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Abstract
The analysis of the vco bands in the IR spectra of the 3-(2′-chlorocyclopentylthio)- and 3-(2′-chloro-1′-methylpropen-1′-ylthio)-thiochromones and their 3-sulfinyl and 3-sulfonyl derivatives, supported by molecular mechanics calculations of 3-methylthio, 3-methylsulfinyl- and 3-methylsulfonyl-thiochromones, taken as model compounds, indicates the existence of a single conformer for the whole series except for the case of the 3-(2′-chloro-1′-methylpropen-1′-ylsulfinyl)-thiochromone where three conformers can be observed. The negative carbonyl frequency shifts (Δv) for the 3-sulfinyl- and 3-thio-thiochromone derivatives and the positive carbonyl shifts for the 3-sulfonyl-thiochromone derivatives are interpreted on the grounds of an interplay of the πco → 3dSOn orbital interaction with the inductive and field effects of the 3-substituents. The orbital interaction is responsible for the shorter S…O intramolecular contact observed previously from X-ray data of some 3-thio-thiochromone derivatives. The bathochromically shifted n π →CO* band for the 3-thio- and 3-sulfinyl-thiochromone derivatives and the slight hypsochromic effect for the 3-sulfonyl-thiochromone derivatives are discussed in terms of the relative energy level stabilization of the πCO* orbital (due to the πco*/3dSOn orbital interaction) and the n o(co) lone pair (due to the electron-attracting inductive effect of the 3-sulfur-substituent). The IR and UV spectra of thiochroman-4-one show the existence of a mixture of two conformers, the twist-boat being the one where the transannular interaction between the 3p(s) π →co* orbitals takes place.