Studies on ¹³C Magnetic Resonance Spectroscopy. XVI. On the Introduction of the Entropy Term ΔS° for the Elucidation of Ipso- and α-¹³C Substituent-Induced Chemical Shifts by an Empirical Approach

Abstract

^<13>C Substituent-induced chemical shifts (SCS) at the ipso and α positions of monosubstituted benzenes or methanes can be expressed by a linear combination of three kinds of empirical parameters, σ_i, σ_π, and ΔS°. ΔS° represents the difference of the entropy (due to the substituent) between the standard entropy S°of the substituted compound and that of the parent one. The following results were obtained. SCS_ipso=63.9·σ_i-3.9·σ_π+0.6ΔS°₊-0.4·ΔS°_-+3.8 (r=0.927, SD=4.9 ppm, n=13) SCS_α=162.1·σ_i-10.1·σ_π+1.2·ΔS°₊-0.3·ΔS°_-+2.3 (r=0.976, SD=4.9ppm, n=13) where + and - subscripts of ΔS° indicate electron-donating and -attracting groups, respectively. This treatment is also applicable for elucidation of the ¹³C SCS of analogous positions of many substituted aromatics and heteroaromatics, including ortho-disubstituted benzenes, and provides a reasonable and explicit chemical basis for the origin of the ¹³C SCS_ipso and SCS_α. The same treatment also provides an explicit chemical basis for Mulliken's electronegativity scale and the steric substituent constant, E_s.