Influence of substitution on the strength and nature of CH···N hydrogen bond in XCCH···NH3 complexes

The effect of substitution on the strength and nature of CH···N hydrogen bond in XCCH···NH₃ (X = F, Cl, Br, OH, H, Me) and NCH···NH₃ complexes were investigated by quantum chemical calculations. Ab initio calculations were performed using MP2 method with a wide range of basis sets. With tacking into account the BSSE and ZPVE, the values of BEs decrease. Replacement of the nonparticipatory hydrogen atom of HCCH by the electronegative atoms (F, Cl, and Br), lead to the BEs increases. The BE corresponding to the replacement of the nonparticipatory hydrogen atom of HCCH by the OH and CH₃ groups decreases. A far greater enhancement of the interaction energy arises from replacement of HCCH by the more acidic HCN. The natural bond orbital analysis and the Bader's quantum theory of atoms in molecules were also used to elucidate the interaction characteristics of these complexes. The electrostatic nature of H‐bond interactions is predicted from QTAIM analysis. In addition, the relationship between the isotropic and anisotropic chemical shifts of the bridging hydrogen and binding energy of complexes as well as electron density at N···H BCPs were investigated. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

DFT study of adenine–cytosine mismatch in quaternary systems involving DNA bases

Structural Chemistry - In the present work, the adenine–cytosine mismatch is theoretically investigated in quaternary systems involving DNA bases. The calculations, in gas phase, are...     

The influence of substituents on cooperativity between CH···<Emphasis Type="Italic">π</Emphasis> and N···H hydrogen bonds in a T-shaped configuration: X-benzene⊥(FH···pyrazine···HF) complexes as a working model

In the present work, the influence of substituents on cooperativity between CH···π and N···H hydrogen bonds is theoretically investigated in the T-shaped configuration of X-benzene⊥(FH···pyrazine···HF) complexes. The calculations are performed at MP2(FC)/6-311++G(d,p) level of theory. The results indicate that the absolute value of binding energy increases as the substituent changes from strong electron acceptor to strong electron donor. Moreover, cooperative and synergistic energies are computed in the complexes. The energetic data reveal a direct correlation between the electron-donating power of substituents and favorable influences of CH···π and N···H hydrogen bonds on each other. The results of AIM and NBO analyses show that the N···H hydrogen bond is strengthened by going from electron acceptor substituents to electron donor ones. It is also found that Hammett constants can be used to predict cooperative and synergistic effects between CH···π and N···H hydrogen bonds.     

Darstellung und Struktur von Tetrafluoro(η5-pentamethylcyclopentadienyl)niob und Tetrafluoro(η5-cyclopentadienyl)niob

Preparation and Structure of Tetrafluoro(η⁵-pentamethylcyclopentadienyl) Niobium and Tetrafluoro(η⁵-cyclopentadienyl) Niobium A facile preparation method for (η⁵-C₅Me₅)NbF₄ 3 and (η⁵-C₅H₅)NbF₄ 4 is reported by using AsF₃ as a fluorinating agent. Single crystals obtained from AsF₃ contain the solvent molecule as well as HF. The composition of the crystal is [(η⁵-C₅Me₅)NbF₄(AsF₃)₂]₂ · [(η⁵-C₅Me₅)NbF₄(HF)AsF₃]₂ 5 . The X-ray crystal structure of 5 will be reported. 5 crystallizes triclinic with one furmula in the space group P1 and lattice constants a = 843.1(4), b = 1154.9(6), c = 1910.2(10) pm, α = 91.68(3)°, β = 99.30(3)°, γ = 104.44(2)°.     

Synthesis and Dynamic NMR Spectroscopic Study of Dialkyl 4-Ethoxy-1-(1-naphthyl)-5-oxo-4,5-dihydro-1<Emphasis Type="Italic">H</Emphasis>-pyrrole-2,3-dicarboxylates

Summary. Protonation of the reactive 1:1 intermediate produced in the reaction between triphenylphosphine and dialkyl acetylenedicarboxylates with ethyl 2-(1-naphthylamino)-2-oxoacetate, leads to a vinylphosphonium salt, which undergoes intramolecular Wittig reaction to produce dialkyl 4-ethoxy-1-(1-naphthyl)-5-oxo-4,5-dihydro-1H-pyrrole-2,3-dicarboxylates in excellent yields. A dynamic NMR effect is observed in the ¹H NMR spectra of the title compounds as a result of restricted rotation around the single bond linking the naphthalene moiety and the heterocyclic system, which is attributed to the peri interaction between the pyrrole residue and the peri CH group. The free energy of activation (G ) for this process is 58±2kJmol^–1.Received December 6, 2002; accepted December 11, 2002 Published online June 12, 2003