On a definition of bond energies based on localized molecular orbitals

A theoretical model is presented for defining bond energies based on localized molecular Orbitals. These bond energies are obtained by rearranging the total SCF energy including the nuclear repulsion term to a sum over orbital and orbital interaction terms and then to total orbital terms, which can be interpreted as the energies of localized orbitals in a molecule. A scaling procedure is used to obtain a direct connection with experimental bond dissociation energies. Two scale parameters are employed, the C-C and the C-H bond dissociation energy in C₂H₆ for A-B and C-H type bonds, respectively. The implications of this scaling procedure are discussed. Numerical applications to a number of organic molecules containing no conjugated bonds gives in general a very satisfactory agreement between experimental and theoretical bond energies.     

Eichinvariante Berechnung des Diamagnetismus und der chemischen Verschiebung

The procedure derived in^1–6 was used to calculate the magnetic susceptibilities and the magnetic shielding of a series of diamagnetic molecules (CH₄, C₂H₆, C₂H₄, C₂H₂, and HCN). The model requires only the knowledge of the unperturbed electron density. An incrementary treatment of the susceptibility-and shielding tensors is based on a partition of the electron density into localized parts.The values obtained by this procedure are in good agreement with experimental results in the case of susceptibilities and magnetic protonshieldings. The results for the¹³C-shielding however are not satisfactory.

     

Anharmonic oscillator model of overtone spectra for D3h symmetry molecules

The bond stretching vibrations of XF₅ molecules with D_3h symmetry are treated computationally on the Morse oscillator model in which the bond oscillators are coupled harmonically. Each calculation involves four parameters for two types of Morse potential and three parameters for the kinetic-energy, potential coupling terms. The eigenvalue formula for overtone and combination states up to three are presented and can be used to predict all the vibrational energy levels from local mode molecules through normal mode molecules. For PF₅, AsF₅ and VF₅, the coupled Morse oscillator model gives a prediction in good agreement with the experimental data.     

Theoretical study of the neutral hydrolysis of methyl formate via a concerted and stepwise water-assisted mechanism using free-energy curves and molecular dynamics simulation

A procedure previously described by us is used for the theoretical study of chemical reactions in solution by means of molecular dynamics simulation, with solute–solvent interaction potentials LJ (12-6-1) derived from ab initio quantum calculations. We apply the procedure to the case of the neutral hydrolysis of methyl formate, HCOOCH₃ + 3H₂O → HCOOH + CH₃OH + 2H₂O in aqueous solution, via concerted and stepwise water-assisted mechanisms. We use the solvent as reaction coordinate, and the free-energy curves for the calculation of the activation energies. The theoretical calculation for the thermodynamics of this hydrolysis reaction in aqueous solution, assisted by three water molecules, is in agreement with the available experimental information. In particular our study gives values of ΔG ^≠ = 28.88 and 28.17 kcal/mol for the concerted and stepwise mechanisms, close to the experimental activation barrier of 28.8 kcal/mol, and a significant improvement over the values of 48.05 and 45.66 kcal/mol found in another similar study using the PCM model.     

Calculated molecular double-differential cross-sections for ionisation under proton impact

This paper describes an approach (DDCS-MT), based on the plane wave Born approximation, intended to calculate double-differential ionisation cross-sections (DDCS) under heavy charged particle impact. A procedure is proposed in order to adapt this formalism to the treatment of molecules, which consists essentially to break up each molecular orbital into its atomic components and to sum up the partial DDCS. The increase of the binding energies in respect to the ion energy, as well as the post-collisional interaction between the passing ion and the ejected electron were taken into account. The method proposed led generally to a satisfactory agreement with measurements from the literature for electron ejection energies larger than 10 to 20 eV, in the case of a variety of low-Z molecules (NH₃, CH₄, CH₃NH₂, N₂, O₂, CO₂) bombarded by 0.25-to 2.0-MeV protons. The whole procedure is free from any fitting parameter and requires no experimental ionisation cross-sections as input data.     

Modeling of gamma-ray radiolysis data at moderate and low solute concentrations in aqueous solutions

A radiation chemical model developed for use in pulse radiolysis studies employing high energy electrons has been applied to the calculation of yields in aqueous chemical systems irradiated with ⁶⁰Co γ-rays. The fit between model calculations and experimental data is almost within experimental error in most instances. Trends are accurately predicted where quantitative agreement is not found. Predictions of molecular yields of H₂ and H₂O₂ in the presence of scavengers of precursors of these molecules are in good agreement if an initial molecular yield is assumed for both molecules. Agreement between model predictions and experimental data is reasonable at low solute concentrations where back reactions of spur intermediates are important. The model appears to be useful in modeling steady state γ radiolysis as well as pulse radiolysis data. Implications of the model in predicting possible errors in rate constant determinations are discussed.     

The infrared spectra of polycyclic aromatic hydrocarbons containing a five-membered ring: symmetry breaking and the B3LYP functional

The infrared spectra of six molecules, each of which contains a five-membered ring, and their cations are determined using density functional theory; both the B3LYP and BP86 functionals are used. The computed results are compared with the experimental spectra. For the neutral molecules, both methods are in good agreement with experiment. Even the Hartree–Fock (HF) approach is qualitatively correct for the neutral species. For the cations, the HF approach fails, as found for other organic ring systems. The B3LYP and BP86 approaches are in good mutual agreement for five of the six cation spectra, and are in good agreement with experiment for four of the five cations where the experimental spectra are available. It is only for the fluoranthene cation where the BP86 and B3LYP functionals yield different results; the BP86 approach yields the expected C _{2 v} symmetry, while the B3LYP approach breaks symmetry. The experimental spectra support the BP86 spectra over the B3LYP spectra, but the quality of the experimental spectra does not allow a critical evaluation of the accuracy of the BP86 approach for this difficult system. Received: 9 February 1999 / Accepted: 31 March 1999 / Published online: 14 July 1999     

Ab initio calculation of the first order term of the intermolecular energy near the van der Waals minimum

The first order term of the intermolecular energies between two hydrogen molecules and between Li⁺ and H₂ has been computed by three different methods: two of them are based on a perturbative procedure, including or neglecting the overlap between the orbitals of the interacting molecules or atoms in the calculation of the electrostatic and exchange terms. We can then study the effect of the overlap on each of these terms. The third method is the SCF supermolecule treatment which provides results in very good agreement with the perturbative procedure including the overlap. TheT configuration in the case of two hydrogen molecules and theC _2v configuration for Li⁺+H₂ are stable with respect to the first order term.     

Molecular solutes in nematic liquid crystals: Orientational order and electric field gradients

The difference between liquid-crystal and gas-phase values for the nuclear quadrupole coupling constant in D₂ and HD is used to obtain the mean electric field gradient in various liquid crystals. Order parameters for small molecules dissolved in liquid crystals are calculated assuming that the orientational order arises from the interaction of the molecular quadrupole moment with the average field gradient. The results obtained are in good agreement with experimental values for hydrogen and several other solutes.     

Monte Carlo simulation of RRKM unimolecular decomposition in molecular beam experiments. V. product ox angular and energy distributions from O(3P)+X2 (X = Br,I)

Statistical simulation was applied to the unimolecular decomposition of the collision complexes formed in the crossed beam experiments on O(³P) + Br₂ by Fernie et al. and O(³P) + I₂ by Durkin et al. The simulation procedure used the fundamentals of RRKM theory and included exact angular momentum conservation. The impact parameter distributions were varied to obtain the best fits. Good agreement with experimental laboratory angular distributions measured with O atoms seeded in both He and Ne was found for impact parameter distributions which were peaked at quite small values, in most cases between 2 and 3 Å. Product OX molecules were found to be rotationally excited and inverted with a mean rotational energy close to twice the value expected without angular momentum restrictions. The differences found between the calculated and the experimental angular distributions do not support any assumptions about osculating or short-lived complexes. The normal exoergicity ΔD₀ of 27 kJ/mol for the O + I₂ reaction agrees well with the experiments by Dunkin et al.     

Correlation of the total cross section for electron scattering by molecules with 10–22 electrons, and some molecular parameters at intermediate energies

Accurate experimental values of the total cross section (σ_T) for electron scattering by molecules (CH₄, NH₃, N₂, CO and CO₂) in the energy range 0.5 to 5 keV have been analysed to obtain correlations with molecular properties. A formula for σ_T, as a function of the number of target electrons and molecular polarizabilities, has been deduced. This empirical expression reproduces to within 6% our experimental results for the abovementioned molecules and has been used to obtain results for other molecular targets with 10 to 22 electrons. Total cross sections show reasonable agreement with the experimental and theoretical data for this energy range.     

CI calculations of electron and x-ray scattering cross sections of non-linear molecules: H2O and NH3

Differential cross sections for high-energy electron and X-ray scattering on H₂O and NH₃ molecules have been calculated in the first Born approximation using approximate Hartree—Fock and configuration-interaction wavefunctions. For non-linear molecules, the first theoretical inelastic and total cross sections with explicit inclusion of electron correlation are presented. A comparison between the theoretical and experimental scattering functions is made. Apart from minor discrepancies which can be due to deficiencies in the theoretical as well as the experimental procedures, agreement between theory and experiment is satisfactory.     

The diffraction of H2, D2 from solid LiF

The collision of H₂ and D₂ molecules with the (001) face of LiF is studied theoretically using the close coupling formalism. The method includes excited states of the diatomic molecule but neglects all couplings to phonons or the possibility of fragmentation of the incident molecule. Using a simple model potential a series of calculations for comparison with experimental data is carried out. Excellent agreement is obtained with the trends obeserved experimentally, and the quantitative agreement is also good.     

Geometry optimization and electronic structures of molecules H_3AXAH_3

The geometries of molecules H_3AXAH_3(X=O,S,Se and A=C,Si)have been optimizedusing STO-3G ab initio calculations and gradient method and the results are in good agreement withreported experimental values.From the STO-3G optimized geometries,we have also calculated theelectronic structures of these molecules using 4-31G and 6-31G basis sets to obtain the MO energies.atomic net charges and dipole moments.The ionization potentials calculated by 6-31G basis set are ingood agreement with experimental values.     

Experimental and computed bond lengths: The importance of their differences

State-of-the-art experimental electron diffraction and computational information on the structure of alkaline earth dihalide molecules are in agreement for the shape of these symmetric triatomic molecules (linear/bent/quasi-linear). However, the computed and measured bond lengths show differences that are not only considerably larger than the experimental error but also have the wrong sign. The physical meaning of experimental bond lengths depends on the physical techniques used in their determination and the ways of averaging over molecular vibrations. The choice of model potentials in the elucidation of experimental information is also important, especially for floppy molecules. When improved computational bond lengths become available, their comparison with experimental information will have to take account of the physical meaning of the experimentally determined bond lengths. The computed equilibrium distance (r_e) should be smaller than the experimental distance-average bond length (r_g). The differences may range from a few thousandths of an Å to a few hundredths with increasing temperature and, especially, with increasing floppiness of the molecule. For truly accurate comparison, experimental bond lengths should be compared with computed ones only following necessary corrections, bringing all information involved in the comparison to a common denominator. © 1992 John Wiley & Sons, Inc.     

An experimental study of the energetics of fragmentation of H3

Spatial beam profile measurements indicate that transient H₃ molecules, formed in electron capture collisions of 3–4 keV H⁺₃ ions with Mg atoms, fragment to H atoms and H₂ molecules with up to five vibrational quanta. The experimental H₃ fragmentation energy (?2.2 ± 0.1 eV) is in good agreement with theoretical predictions.     

Diffusion coefficients of the system α-cyclodextrin-n-butylurea-water at 25°C

Diffusion coefficients were measured for the ternary system -cyclodextrin(I)-n-butylurea(2)-water at three average concentrations. The cross coefficient D₁₂ was found to be almost zero and D₂₁ large and negative. These results are in agreement with the presence of an inclusion complex whose mobility is close to that of the host cyclodextrin molecules. The values of the four experimental diffusion coefficients are used to compute a value of the inclusion constant which is in reasonably good agreement with the calorimetric value.     

Rotational excitation in electron-molecule scattering at intermediate collision energy: A two-centre scattering model

A simple two-centre scattering model is discussed, which leads to compact closed form differential cross sections for rotationally inelastic scattering from diatomic molecules. The model elucidates in a simple way the rotational rainbow structure of the cross sections for both initially rotating and nonrotating molecules. Surprisingly good agreement with extensive computations and experimental measurements fore-Na₂ scattering at 150–300 eV is observed.