Intermediate molecule, definition

The textbook definition of a reactive intermediate is a short-lived, high-energy, highly reactive molecule that determines the outcome of a chemical reaction. Well-known examples are radicals and carbenes such species cannot be isolated in general, but are usually postulated as part of a reaction mechanism, and evidence for their existence is usually indirect. In thermal reactivity, for example, the Wheland intermediate (Scheme 9.1) is a key intermediate in aromatic substitution. 

A photochemical reaction coordinate has two branches an excited state branch and a ground state branch that is reached after decay at a conical intersection. Thus a conical intersection between ground and excited states of a molecule is a precursor to ground state reactivity, and conforms to the above definition of a reactive intermediate. The main focus of our article will be to develop this idea. In Figure 9.1b, we show the energy profile for a photochemical reaction with a conical intersection... 

The discussion in this chapter is limited to cyanine-like NIR conjugated molecules, and further, is limited to discussing their two-photon absorption spectra with little emphasis on their excited state absorption properties. In principle, if the quantum mechanical states are known, the ultrafast nonlinear refraction may also be determined, but that is outside the scope of this chapter. The extent to which the results discussed here can be transferred to describe the nonlinear optical properties of other classes of molecules is debatable, but there are certain results that are clear. Designing molecules with large transition dipole moments that take advantage of intermediate state resonance and double resonance enhancements are definitely important approaches to obtain large two-photon absorption cross sections. 

The chemical behavior of ions, ion pairs, and polarizable molecules partakes of the same indistinctness as the definitions of these species. Any attempt to make a complete catalog of the reactions of ions will almost certainly include borderline reactions whose intermediates are in fact ion-pairs or even covalent molecules. For many purposes the identification of a reaction as carbonium ion-like, or what the Germans would call Krypto-ionenreaktion, is as useful as the certain knowledge that the intermediate is actually a carbonium ion. Many of the ionic reaction mechanisms in the literature do not represent actual free ions and were not so intended by their authors. The ionic representation is often merely a convenient simplification if it is an oversimplification it is one that is easily rectified when the pertinent data become available. The value of such approximate mechanisms is that... 

As Skinner has pointed out [7], there is no evidence for the existence of BFyH20 in the gas phase at ordinary temperatures, and the solid monohydrate of BF3 owes its stability to the lattice energy thus D(BF3 - OH2) must be very small. The calculation of AH2 shows that even if BFyH20 could exist in solution as isolated molecules at low temperatures, reaction (3) would not take place. We conclude therefore that proton transfer to the complex anion cannot occur in this system and that there is probably no true termination except by impurities. The only termination reactions which have been definitely established in cationic polymerisations have been described before [2, 8], and cannot at present be discussed profitably in terms of their energetics. It should be noted, however, that in systems such as styrene-S C/4 the smaller proton affinity of the dead (unsaturated or cyclised) polymer, coupled, with the greater size of the anion and smaller size of the cation may make AHX much less positive so that reaction (2) may then be possible because AG° 0. This would mean that the equilibrium between initiation and termination is in an intermediate position. 

I consider there to be a sharp distinction between the most polar form of a molecule and its ionically dissociated form. The reason for this is empirical An ion is defined as a species carrying a charge equal to an integral multiple of the electronic charge, and this definition implies that it will have a characteristic predictable electronic spectrum and, under suitable conditions, mobility in an electric field. There is so far no evidence which would compel one to abandon this definition, and I think it is important to distinguish clearly in this context between reaction intermediates (chain carriers, active species) of finite life-time, and transition states. 

Dotz reaction is proposed. According to our calculations the addition of the alkyne molecule to the carbene complex takes place before CO loss in the initial steps of the reaction. Further, our study shows that a novel proposal involving a chromahexatriene intermediate entails lower energy barriers and more stable intermediates than the previous reaction mechanisms postulated by Dotz and Casey. The novel findings query revision of the classically assumed paths and put forward that additional experimental and theoretical studies are necessary to definitely unravel the reaction mechanism of this intringuing reaction. 

He suggested that the ionic formulas, like the nonionic formulas, "represent formulations of extremes" and that no bond across the ring is required. Using the hypothesis of the motions of valence electrons, as developed by Stark and Kossel, Arndt suggested the possibility of intermediate valence states (Zwitterstufen) as well.32 Independently, Robinson proposed possible electronic shifts in pyrones and similar systems, but he did not state the idea of a definite "intermediate state" of the molecule between the ionic and uncharged formulas.33... 

A series of observations in which the presence of an apparently unaffected substance initiated the progress of a chemical reaction prompted the Swedish chemist Berzelius in 1835 to introduce the term catalyst , but it was only towards the end of the last century that a dear definition could be given by Ostwald, namely a catalyst is a substance which affects the rate of a chemical reaction without appearing in the final products . This is achieved through the formation of intermediate compounds between the catalyst and the molecules involved in the reaction whereby an alternative path is offered which may be passed through with higher probability, i.e. 

Nitroarenes are recognized from their characteristic neutral losses due to the NO2 substituent. Normally, all theoretically possible fragment ions, the plausible [M-N02] and [M-O] ions as well as the unexpected [M-NO] ion, are observed. It is worth noting that molecular ions are 1,2-distonic by definition, because nitroarene molecules are best represented as zwitterion (Chap. 6.3). The molecular ion may either dissociate directly by loss of an oxygen atom or a NO2 molecule or it may rearrange prior to loss of NO. For the latter process, two reaction pathways have been uncovered, one of them involving intermediate formation of a nitrite, and the other proceeding via a three-membered cyclic intermediate. [208]... 

The homogeneous catalytic reaction occurs in the multi-component liquid phase P. The chemical constituents of the liquid phase include H, e", atoms, ions, and molecules etc. which are dissolved/solvated in one or more molecular or ionic solvents. Primary examples of the ions and molecules present are the dissolved organic and organometallic reagents, intermediates and products. By definition, all the molecular and ionic species involved directly in the homogeneous catalysis are soluble in this liquid phase P. The set of all dissolved species in the phase will be denoted by Eq. (3). 

We must note that we are dealing here not with static molecules, as no molecule is stationary even at the absolute zero of temperature, but rather with non-reacting molecules. This will be extended, however, to include mass spectrometry and the reactions which proceed within the mass spectrometry tube, as these are used to define the structure of the parent molecule. Obviously, though, such reactions have an importance of their own which is not neglected. Details of species involved as reactive intermediates, which may exist long enough for definition by physical techniques, will also be considered. For example, the section on ESR (Section 2.04.3.7) necessarily looks at unpaired electron species such as neutral or charged radicals, while that on UV spectroscopy (Section 2.04.3.3) considers the structure of electronically excited heterocyclic molecules. 

Asymmetric synthesis is a term first used in 1894 by E. Fischer and defined4 in 1904 by W. Markwald as a reaction which produces optically active substances from symmetrically constituted compounds with the intermediate use of optically active materials but with the exclusion of all analytical processes . A modem definition was proposed 5) by Morrison and Mosher An asymmetric synthesis is a reaction in which an achiral unit in an ensemble of substrate molecules is converted by a reactant into a chiral unit in such a manner that the stereosiomeric products (enantiomeric or diastereomeric) are formed in unequal amounts. This is to say, an asymmetric synthesis is a process which converts a prochiral6) unit into a chiral unit so that unequal amounts of stereoisomeric products result . When a prochiral molecule... 

Reference electrode, 1104, 1108, 1113 potential, 819, 874 Refractive index, determination with ellipsometry, 1148. 1151 Reflection coefficient, 1151 Residence time, definition, 1310 Reversal techniques, determination of intermediate radicals, 1416 Reversible adsorption of organic molecules, 969, 970... 

At sufficiently high pressure, collisions with other molecules M in the system will stabilize the excited intermediate C ( ), producing a Boltzmann distribution of populations in the various n levels, characteristic of the temperature T. From the previous definition of the Boltzmann distribution function K(n, T),... 

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