Thermochemistry, computational studies

Computational Studies of the Thermochemistry of the Atmospheric Iodine Reservoirs HOI and IONO2... 

Glukhovtsev, M. N. Laiter, S. Pross, A. (1995b). Thermochemistry of Cyclobutadiene and Tetrahedrane A High-Level Computational Study, J. Phys. Chem., 99, pp. 6828-6831. 

In this paper, we describe a computational study of the cracking reactions of propane and the production of olefins, especially propene, as a preUminary step in our research of gas-phase production of propylene oxide. The purpose of the smdy is twofold. On the one hand, we aim to compare different computational schemes applied to a subset of reactions for which experimental data exist On the other hand, we want to obtain precise estimates of the thermochemistry and kinetics of the radical chain initiation, propagation and termination reactions involved in the mechanism. Previous computational and experimental studies on this area of research have been performed by several authors, which results we will use to compare to our own. 

Strickland, A.D. and CaldweU, R.A., Thermochemistry of strained cycloalkenes experimental and computational studies, /. Phys. Chem., 97, 13394-13402,1993. 

Also, in 2007, El-Nahas et al. reported a computational study of the thermochemistry and decomposition kinetics of MB and its C5H10O2 isomer, ethyl propanoate (EP). For each compound, they identified six unimolecular decomposition paths, both molecular and homolytic, finding that concerted six-center eliminations were the lowest energy paths ... 

Historically, some of those approaches have been developed with a considerable degree of independence, leading to a proliferation of thermochemical concepts and conventions that may be difficult to grasp. Moreover, the past decades have witnessed the development of new experimental methods, in solution and in the gas phase, that have allowed the thermochemical study of neutral and ionic molecular species not amenable to the classic calorimetric and noncalorimetric techniques. Thus, even the expert reader (e.g., someone who works on thermochemistry or chemical kinetics) is often challenged by the variety of new and sophisticated methods that have enriched the literature. For example, it is not uncommon for a calorimetrist to have no idea about the reliability of mass spectrometry data quoted from a paper many gas-phase kineticists ignore the impact that photoacoustic calorimetry results may have in their own field most experimentalists are notoriously unaware of the importance of computational chemistry computational chemists often compare their results with less reliable experimental values and the consistency of thermochemical data is a frequently ignored issue and responsible for many inaccuracies in literature values. 

There is a wide variety of ab initio techniques available for the study of radical thermochemistry, ranging from quite cheap and approximate methods to much more expensive and accurate approaches. The quality of results yielded by these procedures depends on the size of the basis set used and on the degree of electron correlation included. In practice, it is necessary to strike a balance between the required accuracy and the computational cost that can be afforded. 

Use of medium-scale heat flow calorimeter for separate measurement of reaction heat removed via reaction vessel walls and via reflux condenser system, under fully realistic processing conditions, with data processing of the results is reported [2], More details are given elsewhere [3], A new computer controlled reaction calorimeter is described which has been developed for the laboratory study of all process aspects on 0.5-2 1 scale. It provides precise data on reaction kinetics, thermochemistry, and heat transfer. Its features are exemplified by a study of the (exothermic) nitration of benzaldehyde [4], A more recent review of reaction safety calorimetry gives some comment on possibly deceptive results. [5],... 

Computational chemistry procedures describing the geometry and thermochemistry of gaseous ions are often used to predict and support experimental data. This approach has also been employed to investigate the structure and energetics of some organozinc ions at different levels of theory. Cations and anions having a Zn—C bond which were studied in silico are listed in Tables 6 and 7, respectively. 

J. M. L. Martin, in Computational Thermochemistry, K. K. Irikura and D. J. Frurip, Eds., ACS Symposium Series 677, American Chemical Society, Washington, DC, 1998, pp. 212-236. Calibration Study of Atomization Energies of Small Polyatomics. 

Drs. Larry A. Curtiss, Paul C. Redfern, and David J. Frurip present a tutorial on how to compute enthalpies of formation in Chapter 3. Often a computational chemist will want to know how stable a molecule is. The techniques described in this chapter can answer this question. The authors, who have studied what has been called computational thermochemistry, describe ab initio molecular orbital methods (including the highly accurate and popular Gn methods), density functional methods, semiempirical molecular orbital methods, and empirical methods (such as based on bond energies). These methods are richly illustrated with detailed, worked out examples. 

Nowadays ab initio quantum chemical calculations can provide results approaching benchmark accuracy for small molecules in the gas phase [1], and they have proven to be very useful fo complement experimental studies. Small molecules in the gas phase are typically addressed by high-level methods such as CCSD(T), QCISD(T) and MRCI, which in many cases are equally accurate than experiments [2]. A wide variety of properties such as structures [3] thermochemistry [4] spectroscopic quantities [5,6], and kinetics [7] can be effectively computed. 

The importance of the ambient ionization on the cluster formation is well established [34-38,123-127]. The thermochemistry of the hydration of simple and common atmospheric ion HsO" " has been studied using the computational quantum methods in the past. Table 21.9 presents the comparison of calculated hydration... 

Determining AfH°29S (hydrocarbons) by combustion, despite its seeming simplicity, is not an easy job. Combustion thermochemistry requires meticulous control of experimental conditions (Steele et al., 2002). The difficulty of this task and its expense in time and money mean that researchers in need of AfH°29% data are likely to find vast gaps in the thermochemical record. At present, chemists are synthesizing new compounds far more rapidly than thermochemical properties are being measured, hence the reliance in many contemporary studies and in industrial laboratories on computational methods. The simplest of these... 

An area that needs further systematic study is the thermochemistry of gaseous oxides. Utilization of REMPI, MATI, and theoretical computations should lead to a systematic understanding of excitation energies and intrinsic bond energies that will enable better predictions to be made of corresponding trends for the actinide monoxides. 

The hrst hve chapters (Part 1) present an overview of some methods that have been used in the recent hterature to calculate rate constants and the associated case studies. The main topics covered in this part include thermochemistry and kinetics, computational chemistry and kinetics, quantum instanton, kinetic calculations in liquid solutions, and new applications of density functional theory in kinetic calculations. The remaining hve chapters (Part II) are focused on apphcations even though methodologies are discussed. The topics in the second part include the kinetics of molecules relevant to combustion processes, intermolecular electron transfer reactivity of organic compounds, lignin model compounds, and coal model compounds in addition to free radical polymerization. 

The process of detonation reflection and shock reflection within a closed vessel has been studied both experimentally and numerically. Measured peak reflected detonation pressures are approximately 2.5 times higher than the CJ pressure. This finding is in agreement with both the Zel dovich-Stanyukovich approximate analysis and a computation using realistic thermochemistry. Numerical simulations without added dissipation provide a satisfactory representation of the pressure waveforms in the region near the end-wall after detonation reflection. 

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