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At least 127 records · Page 7Linked to original sources
PubMed2004

Vibrational frequencies and structural determination of triethynylmethylsilane.

The normal mode frequencies and corresponding vibrational assignments of Triethynylmethylsilane (CH3Si(CCH)3) are examined theoretically using the Gaussian98 set of quantum chemistry codes. Each of the vibrational modes was assigned to one of nine types of motion predicted by a group theoretical analysis (Si-C stretch, C triple bond C stretch, C-H stretch, C triple bond C-H bend, Si-C triple bond C bend, C-Si-C bend, H-C-H bend, CH3 wag, and CH3 twist) utilizing the C3v symmetry of the molecule. A set of uniform scaling factors was derived for each type of motion. Predicted infrared and Raman intensities are reported.

Molecular Structure↗
PubMed2003

Ab initio study of phenyl benzoate: structure, conformational analysis, dipole moment, IR and Raman vibrational spectra.

The molecular structure (bond distances and angles), conformational properties, dipole moment and vibrational spectroscopic data (vibrational frequencies, IR and Raman intensities) of phenyl benzoate were calculated using Hartree-Fock (HF), density functional (DFT), and second order Møller-Plesset perturbation theory (MP2) with basis sets ranging from 6-31G* to 6-311++G**. The theoretical results are discussed mainly in terms of comparisons with available experimental data. For geometric data, good agreement between theory and experiment is obtained for the MP2, B3LYP and B3PW91 levels with basis sets including diffuse functions. The B3LYP/6-31+G* theory level estimates the shape of the experimental functions for phenyl torsion around the Ph-O and Ph-C bonds well, but reproduces the height of the rotational barriers poorly. The B3LYP/6-31+G* harmonic force constants were scaled by applying the scaled quantum mechanical force field (SQM) technique. The calculated vibrational spectra were interpreted and band assignments were reported. They are in excellent agreement with experimental IR and Raman spectra.

Benzoates↗
PubMed2006

Vibrational spectra and analysis of acetohydrazide CH3-CO-NH-NH2.

The structural stability of acetohydrazide CH(3)-CO-NH-NH(2) was investigated by DFT-B3LYP and ab initio MP2 calculations with 6-311+G** basis set. The C-N rotational barrier in the molecule was calculated to be about 26 kcal/mol that suggested the planar sp(2) nature of the nitrogen atom of the central NH moiety. The N atom of the terminal NH(2) group was predicted to highly prefer the pyramidal sp(3) structure with an inversion barrier of about 7-8 kcal/mol. The molecule was predicted to have a trans-syn (N-H bond is trans with respect to CO bond and NH(2) moiety is syn to C-N bond) conformation as the lowest energy structure. The vibrational frequencies were computed at B3LYP level of theory and normal coordinate calculations were carried out for the trans-syn acetohydrazide. Complete vibrational assignments were made on the basis of normal coordinate analyses and experimental infrared and Raman data.

Azides↗
PubMed2004

Vibrational frequencies and structural determination of 1,3-dichloro-1,3-diazetidine-2,4-dione.

The normal mode frequencies and corresponding vibrational assignments of 1,3-dichloro-1,3-diazetidine-2,4-dione are examined theoretically using the Gaussian 98 set of quantum chemistry codes. Each of the vibrational modes was assigned to one of six types of motion predicted by a group theoretical analysis (C=O stretch, N-C stretch, N-Cl stretch, N-C-N bend, N-Cl bend, and C=O bend) utilizing the C2h symmetry of the molecule. Uniform scaling factors was derived for each type of motion. Predicted infrared and Raman intensities are reported.

Azetidines↗
PubMed2006

Raman and infrared spectroscopy of the manganese arsenate mineral allactite.

The mineral allactite [Mn(7)(AsO(4))(2)(OH)(8)] is a basic manganese arsenate which is highly pleochroic. The use of the 633 nm excitation line enables quality spectra of to be obtained irrespective of the crystal orientation. The mineral is characterised by a set of sharp bands in the 770-885 cm(-1) region. Intense and sharp Raman bands are observed at 883, 858, 834, 827, 808 and 779 cm(-1). Collecting the spectral data at 77K enabled better band separation with narrower bandwidths. The observation of multiple AsO(4) stretching bands indicates the non-equivalence of the arsenate anions in the allactite structure. In comparison the infrared spectrum shows a broad spectral profile with a series of difficult to define overlapping bands. The low wavenumber region sets of bands which are assigned to the nu(2) modes (361 and 359 cm(-1)), the nu(4) modes (471, 452 and 422 cm(-1)), AsO stretching vibrations at 331 and 324 cm(-1), and bands at 289 and 271 cm(-1) which may be ascribed to MnO stretching modes. The observation of multiple bands shows the loss of symmetry of the AsO(4) units and the non-equivalence of these units in the allactite structure. The study shows that highly pleochroic minerals can be studied by Raman spectroscopy.

Arsenates↗
PubMed2004

Raman spectroscopy of graphite.

We present a review of the Raman spectra of graphite from an experimental and theoretical point of view. The disorder-induced Raman bands in this material have been a puzzling Raman problem for almost 30 years. Double-resonant Raman scattering explains their origin as well as the excitation-energy dependence, the overtone spectrum and the difference between Stokes and anti-Stokes scattering. We develop the symmetry-imposed selection rules for double-resonant Raman scattering in graphite and point out misassignments in previously published works. An excellent agreement is found between the graphite phonon dispersion from double-resonant Raman scattering and other experimental methods.

Biophysics↗
PubMed1986

Resonance Raman study on cytochrome c peroxidase and its intermediate. Presence of the Fe(IV) = O bond in compound ES and heme-linked ionization.

Resonance Raman spectra of ferrous and ferric cytochrome c peroxidase and Compound ES and their pH dependences were investigated in resonance with Soret band. The Fe(IV) = O stretching Raman line of Compound ES was assigned to a broad band around 767 cm-1, which was shifted to 727 cm-1 upon 18O substitution. The 18O-isotopic frequency shift was recognized for Compound ES derived in H218O, but not in H216O. This clearly indicated occurrence of an oxygen exchange between the Fe(IV) = O heme and bulk water. The Fe(IV) = O stretching Raman band was definitely more intense and of higher frequency in D2O than in H2O as in Compound II of horseradish peroxidase, but in contrast with this its frequency was unaltered between pH 4 and 11. The Fe(II)-histidine stretching Raman line was assigned on the basis of the frequency shift observed for 54Fe isotopic substitution. From the intensity analysis of this band, the pKa of the heme-linked ionization of ferrocytochrome c peroxidase was determined to be 7.3. The Raman spectrum of ferricytochrome c peroxidase strongly suggested that the heme is placed under an equilibrium between the 5- and 6-coordinate high-spin structures. At neutral pH it is biased to the 5-coordinate structure, but at the acidic side of the transition of pKa = 5.5 the 6-coordinate heme becomes dominant. F- was bound to the heme iron at pH 6, but Cl- was bound only at acidic pH. Acidification by HNO3, H2SO4, CH3COOH, HBr, or HI resulted in somewhat different populations of the 5- and 6-coordinate forms when they were compared at pH 4.3. Accordingly, it is inferred that a water molecule which is suggested to occupy the sixth coordination position of the heme iron is not coordinated to the heme iron at pH 6 but that protonation of the pKa = 5.5 residue induces an appreciable structural change, allowing the coordination of the water molecule to the heme iron.

Computers↗