O-H bond dissociation enthalpies in oximes: order restored

The 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 (alphaH2 = 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., phi(Calpha-C=N-O*) = 90 degrees, 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.     

Critical re-evaluation of the O-H bond dissociation enthalpy in phenol

The gas-phase O-H bond dissociation enthalpy, BDE, in phenol provides an essential benchmark for calibrating the O-H BDEs of other phenols, data which aids our understanding of the reactivities of phenols, such as their relevant antioxidant activities. In a recent review, the O-H BDE for phenol was presented as 90 +/- 3 kcal mol(-1) (Acc. Chem. Res. 2003, 36, 255-263). Due to the large margin of error, such a parameter cannot be used for dynamic interpretations nor can it be used as an anchor point in the development of more advanced computational models. We have reevaluated the existing experimental gas-phase data (thermolyses and ion chemistry). The large errors and variations in thermodynamic parameters associated with the gas-phase ion chemistry methods produce inconsistent results, but the thermolytic data has afforded a value of 87.0 +/- 0.5 kcal mol(-1). Next, the effect of solvent has been carefully scrutinized in four liquid-phase methods for measuring the O-H BDE in phenol: photoacoustic calorimetry, one-electron potential measurements, an electrochemical cycle, and radical equilibrium electron paramagnetic resonance (REqEPR). The enthalpic effect due to solvation, by, e.g., water, could be rigorously accounted for by means of an empirical model and the difference in hydrogen bond interactions of the solvent with phenol and the phenoxyl radical. For the REqEPR method, a second correction is required since the calibration standard, the O-H BDE in 2,4,6-tri-tert-butylphenol, had to be revised. From the gas-phase thermolysis data and three liquid-phase techniques (excluding the electrochemical cycle method), the present analysis yields a gas-phase BDE of 86.7 +/- 0.7 kcal mol(-1). The O-H BDE was also estimated by state-of-the-art computational approaches (G3, CBS-APNO, and CBS-QB3) providing a range from 86.4 to 87.7 kcal mol(-1). We therefore recommend that in the future, and until further refinement is possible, the gas-phase O-H BDE in phenol should be presented as 86.7 +/- 0.7 kcal mol(-1).     

O-H bond dissociation enthalpies of oximes: a theoretical assessment and experimental implications

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.     

Experimental and computational bridgehead C-H bond dissociation enthalpies

Bridgehead C-H bond dissociation enthalpies of 105.7 ± 2.0, 102.9 ± 1.7, and 102.4 ± 1.9 kcal mol(-1) for bicyclo[2.2.1]heptane, bicyclo[2.2.2]octane, and adamantane, respectively, were determined in the gas phase by making use of a thermodynamic cycle (i.e., BDE(R-H) = ΔH°(acid)(H-X) - IE(H(·)) + EA(X(·))). These results are in good accord with high-level G3 theory calculations, and the experimental values along with G3 predictions for bicyclo[1.1.1]pentane, bicyclo[2.1.1]hexane, bicyclo[3.1.1]heptane, and bicyclo[4.2.1]nonane were found to correlate with the flexibility of the ring system. Rare examples of alkyl anions in the gas phase are also provided.     

Accurate bond dissociation enthalpies by using doubly hybrid XYG3 functional

In this work, we examine the performance of XYG3, a newly developed doubly hybrid density functional (Zhang, Xu, and Goddard III, Proc Natl Acad Sci USA 2009, 106, 4963), to calculate covalent bond dissociation enthalpy (BDE). We use 5 atoms, 32 molecular radicals, and 116 closed-shell molecules to set up 142 bond dissociation reactions. For the total of 148 heats of formation (HOFs) and 142 BDEs, XYG3 leads to mean absolute deviations (MADs) of 1.45 and 1.87 kcal/mol, respectively. In comparison with some other functionals, MADs for HOFs are 2.31 (M06-2X), 2.98 (B2PLYP-D), 3.04 (BMK), 3.96 (B3LYP), 4.47 (B2PLYP), 5.42 (B2GP-PLYP), 6.46 (PBE0), and 29.93 kcal/mol (B3P86), and the corresponding errors for BDEs are 2.06 (M06-2X), 2.25 (BMK), 2.51 (B2PLYP-D), 2.89 (B2GP-PLYP), 3.30 (B3P86), 3.44 (B2PLYP), 3.87 (PBE0), and 6.14 kcal/mol (B3LYP).     

Understanding selenocysteine through conformational analysis,proton affinities,acidities and bond dissociation energies

Density functional methods have been employed to characterize the gas phase conformations of selenocysteine. The 33 stable conformers of selenocysteine have been located on the potential energy surface using density functional B3LYP/6‐31+G* method. The conformers are analyzed in terms of intramolecular hydrogen bonding interactions. The proton affinity, gas phase acidities, and bond dissociation energies have also been evaluated for different reactive sites of selenocysteine for the five lowest energy conformers at B3LYP/6‐311++G*//B3LYP/6‐31+G* level. Evaluation of these intrinsic properties reflects the antioxidant activity of selenium in selenocysteine. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

Determination of the substituent effect on the O-H bond dissociation enthalpies of phenolic antioxidants by the EPR radical equilibration technique

The bond dissociation enthalpies (BDE) of several phenols containing electron-withdrawing substituents in the para position have been determined by means of the EPR radical equilibration technique. It has been found that CN, NO(2), CHO, COOR, and COOH induce an increase of the BDE value of the O-H bond, thus producing a worsening of the antioxidant activity of phenols, while Cl, Ph, and CH[double bond]CHPh show an opposite effect. The contributions of these substituents for the calculation of the BDE values in polysubstituted phenols by using the group additivity rule have also been derived. It is shown that this rule provides quite reliable predictions of bond strengths, so that the method can be conveniently used to estimate new data on substituted phenols.     

AM1 calculations of O-H bond dissociation energies in polyoxides

Energies of homolytic cleavage of O-H bonds in 33 compounds of the general formula Ro _n H (n = 2, 3, and 4) were calculated by the AMI method. For hydrotrioxides and hydrotetroxides, the bond dissociation energies are virtually independent of the nature of the substituent R:D(RO _n -H) = 92.3±0.8 kcal mol^–1 (n = 3 and 4).     

The kinetics of the reactions of 2,6-di-t-butyl-4-methylphenol and 2,4,6-trimethylphenol with nitrogen dioxide in solution

The reaction of 2,6-di-t-butyl-4-methylphenol with nitrogen dioxide to form 2,6-di-t-butyl-4-methyl-4-nitrocyclohexa-2,5-dienone has a first order dependence of rate on the concentration of the species N₂O₄. By contrast the observed rate of the corresponding reaction of 2,4,6-trimethylphenol Is independent of the concentration of nitrogen containing species.     

A computational study of bond dissociation enthalpies in haloethenes

Carbon–hydrogen bond dissociation enthalpies (BDEs) were computed for all haloethenes, C₂H_4−nX_n (n=0–3, X=F, Cl, Br, I), at the B3LYP/6-311+G(3df,2p) level using isodesmic reactions. It was found that C–H bond strengths in the monohaloethenes varied substantially, by as much as 18 kJ mol⁻¹, dependent upon the bond's stereochemical position relative to the halogen. BDEs in the dihaloethanes varied in the order CX₂CH–H>(E)-CHXCX–H>(Z)-CHXCX–H. Trends in the computed bond enthalpies were discussed and explained on the basis of relative steric repulsions and hyperconjugative delocalization interactions, as determined from Natural Bond Orbital analysis.     

Comparison of experimental and computationally predicted sulfoxide bond dissociation enthalpies

The accurate estimation of S-O bond dissociation enthalpies (BDE) of sulfoxides by computational chemistry methods has been a significant challenge. One of the primary causes for this challenge is the well-established requirement of including high-exponent d functions in the sulfur basis set for accurate energies. Unfortunately, even when high-exponent d functions were included in Pople-style basis sets, the relative strength of experimentally determined S-O BDE was incorrectly predicted. The aug-cc-pV(n+d)Z basis sets developed by Dunning include an additional high-exponent d function on sulfur. Thus, it was expected that the aug-cc-pV(n+d)Z basis sets would improve the prediction of sulfoxide S-O BDE. This study presents the S-O BDE predicted by B3LYP, CCSD, CCSD(T), M05-2X, M06-2X, and MP2 combined with aug-cc-pV(n+d)Z, aug-cc-pVnZ, and Pople-style basis sets. The accuracy of these predictions was determined by comparing the computationally predicted values to the experimentally determined S-O BDE. Values within experimental error were obtained for dialkyl sulfoxides when the S-O BDEs were estimated using an isodesmic oxygen transfer reaction at the M06-2X/aug-cc-pV(T+d)Z level of theory. However, the S-O BDE of divinyl sulfoxide was overestimated by this method.     

Gas-phase reactivity of rare earth cations with phenol: Competitive activation of C-O, O-H, and C-H bonds

The gas-phase reactions of Sc⁺, Y⁺, and Ln⁺ (Ln=La-Lu, except Pm) ions with phenol were studied by Fourier transform ion cyclotron resonance mass spectrometry. All the ions except Yb⁺ were observed to react with the organic substrate, activating O-H, C-O, and/or C-H bonds, with formation of MO⁺, MOH⁺, and/or MOC₆H ₄ ⁺ ions as primary products. The product distributions and the reaction efficiencies obtained showed the existence of important differences in the relative reactivity of the rare earth metal cations, which are discussed in terms of factors like the electron configurations of the metal ions, their oxophilicity, and the second ionization energies of the metals. The primary product ions participated in subsequent reactions, yielding species such as M(OH)(OC₆H₅)⁺, which lead mainly to M(OC₆H₅)₂(HOC₆H₅) _n ⁺ ions, where n=0–2. Formation of M(OC₆H₅)(HOC₆H₅) _n ⁺ species was also observed in the case of the metals that have high stabilities of the formal oxidation state 2+, Sm and Eu.     

Substituent effects on the bond dissociation enthalpies of aromatic amines

Bond dissociation enthalpy differences, Z-X DeltaBDE = BDE(4-YC(6)H(4)Z-X) - BDE(C(6)H(5)Z-X), for Z = CH(2) and O are largely independent of X and are determined mainly by the stabilization/destabilization effect of Y on the 4-YC(6)H(4)Z(*) radicals. The effects of Y are small (< or =2 kcal/mol for all Y) for Z = CH(2), but they are large for Z = O, where good correlations with sigma(p)(+)(Y) yield rho(+) = 6.5 kcal/mol. For Z = NH, two sets of electrochemically measured N-H DeltaBDEs correlate with sigma(p)(+)(Y), yielding rho(+) = 3.9 and 3.0 kcal/mol. However, in contrast to the situation with phenols, these data indicate that the strengthening effect on N-H BDEs of electron-withdrawing (EW) Y's is greater than the weakening effect of electron-donating (ED) Y's. Attempts to measure N-H DeltaBDEs in anilines using two nonelectrochemical techniques were unsuccessful; therefore, we turned to density functional theory. Calculations on 15 4-YC(6)H(4)NH(2) gave N-H DeltaBDEs correlating with sigma(p)(+) (rho(+) = 4.6 kcal/mol) and indicated that EW and ED Y's had comparable strengthening and weakening effects, respectively, on the N-H bonds. To validate theory by connecting it to experiment, the N-H DeltaBDEs of four 4,4'-disubstituted diphenylamines and five 3,7-disubstituted phenothiazines were both calculated and measured by the radical equilibration EPR technique. For all compounds, theory and experiment agreed to better than 1 kcal/mol. Dissection of N-H DeltaBDEs in 4-substituted anilines and O-H DeltaBDEs in 4-substituted phenols into interaction enthalpies between Y and NH(2)/OH (molecule stabilization/destabilization enthalpy, MSE) and NH*/O* (radical stabilization/destabilization enthalpy, RSE) reveals that for both groups of compounds, ED Y's destabilize the molecule and stabilize the radical, while the opposite holds true for EW Y's. However, in the phenols the effects of substituents on the radical are roughly 3 times as great as those in the molecule, whereas in the anilines the two effects are of comparable magnitudes. These differences arise from the stronger ED character of NH(2) vs OH and the weaker EW character of NH* vs O*. The relatively large contributions to N-H BDEs in anilines arising from interactions in the molecules suggested that N-X DeltaBDEs in 4-YC(6)H(4)NH-X would depend on X, in contrast to the lack of effect of X on O-X and CH(2)-X DeltaBDEs in 4-YC(6)H(4)O-X and 4-YC(6)H(4)CH(2)-X. This suggestion was confirmed for X = CH(3), H, OH, and F, for which the calculated NH-X DeltaBDEs yielded rho(+) = 5.0, 4.6, 4.0, and 3.0 kcal/mol, respectively.     

Estimation of O-H bond dissociation energies in alcohols and acids from kinetic data

The O-H bond dissociation energies (D _O-H) in five alcohols and six acids have been determined from experimental data (rate constants of radical reactions). The ratio of the rate constants of the reactions R¹O˙+RH→R¹OH+R˙ and R ⁱ O˙+RH→R ⁱ OH+R˙ and the intersecting parabolas method are used in the estimation procedure. The D _O-H values are used to calculate the activation energies and rate constants for hydrogen abstraction from 2-methylbutane, butene-1, and cumene by alkoxyl and carboxyl radicals. The geometric parameters of the transition state are calculated for these reactions.     

N-H and alpha(C-H) bond dissociation enthalpies of aliphatic amines

Bond dissociation enthalpies (BDEs) of a large series of aliphatic amines (21) were measured by means of photoacoustic calorimetry. Despite the different structures studied in the primary, secondary, and tertiary amine series, the alpha(C-H) BDEs were found to be very similar for unconstrained amines with values very close to 91 kcal/mol. alphaC- and N-alkylation or introduction of an hydroxy group only slightly affect the BDEs, a fact in perfect agreement with calculations performed at different CBS levels. This demonstrates the predominance of the two-orbital-three-electron interaction involving the N and alphaC(*) orbitals. On the other hand, the N-H BDE decreases when going from primary to secondary amines. This result is interpreted in term of a hyperconjugation in sigmaC-C bonds, which leads to a stabilization of the aminyl radical. For cyclized amines, the BDEs depend on the relative geometry of the singly occupied alphaC(*) orbital with respect to that of the N atom, disfavoring the two-orbital-three-electron interaction. However, such structures can exhibit through-bond interaction. For a crowded structure such as triisopropylamine, for which the alphaC(*) orbital is not coplanar with the nitrogen one, the relaxation of a strain energy allows the BDE to be comparable to flexible structures.     

A new method for estimation of homolytic C-H bond dissociation enthalpies

In this work we have quantitatively analyzed substituent effects on the homolytic bond dissociation enthalpy of 79 different compounds using a method based on discrete distance dependent atomic contributions to a molecular property. The resulting empirical relationship can be used to predict C-H bond dissociation enthalpies (within +/-10 kJ mol(-1)) for molecules where resonance contributions and captodative stabilization are insignificant. For species where captodative stabilization of the corresponding C-centered radical is possible, the method clearly overestimates the C-H bond dissociation enthalpy.     

Correlating acidities, electron affinities, and bond dissociation energies. Measure one, get all three!

Bond energies, acidities, and electron affinities are related in a thermodynamic cycle, and by measuring two of these quantities one can derive the third. This procedure is most often used to determine bond dissociation energies. In this work, we show that Delta H(o)(acid)(HX), EA(X(*)), and BDE(HX) are all linearly related for specific classes of compounds (e.g., alcohols and carboxylic acids), and therefore, only one of these quantities is needed to determine the others. This should simplify thermodynamic determinations and enable hundreds of new values to be obtained from already existing data in the literature. The intercepts obtained from plots of Delta H(o)(acid)(HX) vs EA(X(*)) provide bond energies which correspond to BDE(HX) when EA(X(*)) = 0. The results for eight different types of compounds are provided and some of these values are discussed. Sensitivity factors are obtained from the slopes of these plots, and their sign (+ vs -) can be predicted by considering physical effects such as hybridization and resonance.     

Computational study on the bond dissociation enthalpies in the enolic and ketonic forms of beta-diketones: their influence on metal-ligand bond enthalpies

A computational study on the thermodynamic properties of 13 beta-diketones is presented. The B3LYP//6-311+G(2d,2p)//B3LYP/6-31G(d) theoretical approach was employed to compute the O-H and C-H bond dissociation enthalpies and enthalpy of tautomerization and to estimate standard gas-phase enthalpies of formation for the radicals and for the parent molecules. The gas-phase enthalpies of formation for the neutral molecules are in excellent agreement with available experimental data, supporting the estimates made for the radicals. The latter are very important for the clarification of the thermochemistry of many beta-diketonato metal complexes previously reported in the literature. Importantly, when substituents R = -CHR' are attached to the beta-diketone's scaffold, C-H homolytic bond cleavage is always favored with respect to O-H bond scission.