NMR spectrocopy of organolithium compounds,part XXXIV: Cyclopropyllithium and lithium bromide (1:1) in diethylether/tetrahydrofuran—Identification of a fluxional mixed tetramer

Cyclopropyllithium, C₃H₅Li ( 1 ), was studied in the presence of one equivalent lithium bromide (LiBr) in diethylether (DEE)/tetrahydrofuran (THF) mixtures and in THF as solvents. Increasing the THF concentration in DEE/THF leads in the ⁶Li NMR spectrum to a main signal (S1) at δ0.85 (rel. to ext. LiBr/THF) and a second resonance (S2) at δ0.26 aside from a minor component at δ0.07. In pure THF, the ratio of these signals was 66: 28:6. ⁶Li and ¹³C NMR allowed to identify the main signal as belonging to a mixed dimer, 1 •LiBr, and the signal at 0.26 ppm to a fluxional mixed tetramer, 1 ₂•(LiBr)₂. ¹J(¹³C,⁶Li) coupling constants of 11.0 and 9.8 Hz were measured at 168 K for S1 and S2, respectively, and chemical exchange between both signals was detected by 2D ⁶Li,⁶Li exchange spectroscopy and analyzed by temperature-dependent 1D ⁶Li line-shape calculations. These yielded the equilibrium constants K_eq for the chemical exchange Li₄(C₃H₅)₂Br₂ ⇌ 2 Li₂C₃H₅Br. Their temperature dependence leads to van't Hoff parameters of ΔH° = 4.6 kJ/mol, ΔS° = 41.4 J/mol K, and ΔG°₂₉₈ = −7.8 kJ/mol. From the rate constants k, Eyring parameters of ΔH^* = 42.0 kJ/mol, ΔS^* = 33.0 J/mol K, and ΔG^*₂₉₈ = 32.2 kJ/mol were calculated for the forward reaction Li₄(C₃H₅)₂Br₂ → 2 Li₂C₃H₅Br and ΔH^* = 37.5 kJ/mol, ΔS^* = −8.4 J/mol K, and ΔG^*₂₃₈ = 40.0 kJ/mol for the reverse reaction 2Li₂C₃H₅Br → Li₄(C₃H₅)₂Br₂.     

Complexation of Anions Including Nucleotide Anions by Open‐Chain Host Compounds with Amide,Urea, and Aryl Functions

A systematical evaluation of association constants between halide, phosphate, and carboxylate anions with N‐methylformamide ( 1 ) and the related bidentate receptors 2 – 6 (derived from, e.g., phthalic acid or ethylenediamine) in CDCl₃ as solvent yielded increments of complexation free‐energy ΔΔG for each single H‐bond, which varied like, e.g., 5.1 kJ/mol (for Cl⁻), 4.0 kJ/mol (for Br⁻), 4.0 kJ/mol (for I⁻) (with values taken from Tables 1 and 2), in line with expected H‐bond strength. The observed complexation induced NH‐NMR shift (CIS) values also showed a regular change, in the case of 1 , e.g., from 5.0 to 2.8 to 2.1 ppm (Table 1), with about half of these values with the bidentate ligands (Tables 2 and 3). Tridentate hosts led to a substantial binding increase, if strain‐free convergence of all NH donor functions towards the anion was possible. The tris[urea] ligand 10 yielded, even in the polar solvent DMSO, with Cl⁻ a ΔG of −21.5 kJ/mol and with Br⁻ of −10⋅5 kJ/mol, whereas with I⁻, no association was detectable. The results demonstrated that small, inexpensive, and conformationally mobile host compounds can exhibit high affinities as well as descrimination with anions, as much as more preorganized receptors do which require multistep synthesis. The corresponding adamantyl derivative 13 allowed measurements also in CDCl₃, with K=4.3⋅10⁴ M ⁻¹ for chloride (Table 7). Complexes with nucleotide anions were again particularly strong with the tridentate urea‐based ligands, the latter being optimal ligands for chloride complexation. For the association of 10 with AMP²⁻ and GMP²⁻in (D₆)DMSO, the association constants were 3⋅10⁴ M ⁻¹ (Table 8) and almost the same as with Cl⁻. In the case of the urea derivatives 17 , 18 , and 21 , containing only one phenyl or pyrenyl substituent, however, the ΔG values decreased in the order A>C>T>G (e.g. −13.6, −11.6, −7.6, −10.5 kJ/mol in the case of 17 , resp.; Table 8). In H₂O, the pyrenyl‐substituted urea derivatives allow measurements with fluorescence, and, unexpectedly, show only smaller nucleobase discrimination, with constants around 3⋅10³ M ⁻¹.     

Trans-cis isomerization of 4-(2-(9-anthryl)vinyl) pyridine.Molecular structures and ~1H NMR kinetic studies

Both cis- and trans-isomers of 4-(2-(9-anthryl)vinyl)pyridine were isolated and their molecular structures established by X-ray crystallographic method. Variable temperature 1H NMR spectroscopy was used to study the trans to cis isomerization of the title compound. The kinetic study of the reaction was based on the ratio of the NMR integration heights in toluene-d8 of the double doublet due to the cis-isomer at δ 8.51 to that of the multiplet at δ 8. 15 which was kept constant during the whole experiment. The isomerization process was found to be first order and the Arrhenius activation parameters Ea , In A ,△ H≠ and △ S≠ were calculated as 27.84kJ/mol, 6.71, 25.23 kJ/mol and - 197.89 J/(K·mol) , respectively. Besides,conformational analyses of both compounds based on molecular modelling were carried out and the results were used to compare with the experimental data.     

Solution-Structure and Aggregation Behavior of Two Dilithiostyrene Derivatives

The solution structure and the aggregation behavior of (E)-2-lithio-1-(2-lithiophenyl)-1-phenylpent-1-ene ( 1 ) and (Z)-2-lithio-1-(2-lithiophenyl)ethene ( 2 ) were investigated by one- and two-dimensional ¹H-, ¹³C-, and ⁶Li-NMR spectroscopy. In Et₂O, both systems form dimers which show homonuclear scalar ⁶Li,⁶Li spin-spin coupling. In the case of 2 , extensive ⁶Li,¹H coupling is observed. In tetrahdrofuran and in the presence of 2 mol of N,N,N′,N′-tetramethylethylylenediamine (tmeda), the dimeric structure of 1 coexists with a monomer. The activation parameters for intra-aggregate exchange in the dimers of 1 and 2 ( 1 (Et₂O): ΔH≠ = 62.6 ± 13.9 kJ/mol, ΔS≠ = 5.8 ± 14.0 J/mol K, ΔG≠(263) = 61.1 kJ/mol; 2 (dimethoxyethane): ΔH≠ = 36.9 ± 6.5 kJ/mol, ΔS≠ = ?61 ± 25 J/mol K, ΔG≠(263) = 54.0 kJ/mol) and the thermodynamic parameters for the dimer-monomer equilibrium for 1 (ΔH°; = 26.7 ± 5.5 kJ/mol, ΔS° = 63 ± 27 J/mol K), where the monomer is favored at low temperature, were determined by dynamic NMR studies.     

Synthesis and characterization of new soluble thermally stable poly(azomethine‐ether‐imide)s: discerning the possibility for high temperature applications

A new series of polyimides was synthesized by the condensation of monomers (azomethine‐ether diamine, DA‐1 and DA‐2) with pyromelliticdianhydride (PMDA), 3,4,9,10‐perylenetetracarboxylic dianhydride (PD) and 3,3′4,4′‐benzophenonetetracarboxylic dianhydride (BD). The structural explications of monomers and polyimides was conducted by FT‐IR, ¹H NMR and elemental analysis. All polyimides were found soluble in polar aprotic solvents and found to be semicrystalline in nature confirmed by XRD. The inherent viscosities were found in the range of 0.67–0.77 g/dl. %. Average molecular weight (M_W) and number average molecular weight (Mn) of the polyimides were found in the range of 5.72 × 10⁵ g/mol–6.58 × 10⁵ g/mol and 3.79 × 10⁵ g/mol 4.11 × 10⁵ g/mol respectively. The polyimides exhibited excellent thermal properties having a glass transition temperature T_g in the range of 230–290°C and the 10% weight loss temperature was above 450°C. The values of thermodynamic parameters, activation energy, enthalpy and entropy fall in the range of 45.2–53.90 kJ/mol, 43.5–52.30 kJ/mol and 0.217 kJ/mol k to 0.261 kJ/mol k respectively. Copyright © 2015 John Wiley & Sons, Ltd.     

Refined ab initio 6-31G split-valence basis set optimization of the molecular structures of biphenyl in twisted,planar, and perpendicular conformations

Improved full ab initio optimizations of the molecular structure of biphenyl in twisted minimum energy, coplanar, and perpendicular conformations by use of Poles's GAUSSIAN 82 program have been performed in the 6-31G basis set. These lead to geometries and energies of much higher reliability than our earlier STO-3G results. The torsional angle Φ_min obtained now is 45.41° in close agreement with the recent experimental value of 44.4° ± 1.2°. Calculated CC distances may be converted to experimental ED r_g-values by means of independently determined linear regression correlations with very high statistical confidence, although they agree better with experimental x ray data for coplanar biphenyl without this correction. Calculated intramolecular angles are very similar for both STO-3G and 6-31G basis sets. The calculated torsional energy barrier towards Φ = 90° (ΔE⁹⁰) is 6.76 kJ/mol in close agreement with the experimental-31G value of 6.5 ± 2.0 kJ/mol. For coplanar biphenyl with D_2h-symmetry the calculated torsional energy barrier ΔE⁰ is 13.26 kJ/mol which is surprisingly much higher than the experimental value of 6.0 ± 2.1 kJ/mol. This discrepancy could not be resolved by optimizations assumed for two kinds of distortions of planarity of orthohydrogens from the molecular plane of the coplanar carbon atoms. But for the twisted minimum energy conformation asymmetric bending of ortho-H atoms lead to a torsional angle Φ_min = 44.74° together with a dihedral angle towards ortho-H of 1.22°, and consequently even to an increase of torsional energy barriers to ΔE⁰ = 13.51 and ΔE⁹⁰ = 6.91 kJ/mol.