Conjugation in radical cations of benzene, ethylene, and acetylene derivatives. Nonuniversality of the resonance parameters of group 14 organoelement substituents

Abstract

The first vertical ionization potentials I t of molecules R~X (X = Ph, H2C=CH , and HC~-C) depend on the joint influence of the inductive, resonance, and polarizability effects of substituents X, which are characterized by parameters al, ~R +, and aa, respectively. The mechanism of conjugation in radical cations formed upon ionization of I~X is changed as compared to neutral R~X molecules, while the substituent X becomes polarized. The conjugation and polarizability effects are strenthened in the sequence Ph < H2C=CH < HC~C as I~ changes from Ph to H2C=CH and HC-~C. The a~. + parameters of Si-, Ge-, and Sn-containing substituents X are dependent on the type of R, but are connected by linear dependences in the series of benzene, ethylene, and acetylene derivatives.Key words: ionization potential, radical cations, conjugation, polarizability, group 14 substituents, non-universality of resonance parameters.The mechanism of conjugation of organoelement substituents MR 3 (M = Si, Ge, Sn, and Pb; R are organic radicals) with indicator centers Rn (Ph, H2C=CH, HC=C, furyl, thienyl, etc.) in compounds R~MR 3 is more complex than that in organic derivatives R,tR. t The substituent MR 3 exhibits simultaneously acceptor (d,Tt-conjugation, i.e., the interaction of nd orbitals of M and a*-orbitals of the M--C bond with 1~) and donor (a,rt-conjugation, Le., the interaction of a-orbitals of the M--C bond with R~) properties toward the R.~ fragment. Only a,n-conjugation occurs in coml~ounds R~CH2M R3. tThe quantitative characteristics of conjugation, i.e., parameters ~R ofsubstituents MR 3 and CH2MR3,1'2 are not universal al~d depend on both the type of fragment R~ and charges on its a(oms, hi the case of a small charge on R~ (0.01e; H-complexes of R~MR 3 and R~CHaMR 3 with phenol) and a large charge (>0.9e; the excited state of R~MR 3 complexes with iodine and tetmcyanoethylene), the degree of conjugation in isolated neutral molecules is characterized by parameters aK ~ erR, and erp+, respectively, which can differ in both magnitude and sign. The main reason for these distinctions is the dependence of a,~-conjugation on the charge on R~. I-6 These problems were studied in detail for benzene derivatives (R~ = Rh) t-7 and only fragmentarily for compounds with other type of R~ 1.3-s,s because of scarce information on the experimental values of aR ~ erR, and aR +. Recently, s we have shown that information on the aR + parameters of substituents can be obtained from the first vertical ionization pote~tials /~ of organoeleme~at compounds.