“…The electronic structure of iminoxyl radicals has been a controversial problem. From EPR spectroscopy, Thomas 50 found significant spin density on nitrogen in an orbital with considerable s character and therefore deduced a σ-structure for iminoxyl radicals, a conclusion confirmed by subsequent workers. ,, Iminoxyl radicals are described as …”
Section: Resultsmentioning
confidence: 78%
“…(2) What are the effects of various substitutions on the oxime O−H BDEs? This particular question has not been adequately studied previously, not only due to the lack of reliable experimental data but also because of the fact that the electronic structure of aryl- and alkyl-substituted iminoxyl radicals has been controversial for many years. ,,,,− …”
Section: Introductionmentioning
confidence: 99%
“…This particular question has not been adequately studied previously, not only due to the lack of reliable experimental data but also because of the fact that the electronic structure of aryl-and alkyl-substituted iminoxyl radicals has been controversial for many years. 40,42,43,45,[50][51][52] 2. Computational Methods BDE is defined as the enthalpy change of the following reaction in the gas phase at 298.15 K and 1 atm Specifically, the O-H BDE in the molecule RR′CdNOH is estimated from the expression…”
By using a multilayer composite ab initio method ONION-G3B3, we calculated O-H bond dissociation enthalpies (BDEs) of 58 oximes that were measured experimentally. Experimental BDEs derived from thermal decomposition kinetics and calorimetric measurements were found to be consistent with the theory. However, the electrochemical method was found to give questionably high BDEs possibly due to errors in the measurement of pKa's or redox potentials. Subsequently, the performances of a variety of DFT functionals including B3LYP, B3P86, B3PW91, BHandH, BHandHLYP, BMK, PBE1PBE, MPW1KCIS, mPWPW91, MPW1B95, and MPW1K were tested to calculate oxime O-H BDEs, where ROBHandHLYP was found to be the most accurate. By using this method, we calculated O-H BDEs of over 140 oximes in a systematic fashion. All of the calculated O-H BDEs fell in the range from 76.8 to 89.8 kcal/mol. An amino group on the azomethine carbon was found to strengthen the O-H bond, whereas bulky alkyl substituents on oximes decreased O-H BDEs due to their large steric-strain-relieving effects in the process of O-H bond cleavage. Para substituents had little effect on the BDEs of benzaldoximes and phenyl methyl ketoximes. Finally, on the basis of a spin distribution calculation, aryl-, alkyl-, and carbonyl-substituted iminoxyl radicals were found to be sigma-radicals, whereas amino-substituted iminoxyl radicals were of pi-structure.
“…The electronic structure of iminoxyl radicals has been a controversial problem. From EPR spectroscopy, Thomas 50 found significant spin density on nitrogen in an orbital with considerable s character and therefore deduced a σ-structure for iminoxyl radicals, a conclusion confirmed by subsequent workers. ,, Iminoxyl radicals are described as …”
Section: Resultsmentioning
confidence: 78%
“…(2) What are the effects of various substitutions on the oxime O−H BDEs? This particular question has not been adequately studied previously, not only due to the lack of reliable experimental data but also because of the fact that the electronic structure of aryl- and alkyl-substituted iminoxyl radicals has been controversial for many years. ,,,,− …”
Section: Introductionmentioning
confidence: 99%
“…This particular question has not been adequately studied previously, not only due to the lack of reliable experimental data but also because of the fact that the electronic structure of aryl-and alkyl-substituted iminoxyl radicals has been controversial for many years. 40,42,43,45,[50][51][52] 2. Computational Methods BDE is defined as the enthalpy change of the following reaction in the gas phase at 298.15 K and 1 atm Specifically, the O-H BDE in the molecule RR′CdNOH is estimated from the expression…”
By using a multilayer composite ab initio method ONION-G3B3, we calculated O-H bond dissociation enthalpies (BDEs) of 58 oximes that were measured experimentally. Experimental BDEs derived from thermal decomposition kinetics and calorimetric measurements were found to be consistent with the theory. However, the electrochemical method was found to give questionably high BDEs possibly due to errors in the measurement of pKa's or redox potentials. Subsequently, the performances of a variety of DFT functionals including B3LYP, B3P86, B3PW91, BHandH, BHandHLYP, BMK, PBE1PBE, MPW1KCIS, mPWPW91, MPW1B95, and MPW1K were tested to calculate oxime O-H BDEs, where ROBHandHLYP was found to be the most accurate. By using this method, we calculated O-H BDEs of over 140 oximes in a systematic fashion. All of the calculated O-H BDEs fell in the range from 76.8 to 89.8 kcal/mol. An amino group on the azomethine carbon was found to strengthen the O-H bond, whereas bulky alkyl substituents on oximes decreased O-H BDEs due to their large steric-strain-relieving effects in the process of O-H bond cleavage. Para substituents had little effect on the BDEs of benzaldoximes and phenyl methyl ketoximes. Finally, on the basis of a spin distribution calculation, aryl-, alkyl-, and carbonyl-substituted iminoxyl radicals were found to be sigma-radicals, whereas amino-substituted iminoxyl radicals were of pi-structure.
“…The difference in ratios is ascribed to a diminished steric influence of the asymmetric iminoxy function as compared to the oxime (an example is given in ref. 6).…”
XL3STKACTAlkyl S,4,6-tri-O-acetyl-2-deoxy-2-hydroxyimino-~-D-ur-trhirlcland -/_l*_\-o-hexopyranosides were oxidised with lead(W) acetate in solution, yielding the corresponding Z-and E-isomeric iminoxy radicals (: C= N'O), which gave very intense and persistent electron paramagnetic resonance (e-p-r.) signals. Applying previous results for cyclohexane iminoxy radicals and the differences between the spectra of the err-rtbirlo and I~s-0 compounds, it has been possible to assign the observed, hydrogen hyperfine structure for both Z-and E-radicals to the hexopyranoid-ring hydrogen atoms at C-1,3,4, and 5 or 6. From Z-and E-oximes, the same Z/E-equilibrium mixture of iminoxy radicals is obtained. Also, conclusions on the conformation of the hexopyranoid ring have been drawn. based on solvent and temperature effects. The specificity and sensitivity of the novel e-p-r. method have been used to characterise g-D-oximes in pg-amounts in t.1.c. The presence of other (mainly /I') oximes has also been demonstrated.
“…The previous EPR studies revealed that oxime radicals should be represented by resonance structures A and B (Figure 1). 6, 8a–e Later computational studies showed that the single‐electron spin densities are located on the O atom (0.54–0.58) and on the N atom (0.45–0.47) in oxime radicals 10. Our own computational study also gave the similar spin densities on O and N atoms in the ketoxime radical C .…”
Mit Diethyl‐DEAD und TEMPO gelang die intramolekulare Addition von Oximradikalen an CC‐Bindungen zur Bildung von 4,5‐Dihydroisoxazolen. Die Reaktion verläuft über eine 5‐exo‐trig‐Cyclisierung unter C‐O‐Kupplung. γ,δ‐Ungesättigte Ketoxime reagierten ebenfalls und ergaben cyclische Nitrone. Die Reaktion ist ein metallfreier Ansatz für die vicinale Difunktionalisierung nichtaktivierter Alkene.
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