Phenyl radical geometry structure

In general, the accuracy of a simulated spectrum depends on the quality of the description of both the initial and the final electronic states of the transition. This is obviously related to the proper choice of a well-suited computational model a reliable description of equilibrium structures, harmonic frequencies, normal modes, and electronic transition energy is necessary. In the study of the A Bj Aj electronic transition of phenyl radical the structural and vibrational properties have been obtained with the B3LYP/TDB3LYP//N07D model, designed for computational studies of free radicals. Unconstrained geometry optimizations lead to planar... 

Analgesics of the 4-arylpiperidine class mimic the geometry of a large portion of morphine when in the axial phenyl chair conformation 33a and were thus earlier believed capable of accommodation at the proposed receptor. This view later fell into disrepute as a result of conformational considerations and particularly the radically different structure-activity relationships in regard to N-substituents and the influence of phenolic hydroxyl between the 4-arylpiperidine and morphine groups. There is evidence that reversed esters of pethidine and other 4-phenylpiperidines with C-4 oxygen functions bind in the equatorial 4-phenyl chair conformation 33b, and receptor modifications necessary to allow uptake of both ax and eq 4-phenyl chairs are discussed... 

FIGURE 6.3 Ground-state optimized geometry structures (bond in A angles in degrees) for vinyl, propanyl and phenyl radicals. 

TABLE 6.18 Geometry Structure of Phenyl Radical in the Ground Aj and First... 

The ability of radical stabilizing substituents to have a large effect on R in the TS geometry is what makes the Cope TS chameleonic and the chameleonic nature of the TS is responsible for the cooperative and competitive phenyl substituent effects that have been both calculated [27,28] and observed [5,29,30]. Multiple phenyl substituents, attached where they either all stabilize structure A or all stabilize structure C, distort the TS geometry toward one of these two diradical extremes, thus enhancing the ability of all the substituents to stabilize the TS. In contrast, when one set of substituents stabilizes one of these resonance structures and another substituent stabilizes the other, the result is a compromise TS geometry, with a value of R that is optimal for neither set of substituents. 

A related study on planarized borataanthracene radical anion 42 [45] showed that it maintained a planar geometry in the reduced state, reflected in its structural and photophysical properties. Unlike the unconstrained 9-phenyl-9-borataanthracene 31 [35], 42 did not show any interactions with counterions owing to broad delocalization of the negative charge between the borataanthracene component and the planarized phenyl ring. Electronic spectra of 42 were red-shifted with enhanced vibronic structure compared to 9-Mes-9-borataanthracene. While the fluorescence behavior had not previously been observed in unconstrained borataanthracene derivatives, 42 exhibited intense emission = 584 nm, O = 0.45). 

---