Bistra A. Stamboliyska

The infrared spectra and structure of o-sulfobenzimide (saccharin) and of its nitranion: An ab initio force field treatment

The structure of o-sulfobenzimide (saccharin) and of its nitranion has been studied on the basis of both infrared spectra and ab initio force field calculations. A good agreement has been found between the theoretical and experimental spectroscopic characteristics of the particles studied. The theoretical method used gives a good description of the strong spectral changes caused by the conversion of the saccharin molecule into the corresponding nitranion. The structural changes which accompany this conversion are essential and they spread over the whole sulfocarboximide group and the adjacent bonds. The nitranionic charge is delocalized over the phenylene group (0.29 e−), sulfonyl group (0.26 e−), nitranionic center (0.25 e−), and carbonyl group (0.20 e−).

IR spectra and structure of benzylidenemalononitrile and its cyanide, methoxide and heptylamine adducts: experimental and ab initio studies

Abstract The potassium cyanide, alkali-metal methoxide and heptylamine adducts of benzylidenemalononitrile were prepared as dimethyl sulphoxide (DMSO) and DMSO- d 6 solutions; their structures were studied by IR spectroscopy and ab initio force field calculations. The cyanide and methoxide adducts have a carbanionic structure, whereas heptylamine forms a zwitterion. The IR spectra of the adducts studied are characterized by very intense, low-frequency ν CN bands with a strong ν CN s – ν CN as splitting. The changes in the structure and force field of benzylidenemalononitrile accompanying its conversion into the adducts studied are essential and are spread over the whole molecule. The anionic charge is localized mainly within the dicyanomethide groups of the adducts.

The infrared spectra and structure of acetylsalicylic acid (aspirin) and its oxyanion: an ab initio force field treatment

Abstract The structures of acetylsalicylic acid (aspirin) (I) and its oxyanion (II) have been studied by means of infrared spectra and ab initio 3–21 G force field calculations. The 3100-1100 cm −1 region bands of both the aspirin molecule and its oxyanion have been assigned. The theoretical infrared data for the free aspirin anion are in good agreement with the experimental data for aspirin alkali-metal salts in dimethyl sulfoxide- d 6 . The theoretical geometrical parameters for the isolated aspirin molecule are close to the literature X-ray diffraction data for its dimer in the solid state, except for those of the carboxy group, which participates directly in hydrogen bond formation. The changes in both the spectral and geometrical parameters, caused by the conversion of the aspirin molecule into the anion, are essential, but they are localized mainly within the carboxy group and the adjacent C-Ph bond. This is also true for the changes in the corresponding bond indices and electronic charges.

Vibrational spectra and structure of benzil and its 18O- and d10-labelled derivatives: a quantum chemical and experimental study

Geometry and vibrational spectroscopic data of benzil-d0 benzil-d10 and benzil-18O calculated at various levels of theory (RHF/6-31G*, B3LYP/6-31G*, BLYP/6-31G*) are reported. The theoretical results are discussed mainly in terms of the comparisons with infrared (4000-100 cm(-1)) and Raman (4000-50 cm(-1)) spectral data. The calculated isotopic frequency shifts, induced by the 18O- and d10-labeling, are in a good agreement with the measured values. A complete vibrational assignment was made with the help of ab initio force field calculations. The data thus obtained were used for reassigning some vibrational frequencies. The results of the optimized molecular structure obtained on the basis of RHF and the DFT calculations are presented and compared with the experimental X-ray diffraction for the benzil-d0 single crystal. It turns out that the best structural parameters are predicted by the B3LYP/6-31G* method.

Vibrational spectra and structure of benzophenone and its 18O and d10 labelled derivatives: an ab initio and experimental study

Infrared (4000-100 cm(-1)) and Raman (4000-10 cm(-1)) spectra of benzophenone, benzophenone-d10 and benzophenone-(18)O have been studied in the solid state and in solution and their fundamental frequencies have been assigned using isotopic frequency shifts and differential infrared linear dichroic spectra of oriented polycrystalline layers (4000-400 cm(-1)). Ab initio MO calculations have been carried out for the three benzophenone isotopomers at the HF/3-21G, 6-31G and 6-31G** levels and the computed vibrational frequencies have been compared with the experimental ones. Best agreement is achieved with the 6-31G data, the mean deviation being 25.4 cm(-1). The calculated isotopic frequency shifts induced by the (18)O and d10 labelling, are also in a good accordance with the measured ones. All geometry parameters calculated for the isolated molecule are in good agreement with the X-ray data for the benzophenone single crystal.

Quantification of the push-pull character of donor-acceptor triazenes.

The push-pull character of two series of donor-acceptor triazenes has been quantified by (13)C and (15)N chemical shift differences of the partial N(1)=N(2) and N(3)=C(4) double bonds in the central linking C=N-N=N-C unit and by the quotient of the occupations of both the bonding pi and antibonding orbitals pi* of these partial double bonds. Excellent correlations of the two estimates, to quantify the push-pull effect, with the bond lengths strongly recommend the occupation quotients pi*/pi, the (15)N chemical shift differences Deltadelta[N(1),N(2)], and the corresponding bond lengths as reasonable sensors for quantifying charge alternation along the C=N-N=N-C linking unit, for the donor-acceptor quality of the triazenes 1 and 2 and for the molecular hyperpolarizability beta0 of these compounds. Within this context, certain substances can be strongly recommended for NLO application.

Alkaloids derived from genus Veratrum and Peganum of Mongolian origin as multidrug resistance inhibitors of cancer cells

Alkaloids comprise one of the largest groups of plant secondary metabolites including vinca alkaloids. The ability of six alkaloids from Veratrum lobelianum, one from Veratrum nigrum and three from Peganum nigellastrum to modify transport activity of MDR1 was studied. Flow-cytometry in a multidrug-resistant human MDR1-gene-transfected mouse lymphoma cells (L5178Y) was applied. The inhibition of multidrug resistance was investigated by measuring the accumulation of rhodamine-123 in cancer cells. Veralosinine and veranigrine were the most effective resistance modifiers. In a checkerboard method veralosinine and veranigrine enhanced the antiproliferative effects of doxorubicin on MDR cells in combination. The structure-activity relationships were discussed.

Vibrational spectrum of the hydrogen-bonded complex H2CO…HOCHNH: an ab initio study

Abstract The vibrational spectra for free and complexed H 2 CO and HNCHOH have been studied by ab initio calculations employing the 6-31G basis set. The changes in the vibrational characteristics (vibrational frequencies, infrared intensities and Raman activities) upon hydrogen bonding have been estimated. In agreement with the behaviour always observed with H-bond formation, the ab initio calculations show that the most sensitive to the complexation are the ν OH and ν CO stretching vibrations for both monomer bonds involved in the hydrogen bonding. These vibrations are shifted to lower frequency, corresponding to bond weakening. Their infrared intensities increase with the complex formation. The vibrational characteristics of the remaining vibrations are changed with smaller magnitudes.

Experimental and ab initio MO studies on the IR spectra and structure of cyanoacetamide, its carbanion and dianion

Abstract The structures of cyanoacetamide NC–CH2–CONH2, its carbanion NC− C H−CONH 2 and dianion NC− C H−CO N H , and of their perdeutero and 15N labelled analogues have been studied by means of both IR spectra and ab initio HF 6-31G and 6-31+G(d) calculations. Both the spectral and structural changes, accompanying the conversion of cyanoacetamide into the carbanion, spread over the whole molecule; the carbanion → dianion changes are localized mainly within the carboxamido group. The theory predicts qualitatively well the strong frequency decreases (165 and 247 cm−1) and the strong intensity increases (29 fold and 1.7 fold) of the CN and CO stretching bands respectively, resulting from the conversions studied. The first (carbanionic) electric charge is delocalized over the methide (0.36 e−), carboxamide (0.33 e−) and cyano (0.31 e−) groups; the greater part (0.74 e−) of the second (nitranionic) charge remains localized within the carboxamido group. The possible conformers of the species studied are also discussed. The anionic species exist as ion pairs in dimethyl sulfoxide solutions.

Experimental and ab initio MO studies on the IR spectra and structure of pyridinium dicyanomethylide and trimethylammonium dicyanomethylide.

The structures of the title ylides have been studied by both quantitative infrared (IR) spectra and ab initio HF and MP2 force field calculations. Good agreement has been found between theoretical and experimental data for their spectral and structural characteristics. According to both IR data and geometry parameters the dicyanomethide groups in the ylides studied have a pronounced carbanionic character. The analysis of the calculated net electric charges however shows only moderate (below 0.6 e-) intramolecular charge transfers between the pyridinium (trimethylammonium) and dicyanomethide fragments of the species studied.