Substituent Effects in S- and Se-Centered Radical Cations

The first ionization potentials of molecules XZY and 1,4-XC₆H₄ZR (X, Y are inorganic, organo- metallic, or organic substituents; Z = S, Se), as well as the energies of charge-transfer bands in the electronic spectra of tetracyanoethylene complexes of these molecules are determined by the inductive, resonance, and polarization effects of substituents X and Y. Z-Centered radical cations formed both from individual molecules in the gas phase and from those incorporated in tight radical ion pairs in solutions are closely allied in their electronic structure. The resonance parameters ⁺ _R of organosilicon, organogermanium, and organotin substituents bound to the radical cation center Z+^· were determined.     

Effect of substituents on ionization potentials of phosphorus compounds. Conjugation in radical cations

The first vertical ionization potentials (I) of phosphorus compounds P(X_i)₃, OP(X_i)₃, SP(X_i)₃, (4-XC₆H₄)₃P, and PCX are related to the inductive, resonance, and polarizability parameters of inorganic, organic, and organometallic substituents X by dependences of the type I = I _H + a_I + b_R ⁺ + c, where I _H is the I value for X = H. The I values are also affected by hyperconjugation. The ratio of the contributions of the resonance (b_R ⁺) and polarizability (c) effects to the I value is determined by the degree of delocalization of the unpaired electron and the positive charge in the radical cations formed upon ionization of neutral molecules. The _R ⁺ resonance parameters of organosilicon, organogermanium, and organotin substituents bound to the P ^·+ radical cation center were calculated for the first time.     

Electronic absorption spectra of charge-transfer complexes and effects of substituents in radical cations

The effects of X substituents on the energies of charge-transfer bandshv _CT in electronic absorption spectra of charge-transfer complexes of π-, n-, or σ-donors (DX) with π- or σ-acceptors (A) as well as on the ionization potentialsI _D of individual DX molecules are described by the equationhv _CT(l _D)=a +bσ₁ +cσ _R ⁺ +dσ_α. When DX and A are fixed, the inductive (bσ₁), resonance (cσ _R ⁺ ), and polarization (dσ_α) contributions tohv _CT andI _D are virtually identical. The electronic structure of the D^.+X donor component of the compact [A^.−, D^.+X] radical-ionic pair in a solution is similar to that of the radical cation generated upon photoionization of the individual DX molecule in the gaseous phase. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1002–1006, June, 2000.     

Ionization potentials of non-metal monosulfides. Conjugation in radical cations containing Group IV elements

The first vertical ionization potentialsI(n_s) of 69 monosulfides XSY (X, Y=H, Hal, organic, or heteroorganic substituent) are related to the inductive σ_I resonance (σ _R ⁺ ) and polarizability (σ_α) constants of the substituents by dependences of theI(n_S)=a+bΣσ_I+bΣσ_R+bΣσ_α type. TheI(n_s) values are also affected by hyperconjugation which increases on going from XSH to XSY (Y≠H) compounds. The first calculations of the σ _R ⁺ parameters characterizing the conjugation of Si-, Ge-, Sn-, and Pb-containing substituents with the S^.+ radical cation center are reported. The reasons for weakening of resonance donor properties of heteroorganic substituents of the +M-type in the systems studied as compared to those of the same substituents in the corresponding aromatic radical cations are considered. Translated fromIzvestiya Akademii Nauk. Seriya Khmicheskaya, No. 1, pp. 25–31, January, 2000.     

Substituent effects in radical cations of linear oligosilanes

The dependence was analyzed of the first ionization potentialI(Si−Si) corresponding to detachment of an electron from the σ(Si−Si) highest occupied molecular orbital on the parameters of organic (X=Me, Et, Bu ^t , Ph, CH=CH₂), inorganic (X=F, Cl, Br), and organosilicon (X=SiR₃; R is organic radical) substituents in di-, tri-, and tetrasilanes X₃SiSiX₃. It was found by correlation analysis that out of the three possible effects of substituents X (the inductive, polarizability, and resonance effects), only the first two of them affect theI(Si−Si) values. This means that no conjugation between the substituent X and the radical cation center occurs in X₃Xi±SiX₃. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 253–257, February, 2000.     

Ionization potentials of non-metal monosulfides. Conjugation in radical cations containing Group IV elements

The first vertical ionization potentialsI(n_s) of 69 monosulfides XSY (X, Y=H, Hal, organic, or heteroorganic substituent) are related to the inductive σ_I resonance (σ _R ⁺ ) and polarizability (σ_α) constants of the substituents by dependences of theI(n_S)=a+bΣσ_I+bΣσ_R+bΣσ_α type. TheI(n_s) values are also affected by hyperconjugation which increases on going from XSH to XSY (Y≠H) compounds. The first calculations of the σ _R ⁺ parameters characterizing the conjugation of Si-, Ge-, Sn-, and Pb-containing substituents with the S^.+ radical cation center are reported. The reasons for weakening of resonance donor properties of heteroorganic substituents of the +M-type in the systems studied as compared to those of the same substituents in the corresponding aromatic radical cations are considered. Translated fromIzvestiya Akademii Nauk. Seriya Khmicheskaya, No. 1, pp. 25–31, January, 2000.     

Substituent Effects in I-Centered Radical Cations

The inductive, resonance, and polarization effects of substituents on the ionization potential of iodine n orbitals in IX, ISnR₃, and ICCX molecules, and also on the energy of the charge-transfer band in the UV spectra of the complexes of IX and ISnR₃ with tetracyanoethylene and iodine were studied. The radical cations generated by photoionization of individual molecules in the gas phase and occurring as components of contact radical ion pairs (excited state of charge-transfer complexes) in solution have similar electronic structure. The resonance parameters ⁺ _R of organosilicon, organogermanium, and organotin substituents bound to the radical cation centers I^{+ ·} and I^{+ ·}CC were calculated for the first time.     

Substituent Effects in P- and As-centered Radical Cations

The first vertical ionization potentials of isostructural P(III) and As(III) compounds EX₃ (E = P, As) whose highest occupied molecular orbital is preferentially localized on the lone electron pairs of atom E depend on the inductive, resonance, and polarization effects of substituents X. Hyperconjugation in fragments like As^{+ ·}-C-H is the only resonance effect in radical cations As^{+ ·}X₃. The same effect in similar P-centered radical cations is weaker.     

First ionization potentials of amines and electronic effects in their radical cations

The possibility of application of linear free energy relationships for studying the effects of substituents on the first vertical ionization potentials of amines, I(n_N), was substantiated. The I(n_N) values depend on the inductive, resonance, and polarizability effects of substituents and are also affected by hyperconjugation. The _R ⁺ resonance parameters of substituents MR₃ (M = Si, Ge, Sn) and CH₂SiMe₃ bound to the N ^·+ radical cation center were calculated for the first time.