The substituent effect (SE) is one of the most important
topics
in organic chemistry and related fields, and Hammett constants (σ)
are commonly used to describe it. The results of the computational
studies carried out for Y–R–X systems (reaction sites
Y = NO2, O–; substituents X = NO2, CN, Cl, H, OH, NH2; spacers R = polyene, polyyne,
acene withn= 1–5 repeatable units) show
that the substituent properties depend significantly onn, the type of R, and Y. Results of the analysis of the substituent
effect stabilization energy and geometrical parameters of the Y–R–X
systems reveal that (i) the SE strength and its inductive and resonance
components decay with the increase in spacer length, its weakening
depends on the Y and R type; quantitative relations describing decay
are presented; (ii) the ratio between inductive and resonance effect
strength changes withnand depends on Y; (iii) differences
in the substituents’ properties are examples of reverse SE;
(iv) in general, structural parameters are mutually well correlated
as well as with the SE descriptors; (v) due to the strong O–resonance effect, the changes in π-electron delocalization
within R are well correlated with the SE strength only for Y = O–systems.