Electronegativity, spectroscopic

By 1967 the kinetic model for nondissociative thermal electron attachment and revised values for the electron affinities of 16 aromatic hydrocarbons and 7 aromatic carbonyl compounds were reported [24-26]. The ECD Ea values were correlated to theoretical calculations, electronegativities, spectroscopic data, and reduction potentials. The majority of these remain the most precise electron affinities for such compounds. Some values are assigned to excited states based on the multistate model of the ECD postulated in the 1990s [27, 28]. The electron affinities of atoms, molecules, and radicals were reviewed in 1966 [24]. The relative Ea of nitrobenzene, CS2, and SO2 were measured by the thermal charge transfer techniques and the Ea of O2 by photodetachment [30-32]. 

TABLE 5. Structural determination of organometallic compounds containing tin atoms attached to single-bonded electronegative atoms, by NMR and Mdssbauer spectroscopic methods... 

The mechanical, nonbonded, and electrostatic expressions described above are not sufficient to describe some structural and spectroscopic effects. Three specific structural and spectroscopic phenomena have been incorporated into MM3. They are the electronegativity, anomeric, and Bohlmann effects, which essentially can be traced to molecular orbital origins. These chemical effects are a part of the MM3 program.74,75... 

Huckel s 4n+2 //-electron rule is a necessary but not a sufficient condition for aromaticity. Coplanarity and electronegativity restrictions of constituent atoms represent the most important restrictions. Phos-phole is a marginally aromatic five-membered heterocycle76 (see further examples and discussion). Mesoionic compounds, mesomeric betaines, and 2H-and 4i+-pyrone have all been considered to be weakly aromatic or non-aromatic, though their conjugated acids are aromatic. Spectroscopic data evidenced the aromaticity of dioxolium and oxathiolium cations 59 (Scheme 28) and mesoionic oxathioles not in the classical sense but by their ring currents and chemical stability.77... 

Trisubstituted carbon-centred radicals chemically appear planar as depicted in the TT-type structure 1. However, spectroscopic studies have shown that planarity holds only for methyl, which has a very shallow well for inversion with a planar energy minimum, and for delocalized radical centres like allyl or benzyl. Ethyl, isopropyl, tert-butyl and all the like have double minima for inversion but the barrier is only about 300-500 cal, so that inversion is very fast even at low temperatures. Moreover, carbon-centred radicals with electronegative substituents like alkoxyl or fluorine reinforce the non-planarity, the effect being accumulative for multi-substitutions. This is ascribed to no bonds between n electrons on the heteroatom and the bond to another substituent. The degree of bending is also increased by ring strain like in cyclopropyl and oxiranyl radicals, whereas the disubstituted carbon-centred species like vinyl or acyl are bent a radicals [21]. 

Polymerization in electrostatic systems like the ones mentioned above is stericaUy inhibited by alkyl substitution at the a-carbon which must assume a coordination number greater than 4. Coates and Glockhng have treated this inhibition of polymerization in terms of decreased electronegative character of the branched alkyl groups. Therefore, stimulated by the idea that f-afkylhthium compounds may exist as low polymers or even as monomeric molecules, Weiner and coworkers and Kottke and Stalke have isolated f-butyllithium as a pure substance for the first time and characterised it by spectroscopic methods and X-ray diffraction. The colourless crystalline solid was found to be tetrameric over a range of concentrations in both benzene and hexane ... 

The chemistry of lithium carbenoids, organohthium compounds with an electronegative heteroatom in the a-position, has been developed in two respects during the last two decades. First, the ambiphilic character of these partly short-lived and mostly thermally sensitive compounds is well understood today, due to structure determinations, spectroscopic investigations and theoretical calculations. Second, many of the lithium carbenoids, formerly considered as exotic species, have developed into useful reagents that proved to be particularly fruitful in stereoselective syntheses. 

Af,Af-Dialkoxyamides (Ic) are the least studied class of stable A-heteroatom-substituted hydroxamic esters. The combined electronegativity of two alkoxyl oxygen atoms is less than that of acyloxyl and alkoxyl oxygen in A-acyloxy compounds but properties are largely typical of anomeric amides. There is good spectroscopic evidence that the nitrogen... 

Fig. 4.5. Allen spectroscopic electronegativity versus cation bonding strength.

The effective metal valences that were determined by high-energy spectroscopic studies in some of these compounds show that scandium,111 the smallest and most electronegative metal of the series, seems to transfer less electrons to the cage than... 

More recently Men33 has proposed that electronegativity be defined in terms of the average one-electron energy of valence shell electrons in ground-state free atoms which may be obtained spectroscopically. This quantity is termed the configuration energy ... 

Allen, L. C.J.Arn. Chem. Stic. 1989. 111,9003-9014, Acc. Client. Res. IWO,23, 173-176 A Am. Chem. Stic. 1992.114,1510-1511 Can. J. Chem. 1992,70.631-635. Note that Allen s spectroscopic electronegativity, like Mulliken s. will normally be expressed in terms of electron volts, but can he converted to the Pauling scale if desired. 

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