Accurate determination of the dissociation energies of alkali halides

Photofragmentation spectroscopy offers a possibility to determine the dissociation energy of diatomic molecules with an accuracy of about 0.01 eV, which is half an order of magnitude better than any other applicable method. In this letter the method will be compared with other known techniques and results will be given for NaBr, KBr, NaI ad KI.     

Calculation of bond dissociation energies of diatomic molecules using bond function basis sets with counterpoise corrections

Bond function basis sets combined with the counterpoise procedure are used to calculate the molecular dissociation energies D_e of 24 diatomic molecules and ions. The calculated values of D_e are compared to those without bond functions and/or counterpoise corrections. The equilibrium bond lengths r_e and harmonic frequencies o_e are also calculated for a few selected molecules. The calculations at the fourth-order Møller-Plesset approximation (MP 4) have consistently recovered about 95–99% of the experimental values for D_e; compared to as low as 75% without use of bond functions. The calculated values of r_e are typically 0.01 Å larger than the experimental values, and the calculated values of o_e are over 95% of the experimental values. © 1996 John Wiley & Sons, Inc.     

Relationship between bond dissociation energies and activation energies for bond scission reactions

Bond dissociation energies are frequently derived from values of the high pressure activation energy for bond scission reactions. The value derived depends on the transition state structure chosen for the reaction. We consider several models of the transition state and show that the variation in derived BDE values can be quite substantial, 3 to 6 kcal/mol at the high temperatures of pyrolysis kinetics. Application of the restricted Gorin model of the transition state results in BDE values in good agreement with current thermochemistry, while the other models tested result in lower to much lower values. © 1994 John Wiley & Sons, Inc.     

Bond dissociation energies in nitramines

The enthalpies of formation in the standard state and in the gas phase were recommended for a series of secondary nitramines and n-butyldinitramine on the basis of the experimental enthalpies of combustion and vaporization and literature data. An analysis of the main thermochemical values (the enthalpies of formation in the gas phase and the enthalpies of atomization) showed that the energy properties of the nitramine group are independent of the structure of the molecules studied and of the number of functional groups in them. The enthalpies of formation of the alkylnitramine radicals were determined. The values obtained make it possible to calculate the bond dissociation energies in the nitramines and their radicals of different structures.     

Calculation of bond energies in diatomic molecules

Theoretical Chemistry Accounts - An extrapolation method is proposed for an approximative evaluation of covalent bonding powers of some elements from their electronegativity values. Using these...     

Bond dissociation energies in second-row compounds

Heats of formation at 0 and 298 K are predicted for PF3, PF5, PF3O, SF2, SF4, SF6, SF2O, SF2O2, and SF4O as well as a number of radicals derived from these stable compounds on the basis of coupled cluster theory [CCSD(T)] calculations extrapolated to the complete basis set limit. In order to achieve near chemical accuracy (+/-1 kcal/mol), additional corrections were added to the complete basis set binding energies based on frozen core coupled cluster theory energies: a correction for core-valence effects, a correction for scalar relativistic effects, a correction for first-order atomic spin-orbit effects, and vibrational zero-point energies. The calculated values substantially reduce the error limits for these species. A detailed comparison of adiabatic and diabatic bond dissociation energies (BDEs) is made and used to explain trends in the BDEs. Because the adiabatic BDEs of polyatomic molecules represent not only the energy required for breaking a specific bond but also contain any reorganization energies of the bonds in the resulting products, these BDEs can be quite different for each step in the stepwise loss of ligands in binary compounds. For example, the adiabatic BDE for the removal of one fluorine ligand from the very stable closed-shell SF6 molecule to give the unstable SF5 radical is 2.8 times the BDE needed for the removal of one fluorine ligand from the unstable SF5 radical to give the stable closed-shell SF4 molecule. Similarly, the BDE for the removal of one fluorine ligand from the stable closed-shell PF3O molecule to give the unstable PF2O radical is higher than the BDE needed to remove the oxygen atom to give the stable closed-shell PF3 molecule. The same principles govern the BDEs of the phosphorus fluorides and the sulfur oxofluorides. In polyatomic molecules, care must be exercised not to equate BDEs with the bond strengths of given bonds. The measurement of the bond strength or stiffness of a given bond represented by its force constant involves only a small displacement of the atoms near equilibrium and, therefore, does not involve any reorganization energies, i.e., it may be more appropriate to correlate with the diabatic product states.     

Crystal lattice energies of alkali metal halides

An empirical relation for crystal binding energy in terms of the valence level splitting of the anion and the radius of the cation has been proposed. The results are much closer to the experimental values and comparable with those obtained by different methods. The anomalously high binding energy of fluorides has been discussed.     

Bond length based bond orders for borane cluster compounds

The known molecular structures of several closo borane cluster compounds, together with established electron-counting rules, are used to derive a relationship between bond length and bond order for boronboron two-centre bonds. This relationship is useful in deducing the distribution of electronic charge, and the approach can be extended to other homonuclear clusters.     

Thermochemistry and bond dissociation energies of ketones

Ketones are a major class of organic chemicals and solvents, which contribute to hydrocarbon sources in the atmosphere, and are important intermediates in the oxidation and combustion of hydrocarbons and biofuels. Their stability, thermochemical properties, and chemical kinetics are important to understanding their reaction paths and their role as intermediates in combustion processes and in atmospheric chemistry. In this study, enthalpies (ΔH°(f 298)), entropies (S°(T)), heat capacities (C(p)°(T)), and internal rotor potentials are reported for 2-butanone, 3-pentanone, 2-pentanone, 3-methyl-2-butanone, and 2-methyl-3-pentanone, and their radicals corresponding to loss of hydrogen atoms. A detailed evaluation of the carbon-hydrogen bond dissociation energies (C-H BDEs) is also performed for the parent ketones for the first time. Standard enthalpies of formation and bond energies are calculated at the B3LYP/6-31G(d,p), B3LYP/6-311G(2d,2p), CBS-QB3, and G3MP2B3 levels of theory using isodesmic reactions to minimize calculation errors. Structures, moments of inertia, vibrational frequencies, and internal rotor potentials are calculated at the B3LYP/6-31G(d,p) density functional level and are used to determine the entropies and heat capacities. The recommended ideal gas-phase ΔH°(f 298), from the average of the CBS-QB3 and G3MP2B3 levels of theory, as well as the calculated values for entropy and heat capacity are shown to compare well with the available experimental data for the parent ketones. Bond energies for primary, secondary, and tertiary radicals are determined; here, we find the C-H BDEs on carbons in the α position to the ketone group decrease significantly with increasing substitution on these α carbons. Group additivity and hydrogen-bond increment values for these ketone radicals are also determined.     

Cycloalkane and cycloalkene C-H bond dissociation energies

Both C-H bond dissociation energies for cyclobutene were measured in the gas phase (BDE = 91.2 +/- 2.3 (allyl) and 112.5 +/- 2.5 (vinyl) kcal mol-1) via a thermodynamic cycle by carrying out proton affinity and electron-binding energy measurements on 1- and 3-cyclobutenyl anions. The results were compared to those for an acyclic model compound, cis-2-butene, and provide the needed information to experimentally establish the heat of formation of cyclobutadiene. Chemically accurate G3 and W1 calculations also were carried out on cycloalkanes, cycloalkenes, and selected reference compounds. It appears that commonly cited bond energies for cyclopropane, cyclobutane, and cyclohexane are 3 to 4 kcal mol-1 too small and their pi bond strengths, as given by BDE1 - BDE2, are in error by up to 8 kcal mol-1.     

Studies on the full vibrational energies and dissociation energies of some heteronuclear diatomic molecules

A parameter-free analytical formula for dissociation energies of diatomic molecules is proposed by Fan and Sun (2009) [20] based on LeRoy and Bernstein's vibrational energy expression near dissociation limit. Using three highest vibrational energies which may be generated by the algebraic method (AM) presented in our previous study and by some other physical methods, the new formula is applied to study the molecular dissociation energies of 10 electronic states of KH, ⁷LiD, ⁷LiH, ⁶LiH, NaK, NaLI and NaRb heteronuclear diatomic molecules which have regular (Morse-like) potentials in this work. The results show that the AM energies and dissociation energies have excellent agreement with experimental values.     

Bond dissociation energies and radical stabilization energies associated with model peptide-backbone radicals

Bond dissociation energies (BDEs) and radical stabilization energies (RSEs) have been calculated for a series of models that represent a glycine-containing peptide-backbone. High-level methods that have been used include W1, CBS-QB3, U-CBS-QB3, and G3X(MP2)-RAD. Simpler methods used include MP2, B3-LYP, BMK, and MPWB1K in association with the 6-311+G(3df,2p) basis set. We find that the high-level methods produce BDEs and RSEs that are in good agreement with one another. Of the simpler methods, RBMK and RMPWB1K achieve good accuracy for BDEs and RSEs for all the species that were examined. For monosubstituted carbon-centered radicals, we find that the stabilizing effect (as measured by RSEs) of carbonyl substituents (CX=O) ranges from 24.7 to 36.9 kJ mol(-1), with the largest stabilization occurring for the CH=O group. Amino groups (NHY) also stabilize a monosubstituted alpha-carbon radical, with the calculated RSEs ranging from 44.5 to 49.5 kJ mol(-1), the largest stabilization occurring for the NH2 group. In combination, NHY and CX=O substituents on a disubstituted carbon-centered radical produce a large stabilizing effect ranging from 82.0 to 125.8 kJ mol(-1). This translates to a captodative (synergistic) stabilization of 12.8 to 39.4 kJ mol(-1). For monosubstituted nitrogen-centered radicals, we find that the stabilizing effect of methyl and related (CH2Z) substituents ranges from 25.9 to 31.7 kJ mol(-1), the largest stabilization occurring for the CH3 group. Carbonyl substituents (CX=O) destabilize a nitrogen-centered radical relative to the corresponding closed-shell molecule, with the calculated RSEs ranging from -30.8 to -22.3 kJ mol(-1), the largest destabilization occurring for the CH=O group. In combination, CH2Z and CX=O substituents at a nitrogen radical center produce a destabilizing effect ranging from -19.0 to -0.2 kJ mol(-1). This translates to an additional destabilization associated with disubstitution of -18.6 to -7.8 kJ mol(-1).     

The effect of bond functions on dissociation energies

The procedure employing bond functions recently suggested by Wright and Buenker has been applied to the N₂ X ¹Σ_g⁺ potential curve within the CAS SCF+MRSD Ci treatment of electron correlation. The basis set used herein is identical to that employed by these authors in their SCF+CI calculations. The D_e and the shape of the resulting potential curve, as judged by the computed vibrational levels, is not so accurate as would be expected from the results reported by Wright and Buenker. Our results indicate that using the CI superposition errors associated with bond functions to cancel basis set incompleteness depends on the treatment of the electron correlation.     

木质素模化物键离解能的理论研究

采用密度泛函理论B3P86方法,在6-31G(d,p)基组水平上,对木质素结构中的6种连接方式(β-O-4、α-O-4、4-O-5、β-1、α-1、5-5)的63个木质素模化物的醚键(C-O)和C-C键的键离解能E_B进行了理论计算研究。分析了不同取代基对键离解能的影响以及键长与键离解能的相关性。计算结果表明,C-O键的键离解能通常比C-C键的小,在各种醚键中C_α-O键的平均键离解能最小,为182.7 kJ/mol;其次是β-O-4连接中的C_β-O键,苯环和烷烃基上的取代基对醚键的键离解能有较强的弱化作用,C-O键的键长和键离解能的相关性较差。与C-O键相比,C-C键的键离解能受苯环上取代基的影响很小,而烷烃基上的取代基对C-C键的键离解能有较大的影响,C-C键的键离解能和键长之间存在较强的线性关系,C-C键的键长越长,其键离解能越小。     

Determination of bond dissociation energies using mass spectrometry

Fourier transform ion cyclotron resonance mass spectrometry (FT‐ICR/MS) offers the opportunity for gas phase cluster formation reactions at very low pressures and at temperatures that are different from room temperature. Reactions take place with single positive‐charge metal ions that are normally +2, +3, +4, etc., charged in solution. The ions formed are detected by measuring the current induced by their cyclotron rotation, but they cannot be physically separated and collected. Collision‐induced dissociation (CID) is widely used for ion‐structure determination via the fragmentation of the excited ions. CID study aims to determine the relationship between the V_pp [peak‐to‐peak voltage of the radiofrequency (rf) pulse] and the mass‐to‐charge (m/z) ratio, which will be used for the calculation of the center‐of‐mass translational kinetic energy (Ek_cm) of the excited ion under investigation. CID studies are restricted to stable ions with relatively high abundance. Nevertheless, with the evolution of computational chemistry, such problems can be overcome whereby CID calculations will be used to provide the substantial parameters for computer software, such as the Gaussian 03 program, for the structure determination of the less stable Ni_xS anions. The latter constitutes the core for our current research. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Remote substituent effects on homolytic bond dissociation energies

In the study we tried to answer two questions. First, does X-Z homolytic bond dissociation energy (BDE) of Y-C6H4-X-Z obey the Hammett relationship? Second, if it does what factors determine the magnitude and sign of the slope (rho+) of Hammett regression against substituent sigma(p)(+) constants? We collected a large number of X-Z BDEs for over one-thousand Y-C6H4-X-Z systems using the RMP2/6-311++G**//UB3LYP/6-31G* method. We found that remote substituent effects on X-Z BDEs are determined by both the ground effect (i.e. stabilization/destabilization of X-Z by the substituents) and the radical effect (i.e. stabilization/destabilization of X. by the substituents). The ground or radical effect is determined by the electron demand of X-Z or X. in the same way as the deprotonation enthalpy of HOOC-C6H4-X-Z or HOOC-C6H4-X. is affected by X-Z or X. . As a result, rho+ (BDE) for X-Z bond homolysis can be quantitatively predicted by using the change in deprotonation enthalpy from HOOC-C6H4-X-Z to HOOC-C6H4-X. .     

Bond dissociation energies and radical stabilization energies: an assessment of contemporary theoretical procedures

Various contemporary theoretical procedures have been tested for their accuracy in predicting the bond dissociation energies (BDEs) and the radical stabilization energies (RSEs) for a test set of 22 monosubstituted methyl radicals. The procedures considered include the high-level W1, W1', CBS-QB3, ROCBS-QB3, G3(MP2)-RAD, and G3X(MP2)-RAD methods, unrestricted and restricted versions of the double-hybrid density functional theory (DFT) procedures B2-PLYP and MPW2-PLYP, and unrestricted and restricted versions of the hybrid DFT procedures BMK and MPWB1K, as well as the unrestricted DFT procedures UM05 and UM05-2X. The high-level composite procedures show very good agreement with experiment and are used to evaluate the performance of the comparatively less expensive DFT procedures. RMPWB1K and both RBMK and UBMK give very promising results for absolute BDEs, while additionally restricted and unrestricted X2-PLYP methods and UM05-2X give excellent RSE values. UM05, UB2-PLYP, UMPW2-PLYP, UM05-2X, and UMPWB1K are among the less well performing methods for BDEs, while UMPWB1K and UM05 perform less well for RSEs. The high-level theoretical results are used to recommend alternative experimental BDEs for propyne, acetaldehyde, and acetic acid.     

Atomic contributions to bond dissociation energies in aliphatic hydrocarbons

This paper explores the atomic contributions to the electronic vibrationless bond dissociation enthalpy (BDE) at 0 K of the central C-C bond in straight-chain alkanes (C(n)H(2n+2)) and trans-alkenes (C(n)H(2n)) with an even number of carbon atoms, where n=2, 4, 6, 8. This is achieved using the partitioning of the total molecular energy according to the quantum theory of atoms in molecules by comparing the atomic energies in the intact molecule and its dissociation products. The study is conducted at the MP2(full)6-311++G(d,p) level of theory. It is found that the bulk of the electronic energy necessary to sever a single C-C bond is not supplied by these two carbon atoms (the alpha-carbons) but instead by the atoms directly bonded to them. Thus, the burden of the electronic part of the BDE is primarily carried by the two hydrogens attached to each of the alpha-carbons and by the beta-carbons. The effect drops off rapidly with distance along the hydrocarbon chain. The situation is more complex in the case of the double bond in alkenes, since here the burden is shared between the alpha-carbons as well as the atoms directly bonded to them, namely, again the alpha-hydrogens and the beta-carbons. These observations may lead to a better understanding of the bond dissociation process and should be taken into account when locally dense basis sets are introduced to improve the accuracy of BDE calculations.     

Bond dissociation energies for radical dimers derived from highly stabilized carbon-centered radicals

[reaction: see text] The temperature dependence of the dissociation of dimers formed from highly stabilized carbon-centered radicals has been examined. Analysis of the data yields the bond dissociation energy (BDE) for the central head-to-head C-C bond in these compounds. For example, for the dimer derived from 3-phenyl-2-coumaranone, BDE is 23.6 kcal/mol and the C-C bond length 1.596 A, a rather long value for a sigma bond.     

The effect of bond functions on molecular dissociation energies

Multi-reference Cl calculations are reported for the ground states of HCl and N₂ at their equilibrium distances, and for their separated atoms. Basis sets of double-zeta and double-zeta plus polarization quality are systematically augmented by additional sets of functions located at the bond centers. It is shown that use of bond functions can lead to either an underestimate or an overestimate of the the bond energy. Optimum basis sets for each molecule were obtained, giving D_e values of 4.59 eV for HCl (expt. 4.62 eV) and 9.96 eV for N₂ (expt. 9.905 eV) at the estimated full Cl level. The quality of the potential curves obtained with these basis sets is discussed.