Spectroscopic calculation of CH bond dissociation energy in the series of chloro derivatives of methane, ethane, and propane

The bond-dissociation energy of CH bonds in chloro derivatives of methane, ethane, and propane has been determined by spectroscopic and quantum chemical methods. Spectroscopic values for CH bond dissociation energy were computed, basing on fundamental absorption bands in the anharmonic approximation, by the variational method with the use of the Morse anharmonic basis. Quantum chemical computations were performed using the basis 6-311G(3df, 3pd)/B3LYP. There are discussed the obtained regularities of changes in the bond dissociation energy when the structure of a molecule is changed.     

Spectroscopic calculation of the bond-dissociation energy of CH bonds in fluoro derivatives of methane,ethane, ethene,propene, and benzene

The bond-dissociation energy of CH bonds in fluoro derivatives of methane, ethane, ethene, propene, and benzene is determined by spectroscopic and quantum chemical methods. The spectroscopic values of the bond-dissociation energy of CH bonds are calculated in terms of fundamental absorption bands in the anharmonic approximation by the variational method using the Morse anharmonic basis. The quantum chemical calculations are performed using the 6-311G(3df, 3pd)/B3LYP basis set. The obtained regularities in variations of the CH bond dissociation energy values upon changes in the molecule structure are discussed.     

Spectroscopic calculations of CH and OH bond dissociation energies for aldehydes, ketones, acids, and alcohols

CH and OH bond dissociation energies were calculated by the spectroscopic and quantum-chemical methods for aldehydes, ketones, acids, and alcohols. The spectroscopic values of CH and OH bond dissociation energies were obtained from the fundamental absorption bands by the variational method in an anharmonic approximation using the Morse-anharmonic basis set. Quantum-chemical calculations were carried out using the 6-311G(3df,3pd)/B3LYP basis set. It is discussed how the bond dissociation energies change with the structure of the molecule.     

Spectroscopic calculations of CH bond dissociation energies for ethene, propene, and benzene chlorine derivatives

CH bond dissociation energies have been determined by spectroscopic and quantum-chemical calculations for ethane, propene, and benzene chlorine derivatives. The spectroscopic values of CH bond dissociation energies were obtained from the fundamental absorption bands in an anharmonic approximation using the variation method and the Morse harmonic basis. Quantum-chemical calculations were carried out using the 6-311G(3df,3pd)/B3LYP basis. The resulting tendencies of variation of bond dissociation energies are discussed in relation to changes in the structure of the molecule.     

Spectroscopic calculation of CH bond dissociation energies for the bromo derivatives of alkanes,alkenes, and arenes

The CH bond dissociation energies were determined for the bromo derivatives of methane, ethane, propane, cyclopropane, ethane, propene, and benzene by the spectroscopic and quantum-chemical methods. The spectroscopic values of the CH bond dissociation energies were obtained from the fundamental absorption bands by the variational method in an anharmonic approximation using the Morse-harmonic basis. Quantum-chemical calculations were performed using the 6-311G(3df, 3pd)/B3LYP basis. The resulting tendencies of variation of the bond dissociation energies due to changes in the molecular structure are discussed.     

Spectroscopic Calculation of CH Bond Dissociation Energies for Aliphatic Nitriles

The CH bond dissociation energies were calculated spectroscopically for acetonitrile, propionitrile, isobutyronitrile, and trimethylacetonitrile from the fundamental absorption bands in the anharmonic approximation using the variational technique with the Morse-harmonic basis set. A similar calculation was carried out for various associates of acetonitrile. Due to this, CH bond dissociation energies were determined for the liquid state of acetonitrile.Original Russian Text Copyright © 2004 by L. A. Gribov, I. A. Novakov, A. I. Pavlyuchko, V. V. Korolkov, and B. S. Orlinson__________Translated from Zhurnal Strukturnoi Khimii, Vol. 45, No. 5, pp. 816–821, September–October, 2004.     

“Spectroscopic” Calculation of CH and NH Bond Dissociation Energies for a Series of Primary Amines

The CH and NH bond dissociation energies were calculated by the spectroscopic method for a series of primary amines. The energies were calculated from the fundamental absorption bands in an anharmonic approximation using the Morse harmonic basis set. The results are compared with data obtained from ab initio calculations in different basis sets and experimental values. It is shown that, unlike quantum-chemical calculations, the spectroscopic method correctly reproduces the effect of the molecular structure on the CH and NH bond dissociation energies in amines.Original Russian Text Copyright © 2004 by L. A. Gribov, I. A. Novakov, A. I. Pavlyuchko, V. V. Korolkov, and B. S. Orlinson__________Translated from Zhurnal Strukturnoi Khimii, Vol. 45, No. 6, pp. 999–1007, November–December, 2004.     

On Feasibility of “Spectroscopic” Calculations of XH Bond Dissociation Energies for Polyatomic Molecules from Fundamental Vibration Frequencies Using the Anharmonic Molecular Model

This paper discusses the applicability of the variational technique using a minimal Morse-harmonic basis set to calculations of the fundamental spectrum and the potential function parameters for polyatomic molecules. The potential function is assumed to be the sum of the Morse function for XH bonds and the harmonic function for the skeletal and deformation vibrations. The initial approximation for the potential function is found by ab initio calculations and refined by solving the inverse mechanical problem (selecting the scaling indices). The thus selected harmonic part of the potential function gives equally good agreement between the experimental and calculated transition frequencies in both harmonic and anharmonic approximations. The anharmonic (Morse) term of the potential function (bond dissociation energy) is selected by solving the inverse mechanical problem until the best agreement between the experimental and calculated CH bond stretching frequencies has been achieved. Problem solving ends with the construction of a transmission curve in the IR spectrum. Variations of the dipole moment of the molecule induced by vibrations are found by ab initio calculations.     

“Spectroscopic” Calculations of CH Bond Dissociation Energies for Ethane, Propane, Butane, Isobutane, Pentane, Hexane, and Neopentane Using Fundamental Vibration Frequencies

The fundamental spectrum and the parameters of the potential function of a number of saturated hydrocarbon molecules are calculated in an anharmonic approximation. The calculation is performed by the variational technique using a minimal Morse-harmonic basis. The potential function is taken as the sum of the Morse function for CH bonds and the harmonic function for the skeletal and deformation vibrations. The initial approximation for the potential function is found by ab initio calculations in a 6-31G basis and refined by solving the inverse problem. The calculated CH bond dissociation energies depend significantly on the molecular structure and on the position of CH bonds in the molecule. These energies correlate well with the experimental cleavage energies of these bonds. The changes in the dipole moment of the molecule induced by vibrations were found by ab initio calculations in a 6-31G basis. The calculated IR transmission curves are in good agreement with the experimental curves.     

Structural changes, P-P bond energies, and homolytic dissociation enthalpies of substituted diphosphines from quantum mechanical calculations

The molecular structures of the diphosphines P(2)[CH(SiH(3))(2)](4), P(2)[C(SiH(3))(3)](4), P(2)[SiH(CH(3))(2)](4), and P(2)[Si(CH(3))(3)](4) and the corresponding radicals P[CH(SiH(3))(2)](2), P[C(SiH(3))(3)](2), P[SiH(CH(3))(2)](2), and P[Si(CH(3))(3)](2) were predicted by theoretical quantum chemical calculations at the HF/3-21G*, B3LYP/3-21G*, and MP2/6-31+G* levels. The conformational analyses of all structures found the gauche conformers of the diphosphines with C(2) symmetry to be the most stable. The most stable conformers of the phosphido radicals were also found to possess C(2) symmetry. The structural changes upon dissociation allow the release of some of the energy stored in the substituents and therefore contribute to the decrease of the P-P bond dissociation energy. The P-P bond dissociation enthalpies at 298 K in the compounds studied were calculated to vary from -11.4 kJ mol(-1) (P(2)[C(SiH(3))(3)](4)) to 179.0 kJ mol(-1) (P(2)[SiH(CH(3))(2)](4)) at the B3LYP/3-21G* level. The MP2/6-31+G* calculations predict them to be in the range of 52.8-207.9 kJ mol(-1). All the values are corrected for basis set superposition error. The P-P bond energy defined by applying a mechanical analogy of the flexible substituents connected by a spring shows less variation, between 191.3 and 222.6 kJ mol(-1) at the B3LYP/3-21G level and between 225.6 and 290.4 kJ mol(-1) at the MP2/6-31+G* level. Its average value can be used to estimate bond dissociation energies from the energetics of structural relaxation.     

Reactivity of Pt- and Pd-bound nitriles towards nitrile oxides and nitrones: substitution vs. cycloaddition

Reactions of the nitrone CH3CH=N(CH3)O and the nitrile oxide CH3C[triple bond]NO with the nitrile complexes trans-[MCl2(N[triple bond]CCH3)2] (M = Pt, 1; Pd, 2) were investigated by theoretical methods at B3LYP and, for some processes, CCSD(T) levels of theory. The mechanisms of substitutions and cycloadditions were studied in detail. The former occur via a concerted asynchronous mechanism of dissociative type. The calculations of the metal-ligand bond energies in the starting complexes and substitution products and the analysis of structural features of the transition states indicate that the M-N bond dissociation (rather than M-O bond formation) is the step, which controls the reactivity of and in substitutions. The different chemical behaviours of the Pt and Pd complexes towards the 1,3-dipoles were investigated. The exclusive isolation of cycloaddition rather than substitution products in any solvents in the case of is both kinetically and thermodynamically controlled.The switch of the reaction mode from cycloaddition to substitution for 2 in CH2Cl2 solution is caused by the significantly lower Pd-N bond energy in comparison with the Pt-N bond energy, consistent with the higher lability of the Pd complexes. The different chemical behaviour of 2 in CH3CN and CH2Cl2 solvents is accounted for by the great excess of acetonitrile in the CH3CN solution rather than a different solvation character. The relative variation of Wiberg bond indices along the reaction path is proposed as a quantitative criterion for the classification of the reaction mechanism.     

Free radical abstraction of H atom from peroxides with concerted dissociation of O-O bond

Quantum chemical calculations of the dissociation energy of the C-H bond in the ??-hydroperoxide fragment of Me₂CHOOH were carried out. It was shown that abstraction of H atom is accompanied by dissociation of the O-O bond. Density functional calculations of transition states of the reactions of ^·CH₃, CH₃OO^·, and HO₂ ^· radicals with the C-H bond in the ??-hydroperoxide fragment of Me₂CHOOH were carried out. It was established that H atom abstraction is accompanied by concerted dissociation of the O-O bond. For 45 peroxides R¹R²CHOOH, R¹R²CHOOR³, and R¹R²CHOOC(O)R³ (R¹, R² = H, Me, Et, Ph, H₂C=CH), the enthalpies of H atom abstraction from the C-H bond in the a-hydroperoxide fragment with fragmentation of the peroxides at the O-O bond were calculated. The kinetic parameters for 12 classes of radical abstraction reactions with fragmentation of molecules were calculated from experimental data within the framework of the model of intersecting parabolas. The activation energies and reaction rate constants of H atom abstraction from C-H bonds of a-peroxide fragments involving peroxyl and alkyl radicals were determined for 45 peroxides of different structure.     

Overtone-induced dissociation and isomerization dynamics of the hydroxymethyl radical (CH2OH and CD2OH). I. A theoretical study

The dissociation of the hydroxymethyl radical, CH(2)OH, and its isotopolog, CD(2)OH, following the excitation of high OH stretch overtones is studied by quasi-classical molecular dynamics calculations using a global potential energy surface (PES) fitted to ab initio calculations. The PES includes CH(2)OH and CH(3)O minima, dissociation products, and all relevant barriers. Its analysis shows that the transition states for OH bond fission and isomerization are both very close in energy to the excited vibrational levels reached in recent experiments and involve significant geometry changes relative to the CH(2)OH equilibrium structure. The energies of key stationary points are refined using high-level electronic structure calculations. Vibrational energies and wavefunctions are computed by coupled anharmonic vibrational calculations. They show that high OH-stretch overtones are mixed with other modes. Consequently, trajectory calculations carried out at energies about ~3000 cm(-1) above the barriers reveal that despite initial excitation of the OH stretch, the direct OH bond fission is relatively slow (10 ps) and a considerable fraction of the radicals undergoes isomerization to the methoxy radical. The computed dissociation energies are: D(0)(CH(2)OH → CH(2)O + H) = 10,188 cm(-1), D(0)(CD(2)OH → CD(2)O + H) = 10,167 cm(-1), D(0)(CD(2)OH → CHDO + D) = 10,787 cm(-1). All are in excellent agreement with the experimental results. For CH(2)OH, the barriers for the direct OH bond fission and isomerization are: 14,205 and 13,839 cm(-1), respectively.     

Anharmonic Effect of N‐Propyl Peroxy Dissociation

The anharmonic and harmonic dissociation rate constants of alkylperoxy (RO₂) in different pathways, as well as those for the reactions of the n‐propyl peroxy radical, were calculated using the Rice–Ramsperger–Kassel–Marcus (RRKM) theory. When the temperature/total energy increased, the rate constants of the different pathways varied independently, causing changes in the dominating/leading products. Anharmonic rate constants were larger than harmonic rate constants, and their difference increased with increasing temperature/energy. Therefore, the anharmonic effect cannot be neglected. The rate‐determining steps of CH₃CH₂CH₂OO dissociation are discussed. Then the anharmonic effect was found clearly for CH₃CH₂CH₂OO dissociation, especially for the hydroperoxyalkyl radical (QOOH) dissociation. At low temperature, the rate constants were similar to those found from experiment, which indicated the RRKM theory was suitable for calculating the dissociation rates of RO₂ species.     

Effect of the molecular structure on the strength of the C-NO<Subscript>2</Subscript> bond in a series of monofunctional nitrobenzene derivatives

Geometric parameters and formation enthalpy and the enthalpy of the radicals formed during the homolytic breakage of C-NO₂ bond in 37 aromatic nitro compounds were calculated using different bases of the hybrid density functional method B3LYP, as well as the composite CBS-QB3 methods. On the basis of thermochemical data, were calculated the C-NO₂ bond dissociation energy and the activation energy of the radical gas-phase decomposition. Donor substituents were shown to cause an increase in the C-NO₂ bond dissociation energy, while the acceptors decrease it. The values of activation energies of gas-phase decomposition of aromatic nitro compounds calculated basing on the C-NO₂ bond dissociation energy are in good agreement with experiment.     

Anharmonic Effect in CH<Subscript>3</Subscript>CH<Subscript>2</Subscript>C(=O)OCH<Subscript>2</Subscript>CH<Subscript>3</Subscript> Decomposition

In this paper, using the B3LYP functional and CCSD(T) method with 6-311++G** basis set, the harmonic and anharmonic rate constants in the unimolecular dissociation of ethyl propanoate have been calculated using Rice–Ramsperger–Kassel–Marcus theory. The anharmonic rate constants of the title reaction have also been examined, the comparison shows that, the anharmonic effect especially in the case of high total energies and temperature for channels 3 to 6 is significant, so that the anharmonic effect cannot be neglected for unimolecular dissociation reaction of CH₃CH₂C(=O)OCH₂CH₃ both in microcanonical and canonical systems.     

CH local mode excitations and Fermi resonance in trichloroethylene

The CH stretching overtone spectrum of liquid-phase trichloroethylene is studied using the dual beam thermal lens technique (ΔV_s = 6) and conventional absorption method (ΔV_s = 2-5). The high value of the mechanical frequency is attributed to CH bond strengthening resulting from the electron-withdrawing property of the halogen atoms. The empirical relation between CH bond length and fifth overtone energy predicts that the CH bond in trichloroethylene is 0.002 Å smaller than that in ethylene. The ΔV_s = 3 region shows Fermi resonance between pure overtone and stretch—bend combination states. The magnitude of the Fermi resonance matrix element is close to that reported for dichloromethane.     

MO theoretical studies of electron pair decorrelation processes. I. Pair correlation energy and single bond dissociation

The homolytic dissociation of a single bond involves the decorrelation of one electron pair. Thus, the contribution of electron correlation to dissociation energies is large. In the present paper a new procedure is presented which allows the computation of the (within the given basis) complete correlation energy of one optimized electron pair. The method which requires only modest computational effort has been applied to the calculation of dissociation energies of a number of bonds of different types. The results show that the correlation of the electron pair of the bond which is broken contributes about 50–80% to the change of the total correlation energy occuring during the dissociation process which amounts to 20–70 kcal/mol. The fraction of correlation contributed by the bond electron pair as well as the relative importance of the left-right correlation within the bond depend very much on the type of the bond. In the case of CC and CH single bonds our method yields dissociation energies which are low by only about 5 kcal/mol. Thus, the method seems to be well suited for the calculation of potential surfaces of non-concerted organic chemical reactions which involve diradicals as intermediates.     

Ab initio valence-bond calculations of H2O

The first and second bond dissociation energies for H₂O have been calculated in anab initio manner using a multistructure valence-bond scheme. The basis set consisted of a minimal number of non-orthogonal atomic orbitals expressed in terms of gaussian-lobe functions. The valence-bond structures considered properly described the change in the molecular system as the hydrogen atoms were individually removed to infinity. The calculated equilibrium geometry for the H₂O molecule has an O-H bond length of 1.83 Bohrs and an HOH bond angle of 106.5°. With 49 valence-bond structures the energy of H₂O at this geometry was ?76.0202 Hartrees. The calculated equilibrium bond length for the OH radical was 1.86 Bohrs and the energy, using the same basis set, was ?75.3875 Hartrees. After correction for zero point energies the calculated bond dissociation energies are: H₂O → OH + H, D₁=75.38 kcal/mole and OH → O+H, D₂=54.79 kcal/mole.     

Theoretical predictions of the nitrogen heterocyclic compounds with metal and noble gas (metal = Cu,Ag, Au)

Here, we report a new type of Ng-containing compounds formed between the Ng-M group and nitrogen heterocyclic compounds, (CH₂)_nHNCuNg⁺ (n = 2, 3), (CH)₄NMNg, and (CH)₅NCuNg⁺ (M = Cu, Ag, Au; Ng = Ar, Kr, Xe). Quantum chemistry computations were carried out to optimize their geometric structures and calculate the dissociation energies, dissociation enthalpy, and dissociation free energy change. The stability of these Ng-bonding complexes was inspected by investigating the three dissociation processes of the these compounds into (a) Ng, M, and nitrogen heterocycle C_nN; (b) C_nN + MNg⁺; and (c) C_nNM + Ng, which are all endothermic and nonspontaneous, these dissociation processes are also turned out to be endergonic in nature at standard state. The natural bond orbital, atoms in molecules, and energy decomposition analysis based on the molecular wavefunction show that the M-Ng and M-N bonds have some covalent and electrostatic characters.