R-effect

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In all these cases the S.I.R. effect has a greater influence on the energy than on the entropy of activation (see Section VII). 

Mukhegee, R., Effective Design Air-Cooled Heat Exchangers, Chem. Eng. Prog, p. 26, Eeb. (1997). 

Dl. De Bortoli, R. A., and Masnovi, R., Effect of dissolved hydrogen on burnout for water flowing vertically upwards in round tubes at 2000 psia, WAPD-TH-318 (1957). 

The special substituent constants for + R para-substituents are denoted by a, and those for — R para-substituents are denoted by a+ 54. They are based respectively on the reaction series discussed above. Selected values are given in Table 1. Characteristic a or a+ values are sometimes distinguished for meta-substituents also, but only for a minority of substituents which show very marked + R or — R effects do these differ significantly from ordinary a values. The range of applicability of the Hammett equation is greatly extended by means of a and cr+, notably to nucleophilic (by a ) and to electrophilic (by cr+) aromatic substitution. 

Before we leave the enhancement of the + R effect of S02Me by octet expansion, it is appropriate to mention its stereochemical aspect. This was examined by Kloosterziel and Backer in the later of their two papers to which reference has already been made78. They measured the pKa value of 4-methylsulfonyl-3,5-xylenol 25 and compared its acidity with that of 4-methylsulfonylphenol. Parallel measurements were made with 4-CN and 4-N02... 

A good correlation with ordinary Hammett a values was based on 16 well-behaved substituents, and p-SOMe conformed well to this. Various other substituents showed deviations which were attributed to enhanced + R effects. These included p-SPh and this was explained in terms of 7t(pd) bonding, which was thus taken to play no part in the effect of p-SOMe on the methyl hyperfine splitting. More recently several 4-substituted benzyl radicals of the type RSO C6H4CH2 (n — 0,1 or 2 R = Me, Ph, Tol, COMe or OMe) have been examined by ESR spectroscopy249. The ability to delocalize spin density onto the substituent decreases in general as n increases and the effect of R depends on the oxidation state of sulfur. These authors have devised a new scale of substituent effects (sigma dot... 

The symbol and sign conventions used for substituent effects in this chapter are those most frequently used by writers on correlation analysis in organic chemistry. / or R effects which withdraw electrons from the ring are regarded as positive. See Ref. 48, pp. 229-230 for a more detailed consideration of symbol and sign conventions. 

Martin WR, Sloan JW Relationship of CNS tryptaminergic processes and the action of LSD-like hallucinogens. Pharmacol Biochem Behav 24 393-399, 1986 McCall R Effects of hallucinogenic drugs on serotoninergic neuronal systems. Pharmacol Biochem Behav 24 359-363, 1986... 

The dual substituent parameter treatment attributes substituent effects to an additive blend of polar (/) and pi delocalization (R) effects, each of which may be represented as a op product (3) ... 

Antony, P., Puskas, J.E., Ott, H., Altstadt, V., Kovar, M., and Norton, P.R. Effect of Hard and Soft Segment Composition on the Morphology and Mechanical Properties of Polystyrene-Polyisobutylene Thermoplastic Elastomeric Block Copolymers. Proceedings of the Polymer Processing Society Meeting, May 21-24, Montreal, Canada, 2001. 

Hattori Y, Levi R Effect of PGDj on cardiac con- 43 tractihty a negative inotropism secondary to coronary vasoconstriction conceals a primary positive inotropic action. J Pharmacol Exp Ther 1986 237 ... 

Warach S, Pettigrew LC, Dashe IF, PuUicino P, Lefkowitz DM, Sabounjian L, Harnett K, Schwiderski U, Gammans R. Effect of citicoline on ischemic lesions as measured by diffusion-weighted magnetic resonance imaging. Citicoline 010 investigators. Arm... 

Aman, R., Schieber, A., and Carle, R., Effects of heating and illumination on trans-cis isomerization and degradation of (3-carotene and lutein in isolated spinach chlo-roplasts, J. Agric. Food Chem., 53, 9512, 2005. 

Yong Ju, Z. and Howar, L.R., Effects of solvent and temperature on pressurized liquid extraction of anthocyans and total phenolics from dried red grape skin, J. Agric. Food Chem., 51, 5207, 2003. 

Chatteijee, S., Padwal-Desai, S.R., and Thomas, R, Effect of y-irradiation on the antioxidant activity of turmeric Curcuma longa L.) extracts. Food Res. Int., 32, 487, 1999. 

Jaffe (1953) showed that while many rate or equilibrium data conform well to the Hammett equation (as indicated by the correlation coefficient), many such data are outside the scope of the equation in its original form and mode of application. Deviations are commonly shown by para-substituents with considerable -I-1 or —R effect. Hammett himself found that P-NO2 (+R) showed deviations in the correlation of reactions of anilines or phenols. The deviations were systematic in that a a value of ca. 1.27 seemed to apply, compared with 0.78 based on the ionization of p-nitrobenzoic acid. Other examples were soon discovered and it became conventional to treat them similarly in terms of a duality of substituent constants . 

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