Spin Density Distribution in 2,4,6‐Triphenylphenoxyl

Abstract

The distribution of the unpaired electron over the oxygen and the 24 carbon atoms in the free 2,4,6-triphenylphenoxy radical was determined by electron spin resonance spectroscopy andquantum-mechanical approximation methods. The hyperfine splitting was evaluated with the aid of the spectra of triphenylphenoxyls deuterated in some or all of the substituent phenyl groups. The results of the quantum-mechanical approximations were checked by recording the ESR spectra of triphenylphenoxyls labeled with 13C in positions I , 2,3, or 4 of the central ring. The spin density distribution permits a first discussion of the 1 7 0 -coupling constants of correspondingly labeled triphenylphenoxyl and other organic free radicals.The dehydrogenation of 2,4,6-triphenylphenoI 111 and many other 2,4,6-triarylphenols[zl in an alkaline or a neutral medium leads to phenoxy radicals, which are unusually stable: they do not react with oxygen and, despite their high oxidation potentials 131, they can be kept in organic solvents for a long time without any change. As in the case of 2,4,6-tri-t-butylphenoxyl[41, blocking of the reactive 2, 4, and 6 positions with aryl groups suppresses the susceptibility of the ring to further addition and substitution. Since the steric hindrance with phenyl groups is less than with t-butyl groupsanother factor must be present in 2,4,6-triphenylphenoxyl to compensate for the decrease in steric hindrance. This effect is due to the participation of the phenyl groups in the 7c-electron system thus enabling the unpaired electron to occupy 25 positions, as compared to only seven in phenoxyl or trialkylphenoxyls.The distribution of the unpaired electrons over the various atoms can be determined from the ESR spectra[sl, and in particular from the resulting coupling constants and empirical relations between the latter and the spin densities. The ESR spectra with proton hyperfine structure can be used to determine the spin densities only on carbon atoms carrying hydrogens, and, for the other ("blind" [71) atoms, one can use quantum-mechanical approximations in such a way that the calculated results agree with the experimental values for the hydrogen-carrying carbons. However, it has been shown on p-benzosemiquinone [71, that this technique may lead to incorrect values for the blind atoms. We have therefore checked the calculated spin densities of the blind atoms with triphenylphenoxyl labeled in known positions with 13C. From the experimental spin density distribution and the 170-coupling constant 181, we could make an inference concerning the connection between the 170-coupling constant and the spin density on oxygen.

Modulation of Open‐shell Characters of Amine‐inserted Diphenoquinones via Structural Modification

“…The spectral shapes were mainly attributed to the hyperfine interaction from the central nitrogen atom (j a N j)a nd the four hydrogen atoms on the phenoxy rings (j a H j)f or all compounds. The values of j a H j were predicted about ah alf of the value of the hydrogens of 3a nd 5positions of 2,4,6-triphenylphenoxy radical( 0.175 mT), [20] verifying the much larger exchange interactions than the hyperfine interactions in 2a-d.T he j a N j values of 2a-d were smaller than those of the radical cations of triarylamine derivatives (about 0.9mT) [21,22] and 10-phenyl-10H-phenothiazine( 0.7mT). [23] Most importantly,t he j a N j values become smaller on going from 2a to 2d.The DFT calculations for the triplet states of 2a-d predicted that the spin density on the nitrogen atom decreased in the order of 2a> 2b> 2c> 2d and on the oxygen atoms of the phenoxy groups increased in the order of 2a< 2b< 2c< 2d (Table S7).…”

“…The spectral shapes were mainly attributed to the hyperfine interaction from the central nitrogen atom (| a N |) and the four hydrogen atoms on the phenoxy rings (| a H |) for all compounds. The values of | a H | were predicted about a half of the value of the hydrogens of 3 and 5 positions of 2,4,6‐triphenylphenoxy radical (0.175 mT), verifying the much larger exchange interactions than the hyperfine interactions in 2 a – d . The | a N | values of 2 a – d were smaller than those of the radical cations of triarylamine derivatives (about 0.9 mT) and 10‐phenyl‐10 H ‐phenothiazine (0.7 mT) .…”