Do Hydroxyl Radical−Water Clusters, OH(H2O)n, n = 1−5, Exist in the Atmosphere?

It has been speculated that the presence of OH(H2O)n clusters in the troposphere could have significant effects on the solar absorption balance and the reactivity of the hydroxyl radical. We have used the G3 and G3B3 model chemistries to model the structures and predict the frequencies of hydroxyl radical/water clusters containing one to five water molecules. The reaction between hydroxyl radical clusters and methane was examined as a function of water cluster size to gain an understanding of how cluster size affects the hydroxyl radical reactivity.

Thermodynamics of the Hydroxyl Radical Addition to Isoprene

Oxidation of isoprene by the hydroxyl radical leads to tropospheric ozone formation. Consequently, a more complete understanding of this reaction could lead to better models of regional air quality, a better understanding of aerosol formation, and a better understanding of reaction kinetics and dynamics. The most common first step in the oxidation of isoprene is the formation of an adduct, with the hydroxyl radical adding to one of four unsaturated carbon atoms in isoprene. In this paper, we discuss how the initial conformations of isoprene, s-trans and s-gauche, influences the pathways to adduct formation. We explore the formation of pre-reactive complexes at low and high temperatures, which are often invoked to explain the negative temperature dependence of this reaction’s kinetics. We show that at higher temperatures the free energy surface indicates that a pre-reactive complex is unlikely, while at low temperatures the complex exists on two reaction pathways. The theoretical results show that at low temperatures all eight pathways possess negative reaction barriers, and reaction energies that range from −36.7 to −23.0 kcal·mol−1. At temperatures in the lower atmosphere, all eight pathways possess positive reaction barriers that range from 3.8 to 6.0 kcal·mol−1 and reaction energies that range from −28.8 to −14.4 kcal·mol−1.

Exploration of the Potential Energy Surfaces, Prediction of Atmospheric Concentrations, and Prediction of Vibrational Spectra for the HO2···(H2O)n (n = 1−2) Hydrogen Bonded Complexes

The hydroperoxy radical (HO2) plays a critical role in Earth's atmospheric chemistry as a component of many important reactions. The self-reaction of hydroperoxy radicals in the gas phase is strongly affected by the presence of water vapor. In this work, we explore the potential energy surfaces of hydroperoxy radicals hydrogen bonded to one or two water molecules, and predict atmospheric concentrations and vibrational spectra of these complexes. We predict that when the HO2 concentration is on the order of 108molecules·cm-3 at 298 K, that the number of HO2···H2O complexes is on the order of 107molecules·cm-3 and the number of HO2···(H2O)2 complexes is on the order of 106molecules·cm-3. Using the computed abundance of HO2···H2O, we predict that, at 298 K, the bimolecular rate constant for HO2···H2O + HO2 is about 10 times that for HO2 + HO2.

Synthesis of Macrocyclic Polymers Formed via Intramolecular Radical Trap-Assisted Atom Transfer Radical Coupling

The synthesis of cyclic polystyrene (Pst) with an alkoxyamine functionality has been accomplished by intramolecular radical coupling in the presence of a nitroso radical trap Linear alpha,omega-dibrominated polystyrene, produced by the atom transfer radical polymerization (ATRP) of styrene using a dibrominated initiator, was subjected to chain-end activation via the atom transfer radical coupling (ATRC) process under pseudodilute conditions in the presence of 2-methyl-2-nitrosopropane (MNP). This radical trap-assisted, intramolecular ATRC (RTA-ATRC) produced cyclic polymers in greater than 90% yields possessing < G > values in the 0.8-0.9 range as determined by gel permeation chromatography (GPC). Thermal-induced opening of the cycles, made possible by the incorporated alkoxyamine, resulted in a return to the original apparent molecular weight, further supporting the formation of cyclic polymers in the RTA-ATRC reaction. Liquid chromatography-mass spectrometry (LC-MS) provided direct confirmation of the cyclic architecture and the incorporation of the nitroso group into the macrocycle RTA-ATRC cyclizations carried out with faster rates of polymer addition into the redox active solution and/or in the presence of a much larger excess of MNP (up to a 250:1 ratio of MNP:C-Br chain end) still yielded cyclic polymers that contained alkoxyamine functionality.

The Effects of Pi-Pi Stacking on the Controlled Radical Polymerization of Vinyl Aromatics

The atom transfer radical polymerization (ATRP) of styrene (St) was conducted in the presence of varying equivalence (eq) of hexafluorobenzene (HFB) and octafluorotoluene (OFT) to probe the effects of pi-pi stacking on the rate of the polymerization and on the tacticity of the resulting polystyrene (PSt). The extent of the pi-pi stacking interaction between HFB/OFT and the terminal polystyrenic phenyl group was also investigated as a function of solvent, both non-aromatic solvents (THF and hexanes) and aromatic solvents (benzene and toluene). In all cases the presence of HFB or OFT resulted in a decrease in monomer conversion indicating a reduction in the rate of the polymerization with greater retardation of the rate with increase eq of HFB or OFT (0.5 eq to 1 eq HFB/OFT compared to St). Additionally, when aromatic solvents were used instead of non-aromatic solvents the effect of the HFB/OFT on the rate was minimized, consistent with the aromatic solvent competitively interacting with the HFB/OFT. The effects of temperature and ligand strength on the ATRP of St in the presence of HFB were also probed. It was found that when using N,N,N’,N’,N’’-pentamethyldiethylenetriamine (PMDETA) as the ligand the effects of HFB at 38o were the same as at 86oC. When tris[2-(dimethylamino)ethyl]-amine (Me6TREN) was used as the ligand at 38o there was a decrease in monomer conversion similar to the analogous PMDETA reaction. When the polymerization was conducted at 86oC there was no effect on the monomer conversion with HFB present compared to when HFB was absent. To investigate the pi-pi stacking effect even further, the reverse pi-pi stacking system was observed by conducting the ATRP of pentafluorostyrene (PFSt) in the presence of varying eq of benzene and toluene, which in both cases resulted in an increase in monomer conversion compared to when benzene or toluene were absent; in summary the rate of the ATRP of PFSt increases when benzene or toluene waas present in the reaction. The pi-pi stacking interaction between the HFB/OFT and the dormant alkyl bromide of the polymer chain was verified by 1H-NMR with 1-bromoethylbenzene as the alkyl bromide. Also verified by 1H-NMR was the interaction between HFB/OFT and St and the interaction between PFSt and benzene. In all 1H-NMR spectra a perturbation in the aromatic and/or vinyl peaks was observed when the pi-pi stacking agent was present compared to when it was absent. The tacticity of the PSt formed in the presence of 1 eq of HFB was compared to the PSt formed in the absence of HFB by observing the C1 signal in their 13C-NMR spectra, but no change in shape or chemical shift of the signal was observed indicating that there was no change in tacticity.

Dimerization of Poly(methyl methacrylate) Chains Using Radical Trap-Assisted Atom Transfer Radical Coupling

End-brominated poly(methyl methacrylate) (PMMABr) was prepared by atom transfer radical polymerization (ATRP) and employed in a series of atom transfer radical coupling (ATRC) and radical trap-assisted ATRC (RTA-ATRG) reactions. When coupling reactions were performed in the absence of a nitroso radical trap-traditional ATRC condition-very little coupling of the PMMA chains was observed, consistent with disproportionation as the major termination pathway for two PMMA chain-end radicals in our reactions. When 2-methyl-2-nitrosopropane (MNP) was used as the radical trap, coupling of the PMMA chains in this attempted RTA-ATRC reaction was again unsuccessful, owing to capping of the PMMA chains with a bulky nitroxide and preventing further coupling. Analogous reactions performed using nitrosobenzene (NBz) as the radical trap showed significant dimerization, as observed by gel permeation chromatography (GPC) by a shift in the apparent molecular weight compared to the PMMABr precursors. The extent of coupling was found to depend on the concentrion of NBz compared to the PMMABr chain ends, as well as the temperature and time of the coupling reaction. To a lesser extent, the concentrations of copper(I) bromide (CuBr), nitrogen ligand (N,N,N',N',N"-pentamethyldiethylenetriamine = PMDETA), and elemental copper (Cu) were also found to play a role in the success of the RTA-ATRC reaction. The highest levels of dimerization were observed when the coupling reaction was carried out at 80 degrees C for 0.5h, with ratio of 1:4:2.5:8:1 equiv of NBz: CuBr:Cu:PMDETA:PMMABr.