R. G. Keesee

Thermochemical Data on Gas‐Phase Ion‐Molecule Association and Clustering Reactions

A comprehensive tabulation of the standard enthalpy change, ΔH°, entropy change, ΔS°, and free energy change, ΔG°, for the formation of ion clusters from ion‐molecule association reactions is given. The experimental methods which are used to derive the data are briefly discussed. For some experiments, dissociation energies of ion clusters are reported and listed under the category of ΔH°. The relationship between ΔH° and dissociation energy is discussed in the text.

Noctilucent clouds: Simulation studies of their genesis, properties and global influences

Abstract The formation, evolution and properties of noctilucent clouds are studied using a timedependent one-dimensional model of ice particles at mesospheric altitudes. The model treats ice crystals, meteoric dust, water vapor and air ionization as fully interactive cloud elements. For ice particles, the microphysical processes of nucleation, condensation, coagulation and sedimentation are included; the crystal habits of ice are also accounted for. Meteoric dust is analyzed in the manner of Hunten et al. (1980). The simulated particle sizes range from 10 A to 2.6μm. The chemistry of water vapor and the charge balance of the mesosphere are also analyzed in detail. Based on model calculations, including numerous sensitivity tests, several conclusions are reached. Extremely cold mesopause temperatures ( 2 O. Ample cloud condensation nuclei are always present in the mesosphere; at very low temperatures, either meteoric dust or hydrated ions can act as cloud nuclei. To be effective, meteoric dust particles must be larger than 10–15 A in radius. When dust is present, water vapor supersaturations may be held to such low values that ion nucleation is not possible. Ion nucleation can occur, however, in the absence of dust or at extremely low temperatures ( −3 ) of large ice particles (>0.05 μm radius) and cloud optical depths (at 550 nm) ∼10 −4 , ion nucleation generally leads to a large number (∼10 3 cm −3 ) of smaller particles and optical depths ∼10 −5 ). However, because calculated nucleation rates in noctilucent clouds are highly uncertain, the predominant nucleus for the clouds (i.e., dust or ions) cannot be unambiguously established. Noctilucent clouds require several hours-up to a day-to materialize. Once formed, they may persist for several days, depending on local meteorological conditions. However, the clouds can disappear suddenly if the air warms by 10–20 K. The environmental conditions which exist at the high-latitude summer mesopause, together with the microphysics of small ice crystals, dictate that particle sizes will be ≲ 0.1 μm radius. The ice crystals are probably cubic in structure. It is demonstrated that particles of this size and shape can explain the manifestations of noctilucent clouds. Denser clouds are favored by higher water vapor concentrations, more rapid vertical diffusion and persistent upward convection (which can occur at the summer pole). Noctilucent clouds may also condense in the cold “troughs” of gravity wave trains. Such clouds are bright when the particles remain in the troughs for several hours or more; otherwise they are weak or subvisible. Model simulations are compared with a wide variety of noctilucent cloud data. It is shown that the present physical model is consistent with most of the measurements, as well as many previous theoretical results. Ambient noctilucent clouds are found to have a negligible influence on the climate of Earth. Anthropogenic perturbations of the clouds that are forecast for the next few decades are also shown to have insignificant climatological implications.

The properties of ion clusters and their relationship to heteromolecular nucleation

Ion induced heteromolecular nucleation may be formulated in terms of either a kinetic or a steady‐state thermodynamic model. In the case of the former, nucleation is expressed in terms of the rate constants of the individual association reactions leading to the formation of the ion cluster prenucleation embryos. The thermodynamic approach, on the other hand, leads implicitly to the concept of an energy barrier to nucleation. The two formulations are examined in detail and shown to be complementary. An assessment of the validity of the classical charged liquid drop expression, referred to as the Thomson equation, is made by comparing predicted and experimental values. Although the equation is shown to be useful for calculating the enthalpies of ligand attachment to ions at moderate and larger cluster sizes, in the case of entropies it is only moderately successful for hydration reactions and totally fails for ammonia clustering about ions. It is concluded that the Thomson equation is inadequate for treating the general heteromolecular phenomenon and that methods which are able to effectively take large numbers of configurations into account offer the most promise in describing the molecular properties of clusters. Experimental entropy data indicate that the structures of certain ion clusters are more ordered than accounted for by the classical charged liquid drop formulations. Further examination of these data in light of the Sakur–Tetrode equation indicates the existence of low lying excited internal vibrational modes in ion clusters. These considerations suggest that vibrational frequencies on the order of 1.7×1012 sec−1 are present in clusters of two or more ligands.

Predictions of the electrical conductivity and charging of the aerosols in Titan's atmosphere

Abstract The electrical conductivity and electrical charge on the aerosols in atmosphere of Titan are computed for altitudes between 0 and 400 km. Ionization of methane and nitrogen due to galactic cosmic rays (GCR) is important at night where these ions are converted to ion clusters such as CH + 5 CH 4 , C 7 H + 7 , C 4 H + 7 , and H 4 C 7 N + . The ubiquitous aerosols observed also play an important role in determining the charge distribution in the atmosphere. Because polycyclic aromatic hydrocarbons (PAHs) are expected in Titans atmosphere and have been observed in the laboratory and found to be electrophilic, we consider the formation of negative ions. During the night, the very smallest molecular complexes accept free electrons to form negative ions. This results in a large reduction of the electron abundance both in the region between 150 and 350 km over that predicted when such aerosols are not considered. During the day time, ionization by photoemission from aerosols irradiated by solar ultraviolet (UV) radiation overwhelms the GCR-produced ionization. The presence of hydrocarbon and nitrile minor constituents substantially reduces the UV flux in the wavelength band from the cutoff of CH 4 at 155 to 200 nm. These aerosols have such a low ionization potential that the bulk of the solar radiation at longer wavelengths is energetic enough to produce a photoionization rate sufficient to create an ionosphere even without galactic cosmic ray (GCR) bombardment. At altitudes below 60 km, the electron and positive ion abundances are influenced by the three-body recombination of ions and electrons. The addition of this reaction significantly reduces the predicted electron abundance over that previously predicted. Our calculations for the dayside show that the peaks of the charge distributions move to larger values as the altitude increases. This variation is the result of the increased UV flux present at the highest altitudes. Clearly, the situation is quite different than that for the night where the peak of the distribution for a particular size is nearly constant with altitude when negative ions are not present. The presence of very small aerosol particles (embryos) may cause the peak of the distribution to decrease from about 8 negative charges to as little as one negative charge or even zero charge. This dependence on altitude will require models of the aerosol formation to change their algorithms to better represent the effect of charged aerosols as a function of altitude. In particular, the charge state will be much higher than previously predicted and it will not be constant with altitude during the day time. Charging of aerosol particles, whether on the dayside or nightside, has a major influence on both the electron abundance and electrical conductivity. The predicted conductivities are within the measurement range of the HASI PWA instrument over most but not all, of the altitude range sampled.

Properties of clusters in the gas phase: V. Complexes of neutral molecules onto negative ions

Ion–molecules association reactions of the form A−(B)n−1+B=A−(B)n where studied over a range of temperatures in the gas phase using high pressure mass spectrometry. Enthalpy and entropy changes were determined for the stepwise clustering reactions of (1) sulfur dioxide onto Cl−, I−, and NO2− with n ranging from one to three or four, and onto SO2− and SO3− with n equal to one; and (2) carbon dioxide onto Cl−, I−, NO2−, CO3−, and SO3− with n equal to one. From these data and earlier hydration results, the order of the magnitude of the enthalpy changes on the association of the first neutral for a series of negative ions was found to parallel the gas‐phase basicity of those anions. For any given ion, the relative order of the addition enthalpies among the neutrals was found to be dependent on the polarizabilities of the neutrals and on the covalency in the ion‐neutral bond. Dispersion of charge via covalent bonding was found to significantly affect the succeeding clustering steps.

Gas-phase clusters: spanning the States of matter.

Gas-phase clusters, which are weakly bound aggregates comprised of either atoms or molecules, often display properties that lie between those of the gaseous and condensed states. Interesting questions arise concerning how large a cluster must be before it will display bulk properties. Currently there is extensive research activity directed toward studies of their formation and varying properties and reactivity as a function of the degree of aggregation. Results serve to elucidate at the molecular level the course of change of a system to be followed from the gas to the condensed state, thereby enabling a spanning of the states of matter.

The properties of clusters in the gas phase. IV - Complexes of H2O and HNOx clustering on NOx/-/

Thermodynamic quantities for the gas‐phase clustering equilibria of NO2− and NO3− were determined with high‐pressure mass spectrometry. Comparison of ΔG°n,n+1 values derived from our data shows good agreement with formerly reported values at 296 °K. New data for larger NO2− and NO3− hydrates as well as NO2−(HNO2) were obtained in this study. To aid in understanding the bonding and stability of the hydrates of nitrite and nitrate ions, CNDO/2 calculations were performed, and the results are discussed herein. A correlation between the aqueous phase total hydration enthalpy of a single ion and its gas‐phase hydration enthalpy was obtained. Atmoshperic implications of the data are also briefly discussed.

On the Correlation of Total and Partial Enthalpies of Ion Solvation and the Relationship to the Energy Barrier to Nucleation

Abstract The energetics of ion clustering for the full range of conditions extending from the isolated ion-single molecule cluster in the gas phase, to the bulk condensed state, is examined. Employing concepts taken from the Thomson equation a relationship is developed which correlates the gas phase enthalpies for successive clustering reactions, with the single-ion heats of solvation. The success of the correlation enables the prediction of the heat of solvation of a given ion in a solvent such as water or ammonia from the gas phase determination of the enthalpies for the attachment of only five ligands to a single ion. Employing this new relationship, the contribution of cluster bonding to the energy barrier to nucleation can thereby be readily evaluated for any size cluster.

Predicted electrical conductivity between 0 and 80 km in the Venusian atmosphere

Abstract Calculations of the space charge, ion density, and conductivity in the Venus atmosphere were made. The presence of the cloud particles on Venus causes a profound reduction in the calculated values of the ion density and conductivity compared to the values that are obtained without consideration of the cloud particles. When the cloud particles are included in the calculations, the results for the ion density and conductivity are approximately the same as those of the terrestrial atmosphere at the same pressure-altitude. Because the particles span such a large range of sizes and are abundant over a substantial range of pressure, the space charge varies strongly with altitude and particle size. Differential settling of the particles is expected to produce weak electric fields in the clouds.

Considerations of the rates and lifetimes of intermediate complexes for the association of various ligands to metal ions: Ag+ and Cu+

Rates were measured for the association of CO, CH4, CH3F, NH3, ND3, CH3Cl, and CH3Br onto Ag+ and Cu+ at 298 K. In the order given above, the three‐body association rate constants for Ag+ range from 2.5×10−30 to 3.9×10−27 cm6 s−1. The rate constants for Cu+ are about four to six times larger for a given neutral reactant. The rate constants display trends in the order expected considering the relative bond energies of the clusters, although the enormous range of reactivity is not reflected simply by differences in dipole moments and polarizabilities. There is a very large isotope effect, where the rate constant for ND3 association was found to be about three times greater than NH3 in the case of both ions. (Results for Na+ follow the same trend.) This suggests a coupling of ligand vibrations with orbiting motion which leads to enhanced lifetimes of the cluster intermediates. The trend found for the methyl halides also supports the involvement of this effect.