Density functional study of nitrogen oxides

Theoretical study on PbS, PbO and their anions

An MP2 and density functional study of the oxides of nitrogen

The first-principles approach is applied to study the electronic, vibrational, and superconducting properties of B1-NbC. We used a variety of density functional theory methods that involve local density approximation (PW), generalized gradient approximation; PBE, PBEsol, and GGA functionals corrected with nonlocal correlation functionals (rVV10, vdW-DF2, vdW-DF3). Although the electronic band structures from different methods show an overwhelming agreement, the phonon dispersion curves display significant differences at certain points. The optical phonons make an appreciable part of the electron-phonon coupling constant, λ (up to 23%). This contribution is found to be the largest for PW and the smallest for PBE whereas the nonlocal functionals give intermediate values. This behavior is in perfect analogy with the interaction energies for the three types of methods. The nonlocal functionals, rVV10 and vdW-DF3, fine tuned λ to obtain a value in close agreement with a recently reported experimental value of λ = 0.848 Yan et al. Despite the fact that variations in λ due to nonlocal functionals are not very large, the effects on the superconducting transition temperatures are significant. In addition, the Coulomb screening potential, μ * = 0.1525 gives superconducting transition temperature in the best agreement with the experiment.

We have investigated the N2O–HDO molecular complex using ab initio calculations at the CCSD(T)-F12a/aug-cc-pVTZ level of theory and using cavity ring-down spectroscopy to probe an HDO/N2O/Ar supersonic jet around 1.58 μm. A single a-type vibrational band was observed, 13 cm−1 redshifted compared to the OH+OD excited band in HDO, and 173 vibration-rotation lines were assigned (Trot ≈ 20 K). A weighted fit of existing microwave and present near infrared (NIR) data was achieved using a standard Watson's Hamiltonian (σ = 1.26), producing ground and excited states rotational constants. The comparison of the former with those calculated ab initio suggests a planar geometry in which the OD rather than the OH bond in water is almost parallel to NNO. The equilibrium geometry and dissociation energy (De = –11.7 kJ/mol) of the water–nitrous oxide complex were calculated. The calculations further demonstrate and allow characterising another minimum, 404 cm−1 (ΔE0) higher in energy. Harmonic vibrational frequencies and dissociation energies, D0, were calculated for various conformers and isotopic forms of the complex, in both minima. The absence of N2O–D2O from dedicated NIR experiments is reported and discussed.

The equilibrium geometries, the bond dissociation energies, harmonic vibrational frequencies, and infrared intensities of peroxyacetic nitric anhydride (PAN), peroxyacetyl radical (PA), and NO2 which are important species in environmental chemistry, have been studied using high level of ab initio calculations. The global minimum energy conformation of the PAN molecule is found to be s-cis-PAN. The; predicted enthalpy and entropy changes at 1 atm and 298.15 K for the thermal decomposition of PAN are 25.8 kcal/mol and 42.3 cal/(mol.K), respectively. These values are in excellent agreement with the experimental values. A characteristic IR band of PAN appears at 1232 cm(-1), which is a CH3 rocking mode coupled with CO and OO stretches. The corresponding band of the PA radical appears at 1216 cm(-1), about 10 cm(-1) lower than that of PAN. Thus, determining the concentrations of PAN based on only this band would not be so reliable. We find that the PA radical has a strong IR band at 1150 cm(-1) due to the O-O stretch mode, while the PAN molecule has the corresponding band at 977 cm(-1). Since PAN and the PA radical do not have other IR bands at this region, this characteristic band could be used for identification and measurement of the PA radical and thus for spectroscopic study of thermal decomposition mechanism of PAN.

A theoretical study of diazirine (H2CN2), diazirinyl radical (HCN2) and their related cations <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si11.gif" overflow="scroll"><mml:mrow><mml:mo stretchy="false">(</mml:mo><mml:msub><mml:mrow><mml:mi mathvariant="normal">H</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub><mml:msubsup><mml:mrow><mml:mi mathvariant="normal">CN</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow><mml:mrow><mml:mo>+</mml:mo></mml:mrow></mml:msubsup><mml:mo>,</mml:mo><mml:msubsup><mml:mrow><mml:mi mathvariant="normal">HCN</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow><mml:mrow><mml:mo>+</mml:mo></mml:mrow></mml:msubsup><mml:mo stretchy="false">)</mml:mo></mml:mrow></mml:math>: molecular structure, energetics and ionization potential

We present a new quantum chemical method for the calculation of the equilibrium geometry and the harmonic vibrational frequencies of molecular systems in dense medium at high pressures (of the order of GPa). The new computational method, named PCM-XP, is based on the polarizable continuum model (PCM), amply used for the study of the solvent effects at standard condition of pressure, and it is accompanied by a new method of analysis for the interpretation of the mechanisms underpinning the effects of pressure on the molecular geometries and the harmonic vibrational frequencies. The PCM-XP has been applied at the density functional theory level to diborane as a molecular system under high pressure. The computed harmonic vibrational frequencies as a function of the pressure have shown a satisfactory agreement with the corresponding experimental results, and the parallel application of the method of analysis has reveled that the effects of the pressure on the equilibrium geometry can be interpreted in terms of direct effects on the electronic charge distribution of the molecular solutes, and that the effects on the harmonic vibrational frequencies can be described in terms of two physically distinct effects of the pressure (curvature and relaxation) on the potential energy for the motion of the nuclei.

The radical scavenging activities of the two flavonoids, aureusidin and bracteatin, have been explored by using density functional theory (DFT) with the B3LYP exchange correlation functional. These compounds are characterized by a high antioxidant activity which confers to them pharmacological properties that are useful for the treatment of several diseases. The minimum energy conformations obtained from the energy scan is a further geometry optimization performed at the B3LYP‐DFT level with 6‐311G** basis set. Harmonic vibrational frequencies have been performed on the optimized structures to ensure that the geometries obtained were real minima. For radicals, the geometry optimizations and frequency calculations were also done at the same mode of theory. Single point energy was calculated at the same level of theory in the gas phase and in two solvents with different polarities (water and benzene) with the aim of computing the bond dissociation enthalpy (BDE) for the OH bonds and the ionization potentials (IPs). The OH BDE parameter calculated for each OH group seems to be the best indicator of the antiradical property of these compounds. This demonstrates the importance of the H‐atom transfer mechanism to explain their capacity to scavenge the free radicals. The active sites are identified. BDE for these systems do not follow the same trends in gas and solution phases. The values of reactivity‐based descriptors such as total energy, orbital energy gap, maximum hardness principle, and electronegativity are also related with the antioxidant activity. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011

The effects of relativity on the bond lengths, vibrational frequencies, dissociation energies, and dipole moments of the ground states of the group IB hydrides MH, chlorides MCl, and dimers M2 (M=Cu, Ag, and Au) have been studied by relativistic density functional theory (DFT) with the B88 plus one-parameter progressive (BOP) exchange–correlation functional. The relativistic effects were included through a scalar relativistic scheme by the elimination of the small components (RESC) of the four-component Dirac spinors. Comparisons were made between all-electron results using the nonrelativistic Hamiltonian, results with quasi-relativistic effective core potentials (ECP), and results with a spin-free RESC scheme. The RESC approach clearly works very well. The bond distances, vibrational frequencies, and dissociation energies show a good agreement with the experiment. The expected trends of bond length decrease, harmonic vibrational frequency increase, and dipole moment decrease with relativity are found. Although the dissociation energy increases with the relativity for hydrides and dimers, the reverse trend is observed for chlorides. A quasi-relativistic ECP also works well for hydrides. However, ECP gives rather poor description for chlorides and dimers. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 75: 757–766, 1999

Vibrational spectra and fundamental structural assignments from HF and DFT calculations of methyl benzoate

Comparison of experimental and ab initio HF and DFT vibrational spectra of benzimidazole

Vibrational spectroscopy investigation using ab initio and density functional theory analysis on the structure of 5-amino- o-cresol

FT-Raman and FT-IR spectra, ab initio and density functional studies of 2-amino-4,5-difluorobenzoic acid

Vibrational spectroscopy investigation using ab initio and density functional theory analysis on the structure of 2-amino-5-methylphenol

Ab initio and density functional (BHLYP, B3LYP, and BP86) quantum mechanical methods were applied to predict the structures, properties, and stabilities (M = Li, Na, K) three-membered ring systems. These structures derive from the previously theoretically and experimentally characterized parent homonuclear cyclogallenes M2(GaH)3. Basis sets of triple-ζ plus double polarization augmented with diffuse functions (TZ2P+diffuse) quality were employed at self-consistent field (SCF) and density functional levels of theory. Equilibrium geometries, harmonic vibrational frequencies, and chemical shifts are reported to facilitate experimental verification of the computed structures. All title compounds are minima and their geometries as well as their properties compare well with the cyclogallene three-membered ring compounds M2(GaH)3. Singly and even may well be feasible experimental targets. The large negative nucleus independent chemical shifts (NICS), used as a criterion for aromatic cyclic delocalization, sugges...