Mesomeric state

Metallic materials consist of one or more metallic phases, depending on their composition, and very small amounts of nonmetallic inclusions. In the metallic state, atoms donate some of their outer electrons to the electron gas that permeates the entire volume of the metal and is responsible for good electrical conductivity (10 S cm ). Pure elements do not react electrochemically as a single component. A mesomeric state can be approximately assumed... 

Valence bond theory was initially the most widely accepted approach, probably because it depended on familiar concepts of mesomeric effects in conjugated systems. The theory assumed that the true wave function for the mesomeric state of a molecule is a linear sum of those of the contributing canonical forms. The technique was never successful for quantitative calculation of the absorption spectra of dyes, however, because of the difficulties encountered when introducing the numerous canonical structures necessary for computational precision. 

We are here picturing the production of real states [mesomeric forms] from unreal states [i.e., the ordinary structural formulas] and not the deformation of real states by some external disturbance, as is the case in most of the physical problems to which perturbation theory is commonly applied.. . . There can be no physical separation. . . between resonance vibrations and other electronic vibrations it follows that the unperturbed structures. .. are only of the nature of intellectual scaffolding, and that the actual state is the mesomeric state. Chemical evidence in support of these ideas is extensive.43... 

Whilst no difference can be detected between the chemical properties of the two forms, the physical characteristios indicate that benzene does not actually exist as (6) or (7) but as a resonance hybrid of the two, a condition described as a mesomeric state. The energy of the mesomeric state is less than that of either of the conventional formulae (6) or (7), the difference being known as the resonance energy. 

Here one may consider the mesomeric state, a condition of maximum stability for compounds, containing n electrons as a double bond. This double bond may be conjugated with another double bond or connected with an atom having an available lone pair of electrons (O, N, S, halogens). Then one or more of the unperturbed structures of the molecule (the individual structures, which are capable of representation) show dipole forces, the strength of which is intermediate between normal dipole... 

In terms of spin behavior, through applying the Pauli principle, the total spin function for the two molecular mesomeric states is obtained in two basic combinations ... 

The best-known equation of the type mentioned is, of course, Hammett s equation. It correlates, with considerable precision, rate and equilibrium constants for a large number of reactions occurring in the side chains of m- and p-substituted aromatic compounds, but fails badly for electrophilic substitution into the aromatic ring (except at wi-positions) and for certain reactions in side chains in which there is considerable mesomeric interaction between the side chain and the ring during the course of reaction. This failure arises because Hammett s original model reaction (the ionization of substituted benzoic acids) does not take account of the direct resonance interactions between a substituent and the site of reaction. This sort of interaction in the electrophilic substitutions of anisole is depicted in the following resonance structures, which show the transition state to be stabilized by direct resonance with the substituent ... 

Pfleiderer has suggested that the 7-hydroxy group exists as such in these compounds because this form is stabilized by mesomerism of type 191 (R — H). The tautomerism of 7-hydroxypteridine-2,4-dione in the excited electronic state has been studied on the basis of its fluorescence spectrum. 

Bagal (1974) studied the influence of substituents on the ground and first excited states of arenediazonium ions. With regard to compounds in which mesomeric structures such as 4.1b are important, these authors are skeptical about the validity of the PP method. Later, Bagal et al. (1982) used CNDO/2. The calculated 7r-electron densities at all the nitrogen and carbon atoms were similar to those in the earlier PP results. 

Although the reaction mechanism of this type of reactions is not fully elucidated, it is easily anticipated that no intramolecular special stabilization effect for the carbanion generated from decarboxylation is expected, different from the case of malonic acid-type compounds. Moreover, cinnamic acid derivatives that have both the electron-donating and withdrawing substituents have been reported to undergo this reaction. This fact suggests that the enzyme itself stabilizes the transition state without the aid of mesomeric and inductive effects of the other part of the substrate molecule itself. If such unknown mechanism also works for other... 

Mesomeric, like inductive, effects are permanent polarisations in the ground state of a molecule, and are therefore manifested in the physical properties of the compounds in which they occur. The essential difference between inductive and mesomeric effects is that while inductive effects can operate in both saturated and unsaturated compounds, mesomeric effects can operate only in unsaturated, especially in conjugated, compounds. The former involve the electrons in a bonds, the latter those in tt bonds and orbitals. Inductive effects are transmitted over only quite short distances in saturated chains before dying away, whereas mesomeric effects may be transmitted from one end to the other of quite large molecules provided that conjugation (i.e. delocalised tt orbitals) is present, through which they can proceed. 

Fig. 1 Mesomeric structures of a para-substituted benzene intramolecular charge transfer (ICT) complex in the ground state and in the dipolar excited state...

Fig. 3 Protonation states, isomerism and mesomerism of the HBI chromophore (p-hydroxybenzi-lidene-imidazolinone). The chromophore is shown in its most stable Z ( cw ) conformation, conventionally associated to a 0° value of the dihedral angle t, while the E ( trans ) conformation corresponds to t = 180°. For model compound HBDI (4 -hydroxy-benzylidene-2,3-dimethyl-imidazolinone), Ri = R2 = CH3, for chromophore in GFP, Ri, and R2 stand for the peptidic main chains toward N-terminus and C-terminus, respectively, (a) Possible protonation states of HBI (a) neutral, (b) anionic, (c) enolic, (d) cationic, and (e) zwitterionic. (b) Two resonance structures of the anionic form of HBI...

D) Mesomeric transition state of charge by formation of an oxonium ion... 

The optical properties of organic dyes (Fig. ld-f, Table 1) are controlled by the nature of the electronic transition(s) involved [4], The emission occurs either from an electronic state delocalized over the whole chromophore (the corresponding fluorophores are termed here as resonant or mesomeric dyes) or from a charge transfer (CT) state formed via intramolecular charge transfer (ICT) from the initially excited electronic state (the corresponding fluorophores are referred to as CT dyes) [4], Bioanalytically relevant fluorophores like fluoresceins, rhodamines, most 4,4 -difluoro-4-bora-3a,4a-diaza-s-indacenes (BODIPY dyes), and cyanines (symmetric... 

In 1934 Ingold defined the chemical term "mesomerism" to mean stable intermediate states explained physically by quantal resonance.41 He stressed a preference for the word "mesomerism" over the term "tautomerism" or "resonance" on the grounds that it did not lead to the false impression that alternate structural formulas are passing into each other very rapidly, like tautomerides. If such rapid motion were going on, he argued, every molecule must spend most or all of its time in transforming itself, and the term molecular "state" simply loses its meaning.42... 

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