6-Radicals, hyperfine splittings, table

All structural information discussed now favours pyramidal silyl radicals. The XSiX angle appears to be nearly tetrahedral or larger, evidence being obtained from esr hyperfine splitting. Table 24 presents the symmetry and selection rules for vibrational spectra for silyl radicals of different composition and different geometry. 

The spin adducts of free radicals and MNP or DMPO were observed by means of an ESR spectrometer. The data of hyperfine splitting constants were compiled in Tables 9 and 10 [40-42,44,45]. ESR studies on the initial free radicals revealed that the monoalkylamino radical RHN-, dialkylamino radical R2N-, and aminomethyl radical -CH2N< or aminoethylidene radical >N( CHCH3) were obtained from the corresponding primary, secondary, and cyclic tertiary amine. In case of a tertiary diamine such as TMEDA, formation of... 

TABLE 3. Hyperfine splitting constants (gauss) of some sulfur-cpntaining aromatic radical anions obtained in 1,2-dimethoxyethane with K metal as the reductant at — 80 °C... 

TABLE 4. Hyperfine splitting constants (gauss) of thioxanthone S-(mono or dijoxide radical anions as free or tight contact ion-pair ... 

The ESR spectra of a large variety of sulfonyl radicals have been obtained photolytically in liquid phase over a wide range of temperature. Some selected data are summarized in Table 2. The magnitudes of hyperfine splittings and the observations of line broadening resulting from restricted rotation about the C—S bond have been used successfully in conjunction with INDO SCF MO calculations to elucidate both structure and conformational properties. Thus the spin distribution in these species is typical of (T-radicals with a pyramidal center at sulfur and in accord with the solid-state ESR data. 

In Table 3 are listed the proton hyperfine splittings (o ) for. the anion and cation radicals of heptafulvalene, calculated using McConnell s... 

Spin-density distributions are inherent features of free radicals. Esr experiments take place when the radical is in its electronic ground state and the measurement of the spin distribution constitutes only a minute perturbation of the system. This feature and the fact that esr hyperfine splitting can be measured with high precision makes the esr method ideally suited for the study of substituent effects. Therefore, if spin delocalization is accepted as a measure of stabilization, the data in Table 6 provide quantitative information. However, these are percentage values and not energies of stabiliza-... 

Table 9.4 Isotropic hyperfine splittings (G) in the methyl and fluoromethyl radicals...

The increasing s-character of the singly occupied molecular orbital (SOMO) necessary for deviation of geometry of the radical from planarity is evident in the hyperfine splitting observed. There is an increase in the hyperfine splitting constant ( ) value which accompanies the increase in s-character and the concomitant decrease in planarity of the radical95 (see Table 9). 

TABLE 28. Isotropic hyperfine splittings (Gauss) of the cyclopropyl radical computed at the UMP2/DZ + P level250... 

One remarkable feature of these results (Baird and Thomas, 1961) is that the a-protons in the radicals PhNHO and Me3C. NHO give no detectable hyperfine splitting, although the components are quite narrow. It is postulated that either the oxime tautomer is more stable, or that there is a rapid tautomeric equilibrium. In view of the internal consistency of these results (Table 8) it seems probable that the radicals are all of the same form, in which case all that is needed is an acid-base equilibrium sufficiently fast that the lines are not appreciably broadened. 

ESR spectroscopy is perhaps the best method for the unequivocal detection and observation of free radicals, and ESR 13C hyperfine splitting (hfs) constants are considered to be a very useful indicator of a radical s geometry because non-planarity introduces s character into the orbital that contains the unpaired electron. The methyl radical s 13Ca value of 38 G is consistent with a planar structure. Fluoromethyl radicals exhibit increased 13Ca values, as shown in Table 1, thus indicating increasing non-planarity, with trifluoromethyl radical s value of 272 G lying close to that expected for its sp3 hybridization [4]. 

Table 1. ESR hyperfine splitting constants for fluorinated methyl radicals [4]...

Table 5 Hyperfine splitting constants of SCB anion radicals...

The SiF3 radicals obtained from radiolysis of HSiF3 in SF6 at 77 K have been investigated by esr the results are presented in Table 27. The large value of the isotropic 29Si hyperfine splitting indicates a strongly pyramidal structure, probably due to the polarity of the SiF bond. 

Table 2 Dihedral angles of 3-methylene protons of tyrosyl radicals in PSII and R2 and the hyperfine splitting in the EPR spectra...

Table I H and C Hyperfine Splittings (Gauss G) for Cyclopropyl and 1-Methyl-cyclopropyl Radical in Cyclopropane at -70 C...

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