Surface-enhanced raman spectra and molecular orientation of phthalazine adsorbed on a silver electrode

SERS from phthalazine adsorbed on an Ag electrode was investigated under several conditions of applied voltage and solution concentration. Spectral assignments of the Raman bands were successfully performed and two differently oriented adsorbates, i.e. flat and end-on species, were identified. The contribution of the image field to the SERS intensity was considerable.     

Simple ab initio calculations using a floating basis

Calculations are described on three rotamers of hydrogen disulphide (transgauche- and cis-HSSH) using an ab initio Floating Gaussian Orbital model. The optimised geometrical and electronic structures of each rotamer are discussed in terms of several electronic properties, a population and orbital analysis and an extensive partitioning of the electronic energy amongst the orbitals. The so-called Gauche Effect in HSSH is discussed in connexion with the various models proposed to account for this particular structural feature.     

Propionaldehyde: ab initio and semi-empirical calculations

The internal rotation of propionaldehyde about the 1–2 bond has been studied by means of ab initio calculations. The most stable conformer has methyl and carbonyl eclipsed. Increasing the 1–2 dihedral angle to 60°, 120°, and 180° gives energies of 1.7, 0.4, and 0.7 Kcal/mol, respectively. The agreement with force field calculations and with experiment is reasonable.     

Geometry optimised ab initio calculations on cyclopropenone and dimethylcyclopropenone

Geometry optimised ab initio calculations are reported for cyclopropenone (I) and its dimethyl derivative (II). The effect of the methyl substituents in this strained ring system with an exocyclic heteroatom is to increase the bond lengths and the electronic delocalisation. For cyclopropenone the results are compared, firstly, with previous ab initio calculations, without geometry optimisation, secondly, with a semi-empirical calculation with partial optimisation, and finally, with experimental results. The calculated geometry for this molecule agrees with the experimental results.     

pH dependent SERS and solvation studies of tyrosine adsorbed on silver colloidal nano particles combined with DFT calculations

The surface enhanced Raman spectra of tyrosine in colloidal Ag solution have been recorded over a range of pH. A line shape analysis of the bands at 1359, 1505 and 1577 cm⁻¹ was performed between pH 3.5 and 8.5. The variation of spectral linewidth (FWHM) of the band at 1359 cm⁻¹ with pH is explained in terms of two mechanisms in solution: (i) the fluctuation of the pH of a microscopic volume in a solution with an overall uniform pH and/or (ii) the role of changing viscosity and solvation at different pH values due to the intermolecular ionic interactions between different charged states of the tyrosine molecule. The blue shift in three bands with increasing pH has been explained in terms of charge transfer between the different charged states of tyrosine and metal ions upon chemisorption. The experimental spectra are compared with ab initio/DFT calculations of vibrational wavenumbers, bond geometries, binding energy and charge distributions obtained by means of Hartree–Fock (HF) analysis, the nonlocal density functional method (BLYP) and the hybrid functional method (B3LYP). Two basis sets, CEP-31G and lanl2DZ, were used for all calculations.     

Stereoelectronic structure of methoxyphenyltrichlorostannanes by the results of ab initio calculations

The RHF/3-21G* and MP2/3-21G* levels were implemented for the study of the stereoelectronic structure of 2- and 4-methoxyphenyl- and 2,6-dimethoxyphenyltrichlorostannanes. In the 4-methoxyphenyltrichlorostannane the Sn atom is tetracoordinated, while in 2-methoxy- and 2,6-dimethoxyphenyltrichlorostannanes it is pentacoordinated due to the intramolecular interaction of the O atom with it. In this case a closure occurs of a 4-membered ring. In the last molecule, the Sn atom interacts almost exclusively with only one of the O atoms. The intramolecular interaction between the Sn and O atoms leads to the electron density transfer from C and H atoms of the methoxy groups as well as from the Sn atom to the atoms of its coordination polyhedron. As a result the electron density increases also on the O atom involved in this interaction.     

SERS and tunneling spectroscopy investigation of iron-protoporphyrin IX adsorbed on a silver tip

Iron-protoporphyrin IX adsorbed on a scanning tunneling microscopy silver tip is investigated by combining surface enhancement Raman (SERS) and tunneling spectroscopies down to single molecule regime. Both the Raman signals and the tunneling current intensity reveal fast switching between the iron oxidation states and present analogous, significant fluctuations in time. The results point out the occurrence of a strong electronic coupling between the molecule and the metal, and may contribute to elucidate the dynamical phenomena at the molecule-metal interface, deserving some nanotechnological interest.     

硝基烷系列分子的从头计算研究

硝基甲烷作为最简单的硝基类爆炸物,Marynick等曾以各种半经验分子轨道法和从头计算法进行过研究.我们也对其进行过全电子自洽场从头计算,获得有益启示.本文将类似计算用于硝基乙烷、1-和2-硝基丙烷等,发现用其电子结构可阐明各烷基的供电子能力、缩合反应能力、核磁共振谱和热安定性等诸多实验事实.还进行了相应构型和ρ指数下的CNDO/2计算,得到与从头计算平行的结果.     

Raman spectroscopy of cobalt phthalocyanine adsorbed on a silver electrode

Raman spectra of cobalt tetrasulfonated phthalocyanine adsorbed on a silver electrode in aqueous electrolytes have been recorded in situ. It is shown that the entensity of the Raman bands is directly related to the amount of charge transfered during the electrochemical activation of the silver. The strong potential dependence of distinct Raman bands is discussed with respect to the resonance properties of the adsorbate, taking into account the orientation of the molecule on the surface.     

Excited-state structure and vibrations of p-diaminobenzene studied by ab initio calculations

The structures and vibrations of p-diaminobenzene (PDAB) in the S₀ and S₁ states have been studied by ab initio quantum-chemical calculations. Results from geometry optimization show that the two stable cis and trans conformers of PDAB are non-planar in the S₀ state. Upon electronic excitation to the S₁ state, enhanced interaction between the ring and the amino substituent causes the molecule to become planar and contract along the long in-plane axis. A detailed analysis of the normal vibrations of PDAB in both states has been done on the basis of the motions of individual atoms as well as reduced masses, force constants and frequencies. The computed frequencies are in reasonably good agreement with the available experimental data.     

Voltage-dependent excitation profiles of pyridine adsorbed on a silver electrode

Excitation profiles of Raman modes of pyridine adsorbed on an Ag electrode were measured at three applied voltages. A resonant type of contribution was observed, the resonance energy was voltage dependent. Our observations were attributed to the voltage-dependent effects of charge transfer transitions.     

Molecular orientation of phthalimide adsorbed on colloidal silver

Surface Enhanced Raman Scattering (SERS) obtained for phthalimide (PIMH) and potassium phthalimide (PIMK) are identified with the phthalimide anion adsorbed on the surface of colloidal silver. The molecular orientation on the surface is discussed in terms of the relative intensities of vibrational modes assigned to irreducible representations of the _C2ν point group.     

Empirical force field and ab initio calculations on allyl cations

Allyl cation geometries optimized using an extended version of MMP2, newly parameterized for localized and delocalized classical cations, compare favorably with those obtained at the MP2(full) /6–31G^* level. Hence, the force field should provide good starting structures for ab initio calculations. The π-electron densities obtained by these two very different methods are quite similar. The relative energies of various isomers at MP4/6–31G^*//MP2(full)/6–31G^* are reproduced well by the force-field calculations. The heats of formation calculated by MMP2, as well as those predicted from the ab initio data, agree with experimentally determined values. The force-field method provides interpretive capabilities. Energy differences between isomers can be separated into electronic and steric contributions, reasonable estimates of resonance energies are given, and nonbonded resonance energies in delocalized cations can be evaluated. The stabilizing 1–3 π-interactions in allyl cations are quite significant, but are reduced by alkyl groups hyperconjugatively and sterically. © 1997 by John Wiley & Sons, Inc.     

Excited-state ab initio calculations and multidimensional Franck-Condon simulations on guanine

The guanine enol and keto N7H and N9H tautomers have been optimized at the CASSCF/cc-pVDZ levels of theory. Except for the enol N7H tautomer, CASSCF predicts distorted nonplanar S1 state geometries. Among the vibronic simulations carried out with the optimized structures only the enol N7H tautomer qualitatively mirrors the appearance of the experimental R2PI spectrum. Refined symmetry-adapted cluster configuration interaction (SACCI) geometries of the enol N7H tautomer produce simulations in good agreement with experiment and support the assignment of the first vibronic band and associated vibronic features of the R2PI spectrum to this tautomer. The sharp spectral features and the fact that Franck-Condon simulations based on the harmonic approximation allow for a faithful reproduction of the spectral signature associated with the enol N7H tautomer indicate that within the simulated energy window the S1 potential energy surface of this isomer is fairly harmonic and free from conical intersections involved in the S1 state lifetime-shortening relaxation processes of other DNA bases and possibly the remaining tautomers of guanine.     

MP2, DFT and ab initio calculations on thioxanthone

Density functional theory (DFT), HF and MP2 calculations have been carried out to investigate thioxanthone molecule using the standard 6-31+G(d,p) basis set. The results of MP2 calculations show a butterfly structure for thioxanthone. The calculated results show that the predicted geometry can well reproduce the structural parameters. The predicted vibrational frequencies were assigned and compared with experimental IR spectra. A good harmony between theory and experiment is found. The theoretical electronic absorption spectra have been calculated using CIS method. ¹³C and ¹H NMR of the title compound have been calculated by means of B3LYP density functional method with 6-31+G(d,p) basis set. The comparison of the experimental and the theoretical results indicate that density functional B3LYP method is able to provide satisfactory results for predicting NMR properties.     

Extended ab initio calculations on cyclopropane,cyclopropene and cyclopropenone

The three-membered ring systems, cyclopropane, cyclopropene and cyclopropenone, have been investigated using a (9,5,1 ) Gaussian basis set. A detailed study of the chemical binding in these systems has been of particular interest. Results for the molecular ground states have been compared with predictions made previously using smaller basis sets and also with pertinent experimental findings.     

Vibrational spectroscopic studies and ab initio calculations of sulfanilamide

FT-Raman and FT-IR spectra of sulfanilamide were recorded and analyzed. The vibrational frequencies of the compound have been computed using the Hartree-Fock/6-21G(*) basis and compared with the experimental values. The assignments of the observed bands were made on the basis of available literature.