The adsorption of nitrobenzene at the surface of hexagonal ice was studied by grand canonical Monte Carlo (GCMC) simulations at 200 K by employing TIP5P water model and our modified force field for nitrobenzene. We found that the number of adsorbed nitrobenzene molecules gradually increases with relative pressure before the condensation point and the condensation precedes the monolayer adsorption saturation. The adsorption follows the Langmuir shape only up to a very low coverage. At this low coverage, the adsorption of the molecules occurs independently from each other to adsorption sites (called α sites), where adsorbed nitrobenzene molecules lie almost in parallel with the ice surface to facilitate strong electrostatic interactions with ice surface. More importantly, in the α-type adsorption, a typical O–H···π bond for the adsorption of aromatics on the ice surface is not preferable for nitrobenzene. With increasing surface coverage, additional adsorbed molecules do not take unoccupied α sites due to attractive interactions among adsorbates, inducing a deviation of the adsorption isotherm from the Langmuir shape. In addition, the calculated adsorption energy (−75.98 kJ/mol) for nitrobenzene agrees well with the value (−71.35 kJ/mol) from our validating quantum chemistry calculations, implying the reliability of the results from GCMC simulations.

Monoethanolamine (MEA), a potential atmospheric pollutant from the capture unit of a leading CO2 capture technology, could be removed by participating H2SO4-based new particle formation (NPF) as simple amines. Here we evaluated the enhancing potential of MEA on H2SO4-based NPF by examining the formation of molecular clusters of MEA and H2SO4 using combined quantum chemistry calculations and kinetics modeling. The results indicate that MEA at the parts per trillion (ppt) level can enhance H2SO4-based NPF. The enhancing potential of MEA is less than that of dimethylamine (DMA), one of the strongest enhancing agents, and much greater than methylamine (MA), in contrast to the order suggested solely by their basicity (MEA < MA < DMA). The unexpectedly high enhancing potential is attributed to the role of −OH of MEA in increasing cluster binding free energies by acting as both a hydrogen bond donor and acceptor. After the initial formation of one H2SO4 and one MEA cluster, the cluster growth mainly proceeds by first adding one H2SO4, and then one MEA, which differs from growth pathways in H2SO4–DMA and H2SO4–MA systems. Importantly, the effective removal rate of MEA due to participation in NPF is comparable to that of oxidation by hydroxyl radicals at 278.15 K, indicating NPF as an important sink for MEA.

•We studied nitrosation reactions of piperazine by NO2− and N2O3 in presence of CO2.•A novel three-step nitrosation mechanism initiated by N2O3 was revealed.•Contribution of various nitrosating agents to nitrosamine formation was discussed.Quantum chemistry calculations and kinetic modeling were performed to investigate the nitrosation mechanism and kinetics of diamine piperazine (PZ), an alternative solvent for widely used monoethanolamine in postcombustion CO2 capture (PCCC), by two typical nitrosating agents, NO2− and N2O3, in the presence of CO2. Various PZ species and nitrosating agents formed by the reactions of PZ, NO2−, and N2O3 with CO2 were considered. The results indicated that the reactions of PZ species having NH group with N2O3 contribute the most to the formation of nitrosamines in the absorber unit of PCCC and follow a novel three-step nitrosation mechanism, which is initiated by the formation of a charge-transfer complex. The reactions of all PZ species with NO2− proceed more slowly than the reactions of PZ species with ONOCO2−, formed by the reaction of NO2− with CO2. Therefore, the reactions of PZ species with ONOCO2− contribute more to the formation of nitrosamines in the desorber unit of PCCC. In view of CO2 effect on the nitrosation reaction of PZ, the effect through the reaction of PZ with CO2 shows a completely different tendency for different nitrosating agents. More importantly, CO2 can greatly accelerate the nitrosation reactions of PZ by NO2− through the formation of ONOCO2− in the reaction of CO2 with NO2−. This work can help to better understand the nitrosation mechanism of diamines and in the search for efficient methods to prevent the formation of carcinogenic nitrosamines in CO2 capture unit.Download high-res image (297KB)Download full-size image

The unexpected diverse effect of alternatives for banned chemicals has stimulated scientific and public concern on their environmental risk. As an alternative of polybrominated diphenyl ethers (PBDEs), 1,2-bis(2,4,6-tribromophenoxy) ethane (BTBPE) is currently one of the most commonly applied novel brominated flame retardants with wide market prospects. Due to its frequent and high detection in the atmosphere, revealing the atmospheric fate of BTBPE is of great significance. Here, the mechanism and kinetics of the ·OH-initiated atmospheric reaction of BTBPE have been investigated by combined quantum chemical calculations and kinetics modeling. The results indicate that ·OH addition and hydrogen abstraction pathways in the initiated reactions, are competitive with a rate constant ratio of 3 : 1, and the intermediates formed would react with O2/NO to finally form peroxy radicals and OH-BTBPE which tends to be more toxic. The calculated overall reaction rate constant is 1.0 × 10−12 cm3 per molecule per s, translating into 11.8 days atmospheric lifetime of BTBPE. This clarifies that BTBPE as a substitute for PBDEs still has atmospheric persistence.

We present an exploration of proton transfer dynamics in a monosaccharide, based upon ab initio molecular dynamic (AIMD) simulations, conducted “on-the-fly”, in β-d-galactose-H+ (βGal-H+) and its singly hydrated complex, βGal-H+-H2O. Prior structural calculations identify O6 as the preferred protonation site for O-methyl α-d-galactopyranoside, but the β-anomeric configuration favors the inversion of the pyranose ring from the 4C1 chair configuration, to 1C4, and the formation of proton bridges to the (axial) O1 and O3 sites. In the hydrated complex, however, the proton bonds to the water molecule inserted between the O6 and Ow sites, and the ring retains its original 4C1 conformation, supported by a circular network of co-operatively linked hydrogen bonds. Two distinct proton transfer processes, operating over a time scale of 10 ps, have been identified in βGal-H+ at 500 K. One of them leads to chemical reaction and the formation of an oxacarbenium ion (accompanied by the loss of an H2O molecule). In the hydrated complex, βGal-H+-H2O, this reaction is suppressed, and the proton transfer, which involves multiple jumps between the sugar and the H2O, creates an H3O+ ion, relevant, perhaps, to the reactivity of protonated sugars both in the gas and condensed phases. Anticipating future spectroscopic investigations, the vibrational spectra of βGal-H+ and βGal-H+-H2O have also been computed through AIMD simulations conducted at average temperatures of 300 and 40 K and also through vibrational self-consistent field (VSCF) calculations at 0 K.

Amines have been considered as promising candidates for post-combustion CO2 capture. A mechanistic understanding for the chemical processes involved in the capture and release of CO2 is important for the rational design of amines. In this study, the structural effects of amines on the kinetic competition among three typical products (carbamates, carbamic acids and bicarbonate) from amines + CO2 were investigated, in contrast to previous thermodynamic studies to tune the reaction of amines with CO2 based on desirable reaction enthalpy and reaction stoichiometry. We used a quantum chemical method to calculate the activation energies (Ea) for the reactions of a range of substituted monoethanolamines with CO2 covering three pathways to the three products. The results indicate that the formation of carbamates is the most favorable, among the three considered products. In addition, we found that the Ea values for all pathways linearly correlate with pKa of amines, and more importantly, the kinetic competition between carbamate and bicarbonate absorption pathways varies with pKa of the amines, i.e. stronger basicity results in less difference in Ea. These results highlight the importance of the consideration of kinetic competition among different reaction pathways in amine design.Download full-size image