Search results for "ab initio"
showing 10 items of 990 documents
Synthesis and Structural Characterization of Substituted 2-Phenacylbenzoxazoles
2013
1 H and 13C NMR spectra of eleven 2-phenacylbenzoxazoles (ketimine form) show that their CDCl3-solutions contains also (Z)-2-(benzo[d]oxazol-2-yl)-1-phenylethenols (enolimine form). Intramolecular hydrogen bonding in the latter tautomer was found to be significantly weaker than that one in respective (Z)-2-(2-hydroxy-2-phenylvinyl)pyridines. Integrals of the 1 H NMR signals were used to evaluate the molar ratio of the tautomers. Strong electron-donating substituents were found to stabilize the ketimine tautomer. pKT (negative logarithm of the equilibrium constant, KT = [ketimine]/[enolimine]) was found to be linearly dependent on the Hammett substituent constant σ. The results of the MP2 ab…
A linear open-chain piperazine-pyridine ligand and its meso-helical Co complex
1998
Abstract An oligomeric ligand (HPPy) 2 P, N , N ′-bis[2-(6-( N -( N ′-(2-hydroxyethyl)piperazinyl)methyl)]piperazine, was designed to resemble structurally open-chain aza-crowns. Owing to the all- trans configuration of piperazine and pyridine free electron pairs, it should adopt a near linear overall structure in solvent. Theoretical calculations at ab initio level confirm the overall linear structure of free ligand. The crystal structure of the complex [(HPPy) 2 P][Co(NO 3 ) 2 ] 2 shows a contraction from ∼3 to 2 nm structure. Each coordination site creates either a Δ or Δ configuration around the metal ion, thus causing a ligand with an even number of coordination centres to be meso -hel…
Solid state conformational and theoretical study of complexes containing the (CxN)Pd moiety (CxN = 2-(phenylazo)phenyl-C,N and its derivatives)
2003
Palladium complexes having the 2-(phenylazo)phenyl-C,N ligand exhibit a planar chelating ring with NN and N–C distances longer and shorter respectively than those found in trans-azobenzene. The ligand is not planar upon complexation, the mean angle between the phenyl ring and the chelating one found in the Cambridge Structural Database being of 45.6°. We have quantified and characterised the kind of distortion from planar coordination around metallic centers. The method employed makes use of two improper torsion angles, tetrahedral distortion being most frequently found in phenylazophenyl palladium complexes. Crystal structures of three succinimidate complexes having the title moiety are re…
1H,13C and15N NMR spectroscopic, x-ray structural andab initio/HF studies on nitramino andN-alkylamino-4-nitro derivatives of pyridineN-oxides and py…
2003
The 1H, 13C and 15N NMR spectra in DMSO-d6 were measured for eight nitraminopyridine N-oxides, ten 4-nitropyridine N-oxides, four 2-nitraminopyridines and five 4-nitropyridines. Their chemical shift assignments are based on PFG 1H,X (X = 13C and 15N) HMQC and HMBC experiments. The relative energies for the tautomers of two nitraminopyridine N-oxides were determined by ab initio HF/6–311G** calculations. A single-crystal x-ray structural analysis was made for 4-methyl-2-nitraminopyridine: C6H7O2N3, M = 153.15, triclinic, space group P-1 (No. 2), a = 7.0275(4), b = 6.8034(3), c = 8.6086(5) A, α = 103.620(2), β = 90.309(2), γ = 122.215(3)°, V = 334.11(3) A3, Z = 2. Copyright © 2003 John Wiley …
Structure and properties of hydroxyl radical modified nucleic acid components II. 8-Oxo-adenine and 8-oxo-2′-deoxy-adenosine
1997
Abstract The tautomerism of the 8-oxo-adenine (8-oxo-A) and 8-oxo-2′-deoxy-adenosine (8-oxo-dA) was analysed on the basis of semiempirical, SCF ab initio and DFT density functional quantum chemistry calculations. The results of full gradient geometry optimisation of all possible 8-oxo-A and 8-oxo-dA structures lead to the conclusion that the most stable form is 8-keto-6-amino-tautomer. The second stable tautomer corresponds to 8-hydroxy-isomer. Such an order was unchanged after solvating process. In all studied solvents: water, methanol, acetone, cyclohexane the keto-tautomer proceeds the enol one. The preferred N-glycoside torsion angle corresponds to syn rotamer of 8-oxo-dA in all studied…
A Hydrogen-Bonded Supramolecular meso-Helix
2003
[EN] A new one-dimensional hydrogen-bonded polymer with a unique meso-helical structure has been prepared from the spontaneous self-assembly in the solid-state of meta-substituted phenylene dioxamic acid diethyl ester monomers. The helical nature of this molecule and its self-complementary character, through intermolecular hydrogen bonding between oxamic acid ester functions, are the two main factors responsible for the crystalline aggregation process, as confirmed by both experimental X-ray crystallographic data and theoretical ab initio calculations.
Adducts of Tellurium Tetrachloride with Allyl Alcohol and Allyl Acetate: 1,2- vs 1,3-Addition and Structure and Dynamics of Te···O Interactions in D…
2002
The compounds Cl 3 Te[CH 2 CH(Cl)CH 2 O(H)...].Cl 2 Te[-CH 2 CH(Cl)CH 2 O-] (1) and Cl 3 Te-[CH 2 CH(CH 2 Cl)OC(CH 3 )=O...] (2) were prepared by the reaction of TeCl 4 with allyl alcohol and allyl acetate, respectively. Their molecular and crystal structures were investigated by single-crystal X-ray analysis, 1 H- 1 H-NOESY experiments, IR spectroscopy, and ab initio geometry optimization. 1 is a composite compound, whose subunits Cl 2 Te[-CH 2 CH(Cl)-CH 2 O-] (1A) and Cl 3 Te[CH 2 CH(Cl)CH 2 O(H))...] (1B) are linked in the solid state via Te...Cl-Te and O...H-O bridges. Both Te atoms are involved in similar five-membered rings, having a covalent Te-O bond in one case (1A) and a dative Te…
Rotational Isomerism in Acetic Acid: The First Experimental Observation of the High-Energy Conformer
2003
The high-energy conformer of acetic acid (cis-AA) is produced in an Ar matrix by vibrational excitation of the OH stretching overtone of the ground conformational state (trans-AA). IR-absorption spectroscopy provides a clear identification of the reaction product. cis-AA converts back to trans-AA in a time scale of minutes at 8 K by tunneling. http://dx.doi.org/10.1021/ja038341a
Comparison of calculated DFT/B3LYP and experimental 13C and 17O NMR chemical shifts, ab initio HF/6-31G* optimised structures, and single crystal X-r…
2003
Abstract Single crystal X-ray structures (monoclinic space group P2 1 ) for methyl 3-oxo-5β-cholan-24-oate and methyl 3,12-dioxo-5β-cholan-24-oate have been solved and compared with HF/6-31G* optimised structures. In the crystalline packings the side chains are connected with weak OC(sp 3 ) H ⋯ O -type of interactions between C25– H and C24– O –C25 and the keto ends with weak C(sp 3 ) H ⋯ O C-type of interactions between C4– H and O C3. The orientations of the side chains, which steric configurations are of great importance to the biological activity of the molecules, are compared with the experimental structure of methyl 3α-hydroxy-5β-cholan-24-oate. Probable reasons for the observed dif…
Weak intermolecular interactions promote blue luminescence of protonated 2,2′-dipyridylamine salts
2014
In this work we demonstrate that protonation and π-stacking can be exploited to convert non-luminescent 2,2′-dipyridylamine into blue-emitting derivatives. We have synthesized a series of luminescent 2,2′-dipyridylamine (dpa) salts, i.e., (dpaH)X·nSolv (dpa = 2,2′-dipyridylamine, X = HF2, n = 0.5, Solv = H2O 1; X = Cl, n = 2, Solv = H2O 2; X = Br, n = 2, Solv = H2O 3; X = I n = 1, Solv = H2O 4a; X = I n = 1, Solv = CHCl34b), (dpaH)2[SiF6]·H2O 5 and (dpaH)X (X = I36; SbF67; BF48) and characterized their emission properties, both in the solid-state and in solution. To rationalize our observations and relate the luminescence properties to the structure in the solid state and in solution, we ha…