Neutral/ionic clusters

The cluster approach opens the way for a direct quantum-chemical calculation of the influence of the nearest environment on OH vibration frequencies (12). In such a computation, the scheme of the neutral ionic cluster (12,144) is used that allows one to construct easily a consistent set of the required cluster structures. Two examples are clusters 7a and 7b. Cluster 7a models the hydroxyl group bonded with the A1 atom in tetrahedral coordination. Cluster 7b simulates the bridged hydroxyl group linking two... 

A gas phase ionic cluster can be described as a core ion solvated by one or more neutral atoms or molecules... 

Ethylene—Dicarboxylic Acid Copolymers. Partial neutralization of copolymers containing carboxyls in pairs on adjacent carbons, eg, ethylene—maleic acid, has been described (11). Surprisingly, there is no increase in stiffness related to neutralization. Salts with divalent metal cations are not melt processible. The close spacing of the paired carboxyl groups has resulted in ionic cluster morphology which is distinct from that of the commercial ionomer family. 

Ethylene ionomers consist of copolymers of ethylene and an organic add, such as methacrylic acid, the acid moieties of which have been neutralized to form a metal salt. The metal salts from neighboring chains tend to form clusters, such as the one shown schematically in Fig. 18.3. The net result is the overall structure shown in Fig. 18.2 g), in which the ionic clusters form weak crosslinks between adjacent chains. Ionomers also contain short and long chain branches, which are similar to those found in low density polyethylene. 

Species concentrations are shown in Figure 12. At 34 GPa (2.0g/cc), H2O is the predominant species, with H30+ and OH having mole fractions of ca. 5%. In addition, some aggregation has occurred in which neutral and ionic clusters containing up to six oxygens have formed. The concentrations of OH and H30+ are low for all densities investigated and nonexistent at 95 and 115 GPa (2.8 and 3.0g/cc, respectively). The calculated lifetimes for these species are well below 10 fs for the same thermodynamic conditions (less than 8 fs at 34 GPa). At pressures of 95 and 115 GPa, the increase in the O-H bond distance leads to the formation of extensive bond networks (Figure 13). These networks consist entirely of O-H bonds, whereas 0-0 and H-H bonds were not found to be present at any point. 

Neutral clusters from supersonic beam 157 Spectroscopy of neutral clusters 158 Sources of ionic clusters 167 Mass spectrometry of ionic clusters 170... 

As for neutral cluster sources, an ever growing array of techniques are being implemented to generate ionic clusters and measure their properties. Mass spectrometry is useful for quantitative determination of atoms or molecules and... 

However, careful investigations of the evaporative processes in 1-naphthol-(NH3)n are crucial for understanding the spectroscopy and the dynamics of this system [27]. These evaporation processes are expected to be particularly important since an exothermic reaction occurs in the ionic clusters, even at vertical ionization threshold proton transferred structure is much more favorable in the ion than in the neutral. 

At a quantitative level, near criticality the FL theory overestimates dissociation largely, and WS theory deviates even more. The same is true for all versions of the PMSA. In WS theory the high ionicity is a consequence of the increase of the dielectric constant induced by dipolar pairs. The direct DD contribution of the free energy favors pair formation [221]. One can expect that an account for neutral (2,2) quadruples, as predicted by the MC studies, will improve the performance of DH-based theories, because the coupled mass action equilibria reduce dissociation. Moreover, quadrupolar ionic clusters yield no direct contribution to the dielectric constant, so that the increase of and the diminution of the association constant becomes less pronounced than estimated from the WS approach. Such an effect is suggested from dielectric constant data for electrolyte solutions at low T [138, 139], but these arguments may be subject to debate [215]. We note that according to all evidence from theory and MC simulations, charged triple ions [260], often assumed to explain conductance minima, do not seem to play a major role in the ion distribution. 

Mass spectrometry has been an extremely useful tool for the characterization of neutral organometallic clusters, except for those few cases which have extremely high molecular weights or possess such ligands as PPh3 that reduce the volatility of the compound. Mass spectrometry has not been a useful characterization technique for ionic clusters because these compounds are insufficiently volatile to permit study by conventional electron-impact techniques. However, there is some hope that with the development of field-desorption techniques ionic clusters as well as neutrals will be capable of being analyzed by mass spectrometry (146). 

The processes by which ions are lost in the stratosphere and the troposphere are not completely understood due to a sparcity of laboratory data on ionic recombination. It is most likely that mutual neutralization of cluster ions [reaction (9)] will be the primary loss mechanism in the upper stratosphere, with the process of collision-enhanced (ternary) recombination becoming increasingly important at lower altitudes (Sect. 3.2.5). In the presence of aerosols (liquid or solid droplets), loss of both positive and negative cluster ions from the gas phase can occur by attachment to the aerosol surfaces 85 86) (see Sect. 4). 

The abundant experimental data on ionic clusters reacting with neutral molecules has been used to test some of these collision theories. In the next subsection, we briefly review several papers where comparisons between measured and theoretical rate coefficients have been made, and we summarize some of the important conclusions concerning the reactivity of clusters. 

The protonated cluster ions were observed to be the major product ions in the CMS of ammopia, water, etc., although unprotonated cluster ions have also been observed in these cases, depending on the choice of carrier gas, method of ionization, and electron energy utilized in the investigation (Shinohara et al. 1985, 1986). The inability to observe unprotonated cluster ions is usually attributed to poor Franck-Condon factors for the vertical ionization transitions. These poor Franck-Condon factors arise from the large differences in the configuration of the neutral and ionic clusters (Stace 1987a). 

The scope of this paper is to provide an overview of methods used to study properties of electrically neutral molecular clusters initiating particle formation in the troposphere, with focus on quantum chemistry. The review of results is intended to be complete with regard to water-sulfuric acid-ammonia clusters. Concerning studies on clusters including other molecular species, we review representative examples and newest publications. Ionic clusters and clusters involving iodine, related to coastal nucleation, are mentioned in passing. 

The reactions of vdW molecules and clusters can be divided into intra- and intercluster processes, and further into neutral and ionic cluster reactions. The latter were recently reviewed by Mark and Castleman. Therefore the scope of this contribution will be limited to neutral species only. We distinguish between intra- and intercluster reactions. In intracluster processes reactions are induced inside a cluster, usually by light. Examples of such reactions are the reaction of excited mercury atoms with various molecules attached to them, reactions that follow photodissociation in the cluster, and charge transfers inside a large cluster. In intercluster reactions the cross molecular beam technique is usually applied in order to monitor scattered products and their internal energy. The intercluster reactions may be divided into three major categories recombination processes, vdW exchange reactions, and reactions of clusters with metal atoms. 

To produce ionic clusters, the neutral clusters are ionized by an ultraviolet laser and then irradiated by light from an infrared laser173. 

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