Computing molecular complexes in Earth's and other atmospheres

There has been a considerable research interest in molecular aggregates, co mplexes or clusters, relevant to Earth's and other planetary atmospheres. C onsequently, a considerable amount of data has been obtained in laboratory observations and also in computations. This report surveys our recent and o ngoing computations of several such systems of atmospheric significancy, bo th systems with relatively weak and relatively strong bonding interactions. Among them, several types of homo- and hetero-dimers with importance to Ea rth's atmosphere: (H2O)(2), (N-2)(2), N-2-O-2, (O-2)(2), or (O-3)(2). Dimer of carbon dioxide (CO2)(2) is computed owing to its supposed significancy in the atmosphere of Venus. Systems with stronger bonding are represented b y ClONO2H+ and 2,3,7,8-tetrachlorodibenzo-p-dioxin. The report also discuss es computational tools, combining advanced quantum-chemical methods with st atistical-mechanical treatments. The structure, energetics, and vibrations of the complexes are evaluated at correlated ab initio levels. The computat ions typically show several minimum-energy structures and their relative po pulations are sensitive to temperature. The computed dimerization equilibri um constants are of a special interest as a critical stability measure and an input information for evaluations of the altitude population profiles in the atmosphere. A special attention is paid to the temperature enhancement of clustering degree in saturated vapors. This interesting paradox represe nts a product of atmospheric studies though it is actually a phenomenon of a more general physico-chemical validity. (C) 2001 Published by Elsevier Sc ience Ltd. All rights reserved.