An ab initio study of the structure, torsional potential energy function, and electric properties of disilane, ethane, and their deuterated isotopomers

Highly accurate ab initio computations of the molecular structure and properties, torsional potential energy function, and harmonic force field of disilane and ethane have been carried out. Equilibrium parameters as well as vibrational corrections have been evaluated. In addition, for these systems a vibrational averaging procedure has been employed for calculating the dipole moment of molecules which have no permanent dipole moment, i.e., SiH(3)SiD(3) and CH(3)CD(3). The molecular and spectroscopic properties calculated for ethane and its isotopomers provide a calibration against known experimental data, allowing us to estimate the reliability of our computed results for disilane for which there is much less experimental data. The goal of the present study is to predict the molecular parameters, with estimated uncertainties, that determine the microwave spectrum of SiH(3)SiD(3).     

On the structure and torsional potential of trifluoromethoxybenzene: an ab initio and density functional study

The torsional potential of trifluoromethoxybenzene around the aryl–O bond was investigated with the aid of large-scale ab initio calculations performed at the Møller–Plesset second order (MP2) level, with several post-MP2 methods, and with a hybrid density functional method (B3LYP). Contrary to several recent reports, we do not find substantial qualitative differences between MP2 and B3LYP results, provided sufficiently large basis sets are used. The results are confronted with analogous MP2 and B3LYP data for methoxybenzene, for hypothetical anions as obtained by deprotonation at the para-position, and for ethylbenzene. The trends in the calculated torsional potentials, barrier heights and energy differences between conformers are discussed and correlated with selected structural parameters.     

Gas phase chemical equilibrium in dinitrogen trioxide and dinitrogen tetroxide

The ideal gas chemical thermodynamic properties for NO, NO₂, N₂O₃, and N₂O₄ for the temperature range 50 to 5000 K were evaluated by the statistical thermodynamic method using the most recent molecular parameters. In the calculations for NO and NO₂, the effects of anharmonicity and vibration—rotation interaction were included. The contributions due to centrifugal distortion were also included for NO₂. For evaluation of the thermodynamic properties for N₂O₃ and N₂O₄ molecules, the rigid-rotor and harmonic-oscillator model were adopted. A free internal rotation was assumed for N₂O₃ and an internal rotation barrier height (V₂) of 1.58 kcal mol⁻¹ was derived for N₂O₄. The thermodynamic properties due to hindered internal rotation were clculated using a partition function formed by summation of internal rotation energy levels. The thermodynamic properties for two equilibrium mixtures: NO₂---N₂O₄ and N₂O₃---NO---NO₂---N₂O₄ were also calculated. The effects of temperature and pressure on heat capacities and compositions of these two mixtures are illustrated graphically and the calculated heat capacities and equilibrim constants are in good agreement with available experimental values.     

An ab initio potential energy surface and predissociative resonances of HArF

A three-dimensional potential energy surface of the ground electronic state HArF is constructed from more than 2000 ab initio points at the multireference averaged quadratic coupled-cluster level employing an augmented large basis set. The calculations indicate that the linear HArF molecule is metastable with a barrier of 0.643 eV in the atomization (HArF --> H + Ar + F) channel and a barrier of 1.017 eV in the dissociation (HArF --> Ar + HF) channel. Variational calculations of low-lying predissociative resonances of both HArF and DArF are performed on the three-dimensional potential energy surface using a complex-symmetric Lanczos propagation method, which yields both positions and widths of the resonance states. The resonance lifetime generally decreases with energy, but strong mode selectivity exists. Reasonably good agreement with experiment confirms the accuracy of our potential. These calculations provide valuable information on the stability and dynamics of HArF/DArF in its ground electronic state.     

An ab initio potential energy surface and vibrational energy levels of ZnH2

A three‐dimensional potential energy surface of the electronic ground state of ZnH₂ (${X}^1\sum _g^ +$ ) molecule is constructed from more than 7500 ab initio points calculated at the internally contracted multireference configuration interaction with the Davidson correction (icMRCI+Q) level employing large basis sets. The calculated relative energies of various dissociation reactions are in good agreement with the previous theoretical/experimental values. Low‐lying vibrational energy levels of ZnH₂, ZnD₂, and HZnD are calculated on the three‐dimensional potential energy surface using the Lanczos algorithm, and found to be in good agreement with the available experimental band origins and the previous theoretical values. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2010     

An ab initio study of van der Waals potential energy parameters for silver clusters

We employ ab initio calculations of van der Waals complexes to study the potential energy parameters (C(6) coefficients) of van der Waals interactions for modeling of the adsorption of silver clusters on the graphite surface. Electronic structure calculations of the (Ag(2))(2), Ag(2)-H(2), and Ag(2)-C(6)H(6) complexes are performed using a coupled-cluster approach that includes single, double, and perturbative triple excitations (CCSD(T)), M?ller-Plesset second-order perturbation theory (MP2), and spin-component-scaled MP2 (SCS-MP2) methods. Using the atom pair approximation, the C(6) coefficients for silver-silver, silver-hydrogen, and silver-carbon atom systems are obtained after subtracting the energies of quadrupole-quadrupole interactions from the total electronic energy.     

The ab initio ground-state potential energy function of beryllium monohydride, BeH

The accurate ground-state potential energy function of beryllium monohydride, BeH, has been determined from large-scale ab initio calculations using the multi-reference averaged coupled-pair functional (MR-ACPF) method in conjunction with the correlation-consistent core-valence basis sets up to septuple-zeta quality. The effects of electron correlation beyond the MR-ACPF level of approximation were taken into account. The scalar relativistic and adiabatic (the diagonal correction) effects, as well as some of the nonadiabatic effects, were also discussed. The vibration-rotation energy levels of three isotopologues, BeH, BeD, and BeT, were predicted to sub-cm(-1) accuracy.     

An ab initio study of the electronic structure of diimide

Ab initio calculations on the structure and geometry of the three isomers of N₂H₂ (trans-diimide, cis-diimide, and 1,1-dihydrodiazine) were performed both on HF and CI level using gaussian basis sets with polarization functions. The trans and cis isomers have singlet ground states; the trans isomer is found to be lower in energy than the cis isomer by 6.9 kcal/mol (HF) and 5.8 kcal/mol (CI), respectively. The barrier for the trans-cis isomerization is predicted to be 56 (HF) and 55 (CI) kcal/mol. H₂ N=N has a triplet ground state with a non-planar equilibrium geometry and a rather long NN bond of 1.34 Å. Its lowest singlet state, however, is planar with an NN double bond of 1.22 Å; it is found to lie about 3 kcal/mol above the triplet and 26 kcal/mol above the singlet ground state of trans-diimide.     

Potential energy function for cation–peptide interactions: An ab initio study

A potential energy function is developed to represent the interaction of small monovalent cations, Li⁺, Na⁺, and K⁺, with the backbone of polypeptides. The results are based on ab initio calculations up to the 6-31G* level of the interactions of the ions with acetamide and N-methylacetamide. Basis set superposition errors are corrected with the counterpoise method. A systematic overestimate of the bond polarities is taken into account by an empirical scaling procedure that uses the ratio of the experimental to ab initio dipole moment. The calculated binding energies obtained with this procedure show consistent convergence with different basis sets and are in good agreement with experimental data on cation–water and cation–dimethylformamide systems. Investigations of the calculated ab initio potential energy surface indicate that the cation–peptide interaction is dominated by electrostatics and includes a nonnegligible contribution from polarization of the peptide group by the ion. The induced polarization results in a steeper-than-Coulombic interaction and cannot be described by fixed ion–peptide partial charges electrostatics. Atomic polarizabilities located on the atoms of the ligand molecule are introduced to account for the induced polarization in the empirical energy function. A ～1/r⁴ attractive interaction appears in the potential function. The resulting radial and angular dependence of the potential energy surface is well reproduced. © 1995 by John Wiley & Sons, Inc.     

Vibrational spectra, ab initio calculations, and ring-puckering potential energy function for gamma-crotonolactone

The infrared and Raman spectra of liquid and vapor gamma-crotonolactone have been collected. Both the experimental data and ab initio calculations show that the molecule is rigidly planar in its electronic ground state. This conclusion agrees with the previously reported microwave studies and is attributed to the conjugation between the C=C and C=O double bonds of the ring. The ring-puckering potential energy function was generated from ab initio calculations and was confirmed by the vapor-phase Raman spectra to be nearly harmonic. Density functional theory (DFT) calculations predict a harmonic ring-puckering frequency of 203 cm(-1) as compared to the observed vapor-phase Raman value of 208 cm(-1). The DFT calculations were also used to compute the infrared and Raman spectra of gamma-crotonolactone, and these agree very well with the experimental spectra.     

Electronic structure of NH+: An ab initio study

We report ab initio self‐consistent field MRSD‐CI electronic structure calculations of the NH⁺ cation. A basis set of DZ + POL quality augmented with Rydberg and bond functions was employed together with an extensive treatment of electron correlation. More than 50 electronic states of NH⁺ are reported, including doublets, quartets, and sextets. Leading configurations, vertical ionization energies of NH, vertical excitation energies of NH⁺, and potential energy curves are reported. Spectroscopic properties calculated for the known bound electronic states of NH⁺ are found in good agreement with experiment. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

An ab initio study of the structure and torsional modes of the H2SO4 molecule

Ab initio calculations at the STO—3G and 4—31G levels have been carried out for the H₂SO₄ molecule as a function of the pair of twist angles of the HO bonds about the respective OS bonds. Values for the remaining bond angles and lengths were taken from the recent microwave structural determination by Kuczkowski et al. The results indicate a minimum energy for a structure with a (sc, sc) conformation and C₂ symmetry, where sc denotes synclinal, or gauche. This structure corresponds to that observed. At a higher energy of 11.5 kJ mol^?1 (4—31G) there is a structure with a (+sc, ?sc) conformation and C_s symmetry. The torsional modes corresponding to the a and b irreducible representations of the C₂ point group are estimated to have frequencies of 280 and 265 cm^?1, respectively.     

An ab initio study of the structure of the 2-chloroethyl radical

Minimal and split-valence shell basis set calculations, both with and without d orbitais, predict the radical centre to be pyramidal, with the planar radical only 0.3 kcal mol^?1 higher. The barrier to internal rotation is 2 kcal mol^?1. There is no evidence of bridging from chlorine.     

An ab initio correlated study of the potential energy surface for the HOBr.H2O complex

The potential energy surface (PES) for the HOBr.H(2)O complex has been investigated using second- and fourth-order M?ller-Plesset perturbation theory (MP2, MP4) and coupled cluster theory with single and doubles excitations (CCSD), and a perturbative approximation of triple excitations (CCSD-T), correlated ab initio levels of theory employing basis sets of triple zeta quality with polarization and diffuse functions up to the 6-311++G(3dp,3df ) standard Pople's basis set. Six stationary points being three minima, two first-order transition state (TS) structures and one second-order TS were located on the PES. The global minimum syn and the anti equilibrium structure are virtually degenerated [DeltaE(ele-nuc) approximately 0.3 kcal mol(-1), CCSD-T/6-311++G(3df,3pd) value], with the third minima being approximately 4 kcal mol(-1) away. IRC analysis was performed to confirm the correct connectivity of the two first-order TS structures. The CCSD-T/6-311++G(3df,3pd)//MP2/6-311G(d,p) barrier for the syn<-->anti interconversion is 0.3 kcal mol(-1), indicating that a mixture of the syn and anti forms of the HOBr.H(2)O complex is likely to exist.     

An ab initio intermolecular potential energy surface for the F(2) dimer

Two analytical representations for the potential energy surface of the F(2) dimer were constructed on the basis of ab initio calculations up to the fourth-order of M?ller-Plesset (MP) perturbation theory. The best estimate of the complete basis set limit of interaction energy was derived for analysis of basis set incompleteness errors. At the MP4/aug-cc-pVTZ level of theory, the most stable structure of the dimer was obtained at R = 6.82 au, theta(a) = 12.9 degrees , theta(b) = 76.0 degrees , and phi = 180 degrees , with a well depth of 716 microE(h). Two other minima were found for canted and X-shaped configurations with potential energies around -596 and -629 microE(h), respectively. Hexadecapole moments of monomers play an important role in the anisotropy of interaction energy that is highly R-dependent at intermediate intermolecular distances. The quality of potentials was tested by computing values of the second virial coefficient. The fitted MP4 potential has a more reasonable agreement with experimental values.     

An ab initio LCAO-MO-SCF study of the electronic structure of phosphirane and thiirane

Wave functions have been determined for the C₂H₄PH and C₂H₄S cyclic molecules, using (951/52/3) and (95/52/3) uncontracted Gaussian basis sets for each molecule. From Mulliken population analyses and electron-density plots, it is shown that the valence orbitals of C₂H₄PH and C₂H₄S are closely related and that these are similar to the respective orbitals of cyclopropane.

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Zusammenfassung Für die zyklischen Moleküle C₂H₄PH und C₂H₄S wurden mit den nichtkontrahierten Basissätzen ((951/52/3) und (95/52/3)) von Gaußfunktionen Wellenfunktionen bestimmt. Die Mullikenschen Populationsanalysen sowie Diagramme der Elektronendichte zeigen, daß die Valenzorbitale von C₂H₄PH und C₂H₄S in enger Beziehung stehen und daß diese den entsprechenden Orbitalen des Cyclopropans ähnlich sind.

     

An ab initio study of formamide

Calculated energy and molecular properties of the ground and low-energy excited states of formamide are presented at the ground state geometry. Satisfactory results are obtained except for the ¹* energy which remains too high by 1 eV (which is nevertheless a large improvement over previous calculations). The predicted triplet energies lie at 5.4 eV (³ n*) and 5.8 eV (³*).     

An ab initio study of the structure,dissociation energy,and heat of formation of Na2S

The equilibrium geometries and fundamental frequencies of Na₂S are calculated at HF, MP2(FC, FU), and MP3 with the 6–31G(d) basis set and at HF and MP2(FC, FU) with the 6–31G(d) basis set, respectively. The total energy at MP2(FU)/6–31G(d)-optimized geometry is computed at MP4 with 6–311G(d, p), 6–311 + G(d, p), and 6–311G(2df, p), at QCISD(T)/6–311G(d, p), and at MP2/6–311G(3df, 2p) levels, respectively. The dissociation energy, the atomization energy, and the heat of formation for Na₂S are evaluated using the G1 and G2 models. The calculated results indicated that Na₂S in its ground state was a bent structure (C_2v). Electron correlation corrections on the bending angle are very significant. The equilibrium geometrical parameters are R_e(Na-S) = 2.45 Å and ∠Na-S-Na = 111.13° at the MP2(FU)/6–31G(d) level. The theoretically estimated dissociation energy, total atomization energy, and heat of formation are 67.07, 117.55, and 0.35 kcal mol⁻¹, respectively, at 298.15 K. © 1997 John Wiley & Sons, Inc.     

Some problems est the calculation of ab initio potential energy curves

The non-adiabatic and adiabatic approaches to the calculation of molecular energy levels are presented. The concept of the potential energy curve, its accuracy and limitations are discussed. Simplified approaches to calculations of the intermolecular potential energy curves are also considered.