, F2P(S)N3, in solid noble gas matrices enables a first-time spectroscopic (IR and UV-Vis) identification of thiophosphoryl nitrene, F2P(S)N, in its singlet ground state. Upon visible light irradiation (≥ 495 nm), it converts into the thionitroso isomer F2P-N=S, which can also be produced in the gas phase from flash vacuum pyrolysis of F2P(S)N3. Further irradiation of F2P-NS with UV light of 365 nm leads to the reformation of F2P(S)N and isomerizn ation to a thiazyl species F2P-S≡N.
Journal of the American Chemical Society 08/2015; 137(34). DOI:10.1021/jacs.5b07302 · 12.11 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Das Methylsulfonyloxyl-Radikal, CH3SO3, ein Schlüsselintermediat der atmosphärischen Oxidation von Dimethylsulfid (DMS), wurde durch Blitzpyrolyse von CH3SO2OOSO2CH3 erzeugt und nachfolgend in festen Edelgas-Matrizen isoliert. Das Radikal wurde charakterisiert über seine UV/Vis- und IR-Spektren, sowie seine Tautomerisierung zum CH2SO3H unter Bestrahlung mit Licht von λ≥360 nm.
[Show abstract][Hide abstract] ABSTRACT: We investigate the lowest electronic states of doublet and quartet spin multiplicity states of HNS− and HSN− together with their parent neutral triatomic molecules. Computations were performed using highly accurate
methods with a large basis set. One-dimensional cuts of the full-dimensional potential energy surfaces (PESs) along the interatomic distances and bending angle are presented for each isomer. Results show that the ground anionic states are stable with respect to the electron detachment process and that the long range parts of the PESs correlating to the SH− + N, SN− + H, SN + H−, NH + S−, and NH− + S are bound. In addition, we predict the existence of long-lived weakly bound anionic complexes that can be formed after cold collisions between SN− and H or SH− and N. The implications for the reactivity of these species are discussed; specifically, it is shown that the reactions involving SH−, SN−, and NH− lead either to the formation of HNS− or HSN− in their electronic ground states or to autodetachment processes. Thus, providing an explanation for why the anions, SH−, SN−, and NH−, have limiting detectability in astrophysical media despite the observation of their corresponding neutral species. In a biological context, we suggest that HSN− and HNS− should be incorporated into H2S-assisted heme-catalyzed reduction mechanism of nitrites in vivo.
The Journal of Chemical Physics 07/2015; 143(143):34303-134309. DOI:10.1063/1.4926941 · 2.95 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Electronic structure calculations of the pertinent stationary points on the potential energy surface show that carboxylic acids can act effectively as catalysts in the hydration of acetaldehyde. Barriers to this catalyzed process correlate strongly with pKa of the acid, providing the potential to provide predictive capacity of the effectiveness of carboxylic acid catalysts. Transition states for the acid-catalyzed systems take the form of pseudo-6-membered rings through the linear nature of their hydrogen bonds, which accounts for their relative stability compared to the more strained direct and water-catalyzed systems. When considered as a stepwise reaction of a dimerization followed by reaction/complexation, it is likely that collisional stabilization of the pre-reactive complex is more likely than reaction in the free gas phase, although the catalyzed hydration does retain the potential to proceed on water surfaces or in droplets. Lastly, it is observed that post-reactive diol-acid complexes are significantly stable (~12-17 kcal/mol) relative to isolated products, suggesting the possibility of long-lived hygroscopic species that could act as a seed molecule for condensation of secondary organic aerosols.
The Journal of Physical Chemistry A 03/2015; 119(19). DOI:10.1021/jp510704j · 2.69 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Employing first-principles density functional theory (DFT), the structures and electronic and mechanical properties of Al(111)/ZrB2(0001) heterojunctions are investigated. It is found that both B-terminated ZrB2(0001) and Zr-terminated ZrB2(0001) can form heterojunction interfaces with Al(111) surface. The heterojunction with B-terminated ZrB2(0001) is demonstrated to be most stable by comparing the surface adhesion energies of six different heterojunction models. In the stable configurations, the Al atom is found projecting to the hexagonal hollow site of neighbouring boron layer for the B-terminated ZrB2(001), and locating at the top site of the boron atoms for Zr-terminated ZrB2(001) interface. The mechanisms of interface interaction are investigated by density of states, charge density difference and band structure calculations. It is found that covalent bonds between surface Al atoms and B atoms are formed in the B-terminated heterojunction, whereas the Al atoms and Zr atoms are stabilised by interface metallic bonds for the Zr-terminated case. Mechanical properties of Al/ZrB2 heterojunctions are also predicted in the current work. The values of moduli of Al/ZrB2 heterojunctions are determined to be between those of single crystal Al and ZrB2, which exhibit the transition of mechanical strength between two bulk phases. DFT calculations with the current models provide the mechanical properties for each heterojunction and the corresponding contributions by each type of interface in the composite materials. This work paves the way for industrial applications of Al(111)/ZrB2(0001) heterojunctions.
[Show abstract][Hide abstract] ABSTRACT: Conspectus Oxidation reactions are ubiquitous and play key roles in the chemistry of the atmosphere, in water treatment processes, and in aerobic organisms. Ozone (O3), hydrogen peroxide (H2O2), hydrogen polyoxides (H2Ox, x > 2), associated hydroxyl and hydroperoxyl radicals (HOx = OH and HO2), and superoxide and ozonide anions (O2(-) and O3(-), respectively) are the primary oxidants in these systems. They are commonly classified as reactive oxygen species (ROS). Atmospheric chemistry is driven by a complex system of chain reactions of species, including nitrogen oxides, hydroxyl and hydroperoxide radicals, alkoxy and peroxy radicals, and ozone. HOx radicals contribute to keeping air clean, but in polluted areas, the ozone concentration increases and creates a negative impact on plants and animals. Indeed, ozone concentration is used to assess air quality worldwide. Clouds have a direct effect on the chemical composition of the atmosphere. On one hand, cloud droplets absorb many trace atmospheric gases, which can be scavenged by rain and fog. On the other hand, ionic species can form in this medium, which makes the chemistry of the atmosphere richer and more complex. Furthermore, recent studies have suggested that air-cloud interfaces might have a significant impact on the overall chemistry of the troposphere. Despite the large differences in molecular composition, concentration, and thermodynamic conditions among atmospheric, environmental, and biological systems, the underlying chemistry involving ROS has many similarities. In this Account, we examine ROS and discuss the chemical characteristics common to all of these systems. In water treatment, ROS are key components of an important subset of advanced oxidation processes. Ozonation, peroxone chemistry, and Fenton reactions play important roles in generating sufficient amounts of hydroxyl radicals to purify wastewater. Biochemical processes within living organisms also involve ROS. These species can come from pollutants in the environment, but they can also originate endogenously, initiated by electron reduction of molecular oxygen. These molecules have important biological signaling activities, but they cause oxidative stress when dysfunction within the antioxidant system occurs. Excess ROS in living organisms can lead to problems, such as protein oxidation-through either cleavage of the polypeptide chain or modification of amino acid side chains-and lipid oxidation.
Accounts of Chemical Research 02/2015; 48(3). DOI:10.1021/ar500412p · 22.32 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Surface level ozone destruction in polar environments may be initiated by oxidation of bromide ions by ozone, ultimately leading to Br2 production. While ab initio calculations are used to support development of atmospheric chemistry models, errors can occur in study of the bromide-ozone reaction due to inappropriate treatment of the many-electron species and the charge nature of the reaction. In this work, a high level ab initio study is used to take into account the electronic correlation and the polarization effects. Our results show three possible pathways for the reaction. In particular, we find that this process, while endothermic on a singlet spin state surface, can be energetically feasible on the triplet surface. The triplet surface can be reached through photo-excitation of ozone or by the spin crossing of the potential energy surface. Since this process is known to occur in the dark, it may be that it occurs after intersystem crossing to a triplet surface. This paper also provides a starting point calibration for any future ab initio calculation studies of the bromide-ozone reaction, from the gas to the condensed phase.
The Journal of Physical Chemistry A 02/2015; 119(19). DOI:10.1021/jp5101279 · 2.69 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We report molecular dynamics (MD) simulation evidence of a new family of two-dimensional (2D) clathrate hydrates. Particular attention is placed on the effect of size and hydrophilicity of guest-molecule on the formation of 2D clathrate hydrates. Among MD simulations undertaken, spontaneous formation of bilayer (BL) clathrate hydrates in nanoslits are found with five different hydrophobic guest molecules, namely, ethane (C2H6), ethene (C2H4), allene (C3H4), carbon dioxide (CO2) and hydrogen (H2) molecules, respectively. Our simulations suggest that the host cages in water framework are likely BL-hexagonal cages with single occupancy for H2, or BL-heptagonal cages for CO2. With further increase of guest size, the host cages for C2H6, C2H4, and C3H4 are BL-octagonal cages with single occupancy, and their long molecular axis tends to be normal to the surface of clathrate hydrates. In addition, for hydrophilic guest molecules such as NH3 and H2S which can form strong hydrogen bonds with water, we find that most guest molecules can preferentially displace water molecules from lattice sites of water framework, instead of being separately trapped within water cages. Structural analogy between the 2D and 3D clathrates enlightens us to predict stability of several bulk gas hydrates, namely, “ethane clathrate III”, “CH4 ice-i” and “H2 ice-i”. Our findings not only can enrich clathrate structures in the hydrate family but also may improve understanding of the hydrate formation in microporous media.
[Show abstract][Hide abstract] ABSTRACT: The hydrolysis of ketene (H2C=C=O) to form acetic acid involving two water molecules and also separately in the presence of one to two water molecules plus formic acid (FA) has been investigated. Our results show that while the currently accepted indirect mechanism, involving addition of water across the carbonyl C=O bond of ketene to form an ene-diol followed by tautomerization of the ene-diol to form acetic acid is the preferred pathway when water alone is present, with formic acid as catalyst, addition of water across the ketene C=C double bond to directly produce acetic acid, becomes the kinetically favored pathway for temperatures below 400K.We find that the overall barrier for ketene hydrolysis involving one water molecule plus formic acid (H2C2O + H2O + FA) is not only significantly lower than that involving two water molecules (H2C2O + 2H2O), but that FA is able to reduce the barrier height for the direct path, involving addition of water across the C=C double bond, so that it is essentially identical with(6.4 kcal/mol) that for the indirect ene-diol formation path involving addition of water across the C=O bond. For the case of ketene hydrolysis involving two water molecules plus formic acid (H2C2O + 2H2O + FA), the barrier for the direct addition of water across the C=C double bond is reduced even further, and is 2.5 kcal/mol lower relative to the ene-diol path involving addition of water across the C=O bond. In fact, the hydrolysis barrier for the H2C2O + 2H2O + FA reaction through the direct path is sufficiently low (2.5 kcal/mol) for it to be an energetically accessible pathway for acetic acid formation under atmospheric conditions. Given the structural similarity between acetic and formic acid, our results also have potential implications for aqueous phase chemistry. Thus in an aqueous environment, even in the absence of formic acid, though the initial mechanism for ketene hydrolysis is expected to involve addition of water across the carbonyl bond as is currently accepted, the production and accumulation of acetic acid will likely alter the preferred pathway to one involving addition of water across the ketene C=C double bond as the reaction proceeds.
The Journal of Physical Chemistry A 01/2015; 119(19). DOI:10.1021/jp5076725 · 2.69 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: One route to break down halomethanes is through reactions with radical species. The capability of the artificial force-induced reaction algorithm to efficiently explore a large number of radical reaction pathways has been illustrated for reactions between haloalkanes (CX3Y; X=H, F; Y=Cl, Br) and ground-state (2Σ+) cyano radicals (CN). For CH3Cl+CN, 71 stationary points in eight different pathways have been located and, in agreement with experiment, the highest rate constant (108 s−1 M−1 at 298 K) is obtained for hydrogen abstraction. For CH3Br, the rate constants for hydrogen and halogen abstraction are similar (109 s−1 M−1), whereas replacing hydrogen with fluorine eliminates the hydrogen-abstraction route and decreases the rate constants for halogen abstraction by 2–3 orders of magnitude. The detailed mapping of stationary points allows accurate calculations of product distributions, and the encouraging rate constants should motivate future studies with other radicals.