Resonance constant

Inductive and resonance constants from reactivity studies. . . . ... 

Exner devoted some attention to SOPh and S02Ph in his extensive compilation of substituent constants of 197879. For the former, however, values based directly on chemical reactivity were found to be few, as is already apparent in the present section, and so Exner considered it useful to give values based on appropriate summation of inductive and resonance constants (see Section III.B). These were considered to be normal values, i.e. 7m° and a°, of 0.51 and 0.50 respectively. Corresponding values for S02Ph were 0.59 and 0.66 respectively, both slightly smaller (for no obvious reason) than the chemically based values of am and ap (and a value supposed to be of a°) quoted earlier in this section. 

Inductive and resonance constants from reactivity studies... 

Oj and aR are respectively the inductive and resonance constants of Taft s analysis of ordinary composite Hammett constants (values obtained by Charton were used) and v is the steric substituent constant developed by Charton161,162. The intercept term, h, notionally corresponds to log k for H as an orfho-substituent, but is not found in practice always to agree closely with the observed value of log k for the parent system. 

Rough Hammett correlations of with the resonance constants R for para-substituted isomers (ppar = —0.25) and with om+ for meto-substituted... 

A classical Hansch approach and an artificial neural networks approach were applied to a training set of 32 substituted phenylpiperazines characterized by their affinity for the 5-HTiA-R and the generic arAR [91]. The study was aimed at evaluating the structural requirements for the 5-HTiA/ai selectivity. Each chemical structure was described by six physicochemical parameters and three indicator variables. As electronic descriptors, the field and resonance constants of Swain and Lupton were used. Furthermore, the vdW volumes were employed as steric parameters. The hydrophobic effects exerted by the ortho- and meta-substituents were measured by using the Hansch 7t-ortho and n-meta constants [91]. The resulting models provided a significant correlation of electronic, steric and hydro-phobic parameters with the biological affinities. Moreover, it was inferred that the... 

Fig. 15.22 Display of predicted Hansch parameters of the sum of inductive and resonance constants (F + R) versus lipophilicity (jr) for 302 commercially available carboxylic acids, acid chlorides, sulphonyl chlorides and isocyanates. Symbol size corresponds to larger molecular size (MR). Circles represent the selected R groups and triangles represent the unselected compounds. Colors convey the segmentation of the diversity reagents into nine sectors for selection.

For that purpose the Swain nonresonance field constants F and resonance constants R were used (68JA4328). Xanrorc was calculated from % (chemical yield)/100 X %Sn(ANRORC)/100. The Xanrorc of the thiocyanate and the sulfonylphenyl are not included in Table II.6, since the F and R values are unknown. The 2-sulfonylmethyl substituent is also not included, as the F and R values of the conjugate base (the species present in this strong medium) are unknown. 

Definitions ctr, resonance constant cti, field constant a = or + ai. Reaction mole ratio of Ib/Ph-R/aluminum chloride = 1 20 0.1. 

An approach to the correlation analysis of substituent effects in chemical reactions and processes. In this treatment, there are two parameters, a field constant (F) and a resonance constant (R). The approach has received some criticism ... 

TABLE 6. The inductive and resonance constants of COX groups as determined through 19F NMR measurements92... 

Note Log P is the partition coefficient Hammett sigma constants at the meta and para positions, respectively F and R are polar and resonance constants, respectively, proposed by Swain and Lupton (1968). 

Hammett s equation was also established for substituted phenols from the elementary hydroxyl radical rate constants. The Hammett resonance constant was used to derive a QSAR model for substituted phenols. The simple Hammett equation has been shown to fail in the presence of electron-withdrawing or electron-donating substituents, such as an -OH group (Hansch and Leo, 1995). For this reason, the derived resonance constants such as o°, cr, and o+ were tested in different cases. In the case of multiple substituents, the resonance constants were summed. Figure 5.24 demonstrates a Hammett correlation for substituted phenols. The least-substituted compound, phenol, was used as a reference compound. Figure 5.24 shows the effects of different substituents on the degradation rates of phenols. Nitrophenol reacted the fastest, while methoxyphenol and hydroxyphenol reacted at a slower rate. This Hammett correlation can be used to predict degradation rate constants for compounds similar in structure. 

The QSAR models for phenols using resonance constants are apparently more accurate than QSAR models using Hammett constants. The R2 values for QSAR models using resonance constants, oresonancer and Hammett constants, a, were 0.9492 and 0.5473, respectively. The R2 value demonstrates that the QSAR model using resonance constants has a better fit. Figure 5.25 shows the reaction pathway for hydroxyl radical attack on phenols. 

Conjugation of the nitrogen lone pair with the adjacent phosphoryl or carbonyl function was tested by the 13C NMR parameters of the N-phosphorylated and N-acetylated aniline, as well as of their complexes with Lewis acids. (h ). The inductive and resonance constants for the neutral and charged amide groups were determined using the dual substituent parameter (dsp) approach. ( 5). Results are given in the Table. 

They allow one to calculate the resonance constants effective charge component of the donor centre, resulting from conjugation31,109-115. 

The results of two-parameter correlations of the 13C and 15 N NMR chemical shifts of 2-substituted 5(6)-nitrobenzimidazoles with the induction and resonance constants (a, ak) are given in Table 3.26 [688, 689], The electronic influence of substituents on the chemical shifts of carbons (and protons) in positions 4 and 7 are mainly transmitted by a resonance mechanism, C-4 being more sensitive than C-7 to the substituent effect. For positions 5 and 6 a slightly smaller contribution from the resonance component to the total transmission of substituent effects is observed. However, analysis of correlation between 5 15 N and substituent parameters indicates an approximately equal influence of the induction and resonance substituent effects on the nitro group shielding (see Table 3.26) [688, 689], When other sets of substituent constants (F and R, c, and ck, etc.) are used in the correlation the percentage ratios of resonance and inductive contributions remain unchanged. 

Some differences in the change of 13C NMR chemical shifts of nitrobenzothiazoles compared with nitrobenzimidazoles and nitrobenzoxazoles have been discussed. In Table 3.30 the 13C NMR chemical shifts of nitrobenzothiazoles are presented. The nitro group introduction into position 2 leads to a 10 ppm down field shift of the ipso-carbon resonance, whereas a similar effect of the ipso-substitution in the phenylene fragment of benzothiazoles is 20 ppm [778-781], The results of regression analysis of the 13C NMR chemical shifts of benzothiazoles in terms of the inductive and resonance constants of substituents (F and R, c, and cR, c, and aR°) provide evidence for the fact that the substituent effect transmission from positions 2-6 is approximately 30% weaker than in the opposite direction [779], As stated previously, an analogous picture is observed for benzimidazoles. 

The quantity aj in equation 22 is an inductive X substituent constant and OR+ is an electrophilic resonance constant. The constant ctr+ accounts for the influence of X on the reaction centre D+", which has large positive charge arising in the processes shown in equations 20 and 21. The constant denotes an electrostatic attraction between the positive charge of the radical-cation and the dipole moment induced by this charge in the X substituent. The coefficients k, I, m and n depend on the type of donor centre D. When correlating the Ip and hvci values according to equation 22, these coefficients are equal for D = const. ... 

Donors and acceptors in ID it systems. Both the structure of the bridge and the strength of donor and acceptor groups decide on the extent of coupling. These strengths may be conveniently defined in terms of the Hammett cr values of classical correlation analysis, which have been split also into electrostatic , resonance and even more contributions (Charton, 1987). Especially suitable for NLO are obviously the resonance constants... 

Table 14. Comparison of Inductive and Resonance Constants for Tetra- and Pentacoordinate Organosilicon Groups ...

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-... 

The QSAR of these compounds has been analyzed with the CP-MLR [51] procedure with physicochemical substituent constants [41] and several indicator parameters signifying their sub-structural features [181]. hi this, the HIV-1 RT inhibitory activity of the compounds listed in Table 16 has been found to be correlated with the CLOGP, Swain and Lupton s resonance constant of the 4" position substituent 3I411) and some indicator parameters as shown. 

Most of the variance in these data was explained by the Hammett through-resonance constant (ctr). It implied that electron-re-leasing substituents enhanced inhibitory potency. Later, expanded and extensive studies on this system revealed that inhibition of the bacterial enzyme was related to mostly... 

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