O−H Bond Dissociation Enthalpies of Oximes:  A Theoretical Assessment and Experimental Implications

Abstract

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.

Oxime Radical Promoted Dioxygenation, Oxyamination, and Diamination of Alkenes: Synthesis of Isoxazolines and Cyclic Nitrones

“…[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. It means that oxime radicals can behave as nitrogen-centered radicals as well as oxygen-centered radicals in the reaction. This result also proves that oxime radicals are s radicals rather than p radicals.…”

“…[10] We anticipated that TEMPO might convert oximes into oxime radicals by a hydrogen atom abstraction (HAT) process. Previous studies indicate that the OÀH bond in an oxime has a relatively low bond dissociation energy (BDE, an average of about 83 kcal mol À1 ).…”

The Only Stable Organic Sigma Radicals: Di‐ tert ‐Alkyliminoxyls

“…More recently, the Mulliken spin densities on the oxygen and nitrogen have been calculated to be: O/N = +0.57/+0.45 for H 2 C=NO • , +0.57/+0.46 for Me 2 C=NO • , and +0.53/+0.48 for (Me 3 C) 2 C=NO • . 15 The oxygen and nitrogen spin densities for aromatic iminoxyls have been computed to be very similar, 16 for example, +0.58/+0.46 for PhCH=NO • and +0.57/+0.46 for Ph 2 C=NO • .…”

“…27 However, in 2004 "order was restored" and the correctness of the 1973 prediction 24 (revised downward by about 2 kcal/mol, vide supra) was confirmed both by experiment and by theory. 25,16 For example, the following R,R C=NO-H bond dissociation energies have been recommended 25 : 83.4, 84.6, and 81.6 kcal/mol for R = R = hydrogen, methyl, and phenyl, respectively.…”

Testing the ONIOM G2R3 model against donor–acceptor dissociation energies of group 13–15 complexes: Accuracy comparable to CCSD(T)/aug‐CC‐pVTZ at a fraction of the resource cost

“…Guo and coworkers 17 applied the ONIOM 1 composite concept to develop a twolayer ONIOM-G3B3 method, which gave deviations from experiment averaging \2 kcal mol 21 for BDEs in a database of 63 organic molecules, at a cost of requiring several more calculations per molecule than the OG2R3 approach. They have since applied this to interesting problems in organic chemistry 18 and biochemistry. 17 Gille and Gilbert 19 applied the OG2R3 approach to group 13-15 Lewis acid-base complexes, including Stephan's frustrated Lewis pair 20 (FLP) (F 5 C 6 ) 3 B/P(t-Bu) 3 .…”