Mono-substituent effect

An EPR study of the monomeric 02 adducts of the Schiff base complexes of Co(bzacen)(py) (71a) and the thiobenzoyl analog Co(Sbzacen)(py) (71b) characterized the five-coordinate mono (pyridine) precursors and the six-coordinate 02 adducts.327 Increased covalency in the Co—S bonds was seen in the EPR parameters, indicative of 7r-backbonding. Substituent effects on the aromatic rings had no effect on the EPR spectra, but these were reflected in the observed redox potentials. Furthermore, the S-donors stabilize the Co ion in lower oxidation states, which was consistent with destabilization of the 02 adducts. 

The H-NMR spectra of amino, nitro and nitroso compounds have been reviewed16,17, and the effects of these substituents on the proton chemical shifts have been investigated16. Table 4 gives these substituent effects for mono-substituted benzenes. 

TABLE 4. Substituent effects on H shifts in mono-substituted benzenes0... 

TABLE 5. Substituent effects on 13C shifts in mono-substituted benzenes19,21-23... 

Some stabilization energies are collected in Table 3. The presentation provides an easy overview of the stabilization in singly and doubly substituted methyl radicals. Values in parentheses for doubly substituted radicals represent the sum of the stabilization energies derived from mono-substituted radicals. By comparing the values calculated directly for the doubly substituted radical, information is obtained on antagonistic, additive or synergetic substituent effects. Apart from methyl and ethyl radicals, all other radicals are stabilized. Some points merit comment. 

The new absorptions in the spectra of crosslinked rubber are assigned on the basis of 13C solution NMR chemical shifts for a variety of model compounds, such as pentenes and mono-, di- and tri-sulfidic compounds, by using the 13C chemical shift substituent effect. From the calculated values for particular structural units, the experimental spectra of a sulfur vulcanized natural rubber 194,195,106), natural rubber cured by accelerated sulfur vulcanization 197 y-irradiation crosslinked natural rubber198 and peroxide crosslinked natural rubber and cis-polybutadiene 193 1991 are assigned. 

Another extension is in the field of alicyclic compounds (7, 112). In steroid compounds it has been proved that long-distance effects, e.g. from position 17 to 3 and vice versa, affect polarographic half-wave potentials. Finally it has been demonstrated (7, 113) that Hammett and Taft substituent constants can be used as a first approximation to express the substituent effects in numerous types of mono- and polycyclic heterocyclic compounds. 

FIGURE 18. First vertical ionization energies IE] (nS) (eV) (a) Comparison between thiomethyl and methoxy derivatives and (b) derivatives of H2S and its alkyl, silyl, trimethylsilyl and (trimethylsilyl)methyl derivatives (— — identical mono- and di-substitution, alkyl/silyl comparison,------- /3-silyl substituent effects)... 

C NMR spectra of five separate series of mono-, di-, and polysubstituted chloro-, bromo-, iodo-, benzoyl-, and nitrocarbazoles substituted at the T,3-, 6-, and 8-positions have been recorded <2005JHC867>. C chemical shifts of carbons ipso, ortho, and para show good linear correlations with net atomic charge density values, suggesting that charge density nicely predicts the substituent effects. 

Scheme 37. The symmetry criterion for cation radical formation via outer sphere electron transfer. Substituent effects in mono- and disubstituted stilbenes are multiplicative log krei correlates with (t+ with p = -4.16 the oxidation potentials of these same stilbene derivatives correlate with the same substituent parameters with p = -5.02.

Membrane permeant substituted-aryl phosphoralaninate derivatives (6) of the anti-HIV drug d4T have been synthesised. All of the derivatives were significantly more potent than d4T against HIV in cell culture however steric and electronic effects altered only slightly the inhibitor potency while the position of the mono-substituent had no obvious effect. 

The spectra of isomeric mono-chloro and mono-methyl quinolines have been investigated and substituent effects have been correlated with the molecular dimensions of the substituted quinoline molecules31. 

There have also been attempts to predict the 13C chemical shifts of coumarins. The first to do so was Sojka (15), who found a fairly good correlation between the carbon shifts of coumarin and its protonated derivative with the n charge densities calculated by the CNDO/2 method (61). Shortly afterwards, Gunther et al. (14) reported that substituent effects (SCS) in various mono- and di-methoxycoumarins correlate well with the HMO atom-atom polarizabilities, 7t,j (62) ... 

The smallest values of /(C,C) couplings so far reported for mono-substituted benzenes are V(C1,C2) = 29.5 and /(C1,C2) = 27.8 Hz. Additivity of substituent effects on /(C,C) couplings in several substituted nitrobenzenes was reported. Such couplings were also calculated with the INDO-SCPT method and in general a satisfactory agreement with experimental data was found." ... 

Fuiidamental studies on the aromatic properties of the azoles was r orted during period. In a study of the role of zwitterions in the tautomerism of hydroxy anr mercaptoheterocycles, the introduced aromaticity index (1 ) for the l,z,3-triazoK thiolates (5, 5 ) was 99.8 and 99, respectively. These values are higher than the predictei value of 90 which indicates considerably more aromaticity than expected for this syster (94H(37)249]. The substituent effects of the trifluorometnyl group on the structure anc basicity of the 1,2,4-triazole nucleus was determined from studies with the mono- an<... 

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