A HF/CI-SD study of the low-lying states of nitroprusside ion

Since the discovering of two photoexcited metastable states of crystalline sodium nitroprusside, Na2Fe(CN)5NO.2H2O (SNP), showing rather long lifetimes at temperatures below 160 K, much effort has been devoted toward the study of its electronic structure. Despite this tremendous effort the nature of the frontier orbitals and the related low energy excitations remains controversial. Early calculations, EHT, showed the HOMO as mainly the metallic 3d orbital while the LUMO had a major p* (NO) contribution. However INDO calculations, clearly set the metal d orbital many electron-volts deep in core. The vertical electronic spectrum have been estimated through ab initio HF/CI-SD with a double-zetaF quality basis set. The ab initio results support Bottomley and Greins interpretations and assign the first electronic transitions to ligand-to-ligand charge-transfer excitation from trans-cyano to nitrosyl ligands. The corresponding oscillator strengths have been calculated showing comparable intensity with the experimental results. The excitation energy of the metal ® NO charge-transfer transition, 8e ® 13e (d xz,d yz ® p* NO) have been estimated to be at 4.52 eV and show a rather intense absorption band. The second CT excitation, 1b2 ® 13e (d xy ® p* NO), pointed by previous works as a typical CT band, exhibits a small intensity at 5.04 eV. In the calculations it was observed that SCF orbital ordering are rather dependent on the metal basis set used. Metallic minimal basis set show results in close agreement with EHT early calculations while double-zeta basis set pushes the metallic d orbitals deep away from the HOMOs. The HF orbital ordering has been used to interpret photochemical and thermoanalysis experiments on SNP and the results seem to fit properly with the calculated properties

Desenvolvimento de metodos ab-initio para estudos envolvendo quebra de simetria em sistemas moleculares de grande porte

Orientadores : Yuji Takahata, Marco Antonio Chaer NascimentoTese (doutorado) - Universidade Estadual de Campinas, Instituto de QuimicaDoutorad

O uso de pseudopotenciais e modelos HF/MP2/DFT na previsão de freqüências vibracionais em complexos metálicos The use of pseudopotentials and HF/MP2/DFT models for the prediction of vibrational frequencies of metal complexes

<abstract language="eng">Four different pseudopotentials and three methodologies were employed in the calculation of the geometry and the frequencies of metal complexes like [M(NH3)2X2] [X=halogen, M=Zn, Cd], and [Hg(NH3)2]Cl2. The vibrational assignments were carefully checked and compared to the theoretically calculated ones. Graphical procedures were employed to estimate family errors and their average behavior. The calculated results show the SBK-X basis set with the best results for the geometries and calculated frequencies, for individual species and statistical results. Its use is recommend, mainly if the neighborhood atoms are described with similar pseudopotentials. Excellent results were also obtained with the Hay and Wadt pseudopotential

O modelo AM1 na previsão de frequências vibracionais The vibration frequencies predicted by the AM1 model

<abstract language="eng">We analyse vibrational frequencies of 168 compounds with the AM1 model concerning its experimentally observed gaseous frequencies. Stretching of CH, NH, OH and CO bonds, its related bending frequencies, and the CC frame movements are the studied vibrations. The results show problems with the AM1 vibrational splittings. Often symmetric stretching frequencies, like in CH3, CH2 and NH3, appear switched with the corresponding antisymmetrical ones. Among the studied vibrations many stretchings are overestimated, while bendings oscillate around experimental values. Fluorine stretchings, NN, OO, CH, double and triples CC bonds and cyclic hydrocarbon breathing modes are always overestimated while torsions, umbrella modes and OH/SH stretching are, in average, underestimated. Graphical analysis show that compounds with the lowest molecular masses are the ones with the largest difference to the experimental values. From our results it is not possible to fit confortably the calculated frequencies by a simple linear relationship of the type, n(obs)=a*n(AM1). Better aggreement is obtained when different curves are adjusted for the stretching and bending modes, and when a complete linear function is used. Among our studies the best obtained statistical results are for CH, NH and OH. The conclusions obtained in this work will improve the AM1 calculated frequencies leading to accurate results for these properties

Vibrational study of dialkylphosphonates: di-n-propyl- and di-i-propylphosphonates by semiempirical and ab initio methods

11 p. : il.Fourier transform infrared and Fourier transform Raman spectra of n-C3H7 and i-C3H7 dialkylphosphonates have been obtained. Semiempirical AM1 and the ab initio orbital molecular RHF/6-31G∗ theories have been used to study the molecular geometry, and the harmonic vibrational spectra with the purpose to assist the experimental assignments of these compounds. An extensive discussion on the assignment of the C–C, C–O, P–O and P=O stretching is carried out based on experimental data of compounds which have the propyl and isopropyl groups, as well as comparing the vibrational spectra of propane. Most of the RHF/6-31G∗ and AM1 results, once applied the appropriate scaling factor, showed an excellent agreement with the experimental wavenumbers. A few calculated frequencies related to CC and CO stretching do not agree well with the experimental trends

Configuration interaction simulation of the NEXAFS photoabsorption spectrum of naphthalene

The near-edge X-ray absorption fine structure (NEXAFS) spectrum of naphthalene was analysed theoratically using the final state approximation rule. ROHF/SCF calculations were carried out for a localized core-hole cationic state under the Bagus-Schaffer scheme for each channel of inequivalent excitations. CI calculations were performed allowing single and double excitations from carbon 1s electrons to the first ten virtual orbitals. The energies and oscillator strengths were calculated. The final spectrum was generated by convolution of Gaussian peaks with 0.5 eV as FWHM of the statistically-averaged calculated transitions. The theoretical result shows remarkable agreement with the experimental high-resolution NEXAFS data measured at BESSY II. Work is in progress to get better control of the variational wave function collapse problem at CI level through the use of symmetry breaking core-hole excited states (MEG-CI). This methodology might offer a clear and transparent way to take into account the relaxational and correlation energy, the virtual orbital specificity and the required restricted configuration interaction needed to the NEXAFS prediction problem