Structure of Solids Thermodynamic Approach

A high-value approach for the structure determination of an enantiotropically-related dimorphic system having low solid-solid conversion temperatures has been presented [67]. The crystal structure of the thermodynamically more stable form at room temperature was determined by single-crystal XRD (polymorph 1, Z =4, Z= 16). The crystal structure of the other form (polymorph 2, Z = 1, Z=4) was determined using iterative PXRD structure solution methods, assisted by SSNMR experiments (dipolar connectivity and CS measurements). DFT geometry optimizations were used in tandem with Rietveld refinement and NMR CS calculations to improve and verify the structure for polymorph 2. 

The English translation of this book by D. Smith and N. G. Adams provides a detailed account of theoretical approaches and experimental techniques of adsorption. The subject matter, essentially comprising physical chemistry, includes defined substances, defined surfaces and their preparation, methods for studying the texture of adsorbents, methods of studying adsorption, the surface structure of solids, theories of adsorption forces, adsorption kinetics and thermodynamics, theories of adsorption equilibria, the mechanisms of physical adsorption and chemisorption, adsorption from flowing gases and liquids, practical applications of adsorption, adsorption from solutions and the relationship between adsorption and catalysis. 

It would be an advantage to have a detailed understanding of the glass transition in order to get an idea of the structural and dynamic features that are important for photophysical deactivation pathways or solid-state photochemical reactions in molecular glasses. Unfortunately, the formation of a glass is one of the least understood problems in solid-state science. At least three different theories have been developed for a description of the glass transition that we can sketch only briefly in this context the free volume theory, a thermodynamic approach, and the mode coupling theory. 

Physical inorganic chemistry is an enormous area of science. In the broadest sense, it comprises experimental and theoretical approaches to the thermodynamics, kinetics, and structure of inorganic compounds and their chemical transformations in solid, gas, and liquid phases. When I accepted the challenge to edit a book on this broad topic, it was clear that only a small portion of the field could be covered in a project of manageable size. The result is a text that focuses on mechanistic aspects of inorganic chemistry in solution, similar to the frequent association of physical organic chemistry with organic mechanisms. 

Representations of lattice imperfections as individual and randomly-distributed entities within crystalline sohds have provided satisfactory theoretical models for the explanation of a number of phenomena. This approach [81], however, has not so far been shown to be capable of accounting for observed reactivities, structures and thermodynamic properties of a munber of the more highly defective and nonstoichiometric phases. Evidence has accumulated which indicates that properties of nonstoichiometric solids are determined by structures and mechanisms which depend on the interactions of imperfections which are distributed in a regular... 

The most preferred approach for studying the thermodynamics of adsorption on solids considers the adsorbed phase as a distinct phase located on the surface of the solid, which is considered to be inert. Here, the concept of inertness of the adsorbent presupposes that no chemical reactions between it and the adsorbate are possible, and that the structure of the solid is rigid. Thus, in this formalism, the properties of the adsorbent and the gas phase are not explicitly included in the calculation. According to the law of conservation of energy, the total energy U is a constant provided that the system is isolated and its volume remains constant. Thus,... 

The complex nature of the HDS and HDN problems requires a broad, transdisciplinary approach in order to try to answer the most varied questions related to these important classes of reactions. The key issues include the practical aspects related to process and product engineering, a precise knowledge of the nature and the composition of petroleum and of refinery fractions, and the thermodynamics and detailed kinetics of the different processes involved. Also, a number of more fundamental solid-state and surface chemistry considerations regarding the preparation, the characterization, and the resulting properties of HDS and HDN catalysts, as well as the complicated reaction mechanisms involved for the various important families of substrates, need to be understood in depth. Even though some very impressive achievements have been disclosed over the last 30-40 years, it seems that some of the major new discoveries desired today may have been held back by the lack of a better understanding of some key issues. Of particular importance are the nature and the structure of HDS-HDN active sites on metal sulfide catalysts, and the intimate details of the elementary reactions implicated in the commonly accepted catalytic schemes. 

First approaches to the quantitative understanding of defects in stoichiometric crystals were published in the early years of the last century by Frenkel in Russia and Schottky in Germany. These workers described the statistical thermodynamics of solids in terms of the atomic occupancies of the various crystallographic sites available in the structure. Two noninteracting defect types were envisaged. Interstitial defects consisted of atoms that had been displaced from their correct positions into normally unoccupied positions, namely, interstitial sites. Vacancies were positions that should have been occupied but were not. 

---