Oxygen-carbon bond dissociation enthalpies of benzyl phenyl ethers and anisoles. An example of temperature dependent substituent effects

For some time it has been assumed that the direction and magnitude of the effects of Y-substituents on the Z-X bond dissociation enthalpies (BDE's) in compounds of the general formula 4-YC(6)H(4)Z-X could be correlated with the polarity of the Z-X bond undergoing homolysis. Recently we have shown by DFT calculations on 4-YC(6)H(4)CH(2)-X (X = H, F, Cl, Br) that the effects of Y on CH(2)-X BDE's are small and roughly equal for each X, despite large changes in C-X bond polarity. We then proposed that when Y have significant effects on Z-X BDE's it is due to their stabilization or destabilization of the radical. This proposal has been examined by studying 4-YC(6)H(4)O-X BDE's for X = H, CH(3), and CH(2)C(6)H(5) both by theory and experiment. The magnitudes of the effects of Y on O-X BDE's were quantified by Hammett type plots of DeltaBDE's vs sigma(+) (Y). Calculations reveal that changes in O-X BDE's induced by changing Y are large and essentially identical (rho(+) = 6.7-6.9 kcal mol(-)(1)) for these three classes of compounds. The calculated rho(+) values are close to those obtained experimentally for X = H at ca. 300 K and for X = CH(2)C(6)H(5) at ca. 550 K. However, early literature reports of the effects of Y on O-X BDE's for X = CH(3) with measurements made at ca. 1000 K gave rho(+) approximately 3 kcal mol(-)(1). We have confirmed some of these earlier, high-temperature O-CH(3) BDE's and propose that at 1000 K, conjugating groups such as -OCH(3) are essentially free rotors, and no longer lie mainly in the plane of the aromatic ring. As a consequence, the 298 K DFT-calculated DeltaBDE for 4-OCH(3)-anisole of -6.1 kcal mol(-)(1) decreases to -3.8 kcal mol(-)(1) for free rotation, in agreement with the ca. 1000 K experimental value. In contrast, high-temperature O-CH(3) DeltaBDE's for three anisoles with strongly hindered substituent rotation are essentially identical to those that would be observed at ambient temperatures. We conclude that substituent effects measured at elevated temperatures may differ substantially from those appropriate for 298 K.     

From ballistic motion to localization: a phase space analysis

We introduce phase space concepts to describe quantum states in a disordered system. The merits of an inverse participation ratio defined on the basis of the Husimi function are demonstrated by a numerical study of the Anderson model in one, two, and three dimensions. Contrary to the inverse participation ratios in real and momentum space, the corresponding phase space quantity allows for a distinction between the ballistic, diffusive, and localized regimes on a unique footing and provides valuable insight into the structure of the eigenstates. Received 5 March 2002     

Specification of Molecular Chirality

The topological analysis of chiral molecular models has provided the framework of a general system for the specification of their chirality. The application, made in and before 1956, of this system to organic-chemical configurations is generally retained, but is redefined with respect to certain types of structure, largely in the light of experience gained since 1956 in the Beilstein Institute and elsewhere. The system is now extended to deal, on the one hand, with organic-chemical conformations, and, on the other, with inorganic-chemical configurations to ligancy six. Matters arising in connexion with the transference of chiral specifications from model to name are considered, notably that of the symbiosis in nomenclature of expressions of the general system and of systems of confined scope. For corrigendum see DOI: 10.1002/anie.196605111     

Isomerization of triphenylmethoxyl and 1,1-diphenylethoxyl radicals. Revised assignment of the electron-spin resonance spectra of purported intermediates formed during the ceric ammonium nitrate mediated photooxidation of aryl carbinols

Grossi and Strazzari have reported (J. Org. Chem. 2000, 65, 2748-2754) that the ceric ammonium nitrate modulated photooxidation of triphenylmethanol and 1,1-diphenylethanol yielded ESR spectra of the putative spiro-cyclohexadienyl intermediates in the O-neophyl rearrangements of the corresponding alkoxyl radicals, Ph2(R)CO* (R = Ph, CH3), to the phenoxymethyl radicals, Ph(R)C*OPh. Both ESR spectra are reassigned to the phenoxyl radical, C6H5O*, and the probable mechanism by which phenoxyl is formed in these systems is presented.     

Detection of interaction-induced nonlocal effects using perfectly transmitting nanostructures

We consider one-dimensional transport through an interacting region in series with a point-like one-body scatterer. When the conductance of the interacting region is perfect, independently of the interaction strength, a nonlocal interaction effect yields a total conductance of the composed system that depends on the interaction strength and is lower than the transmission of the one-body scatterer. This qualitative nonlocal effect allows to probe the dressing cloud of an interacting system in ideal noninteracting leads. The conductance correction increases with the strength of the interaction and the reflection of the one-body scatterer (attaining relative changes >50%), and decreases with the distance between the interacting region and the one-body scatterer. Scaling laws are obtained and possible experimental realizations are suggested.     

Cross measurements of linear momentum transfer and energy dissipation in collisions between 290 MeV20Ne and238U

The²⁰Ne + U reactions are investigated at 290 MeV bombarding energy. The linear momentum transfer and excitation energy are deduced eventwise from the respective measurements of the folding angle between correlated fission fragments and the neutron multiplicity. A simple incomplete fusion picture is shown to essentially account for the data. The sensitivity of the two measurements in order to infer the violence of a collision is discussed in details.     

Observation of the nuclear rainbow scattering for12C+12C atE Lab =300 MeV

The elastic and inelastic scattering of¹²C on¹²C has been measured in the angular range between 2.8° and 70.4° in the c.m. system atE _Lab =300 MeV. Optical model calculations have been performed with Woods-Saxon and folded potentials, the ground state and the first 2⁺-state were coupled in the calculations. The large cross sections of the elastic scattering at large angles is related to the nuclear rainbow scattering, which is centered at about 56°. This requires a potential depth of 100 MeV at a distance of 3 fm, the fit to the data is sensitive down to this region. The calculations with the folded potential show a better agreement with the data than those with the Woods-Saxon shape. The total reaction cross section of 1,420 mb, obtained from the optical model analysis, corresponds to the geometrical value; no transparency is observed.     

Interaction of 80 MeV12C with88Sr; Neutron transfer,inelastic scattering and spin alignment of the 2+ State of12C

Level structure of¹⁰²Cd has been studied via both¹⁰²In decay and nuclear reactions. Using⁹²Mo+¹⁴N at 86 MeV, the nucleus¹⁰²In was identified and its decay (T _1/2=24 ±4s) studied with the aid of on-line mass separation techniques. Levels of¹⁰²Cd were also populated with⁹²Mo+¹²C at 50 MeV and¹⁰²Pd+³He at 35 MeV reactions and investigated by means of standard in-beam techniques. A level scheme including states up to 4.5 MeV excitation energy is proposed and then discussed in the frame of available calculations.