title: Structure and C-S bond cleavage in aryl 1-methyl-1-arylethyl sulfide radical cations.
abstract: Steady state and laser flash photolysis (LFP) of a series of p-X-cumyl phenyl sulfides (4-X-C(6)H(4)C(CH(3))(2)SC(6)H(5): 1, X = Br; 2, X = H; 3, X = CH(3); 4, X = OCH(3)) and p-X-cumyl p-methoxyphenyl sulfides (4-X-C(6)H(4)C(CH(3))(2)SC(6)H(4)OCH(3): 5, X = H; 6, X = CH(3); 7, X = OCH(3)) has been carried out in the presence of N-methoxy phenanthridinium hexafluorophosphate (MeOP(+)PF(6)(-)) under nitrogen in MeCN. Steady state photolysis showed the formation of products deriving from the C-S bond cleavage in the radical cations 1(+•)-7(+•) (2-aryl-2-propanols and diaryl disulfides). Formation of 1(+•)-7(+•) was also demonstrated by LFP experiments evidencing the absorption bands of the radical cations 1(+•)-3(+•) (λ(max) = 530 nm) and 5(+•)-7(+•) (λ(max) = 570 nm) mainly localized in the arylsulfenyl group and radical cation 4(+•) (λ(max) = 410, 700 nm) probably mainly localized in the cumyl ring. The radical cations decayed by first-order kinetics with a process attributable to the C-S bond cleavage. On the basis of DFT calculations it has been suggested that the conformations most suitable for C-S bond cleavage in 1(+•)-4(+•) and 7(+•) are characterized by having the C-S bond almost collinear with the π system of the cumyl ring and by a significant charge and spin delocalization from the ArS ring to the cumyl ring. Such a delocalization is probably at the origin of the observation that the rates of C-S bond cleavage result in very little sensitivity to changes in the C-S bond dissociation free energy (BDFE). A quite large reorganization energy value (λ = 43.7 kcal mol(-1)) has been calculated for the C-S bond scission reaction in the radical cation. This value is much larger than that (λ = 12 kcal mol(-1)) found for the C-C bond cleavage in bicumyl radical cations, a reaction that also leads to cumyl carbocations.
authors.affiliation: Istituto CNR di Metodologie Chimiche-IMC, Sezione Meccanismi di Reazione c/o Dipartimento di Chimica, Università di Roma La Sapienza, P.le A. Moro 5, 00185 Rome, Italy.
authors.forename: Enrico
authors.initials: E
authors.lastname: Baciocchi
authors.forename: Marta
authors.initials: M
authors.lastname: Bettoni
authors.forename: Tiziana
authors.initials: T
authors.lastname: Del Giacco
authors.forename: Osvaldo
authors.initials: O
authors.lastname: Lanzalunga
authors.forename: Marco
authors.initials: M
authors.lastname: Mazzonna
authors.forename: Paolo
authors.initials: P
authors.lastname: Mencarelli
mesh_terms: D002412:Cations / Q000737:chemistry*; D005609:Free Radicals / Q000737:chemistry*; D007834:Lasers; D008027:Light; D009682:Magnetic Resonance Spectroscopy; D015394:Molecular Structure; D010782:Photolysis; D013438:Sulfhydryl Compounds / Q000737:chemistry*; D013440:Sulfides / Q000737:chemistry*
chemical_list: D002412:Cations; D005609:Free Radicals; D013438:Sulfhydryl Compounds; D013440:Sulfides