In monosubstituted benzene

Early work by these authors established a relationship between Amono1/2, the square root of the integrated absorbance of the v16 ring bands in monosubstituted benzenes, and aR° for the substituents. The equation was usually written as follows126 ... 

In monosubstituted benzenes there are conventionally supposed to be the substituent on the carbon with the largest amplitude of and and with the node of and in order to simplify the following arguments. The substitution does not perturb and but does perturb and... 

Cocchi, M., Menziani, M.C., De Benedetti, P.G. and Cruciani, G. (1992) Theoretical versus empirical molecular descriptors in monosubstituted benzenes. A chemometric study. Chemometrics and Intelligent Laboratory System, 14, 209-224. 

We would therefore expect to find in monosubstituted benzenes a second ionization potential, corresponding to the unpertiu-bed orbital, at a value not very different from that of benzene ( 9-2 e.v.), as well as the higher value, corresponding to the lowest 7r-level (tti), and differing from the benzene iri-value by an amount similar to the first ionization potential difference. This has in fact been found in those compounds which have so far been studied by photoelectron spectroscopy (see Table 8). The simple monoalkylbenzenes and styrene have second ionization potentials in the range 9-0-9-1 e.v. Phenylacetylene has a second... 

Meta carbon atoms (C-3.5) remain almost unaffected by all kinds of substituents. In monosubstituted benzenes they usually resonate at 129 + 1 ppm (Table 4.53). Recalling Section 3.1.3.6, electron releasing substituents (( + )-M substituents, electron donors) increase n electron densities in ortho and para positions and thereby induce a shielding relative to benzene (<5c(t) p) < 128.5 ppm). Electron withdrawing groups (( —)-M substituents, electron acceptors) decrease it electron densities in o and p positions thus, a deshielding relative to benzene is observed (<5c 128.5 ppm). 

Most of the substituent increments presented in Table 4.82 can be derived from 3C shifts of benzenoid carbons in monosubstituted benzenes as listed in Table 4.53. Additional substituent increments are available for fused aromatic rings such as naphthalene and... 

Table 4.82. Empirical Substituent Increments in Monosubstituted Benzenes [383].

In the benzene molecule, all the carbon atoms are identical. So in monosubstituted benzene derivatives, there is no need to number the carbon atoms. Thus there is only one possible fluorobenzene structure, it makes no difference to which carbon atom the fluorine atom is attached. 

A number of workers have reported the NMR spectra of mono-substituted phenyl groups and correlated the shifts vdth molecular parameters. It has been shown for organic compounds that the chemical shift of the para-carbon in monosubstituted benzenes is linearly related to the total TT-electron density at the para position in these compounds. Also the shift separation of the meta- and para-carbons appears to be linearly related to the rr-electron density on the para-carbon due to resonance interaction with the substituent 49, 156). Spiesecke and Schneider have reported a good linear relationship between the para-carbon chemical shift of monosubstituted benzenes and the Hammett, o-para constant, but no such relationship appears to exist for the other carbon chemical shifts, except between the chemical shift for the substituted carbon atom (corrected for magnetic anisotropy effects of the substituent) and the electronegativity of the substituent 210). 

The chemical shifts of meta and para (but not ortho carbon atoms in monosubstituted benzenes correlate well with the inductive and resonance constants of the substituents " . Examination of and a° constants obtained from NMR data (Table 14) shows that for the groups containing a penta-coordinate silicon atom the cr-donor component is enhanced and the rc-acceptor one is lowered. Similar conclusions have been reached by other measurements e.g., the in-... 

FIGURE 11.19. Bond angle variations in monosubstituted benzene derivatives (Ref. 76). Note how the angle at the site of substitution varies with the electron-withdrawing power of the substituent. 

Norskov-Lauritsen, L., and Biirgi H.-B. Cluster analysis of periodic distributions application to conformational analysis. J. Comput. Chem. 6,216-228 (1985). Domenicano, A., Murray-Rust, P., and Vaciago, A. Molecular geometry of substituted benzene derivatives. IV. Analysis of variance in monosubstituted benzene rings. Acta Cryst. B39, 457-468 (1983). 

Direct infrared determination of the resonance interaction in monosubstituted benzenes. /. Am. Chem. Soc., 87, 3260-3261. 

Doub and Vandenbelt [6] observed the following series of band displacements for the and transitions in monosubstituted benzenes ... 

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