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

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^<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 ^<13>C 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 ^<13>C 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.
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1980-10-25