Structure, electronic, atmospheric systems calculations

Detailed structural calculations have been carried out for this system. This is because the neutral isomer, C2HsO, which is implicated in the thermochemistry of ethanol, is of interest in pollution control, atmospheric chemistry, and combustion. Also, there is new information available from photoionization experiments with which to compare theoretical calculations. For details of these comparisons, see Curtiss et al.73 In the earlier theoretical studies of Nobes et al.,74 calculations were performed at the MP2 and MP3 levels with basis sets of double plus polarization (6-13G ) with electron correlation. These studies revealed four stable minima for the system protonated acetaldehyde, CHj-C H-OH <-> CH3-CH=0+H the methoxymethyl cation, CH3OCH2 protonated oxirane, (CH2)2OH+ and vinylox-... 

The general composition of this chapter is as follows. We first present a critical review of the current status of electronic-structure calculations for molecular systems. This is followed by a compilation of the potential-energy curves, derived spectroscopic analysis, and pertinent discussion of the selected atmospheric molecules mentioned in the preceding paragraph. 

Quantum chemical methods are valuable tools for studying atmospheric nucle-ation phenomena. Molecular geometries and binding energies computed using electronic structure methods can be used to determine potential parameters for classical molecular dynamic simulations, which in turn provide information on the dynamics and qualitative energetics of nucleation processes. Quantum chemistry calculations can also be used to obtain accurate and reliable information on the fundamental chemical and physical properties of molecular systems relevant to nucleation. Successful atmospheric applications include investigations on the hydration of sulfuric acid and the role of ammonia, sulfur trioxide and/or ions... 

Unfortunately, quantitatively reliable quantum chemical calculations of nucleation rates for atmospherically relevant systems would require the application of both high-level electronic structure methods and complicated anharmonic thermochemical analysis to large cluster structures. Such computations are therefore computationally too expensive for currently available computer systems, and will likely remain so for the foreseeable future. Instead, a synthesis of different approaches will probably be necessary. In the future, successful nucleation studies are likely to contain combinations of the best features of both classical (Monte Carlo and molecular dynamics) and quantum chemical methods, with the ultimate objective being a chemically accurate, complete configurational sampling. 

Quantum-mechanical ab initio calculations for small molecular systems are widely used these days as an instrument in studying problems in various Helds of chemistry and molecular physics . Most studies deal with ground-state phenomena, i.e. the structure and properties of compounds, thermal reaction pathways and dynamical behavior based on this information. There has been a noticeable increase in excited-state studies in recent years, however, in particular in connection with problems in molecular spectroscopy, in ionization processes or in the detailed study of photochemical reactions, such as photodissociation, energy-transfer and charge-exchange reactions. The calculations are especially powerful for small molecules (for example, for systems up to SO electrons and six atoms other than hydrogen), and hence numerous applications are found in particular in the area of atmospheric and interstellar chemistry and in the study of combustion processes. In these Helds it is often found that experimental and theoretical studies are undertaken in close conjunction and that the two yield complementary data which, taken together, are able to clarify a process. In other instances it is not uncommon that for short-lived species the values obtained from calculations are so far the only ones available. 

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