Homolytic Bond Dissociation Research Articles

An AM1 MO study of bond dissociation energies in substituted benzene and toluene derivatives relative to the principle of maximum hardness

The heats of formation of a series of m- and p-substituted benzene and toluene derivatives, Ar-Y and ArCH 2-Y, and their phenyl or benzyl cations, anions, and radicals were calculated by the semiempirical AM1 MO method. Using this data and either the experimental values for the Y species or those obtained by the ab initio CBS-4 method, the heterolytic and homolytic bond dissociation energies (BDEs) were calculated along with the electron transfer energies for the ions. While the values of the homolytic BDEs were essentially independent of the ring substituents, a plot of the heterolytic BDEs versus the electron transfer energies gave a straight line of unit slope with an intercept at ΔH homo thus confirming that ΔH hot = ΔH ET + ΔH homo. Likewise, a plot of the appropriate HOMO or LUMO energies of the phenyl, benzyl, or Y ions versus ΔH hot gave a linear plot in agreement with the principle of maximum hardness. A positive charge adjacent to the bond being broken increases the ΔH homo value while a negative charge decreases ΔH homo [Display omitted] .

Equilibrium Acidities and Homolytic Bond Dissociation Enthalpies of the Acidic C-H Bonds in As-Substituted Triphenylarsonium and Related Cations(1).

Equilibrium acidities (pK(HA)s) of As-fluorenyltriphenylarsonium, As-phenacyltriphenylarsonium, six As-(para-substituted benzyl)triphenylarsonium [p-GC(6)H(4)CH(2)(+)AsPh(3)] (G = H, Me, CF(3), CO(2)Me, CN, and NO(2)), and six P-(para-substituted benzyl)tri(n-butyl)phosphonium [p-GC(6)H(4)CH(2)(+)P(n-Bu)(3)] (G = H, Me, CF(3), CO(2)Me, CN, and NO(2)) bromide salts, together with the oxidation potentials [E(ox)(A(-))] of their conjugate bases (ylides) have been determined in dimethyl sulfoxide (DMSO) solution. Introduction of an alpha-triphenylarsonium (alpha-Ph(3)As(+)) group was found to increase the adjacent C-H bond acidities by 13-20 pK units (18-27 kcal/mol). The equilibrium acidities for the two series p-GC(6)H(4)CH(2)(+)AsPh(3) and p-GC(6)H(4)CH(2)(+)P(n-Bu)(3) cations were found to be nicely correlated with the Hammett sigma(-) constants of the corresponding para-substituents (G) (Figures 1 and 2). The homolytic bond dissociation enthalpies (BDEs) of the acidic C-H bonds determined by using eq 1 reveal that an alpha-Ph(3)As(+) group increases the BDE value of the adjacent acidic C-H bond by 2-5 kcal/mol, whereas the substituent effects for an alpha-Ph(3)P(+) or alpha-(n-Bu)(3)P(+) group was found to be dependent on the nature of the substituents attached to the alpha-carbon atom. Good linear correlations were obtained for the equilibrium acidities of As-(para-substituted benzyl)triphenylarsonium and P-(para-substituted benzyl)tri(n-butyl)phosphonium cations with the oxidation potentials of their conjugate bases (ylides) as shown in Figures 3 and 4, respectively.

Hydride Affinities of Arylcarbenium Ions and Iminium Ions in Dimethyl Sulfoxide and Acetonitrile

Gibbs free energies for scission of C−H bonds leading to carbanion and proton (mode a), radical pair (mode b), and carbenium ion and hydride (mode c) have been determined for a series of acidic C−H bonds in ca. 45 weak acids. This involved the use of the equilibrium acidities (or homolytic bond dissociation enthalpies), redox potentials in DMSO or MeCN solution, and the appropriate thermodynamic cycles in the two solvents. The introduction of electron-donating groups generally results in small-to-negligible effects on ΔGR- values (mode a scission) but in a relatively large stabilizing influence on the ΔGR+ values for heterolytic cleavage to hydride and carbenium ion (mode c). Electron-withdrawing groups exert large stabilizing effects on ΔGR−, but their effects on ΔGR+ are dependent on the nature of the substituents. The heterolytic processes a are usually favored in solution due to the strong solvation of the proton. Homolytic scission (mode b) is usually more favorable than the corresponding heterolytic process c, but they can be comparable when strong electron-donating groups such as dialkylamino are present. Indeed, heterolytic cleavage to hydride and carbenium ion was ca. 10 kcal/mol more favorable than homolytic cleavage for a series of highly stabilized 2-benzoyl-N,N‘-dialkylperhydropyrimidines.

Effects of sulfenyl, sulfinyl and sulfonyl groups on acidities and homolytic bond dissociation energies of adjacent C?H and N?H bonds

Acidities and bond dissociation energies (BDEs) of the N–H bond in two phenylsulfenylamides, PhSNHBz and PhSNH-t-Bu, and four phenylsulfenylanilides, 4-GC6H4NHSPh, where G = MeO, H, Br and CN, were measured in order to compare the effects of substituents on acidities and BDEs of N—H bonds with those of C—H bonds. The effects of PhS groups on acidities and BDEs in a series of C—H acids were found to be comparable to those on acidities and BDEs of PhS in a similar series of N—H acids. Comparisons were also made of the effects of changing the oxidation state of sulfur in the series PhS, PhSO and PhSO2 on the acidities and BDEs of adjacent N—H and C—H bonds in weak acids. Hammett-type plots of pKHA values for phenyl benzyl sulfones (4-GC6H4CH2SO2Ph) and phenylsulfenylanilides (4-GC6H4NHSPh) were linear vs σp− values. A linear plot was obtained and explained for a plot of BDE of the N—H bonds in remotely substituted phenylsulfonylanilides with σ+ values. Plots of BDEs vs Eox(A−) were also linear for 4-substituted phenylsulfenylanilides (4-GC6H4NHSPh), phenylsulfonylanilides (4-GC6H4NHSO2Ph) and phenyl benzyl sulfones (4-GC6H4CH2SO2Ph). © 1998 John Wiley & Sons, Ltd.

Deprotonating Molecules and Free Radicals to Form Carbon-Centered Anions: A G2 ab Initio Study of Molecular and Free Radical Acidity

Molecules CH3X and free radicals •CH2X can be deprotonated to form carbon-centered anions CH2X- and radical anions HCX•-, respectively. We have studied the geometric and thermochemical changes that accompany such deprotonation processes for a variety of substituents X including the π-donor groups NH2, OH, OCH3, PH2, SH, F, Cl, and Br and the π-acceptor groups BH2, AlH2, CHO, NO2, CN, and NC. Thermochemical properties calculated and discussed include the gas-phase acidities of the molecules and free radicals, the electron affinities of the •CH2X free radicals, various dissociation energies, and the heats of formation of all species. The acidities of •CH2X radicals are predicted to be greater than those of CH3X for π-donor substituents but less for π-acceptor substituents (except CN and NC). The changes that are predicted to occur upon deprotonation in C−X bond lengths, C−X homolytic and heterolytic bond dissociation enthalpies, C−H homolytic bond dissociation enthalpies, and radical stabilization energies may be understood by examining the orbital interactions that take place in each species.

Radical Substituent Effects of α-Fluorine and α-Trifluoromethyl Groups

Homolytic bond dissociation enthalpies (BDEs) of the C−H bonds in 30 fluorinated hydrocarbons have been obtained from ab initio CBS-4 (complete basis set) model calculation at both 0 and 298 K. The calculated C−H BDEs agree with the available literature experimental results within ±2 kcal/mol. Introduction of one or two α-fluorine (α-F) substituent(s) into methane, ethane, or isopropane was found to weaken the sp3 C−H bond strength by 1−4 kcal/mol primarily due to the resonance delocalization of the unpaired electron of the radicals formed by removal of one hydrogen atom. In contrast, introduction of three α-F substituents in trifluoromethane (CHF3) was found to strengthen the sp3 C−H bond by about 1 kcal/mol. The 4−6 kcal/mol bond-strengthening effects of α-trifluoromethyl groups were attributed exclusively to their inductive effects. The substituent effects of fluorine and trifluoromethyl groups have also been examined on the strength of the ethylene sp2 C−H bonds and the acetylene sp C−H bonds. The ground-state effects of the polyfluorine substituents on the C−H BDE values have also been discussed.

A theoretical study on the modes of homolytic bond fragmentation in H nXYH m, HSXY and HS(O)XY systems

Recent studies in the authors' laboratories have involved studies on the formation and reactions of substituted alkoxy radicals derived by the photo-induced homolytic cleavage of the SO bond of substituted alkyl 4-nitrobenzenesulfenates ( 1), and the alkyl radicals formed by the β-scission of the alkoxy radicals. An interesting extension of these studies would be consideration of the homolytic cleavage of the SX and XY bonds in ArSXY and ArS(O)XY systems where X and Y are various combinations of nitrogen, oxygen and sulfur. Ab initio theoretical calculations have been carried out on various parent H-S-X-Y-H and HS(O)XYH systems and on the radical species formed by the homolytic dissociation of the SX and XY bonds using the G2MP2 method in order to determine the relative magnitudes of the homolytic bond dissociation energies (taken as the difference in the sum of the total energies of the product radicals minus the total energy of the parent system, E rexs). For calibration purposes, similar calculations have been carried out on hydrogen peroxide and hydrazine for which experimental homolytic bond dissociation energies have been measured. It is found that, in the latter calculations, the results obtained at the HF/UHF level give very poor correspondence with the experimental values, but the results obtained at the UMP2 and QCISD(T) levels nicely bracket the experimental values. The results of the calculations have given an indication of whether the SX or XY bond prefers to undergo homolytic cleavage. The results of these studies could very well be of significant importance in the areas of both physical and synthetic organic chemistry. In these homolytic cleavage reactions some very interesting radical species are formed, only a few of which have been studied previously experimentally or theoretically. The results of these studies indicate that the preferred mode of homolytic cleavage of the SX and XY bonds is essentially dependent only on the relative stability of the radicals formed in the two modes of homolytic bond cleavage.

Effects of double substitution on the thermodynamic stabilities of nitrogen radicals

The gas-phase homolytic bond dissociation energies (BDE) of the N—H bonds in forty-eight nitrogencontaining compounds (G-NH-G’) were investigated based on a cycle developed earlier utilizing acidity and electrochemical data in solution. From the relative BDE data thus derived, combined with the relevant BDE’s available in the literature, substituent effects on the thermodynamic stabilities of doubly-substituted nitrogen radicals were systematically studied. The experimental results show that the pattern of substituent effect on this type of nitrogen radicals is quite different from that on similarly substituted carbon radicals. It is generally observed that effects of the second substituent on radical stability are usually attenuated to a certain extent compared with that of the first one due to the socalled “saturation effect”. If, however, the second group is of the type that bears a pair of unshared electron (e.g. NH2, OH, etc.) and thus is capable of forming a two-center-three-electron bond with a neighboring nitrogen radical, or, it is such an electron-withdrawing group that can facilitate formation of an “electron-delocalizing channel” (e.g. Ph3P+, see the text), the two substituents (i. e. G and G’) may then act concertedly to result in a “synergistic effect” on the stability of nitrogen radicals.

The Role of Homolytic Bond Dissociation Energy in the Deprotonation of Cation Radicals. Examples in the NADH Analogues Series

The deprotonation of the cation radical of 9-cyanomethylacridane by a series of normal bases is investigated and its pKa and homolytic bond dissociation energy determined experimentally. The latter...

Hybrid density functional theory and quadratic complete basis set ab initio computational study of structural properties, vibrational spectra, and bond dissociation energy for methyl aluminum and methyl gallium

Geometries, harmonic frequencies, and homolytic and heterolytic bond dissociation energies were computed with ab initio and density functional theory (DFT) methods, the target being to determine the stability of GaCH 3 and AlCH 3, generate their infra-red spectra, and evaluate the accuracy of hybrid DFT methods for these and similar molecular systems against highly accurate ab initio methods. For this study, the ab initio methods HF, MP2, CCSD(T), and quadratic complete basis set (CBSQ) were selected. Their computational data were used for comparison with DFT (B3LYP, B3P86, and B3PW91) computed values. The stability and physical properties of the GaCH 3 and AlCH 3 organometallic complexes are discussed on the basis of these computational results.

Discrimination between hydrogen atom and proton abstraction in the quenching of n, π* singlet‐excited states by protic solvents

AbstractThe fluorescence quenching of the azoalkane 2,3‐diazabicyclo[2.2.2]oct‐2‐ene by protic solvents (methanol, methanol‐OD, water, deuterium oxide, acetic acid) has been examined. The pseudo‐unimolecular quenching rate constants (kq) vary from 0.30‐44×106s−1 and decrease upon deuteration of the solvent OH bonds, e.g., the isotope effect for methanol/methanol‐OD is ca. 8.5. This demonstrates that the hydroxylic hydrogens are predominantly responsible for the fluorescence quenching. The activation parameters were determined for methanol and methanol‐OD. The activation enthalpies are unexpectedly low (ΔH‡ = 1.8 kcal mol−1 for methanol) and increase upon deuteration (ΔH‡ = 3.0 kcal mol−1 for methanol‐OD), while the activation entropies remain the same (ΔS‡ ca. 17.5 cal K−1 mol−1. This provides evidence for a fully classical isotope effect related to differences in zero‐point vibrational energies. Tunneling appears to play no significant role. The quenching rate constants display no trend with the acidity of the solvent (pKa values) but with the homolytic bond dissociation energies of the OH bonds. This suggests the involvement of a hydrogen atom rather than a proton transfer. All important aspects (activation enthalpies, isotope effects, etc.) of this novel quenching mechanism of protic solvents are reproduced by MCSCF quantum‐chemical calculations with a complete active space of CAS (12,10) or CAS (8,7). Most importantly, the computed data indicate the occurrence of a conical intersection, i.e., a real surface crossing, which follows the transition state and provides an efficient trigger for radiationless return to the ground‐state energy surface (fluorescence quenching).

Homolytic Bond Dissociation Enthalpies of the C−H Bonds Adjacent to Radical Centers

Homolytic bond dissociation enthalpies (BDEs) at 0 and 298 K of the C−H bonds adjacent to various radical centers have been obtained from ab initio CBS-4 (complete basis set) model calculations and experimental data available in the literature. The BDEs of the C−H bonds adjacent to the radical centers derived from 11 saturated hydrocarbons were found to be 33.5 ± 3 kcal/mol at 298 K. The BDEs of the C−H bonds adjacent to nine allylic and benzylic radical centers were found to be 48 ± 3 kcal/mol at 298 K, but the benzylic C−H BDE of the PhCH2CH2• radical was found to be only 29.7 and 30.5 kcal/mol at 0 and 298 K, respectively. The BDEs of the vinylic C−H bonds adjacent to four vinylic radical centers were found to be 35.5 ± 3.5 kcal/mol at 298 K. The BDEs of the vinylic C−H bonds adjacent to three allylic radical centers were found to be 56.5 ± 3 kcal/mol at 298 K. These results suggest that the radical centers weaken the adjacent C−H bond strengths by about 50−70 kcal/mol. The calculated BDEs agree within...

ChemInform Abstract: Equilibrium Acidities and Homolytic Bond Dissociation Energies (BDEs) of the Acidic H—N Bonds in Hydrazines and Hydrazides.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Equilibrium acidities and homolytic bond dissociation enthalpies ofm- andp-substituted benzaldoximes and phenyl methyl ketoximes

The equilibrium acidities in DMSO of nine p- and m-substituted benzaldoximes and eight p-substituted phenyl methyl ketoximes were measured. Estimates of the homolytic bond dissociation energies (BDEs) of the acidic O—H bonds in these compounds were made by combination of their pKHA values with the oxidation potentials of their conjugate bases, Eox(A−), using the equation BDE = 1.37pKHA + 23.06Eox(A−) + 73.3 kcal (1 kcal = 4.184 KJ). Plots of Eox(A−) vs pKHA for p-substituted benzaldoximes and p-substituted phenyl methyl ketoximes were linear with slopes near unity. Consequently, as required by the above equation, the BDEs estimated for the O—H bonds in these oximes were constant, being 88.3 ± 0.3 and 89.2 ± 0.4 kcal, respectively. © 1998 John Wiley & Sons, Ltd.

Ab initio study of bond strengths in chlorinated ethane molecules and ethyl radicals

All the bond energies in chlorinated ethane molecules and ethyl free radicals have been calculated by ab initio molecular orbital methods. The bond strengths were determined using a direct homolytic bond dissociation reaction. All structures of interest were fully optimized at the MP2/6-31G(d,p) level of theory and were at their global minimum. A single-point energy was calculated for all compounds at the MP4/6-311G(d,p) level of theory. The zero-point energy corrected energy values of the reaction species were used in the calculations. Several trends in bond energies were observed. The results of the calculations showed that the α-substituted Cl atom has an obvious tendency to increase Cβ–H bond strength and decrease the vicinal Cα–H bond strength. The Cl atom has a tendency to make other C–Cl bonds weaker, both in radicals and in molecules. The Cl atom strongly affects the thermal stability of the radicals: all β-chlorinated ethyl radicals have a very weak C–Cl bond which causes them to decompose at low temperatures, whereas α-chlorinated radicals are relatively stable at elevated temperature. When a chlorinated ethyl radical decomposes, the most probable product is vinyl chloride.

Equilibrium acidities and homolytic bond dissociation enthalpies of m‐ and p‐substituted benzaldoximes and phenyl methyl ketoximes

Equilibrium acidities and homolytic bond dissociation enthalpies of m‐ and p‐substituted benzaldoximes and phenyl methyl ketoximes

Equilibrium Acidities and Homolytic Bond Dissociation Energies (BDEs) of the Acidic H−N Bonds in Hydrazines and Hydrazides

The equilibrium acidities in DMSO for phenylhydrazine, five of its p-substituted derivatives, 1,2-diphenylhydrazine, and 1,1,2-triphenylhydrazine were measured and the BDEs of their acidic N−H bonds were estimated by using the following equation: BDE = 1.37pKHA + 23.06Eox(A-) + 73.3 kcal/mol. The α-N−H bonds in the hydrazides CH3CONHNH2, PhCONHNH2, NH2NHCO2Et, and PhSO2NHNH2 were found to be 2 to 4 pKHA units more acidic than the α-N−H bonds in the corresponding amides, and the BDEs were estimated to be 23−27 kcal/mol weaker. Similarly, the BDE of a N−H bond in hydrazine was estimated to be 26 kcal/mol weaker than that of an N−H bond in NH3. Introduction of a RCO group into hydrazine had little or no effect on the BDE, but introduction of RCO into the β-position of PhCONHNH2 caused about an 8 kcal/mol increase in BDE. An increase in BDE was also observed for introduction of an RCO group into aniline. Here the carbonyl group is effectively destabilizing a nitrogen-centered radical by virtue of its strong electron-withdrawing effect. Incorporation of an open-chain carbohydrazide into a ring structure tends to strengthen the acidity of the N−H bond and weaken its BDEs.

Large Effect on Borane Bond Dissociation Energies Resulting from Coordination by Lewis Bases

Ab initio molecular orbital calculations at the G-2 and CBS-4 levels of theory were used to determine homolytic bond dissociation energies (BDE's) for the B−H bonds in a series of donor−acceptor complexes of borane. The B−H bonds of four-coordinate boron were found to be weaker than those of three-coordinate boron. The effect of complexation on BDE varied from very small, in the cases of NH3 and H2O, to as much as 30−50 kcal/mol, in the cases of formaldehyde, CO, and HCN. The BDE's were not found to correlate with the strength of coordination. However, they were closely correlated with the degree to which spin density in the radical was delocalized away from boron and onto the associated Lewis base. The presence of either a π system or, to some extent, a second-row element such as phosphorus or sulfur promoted such delocalization. Delocalization of spin density onto carbon appeared to be particularly favorable, and to correlate with particularly low B−H BDE's. The BDE's in 4-coordinate complexes of borane...

Organometallic Photochemistry: Basic Principles and Applications to Materials Chemistry

An overview is presented of organometallic photochemistry and its applications to materials chemistry. The excited states of typical organometallic complexes include ligand field excited states, metal-to-ligand charge-transfer excited states, ligand-to-metal charge-transfer excited states, Rydberg excited states, and excited states associated with metal-metal bonds. In general, each of these excited states leads to a particular type of photochemical reaction. Ligand field excited states typically lead to metal-ligand bond heterolysis, charge-transfer states to redox processes, and Rydberg states to homolytic metal-ligand bond dissociation. Excitations involving metal-metal bond states lead to metal-metal bond homolysis. Each of these photoprocesses has uses in materials chemistry. Metal-ligand bond dissociations are used to deposit thin films of metals or alloys. The thin films thus produced are used in the manufacture of semiconductors or microcircuitry. Likewise, metal-ligand bond dissociations are used...

ChemInform Abstract: Equilibrium Acidities and Homolytic Bond Dissociation Energies of N‐H and/or O‐H Bonds in N‐Phenylhydroxylamine and Its Derivatives.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.