Molecular Complexes in Earth's, Planetary, Cometary, and Interstellar Atmospheres

The following sections are included:

• Preface

• Contents

• Introduction

Basic ideas are summarized which refer to molecular association thermodynamics and statistical physics of molecular pairs. The principles and methods often used in the calculation of equilibrium composition of associating gases are commented on. Attention is mainly focussed on the rigorous definition of tightly bound, metastable and free states in the phase space of interacting pairs.

Weakly bound complexes present significant challenges to both experimentalist and theoretician. In many cases, large-amplitude motions of the complexes make the deduction of structural parameters from spectroscopic data uncertain. When attempting to calculate those quantities, theoreticians are faced with the considerable cost and difficulty of locating stationary points on shallow potential energy surfaces. Often a useful measure of agreement between theory and experiment is the value of a tunneling splitting. But accurate calculation of barriers with ab initio methods is far from easy and the use of approximate functional forms to mimic the potential can be similarly fraught with difficulties. This chapter describes a number of complexes of acetylene and sulphur dioxide in order to show that nonrigidity may manifest itself in many different ways. In each case the consequence is a rich set of spectra whose interpretation caused controversy over a period of years. Whilst accompanying theory cannot yet offer quantitative agreement with experiment, its value is in identifying global minima and low-lying transition states from which likely interconversion pathways may be deduced. Accompanying group theory can often show that suggested mechanisms are consistent with spectroscopic observations.

Possible spectroscopic manifestations of van der Waals and hydrogen-bonded molecules in planetary atmospheres are reviewed. Special attention is paid to existing experimental spectroscopic patterns which can be interpreted as weakly bound species in the atmosphere. In most cases the spectroscopic signatures of dimers stand on the background formed by collision-induced diffuse absorption. Possible contribution of the weakly bound dimers of hydrogen, nitrogen, oxygen and carbon dioxide to collision-induced absorption phenomena in the atmosphere is considered. The problem of the water vapor continuum absorption is also considered in its relation to the problem of dimeric absorption.

During the last decade, there has been a considerable interest in molecular aggregates, complexes or clusters, and a considerable amount of data has been obtained in laboratory observation, for example in matrix isolations or nozzle beam expansion techniques, and also in computations. It has been soon recognized that such dimers, or even higher aggregates, can be formed in atmospheres of cosmic bodies. In addition to the homo-dimers, hetero-dimers were given some attention, too. It has been obvious that molecular complexes should equally well be significant not only to Earth but also to other sufficiently dense planetary atmospheres and that, in principle, they must be present in any atmosphere (though their amount can be rather negligible). There are several types of molecular complexes of obvious importance to Earth's atmosphere, for example: (H₂O)₂, (N₂)₂, N₂ − O₂ , (O₂)₂, or (O₃)₂. The water dimer is relevant to cometary atmospheres, too, especially tocomet Halley. The paper surveys recent results on the subjects. The stress is onthe computational approach which combines advanced quantum-chemical methods with statistical-mechanical treatments. However, the computational approach is placed in a wider context of contemporary studies. The structure, energetics, and harmonic vibrations of the complexes were evaluated at a correlated ab initio level. The computed dimerization equilibrium constants are of a focused interest as a key stability measure and an input information for the evaluation of the altitude population profiles in atmospheres. The mole fractions of each of the dimers, computed for their equilibrium mixture with N2 and O₂ under the atmospheric conditions, exhibit a monotonous decrease with the altitude. The ozone dimer is treated in a similar way for Antarctic atmospheric conditions. The (CO₂)₂ case is especially important for the atmosphere of Venus, though it may also have some relevancy for Mars. A recent example of observed planetary dimer is supplied by (H₂)₂ in the atmosphere of Jupiter. The computations also predict that Ar–N₂ should be significant in the atmosphere of Titan. The survey pays a systematic attention to the present knowledge of the selected planetary complexes derived from computational chemistry treatments.

The first spectral diffuse interstellar bands were observed as early as 1921, however, their origin is not entirely clarified even at present. Well over a hundred of these broad interstellar features have been listed in the visible and near-infrared spectral regions. It is probable that they come from more than one specific carrier, and various carriers have been considered like dust grains, carbon chains, polycyclic aromatic species, fullerenes, endohedral fullerenes. Recently, highly unsaturated hydrocarbon species have attracted attention as particularly promising candidates for the carriers.

Historically, astrophysical spectral problems had accelerated studies of purely carbonaceous molecules. The laboratory experiments designed to simulate the formation of carbon molecules and graphitic grains in space lead to the discovery and synthesis of fullerenes, respectively, one of the most celebrated events of modern science. Still, there is no final proof that fullerenes are present in interstellar space. Nevertheless, fullerene science has developed dramatically with far reaching consequences for chemistry, molecular physics, and materials science.

The article surveys relevant theoretical and experimental aspects of small carbon aggregates as well as fullerenes. The small clusters had been observed and computed well before the fullerene era. In spite of the long research interest, they are still not well understood. They are apparently present in early stages of the fullerene synthesis and, with a few hydrogen atoms (possibly only one), they can be responsible for the diffuse interstellar bands. Considerably more results have been accumulated on fullerenes, not only on C₆₀ but also on higher fullerenes, giant fullerenes, and then on nanotubes. The tremendous developments, actually rooted in the diffuse interstellar bands and other astrophysical spectral problems, represent an instructive lesson how it is difficult to evaluate interlinks and make predictions in fundamental research.

Small carbon clusters C_n were observed already more than fifty years ago. Since the fullerene discovery they have always been considered as a logical first step in fullerene synthesis. Existence of both linear and cyclic species has been established, the linear even-numbered structures being in triplet electronic state. The linear structures prevail for low n values, then the cyclic species become more important, and finally polyhedral structures. Although C₆₀ and C₇₀ have primarily attracted research interest, higher fullerenes are of growing importance, too. They are experimentally and computationally characterized well over n = 90. Computations play a particularly important role in the field owing to the structural complexity.

Collision-induced absorption plays several roles in planetary atmospheres. It provides the thermal opacity of the giant planets whereby H₂ and He radiate most of the absorbed sunlight and internal heat to space. This process governs the vertical temperature structure in their upper atmospheres. CIA also is expected to have played a role in the formation and early evolution of the giant planets by governing the rate of cooling of the protoplanetary material and the resultant contraction to their present sizes. It may also have played a role in an early massive atmosphere of Triton if this satellite was captured. CIA from H₂, N₂, and CH₄ in Titan's atmosphere is primarily responsible for the warm surface and the increase of atmospheric temperature with depth. On Venus, CIA of CO₂ is thought to contribute to the continuum spectrum of the nightside thermal emission in its atmospheric spectral windows. On Triton and Pluto, the spectral profile of the 2.15 pm band of N₂ ice provides the temperature of the ice which constrains the ice vapor pressure and atmospheric density. Since these atmospheres are supported entirely by the equilibrium vapor pressure, their mass varies sensitively with seasonal changes in the insolation. Finally, CIA may play a significant role in the thermal opacity of Jupiter-sized extrasolar planets. However, the thermal opacity of brown dwarfs and hot super-Jupiter sized extrasolar planets is likely to be dominated by H₂O and CH₄ absorption.

The application of the matrix isolation technique to molecular complexes study is described. Following a short description of the experimental technique three areas are reviewed in which matrix experiments are particularly well suited to determine the spectroscopic photochemical and photophysical properties of molecular complexes. The first one involves the IR spectroscopic properties of molecular complexes and is divided into three sections: charge transfer complexes, hydrogen bonded complexes, structural arrangement of weak complexes. The second one is devoted to infrared photodissociation of hydrogen bonded complexes. The third part is related to visible and UV photochemistry of some molecular complexes. Examples including mainly atmospheric species are chosen to evidence the power of the method for stabilizing a wide range of molecular complexes from those between highly reactive reagents to weakly bound complexes often postulated as reaction or catalytic intermediates at room temperature.

Infrared molecular beam depletion and fragment spectroscopy have been employed to study the absorption behavior of small water complexes [(H₂O)_n, n = 2 − 5] in the region of their stretching vibrations between 3300 and 3800 cm⁻¹ . This spectral range was covered with an injection-seeded optical parametric oscillator. Size-specific information was obtained by dispersing the water clusters with a secondary helium beam and measuring the laser-induced depletion as a function of the scattering angle. Three absorption bands could be assigned to the water dimer (H₂O)₂, with the bonded O-H stretch being localized at 3601 c m⁻¹. For each of the larger water clusters (n = 3 − 5) , which have cyclic structures, two absorption bands could be identified, one belonging to the free O-H stretch and the other being due to the excitation of the O-H ring vibration. The measurements on free gas phase water clusters were complemented by studies on small water complexes attached to large argon clusters or solvated in liquid helium droplets. Such host-guest cluster systems provide the possibility to investigate the molecular aggregates in an intermediate state between the free gas phase and the bulk matrix. The measured frequency shifts are compared with the available matrix data and discussed in terms of a correlation with the critical temperature of the gas providing the solvent. As an example for the interaction between a molecular chromophore and a large water cluster, we present first results on the system CH₃OH·(H₂O)N.