共查询到20条相似文献,搜索用时 0 毫秒
1.
The geometries and vibrational frequencies of two conformers of pyruvic acid have been obtained at the ab initio second order Möller-Plesset level of theory using the 6-311++G** basis set. While the calculated geometries have been compared to the experimental microwave data, the vibrational frequencies have been assigned, using the experimental gas phase IR spectra of 13 isotopes of pyruvic acid by a recently developed scaling procedure (IRPROG). An attempt has been made to explain the stability of the eclipsed conformation over the staggered conformation of pyruvic acid by taking account of the molecular orbitals. 相似文献
2.
Gustavo Portalone Fabio Ramondo Aldo Domenicano Istvn Hargittai 《Journal of organometallic chemistry》1998,560(1-2)
The molecular structure of phenylsilane has been determined accurately by gas-phase electron diffraction and ab initio MO calculations at the MP2(f.c.)/6-31G* level. The calculations indicate that the perpendicular conformation of the molecule, with a Si–H bond in a plane orthogonal to the plane of the benzene ring, is the potential energy minimum. The coplanar conformation, with a Si–H bond in the plane of the ring, corresponds to a rotational transition state. However, the difference in energy is very small, 0.13 kJ mol−1, implying free rotation of the substituent at the temperature of the electron diffraction experiment (301 K). Important bond lengths from electron diffraction are: <rg(C–C)>=1.403±0.003 Å, rg(Si–C)=1.870±0.004 Å, and rg(Si–H)=1.497±0.007 Å. The calculations indicate that the Cipso–Cortho bonds are 0.010 Å longer than the other C–C bonds. The internal ring angle at the ipso position is 118.1±0.2° from electron diffraction and 118.0° from calculations. This confirms the more than 40-year old suggestion of a possible angular deformation of the ring in phenylsilane, in an early electron diffraction study by F.A. Keidel, S.H. Bauer, J. Chem. Phys. 25 (1956) 1218. 相似文献
3.
4.
Ab initio HF and MP2 calculations on prototype model linkages of phosphorothioate DNA backbones illuminate the effects of phosphorothioation on electronic and structural properties of DNA backbone. The replacement of a bridging oxygen atom by sulfur in the phosphodiester linkage is energetically favored over that of replacement of a non-bridging oxygen atom. In phosphorothioate derivatives containing the P(OS)nb moiety, the non-bridging oxygen atom always bears a higher negative charge than the non-bridging sulfur. Additional calculations on protonated (neutral) adducts suggest that phosphorothioation of the phosphodiester linkage lowers its proton affinity. Moreover, protonation of the non-bridging oxygen atom at phosphorous is favored over the protonation of the non-bridging sulfur atom for linkages containing the P(OS)nb moiety. The ab initio calculated structural parameters are compared to the available crystallographic data of small phosphorothioate molecules and phosphorothioate oligodeoxynucleotides. These results have implications upon the biological activity of phosphorothioate DNA analogs. 相似文献
5.
Magdalena M. Glice Andrzej Le Krzysztof Bajdor 《Journal of Molecular Structure》1998,450(1-3):141-153
A comparative analysis of the IR and Raman spectra of aminoglutethimide (AG) dissolved in CCl4, CHCl3 and CH3CN was performed. Most of the absorption bands were assigned to characteristic group vibrations with the use of the IR and Raman spectra of deuterated AG, glutethimide, N-methyl glutethimide and glutarimide. The AG samples very weakly interacting with the environment were studied with the use of the Ar matrix isolation IR spectra. For comparison, the IR and Raman spectra of the crystalline samples formed by hydrogen-bonded AG molecules were recorded. The spectra were analyzed also in terms of normal modes and the harmonic approximation with the use of the ab initio restricted Hartree-Fock theory. It was found that increasing the solute concentration in CCl4 and CHCl3 leads to formation of the autoassociates. In CH3CN the solute–solvent AG–CH3CN dimers occur. Possible structures of the associates were theoretically studied on the model systems: the centrosymmetric glutarimide dimer and the linear AG–CH3CN dimer. By a comparison of the theoretical and experimental spectra we were able to identify several peaks originating from the solute–solvent interactions. 相似文献
6.
7.
E. D. Jemmis K. T. Giju K. Sundararajan K. Sankaran V. Vidya K. S. Viswanathan J. Leszczynski 《Journal of Molecular Structure》1999,510(1-3):59-68
The details of weak C–Hπ interactions that control several inter and intramolecular structures have been studied experimentally and theoretically for the 1:1 C2H2–CHCl3 adduct. The adduct was generated by depositing acetylene and chloroform in an argon matrix and a 1:1 complex of these species was identified using infrared spectroscopy. Formation of the adduct was evidenced by shifts in the vibrational frequencies compared to C2H2 and CHCl3 species. The molecular structure, vibrational frequencies and stabilization energies of the complex were predicted at the MP2/6-311+G(d,p) and B3LYP/6-311+G(d,p) levels. Both the computational and experimental data indicate that the C2H2–CHCl3 complex has a weak hydrogen bond involving a C–Hπ interaction, where the C2H2 acts as a proton acceptor and the CHCl3 as the proton donor. In addition, there also appears to be a secondary interaction between one of the chlorine atoms of CHCl3 and a hydrogen in C2H2. The combination of the C–Hπ interaction and the secondary ClH interaction determines the structure and the energetics of the C2H2–CHCl3 complex. In addition to the vibrational assignments for the C2H2–CHCl3 complex we have also observed and assigned features owing to the proton accepting C2H2 submolecule in the acetylene dimer. 相似文献
8.
By using density functional theory calculations at the PBE+U level, we investigated the properties of hematite (0001) surfaces decorated with adatoms/vacancies/substituents. For the most stable surface termination over a large range of oxygen chemical potentials (${\mu _{\rm{O}} }$ ), the vacancy formation and adsorption energies were determined as a function of ${\mu _{\rm{O}} }$ . Under oxygen‐rich conditions, all defects are metastable with respect to the ideal surface. Under oxygen‐poor conditions, O vacancies and Fe adatoms become stable. Under ambient conditions, all defects are metastable; in the bulk, O vacancies form more easily than Fe vacancies, whereas at the surface the opposite is true. All defects, that is, O and Fe vacancies, Fe and Al adatoms, and Al substituents, induce important modifications to the geometry of the surface in their vicinity. Dissociative adsorption of molecular oxygen is likely to be exothermic on surfaces with Fe/Al adatoms or O vacancies. 相似文献
9.
W.B Tzeng K Narayanan C.Y Hsieh C.C Tung 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》1997,53(14):2595-2604
Hydroquinone (HYQ) in the lowest electronically excited state has been studied by ab initio quantum chemical calculations and resonant two-photon ionization (R2PI) spectroscopy. Calculations at the MP2/6-31G* and CIS/6-31G* levels yield satisfactory results on structures and vibrational frequencies of the cis-HYQ and trans-HYQ in both the S0 and S1 states. Only transitions involving in-plane modes are observed in the R2PI spectrum of HYQ. All spectral bands including some newly observed ones have been successfully assigned with the help of our computed results and analogy with the reported spectra for similar molecules. 相似文献
10.
B. S. Jursic 《Journal of Molecular Structure》1999,490(1-3):81-85
The aromaticity of planar and highly symmetric three-dimensional hydrogen clusters were evaluated with the complete basis set ab initio computational method. The energy of formation of the hydrogen clusters from the hydrogen molecule and hydrogen molecular ions were used in comparison to their relative stabilities. The aromaticity of planar hydrogen clusters, as well as hydrogen clusters in the three dimensions, arranged as highly symmetric regular polyhedra (Plato's polyhedras), were discussed with respect to the Hückel and Möbius aromatic rules. 相似文献
11.
Gamil A. Guirgis Stephen Bell James R. Durig 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》1996,52(14):1861-1873
The infrared (3500-80 cm−1) and Raman (3500-20 cm−1) spectra of 3-fluoro-1-butyne, CH3CHFCCH, have been recorded for the gas and solid. Additionally, the Raman spectrum of the liquid has also been recorded to aid in the vibrational assignment. Ab initio electronic structure calculations of energies, geometrical structures, vibrational frequencies, infrared intensities, Raman activities and the potential energy function for the methyl torsion have been calculated to assist in the interpretation of the spectra. The fundamental torsional mode is observed at 251 cm−1 with a series of sequence peaks falling to lower frequency. The three-fold methyl torsional barrier is calculated to be 1441 ± 20 cm−1 (4.12 ± 0.06 kcal mol−1) where the uncertainty is partly due to the uncertainty in values of the V6 term. A complete vibrational assignment is proposed based on band contours, relative intensities, and ab initio predicted frequencies. Several fundamentals are significantly shifted in the condensed phases compared to values in the vapor state. 相似文献
12.
Gerald H. Lushington Pablo J. Bruna Friedrich Grein 《International journal of quantum chemistry》1997,63(2):511-521
For the 2Σ+ ground states of the ions Li2+, Li2−, and Be2+, the dependence of the magnetic moment (parametrized by g-shifts) on the bond length R was studied at the ROHF level. The Δ g-values were calculated via a perturbative approach (complete to second order in Breit-Pauli interactions) using quadruple-zeta AO basis sets augmented by semidiffuse and polarization functions. All Δ g-values in these systems are negative. The parallel component Δ g∥ generally changes little with R, remaining close to the g-shift of the corresponding 2S atomic dissociation product. For Li2+ and Be2+, the perpendicular component Δ g ⟂ is more sensitive to geometry than is Δ g∥, mainly because of the second-order magnetic coupling with excited 2Π states. For Li2−, Δ g ⟂ and Δ g∥ are similar due to the large size of the 2σu, SOMO, resulting in g-values close to that of a free electron. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 63: 511–521, 1997 相似文献
13.
I. Baraille M. Loudet S. Lacombe H. Cardy C. Pisani 《Journal of Molecular Structure》2003,620(2-3):291-300
The study of the grafting of trialkoxysilane R′Si(OR)3 molecules on amorphous silica has been undertaken at the Hartree–Fock level using a biperiodic model for the surface. Different types of slab cut out from the model system Edingtonite (a tetragonal silica structure with five SiO2 groups per unit cell) have been used to simulate isolated and interacting silanol sites at the amorphous silica surface, while only the simple case of HSi(OH)3 has been considered for the interacting molecule. In a first step, for each type of surface the geometrical parameters have been optimised and the surface formation energy determined. The geometrical structure of the grafted molecule is compared with that of the isolated one. The geometrical strains of the surfaces with either isolated or interacting silanols are also compared before and after the grafting reactions. The calculated values of the chemisorption energies show that the grafting process is favored on isolated silanols only if correlation effects are included. 相似文献
14.
A program to compute many functions dependent on the electron density ρ(r) from the results of ab initio molecular calculations is presented. The program allows the generation of different one-, two-, and three-dimensional grids for further graphical representation or numerical analysis. Other options like extracting separate atom contributions to the function computed or locating maximum and minimum values are also implemented. A number of illustrative applications regarding different ρ(r)-dependent functions are presented and the performance and portability of the program is discussed. 相似文献
15.
Adsorption of polyacrylic acid on aluminium oxide: DRIFT spectroscopy and ab initio calculations 总被引:1,自引:0,他引:1
K. Vermhlen H. Lewandowski H. -D. Narres E. Koglin 《Colloids and surfaces. A, Physicochemical and engineering aspects》2000,170(2-3):181-189
Diffuse reflectance Fourier transform infrared (DRIFT) spectroscopy was used to study the adsorption process of the water-soluble polyacrylic acid (PAA) polymer on hydrous δ-Al2O3. Vibrational assignment of PAA, sodium polyacrylate, (Na–PA) and the PA-oxide surface complex was achieved by comparison of observed band position and intensity in the DRIFT spectra with wavenumbers and intensities from ab initio quantum mechanical calculations. The presented data of polyacrylic acid suggest that IR data calculated ab initio on relatively short oligomers (quantum-mechanical oligomer approach) may provide valuable information regarding the interpretation of polyelectrolyte infrared spectra. Batch adsorption experiments were performed to sorb PAA onto the δ-Al2O3 surface. The results obtained from DRIFT studies were compared with adsorption isotherm experiments in order to relate the level of PAA coverage to the nature of the surface complex. Ab initio molecular orbital calculations on PAA/Al2O3 clusters were used to model possible surface complexes. Strong correlation were found between theoretical and observed DRIFT frequencies of the antisymmetric R-COO− vibration. A number of possible configurations of the polyacrylic acid/aluminate surface complex were tested via ab initio calculations. These possible configurations included different di-aluminium octahedral Al3+ surface models. Results obtained from adsorption isotherm experiments, DRIFT spectra and ab initio calculations indicate that the carboxylate oxygens bridge an Al3+-octahedral dimer [Al2(OH)24(H2O)2(OH)] in a ligand-exchange inner sphere complex. 相似文献
16.
Thomas A. Ford 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2006,64(5):1151-XXXIV
Using a number of potential models for the gas-phase structure of the trimer of carbon monoxide as a guide, ab initio molecular orbital calculations have been carried out on this aggregate in order to determine its probable structure and vibrational spectrum in cryogenic matrices. The Fourier transform infrared spectra of carbon monoxide trapped in argon and nitrogen matrices have been recorded and, on the basis of the results of the theoretical calculations, a search for possible absorptions which may be assigned to trimeric species has been undertaken. 相似文献
17.
Hans Georg von Schnering Mehmet Somer Martin Kaupp Wilder Carrillo-Cabrera Michael Baitinger Andre Schmeding Yuri Grin 《Angewandte Chemie (International ed. in English)》1998,37(17):2359-2361
Solely on the basis of Raman spectra and quantum chemical calculations, the previously unknown cluster anion Si94− (structure shown) was characterized and its structure determined. The anion is formed as a component of solid phases by the thermal decomposition of alkali metal monosilicides. 相似文献
18.
A magnetostructural classification of dimmers, containing the Cu (μ-alkoxo) Cu core, based on data obtained from X-ray diffraction analysis reported in the literature has been performed. In these complexes, the local geometry around the copper ions is generally a square planar and each copper ion is surrounded by one N atom and three O atoms. The influence of the overlap interactions between the bridging ligands and the metal (Cu) d orbitals on the super-exchange coupling constant has been studied by means of ab initio Restricted Hatree–Fock molecular orbital calculations. The interaction between the magnetic d orbitals and highest occupied molecular orbitals of the acetate oxygens has been investigated in homologous μ-acetato-bridged dicopper(II) complexes which have significantly different −2J values (the energy separation between the spin-triplet and spin-singlet states). In order to determine the nature of the fronter orbitals, Extended Hückel molecular Orbital calculations are also reported. Ab initio restricted Hartree–Fock calculations have shown that the acetato bridge and the alkoxide bridge contribute to the magnetic interaction countercomplementarily to reduce antiferromagnetic interaction. 相似文献
19.
High-level ab initio calculations with large basis sets are reported for silene, H2C=SiH2. Correlated harmonic force fields are obtained from coupled cluster CCSD(T) calculations with the cc-pVQZ basis (cc-pVTZ
for H) while the anharmonic force fields are computed at the MP2/TZ2Pf level. There is excellent agreement with the available
experimental data, in particular the equilibrium geometry and the fundamental vibrational frequencies. Many other spectroscopic
constants are predicted for the C
2
v
isotopomers of silene.
Received: 27 May 1998 / Accepted: 23 July 1998 / Published online: 9 October 1998 相似文献
20.
Rosa M. Gomila Antonio Frontera David Quiñonero Antonio Costa 《Tetrahedron letters》2004,45(51):9387-9391
Several porphyrin dimers have been newly designed and synthesized to construct assemblies with 1,4-diazabicyclo[2.2.2]octane (DABCO) as a bidentate binding ligand. Semi empirical (AM1) and ab initio calculations have been used to study the assemblies generated by the organization of dimers and DABCO, including the computation of 1H NMR complexation-induced chemical shifts using the ab initio/GIAO methodology. The diagnostic capacity of the theoretical method has been applied to explain experimental results and geometrical features of the complexes. 相似文献