O-H bond dissociation enthalpies in oximes: Order restored

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DOIResolve DOI: http://doi.org/10.1021/ja047566y
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Journal titleJournal of the American Chemical Society
Pages1066710675; # of pages: 9
SubjectChemical bonds; Dissociation; Electrons; Enthalpy; Hydrogen; Pyrolysis; Alklyloximes; Aromatic rings; Delocalization; Oximes; Oxygen; alkyl group; aromatic compound; azobenzene; benzene; hydrogen; iminoxyl radical; oxime; oxygen; radical; unclassified drug; article; calculation; chemical reaction; chemical structure; conformational transition; decomposition; dissociation; electron spin resonance; electron transport; enthalpy; geometry; heat transfer; hydrogen bond; reaction analysis; stereospecificity; structure analysis
AbstractThe O-H bond dissociation enthalpies (BDEs) of 13 oximes, RR′C=NOH, having R and/or R′ = H, alkyl, and aryl are reported. Experimental anchor points used to validate the results of theoretical calculations include (1) the O-H BDEs of (t-Bu)2C=NOH, t-Bu(i-Pr)C=NOH, and t-Bu(1-Ad)C=NOH determined earlier from the heat released in the reaction of (t-Bu) 2C=NO. with (PhNH)2 in benzene and EPR spectroscopy (Mahoney, L. R.; Mendenhall, G. D.; Ingold, K. U. J. Am. Chem. Soc. 1973, 95, 8610), all of which were decreased by 1.7 kcal/mol to reflect a revision to the heat of formation of (E)-azobenzene (which has significant ramifications for other BDEs) and to correct for the heat of hydrogen bonding of (t-Bu)2C=NOH (α2H = 0.43 measured in this work) to benzene, and (2) the measured rates of thermal decomposition of six RR′C=NOCH2Ph at 423 or 443 K, which were used to derive O-H BDEs for the corresponding RR′C=NOH. Claims (Bordwell, F. G.; Ji, G. Z. J. Org. Chem. 1992, 57, 3019; Bordwell, F. G.; Zhang, S. J. Am. Chem. Soc. 1995, 117, 4858; and Bordwell, F. G.; Liu, W.-Z. J. Am. Chem. Soc. 1996, 118, 10819) that the O-H BDEs in mono- and diaryloximes are significantly lower than those for alkyloximes due to delocalization of the unpaired electron into the aromatic ring have always been inconsistent with the known structures of iminoxyl radicals as are the purported perpendicular structures, i.e., φ(C α-C=N-O.) = 90°, for sterically hindered dialkyl iminoxyl radicals. The present results confirm the 1973 conclusion that simple steric effects, not electron delocalization or dramatic geometric changes, are responsible for the rather small differences in oxime O-H BDEs.
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AffiliationNational Research Council Canada (NRC-CNRC)
Peer reviewedYes
NPARC number21276477
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Record identifier439802e1-d95f-448c-a9f6-d48dfb83f4fd
Record created2015-10-13
Record modified2016-05-09
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