G value of radicals

In ESR, the g-values of radicals reflect their electronic structure. In NMR, especially in proton-NMR, chemical shifts are used for different transition fields due to different situations of different spins. We consider that chemical shifts are due to the shielding of an external magnetic field (B) by electrons in a molecule. When the shielding constant of a proton is written by o, the field felt by this proton becomes B(l-a). We usually adopt... 

Acid Solutions. There are relatively few systems in which the G values of radicals have been measured as functions of acidity. The principal systems where this has been done are Fe2+-02 (6), Fe2+-N20 (31), C0-02 (11), ethyl alcohol-02 ( 45), formic acid-02 ( 39, 40, 41), and Br"-02 (17, 67) systems. All these systems revealed that the yield of the radicals increased with acidity, whereas Gred increases by about 0.9 units from pH 7 to 0.3. 

Polym. Method for determining the G value of radicals bv kinetics of polymerisation... 

The rate of initiation R[ is proportional to the G-value of radical formation in the system, i.e., it is proportional to the dose rate. Therefore, the overall rate of polymerization Rp is proportional to the square root of the dose rate (O Eq. (23.100)). 

The g-factors of radicals 101 and 102 are 2.0065 and 2.0059, respectively. The pyrazolylnitroxides have effective magnetic moments at room temperature corresponding to the standard values for one unpaired electron per molecule (1.71 0.05 B.M.). The values of effective magnetic moments of the nitroxyls practically do not change in the temperature range 5-300K. 

The theory of radiation-induced grafting has received extensive treatment. The direct effect of ionizing radiation in material is to produce active radical sites. A material s sensitivity to radiation ionization is reflected in its G value, which represents the number of radicals in a specific type (e.g., peroxy or allyl) produced in the material per 100 eV of energy absorbed. For example, the G value of poly(vinyl chloride) is 10-15, of PE is 6-8, and of polystyrene is 1.5-3. Regarding monomers, the G value of methyl methacrylate is 11.5, of acrylonitrile is 5.6, and of styrene is >0.69. 

The rate of free radical production from Am and B are dependent on the G value of both ... 

Kemp and coworkers employed the pulse radiolysis technique to study the radiolysis of liquid dimethyl sulfoxide (DMSO) with several amines as solutes [triphenylamine, and N, A, A, N -tetramethyl-p-phenylenediamine (TMPD)]. The radiolysis led to the formation of transient, intense absorptions closely resembling those of the corresponding amine radical cations. Pulse radiolysis studies determine only the product Ge, where G is the radiolytic yield and e is the molar absorption. Michaelis and coworkers measured e for TMPD as 1.19 X 10 m s and from this a G value of 1.7 is obtained for TMPD in DMSO. The insensitivity of the yield to the addition of electron scavenger (N2O) and excited triplet state scavenger (naphthalene) proved that this absorption spectrum belonged to the cation. 

Multifrequency HF-EPR permits precise determination of the g-values of the exchange-coupled organic radical metal ion species, provides parameters for accurate simulation of the EPR spectra, and allows determination of detailed information about the radicals themselves and their environment (Gerfen et al. 1993, Un et al. 1995, Bar et al. 2001, Ivancich et al. 2001). 

The g-tensor principal values of radical cations were shown to be sensitive to the presence or absence of dimer- and multimer-stacked structures (Petrenko et al. 2005). If face-to-face dimer structures occur (see Scheme 9.7), then a large change occurs in the gyy component compared to the monomer structure. DFT calculations confirm this behavior and permitted an interpretation of the EPR measurements of the principal g-tensor components of the chlorophyll dimers with stacked structures like the P 00 special dimer pair cation radical and the P700 special dimer pair triplet radical in photosystem I. Thus dimers that occur for radical cations can be deduced by monitoring the gyy component. 

Since the g-values of organic and organometallic free radicals are usually in the range 1.9-2.1, the free electron value is a good starting point for describing the experiment. 

The g-value of a free electron is a scalar, ge = 2.00232. In a radical species, g becomes a matrix because of the admixture of orbital angular momentum into S through spin-orbit coupling. The components of the g-matrix thus differ from ge to the extent that p-, d-, or f-orbital character has been incorporated, and they differ from one another, depending on which p-, d-, or f-orbitals are involved. 

Formation of cuprene is either by a free-radical chain reaction or by clustering around the parent ion (cluster size 20) followed by neutralization, which is not a chain process. The M /N value for decomposition of acetylene is about 20, giving the corresponding G value as 70-80, which is very large. The G value of benzene production is 5, whereas the G of conversion of monomers into the polymer is 60. 

Due to the high initiation rate and low (room) temperature, chains for oxidation of alkanes are short and many products are formed by disproportionation of peroxyl and hydroperoxyl radicals. The G values of the products of radiolytic oxidation of four alkanes are given in the following table [233] ... 

The triaryl radicals of germanium exhibit lower g values than their trialkyl counterparts. In part, this arises from increased delocalization of the unpaired spin density onto the aryl rings (and the Ar3Ge radicals do show hyperfme coupling to the ring protons). For example, spin densities for the radicals PhmMe3 mGe, calculated by the Hiickel method (Table 2), reveal that there is a linear correlation between the g value of the radical and... 

EPR-IR. The neutral sandwich complexes of Lu and the one-electron oxidized sandwich complexes of Zr have a jr-radical anion that can be observed by EPR spectroscopy. The EPR spectra for compounds 21, 23, 25, 27 and Collman s mixed porphyrin-pz system (30) give a signal for an S = j system with a g value of 2.0037, which is typical for an organic n radical (35). Further evidence for the Jt-radical character in sandwich compounds of phthalocyanines, porphyrins, and porphyrazines may be obtained from infrared (IR) spectroscopy by the presence of diagnostic marker bands (81). These intense bands are found in the IR spectra of Lu(III) compounds (21) (1150 cm-1) (35), 23 (1140 cm-1) (78), and 25 (1261 cm4) (34) and are absent in the Zr(IV) and Ce(IV) compounds 27,29, and 31. 

Thus, polymethacrylic acid undergoes net scission on gamma radiolysis with a G-value of approximately 4, while polyacrylic acid, on the other hand, undergoes net crosslinking with a G-value of approximately 1.2 (7). Crosslinking in polyacrylic acid is favourable because of the formation of main chain radicals. These can react to form crosslinks between polymer chains. 

Figure 2. G-value for radical production for poly(methacrylic acid-co-styrene) as a function of copolymer composition.

Two stable radical intermediates are observed following gamma radiolysis at 303 K. The alpha carbon radical VIII and the side chain radical IX are formed in approximately equal yields, with the total G-value for radical production equal to 3.2. This value is similar to that observed for the poly acids. The observed radicals are those which would be expected on the basis of the aliphatic carboxylic acids and previous studies of the poly amino acids with aliphatic side chains. 

---