A Study of the Charge Distribution in Unconjugated Thiocarbonyl Compounds: a Comparison with Carbonyl and Imine Chromophores

Via a literature search it has been shown, by means of the ira spectra of thioamides, the dipole moments of thiocarbonyl compounds, the basicity of thioamides and thioureas, the acidity of carboxylic acids carrying a substituent with the thiocarbonyl function, LCAO-b MOc method, and proton chemical shifts, that the thiocarbonyl group has a tendency to give greater mesomeric interaction (a) than the carbonyl group (b) with electron donating heteroatoms. A comparison of the charge distribution in unconjugated thiocarbonyl (c), carbonyl (d), and imine (e) chromophores is made by means of surveying the dipole moments of several small molecules, calculating the forces constants from the ir absorption frequencies, measuring the bond distances by x-ray, and observing the proton chemical shifts in the nmrd spectra. The use of the electronic absorption spectra, molecular orbital theory, and mass spectrometry are also included. All data support the hypothesis that the thiocarbonyl chromophore has more ionic contribution to the resonance hybrid than is present in (d) or (e). This is not predictable on the basis of the electronegativity of the heteroatoms