WHY DOES THIOFORMAMIDE HAVE A LARGER ROTATIONAL BARRIER THAN FORMAMIDE

The C-N rotational barrier for thioformamide is known to be larger than that for formamide. The origin of this barrier has been examined with the aid of ab initio molecular orbital calculations. The larger barrier is reproduced, and it is found that the amino group of thioformamide is less ''floppy'' than that of ordinary amides. In addition, the change in charge density at sulfur on rotation of the amino group in thioformamide is much greater than that at oxygen in formamide. It is concluded that the traditional picture of amide ''resonance'' is more appropriate for thioamides than for amides. The small difference in electronegativity between carbon and sulfur and the larger size of sulfur are the major factors that allow charge transfer from nitrogen to sulfur in thioamides. The effect of replacing the carbonyl oxygen of formamide by =NH, =PH, =CH2, and =SiH2 also was examined. The energies associated with group separation reactions were divided into pi components (the rotational barriers) and a components. The latter were found to increase with increasing electronegativity of the substituent, indicating that they resulted from internal Coulombic stabilization. The pi components were about the same for the corresponding first and second row C=Y groups where Y is the terminal atom or group of the double bond, and they increased with increasing electronegativity of Y.