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1Author    Piotr CysewskiRequires cookie*
 Title    Theoretical Studies on the Tautomeric Properties of Diamino-5-formamidopyrimidines  
 Abstract    The results of theoretical geometry prediction of formamidopyrimidine(fapy)-adenine and fapy-guanine tautomers are presented. Among 54 potential tautomeric structures of fapy-adenine the most stable structure corresponds to the diamino-keto isomer. The solvent effect has insignificant influence on the fapy-adenine tautom ers succession. The fapy-guanine has 172 potential isomers. There are three most stable tautom ers of this guanine derivative, which may exchange the order depending on the polarity of the environment. In vapour the most probable is the 4-enol-6-keto-diamino tautomer, while in water environm ent the 4,6-diketo-diamino isomer is dominant. A more polar solvent stabilises more polar fapy-guanine tauto­ mers. The geometric parameters and point-atomic charges corresponding to most probable tau­ tomers are also supplied. 
  Reference    Z. Naturforsch. 53c, 1027—1036 (1998); received April 20/June 19 1998 
  Published    1998 
  Keywords    Fapy-adenine, Fapy-guanine, Tautomerism, Solvent Effect 
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 TEI-XML for    default:Reihe_C/53/ZNC-1998-53c-1027.pdf 
 Identifier    ZNC-1998-53c-1027 
 Volume    53 
2Author    Piotr Cysewski, Ryszard OlińskiRequires cookie*
 Title    Theoretical Description of the Coding Potential of Diamino-5-formamidopyrimidines  
 Abstract    The results of geometry optimisation of possible Watson-Crick-like pairs o f 2,6-diamino-4-oxy-5-formamidopyrimidine (fapy-adenine) or 4,6-diamino-5-formamidopyrimidine (fapy-guanine) were presented. In the absence of the external field the fapy-adenine is able to form pairs with all four canonical nucleic acid bases. However, pairs with guanine, cytosine and thymine the most stable are. Thus, the potential miscoding abilities may be observed. In contrast, in the presence o f the external field the mispairing abilities of fapy-adenine becom e insignificant since the most stable dimers are formed with thymine. The pairing properties of fapy-guanine are complex and depend on its tatomeric form. In the absence of an external field the 4-enol-6-keto-diam ino tautomer of fapyG is able to form stable dimers with thymine and cytosine, while the 4,6-diketo-diamino tautomer forms the most stable pairs with cytosine and guanine. The presence of the water solvent does not significantly alter the pairing abilities of fapy-guanine. However, pairs with thymine are at least as stable as the Watson-Crick GC pair. TTius, in polar conditions the mispairing potential of fapyG will be extended and may be enriched by potential GC —* AT transition. 
  Reference    Z. Naturforsch. 54c, 239 (1999); received October 26/November 28 1998 
  Published    1999 
  Keywords    Fapy-adenine, Fapy-guanine, Pairing, Miscoding, Solvent Effect 
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 TEI-XML for    default:Reihe_C/54/ZNC-1999-54c-0239.pdf 
 Identifier    ZNC-1999-54c-0239 
 Volume    54 
3Author    FayezY. Khalil, M. T. HannaRequires cookie*
 Title    Kinetic Study of the Acid Hydrolysis of Ethyl Hydrogen Succinate in Binary Solvent Mixtures  
 Abstract    The specific rate constants for the acid-catalysed hydrolysis of ethyl hydrogen succinate in water and in a series of acetone-water and dioxane-water mixtures were determined at 30-45 °C. The solvent composition covered the range 0-90% by weight of the organic solvent. The rate was found to decrease to a minimum at about 80% of the organic solvent then to increase again with successive addition of the latter. In dioxane-water medium a slight initial increase in rate on the first addition of dioxane to water was observed and attributed to an increase in the number of "free" water molecules. The activation energy showed only slight dependence on solvent composition. The radius of the activated complex was calculated in the two media. The thermodynamic parameters were evaluated and discussed in terms of solvent composition. The reaction proved to be essentially an ion-dipole rather than an ion-ion interaction. A comparative discussion of the specific solvent effects of the two solvent systems under consideration is presented, based on the respective bond energies as given by the heats of mixing of the solvent components. 
  Reference    Z. Naturforsch. 33b, 1479—1483 (1978); received April 25 1978 
  Published    1978 
  Keywords    Ethyl Hydrogen Succinate, Acid Hydrolysis, Kinetics, Solvent Effects, Thermodynamic Parameters 
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 TEI-XML for    default:Reihe_B/33/ZNB-1978-33b-1479.pdf 
 Identifier    ZNB-1978-33b-1479 
 Volume    33 
4Author    CliffordW. Fonga, HamishG. GrantRequires cookie*
 Title    Solvent Effects on the Carbon - 13 NMR Chemical Shifts and Rotational Barriers of N,N-Dimethylbenzamide - Solvent Enhanced n Polarization  
 Abstract    The 13 C NMR chemical shifts of N,N-dimethylbenzamide in thirty solvents have been measured at high dilution. The solvent induced chemical shifts (s.i.c.s.) of the carbonyl group carbon atom in twenty three solvents and the thermodynamic barriers to rotation about the C-N bond in eleven solvents are linearly related to the solvent parameter, ET(30). Multi-parametric analysis of the carbonyl s.i.c.s. indicates hydrogen bond donor effects are more important than polar effects. Rotational barriers for N,N-dimethylbenz-amide may be determined by measurement of the 13 C chemical shift of the carbonyl group in a particular solvent. The s.i.c.s. of the aromatic ring carbon atoms may be explained by the polarization of the aromatic ji electron system induced by the solvent enhanced polarization of the dimethylcarboxamido moiety. Hydrogen bonding solvents and polar solvents result in two effective dipoles on the dimethylcarboxamido moiety, which polarize the aromatic n electron system differently. 
  Reference    Z. Naturforsch. 36b, 585—595 (1981); received May 19 1980/January 21 1981 
  Published    1981 
  Keywords    Solvent Effects, 13 C NMR Spectra Rotational Barriers, N, N-Dimethylbenzamide 
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 TEI-XML for    default:Reihe_B/36/ZNB-1981-36b-0585.pdf 
 Identifier    ZNB-1981-36b-0585 
 Volume    36 
5Author    J. Manzur, G. GonzálezRequires cookie*
 Title    Solvent Effects on the IR Spectra of N-Methylacetamide  
 Abstract    The solvent effect on the v(N-H) amide I and amide II IR bands of N-methylacetamide were investigated. The frequency shifts could be related to the solvent donor and acceptor num-bers by mean of a two parameters relationship that accounts for the mutual influence between the two interacting sites of the amide. 
  Reference    Z. Naturforsch. 36b, 763—764 (1981); received January 2 1981 
  Published    1981 
  Keywords    Solvent Effect, Amidic Bond, N-Methylacetamide, Hydrogen Bonding Interactions 
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 TEI-XML for    default:Reihe_B/36/ZNB-1981-36b-0763_n.pdf 
 Identifier    ZNB-1981-36b-0763_n 
 Volume    36 
6Author    Waldemar Adam, Omar CuetoRequires cookie*
 Title    3,3-Dimethyl-4 [2-methyl-l-propenyl] -1,2-dioxetane: Its Thermal Stability and Chemiluminescent Properties  
 Abstract    The alkenyl-substituted 1,2-dioxetane (1) was prepared by photosensitized singlet oxygenation of 2,5-dimethyl-2,4-hexadiene and its activation and excitation parameters determined. It is shown that dioxetane (1) is comparable in its thermal stability to alkyl-substituted dioxetanes and an inefficient chemical source of electronic excitation. 
  Reference    Z. Naturforsch. 36b, 1653—1654 (1981); received June 29 1981 
  Published    1981 
  Keywords    1, 2-Dioxetanes, Chemüuminescence, Activation Parameters, Excitation Yields, Solvent Effects 
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 TEI-XML for    default:Reihe_B/36/ZNB-1981-36b-1653.pdf 
 Identifier    ZNB-1981-36b-1653 
 Volume    36 
7Author    Ulrich Nagel, Christoph RollerRequires cookie*
 Title    Enantioselective Catalysis, XVII [1], Enantioselective Catalytic Hydrogenation of Unfunctionalized Ketones  
 Abstract    Three diastereomeric rhodium bisphosphane complexes have been applied to asymmetric hydrogenation of unfunctionalized, non-chelating aliphatic and aromatic ketones. The ee values o f the catalysis products differ considerably for the diastereomerical catalysts. 70% ee were obtained in hydrogenating butyrophenone, and 83.7% ee for pinacoline. The results depend strongly on the solvent used. 
  Reference    Z. Naturforsch. 53b, 267—270 (1998); received January 15 1998 
  Published    1998 
  Keywords    Enantioselective Hydrogenation, Unfunctionalized Ketones, Rhodium, Salt Effect, Solvent Effect 
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 TEI-XML for    default:Reihe_B/53/ZNB-1998-53b-0267.pdf 
 Identifier    ZNB-1998-53b-0267 
 Volume    53 
8Author    Joäo Manuel, Marques Cordeiro, Luiz Carlos, Gomide FreitasRequires cookie*
 Title    Study of Water and Dimethylformamide Interaction by Computer Simulation  
 Abstract    Monte Carlo simulations of water-dimethylformamide (DMF) mixtures were performed in the isothermal and isobaric ensemble at 298.15 K and 1 atm. The intermolecular interaction energy was calculated using the classical 6-12 Lennard-Jones pairwise potential plus a Coulomb term. The TIP4P model was used for simulating water molecules, and a six-site model previously optimised by us was used to represent DMF. The potential energy for the water-DMF interaction was obtained via standard geometric combining rules using the original potential parameters for the pure liquids. The radial distribution functions calculated for water-DMF mixtures show well characterised hydrogen bonds between the oxygen site of DMF and hydrogen of water. A structureless correlation curve was observed for the interaction between the hydrogen site of the carbonyl group and the oxygen site of water. Hydration effects on the stabilisation of the DMF molecule in aqueous solution have been investigated using statistical perturbation theory. The results show that energetic changes involved in the hydration process are not strong enough to stabilise another configuration of DMF than the planar one. 
  Reference    Z. Naturforsch. 54a, 110—116 (1999) 
  Published    1999 
  Keywords    Monte Carlo Simulation, Solvent Effects, Statistical Perturbation Theory, Hydration of Peptides 
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 TEI-XML for    default:Reihe_A/54/ZNA-1999-54a-0110.pdf 
 Identifier    ZNA-1999-54a-0110 
 Volume    54