Byab Initio Research Articles

Estudo das ligações de hidrogênio para dímeros formados pelas moléculas de H2O, NH3, HF, HCl e HBr através de cálculos baseados em primeiros princípios

HYDROGEN BOND STUDY BY AB INITIO CALCULATIONS FOR DIMERS FORMED FROM H2O, NH3, HF, HCl AND HBr MONOMERS. Hydrogen bond energies of fifteen dimers were calculated using the large basis set 6-311++G(3df,3pd), at HartreeFock (HF) level including Moller-Plesset (MP2) calculations. The procedure for obtaining such energies were based on the dimer’s energy rise provoked by increasing in intermolecular distance of the system component units. Deviations from a strictly linear hydrogen bond were investigated and rotational barriers were also computed allowing the calculation of the second order attractive interactions. In order to provide a more objective definition of hydrogen bond, a lower energy limit was proposed in place of the merely empirical parameters employed in the classical definition.

A kinetic, spectral and theoretical investigation on the role of oxygen in the radiolytic oxidation of a sorbityl cyclic acetal

The oxidation process of the cyclic acetal sorbitylfurfural (SF) has been thoroughly examined from the kinetic, spectroscopic and theoretical point of view. Oxidation has been initiated by the radiolitically produced OH radical in the presence of variable oxygen amounts. Two competing reaction pathways are evidenced which lead to quite different products, although they do not affect the acetal ring integrity. The peroxidation of the hydroxylated furanic ring (k 4=(6.1±0.9)×108 M−1 s−1) maintains the ring structurevia HO 2 • elimination (k 6=(1.9±0.4)×105 s−1). Unlike that, the peroxidation of the pseudo-allylic radical (k 5=(1.9±0.9)×109 M−1 s−1), formedvia β-cleavage, fixes the destructured intermediate, leading to a tetroxide, which slowly decomposes through a Russell mechanism (k 8=(2.3±0.6)×102 s−1). It is confirmed that the steady state concentration of the tetroxide is very low, which suggests a molar absorption coefficient for it around 1.2×104 M−1 cm−1 at 265 nm. The end products of the latter pathway have been characterized as carboxylic and butenald-sorbitol derivatives. The kinetic and spectral data of every step of the process have been fitted by the above outlined mechanism. The energetics of the mechanism has been detailed byab initio computations as well, carrying further substantiation to it. Semi-empirical calculations were also employed to describe the spectral properties of each intermediate.

Pressure Induced Metallization in the ε Phase of Solid Oxygen byab initio PseudopotentialPlane-Wave Calculations

We perform an ab initio study on the electronic structure andcharge density of the ε-oxygen under high pressure, which is obtainedby powder x-ray diffraction experiment recently. Our results show thatthe hybridization among the σg*, πu and πg*bands in the ε-oxygen are not significant even at megabar pressure.Pressure-induced metallization occurs due to the band overlapping nearthe Fermi level at about 50 GPa. A new network along the b-axis isformed and the O8 characteristic in the ε phase disappearsabove 50 GPa even though the symmetry remains unchanged.

The˙NO3 Radical and Related Nitrogen Oxides, Characterized byAb Initio Calculations of Thermochemical Properties

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A Reinvestigation of the Structure and Torsional Potential of N2O5 by Gas-Phase Electron Diffraction Augmented byAb Initio Theoretical Calculations

Gaseous N2O5 consists of two NO2 groups bonded to a bridging O-atom to form a nonlinear N−O−N moiety. The NO2 groups undergo slightly hindered internal rotation around the bonds to the bridge so that instantaneous composition of the gaseous system is characterized by molecules with all combinations of torsion angles. In an earlier investigation, an attempt was made to determine the coefficients for an empirical form of the double-rotor torsional potential, and the bond lengths and bond angles measured subject to assumptions that the structure of the O−NO2 groups was invariant to torsion angle and that these groups had C2v symmetry. The system has now been reinvestigated in terms of a more realistic model in which this symmetry restriction was relaxed, account was taken of structural changes in the NO2 groups with torsion angle as predicted by ab initio theory at the B3LYP/6-311+G* level, and a more convenient form of the torsional potential was assumed. The most stable conformation has C2 symmetry with torsion angles τ1 (defined as ∢(N−O−N=O4)) equal to τ2 (defined as ∢(N−O−N=O6)) equal to 33.7°; because of the broad potential minimum in this region, the uncertainty in these angles is difficult to estimate, but is probably 3 – 4°. The results for the bond lengths and bond angles for the most stable conformation are rg(N−O)=1.505(4) A, rg(N=O)=1.188(2) A, ∢α(N−O−N)=112.3(17)°, ∢α(O=N=O)=134.2(4)°, 〈∢α(O−N=O)〉=112.8(2)°. The difference between the symmetry-nonequivalent O−N=O angles is estimated to be ca. 6.7° with the larger angle positioning the two N=O bonds on different NO2 groups nearest each other. These average values are similar to those obtained in the original study. The main difference is found in the shape of the torsional potential, which at τ1/τ2=0/0 has a saddle point in the present work and a substantial peak in the earlier. The implication of the torsion-angle findings for electron-diffraction investigations of this type is discussed.

Dipole moment and tautomeric form of 3(5)-nitropyrazole in dioxane solution

The dipole moments of 3(5)-nitropyrazole, its methyl-substituted derivatives, and H-complexes with dioxane were measured experimentally and estimated byab initio calculations (6-31G* basis set). Comparison of the experimental and calculated dipole moments suggests a shift of the tautomeric equilibrium toward the 3-nitroisomer.

Structure of guest-host complexes of β-cyclodextrin with arenes: a quantum-chemical study

The structure of β-cyclodextrin (β-CD), as well as the structure and energetics of β-CD-naphthalene, β-CD-fluorene, β-CD-phenanthrene, β-CD-cyclohexane (1:1), and β-CD-naphthalene (2:2) inclusion complexes was studied by the semiempirical MNDO/PM3 method. Calculations of a β-CD-naphthalene-cyclohexane (1:1:1) complex were also performed. The minimum heat of formation was found for the symmetric β-CD conformation withC 7 symmetry axis. The structure is stabilized by the ring of interunit H-bonds formed by the protons of the 2-OH groups and the O atoms of the 3′-OH groups of the glucose units. Preferableness of this orientation of interunit H-bonds was confirmed byab initio calculations of the molecule of α-(1–4)-glucobiose (maltose) in the MP2/6-31G(d,p)//6-31G(d,p) approximation. The formation of any inclusion compounds of β-CD with arenes is energetically favorable: the complexation energy varies in the range −9 to −12 kcal mol−1. Among complexes with naphthalene, that of composition 2:2 is the most energetically favorable, which is in agreement with experimental data. In this complex, β-CD exists as a dimer of the “head-to-head” type, in which both partners are linked by a system of H-bonds. The structure of the “head-to-head” dimer of β-CD was simulated byab initio calculations of the H-bonded dimer of α-d-glucose in the RHF/6-31G(d,p) approximation. In the dimer, both components are linked by a pair of H-bonds formed by the protons of the 3-OH groups and the O atoms of the, 2-OH groups. The dimerization energies obtained fromab initio and semiempirical MNDO/PM3 and AM1 calculations differ by about 2.5 times (8.6vs 3.2 and 3.8 kcal mol−1, respectively).

Study on the effects of fluorine and oxygen deficiency on YBa2Cu307 byab initio method

Study on the effects of fluorine and oxygen deficiency on YBa2Cu307 byab initio method

The thermal stability of S-nitrosothiols: experimental studies and ab initio calculations on model compounds

One factor responsible for the enhanced thermal stability of S-nitroso-N-acetylpenicillamine (SNAP), compared with S-nitroso-N-acetylcysteine (SNAC), has been shown, by ab initio calculations on model compounds, to be steric interactions in the dimerisation reaction leading to disulfide formation. Studies using DSC and TGA indicate that the two gem methyl groups in SNAP do not have a substantial effect on the strength of the –S–NO bond.

Accurate electrical and spectroscopic properties ofX 1?+ BeO from coupled-cluster methods

This paper reports a series of coupled-cluster (CC) calculations through CCSDT on the theoretically challenging ground state of the BeO molecule. Along with CC methods, quadratic configuration interaction (QCI) approximations to CC theory have been used (QCISD and QCISD(T)), which show several dramatic failings. Equilibrium electrical properties (μ, α xx , and α zz ) and basic spectroscopic properties (r e, θe,D e, and infrared intensity (I)) have been computed. Basis set and electron correlation effects are analyzed in order to arrive at accurate values of the dipole moment and polarizability, which are not known experimentally. For the dipole moment, we obtain a value of 6.25 D, with an uncertainty of about 0.1 D. For α xx and α zz , we suggest respective values of 32 and 36 atomic units (a.u.) and error bars of about 1 and 2 a.u. With extended basis sets, the spectroscopic propertiesr e, θe, andD e are reproduced to high accuracy, which is the first time this has been achieved for this species byab initio methods. At the highest calculation levels,I is predicted to be very small. AlthoughI has not been measured, some support for this prediction comes from a recent infrared study of BeO-rare gas complexes. The QCI methods are shown to be much more sensitive to basis set, and even with large basis sets yield values of α zz andI which differ from CC results by an order of magnitude and three orders of magnitude, respectively. These differences doubtless arise from the importance of single excitations (T 1) for this molecule, as several terms involvingT 1 are neglected in the QCISD approximation compared with CCSD. We also report CC calculations with Brueckner orbitals, which yield results similar to those obtained with restricted Hartree-Fock orbitals.

Muon level-crossing resonance in Si∶Al

μLCR spectra from single-crystal Si∶Al have been taken at 30 K with the field along the 〈111〉 direction. The spectra show strong resonances near 12 and 50 mT, and weaker resonances near 38 and 42 mT. There is also indication of lines near 15 and 21 mT. The strong signals are due to the well-known zero crossings for Mu* in Si, and the weak signals may be associated with the quadrupole interaction of27Al, as predicted byab initio electronic structure calculations.

Factors contributing to phantom bonds in inorganic molecules: Interpretations and predictions

The validity of various qualitative proposals for interpreting and predicting the existence of short contacts between formally non-bonded atoms, as in cyclodisiloxane and related inorganic ring systems, is critically evaluated. The models range from simple considerations of geometric constraints, lone pair repulsions and π-complex formation to proposals such as the unsupported π-bond model and the σ-bridged-π bond concept. It is pointed out that a unified description based on a combination of closed and open 3-centre 2-electron bonds is possible. The role of hybridisation is emphasized in the short phantom bond computed in an earlier model system. These insights are used to predict structures with exceptionally short Si..Si and B..B phantom bonds. The proposals are confirmed byab initio calculations.

Electronic structure and the hydrogen-shift isomerization of hydrogen nitryl HNO2

Electronic structure of hydrogen nitryl HNO2, a yet not identified entity, and the path of its possible isomerization totrans-HONO have been investigated byab initio SCF and MRD-CI computations using the 6-31G** basis set. HNO2 isC2v-symmetric and its ground state (1A1) is less stable thantrans-HONO by 66 kJ/mol (with the SCF vibrational zero-point energy correction). The lowest two excited singlet states (1A2 and1B1) are nearly degenerate, their vertical excitation energies being predicted to be 4.8 eV. The isomerization path is traced by the CASSCF procedure and the activation barrier height is evaluated by the CI treatment. HNO2 in its ground state isomerizes totrans-HONO by maintaining the planar (Cs-symmetric) structure. The activation energy is calculated to be 171 kJ/mol, which is clearly lower than the calculated H-N bond energy (253 kJ/mol). The transition state seems to be more adequately described as an interacting system of proton and the nitrite anion rather than as a pair of two fragment radicals.

The phosphorescence of benzene obtained byab initio and semi-empirical calculations

Radiative decay and phosphorescence of triplet stare benzene is doubly -orbital and spin- forbidden and is only activated through vibronic coupling among the manifold of triplet states. For this reason the determination of lifetime and transition moments for the decay of triplet benzene has posed a considerable challenge to both theory and experiment. In the present work we have addressed the triplet benzene problem at several levels of theory; by truncated perturbation theory and semiempirical, CNDO/S-CI, calculations; by complete sum-over-state calculations as implemented in recentab initio multiconfiguration quadratic response (MCQR) theory; and by direct MCQR calculations of vibronic phosphorescence. The vibronic coupling is in the two former cases treated by the Herzberg-Teller (H-T) perturbation theory, involving four main mechanisms for the phosphorescent decay of triplet benzene. The results and interpretations given by these approaches as well as their merits and limitations are presented and discussed in some detail. Our calculations indicate that the phosphorescent decay of the3B1u state takes place predominantly through vibronic coupling along thee2g mode. We obtain a phosphorescence that is almost completely out-of-plane polarized, which is in line with more recent measurements by the microwave-induced delayed phosphorescence technique, and could reproduce quite well the intensity ratios for different vibronic bands obtained in that experiment. The final triplet state lifetime is the result of a delicate sum of contributions from several vibronic degenerate and non-degenerate modes. The direct vibronic phosphorescence calculations predict a long lifetime, about one minute — 68 seconds for the best wavefunction — and seem to focus on a doubling of the assumed, albeit not established, “best experimental” value for the radiative lifetime of triplet benzene; ⋍ 30 seconds.

Monosubstituted benzenes. Some features of the electron density distribution obtained byab initio calculations

A comparative analysis was made of the electron density distribution in monosubstituted benzenes by the nonempirical LCAO-MO SCF STO-3G method. It was shown that the effective charge that appears at the carbon atoms of the benzene ring as a result of introduction of the substituent is divided into π and σ components in the same proportions, irrespective of the position of the carbon atoms (ortho, meta, para). It was established that there are also linear relationships between the π charges of theortho, meta, andpara carbon atoms of monosubstituted benzenes. Thus, the changes in all the π and σ charges of theortho, meta, andpara carbon atoms during substitution in benzene are related linearly to each other. The obtained equations in the generalized form reflect the response of the benzene ring to substitution.

Relativistic treatment of excited electronic states of atomic copper

The2Sg (d10s1),2Dg (d9s2),2Pu (d10p1),2,4{F,P}u (d9s1p1) states of copper as well as the1Sg (d10) state of the positive copper ion are studied byab initio methods. Relativistic wavefunctions are determined variationally solving a one-component no-pair equation. This approximation makes it possible to treat all the states in a common set of orbitals. It is found that differential relativistic effects for the excitation energies are independent of the one-particle basis employed. The first-order perturbation estimate using the mass-velocity and Darwin operators depends critically on the description of the 3s and 3p core electrons. Among the various one-particle sets tested,2Dg orbitals, with the (4s, 4p) near-degeneracy effects included in the orbital optimization step, are most appropriate for the correlation treatment. They give an error of 0.3 eV for the2Sg–2Dg separation only slightly inferior to our best result employing parent orbitals for both states. All other states agree with experiment to within 0.2 eV. The first-order spin-orbit splitting of the2Dg state (−2006 cm−1) is in excellent agreement with the experimental value.

The influence of Na+ on neighbouring hydrogen bonds of aliphatic amino acid

The influence of Na+ on hydrogen bonds of the OH ⋯ O and NH ⋯ O type between an aliphatic amino acid (glycine zwitterion) and water is investigated byab initio calculations with minimal Gaussian basis sets. Distortion of the hydration shell caused by Na+, and interaction energies contributing to the over-all stabilization are discussed.

Ab initio calculations, electronegativity equalisation and group electronegativity

A correspondence betweenab initio calculations, the principle of electronegativity equalisation and group electronegativity has been established within the framework of Mulliken population analysis. Using this we have calculated electronegativities of some 37 groups/atoms. These electronegativities show excellent linear correlation with1JCC coupling constants in monosubstituted benzenes and Inamoto’si scale and a satisfactory one with Wells’ group electronegativity data. The correspondence however required a scaling of charge (obtained byab initio calculations) and a proportionality between the electronegativity of the neutral group and its hardness. It is shown that using these electronegativity values it is possible to calculate group charges in molecules where groups under consideration interact with each other through σ bond only.

Ab initio pair potentials for the ionic lithium-formate system

The potential energy surfaces for the interatomic interaction in the Li+HCOO− system have been investigated byab initio methods within the rigid-molecule approximation. Analytical potential expressions were fitted to 133 calculated SCF energies for the Li+-HCOO− interaction, 42 SCF energies for the Li+-Li+ interaction, and 332 SCF energies for the HCOO−-HCOO− interaction. The global minimum on the Li+-HCOO− surface is −170 kcal/mol and corresponds to the lithium ion lying on the C2 axis of the formate ion at 2.2 A from the carbon atom on the oxygen side. The cation-cation and anion-anion interactions are repulsive everywhere, although the potential surface is markedly anisotropic for the HCOO−-HCOO− interaction.

Anab initio pair potential for the interaction between a water molecule and a formate ion

The potential surface for the interaction between a rigid formate ion and a rigid water molecule has been investigated byab initio methods. An analytical potential expression was derived to fit the 591 calculated SCF energies. The global minimum on the surface is −16.3 kcal/mol and corresponds to a bifurcated bonding situation.