Heterolytic Bond Dissociation Research Articles

Alkali chloride cluster ion dissociation examined by the kinetic method: heterolytic bond dissociation energies, effective temperatures, and entropic effects

Branching ratios have been measured as a function of collision energy for the dissociation of mass-selected chloride-bound salt cluster ions, [Rb- 35Cl-M i] +, where M i = Na, K, Cs. The extended version of the kinetic method was used to determine the heterolytic bond dissociation energy (HBDE) of Rb-Cl. The measured value of 480.8 ± 8.5 kJ/mol, obtained under single collision conditions, agrees with the HBDE value (482.0 ± 8.0 kJ/mol), calculated from a thermochemical cycle. The observed effective temperature of the collisionally activated salt clusters increases with laboratory-frame collision energy under both single- and multiple-collision conditions. Remarkably, the effective temperatures under multiple collision conditions are lower than those recorded under single-collision conditions at the same collision energy, a consequence of the inability of the triatomic ions to store significant amounts of internal energy. Laboratory-frame kinetic energy to internal energy transfer (T→V) efficiencies range from 3.8 to 13.5%. For a given cluster ion, the T→V efficiency decreases with increasing collision energy. Many features of the experimental results are accounted for using MassKinetics modeling (Drahos and Vékey, J. Mass Spectrom. 2001, 36, 237).

Comprehensive thermodynamic characterization of the metal-hydrogen bond in a series of cobalt-hydride complexes.

A detailed structural and thermodynamic study of a series of cobalt-hydride complexes is reported. This includes structural studies of [H(2)Co(dppe)(2)](+), HCo(dppe)(2), [HCo(dppe)(2)(CH(3)CN)](+), and [Co(dppe)(2)(CH(3)CN)](2+), where dppe = bis(diphenylphosphino)ethane. Equilibrium measurements are reported for one hydride- and two proton-transfer reactions. These measurements and the determinations of various electrochemical potentials were used to determine 11 of 12 possible homolytic and heterolytic solution Co-H bond dissociation free energies of [H(2)Co(dppe)(2)](+) and its monohydride derivatives. These values provide a useful framework for understanding observed and potential reactions of these complexes. These reactions include the disproportionation of [HCo(dppe)(2)](+) to form [Co(dppe)(2)](+) and [H(2)Co(dppe)(2)](+), the reaction of [Co(dppe)(2)](+) with H(2), the protonation and deprotonation reactions of the various hydride species, and the relative ability of the hydride complexes to act as hydride donors.

Computational characterization of sulfur-cxygen-bonded sulfuranyl radicals derived from alkyl- and (carboxyalkyl)thiopropionic acids: evidence for sigma-type radicals.

Sulfide radical cations are known to stabilize via sulfur-oxygen bond formation with carboxylic acid functions. However, structural information on the resulting sulfuranyl radicals has so far only been provided by ESR spectroscopy for more stable, aromatic-substituted species, while the rather short-lived aliphatic sulfuranyl radicals have largely been characterized by time-resolved UV spectroscopy only. Therefore, we have obtained theoretical structural information on S-O-bonded sulfuranyl radicals from three model compounds, tert-butyl 2-(methylthio)peroxybenzoate, 3-methylthiopropionic acid, and 3,3'-thiodipropionic acid, using density functional theory, semiempirical, and molecular mechanics methods. All S-O-bonded species exist predominantly as three-electron-bonded sigma-radicals with an estimated heterolytic bond dissociation energy of the S therefore O bond on the order of 23-27 kcal mol(-1). Characteristic optical absorption bands, vibrational frequencies, and hyperfine coupling tensors are evaluated to facilitate the identification of such radicals by time-resolved UV, IR, and ESR spectroscopy.

Diene-Containing Half-Sandwich MoIII Complexes as Ethylene Polymerization Catalysts: Experimental and Theoretical Studies

Seventeen-electron compounds of MoIII having the general formula [(eta5-C5R5)Mo(eta4-diene)X2] (R = H, Me: dieney = butadiene, isoprene, or 2,3-dimethylbutadiene: X= Cl, CH3) are a new class of ethylene polymerization catalysts. The polyethylene obtained shows a bimodal distribution, the major weight fraction being characterized by very long (M around 10(6)) and highly linear polymer chains. The newly prepared pentamethylcyclopentadienyl (Cp*) derivatives are more active than the cyclopentadienyl (Cp) derivatives, but much less active than previously investigated niobiumIII compounds having the same stoichiometry. On the other hand, the turnover frequency of the active site leading to the high molecular weight chains is at least 10 times greater than that obtained with the corresponding Nb catalyst. The reason for the low activity is explained by a difficult activation process that is attributed to the low polarity and high strength of the Mo-alkyl bond. This is confirmed by a Mulliken charge analysis of density functional theory (DFT) geometry-optimized [CpM(eta4-C4H6)(CH3)2] (M = Nb, Mo) and by the calculation of the heterolytic bond dissociation energies. DFT calculations have also been carried out on the ethylene insertion coordinate for the [CpM(eta4-C4H6)(CH3)]+ model of the presumed active site. The results indicate an equivalent activation barrier to insertion for the Nb and Mo systems. Differences in optimized geometries for the reaction intermediates are attributed to the presence of the extra electron for the Mo system. This electron opposes the formation of M-H-C agostic interactions, while it strengthens the back-bonding M-ethylene interaction, but otherwise plays no active role in the polymer chain propagation mechanism. According to the calculations, the chain propagation for the Mo system occurs entirely on the spin doublet surface, the minimum energy crossover point with the quartet surface lying at a higher energy than the transition state for insertion on the doublet surface.

Theoretical, Thermodynamic, Spectroscopic, and Structural Studies of the Consequences of One-Electron Oxidation on the Fe−X Bonds in 17- and 18-Electron Cp*Fe(dppe)X Complexes (X = F, Cl, Br, I, H, CH3)

The compounds Cp*Fe(dppe)X ([Fe]X) and the corresponding cation radicals [Fe*]X*+ are available for the series X = F, Cl, Br, I, H, CH3. This has allowed for a detailed investigation of the dependence of the nature of Fe-X bonding on the identity of X and the oxidation state (charge) of the complex. Cyclic voltammetry demonstrates that the electrode potentials for the [Fe]X0/+ couples decrease in the order I > Br > Cl > H > F > CH3. An "inverse halide order" is seen, in which the most electronegative X leads to the most easily oxidized complex. This suggests that F is the best donor among the halides. The halide trend is also reflected in NMR spectroscopic data. Mössbauer spectroscopy data also suggest that the F ligand is a strong donor (relative to H and CH3) in [Fe*]X*+. DFT calculations on CpFe(dpe)X ([Fe]X) model complexes nicely reproduce the trend in the electrode potentials for the [Fe*]X0/+ couples. Analysis of the theoretical data within the halogen series indicates that the energy of the [Fe]X HOMO does not correlate with the extent of its Fe(d(pi))-X(p(pi)) antibonding character, which varies in the order I > Br > Cl > F, but rather depends on the destabilizing electrostatic effect caused by X. This effect varies in the order F > Cl > Br > I. A thermochemical cycle that incorporates the [Fe*]X0/+ and [Fe*]0/+ electrode potentials was used to investigate the effect of the oxidation state of the complex on the homolytic bond dissociation energy (BDEhom), defined for the processes Fe-X --> Fe* + X* and Fe-X*+ --> Fe*+ + X*. For all X, it was found that a one-electron oxidation leads to a weakening of the Fe-X bond. This trend was reproduced by the DFT calculations. On the other hand, IR nu(Fe-X) spectroscopy data showed an increase in the stretching frequencies for X = H and Cl upon oxidation. X-ray crystallographic data showed a shortening of the Fe-Cl bond upon oxidation. The trends in IR and Fe-Cl bond distances were reproduced in the DFT calculations. The combined data therefore suggest that oxidation leads to weaker, but shorter, Fe-X bonds. A second thermochemical cycle was applied to investigate the effect of the one-electron oxidation on the heterolytic bond dissociation energies (BDEhet), defined for the processes Fe-X --> Fe+ + X- and Fe-X*+ --> Fe2+ + X-. In this case, the oxidation led to bond strengthening in all cases. The computed BDE values have been analyzed within Ziegler's transition state methodology and decomposed into two components, one electrostatic and one covalent, describing the interaction between the unrelaxed fragments. In all the computed BDEhom and BDEhet values of the [Fe]X models the electrostatic component is important. This helps to understand their respective variations upon oxidation.

Density functional theory studies on the dissociation energies of metallic salts: relationship between lattice and dissociation energies

AbstractThe formation and physicochemical properties of polymer electrolytes strongly depend on the lattice energy of metal salts. An indirect but efficient way to estimate the lattice energy through the relationship between the heterolytic bond dissociation and lattice energies is proposed in this work. The heterolytic bond dissociation energies for alkali metal compounds were calculated theoretically using the Density Functional Theory (DFT) of B3LYP level with 6‐311+G(d,p) and 6‐311+G(2df,p) basis sets. For transition metal compounds, the same method was employed except for using the effective core potential (ECP) of LANL2DZ and SDD on transition metals for 6‐311+G(d,p) and 6‐311+G(2df,p) calculations, respectively. The dissociation energies calculated by 6‐311+G(2df,p) basis set combined with SDD basis set were better correlated with the experimental values with average error of ca. ±1.0% than those by 6‐311+G* combined with the LANL2DZ basis set. The relationship between dissociation and lattice energies was found to be fairly linear (r>0.98). Thus, this method can be used to estimate the lattice energy of an unknown ionic compound with reasonably high accuracy. We also found that the dissociation energies of transition metal salts were relatively larger than those of alkaline metal salts for comparable ionic radii. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 827–834, 2001

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] .

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.

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.

Modeling Ions through Conjugation Operators

AbstractIn the chemical structures of ions, integer charges are nominally placed on particular atoms. But the properties of ions are affected by the dispersion of these charges to neighboring atoms. In order to account for the dispersion of charge, we propose the use of conjugation operators to model the chemical structures of ions. As a demonstration, an estimation method for the heterolytic bond dissociation energies associated with alcohol anions is derived from this type of model.

The Effects of Protonation on the Structure, Stability, and Thermochemistry of Carbon-Centered Organic Radicals

The effects of protonation on the geometries and stabilization energies of prototypical •CH2X radicals (X = NH2, OH, OCH3, PH2, SH, F, Cl, Br, CN, CHO, and NO2) have been studied with the use of ab initio molecular orbital calculations at the G2 level. The proton affinities at X of the •CH2X radicals and the analogous substituted methanes, CH3X, are compared and the corresponding heats of formation calculated. For π-donor substituents (X = NH2, OH, OCH3, PH2, SH, F, Cl and Br), protonation at X leads to considerable +C :XH character for both CH3XH+ and •CH2XH+, resulting in substantially lower heterolytic bond dissociation enthalpies and longer C−X bonds. Protonation also strengthens the C−H bonds in CH3XH+ and, in combination with the reduced interaction of the lone pair on X with the singly-occupied orbital at the radical center in the radicals, results in negative radical stabilization energies for •CH2XH+. For the π-acceptor substituents (CN, CHO, and NO2), protonation enhances hyperconjugative electr...

Estimation of heterolytic bond dissociation energies by the kinetic method

Estimation of heterolytic bond dissociation energies by the kinetic method

Drago’se, c andt parameters for some mono- and bivalent metal ions. Lack of correspondence with Pearson’s chemical hardness

Drago’se, c andt parameters for a number of chemically important mono- and bivalent metal ions are estimated from the heterolytic bond dissociation energy data of their binary halides, hydrides and methides. An important result is that, for the cationic acids, thec andt parameters are not two independent variables. Attempts to correlate these parameters or any combination of these with Pearson’s chemical hardness have been unsuccessful so far.

Carbon-halogen bond cleavage energies of polyhaloalkyl radical anions in solution

The solution heterolytic bond dissociation energies of seven polyhaloalkyl radical anions ΔH het(R-X −.) were derived from a cycle using homolytic bond energy (BDE) and electrochemical data. A brief quantitative discussion on the structural influence on ΔH het(R-X −.) and on the effect of one-electron reduction on weakening the CX bonds are presented.

The Thio-Wittig Rearrangement of Deprotonated Allyl Methyl Sulfide. A Gas-Phase Unimolecular Isomerization Probed with a Variable Temperature Flowing Afterglow-Triple Quadrupole Device

The thio-Wittig rearrangement of deprotonated allyl methyl sulfide has been examined in the gas phase with a variable temperature flowing afterglow-triple quadrupole device. Collision-induced dissociation studies of a series of thiolate anions (RS-) reveal that methyl deprotonation leads to 3-butene-1-thiolate (2a), the [2,3]-Wittig product, while 1-thiomethylallyl anion (1a) isomerizes to 1-butenyl thiolate (4a), the [1,4]-Wittig product, at elevated temperatures. Activation energies for these processes have been estimated using the Arrhenius equation and are compared to high-level (G2) calculations for the homolytic and heterolytic bond dissociation energies. Stepwise and concerted [1,4] pathways are found to have similar energy requirements, which accounts for some of the mechanistic controversy regarding these transformations in solution. The observed selectivity, [1,2] vs [1,4], is most easily accommodated by a concerted process but can be explained in terms of a stepwise mechanism by considering the spin density and charge location in a radical anion intermediate (6a). Frontier molecular orbital theory, however, leads to the wrong prediction. The [2,3]-Wittig rearrangement appears to proceed via a concerted pathway in the gas phase as has been invoked in the liquid phase. Heats of formation for acrolein (ΔH°f298 = −15.6 kcal mol-1), thioacrolein (ΔH°f298 = 37.9 kcal mol-1), and their radical anions (ΔH°f298 = −17.3 kcal mol-1 and ΔH°f298 = 16.3 kcal mol-1, respectively) are also provided.

Strain and structural effects on rates of formation and stability of tertiary carbenium ions in the light of molecular mechanics calculations

AbstractAn empirical MM2 force‐field was developed for the calculation of steric or strain energies of carbenium ions, and applied to the rationalization of the rates of solvolysis of bridgehead derivatives. The latter constitute a homogeneous series of model compounds for solvolysis, spanning a rate range of ca 20 log units. Their rate constants correlate with the calculated steric energy differences between bridgehead bromides and the corresponding carbenium ions. The rate constants of tertiary derivatives of general structure may similarly be rationalized in terms of strain changes, although the correlation exhibits more scatter than that for the bridgehead derivatives alone. In the bridgehead series, the relative free energies of activation for solvolysis correlate with the heterolytic bond dissociation energies D° (R+ – Br−) in the gas phase. However, this correlation breaks down when simple mono‐ and acyclic substrates are included. This is attributed in part to the proximity of the leaving group in the transition state of solvolysis, which stabilizes the developing positive charge at the cationic centre in comparison with the charge of the free ion. The significance of the force‐field calculations with respect to the structure of bridgehead carbenium ions was tested by comparison of structural data obtained from ab initio calculations. The structures of cations suffering strong distortions owing to C–C hyperconjugation are poorly reproduced by the molecular mechanics calculations, the parameters of which are based on solvolytic reactivity and not on carbenium ions.

Estimation of Heterolytic Bond Dissociation Energies of Alkanes

Gas-phase enthalpies of ionic intermediate compounds can be estimated using a new technique which is based on the analysis of conjugate forms of the compounds. For ionic intermediates, conjugation determines the extent of charge dispersion which is directly related to the thermodynamic stability of an ion. In a simple application, the proposed method estimates the heterolytic bond dissociation energy of alkanes, which can be used to determine the enthalpy of formation of alkyl cations

Ionic Lewis superacids in the gas phase. Part 3. Reactions of gaseous CF +3 with nitrogen bases

The results of a Fourier-transform ion cyclotron resonance mass spectrometric study of the gas-phase reactions of CF + 3 ions toward a number of amines are reported and compared with those of the preceding paper. As with alcohols, the CF + 3 ion exclusively attacks the n-center of the amine giving the corresponding excited onium intermediate that, at variance with that from alcohols, undergoes fragmentation via preferential HF elimination. This process is overwhelmed by simple electron transfer within the collision complex, when thermochemically allowed. With azetidine, pyrrolidine, and piperidine, a cycloreversion reaction in the corresponding adducts with CF + 3 is observed as well. The reactivity patterns of CF + 3 towards amines has been contrasted with that of SiF + 3. The different behavior of the two electrophiles toward n-bases is ascribed to the much larger heterolytic bond dissociation energies of SiX (X = F, O, or N) compared to those of CX. According to its reactivity properties, the CF + 3 ion can be ranked as a gaseous “Lewis superacid” but much weaker than SiF + 3.

Heterolytic Bond Dissociation Energies of Halobenzene Anion Radicals.

Heterolytic Bond Dissociation Energies of Halobenzene Anion Radicals.

Pearson′s chemical hardness, heterolytic dissociative version of Pauling′s bond-energy equation and a novel approach towards understanding Pearson′s hard–soft acid–base principle

The empirical chemical hardness parameters developed recently by Pearson have been examined critically. It was found that these parameters normally characterise a group but not the corresponding ion as postulated. Equation (i) has been found quite successful when the heterolytic dissociation of D(A+B–)–½[D(A+A–)+D(B+B–)]= 2|Δγ|2(i) AB into A++ B– follows the general notion of electronegativity (i.e. the more electronegative part of AB dissociates as the anion), where D(A+B–) is the energy required to dissociate the AB bond into A++ B– and D(A+A–) and D(B+B–) are the heterolytic bond-dissociation energies of the molecules AA and BB respectively. Equation (i) has been used to define a new parameter γ, where |Δγ|=|γA+–γB–|. The γ parameters characterise the ions, and have been used to rank various monovalent anions and cations in a unified way to explain the hard–soft acid–base principle of Pearson. It was found that a reaction of type (II) proceeds from left to right in the gas phase if the AB + CD → AC + BD (ii)|ΔΔγ| value of the right-hand side is greater than that of the left. Out of some 282 gas-phase reactions considered, there are few exceptions. Emphasis has been placed on explaining the course of inorganic reactions. It is felt that the γ parameter can be identified with the chemical hardness of an ion.