¹³C-NMR-Untersuchungen von Substituenteneffekten in mehrfach substituierten Benzen-und Naphthalenverbindungen: Inkrementberechnungen der¹³C-chemischen Verschiebungen

Summary

The aryl¹³C chemical shifts of Cl-substituted 4-amino-, 4-diazonium-N,N-dimethylanilines, N,N-dimethylanilines and differently substituted naphthalenes were assigned by means of different NMR methods. The assignments were compared with chemical shifts obtained by using empirical additivity relationship for mono substituted aromatic substances. As a means of substitutent interactions, the chemical shift difference between calculated and experimental values (Δδ ⁱ_c ) has been used. In the presence of remarkable steric and electronic substituent interactions, large deviations from additivity (Δδ ⁱ_c values up to 15.4 ppm) were found. Which originate primarily from steric interactions between the substitutents. In order to account therefore, correction increments have been developed by employing the Δδ ⁱ_c values obtained from 1,2-disubstituted benzenes or naphthalenes. The¹³C chemical shifts of more than seventy substituted benzenes and naphthalenes have been predicted. The results corroborate that reasonable calculation of chemical shifts in sterically hindered benzenes is possible by using the extended additivity rule. The Δδ ⁱ_c values are much lower and allow reasonable structural assignments.

For external users of this incremental system, a computer program for IBM compatible PC/AT was developed. By means of this program, the¹³C chemical shifts for different benzenes and naphthalenes with or without 1,2-disubstituted correction increments will be calculated and the corresponding spectrum displayed. The program can assist the successful assignment of experimental¹³C chemical shifts.