SN2 Reactions of N2O5 with Ions in Water: Microscopic Mechanisms, Intermediates, and Products.

Reactions of dinitrogen pentoxide (N2O5) greatly affect the concentrations of NO3, ozone, OH radicals, methane, and more. In this work, we employ ab initio molecular dynamics and other tools of computational chemistry to explore reactions of N2O5 with anions hydrated by 12 water molecules to shed light on this important class of reactions. The ions investigated are Cl-, SO42-, ClO4-, and RCOO- (R = H, CH3, C2H5). The following main results are obtained: (i) all the reactions take place by an SN2-type mechanism, with a transition state that involves a contact ion pair (NO2+NO3-) that interacts strongly with water molecules. (ii) Reactions of a solvent-separated nitronium ion (NO2+) are not observed in any of the cases. (iii) An explanation is provided for the suppression of ClNO2 formation from N2O5 reacting with salty water when sulfate or acetate ions are present, as found in recent experiments. (iv) Formation of novel intermediate species, such as (SO4NO2-) and RCOONO2, in these reactions is predicted. The results suggest atomistic-level mechanisms for the reactions studied and may be useful for the development of improved modeling of reaction kinetics in aerosol particles.

Control of Interfacial Cl2 and N2O5 Reactivity by a Zwitterionic Phospholipid in Comparison with Ionic and Uncharged Surfactants.

Gas-liquid scattering experiments reveal that charge-separated but neutral (zwitterionic) surfactants catalyze the oxidation of dissolved Br- to Br2 by gaseous Cl2 at the surface of a 0.3 M NaBr/glycerol solution. Solutions of NaBr dissolved in glycerol with no surfactant were compared with solutions coated with zwitterionic, cationic, and anionic surfactants at dilute surface concentrations of 1.1 to 1.5 × 1014 cm-2 (less than 65% of maximum chain packing). The zwitterionic phospholipid enhances Cl2 conversion of Br- to Br2 by a factor of 1.61 ± 0.15, in comparison with a 14-fold enhancement by a cationic surfactant (tetrahexylammonium) and a five-fold suppression by an anionic surfactant (dodecyl sulfate). Further studies indicate that even an uncharged surfactant, monododecanoylglycerol, enhances Cl2 → Br2 production. Similar behavior is observed for the oxidation of Br- to Br2 by N2O5; it is just slightly suppressed by the phospholipid and strongly enhanced by the cationic surfactant. Collectively, these results suggest that attractions and repulsions between the negative Br- ions and the positive and negative charges of the surfactant headgroups draw Br- ions to the surface or repel them away. At low coverages, ion-induced dipole and dispersion interactions between the CH2 groups and Br- or Cl2 may also enhance reactivity. These results demonstrate that the hydrocarbon chains of loosely packed surfactants do not necessarily block gas-liquid reactions but that positively charged, and even uncharged, groups can instead facilitate reactions by bringing gas-phase and solution-phase reagents together in the interfacial region.

Reactions of N2O5 with Salty and Surfactant-Coated Glycerol: Interfacial Conversion of Br- to Br2 Mediated by Alkylammonium Cations.

Gas-liquid scattering and product-yield experiments are used to investigate reactions of N2O5 with glycerol containing Br- and surfactant ions. N2O5 oxidizes Br- to Br2 for every solution tested: 2.7 M NaBr, 0.03 M tetrahexylammonium bromide (THABr), 0.03 M THABr + 0.5 M NaBr, 0.03 M THABr + 0.5 M NaCl, 0.03 M THABr + 0.01 M sodium dodecyl sulfate (SDS), and 0.01 M cetyltrimethylammonium bromide (CTABr). N2O5 also reacts with glycerol itself to produce mono- and dinitroglycerin. Surface tension measurements indicate that 0.03 M THABr and 2.7 M NaBr have similar interfacial Br- concentrations, though their bulk Br- concentrations differ by 90-fold. We find that twice as much Br2 is produced in the presence of THA+, implying that the conversion of Br- to Br2 is initiated at the interface, perhaps mediated by the charged, hydrophobic pocket within the surface THA+ cation. The addition of 0.5 M NaBr, 0.5 M NaCl, or 0.01 M SDS to 0.03 M THABr lowers the Br2 production rate by 23%, 63%, and 67% of the THABr value, respectively. When CTA+ is substituted for THA+, Br2 production drops to 12% of the THABr value. The generation of Br2 under such different conditions implies that trace amounts of surface-active alkylammonium ions can catalyze interfacial N2O5 reactions, even when salts and other surfactants are present.

Conformation-specific spectroscopy of capped, gas-phase Aib oligomers: tests of the Aib residue as a 310-helix former.

The conformational preferences of a series of capped peptides containing the helicogenic amino acid aminoisobutyric acid (Aib) (Z-Aib-OH, Z-(Aib)2-OMe, and Z-(Aib)4-OMe) are studied in the gas phase under expansion-cooled conditions. Aib oligomers are known to form 310-helical secondary structures in solution and in the solid phase. However, in the gas phase, accumulation of a macrodipole as the helix grows could inhibit helix stabilization. Implementing single-conformation IR spectroscopy in the NH stretch region, Z-Aib-OH and Z-(Aib)2-OMe are both observed to have minor conformations that exhibit dihedral angles consistent with the 310-helical portion of the Ramachandran map (ϕ, ψ = -57°, -30°), even though they lack sufficient backbone length to form 10-membered rings which are a hallmark of the developed 310-helix. For Z-(Aib)4-OMe three conformers are observed in the gas phase. Single-conformation infrared spectroscopy in both the NH stretch (Amide A) and C[double bond, length as m-dash]O stretch (Amide I) regions identifies the main conformer as an incipient 310-helix, having two free NH groups and two C10 H-bonded NH groups, labeled an F-F-10-10 structure, with a calculated dipole moment of 13.7 D. A second minor conformer has an infrared spectrum characteristic of an F-F-10-7 structure in which the third and fourth Aib residues have ϕ, ψ = 75°, -74° and -52°, 143°, Ramachandran angles which fall outside of the typical range for 310-helices, and a dipole moment that shrinks to 5.4 D. These results show Aib to be a 310-helix former in the gas phase at the earliest stages of oligomer growth.

Binding water to a PEG-linked flexible bichromophore: IR spectra of diphenoxyethane-(H2O)n clusters, n = 2-4.

The single-conformation infrared (IR) and ultraviolet (UV) spectroscopies of neutral 1,2-diphenoxyethane-(H2O)n clusters with n = 2-4 (labeled henceforth as 1:n) have been studied in a molecular beam using a combination of resonant two-photon ionization, IR-UV holeburning, and resonant ion-dip infrared (RIDIR) spectroscopies. Ground state RIDIR spectra in the OH and CH stretch regions were used to provide firm assignments for the structures of the clusters by comparing the experimental spectra with the predictions of calculations carried out at the density functional M05-2X/6-31+G(d) level of theory. At all sizes in this range, the water molecules form water clusters in which all water molecules engage in a single H-bonded network. Selective binding to the tgt monomer conformer of 1,2-diphenoxyethane (C6H5-O-CH2-CH2-O-C6H5, DPOE) occurs, since this conformer provides a binding pocket in which the two ether oxygens and two phenyl ring π clouds can be involved in stabilizing the water cluster. The 1:2 cluster incorporates a water dimer "chain" bound to DPOE much as it is in the 1:1 complex [E. G. Buchanan et al., J. Phys. Chem. Lett. 4, 1644 (2013)], with primary attachment via a double-donor water that bridges the ether oxygen of one phenoxy group and the π cloud of the other. Two conformers of the 1:3 cluster are observed and characterized, one that extends the water chain to a third molecule (1:3 chain) and the other incorporating a water trimer cycle (1:3 cycle). A cyclic water structure is also observed for the 1:4 cluster. These structural characterizations provide a necessary foundation for studies of the perturbations imposed on the two close-lying S1/S2 excited states of DPOE considered in the adjoining paper [P. S. Walsh et al., J. Chem. Phys. 142, 154304 (2015)].

Solvent-mediated internal conversion in diphenoxyethane-(H2O)n clusters, n = 2-4.

1,2-diphenoxyethane (DPOE) is a flexible bichromophore whose excited states come in close-lying pairs whose splitting and vibronic coupling can be modulated by solvent. Building on the ground state infrared spectroscopy of DPOE-(H2O)n clusters with n = 2-4 from the adjoining paper [Walsh et al., J. Chem. Phys. 142, 154303 (2015)], the present work focuses on the vibronic and excited state infrared spectroscopies of the clusters. The type and degree of asymmetry of the water cluster binding to DPOE is reflected in the variation in the magnitude of the S1/S2 splitting with cluster size. Excited state resonant ion-dip infrared spectroscopy was performed at the electronic origins of the first two excited states in order to explore how the water clusters' OH stretch spectra report on the nature of the two excited states, and the interaction of the S2 state with nearby S1 vibronic levels mediated by the water clusters. The data set, when taken as a whole, provides a state-to-state view of internal conversion and the role of solvent in mediating conversion of electronic excitation between two chromophores, providing a molecular-scale view of Kasha's rule.

Mimicking the first turn of an α-helix with an unnatural backbone: conformation-specific IR and UV spectroscopy of cyclically constrained ß/γ-peptides.

The folding preferences of two capped, constrained ß/γ-dipeptide isomers, Ac-ßACPC-γACHC-NHBn and Ac-γACHC-ßACPC-NHBn, (designated ßγ and γß, respectively), have been investigated using single- and double-resonance ultraviolet and infrared spectroscopy in the gas phase. These capped ß/γ-dipeptides have the same number of backbone atoms between their N- and C-termini as a capped α-tripeptide and thus serve as a minimal structural unit on which to test their ability to mimic the formation of the first turn of an α-helix. Resonant two-photon ionization and UV-UV hole-burning spectroscopy were performed in the S0-S1 region, revealing the presence of three unique conformations of ßγ and a single conformation of γß. Resonant ion-dip infrared spectra were obtained in the NH stretch region from 3300 to 3500 cm(-1) and in both the amide I and amide II regions from 1400 to 1800 cm(-1). These infrared spectra were compared to computational predictions from density functional theory calculations at the M05-2X/6-31+G(d) level, leading to assignments for the observed conformations. Two unique bifurcated C8/C13 H-bonded ring structures for ßγ and a single bifurcated C9/C13 H-bonded ring structure for Î³ß were observed. In all cases, the H-bonding patterns faithfully mimic the first full turn of an α-helix, most notably by containing a 13-membered H-bonded cycle but also by orienting the interior amide group so that it is poised to engage in a second C13 H-bond as the ß/γ-peptide lengthens in size. The structural characteristics of the ß/γ-peptide version of the 13-helix turn are compared with the α-helix counterpart and with a reported crystal structure for a longer ß/γ-peptide oligomer.

Role of ring-constrained γ-amino acid residues in α/γ-peptide folding: single-conformation UV and IR spectroscopy.

The capped α/γ-peptide foldamers Ac-γACHC-Ala-NH-benzyl (γα) and Ac-Ala-γACHC-NH-benzyl (αγ) were studied in the gas phase under jet-cooled conditions using single-conformation spectroscopy. These molecules serve as models for local segments of larger heterogeneous 1:1 α/γ-peptides that have recently been synthesized and shown to form a 12-helix composed of repeating C12 H-bonded rings both in crystalline form and in solution [Guo, L.; et al. J. Am. Chem. Soc. 2009, 131, 16018]. The γα and αγ peptide subunits are structurally constrained at the Cß-Cγ bond of the γ-residue with a cis-cyclohexyl ring and by an ethyl group at the Cα position. These triamides are the minimum length necessary for the formation of the C12 H-bond. Resonant two-photon ionization (R2PI) provides ultraviolet spectra that have contributions from all conformational isomers, while IR-UV hole-burning (IR-UV HB) and resonant ion-dip infrared (RIDIR) spectroscopies are used to record single-conformation UV and IR spectra, respectively. Four and six conformers are identified in the R2PI spectra of the γα and αγ peptides, respectively. RIDIR spectra in the NH stretch, amide I (C═O stretch), and amide II (NH bend) regions are compared with the predictions of density functional theory (DFT) calculations at the M05-2X/6-31+G* level, leading to definite assignments for the H-bonding architectures of the conformers. While the C12 H-bond is present in both γα and αγ, C9 rings are more prevalent, with seven of ten conformers incorporating a C9 H-bond involving in the γ-residue. Nevertheless, comparison of the assigned structures of gas-phase γα and αγ with the crystal structures for γα and larger α/γ-peptides reveals that the constrained γ-peptide backbone formed by the C9 ring is structurally similar to that formed by the larger C12 ring present in the 12-helix. These results confirm that the ACHC/ethyl constrained γ-residue is structurally preorganized to play a significant role in promoting C12 H-bond formation in larger α/γ-peptides.

Direct measurements of collisionally broadened Raman linewidths of CO2 S-branch transitions.

We report direct measurements of S-branch Raman-coherence lifetimes of CO(2) resulting from CO(2)-CO(2) and CO(2)-N(2) collisions by employing time-resolved picosecond coherent anti-Stokes Raman scattering spectroscopy. The S-branch (ΔJ = +2) transitions of CO(2) with rotational quantum number J = 0-52 were simultaneously excited using a broadband (~5 nm) laser pulse with a full-width-at-half-maximum duration of ~115 ps. The coherence lifetimes of CO(2) for a pressure range of 0.05-1 atm were measured directly by probing the rotational coherence with a nearly transform-limited, 90-ps-long laser pulse. These directly measured Raman-coherence lifetimes, when converted to collisional linewidth broadening coefficients, differ from the previously reported broadening coefficients extracted from frequency-domain rotational Raman and infrared-absorption spectra and from theoretical calculations by 7%-25%.

Solvent Effects on Vibronic Coupling in a Flexible Bichromophore: Electronic Localization and Energy Transfer induced by a Single Water Molecule.

Size and conformation-specific ultraviolet and infrared spectra are used to probe the effects of binding a single water molecule on the close-lying excited states present in a model flexible bichromophore, 1,2-diphenoxyethane (DPOE). The water molecule binds to DPOE asymmetrically, thereby localizing the two electronically excited states on one or the other ring, producing a S1/S2 splitting of 190 cm(-1). Electronic localization is reflected clearly in the OH stretch transitions in the excited states. Since the S2 origin is imbedded in vibronic levels of the S1 manifold, its OH stretch spectrum reflects the vibronic coupling between these levels, producing four OH stretch transitions that are a sum of contributions from S2-localized and S1-localized excited states. The single solvent water molecule thus plays multiple roles, localizing the electronic excitation in the bichromophore, inducing electronic energy transfer between the two rings, and reporting on the state mixing via its OH stretch absorptions.

A threshold-based approach to calorimetry in helium droplets: measurement of binding energies of water clusters.

Helium droplet beam methods have emerged as a versatile technique that can be used to assemble a wide variety of atomic and molecular clusters. We have developed a method to measure the binding energies of clusters assembled in helium droplets by determining the minimum droplet sizes required to assemble and detect selected clusters in the spectrum of the doped droplet beam. The differences in the droplet sizes required between the various multimers are then used to estimate the incremental binding energies. We have applied this method to measure the binding energies of cyclic water clusters from the dimer to the tetramer. We obtain measured values of D(0) that are in agreement with theoretical estimates to within ∼20%. Our results suggest that this threshold-based approach should be generally applicable using either mass spectrometry or optical spectroscopy techniques for detection, provided that the clusters selected for study are at least as strongly bound as those of water, and that a peak in the overall spectrum of the beam corresponding only to the cluster chosen (at least in the vicinity of the threshold) can be located.

Spontaneous hydrogen generation from organic-capped Al nanoparticles and water.

The development of technologies that would lead toward the adoption of a hydrogen economy requires readily available, safe, and environmentally friendly access to hydrogen. This can be achieved using the aluminum-water reaction; however, the protective nature and stability of aluminum oxide is a clear detriment to its application. Here, we demonstrate the spontaneous generation of hydrogen gas from ordinary room-temperature tap water when combined with aluminum-oleic acid core-shell nanoparticles obtained via sonochemistry. The reaction is found to be near-complete (>95% yield hydrogen) with a tunable rate from 6.4x10(-4) to 0.01 g of H2/s/g of Al and to yield an environmentally benign byproduct. The potential of these nanoparticles as a source of hydrogen gas for power generation is demonstrated using a simple fuel cell with an applied load.