Thermolabile Kohlenwasserstoffe, XXII. Thermolysesymm. Tetraalkyldiarylethane und Resonanzstabilisierung der α,α-Dialkylbenzylradikale

A new approach of evaluating bond dissociation energy from eigenvalue of bonding orbital-connection matrix for C-C and C-H bonds in alkane

A new bonding orbital-connection matrix was constructed, in which the diagonal elements were assigned the chemical potentials E(1) and E(2) of two radicals R(i)* and R(j)* obtained by cutting the interested bond, and the off-diagonal elements, representing bonding connections, were assigned values 1. The eigenvalues X(1CC) and X(1CH) of the bonding orbital-connection matrix were obtained for C-C and C-H bonds, respectively. Also a steric effect parameter S(ij) was proposed for C-C bond. Using X(1CC), X(1CH), and S(ij) as bond descriptors, good correlations with the Bond Dissociation Energies (BDEs) of the C-C and C-H bonds were obtained for alkanes. The result shows that the eigenvalue of bonding orbital-connection matrix is a good descriptor for expressing the relative bond energies of C-C and C-H bonds in alkane. This work provides a new physical insight and a principally novel general approach to the evaluation of the bond dissociation energies of carbon-carbon (C-C) and carbon-hydrogen (C-H) bonds in alkane. Also it builds a simple bridge linking the adjacent matrix of radicals R(i)* and R(j)* with the BDE of R(i)-R(j) and R(i)-H. Furthermore, the Heat of Atomization (HA) and Heat of Formation in Gas (HFG(0)) of alkane can be estimated well with the parameters X(1CC), X(1CH), and S(ij).

Thermochemical Stability of alpha-Amino-alpha-carbonylmethyl Radicals and Their Resonance As Measured by ESR

ESR spectra of the captodative alpha-amino-alpha-carbonylmethyl radicals 8 have been recorded. No coalescence temperature for the rotation of the two NMe groups was found at temperatures below the decomposition temperature of the radicals. From known coalescence temperatures and rotational barriers of substituted methyl radicals the rotational barrier of >/=17 kcal mol(-)(1) was estimated for the (*)C-N bond in the radicals 8. Enthalpies DeltaH(diss) and entropies DeltaS(diss) of the homolytic dissociation of 7a,c,d into 8a,c,d have been obtained from equilibrium measurements by ESR. By correcting for substituent interaction enthalpies in 7 (steric and geminal), a radical stabilization enthalpy RSE = -20.7 +/- 1.0 kcal mol(-)(1) was obtained for 8. By addition of the known RSEs of dialkylamino- and carbonyl groups, a RSE = -9.9 kcal mol(-)(1) is predicted for 8. The difference between the experimental and predicted values of 10.8 kcal mol(-)(1) is attributed to a synergistic captodative substituent effect. A linear correlation between the radical stabilization enthalpies of the radicals 8 and of other mono- and disubstituted alkyl radicals and their ESR aH(alpha) coupling constants was found. According to this correlation the reduction of aH(alpha) by 1 G corresponds to an increase in RSE of 1.57 kcal mol(-)(1). The large resonance of the captodative alpha-amino-alpha-carbonylmethyl radicals 3, expressed by their high RSE, their small aH(alpha) coupling constant, and their high rotational barrier, can be rationalized by a strong interaction between the alpha-amino and the alpha-carbonyl groups similar to that in amides and expressed in the resonance structures 6.

Mesolytic Scission of C-C Bonds as a Probe for Photoinduced Electron Transfer Reactions of Quinones

Photoinduced electron transfer reactions of chlorinated benzoquinones are investigated using bibenzylic donors that undergo rapid fragmentation upon oxidation. The fragmentation rates and the quantum yields are used to probe the dynamics of back-electron transfer (BET) in two types of radical ion pairs. The triplet ion pairs formed by interception of excited state quinones give products with high quantum yields. The singlet ion pairs formed by irradiation of the charge-transfer (CT) complexes between the quinones and the donors undergo reactions with significantly lower efficiency. The advantage of the first method (triplet quenching) over the CT-irradiation depends on the energetics of BET. It is large for reactions with relatively small DeltaG(et) for BET and it decreases for reactions with more negative DeltaG(bet). The indirectly obtained rates of BET are in excellent agreement with literature data for similar, but unreactive systems, and the rates of C-C bond scission in radical cations generated in these systems are consistent with the thermodynamics of these processes.