NMR chemical shielding and spin–spin coupling constants across hydrogen bonds in uracil–α-hydroxy-N-nitrosamine complexes

One- and two bond spin–spin coupling constants, ¹ J, ^1h J , and ^2h J across X–H?O hydrogen bonds and shielding constants of bridging hydrogens have been computed for complexes formed from interaction between the α-hydroxy-N-nitrosamine (NP) and four preferential binding sites of the uracil (U) at B3LYP/6-311++G(2d,2p)//MP2/6-311++G(2d,2p) level of theory. All complexes are stabilized by two H_U?O_NP and H_NP?O_U hydrogen bonds. Very good correlations were found between NMR spin–spin coupling constant as well as isotropic shielding constant and the binding energy, H-bond distance, red-shift of vibration frequency, charge transfer energy, and electron density at H-bond critical point.     

Host–guest interactions of thiabendazole with normal and modified cucurbituril: 1H NMR,phase solubility and antifungal activity studies

Guest–host inclusion complexes between thiabendazole (TBZ) and cucurbit[7]uril (Q[7]), symmetrical tetra-methylcucurbit[6]uril (TMeQ[6]) and meta-hexamethyl-substituted cucurbit[6]uril (HMeQ[6]) in aqueous solution were investigated by ¹H NMR spectroscopy and phase solubility studies. The antifungal activities of the inclusion complexes were also determined. Analysis of the ¹H NMR spectra revealed that the host Q[7] selectively binds the benzimidazole ring moiety of the guest molecule and that the thiazole ring is encapsulated into the cavities of TMeQ[6] and HMeQ[6]. Phase solubility diagrams were analysed using rigorous procedures to obtain estimates of the complex formation constants for Q[n]-TBZ complexation. The phase solubility studies showed that TBZ solubility increased as a function of Q[7], TMeQ[6] and HMeQ[6] concentrations. We found that complexation of TBZ with Q[n] increased the inhibitory effect of TBZ on the growth of Fusarium graminearum. Our results thus demonstrate that complexation of TBZ with Q[n] could be used to improve the solubility and antifungal activity of TBZ.     

Total synthesis of novel bioactive natural product paracaseolide A and analogues: computational evaluation of a ‘proposed’ biomimetic Diels–Alder reaction

A short and generally applicable synthesis of bioactive tetracyclic natural product paracaseolide A has been accomplished employing a ‘proposed’ biomimetic Diels–Alder reaction as the key strategic step. The Diels–Alder precursors for this purpose were readily assembled through a versatile Suzuki coupling on preformed α-halo butenolides. The mechanistic aspects of the ‘putative’ biomimetic Diels–Alder reaction have been probed using computational methods, which suggest that this [4+2]-cycloaddition proceeds through a step-wise process and product profile is thermodynamically governed.     

Host–guest interactions of 6-benzyladenine with normal and modified cucurbituril: 1H NMR,UV absorption spectroscopy and phase solubility methods

Guest–host inclusion complexes between 6-benzyladenine (6-BA), cucurbit[7]uril (Q[7]), symmetrical tetramethylcucurbit[6]uril (TMeQ[6]) and meta-hexamethyl-substituted cucurbit[6]uril (HMeQ[6]) in aqueous solution were investigated by ¹H NMR, UV absorption spectroscopy and phase solubility studies. The ¹H NMR spectra analysis revealed that the hosts selectively bound the phenyl moiety of the guests. Absorption spectroscopic analysis defined the stability of the host–guest inclusion complexes. A host:guest ratio of 1:1 was measured quantitatively as (5.63 ± 0.26) × 10⁴, (1.94 ± 0.17) × 10³ and (2.89 ± 0.23) × 10³ mol L^? 1 for the Q[7]-6-BA, TMeQ[6]-6-BA and HMeQ[6]-6-BA systems, respectively. Phase solubility diagrams were analysed through rigorous procedures to obtain estimates of the complex formation constants for Q[n]-6-BA complexation. The formation constants were (1.29 ± 0.24) × 10⁴ L mol^? 1 for Q[7]-6-BA, (3.20 ± 0.17) × 10³ L mol^? 1 for TMeQ[6]-6-BA and (3.52 ± 1.01) × 10³ L mol^? 1 for TMeQ[6]-6-BA. Furthermore, phase solubility studies showed that 6-BA solubility increased as a function of Q[7], TMeQ[6] and HMeQ[6] concentrations. The thermodynamic parameters of the complex formation were also determined. The formation of inclusion complexes between 6-BA and Q[7] was enthalpy controlled, suggesting that hydrophobic and van der Waals interactions were the main driving forces. Our results demonstrated that the complexation of 6-BA with Q[n] could be used to improve the solubility of 6-BA.     

31P and 17O nuclear spin—lattice relaxation in some phosphorylated molecules. Determination of 31P spin—rotation coupon constants, 17O quadrupolar coupling constants and chemical shielding anisotropy of the 31P nucleus

The spin—lattice relaxation time of the ³¹P nucleus was measured for 11 phosphorylated molecules (phosphine oxides, trialkylphosphates and phosphoramides) dissolved in nitromethane at three different frequencies and as a function of the temperature for three compounds. The different contributions to the relaxation rate due to dipolar, chemical shift anisotropy and spin—rotation interactions were determined and the reorientational correlation times of the molecules were deduced when the anisotropy of the chemical shift tensor of the ³¹P nucleus could be (re)determined. The quadrupolar coupling constant of the ¹⁷O nucleus was also determined from the linewidth of the nuclear magnetic resonance signals, for phosphine oxides and triphenylphosphate, giving some information on the electronic distribution into the phosphoryl bond. The spin—rotation coupling constants for trimethylphosphine oxide and triphenylphosphine oxide were deduced and the chemical shift anisotropy Δσ of trialkylphosphates estimated.     

Calculated coupling constants 1J(X–Y) and 1K(X–Y), and fundamental relationships among the reduced coupling constants for molecules HmX–YHn,with X,Y ═ 1H, 7Li, 9Be, 11B, 13C, 15N, 17O, 19F, 31P, 33S,and 35Cl

Equation-of-motion coupled-cluster singles and doubles (EOM-CCSD) calculations have been performed to determine coupling constants ¹J(X–Y) for 65 molecules H_mX–YH_n, with X,Y ═ ¹H, ⁷Li, ⁹Be, ¹¹B, ¹³C, ¹⁵N, ¹⁷O, ¹⁹F, ³¹P, ³³S, and ³⁵Cl. The computed ¹J(X–Y) values are in good agreement with available experimental data. The reduced coupling constants ¹K(X–Y) have been derived from ¹J(X–Y) by removing the dependence on the magnetogyric ratios of X and Y. Patterns are found for the reduced coupling constants on a ¹K(X–Y) surface that are related to the positions of X and Y in the periodic table.     

Solvatochromic,acid–base features and time effect of some azo dyes derived from 1,3-benzothiazol-2-ylacetonitrile: Experimental and semiempirical investigations

The solvatochromism and other spectroscopic properties of seven azo dyes were studied, with a particular respect to the role of the solvent basicity, and interpreted with the aid of experimental findings and semiempirical data. The electronic absorption spectra of the dyes examined in different solvents combined with theoretical calculations showed that most of the investigated compounds coexist in the hydrazone and/or azo-enamine-common anion equilibrium or in the solely anionic form depending upon the nature of the solvent employed. These interesting features open up possibilities for the use of these compounds in analytical chemistry as acid–base indicators. Furthermore, both of intermolecular and intramolecular charge transfer equilibria have been reflected by experimental absorption spectra of compounds 4 and 5. The enthalpies of formation predicted at PM6 (COSMO) and PM6/CI (COSMO) for the ground (S₀) and excited (S₁) states, respectively have been successfully used for the explanation of the observed bathchromic shift in non-polar solvents. The effect of time on the longer wavelength visible band of compound 7 has been thoroughly investigated.     

Host-guest complexes of a water soluble cucurbit[6]uril derivative with some dications of 1,ω-alkyldipyridines: <Superscript>1</Superscript>H NMR and X-ray structures

Interactions between a symmetrical tetramethyl-substituted cucurbit[6]uril (host: TMeQ[6]) and 1,ω-alkylenedipyridine (ω = 2, 4, 6, 8, 10) dicationic guests were investigated using ¹H NMR spectroscopy and single crystal X-ray crystallography. In these inclusion complexes, combined cavity and portal binding in TMeQ[6] were observed, and the length of the bridged alkylene was found to play an important role not only in balancing the overall hydrophilic/hydrophobic interaction between the host and the guest, but also in defining the structure of the resulting inclusion complexes. For the guest 1,2-ethylenedipyridine (Edpy), TMeQ[6] includes a positively charged pyridine ring of Edpy to form an unsymmetrical inclusion complex; for the guest 1,4-butylenedipyridine (Bdpy), TMeQ[6] includes a positively charged pyridine ring of Bdpy, but the different competitive interactions in and between the related inclusion complexes could lead to a fast exchange between the hosts and guests. For the guests with longer bridge chains, such as 1,6-hexamethylenedipyridine (Hdpy) or 1,8-octylenedipyridine (Odpy), a stable pseudorotaxane inclusion complex is formed by combining the hydrophobic cavity and the outer portal dipole-ion interactions. However, for 1,10-decatylenedipyridine (Ddpy), the two TMeQ[6] host molecules include the two end pyridine rings of Ddpy and form a dumbbell inclusion complex. Supported by the National Natural Science Foundation of China (Grant Nos. 20662003 & 20767001), the International Collaborative Project of Guizhou Province (Grant No. 2007400108), the Science Technology Fund of Guizhou Province (Grant No. J-2008-2012) and the Natural Science Youth Foundation of Guizhou University (Grant No. 2007-005)     

Guest–host systems containing anthraquinone dyes with multiple visible transitions giving positive and negative dichroic order parameters: an assessment of principal molecular axes and computational methods

ABSTRACT

Three 1,4-disubstituted anthraquinone dyes with bis(4-n-butylphenyl) substituents connected via amine or amide linking groups have been studied as guest molecules dissolved in the nematic host E7. UV-visible absorption spectroscopy has shown each of the dyes to exhibit multiple absorption bands in the visible region, and dichroic order parameters obtained from polarised spectra of aligned guest–host samples were shown to differ significantly between the bands for each dye, and between the dyes. Time-dependent density functional theory calculations indicated that each dye exhibits several transitions, giving transition dipole moment vectors with a range of orientations, and fully atomistic molecular dynamics simulations of the guest–host mixtures showed differences in the calculated molecular alignments of the dyes. Combining the results from these two sets of calculations enabled a comparison of molecular alignment models based on the moments of inertia and the surface tensors of the dyes. The match between calculated and experimental values was improved significantly when using the surface tensor rather than the moment of inertia model, indicating that the shapes of the molecular surfaces of these dyes are crucial to their alignment. A novel method of calculating polarised UV-visible absorption spectra of dyes in liquid crystal hosts is also presented.     

Voltammetric,spectroscopic and thermal studies on the binding of some heterocyclic azo compounds with α- and β-cyclodextrins: pH effect and association affinity

The binding abilities of α- and β-cyclodextrins (α-CD and β-CD) with some heterocyclic azo compounds (1,1'-(azodicarbonyl)dipiperidine (ADP) and azodicarboxylic dimorpholide (ADM)) were studied at different pHs (4, 7.4 and 10) by UV-Vis spectroscopy and square-wave voltammetry techniques. The association constants (K _i) and stoichiometries of the binding of these azo compounds with α-CD and β-CD were determined by using square-wave voltammetry technique. These bindings were formed with a stoichiometry of 1: 1 in solution. The solid samples, obtained from the mixtures (molar ratio of 1: 1) of these azo dyes and CDs in aqueous phase were analyzed by FT-IR spectroscopy and thermal analysis methods. Thermal analysis results showed that ADP and ADM formed the inclusion complexes with α-CD; however, the binding of the azo dyes with β-CD gave non-inclusion complexes.     

Self-assembly of tetrareduced corannulene with mixed Li–Rb clusters: dynamic transformations,unique structures and record 7Li NMR shifts

Self-assembly processes of the highly reduced bowl-shaped corannulene generated by the chemical reduction with a binary combination of alkali metals, namely Li–Rb, have been investigated by variable-temperature ¹H and ⁷Li NMR spectroscopy. The formation of several unique mixed metal sandwich products based on tetrareduced corannulene, C₂₀H₁₀ ^4– (1 ^4–), has been revealed followed by investigation of their dynamic transformations in solutions. Analysis of NMR data allowed to propose the mechanism of stepwise alkali metal substitution as well as to identify experimental conditions for the isolation of intermediate and final supramolecular products. As a result, two new triple-decker aggregates with a mixed Li–Rb core, [{Rb(THF)₂}₂]//[Li₃Rb₂(C₂₀H₁₀)₂{Li⁺(THF)}] (2) and [{Rb(diglyme)}₂]//[Li₃Rb₃(C₂₀H₁₀)₂(diglyme)₂]·0.5THF (3·0.5THF), have been crystallized and structurally characterized. The Li₃Rb₂-product has an open coordination site at the sandwich periphery and thus is considered transient on the way to the Li₃Rb₃-sandwich having the maximized intercalated alkali metal content. Next, the formation of the LiRb₅ self-assembly with 1 ^4– has been identified by ⁷Li NMR as the final step in a series of dynamic transformations in this system. This product was also isolated and crystallographically characterized to confirm the LiRb₅ core. Notably, all sandwiches have their central cavities, located in between the hub-sites of two C₂₀H₁₀ ^4– decks, occupied by an internal Li⁺ ion which exhibits the record high negative shift (ranging from –21 to –25 ppm) in ⁷Li NMR spectra. The isolation of three novel aggregates having different Li–Rb core compositions allowed us to look into the origin of the unusual ⁷Li NMR shifts at the molecular level. The discussion of formation mechanisms, dynamic transformations as well as unique electronic structures of these remarkable mixed alkali metal organometallic self-assemblies is provided and supported by DFT calculations.     

Second-order Møller-Plesset calculations of total ground-state energies of tetrahedral and octahedral molecules at equilibrium, confronted with some model-dependent scaling properties

Using second-order M?ller-Plesset (MP2) optimized geometries, calculations of total energies of some 20 tetrahedral and octahedral molecules have been carried out. We have then confronted these first-principles results, which of course include a fraction of the electronic correlation energy, with some model-dependent scaling properties. In particular, one scaling relation emerging from the semiclassical Thomas-Fermi electron density method relates the known nuclear-nuclear potential energy, V(nn), at equilibrium to the electron-nuclear term, V(en), and the total kinetic energy.     

Encapsulation of vortioxetine with cyclodextrins via host–guest inclusion complex: Synthesis,characterization, solubility,and in vitro dissolution studies

Drug solubility plays a significant role in the development of drug formulation. The objectives of this work are to improve the solubility and dissolution rate of vortioxetine (VT) by preparing its inclusion complexes (ICs) with β-Cyclodextrin (β-CD) and γ-Cyclodextrin (γ-CD). The ICs were prepared in 1:1 M ratio via recrystallization method and characterized by P-XRD, FT-IR, ¹H NMR, 2D NOESY, and DSC. Further, the crystal structure of VT-β-CD was analyzed by SC-XRD. P-XRD data obtained for ICs describe the crystalline pattern. The DSC analysis shows change in the thermal behavior of VT, CDs and ICs. FT-IR analysis shows shifting of frequencies in ICs when compared with the pristine VT drug and CDs. The 2D NOESY in DMSO-d⁶ indicates weak interaction between the VT and CD molecules. The crystal structure of VT-β-CD consists of one guest VT, one host CD, and nine water molecules in the crystal lattice. The solubility of ICs was significantly improved in distilled water, pH 1.2 acidic, and phosphate buffer pH 6.8 medium, as compared with the solubility of the pristine VT drug. The in vitro dissolution rate of ICs in different dissolution media was investigated, which was higher than that of the commercial product of VT.     

Infrared and Raman studies of hydrogen bonded complexes involving acetone,acetophenone and benzophenone—I. Thermodynamic constants and frequency shifts of the νOH and νCO stretching vibrations

The hydrogen bonded complexes between phenol derivatives and acetone (I), acetophenone (II) and benzophenone (III) have been studied in carbon tetrachloride solution by i.r. spectroscopy. The formation constants, the enthalpies of complex formation, the Δν_OH and Δν_CO values have been determined. For a given phenol derivative, the thermodynamic constants and Δν_OH are ordered according to I > II > III and the influence of a substituent implanted on the phenolic ring can be expressed by the Hammett relationship. The ϱ coefficients of the Hammett equation are related to the complexation enthalpies. The Badger—Bauer relation is valid for the three bases. The comparison with complexes involving other carbonyl bases allows to precise the influence of the substituent implanted on the carbonyl group. The Δν_OH values obey the dual substituent parameter equation using σ_I and σ⁺_R; the ϱ_I/ϱ_R ratio is higher than one. The Δν_CO values are shown to depend on the complexation enthalpy and on the delocalization effect of the substituents.     

Studying (n, 0) and (m,m) GaP nanotubes (n = 3–10 and m = 2–6) through DFT calculations of Ga-69 quadrupole coupling constants

We investigated properties of representative zigzag and armchair gallium phosphide (GaP) nanotubes by performing density functional theory (DFT) calculations. To achieve our purpose, eight models of (n,0) zigzag GaP nanotubes with n = 3–10 and five models of (m,m) armchair GaP nanotubes m = 2–6 were considered. Each model was firstly optimized and quadrupole coupling constants (C_Q) were subsequently calculated for gallium-69 atoms of the optimized structures. The results indicated that the optimized properties including dipole moments, energy gaps, binding energies, and bond lengths could be mainly dependent on the diameters of GaP nanotubes, which are directly determined by n or m indices. Moreover, comparing the values of C_Q parameters indicated that the narrower GaP nanotubes could be considered as more reactive materials than the wider nanotubes, in which the reactivities are very important in determining the applications of nanotubes. And finally, the atoms at the sidewalls of nanotubes could be divided into atomic layers based on the similarities of properties for atoms of each layer, in which the properties of Ga atoms at the edges of nanotubes are significantly different from other layers only for wider nanotubes.     

Synthesis of new cyano-substituted analogues of Tröger's bases from bromo-derivatives. A stereochemical dependence of long-range (nJHH,n = 4, 5, and 6) proton–proton and proton–carbon (nJCH,n = 1, 2, 3, 4, and 5) coupling constants of these compounds

A free-catalyst microwave-assisted cyanation of brominated Tröger's base derivatives ( 2a - f ) is reported. The procedure is simple, efficient, and clean affording the nitrile compounds ( 3a - e, I ) in very good yields. Complete assignment of ¹H and ¹³C chemical shifts of 2a - f, I and 3a - d, I was achieved using gradient selected 1D nuclear magnetic resonance (NMR) techniques (1D zTOCSY, PSYCHE, DPFGSE NOE, and DEPT), homonuclear 2D NMR techniques (gCOSY and zTOCSY), and heteronuclear 2D NMR techniques (gHSQCAD/or pure-shift gHSQCAD, gHMBCAD, bsHSQCNOESY, and gHSQCAD-TOCSY) with adiabatic pulses. Determination of the long-range proton–proton coupling constants ⁿJ_HH (n = 4, 5, 6) was accomplished by simultaneous irradiation of two protons at appropriate power levels. In turn, determined coupling constants were tested by an iterative simulation program by calculating the ¹H NMR spectrum and comparing it to the experimental spectrum. The excitation-sculptured indirect-detection experiment (EXSIDE) and ¹H-¹⁵N CIGARAD-HMBC (constant time inverse-detection gradient accordion rescaled heteronuclear multiple bond correlation) were applied for determination of long-range carbon–proton coupling constants ⁿJ_CH (n = 2, 3, and 4) and for assignment of ¹⁵N chemical shift at natural abundance, respectively. DFT/B3LYP optimization studies were performed in order to determine the geometry of 2c using 6-31G(d,p), 6-311G(d,p), and 6–311 + G(d,p) basis sets. For calculation of ¹H and ¹³C chemical shifts, ⁿJ_HH (n = 2, 3, 4, 5, and 6), and ⁿJ_CH (n = 1, 2, 3, and 4) coupling constants, the GIAO method was employed at the B3LYP/6-31G(d,p), B3LYP/6-31+G(d,p), B3LYP/6-311+G(d,p), B3LYP/6-311++G(2d,2p), B3LYP/cc-pVTZ), and B3LYP/aug-cc-pVTZ) levels of theory. For the first time, a stereochemical dependence magnitude of the long-range ⁿJ_HH (n = 4, 5, and 6) and ⁿJ_CH (n = 1, 2, 3, 4, and 5) have been found in bromo-substituted analogues of Tröger's bases.     

A comparative study of complexation of enalapril with α-, β- and γ-cyclodextrins in aqueous medium: structure elucidation of inclusion complexes using NMR spectroscopic and molecular mechanics methods

Structural studies of complexes of enalapril maleate with α-, β- and γ-cyclodextrins were carried by NMR spectroscopy and computational methods. The formation of complexes of enalapril with all the three cyclodextrins was established by chemical shift changes observed in the cavity protons of cyclodextrins in the presence of enalapril maleate. The stoichiometry of the complexes was determined to be 1:1 by ¹H NMR titrations studies using Scott’s method. Intermolecular cross peaks observed in the 2D ROESY spectra of mixtures of enalapril maleate with three cyclodextrins helped in establishing the probable structures of these inclusion complexes which were supported by molecular mechanics (MM2) studies. Enalapril forms 1:1 inclusion complex with all the studied cyclodextrins through aromatic ring. The mode of approach of aromatic ring to the α-cyclodextrin cavity was found to be different from those of β- and γ-cyclodextrins, which were identical.     

Host–guest complexation of omeprazole, pantoprazole and rabeprazole sodium salts with cyclodextrins: an NMR study on solution structures and enantiodiscrimination power

The application of different cyclodextrins (CDs) as NMR chiral solvating agents (CSAs) for the sodium salts of the proton-pump inhibitors omeprazole, pantoprazole (sesquihydrate) and rabeprazole was investigated. It was proved that the formation of diastereomeric host–guest complexes in D₂O solution between the CDs and those substrates permitted the direct ¹H NMR discrimination of the enantiomers of the sodium salts of these compounds without the need of previous working-up. Rotating frame nuclear overhauser effect spectroscopy (ROESY) was used to ascertain the solution geometries of the host–guest complexes. The results suggested a preferential binding of the benzimidazole moiety of the guest molecules within the macrocyclic cavity of α-CD, whereas the higher dimensions of β- and γ-CD also permitted the inclusion of the highly substituted pyridine moieties. Moreover, the solution stoichiometries and the binding constants of the complexes formed with pantoprazole at room temperature were determined by ¹H and ¹⁹F NMR titration. Diffusion-filtered Spectroscopy was applied to obtain clean spectra without the interference of the HOD signal.     

Host-guest complexes of a water soluble cucurbit[6]uril derivative with some dications of 1,ω-alkyldipyridines:~1H NMR and X-ray structures

Interactions between a symmetrical tetramethyl-substituted cucurbit[6]uril (host:TMeQ[6]) and 1,ω-alkylenedipyridine (ω = 2,4,6,8,10) dicationic guests were investigated using 1H NMR spectroscopy and single crystal X-ray crystallography. In these inclusion complexes,combined cavity and portal binding in TMeQ[6] were observed,and the length of the bridged alkylene was found to play an important role not only in balancing the overall hydrophilic/hydrophobic interaction between the host and the guest,but also ...     

Syntheses of amides via iodine-catalyzed multiple sp~3 C–H bonds oxidation of methylarenes and sequential coupling with N,N-dialkylformamides

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