Kinetic and thermodynamic parameters for the formation of 3,5,5-trimethyl-2-oxomorpholin-3-yl (TM-3). A negative activation energy for radical combination
“…The transformation rate of bands b to c, to our surprise, increases to reach a maximum at T ≈ 328 K, then continuously decreases at higher temperatures. The Arrhenius plot of ln( k ) vs 1/ T shows a curve-up shape with positive slopes (i.e., negative E a values, see Figure ) as T > 328 K. In the literature, the “negative activation energy” was observed in the reactions of radicals with neutral molecules, − radical recombination, decay of triplet biradicals, a proton-transfer reaction, DNA triplet formation, and termolecular reactions . These observations were commonly fit to the following model where the reactant R is in preequilibrium with an intermediate I.…”
A stable, broad ESR signal (g = 1.998,
ΔH
pp = 37.0 G) of the
C60
•- anion radical was generated by
irradiation
of a C60−toluene solution in the presence of organic
salt,
[(ph)3P]2N+(ph)4B-,
and 14.3% methanol. Upon
exposure to molecular oxygen, the broad band gradually diminishes and a
narrow band of g = 2.0008 and
ΔH
pp = 3.32 G (hereafter, band b) grows,
which further transforms to another narrow band of g =
2.0026
and ΔH
pp = 1.67 G (hereafter, band c).
The transformation rate of bands b to c was found to be
negative
temperature dependent, i.e., the higher the temperature, the slower the
transformation rate. At high temperatures
(e.g., 365 K) and in polar solvents (e.g., 30% methanol in toluene),
band c can reversibly transform back to
band b. Microwave power saturation experiments show that band c
has much longer relaxation times than
band b. Both bands b and c resemble the “spike” commonly
observed in the C60
•- anion radical ESR
spectra
and were designated to two isomers of the
C60O2
•- anion radical. A
kinetic model was derived to account for
the negative temperature-dependent transformation rate of bands b to
c.
“…The transformation rate of bands b to c, to our surprise, increases to reach a maximum at T ≈ 328 K, then continuously decreases at higher temperatures. The Arrhenius plot of ln( k ) vs 1/ T shows a curve-up shape with positive slopes (i.e., negative E a values, see Figure ) as T > 328 K. In the literature, the “negative activation energy” was observed in the reactions of radicals with neutral molecules, − radical recombination, decay of triplet biradicals, a proton-transfer reaction, DNA triplet formation, and termolecular reactions . These observations were commonly fit to the following model where the reactant R is in preequilibrium with an intermediate I.…”
A stable, broad ESR signal (g = 1.998,
ΔH
pp = 37.0 G) of the
C60
•- anion radical was generated by
irradiation
of a C60−toluene solution in the presence of organic
salt,
[(ph)3P]2N+(ph)4B-,
and 14.3% methanol. Upon
exposure to molecular oxygen, the broad band gradually diminishes and a
narrow band of g = 2.0008 and
ΔH
pp = 3.32 G (hereafter, band b) grows,
which further transforms to another narrow band of g =
2.0026
and ΔH
pp = 1.67 G (hereafter, band c).
The transformation rate of bands b to c was found to be
negative
temperature dependent, i.e., the higher the temperature, the slower the
transformation rate. At high temperatures
(e.g., 365 K) and in polar solvents (e.g., 30% methanol in toluene),
band c can reversibly transform back to
band b. Microwave power saturation experiments show that band c
has much longer relaxation times than
band b. Both bands b and c resemble the “spike” commonly
observed in the C60
•- anion radical ESR
spectra
and were designated to two isomers of the
C60O2
•- anion radical. A
kinetic model was derived to account for
the negative temperature-dependent transformation rate of bands b to
c.
“…Much of the interest in this area has been aimed to determine the extent to which the combined stabilization provided by the substituents is synergistic. − The determination has not been straightforward, as it has been difficult to delineate the effects of radical stabilization from steric and polar effects, and other factors affecting radical formation. Nevertheless, it now seems clear that there is synergistic stabilization of amino carboxy substituted radicals ,,,− and additive, but not synergistic, stabilization of amido carboxy substituted radicals…”
Section: α-Carbon-centered Radicalsmentioning
confidence: 99%
“…Photochemical reduction of the imines 226a and 226b has been used to produce the dimers 228a and 228b , respectively, through coupling of the corresponding α-carbon-centered amino acid radicals 227a and 227b (Scheme ). ,, The coupling reaction is reversible, and in solution at room temperature, the dimers 228a and 228b exist in equilibrium with the corresponding radicals 227a and 227b . Through spontaneous carbon−carbon bond homolysis the diastereomers of the dimers 228a and 228b interconvert, they undergo oxidation in air to revert to the imines 226a and 226b , and they give mixtures of the imines 226a and 226b and the reduced analogues 229a and 229b as a result of disproportionation of the corresponding radicals 227a and 227b . − 26 …”
Section: Functional Group Transformationsmentioning
“…26 Also, the calculations of Katritzky, Zerner, and Karelson suggest that solvation effects are the sole cause of synergistic effects.' They concluded on this basis that the dipolar nature of the captodative radical (note 4b and 4c) leads to strong solvation.…”
Radical stabilization energies (RSEs) have been estimated for 9-RR'N-fluorenyl radicals, relative to the fluorenyl radical, by combining their ApKHA values with the oxidation potentials of their conjugate bases, AEox(A-). The RSEs for 9-RNH-fluorenyl radicals, with R = H, Bu, or PhCH,, and for the 9-azetidinylfluorenyl radical (13-15 kcal/mol) are all appreciably larger than those for the corresponding 9-RNCH3-fluorenyl radicals (7-8 kcal/mol). The RSEs for a-amino, a-(dimethylamino)-, and a-piperidinylacetophenonyl radicals, relative to the acetophenonyl radical, are all -21 kcal/mol; a-methoxy-and a-ethoxyacetophenonyl radicals have smaller RSEs (1 3 kcal/mol). a-Amine and a-(dimethy1amino)malononitrilyl radicals have RSEs, relative to the malononitrilyl radical, of 16 kcal/mol. Examination of these data indicates that N-alkylation has little or no effect on the stabilities of a-NH2 carbon-centered radicals. The possible role played by synergistic (captodative) effects in determining the size of RSEs for radicals bearing two substituents is discussed in light of the presence of saturation effects.
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