Collect. Czech. Chem. Commun. 1982, 47, 828-837

Dipole moment and electron distribution of the thioamide group

Otto Exnera and Karel Waisserb

a Institute of Organic Chemistry and Biochemistry, Czechoslovak Academy of Sciences, 166 10 Prague 6
b Department of Inorganic and Organic Chemistry, Pharmaceutical Faculty, Charles University, 501 65 Hradec Králové


Dipole moments of seven substituted thiobenzamides Ia-Ig and of two N,N-dimethylthiobenzamides IIa, IIb were measured in dioxan and/or benzene solutions; the infrared spectra proved that association of solutes is negligible under conditions given. The CSNH2 group moment was estimated to 15.6 . 10-30C m at an angle of 75° to the C(1)-C bond, that one of the CSN . (CH3)2 group to 16.5 . 10-30 C m (angle 81°). Analysis in terms of bond moments revealed mesomeric moments m2 of 6.2 or 8.5 . 10-30 C m, respectively, which account for the conjugation within the thioamide group. These values are approximately twice greater than in amides and depend similarly on methyl substitution; there are even minor differences in direction. Also the mesomeric moment of the S=C-O group, redetermined now to 3.3 . 10-30 C m, is greater than in the ester group and even of different direction. The results confirm the importance of n-π conjugation for dipole moments.