Thermochemical and Other Properties

The thermodynamic properties of thiophene,2-methylthiophene, ° and 3-methylthiophene have been computed from careful measurements of the heat capacity of the solid, liquid, and vapor states, the heat of fusion, the heat of vaporization, and the heat of combustion. From the heat of combustion of thiophene and from thermochemical bond energies, the resonance energy of thiophene has been re-estimated to be only 20 kcal/mole. 

The reduction of the C— Br and C—1 group moments from 1.10 and 0.90 in bromo- and iodo-benzene to about 0.80 and 0.50 in 2-bromo- and 2-iodo-thiophene has been ascribed to the larger weight of resonance forms such as (8) and (9) in the thiophene series. The chlorine, nuclear, quadrupole, resonance frequencies of chloro-substituted thiophenes are much higher than those of the corresponding benzene derivatives. This has been ascribed to a relayed inductive effect originating in the polarity of the C—S o-bond in thiophenes. The refractive indices, densities, and surface tension of thiophene, alkyl- and halo-thiophenes, and of some other derivatives have been 

Motoyama, S. Nishimura, E. Imoto, Y. Murakami, K. Hari, and J. Ogawa, Nippon Kagaku Zasshi, 78, 965 (1957) Chem. Abstr. 54, 14224 (1960), 

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A force field is considered transferable from an arbitrary molecule A to another molecule B if the agreement of properties calculated fori (geometrical, vibrational, thermochemical, and other properties) with the respective experimental values is as good as for A. In our calculations we are dealing with force fields which describe entire families of molecules. Within these families, properties of only a fraction of their members are known experimentally while it is our aim to predict the others computationally. It is therefore clear that the problem of transferability is of decisive importance for force field calculations of unknown systems or those with unknown properties or. Traditional vibrational spectroscopic force fields in most cases reproduce well the frequencies of a single molecule or a family of closely related molecules however, they are not transferable to molecules of different strain. Subsequently we comment on this point in somewhat more detail. 

The reactivity of iron(II) cations [FeX]+, where X = H, Me, C3H5, NH2, OH, F, Cl, Br, and I, have been examined by reactions with acetone (177). The C-C bond activation was the major process for the iron halide cations. The [FeF]+ ion promoted C-H bond activation as well as C-C bond activation and C-H bond activation was also promoted by the other [FeX]+ ions. The relative reactivities of the [FeX]+ ions toward acetone have been correlated with the thermochemical and electronic properties of the substituents X. 

The scope of the present work remains essentially that of Special Publication 454, The general aim 1 to assist the reader in locating those publications which contain thermochemical data which can best serve his needs. Equilibrium data Is taken in its most general sense and includes equilibrium constants, enthalpies, entropies, heat capacities, volumes, and partial molar and excess property data. To a much lesser extent, transport and other properties have been included. Unfortunately, much of the data on biochemical systems is scattered throughout much of the literature and there is a need for... 

At the time of publication this seven volume series was the most comprehensive source of thermochemical and other data in existence. Essentially all equilibrium and transport properties and classes of materials were covered. 

Energies, geometries, frequencies, and other properties were calculated using the Gaussian 09 computer code [44]. The thermochemical and kinetic analysis were performed using the program TAMkin [45]. 

Many experimental and theoretical studies of thermochemical and thermophysical properties of thorium, uranium, and plutonium species were undertaken by Manhattan Project investigators. Some of these reports appeared in the National Nuclear Energy Series [1]. These papers, and others in the literature through 1956, formed the basis for Table 11.11 Summary of thermodynamic data for the actinide elements of the first edition of this book. That table, completed by J. D. Axe and E. F. Westrum Jr, listed 126 species, of which the properties of 40 were estimates. A fair measure of the progress in actinide thermodynamics is the number of subsequent research papers and reviews another measure is the 731 species included in Table 17.14 of this chapter, few of which are estimates. 

Thermal analysis gives information on the fundamental behavior and structure of materials based on their thermochemical and thermophysical properties. Differential thermal analysis (DTA), differential scanning calorimetry (DSC), thermogravimetry (TG), dilatometry, and other related dynamic thermal methods serve as analytical tools for characterizing a wide variety of solid materials. Information obtainable by these methods includes phase relationships, identification and measurement of impurities in high-purity materials, fingerprint identifications, thermal histories of the material, and dissociation pressures. 

The physical properties of bismuth, summarized ia Table 1, are characterized by a low melting poiat, a high density, and expansion on solidification. Thermochemical and thermodynamic data are summarized ia Table 2. The soHd metal floats on the Hquid metal as ice floating on water. GaUium and antimony are the only other metals that expand on solidification. Bismuth is the most diamagnetic of the metals, and it is a poor electrical conductor. The thermal conductivity of bismuth is lower than that of any other metal except mercury. 

Don tpanic This is not to imply that such a method may not be very useful and reliable for modeling other properties of molecular systems. For example, as we ll see in the next subsection, model chemistries that are known to be quite reliable for optimizing geometries can be quite poor at predicting absolute thermochemical properties. 

Exp-6 potential models can be validated through several independent means. Fried and Howard33 have considered the shock Hugoniots of liquids and solids in the decomposition regime where thermochemical equilibrium is established. As an example of a typical thermochemical implementation, consider the Cheetah thermochemical code.32 Cheetah is used to predict detonation performance for solid and liquid explosives. Cheetah solves thermodynamic equations between product species to find chemical equilibrium for a given pressure and temperature. From these properties and elementary detonation theory, the detonation velocity and other performance indicators are computed. 

Since they generally have more symmetry than cis isomers, trans isomers in most cases have higher melting points and lower solubilities in inert solvents. The cis isomer usually has a higher heat of combustion, which indicates a lower thermochemical stability. Other noticeably different properties are densities, acid strengths, boiling points, and various types of spectra, but the differences are too involved to be discussed here. 

Benson [15] has created and developed a general method of calculation of the values of thermochemical and kinetic parameters based on a systematic use of the additivity of group properties on the one hand and the activated complex theory on the other. Other methods for estimating a priori kinetic parameters have recourse to structural analogies or semi-empirical correlations. 

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