Radical Stabilization Enthalpy Research Articles

The influence of substituents in governing the strength of the P-X bonds of substituted halophosphines R1R2P-X (X = F and Cl).

In this study, the gas-phase homolytic P-F and P-Cl bond dissociation energies (BDEs) of a set of thirty fluorophosphine (R1R2P-F) and thirty chlorophosphine-type (R1R2P-Cl) molecules have been obtained using the high-level W2 thermochemical protocol. For the R1R2P-F species, the P-F BDEs (at 298K) differ by up to 117.0kJmol-1, with (H3Si)2P-F having the lowest BDE (439.5kJmol-1) and F2P-F having the largest BDE (556.5kJmol-1). In the case of the chlorophosphine-type molecules, the difference in BDEs is considerably smaller (i.e., 72.6kJmol-1), with (NC)2P-Cl having the lowest P-Cl BDE (299.8kJmol-1) and (HO)2P-Cl having the largest (372.4kJmol-1). We have further analyzed the effect of substituents in governing the P-F and P-Cl BDEs by considering the effect of substituents in the parent halogenated precursors (using molecule stabilization enthalpies) and the effect of substituents in the product radicals (using radical stabilization enthalpies). Finally, we have also assessed the performance of a wide range of DFT methods for their ability to compute the gas-phase P-F and P-Cl BDEs contained in this dataset. We find that, overall, the double hybrid functional DSD-PBEB95 offers the best performance for both bond types, with mean absolute deviations of just 2.1 (P-F BDEs) and 2.2 (P-Cl BDEs) kJmol-1.

A Systematic Exploration of B-F Bond Dissociation Enthalpies of Fluoroborane-Type Molecules at the CCSD(T)/CBS Level.

Fluoroborane-type molecules (R1R2B-F) are of interest in synthetic chemistry, but to date, apart from a handful of small species (such as H2BF, HBF2, and BF3), little is known concerning the effect of substituents in governing the strength of the B-F bonds of such species toward homolytic dissociation in the gas phase. In this study, we have calculated the bond dissociation enthalpies (BDEs) of thirty unique B-F bonds at the CCSD(T)/CBS level using the high-level W1w thermochemical protocol. The B-F bonds in all species considered are very strong, ranging from 545.9 kJ mol-1 in (H2B)2B-F to 729.2 kJ mol-1 HBF2. Nevertheless, these BDEs still vary over a wide range of 183.3 kJ mol-1. The structural properties that affect the BDEs are examined in detail, and the homolytic BDEs are rationalized based on molecule stabilization enthalpies and radical stabilization enthalpies. Since polar B-F bonds may represent a challenging test case for density functional theory (DFT) methods, we proceed to examine the performance of a wide range of DFT methods across the rungs of Jacob's Ladder for their ability to compute B-F BDEs. We find that only a handful of DFT methods can reproduce the CCSD(T)/CBS BDEs with mean absolute deviations (MADs) below the threshold of chemical accuracy (i.e., with average deviations below 4.2 kJ mol-1). The only functionals capable of achieving this feat were (MADs given in parentheses): ωB97M-V (4.0), BMK (3.5), DSD-BLYP (3.8), and DSD-PBEB95 (1.8 kJ mol-1).

A quantum chemical study of the gas-phase homolytic S–Br bond dissociation enthalpies of sulfenyl bromides

Abstract This study reports the gas-phase homolytic S–Br bond dissociation enthalpies (at 298 K) for a set of 44 sulfenyl bromide derivatives (RSBr) obtained using the high-level G4 thermochemical protocol. For the species in this set, we find that the BDEs range from 178.0 kJ mol–1 (F3CSSBr) to 239.5 kJ mol–1 (HC(=NH)SBr). We have additionally reported molecule and radical stabilization enthalpies for all species, allowing for insights to be gained as to the effect of substituents in both the sulfenyl bromide reactants and the sulfur-centered radical products. In addition, we have also reported the BDEs of six sufinyl bromides (RS(=O)Br) and six sulfonyl bromides (RSO2Br). For the sulfinyl bromides, the BDEs range from 156.6 kJ mol–1 (F3CS(=O)Br) to 173.8 kJ mol–1 (tBuS(=O)Br), whilst for the sulfonyl bromides, the BDEs range from 211.0 kJ mol–1 (F3CSO2Br) to 226.2 kJ mol–1 (FSO2Br).

Computation of Relative Bond Dissociation Enthalpies (ΔBDE) of Phenolic Antioxidants from Quantum Topological Molecular Similarity (QTMS)

A recently proposed method called quantitative topological molecular similarity (QTMS) generated a model for the computation of the relative substituent effects on the bond dissociation enthalpies (DeltaBDEs) for a set of 39 phenols. The data set includes a diverse set of substituents with monosubstituted and poly-substituted derivatives that exhibit different electronic and steric effects. Many share common structural features with already well-established antioxidants. QTMS reveals the active region of the substituted phenols and identifies the electronic descriptors that best explain the range of DeltaBDEs observed. For substituents in the 4-X position (para) we find that our model requires a correction for radical stabilization enthalpy (RSE). Application of the QTMS methodology yields an unrivalled QSAR with r(2) = 0.98 and q(2) = 0.85 for the bond dissociation enthalpies of this phenolic antioxidant data set.

A detailed investigation of subsitituent effects on N-h bond enthalpies in aniline derivatives and on the stability of corresponding N-centered radicals.

The N-H bond dissociation enthalpies (BDE's) of 40 anilines (pGC(6)H(4)NHY) from series 1 to 4 with alpha-Y and p-G substituents and the stability of related radicals (pGC(6)H(4)Ndot;Y) were studied using ab initio (MP2) and density functional methods (B3LYP) with large basis sets. The results show that both methods reproduce earlier experimental BDEs within 2-3 kcal/mol and satisfactorily predict the alpha and remote substituent effects on BDEs (DeltaBDEs), as they reproduced the experimental DeltaBDEs within less than 1 kcal/mol. Furthermore, the conventional radical stabilization enthalpy (RSE = - DeltaBDE) was found to be invalid to represent the trend of the radical stabilization because the molecule effect (ME) can contribute more to RSE than the radical effect (RE) for certain series (1 and 4). These radicals are in fact stabilized by electron-withdrawing groups (EWGs) but destabilized by electron-donating groups (EDGs), a phenomenon just opposite to the observed O-behavior of many other aromatic heteroatomic radicals studied so far. These radicals are thus assigned as a new radical class, Class counter-O (or O) according to Walter's terminology. Moreover, the excellent multi-parametric Hammett-type correlations indicated that the para substituent effects on BDEs originate mainly from polar effects, but those on radical stability originate from both spin delocalization and polar effects. The atomic charge and spin population variations at a radical center due to p-G substitution were also found to correlate satisfactorily with REs. These results show that the spin delocalization effect should be explicitly considered in accounting for both DeltaBDEs and radical stabilization effects. Finally, an overall subsituent effect scale for radical stability has been proposed, and the overall substituent effect on the N-radicals was found to conform to the Capto-dative Principle.

Estimation of Bond Dissociation Energies and Radical Stabilization Energies by ESR Spectroscopy

Correlations of various indices of the stability and reactivity of carbon-centered radicals with ESR hyperfine splitting constants have been examined. For a large number of mono- and disubstituted radicals there is a moderately good linear correlation of α-proton hyperfine splitting constants (a(Hα)) with radical stabilization enthalpies (RSE) and with BDE(C−H), the C−H bond dissociation energies for the corresponding parent compounds determined from thermodynamic and kinetic studies of C−C homolysis reactions. There is a similarly satisfactory linear correlation of a(Hα) with BDE(C−H) determined by Bordwell's electrochemical and acidity function method. In all cases the correlations fail for nonplanar radicals. As expected, β-proton hyperfine splitting constants (a(HβMe)) for radicals with a freely rotating methyl substituent are less sensitive to deviations from planarity and give better linear correlations with RSE and BDE(C−H). The correlations cover a range of more than 20 kcal/mol and are reliable predictors of RSE and BDE(C−H) for a variety of radicals including captodative species. However, the correlations fail for significantly nonplanar radicals and for radicals with cyclic delocalized systems, e.g., cyclopentadienyl. The ratio a(HβMe)/a(Hα) for suitably substituted radicals provides an index of pyramidalization and allows one to decide for which compounds values of RSE and BDE(C−H) can be confidently estimated.

Are α-centered peptide radicals stabilized by a capto-dative effect?

The kinetics of the thermal C-C-cleavage reaction of the dimer of sarcosine anhydride 5 has been investigated between 295 and 333 °C in mesitylene. From the temperature dependence and from the release of strain on dissociation the cyclic α-peptide radical 6 was calculated to have a radical stabilization enthalpy (RSE) of −6.3 ± 1.3 kcal/mol thus indicating the absence of a synergistic capto-dative effect.

Thermochemical Stability of α-Amino-α-carbonylmethyl Radicals and Their Resonance As Measured by ESR

ESR spectra of the captodative alpha-amino-alpha-carbonylmethyl radicals 8 have been recorded. No coalescence temperature for the rotation of the two NMe groups was found at temperatures below the decomposition temperature of the radicals. From known coalescence temperatures and rotational barriers of substituted methyl radicals the rotational barrier of >/=17 kcal mol(-)(1) was estimated for the (*)C-N bond in the radicals 8. Enthalpies DeltaH(diss) and entropies DeltaS(diss) of the homolytic dissociation of 7a,c,d into 8a,c,d have been obtained from equilibrium measurements by ESR. By correcting for substituent interaction enthalpies in 7 (steric and geminal), a radical stabilization enthalpy RSE = -20.7 +/- 1.0 kcal mol(-)(1) was obtained for 8. By addition of the known RSEs of dialkylamino- and carbonyl groups, a RSE = -9.9 kcal mol(-)(1) is predicted for 8. The difference between the experimental and predicted values of 10.8 kcal mol(-)(1) is attributed to a synergistic captodative substituent effect. A linear correlation between the radical stabilization enthalpies of the radicals 8 and of other mono- and disubstituted alkyl radicals and their ESR aH(alpha) coupling constants was found. According to this correlation the reduction of aH(alpha) by 1 G corresponds to an increase in RSE of 1.57 kcal mol(-)(1). The large resonance of the captodative alpha-amino-alpha-carbonylmethyl radicals 3, expressed by their high RSE, their small aH(alpha) coupling constant, and their high rotational barrier, can be rationalized by a strong interaction between the alpha-amino and the alpha-carbonyl groups similar to that in amides and expressed in the resonance structures 6.

Substituent Effects on the CC Bond Strength, 20. – Geminal Substituent Effects, 14. – Stabilization of the Cyano(dimethylamino)methyl Radical – Synergistic effect Due to Interaction Between α‐Amino and α‐Cyano Groups on the Radical Stabilization Energy

AbstractThe thermolysis reactions of 3a and 2a, b were studied over a temperature range of 40°C and the activation parameters were determined. They were compared with the activation parameters of structurally comparable hydrocarbons of similar strain in order to obtain the radical stabilization enthalpies RSEs of the cyano(dimethylamino)methyl radical 1a. For this comparison the geminal interaction enthalpies of the cyano and the dimethylamino groups in the ground state had to be determined for the series of amino nitriles 4a–4c and 4j–4n by thermochemical methods. The geminal interaction in the ground state varies between −0.6 kcal/mol stabilization in 4a and a destabilization of +10.7 and +7.0 kJ mol−1 for secondary and tertiary α‐dialkylaminonitriles, respectively. A synergistic (i.e. more than additive) stabilization enthalpy of 26 kJ mol−1 of 1a was found in contrast to predictions in the literature. This stabilization is interpreted by conjugation between the substituents, which are separated by the radical center.

Substituent effects on the CC bond strength, 18. Stabilization of alkyl radicals by 4‐pyridyl, 1‐naphthyl or 2‐naphthyl substituents in α‐position

AbstractThe formation of the α‐substituted radicals 7–9 with the 4‐pyridyl (7), 1‐naphthyl (8), and the 2‐naphthyl (9) substituent by thermolysis of their dimers 1–3 in solution was investigated. The activation parameters of the homolysis of the central CC bond were obtained from kinetic measurements of the thermal cleavage of 1–3 in the presence of radical scavengers. These data were compared with those obtained from the thermolysis of alkanes which cleave into α‐alkyl branched radicals, and which lose the same amount of strain during the dissociation. The „strain‐free”︁ radical stabilization enthalpies of the α‐substituted radicals [RSE = 7.9 ± 1.8 (7), 12.0 ± 1.4 (8), and 8.7 ± 1.9 kcal/mol (9)] were obtained in this way.

Thermolabile hydrocarbons, 34. . Kinetics of the generation of 1,3‐di‐tert‐butylallyl radicals by thermal CC bond cleavage reactions

AbstractThe title radicals were generated by thermal decomposition of (E,E)‐meso‐ and (E,E)‐(±)‐5,6‐di‐tert‐butyl‐2,2,9,9‐tetramethyl‐3,7‐decadiene (4a, b), (E)‐5‐(1,1‐dimethylethyl)‐2,2,6‐trimethyl‐6‐phenyl‐3‐heptene (6), and (E)‐5‐(diphenyl‐methyl)‐2,2,6,6‐tetramethyl‐3‐heptene (7). The activation parameters of these reactions were calculated from the rate constants which were followed over a 40°C temperature range. The enthalpies of formation of 4a, b were determined from the enthalpies of combustion and enthalpies of vaporization, those of 6 and 7 by MM2 calculation. From these data and their comparison with those of comparable saturated hydrocarbons of similar strain the radical stabilization enthalpy of the title radical RSE = 11.4 ± 1.5 kcal/mol was determined, which is in good agreement with results obtained from other model compounds (12.6 kcal/mol) previously.

Substituent Effects on the C‐C Bond Strength, 15 [1] Geminal Substituent Effects, 7 [2] . Thermochemistry and Thermal Decomposition of Alkyl‐substituted Tricyanomethyl Compounds

The thermolysis reactions of the tricyanomethyl compounds 10a-c were studied in solution. 2,2-Dicyano-3-methyl-3-phenylbutyronitrile (10a) and 2,2-dicyano-3-methyl-3-(4-nitrophenyl)butyronitrile (10b) decomposed heterolytically into carbenium ions and (CN)3C− anions, while 9-methyl-9-(tricyanomethyl)fluorene (10c) underwent about 11% homolytic C-C bond cleavage into 9-methyl-9-fluorenyl- and tricyanomethyl radicals. The rates of the homolysis were determined by a radical scavenger procedure under conditions of pseudozero order kinetics. From the temperature effect on the rate constants the activation parameters were determined [ΔH (10c) = 155· 2 kJ mol−1, ΔS (10c) = 58· 5 J mol−1 K−1]. Standard enthalpies of formation ΔH (g) were determined for 2,2-dicyanopropionitrile (2) (422.45 kJ mol−1), 2,2-dicyanohexanenitrile (3) (349.74 kJ mol−1), 2,2-dicyano-3-phenylpropionitrile (4) (540.75 kJ mol−1), 2-butyl-2-methylhexanentrile (5) (-133.20 kJ mol−1), 2,2-dimethylpentanenitrile (6) (-45.78 kJ mol−1), and 2-methylbutyronitrile (7) (2.44 kJ mol−1) from the enthalpies of combustion and enthalpies of sublimation/vaporization. From these data and known Δ (g) values for alkanenitriles and -dinitriles, thermochemical increments for ΔH (g) were derived for alkyl groups with one, two, or three cyano groups attached. The comparison of these increments with those of alkanes reveals a strong geminal destabilization, which is interpreted by dipolar repulsions between the cyano groups. - From ΔH (g) of 10c and ΔH of its homolytic decomposition the radical stabilization enthalpy for the tricyanomethyl radical 1RSE (1) = -18 kJ mol−1 was determined. Thus, 1 is destabilized, in comparison with the RSEs of tertiary α-cyanalkyl (23 kJ mol−1) and α,α-dicyanoalkyl (27 kJ mol−1) radicals, which were recalculated from bond homolysis measurements[4] and the new thermochemical data. This change of RSE on increasing the number of α-cyano groups is discussed as the result of the additive contributions by resonance stabilization and increasing destabilization by dipolar repulsion. The amount of the dipolar energies was estimated by molecular mechanics (MM2).

Substituenteneffekte auf die CC‐Bindungsstärke, 14. Kinetische und thermodynamische Stabilität von 2,3‐Bis(dialkylamino)‐1,4‐diketonen — Stabilisierungsenergie capto‐dativ substituierter a‐Dialkylamino‐a‐Carbonylalkyl‐Radikale

Substituent Effects on the Strength of CC Bonds, 14[1]. — Kinetic and Thermodynamic Stability of 2,3‐Bis(dialkylamino)‐1,4‐diketones — Energy of Stabilization of α‐Dialkylamino α‐Carbonylalkyl Radicals with Capto‐dative SubstituentsProf. Dr. H.‐G. Viehe zum 65. Geburtstag gewidmet. The equilibrium constants and rate constants for the dissociation of the 2,3‐bis(dialkylamino)‐1,4‐diketone diastereomers meso‐ and DL‐7a and 7b were measured over a temperature range of 40°C. From the enthalpies of dissociation ΔHDiss and enthalpies of activation ΔH≠ and the strain enthalpies of 7 the bond dissociation enthalpies BDE(CC) of 7 were determined. By comparison with the dissociation enthalpies of Ct‐Ct alkanes[21] the change of these BDEs(CC) by the cap‐to‐dative substitution was determined to be 85.4 kJ mol−1 (20.4 kcal mol−1). The heats of formation ΔH (g) of a series of amino ketones 8 were determined from their heats of combustion and their heats of evaporation. From the ΔH(g) values in combination with MM2 calculations of their strain enthalpies strain‐free increments CHn[N, CO, C2 ‐ n] with n With n = 0, 1, 2 were derived and geminal interaction enthalpies in the ground states were obtained thereof. The radical stabilization enthalpy RSE of 6 was deduced from the ΔBDE(CC) values and the ground state effect to be 73.6 kJ mol−1 (17.6 kcal mol−1). From these data and the radical stabilization enthalpies RSE of α‐aminoalkyl radicals (4.2 kJ mol−1) and α‐carbonyl radicals (28.9 kJ mol−1) a synergetic radical stabilization enthalpy of 40.5 kJ mol−1 (9.7 kcal mol−1) is deduced. This number combines “extra” resonance stabilization and general inductive or anomeric geminal substituent interaction in the radicals. The crystal structure of meso‐7a has been determined by X‐ray diffraction methods.