ON THE NATURE OF ARYLSULFIDE AND ARYLSELENIDE DIMMER RADICAL CATIONS

MARCELO PUIATTI; LYDIA M. BOUCHET; JUAN E. ARGÜELLO; ADRIANA B. PIERINI

Carlos Paz

Conferencia; 13a Conferencia Latinoamericana de Físico-Química Orgánica (CLAFQO-13); 2015

The chemistry of sulfur radical cations (R2S)+. has been subject of several studies not only because they are key intermediates in organic reactions, but they are also important in biological processes. In some cases, the stabilization of these compounds is given by the interaction of the unpaired electron in a p-orbital of the (R2S)+. with a free p-electron pair from the unoxidized atom, featuring a long weak bond of two centers and three electrons (2c-3e) with a bond order of 0.5 (hemi-bond). Its periodic neighbor, selenium, also has this behavior [1]. TheseArZCH3+. (Z=S, Se) could be observed by transient absorption spectroscopy, the monomer shows a maximum at 500 nm, meanwhile a band at~640 nm was assigned to the dimmer [R2SSR2] +. [2].In the present work, the formation of different dimmers of ArSCH3 and ArSeCH3 radical cations ([R2ZZR2]+., Z=S, Se) were studied by means of DFT calculations(scheme 1). Sawaki, et al [2], employed the BLYP method, obtaining three conformers, of the dimmer [Ar(CH3)SSAr(CH3)]+., with similar energies of formation (a,b,d) together with a less stable conformer (d, scheme 1). However, BLYP is not the appropriate functional for dealing with these kind of complexes. By using the M062Xfunctional, suited for working with complexes with sigma-phi, phi-phi interactions, different energies were obtained. The geometries are also different, with a SS 2c-3e bond shorter compared with the reported in ref. [2]. In the case of selenium complexes, it was found that they are stronger as they present lower dimerization energies, also reflected in their higher dimerization equilibrium constant (Kdim).The nature of the ZZ 2c-3e bond, different contributions to the formation energy, the influence of electron donating and electron acceptors groups in the phenyl moiety as well as the steric effect of alkyl substituents will be discussed in order to explain the experimental observations.[1] Zhang, S.; Wang, X.; Su, Y.; Qiu, Y.; Zhang, Z.; Wang, X. Nature Comm.2014, 5, 4127. [2] Yokoi, H.; Hatta, A.; Ishiguro, K.; Sawaki, Y. J. Am. Chem. Soc.1998, 120,12728-12733.