Intermediates Molecular Entities

Depending on the type of dihalogen and the chosen Lewis base, the interaction can vary from very weak, e.g., as in F2- NH3, to about 20 kcal/mol in the complex of trimethylamine with BrF. The XY amine complexes are of outer or intermediate types. The molecular entity XY can still be recog-... 

Most chemists tend to think of infrared (IR) spectroscopy as the only form of vibrational analysis for a molecular entity. In this framework, IR is typically used as an identification assay for various intermediates and final bulk drug products, and also as a quantitative technique for solution-phase studies. Full vibrational analysis of a molecule must also include Raman spectroscopy. Although IR and Raman spectroscopy are complementary techniques, widespread use of the Raman technique in pharmaceutical investigations has been limited. Before the advent of Fourier transform techniques and lasers, experimental difficulties limited the use of Raman spectroscopy. Over the last 20 years a renaissance of the Raman technique has been seen, however, due mainly to instrumentation development. 

A reaction in which an electrophile participates in het-erolytic substitution of another molecular entity that supplies both of the bonding electrons. In the case of aromatic electrophilic substitution (AES), one electrophile (typically a proton) is substituted by another electron-deficient species. AES reactions include halogenation (which is often catalyzed by the presence of a Lewis acid salt such as ferric chloride or aluminum chloride), nitration, and so-called Friedel-Crafts acylation and alkylation reactions. On the basis of the extensive literature on AES reactions, one can readily rationalize how this process leads to the synthesis of many substituted aromatic compounds. This is accomplished by considering how the transition states structurally resemble the carbonium ion intermediates in an AES reaction. 

Any weakly attractive, short-lived complex that is typically formed as an intermediate in a reaction mechanism. When there are only two such molecular entities engaged in the formation of a particular encounter complex, that complex is often called an encounter pair. lUPAC (1979) Pure and Appl. Chem. 51, 1725. 

An intermediate, often transient, appearing in a chemical or enzymatic reaction in which a carbon atom, which had been double-bonded (i.e., in a trigonal structure) in a particular molecular entity, has been transformed to a carbon center having a tetrahedral arrangement of substituents. Tetrahedral intermediates of proteases have been stabilized with cryoenzymological tech-niques ... 

If a particular molecular entity(ies) participates in two or more parallel reactions and the proportion of the resulting products is determined by the relative equilibrium constants for the interconversion of reaction intermediates on or after the rate-determining step(s), then the more prevalent product is said to be thermodynamically controlled (i.e., the more stable product will be the one formed in highest amounts). If the reactions are reversible and the system is allowed to go to equilibrium, the favored product is the thermodynamically controlled species. A synonymous term is equilibrium control. See also Kinetic Control... 

The binding, reaction, or interception of a reactive molecular entity or transitory intermediate in a reaction pathway to convert the substance to a more stable form and/or remove that substance from the system. Trapping may involve binding or reaction with another molecular entity or involve the alteration of some parameter (e.g., thermal trapping) ... 

Encounter complex (or precursor complex) — is a complex of -> molecular entities produced at an - encounter-controlled rate, and which occurs as an intermediate in a reaction mechanism. If the complex is formed from two molecular entities, it is called an encounter pair . A distinction between encounter pairs and (larger) encounter complexes may be relevant in some cases, e.g., for mechanisms involving pre-association. 

Photoaflfinity labelling A technique in which a photochemically reactive molecular entity, specifically associated with a biomolecule, is photoexcited in order to covalently attach a label to the biomolecule, usually via intermediates. 

Transient spectroscopy A technique for the spectroscopic observation of transient species (excited-state molecular entities or reactive intermediates) generated by a pulse of short duration. 

The profile of impurities in a new drug substance may change for a variety of reasons, such as process scale-up changes, synthetic route changes, and changes made to key intermediates. ICH decision trees help classify, qualify, and select limits for new molecular entities (NMEs). If an impurity exceeds the qualification threshold listed below in Table 3 (ICFI Q3A(R)), studies are needed to qualify that impurity in drug substances. 

We will attempt an analysis of the possible misconceptions. To begin with, let us consider only the heterogeneity, independent of whether the surface is bare or chemically modified. The topography of the adsorption centers is usually described as patch-wise, random and intermediate. It seems that the surfaces employed in the radiochemical studies have been almost solely of the random type. In the strict sense of the term, such a surface consists of sites with uncorrelated values of the adsorption energy. However, it seems that the potential barrier between two adjacent adsorption sites cannot be of completely random height it must be somewhat correlated with the depths of the partner wells. As to the adsorbate, we suppose that each molecular entity takes only one site and that the particles occupying adjacent sites do not interact with each other. 

The bio-related redox molecular entities shown in Figure 2.8 thus display a pattern which follows consistently the concepts of two-step electrochemical tunnehng and the formalism discussed above. Working principles of redox switching, rectification, and amplification at the single-molecule level of interfacial electrochemical ET have thus been achieved. This can be compared with biological redox macromolecules addressed below. We consider first briefly two cases of biomolecules or bio-related molecules intermediate in size between... 

Identifiers Uniquely identify the molecular entity in the data repository and provide a descriptive name for identification of molecules in the user interface and reports. A classifier is used to categorize the object in a collection classifiers may be metabolite, intermediate, or reactant. 

Intermediate to Penning ionization, in which the unexcited reaction partner is ionized by the excited one [reactions (4) and (5)], and associative ionization, in which only one molecular entity is formed, is the process (3), which has been called by Berry rearrangement ionization. ... 

Up to now, we have been assuming that the molecular entity being considered is stable, in the sense that it is in a geometry of minimum potential energy, so that all of its 3A — 6 values of A are positive. This geometry need not be the most stable one on the potential energy surface it can be at a local minimum. It follows that even a chemical species that would normally be classified as an unstable intermediate as distinguished from a transition state, is stable in this restricted sense. 

Mesomolecule A defined molecular entity of intermediate size. This term is used to distinguish supermolecules and other large molecules from conventional macromolecules prepared by polymerization processes or from supramolec-ular assemblies. These latter structures may be of similar size but generally have a distribution of molecular weights and stoichiometries. 

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