Calculated thermodynamics of reactions involving NO+·X complexes (where X = H2O, N2 and CO2)

Noncovalent Interactions in Hydrated Nitrosonium Ion Clusters Mediated by Hydrogen-Bonded Water Networks

As important species in the D region of the ionosphere, hydrated nitrosonium ion clusters [NO⁺(H₂O)_n] are also archetypal and concise models to illustrate effects of different solvent shells. We have investigated noncovalent interactions in NO⁺(H₂O)₃ and NO⁺(H₂O)₄ isomers with high levels of ab initio and symmetry-adapted perturbation theory (SAPT) methods. On the basis of our computations, the exchange energies become much more repulsive, whereas the induction energies are significantly more attractive for the noncovalent interactions of NO⁺ with hydrogen-bonded water chains. Combined with analyses of the electron densities for the NO⁺(H₂O)₃ and NO⁺(H₂O)₄ isomers, we propose that the counteracting effect of the exchange and induction energies could be deemed as an index for the tendency to form the HO-NO covalent bond. Moreover, we have also found that the third-order induction terms are very important to evaluate reasonable charge transfer energies with the SAPT computations.

Theoretical study of water-assisted reaction between methane and nitrosonium cation

The water-assisted reaction between CH4 and NO+ has been studied employing post Hartree–Fock and density functional theory methods. Two reaction pathways were considered: (i) a frontside NO+ attack on the C–H bond of methane with retention of configuration and (ii) a backside attack with inversion of configuration. The first pathway leads to a thermodynamically more favorable hydrate of N-protonated nitrosomethane, and the second one leads to its O-protonated isomer. The catalytic effect of a single water molecule is expressed in decreasing the activation energy for the elementary step by about a factor of two for the frontside mechanism and by a factor of four to five for the backside one when compared with the calculated literature data on water-free methane nitrosation. The combination of the activation strain model of reactivity and the energy decomposition analysis reveals that the activation barriers are largely determined by the relative stability of the termolecular reactant complexes. The crucial factor that stabilizes these complexes comes from the electrostatic attraction. The catalytic effect of the key water molecule is decreased with introduction of additional one or two explicit water molecules, which form a coordinate O→N bonding with NO+. On the contrary, an additional water molecule hydrogen bonded to the key catalytic water molecule enhances the catalytic effect.

Anisotropy Measurements from the Near-Threshold Photodissociation of the N2-NO Complex

We have used velocity map ion imaging to measure the angular anisotropy of the NO (A) products from the photodissociation of the N₂-NO complex. Our experiment ranged from 108 to 758 cm^-1 above the threshold energy to form NO (A) + N₂ (X) products, and these measurements reveal, for the first time, a strong angular anisotropy from photodissociation. At 108 cm^-1 above the photodissociation threshold, we observed NO (A) photoproducts recoil preferentially perpendicular to the laser polarization axis with an average anisotropy parameter, β = -0.25; however, as the available energy was increased, the anisotropy increased, and at 758 cm^-1 above the threshold energy, we found an average β = +0.28. The observed changes in the angular anisotropy of the NO (A) photoproduct are qualitatively similar to those observed for the photodissociation of the Ar-NO complex and likely result from changes in the region of the excited state potential energy surface accessed during the electronic excitation. At the lowest available energy, we also noted a large contribution from hotband excitation; however, this contribution decreased as the available energy increased. The outsized contribution at the lowest available energy may result from hotbands having better Franck-Condon overlap with the excited electronic state near threshold. Finally, we contrast the experimental center of mass translational energy distribution with a statistical energy distribution determined from phase space theory. The experimental and statistical distributions show pronounced disagreement, particularly at low kinetic energies, with the experimental one showing less dissociation resulting in high rotational levels of the fragments.

Photodissociation of the N2-NO Complex between 225.8 and 224.0 nm

Our primary goal was to measure the NO (A) photoproduct appearance energy and ground-state dissociation energy of the N₂-NO complex. We recorded velocity map ion images of NO photofragments resulting from the dissociation of the N₂-NO complex excited between ∼225.8 and 224.0 nm, which ranged from the photodissociation threshold to about 342 cm^-1 above the threshold. In the experiment, one photon dissociated the complex through the N₂ (X ¹Σ_g⁺)-NO (A ²Σ⁺) ← N₂ (X ¹Σ_g⁺)-NO (X ²Π) transition, and a second photon nonresonantly ionized the NO (A) photoproduct. The lowest-energy photons near 225.8 nm did not have sufficient energy to photodissociate the lowest excited state of the complex; however, dissociation was observed with increasing photon energy. On the basis of the experiments, we determined the appearance energy for the NO (A) photoproduct to be 44 284.7 ± 2.8 cm^-1. From the appearance energy and the NO A ← X origin band transition, we determined a ground-state dissociation energy of 85.8 ± 2.8 cm^-1. As we increased the photon energy, the excess energy was partitioned into rotational modes of the diatomic products as well as product translational energy. We found good agreement between the average fraction of rotational energy and the predictions of a simple pseudo three atom impulsive model. Finally, at all photon energies, we observed some contribution from internally excited complexes in the resulting P(E_T). The maximum internal energy of these complexes was consistent with the ground-state dissociation energy.

Quantum chemical study of the structure, spectroscopy and reactivity of NO+.(H2O) n=1-5 clusters

Quantum chemical methods including Møller-Plesset perturbation (MP2) theory and density functional theory (DFT) have been used to study the structure, spectroscopy and reactivity of NO⁺(H₂O) _n=1-5 clusters. MP2/6-311++G** calculations are shown to describe the structure and spectroscopy of the clusters well. DFT calculations with exchange-correlation functionals with a low fraction of Hartree-Fock exchange give a binding energy of NO⁺(H₂O) that is too high and incorrectly predict the lowest energy structure of NO⁺(H₂O)₂, and this error may be associated with a delocalization of charge onto the water molecule directly binding to NO⁺ Ab initio molecular dynamics (AIMD) simulations were performed to study the NO⁺(H₂O)₅ [Formula: see text] H⁺(H₂O)₄ + HONO reaction to investigate the formation of HONO from NO⁺(H₂O)₅ Whether an intracluster reaction to form HONO is observed depends on the level of electronic structure theory used. Of note is that methods that accurately describe the relative energies of the product and reactant clusters did not show reactions on the timescales studied. This suggests that in the upper atmosphere the reaction may occur owing to the energy present in the NO⁺(H₂O)₅ complex following its formation.This article is part of the theme issue 'Modern theoretical chemistry'.

Reactivity of the O2+·(H2O)n and NO+·(H2O)n cluster ions in the D-region of the ionosphere

Ab initio molecular dynamics simulations reveal different reactivities of NO⁺·(H₂O)_n and O₂⁺·(H₂O)_n cluster ions in the D-region of the ionosphere.

Computational study of the interaction between NO, NO+, and NO- with H2O

In this computational study the interaction of NO^., NO⁺, and NO^- with H₂O: [NO--H₂O]^., 1 ^., [NO--H₂O]⁺, 1 ⁺ , and [NO--H₂O]^-, 1 ^- was analysed. The optimized geometries indicate that the relative position of NO and H₂O depends on the total charge: (ON^.--H-OH), (NO^---H-OH), and (ON⁺--OH₂). Moreover, atomic spin density along with frontier molecular orbitals help to identify the preferred reduction or oxidation sites on the nitric oxide. Thus, quantum theory of atoms in molecules (QTAIM), electron localization function (ELF), and natural bond-bond polarizability (NBBP) methods aid to quantify the electron delocalization level between NO and H₂O, 1 ⁺ > 1 ^. > 1 ^- , and show the predominantly ionic, and covalent character to inter-molecular, and intra-molecular chemical bonds, respectively. Furthermore, the natural bond orbital (NBO) and localized molecular orbital energy decomposition analysis (LMO-EDA) methods enable energy analyses of the interaction between NO and H₂O in the complexes 1 ^., 1 ⁺ , and 1 ^- . Where, the first method showed that the interaction between the natural bond orbitals in 1 ^- is more favorable, than in 1 ⁺ , and less in 1 ^., however, the second method designates that the total interaction energy is lower for 1 ⁺ in relation to 1 ^- and 1 ^., due mainly to the electrostatic component. As a final point, analysis of the electrostatic potential surfaces provides a clear and direct explanation for the relative position of the monomers. It also shows that the predominant Coulombic attraction between H₂O and the charged NO⁺, and NO^- compounds will be stronger in relation to the neutral NO^.. Graphical abstract ᅟ.

A mass spectrometry study of alkanes in air plasma at atmospheric pressure

The positive APCI-mass spectra in air of linear (n-pentane, n-hexane, n-heptane, n-octane), branched [2,4-dimethylpentane, 2,2-dimethylpentane and 2,2,4-trimethylpentane (i-octane)], and cyclic (cyclohexane) alkanes were analyzed at different mixing ratios and temperatures. The effect of air humidity was also investigated. Complex ion chemistry is observed as a result of the interplay of several different reagent ions, including atmospheric ions O(2)(+*), NO(+), H(3)O(+), and their hydrates, but also alkyl fragment ions derived from the alkanes. Some of these reactions are known from previous selected ion/molecule reaction studies; others are so far unreported. The major ion formed from most alkanes (M) is the species [M - H](+), which is accompanied by M(+*) only in the case of n-octane. Ionic fragments of C(n)H(2n+1)(+) composition are also observed, particularly with branched alkanes: the relative abundance of such fragments with respect to that of [M - H](+) decreases with increasing concentration of M, thus suggesting that they react with M via hydride abstraction. The branched C(7) and C(8) alkanes react with NO(+) to form a C(4)H(10)NO(+) ion product, which upon collisional activation dissociates via HNO elimination. The structure of t-Bu(+)(HNO) is proposed for such species, which is reasonably formed from the original NO(+)(M) ion/molecule complex via hydride transfer and olefin elimination. Finally, linear alkanes C(5)-C(8) give a product ion corresponding to C(4)H(7)(+)(M), which we suggest is attributed to addition of [M - H](+) to C(4)H(8) olefin formed in the charge-transfer-induced fragmentation of M. The results are relevant to applications of nonthermal plasma processes in the fields of air depuration and combustion enhancement.

Gas-Phase Clustering of NO+ with H2S and H2O

The complex NO+·H2S, which is assumed to be an intermediate in acid rain formation, exhibits thermodynamic stability of ∆Hº300 = -76 kJ mol-1, or ∆Gº300 = -47 kJ mol-1. Its further transformation via H-transfer is associated with rather high barriers. One of the conceivable routes to lower the energy of the transition state is the action of additional solvent molecule(s) that can mediate proton transfer. We have studied several NO+·H2S structures with one or two additional water molecule(s) and have found stable structures (local minima), intermediates and saddle points for the three-body NO+·H2S·H2O and four-body NO+·H2S·(H2O)2 clusters. The hydrogen bonds network in the four-body cluster plays a crucial role in its conversion to thionitrous acid.