Effects of hyperconjugation on the electronic structure and photoreactivity of organic sulfonyl chlorides

The electronic structure of organic sulfonyl compounds of the form RSO(2)G (G = -Cl, -OH, -CH(3)) is investigated to evaluate the effect of aryl R groups on photocleavage of the S-G bond. Sulfur K-edge X-ray absorption spectroscopy (XAS) provides a direct measure of the empty low-lying molecular orbitals in these complexes and, in combination with DFT calculations, a detailed description of the bonding in these compounds. The presence of an aryl group bound to the sulfonyl moiety has a significant impact on the spectroscopy and electronic structure of the site. The analysis suggests that the SCl(sigma*) orbital is significantly affected by mixing with the aryl pi* manifold. This mixing is dependent upon the nature of G and is most pronounced in the sulfonyl chlorides, where the energy of the SCl(sigma*) orbital is lowered by approximately 0.5 eV. The observed mixing is best described as excited-state hyperconjugation of the aryl pi system into the SCl(sigma*) orbital. The magnitude of the effect can be estimated directly from the S K-edge XAS spectra. These results are discussed in relation to the observed photochemistry of RSO(2)Cl, which is significantly enhanced when R = aryl as compared to alkyl substituents.