Steric correlation coefficient

The symmetrical cis- and trans-olefins RCH=CHR (Table III) are particularly suited to this evaluation by Equation 10 because the polar reaction constant p and the steric correlation coefficient 8 reflect, in this series of olefins, the effect of varying two substituents. Hence, the expected effects (both on p and 8) are more pronounced than those expected for olefins in which only one substituent is varied. 

The value of d obtained by linear regression is 0.96 with a correlation coefficient of 0.9985. For 2 alkylpyridines 8 is 2.030 (256), which leads to the conclusion that 2-alkylpyridines are twice as sensitive to steric effects as their thiazole analogs. 

The best fits to the linear equation 8, for temperature differentials (from equation 7) versus reactant state steric effects, are obtained for reaction 4 (Table III). A modest correlation for equation 8 is obtained for reaction 1. Essentially no fit to equation 8 is found for reactions 2 and 3 (small correlation coefficients and small N slopes). 

The specific effect of the different end groups is expressed by variations of k (branching, heteroatom) and b (additional n-electrons) X As can be read from Table 9, the correlation coefficient r is close to 1.0 in most cases. In some cases for n = 0 there are deviations which obviously arise from steric hindrance . Thus these data are omitted from the correlation. With cyanines one finds k 65 which some of the violenes exhibit too. Figure 9 again demonstrates the discussed correlation. 

The QSARs developed by Iwamura provide concise recipes for designing dipeptide sweeteners. However, the uniqueness of an LFE-based QSAR is always an issue. For this case the singularity of the QSARs with respect to the steric parameters L, and is suspect. The values of and can be shown to correlate to the lipophilicity of R as measured by log P for 38 compounds with a correlation coefficient of. 943. Also correlates with log P for 40 choices of R with a correlation coefficient of. 908. Overall, it is possible to express log (SP) as ... 

The data used are presented in Table 2, and the results of the correlations in Table 3. The steric term was included to account for the observation that conformation in molecules of the type XCH2CH2Z where Z is constant is a function of both ct,x and ux. Unfortunately as is shown by the partial correlation coefficients, ux is significantly linear in ctx. The results nevertheless support the general validity of equation 4. Further support is provided by the correlation of aliphatic nx values with equation 5. Again, the data used and the results of the correlations are presented in Tables 2 and 3 respectively. 

Quinones et al. (2000) reported the successful use of neural networks to predict the half-life of a series of 30 antihistamines. The input for the network was derived from the output of CODES, a routine that generates descriptors for a structure based on atom nature, bonding, and connectivity. Attempts to correlate the half-life with the physicochemical parameters log Kow, pKa, molecular weight, molar refractivity, molar volume, parachor, and polarity were unsuccessful. In a subsequent study by Quinones-Torrelo et al. (2001), the authors correlated the half-life of 18 antihistamines with their retention in a biopartitioning micellar chromatography system with a resultant correlation coefficient (R2adj) value of 0.89. The correlation is explained in that the retention in this system is dependent on hydrophobic, electronic, and steric properties, which are also important in determining half-life. 

Remark b. Because nucleophilicity is usually recorded in organic solvents and pKa in water (or water/methanol mixtures), this can make a large difference. Answer The nucleophilicities of a series of molecules in different organic solvents are correlated with excellent correlation coefficients (r). Furthermore, correlations of each series with pKa in the case of non-ortho substituents is also very good. That the slope, a, of the Bronsted relation is an estimate of transition state location may be questioned (78T2331). Deviations from the Bronsted a seem to be well established as a measure of steric effects. 

The thermal fading rates were measured in toluene at 10°C for a series of 6-nitroBIPS having additional substituents (nitro, methoxy, chloro, bromo, carbo-methoxy, and carboethoxy) in the 5 - and 8-positions. The rate constants were fitted to an equation of the form log (k/k0) =piOi + p2a2 with a correlation coefficient / =().85. This relatively low correlation coefficient, which is not improved by attempts to include the specific steric effects of the 8 substituent or by including the cross-term p12ai02, implies the complexity of the substituent effects on the fading rate.113... 

The 95% confidence intervals of the coefficients in Equations 11 and 12 indicated little significance of the electronic and steric parameters of the ortho substituents. In fact, better correlations were obtained when they were omitted and further separation of the hydrophobic constants was introduced as in Equations 13 and 14. All the parameters are highly significant except the linear w term for the ortho substituents (tt ). The correlation coefficient and standard deviation improved only slightly when a ( 2-4)2 term was added, as shown in Equation 15 (Figure 6). 

Figure 4. Pictorial representation of 3D-QSAR models. The color code is as follows sterically favourable and unfavourable interactions, green and red regions, respectively favourable and unfavourable influence of high electron density, cyan and yellow zones respectively. To aid interpretation the template 26, idazoxan compounds 35 and 40 have been added to the electrostatic map, whereas clonidine, compounds 5, 8 and 34 are shown in the steric map. n, number of data points q and r, cross-validated and non-cross-validated correlation coefficient, respectively s, standard deviation one, optimal number of components.

Fig. 18.5. Correlation between observed helix-twist angles and sum function 1 (a) and correlation between observed roll angles and sum function (b) for the dodecamer [d(CGCGAATTCGCG)]2. A decrease in helical twist or an opening of the roll angle on the side of the base pair where a clash occurs will relieve the steric hindrance between adjacent purines of opposite strands. For example, the clash at the first G-C base pair step (R-Y, step 2) takes place in the major groove. To relieve the steric hindrance, the twist angle has to be decreased, and at the same time the twist at each of the two neighboring steps is increased by half that amount. Thus, the contributions to X) 1 for tbe three steps 1, 2 and 3 are +1, -2, -t-1. If the clash occurs in the minor groove (e.g. with the neighboring C-G step, step 3), each of the contributions has to be doubled. Thus, the contributions to 1 for the three steps 2, 3 and 4 are +2, -4, +2 etc. (for details, see [39, 40]). The correlation coefficients were 0.94 (a) and 0.92 (b). For the correlations, the contributions of terminal base-pair steps (1 and 11) were omitted to avoid the influence of end effects...

Aldehyde and ketone suprathreshold odor intensity correlated well with log P and HB as shown in Table II. No significant relationship between steric or electronic parameters with aldehyde-ketone suprathreshold data was found with the exception of the Sterimol parameter L which was highly correlated to log P (R=0.95). Aldehyde threshold data weis found to be linearly related to log P as shown in equations 10 eind 13. The same data was poorly correlated with E andvas shown in Table II (eq. 12, 15 and 16). Note that two different aldehyde threshold data sets from two different sources produced very similar equations having slopes, intercepts, correlation coefficients and standard deviations which are not statistically different at the 95% level of confidence (eq. 10 and 13). 

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