Calculations of vibrational frequencies, Raman activities and degrees of depolarization for complexes involving water, methanol and ethanol

Raman activities and degrees of depolarization are reported for 14 complexes involving methanol, ethanol and water using the MP2/aug-cc-pVDZ model. For ethanol both trans and gauche isomers are considered. The red-shifts of the OH stretching and the blue shifts of the bending tau(CO-OH) mode were analyzed for the proton-donor molecules upon hydrogen bond. The shift of the nu(CO) stretching mode of the alcohol molecules are also analyzed and found to be specific giving characterization of the amphoteric relation, being positive for the proton-acceptor and negative for the proton-donor molecule. (c) 2008 Elsevier B.V. All rights reserved.

Realistic calculations of carbon-based disordered systems

Carbon nanotubes rank amongst potential candidates for a new family of nanoscopic devices, in particular for sensing applications. At the same time that defects in carbon nanotubes act as binding sites for foreign species, our current level of control over the fabrication process does not allow one to specifically choose where these binding sites will actually be positioned. In this work we present a theoretical framework for accurately calculating the electronic and transport properties of long disordered carbon nanotubes containing a large number of binding sites randomly distributed along a sample. This method combines the accuracy and functionality of ab initio density functional theory to determine the electronic structure with a recursive Green`s functions method. We apply this methodology on the problem of nitrogen-rich carbon nanotubes, first considering different types of defects and then demonstrating how our simulations can help in the field of sensor design by allowing one to compute the transport properties of realistic nanotube devices containing a large number of randomly distributed binding sites.

The role of carbon impurities on the Si(001)-c(4 x 4) surface reconstruction: Theoretical calculations

In this work we employ the state-of-the-art pseudopotential method, within a generalized gradient approximation to the density functional theory, combined with a recently developed method for the calculation of HREELS spectra to study a series of different proposed models for carbon incorporation on the silicon (001) surface. A fully discussion on the geometry, energetics and specially the comparison between experimental and theoretical STM images and electron energy loss spectra indicate that the Si(100)-c(4 x 4) is probably induced by Si-C surface dinners, in agreement with recent experimental findings. (C) 2009 Elsevier B.V. All rights reserved.

Vibrational properties, crystal X-ray diffraction structure and quantum chemical calculations on a divalent sulfur substituted phthalimide: 1H-Isoindole-1,3(2H)-dione, 2-[(methoxycarbonyl)thio]

Structural and conformational properties of 1H-Isoindole-1,3(2H)-dione, 2-[(methoxycarbonyl)thio] (S-phthalimido O-methyl thiocarbonate) are analyzed using a combined approach including X-ray diffraction, vibrational spectra and theoretical calculation methods. The vibrational properties have been studied by infrared and Raman spectroscopies along with quantum chemical calculations (B3LYP and B3PW91 functional in connection with the 6-311++G** and aug-cc-pVDZ basis sets). The crystal structure was determined by X-ray diffraction methods. The substance crystallizes in the monoclinic P2(1)/c space group with a = 6.795(1), b = 5.109(1), c = 30.011(3) angstrom, beta = 90.310(3)degrees and Z = 4 molecules per unit cell. The conformation adopted by the N-S-C=O group is syn (C=O double bond in synperiplanar orientation with respect to the N-S single bond). The experimental molecular structure is well reproduced by the MP2/aug-cc-pVDZ method. (C) 2010 Elsevier B.V. All rights reserved.

Spin contamination in XOO radicals X = F, Cl, Br, HO: How is the investigation of the HOOO radical affected

We have analysed the effect of spin contamination in the wavefunction of HOOO. At least, two solutions can be found for the HF wavefunction. One, lower in energy, presents a high spin contamination and gives qualitatively incorrect structural parameters. On the other hand, the less contaminated HF reference gives structural parameters that are in better agreement with experiment, and positive spin densities on all atoms. Some of the problems described during previous investigations of HOOO can now be traced to problems in the HF reference. For the first time we report a CCSD(T) estimation of the structure of HOOO cis employing a HF reference with small spin contamination. (C) 2008 Elsevier B.V. All rights reserved.

Azo-hydrazone tautomerism in protonated aminoazobenzenes: Resonance Raman spectroscopy and quantum-chemical calculations

The protonation effect on the vibrational and electronic spectra of 4-aminoazobenzene and 4-(dimethylamino)azobenzene was investigated by resonance Raman spectroscopy, and the results were discussed on the basis of quantum-chemical calculations. Although this class of molecular systems has been investigated in the past concerning the azo-hydrazone tautomerism, the present work is the first to use CASSCF/CASPT2 calculations to unveil the structure of both tautomers as well the nature of the molecular orbitals involved in chromophoric moieties responsible for the resonance Raman enhancement patterns. More specifically both the resonance Raman and theoretical results show clearly that in the neutral species, the charge transfer transition involves mainly the azo moiety, whereas in the protonated forms there is a great difference, depending on the tautomer. In fact, for the azo tautomer the transition is similar to that observed in the corresponding neutral species, whereas in the hydrazone tautomer such a transition is much more delocalized due to the contribution of the quinoid structure. The characterization of protonated species and the understanding of the tautomerization mechanism are crucial for controlling molecular properties depending on the polarity and pH of the medium.

A theoretical study on the XeF(2) molecule

A relativistic four-component study was performed for the XeF(2) molecule by using the Dirac-Coulomb (DC) Hamiltonian and the relativistic adapted Gaussian basis sets (RAGBSs). The comparison of bond lengths obtained showed that relativistic effects on this property are small (increase of only 0.01 angstrom) while the contribution of electron correlation, obtained at CCSD(T) or CCSD-T levels, is more important (increase of 0.05 angstrom). Electron correlation is also dominant over relativistic effects for dissociation energies. Moreover, the correlation-relativity interaction is shown to be negligible for these properties. The electron affinity, the first ionization potential and the double ionization potential are obtained by means of the Fock-space coupled cluster (FSCC) method, resulting in DC-CCSD-T values of 0.3 eV, 12.5 eV and 32.3 eV, respectively. Vibrational frequencies and some anharmonicity constants were also calculated under the four-component formalism by means of standard perturbation equations. All these molecular properties are, in general, ill satisfactory agreement with available experimental results. Finally, a partition in terms of charge-charge flux-dipole flux (CCFDF) contributions derived by means of the quantum theory of atoms in molecules (QTAIM) in non-relativistic QCISD(FC)/3-21G* calculations was carried out for XeF(2) and KrF(2). This analysis showed that the most remarkable difference between both molecules lies on the charge flux contribution to the asymmetric stretching mode, which is negligible in KrF(2) but important in XeF(2). (c) 2008 Elsevier B.V. All rights reserved.

A quantum chemical study on the formation of phosphorus mononitride

The chemical mechanism of the (1)PN formation was successfully studied by using the CCSD(T)/6-311++G(3df,3pd) level of theory. The (1)NH(3) + (3)PH and (4)P + NH(3) reaction paths are not energetically favorable to form the (1)PN molecule. However, the (3)NH + (3)PH, (4)N + (3)PH(3), (4)N + (3)PH, (4)P + (3)NH, and (4)P + (2)NH(2) reaction paths to form the (1)PN molecule are only energetically favorable by taking place through specific transition states to form the (1)PN molecule. The NH(3) + (3)PH, (4)N + (1)PH(3), NH(3) + (4)P, and (4)N + (2)PH(2) reactions are spin-forbidden and the probability of hopping for these reactions was estimated to be 0 by the Landau-Zener theory. This is the first detailed study on the chemical mechanism for the (1)PN formation. (C) 2009 Elsevier B.V. All rights reserved.

CASPT2 Study of the Potential Energy Surface of the HSO(2) System

The importance of the HSO(2) system in atmospheric and combustion chemistry has motivated several works dedicated to the study of associated structures and chemical reactions. Nevertheless controversy still exists in connection with the reaction SH + O(2) -> H + SO(2) and also related to the role of the HSOO isomers in the potential energy surface (PES). Here we report high-level ab initio calculation for the electronic ground state of the HSO(2) system. Energetic, geometric, and frequency properties for the major stationary states of the PES are reported at the same level of calculations:,CASPT2/aug-cc-pV(T+d)Z. This study introduces three new stationary points (two saddle points and one minimum). These structures allow the connection of the skewed HSOOs and the HSO(2) minima defining new reaction paths for SH + O(2) -> H + SO(2) and SH + O(2) -> OH + SO. In addition, the location of the HSOO isomers in the reaction pathways have been clarified.

Characterization and spectroscopic analysis of phenol-ethanol hydrogen bonded clusters

Different hydrogen bonded clusters involving phenol and ethanol are studied theoretically using MP2/aug-cc-pVDZ. Nine different 1: 1 clusters are obtained and analyzed according to their stability and spectroscopic properties. Different isomeric forms of ethanol are considered. Attention is also devoted to the spectral shift of the characteristic pi -> pi* transition of phenol. Using TDHF, CIS, CIS(D) and TDB3LYP in aug-cc-pVDZ basis set, all results agree that a red shift is obtained when phenol is the hydrogen donor and a blue shift is obtained in the opposite case. These results are used to rationalize the red shift observed for phenol in liquid ethanol. (C) 2010 Elsevier B.V. All rights reserved.

NMR Chemical Shielding and Spin-Spin Coupling Constants of Liquid NH(3): A Systematic Investigation using the Sequential QM/MM Method

The NMR spin coupling parameters, (1)J(N,H) and (2)J(H,H), and the chemical shielding, sigma((15)N), of liquid ammonia are studied from a combined and sequential QM/MM methodology. Monte Carlo simulations are performed to generate statistically uncorrelated configurations that are submitted to density functional theory calculations. Two different Lennard-Jones potentials are used in the liquid simulations. Electronic polarization is included in these two potentials via an iterative procedure with and without geometry relaxation, and the influence on the calculated properties are analyzed. B3LYP/aug-cc-pVTZ-J calculations were used to compute the V(N,H) constants in the interval of -67.8 to -63.9 Hz, depending on the theoretical model used. These can be compared with the experimental results of -61.6 Hz. For the (2)J(H,H) coupling the theoretical results vary between -10.6 to -13.01 Hz. The indirect experimental result derived from partially deuterated liquid is -11.1 Hz. Inclusion of explicit hydrogen bonded molecules gives a small but important contribution. The vapor-to-liquid shifts are also considered. This shift is calculated to be negligible for (1)J(N,H) in agreement with experiment. This is rationalized as a cancellation of the geometry relaxation and pure solvent effects. For the chemical shielding, U(15 N) Calculations at the B3LYP/aug-pcS-3 show that the vapor-to-liquid chemical shift requires the explicit use of solvent molecules. Considering only one ammonia molecule in an electrostatic embedding gives a wrong sign for the chemical shift that is corrected only with the use of explicit additional molecules. The best result calculated for the vapor to liquid chemical shift Delta sigma((15)N) is -25.2 ppm, in good agreement with the experimental value of -22.6 ppm.

Dipole polarizability and Rayleigh light scattering by the hydrated electron

Assuming the existence of a confined state of the electron in bulk water the polarizability of the hydrated electron is analyzed. Statistically uncorrelated supermolecular structures composed of seven water molecules (first solvation shell) with an extra electron were extracted from classical Monte Carlo simulation and used in quantum mechanical second-order Moller-Plesset calculations. It is found that the bound excess electron contributes with 274 a.u. to the total dipole polarizability of 345 a.u. for (H(2)O)(7)(-). From the calculated polarizabilities the Rayleigh elastic light scattering properties are inferred and found to considerably enhance activity and light depolarization. (C) 2009 Elsevier B.V. All rights reserved.

Conformational behavior of different possible ways of oligoglycine formation in a solvent-free environment

The possible ways for glycine oligopeptide formation in gas phase, both in the extended P-strand like conformation and folded 2(7)-ribbon like conformations are analyzed using quantum chemical calculations. We focus on the sequential formation of peptide bond through upgradation of the immediate lower order molecule and observe the consequences in other related processes like oligoglycine formation through simultaneous peptide linkage of n glycine monomers and interchange of molecular conformation through peptide linkage. A comparison is made between the structures and binding energies obtained for both conformers. All binding energies are increased by the zero-point energy contribution. The role of electron correlation effects is briefly analyzed. The folded 2(7)-ribbon-like conformations in vacuo are found to be more stable in comparison to the extended structure. (c) 2007 Elsevier B.V. All rights reserved.

Electronic and magnetic properties of Ti and Fe on graphene

The structural, electronic and magnetic properties of Fe and Ti atomic wires and the complete covering when adsorbed on graphene are presented through ab initio calculations based on density functional theory. The most stable configurations are investigated for Fe and Ti in different concentrations adsorbed on the graphene surface, and the corresponding binding energies are calculated. The results show a tendency of the Ti atoms to cover uniformly the graphene surface, whereas the Fe atoms form clusters. The adsorption of the transition metal on the graphene surface changes significantly the electronic density of states near the graphene Fermi region. In all arrangements studied, a charge transfer is observed from the adsorbed species to the graphene surface due to the high hybridizations between the systems.

Energetics and stability of vacancies in carbon nanotubes

In this work we present ab initio calculations of the formation energies and stability of different types of multi-vacancies in carbon nanotubes. We demonstrate that, as in the case of graphene, the reconstruction of the defects has drastic effects on the energetics of the tubes. In particular, the formation of pentagons eliminates the dangling bonds thus lowering the formation energy. This competition leads to vacancies having an even number of carbon atoms removed to be more stable. Finally the appearance of magic numbers indicating more stable defects can be represented by a model for the formation energies that is based on the number of dangling bonds of the unreconstructed system, the pentagons and the relaxation of the final form of the defect formed after the relaxation. (C) 2011 Elsevier Ltd. All rights reserved.