A comparison of some properties of C=O and C=S bonds
Kenneth B. Wiberg* and Yigui Wang
Department of Chemistry, Yale University, P. O. Box 208107, New Haven,
Connecticut 06520-8107, USA
E-mail:
[email protected]
Dedicated to Prof. William F. Bailey on the occasion of his 65th Birthday
Abstract
The properties of compounds with carbonyl and thiocarbonyl groups have been examined theoretically, and the results have been compared with the available experimental data. The properties include the bond dissociation energies, the vibrational force constants, the n•π* transitions and the charge distributions. The origin of the differences in properties are discussed
Keywords: Carbonyl group, thiocarbonyl group, bond dissociation energies, n•π* transitions, charge distributions
Introduction
There are significant and interesting differences between compounds with carbonyl and thiocarbonyl groups. The C=S bond length (~1.6 Å)1 is considerably longer than C=O (~1.25 Å). As a result, one would expect the bond strengths to differ significantly. The π-bond also might be expected to be strongly affected in view of the orbital size mismatch between first and second row atoms. On the other hand, thioamides are known to have larger rotational barriers than amides, where the barrier arises from the interaction of the amide nitrogen with the adjacent C=O or C=S group.2
Another interesting difference is found in the position of the n-π* transition that is shifted into the visible spectrum by the sulfur.3 As a result the thiocarbonyl derivatives have interesting colors with phenyl thiocarbonyl chloride being red (•max = 530 nm). The transitions are shifted by about 2eV (~50 kcal/mol) to the red as compared to the corresponding carbonyl compounds.
There are also significant differences in reactivity. Benzoyl chloride has been found to be nine times more reactive toward methanol than thiobenzoyl chloride.4 The reactions of phenoxycarbonyl chloride and phenoxythiocarbonyl chloride with trifluoromethyl phenyl carbinol in the presence of triethylamine has been examined,5 and whereas the carbonyl chloride reacts normally with the intermediate acylammonium ion to give the ester, the thiocarbonyl
