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.     

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.     

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.     

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.     

S=O homolytic bond dissociation enthalpies in sulfoxides

The S=O bond dissociation enthalpies (BDE) were calculated using high-level ab initio methods including G3, G3B3, CBS-Q, CBS-4M, CCSD(T), and MP2. Based on the comparison of these theoretical values and experimental ones, the performances of a number of density functional theory (DFT) methods were then assessed. It was found that the B3P86 method gave the lowest root of mean square error. We therefore used this method to calculate the S=O BDEs of a number of substituted sulfoxides. The electronic effect of the substituents and the remote substituents effect of aryl-substituted sulfoxides on S=O BDE were investigated. In addition, a molecular orbital analysis of typical molecules was conducted in order to investigate the electronic effect on S=O BDEs. We also predicted several S=O BDE values of heteroaromatic substituted sulfoxides using the B3P86 method.     

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.     

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.     

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.     

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.     

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).     

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.     

Role of hyperconjugation in determining carbon-oxygen bond dissociation enthalpies in alkylperoxyl radicals

[reaction: see text] Theoretical calculations of carbon-oxygen bond dissociation enthalpies in substituted methylperoxyl radicals (YCH(2)OO*) reveal that bond strengths are not governed solely by the stability of YCH2* radicals but are largely affected by hyperconjugation when Y is electron-donating or conjugating. In many cases, this hyperconjugative effect is greater than stabilization of the methyl radical by Y. All electron-withdrawing Y exert small destabilizing effects via inductive withdrawal of electrons from the polarized C-OO* bond.     

C-H bond dissociation enthalpies in norbornane. An experimental and computational study

Gas-phase C-H bond dissociation enthalpies (BDEs) in norbornane were determined by quantum chemistry calculations and the C2-H BDE was experimentally obtained for the first time by time-resolved photoacoustic calorimetry. CBS-Q and CBS-QB3 methods were used to derive the values DH degrees (C1-H) = 449 kJ mol-1, DH degrees (C7-H) = 439 kJ mol-1, and DH degrees (C2-H) = 413 kJ mol-1. The experimental result DH degrees (C2-H) = 414.6 +/- 5.4 kJ mol-1 is in excellent agreement with the theoretical value. The trend DH degrees (C1-H) > DH degrees (C7-H) > DH degrees (C2-H) is discussed.     

Determination of enthalpies of formation of organic free radicals from bond dissociation energies

The values of C−H and C−I bond dissociation energies were used to calculate the enthalpies of formation (δH _f ^o of 20 cyclic and conjugated hydrocarbon radicals (R′). The values of δH _f ^o (R′) were analyzed in terms of the quantitative structure-property correlation based on the additive-group model, and the reliability of these data was shown. Based on the correlation, several strain energies of cycles and energies of conjugation of a lone electron with a ρ-system were calculated. The additive-group method for calculation of δH _f ^o can be extended for radicals of the naphthalyl type. For Part 2, see Ref. 1. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 286–288, February, 1999.     

An intrinsic radical stability scale from the perspective of bond dissociation enthalpies: a companion to radical electrophilicities

Bond dissociation enthalpies (BDEs) of a large series of molecules of the type A-B, where a series of radicals A ranging from strongly electrophilic to strongly nucleophilic are coupled with a series of 8 radicals (CH2OH, CH3, NF2, H, OCH3, OH, SH, and F) also ranging from electrophilic to nucleophilic, are computed and analyzed using chemical concepts emerging from density functional theory, more specifically the electrophilicities of the individual radical fragments A and B. It is shown that, when introducing the concept of relative radical electrophilicity, an (approximately) intrinsic radical stability scale can be developed, which is in good agreement with previously proposed stability scales. For 47 radicals, the intrinsic stability was estimated from computed BDEs of their combinations with the strongly nucleophilic hydroxymethyl radical, the neutral hydrogen atom, and the strongly electrophilic fluorine atom. Finally, the introduction of an extra term containing enhanced Pauling electronegativities in the model improves the agreement between the computed BDEs and the ones estimated from the model, resulting in a mean absolute deviation of 16.4 kJ mol(-1). This final model was also tested against 82 experimental values. In this case, a mean absolute deviation of 15.3 kJ mol(-1) was found. The obtained sequences for the radical stabilities are rationalized using computed spin densities for the radical systems.     

A theoretical study of five nitrates: Electronic structure and bond dissociation energies

Three density-functional methods (B3P86, B3PW91, and B3LYP) are employed to investigate the O–NO₂ bond lengths, frontier orbital energies, and O–NO₂ bond dissociation energies (BDEs) of n-propyl nitrate (NPN), isopropyl nitrate (IPN), 2-ethylhexyl nitrate (EHN), triethylene glycol dinitrate (Tri-EGDN), and tetraethylene glycol dinitrate (Tetra-EGDN). It is found that the O–NO₂ bond lengthens (destabilizes) in the order of IPN, NPN, EHN, Tetra-EGDN, and Tri-EGDN. From the data of frontier orbital energies (E_HOMO, E_LUMO), and energy gaps (ΔE), we estimate the relative thermal stability ordering of five nitrates and their corresponding radicals. The predicted BDEs of O–NO₂ bond in NPN, IPN, EHN, Tri-EGDN, and Tetra-EGDN, are 176.6, 174.5, 168.1, 156.1, and 159.3 kJ mol⁻¹, respectively. Based on the finding that the present results of BDEs are well coincident with the experimental results of apparent activation energies from the literature, we can draw a conclusion that the experimental thermolysis of five nitrates is only unimolecular homolytical cleavage of the O–NO₂ bonds.     

Periodic trends in bond dissociation energies. A theoretical study

Bond dissociation energies (BDEs) of all possible A-X single bonds involving the first- and second-row atoms, from Li to Cl, where the free valences are saturated by hydrogens, have been estimated through the use of the G3-theory and at the B3LYP/6-311+G(3df,2pd)//B3LYP/6-31G(2df,p) DFT level of theory. BDEs exhibit a periodical behavior. The A-X (A = Li, Be, B, Na, Mg, Al, and Si) BDEs show a steady increase along the first and the second row of the periodic table as a function of the atomic number Z(X). For A-X bonds involving electronegative atoms (A = C, N, O, F, P, S, and Cl) the bond energies achieve a maximum around Z(X) = 5. The same behavior is observed when BDEs are plotted against the electronegativity chi(X) of the atom X. Thus, for A-X bonds (A = Li, Be, B, Na, Mg, Al, Si), the BDEs for a fixed A increases, grosso modo, as the electronegativity differences between X and A increase, with some exceptions, which reflect the differences in the relaxation energies of the radicals produced upon the bond cleavage. A similar trend, albeit less pronounced, is found for single A-X bonds, where A = C, N, O, F, P, S, and Cl. However, there is an additional feature embodied in the enhancement of the strength of the A-boron bonds due to the ability of boron to act as a strong electron acceptor. The trends in bond lengths and charge densities at the bond critical points are in line with the aforementioned behavior.     

DFT study of the C-Cl bond dissociation enthalpies and electronic structure of substituted chlorobenzene compounds

Quantum chemical calculations were used to estimate the bond dissociation energies (BDEs) for 13 substituted chlorobenzene compounds. These compounds were studied by employing the hybrid density functional theory methods (B3LYP, B3PW1, B3P86) with 6-31G** and 6-311G** basis sets. It was demonstrated that B3P86/6-311G** method is the best method for computing the reliable BDEs for substituted chlorobenzene compounds which contain the C-Cl bond. It was found that the C-Cl BDE depends strongly on a computational method and basis set used. Substitution effect on the C-Cl BDE of substituted chlorobenzene compounds is further discussed. It is shown that the effects of substitution on the C-Cl BDE of substituted chlorobenzene compounds are very insignificant. Frontier orbital energy gap of studied compounds was also investigated. From the data on frontier orbital energies gap, we estimated the relative thermal stability of substituted chlorobenzene compounds.