Intermolecular noncovalent interactions with carbon in solution

One of the most familiar carbon-centered noncovalent interactions (NCIs) involving an antibonding π*-orbital situated at the Bürgi–Dunitz angle from the electron donor, mostly lone pairs of electrons, is known as n → π* interactions, and if it involves a σ* orbital in a linear fashion, then it is known as the carbon bond. These NCIs can be intra- or inter-molecular and are usually weak in strength but have a paramount effect on the structure and function of small-molecular crystals and proteins. Surprisingly, the experimental evidence of such interactions in the solution phase is scarce. It is even difficult to determine the interaction energy in the solution. Using NMR spectroscopy aided with molecular dynamics (MD) simulation and high-level quantum mechanical calculations, herein we provide the experimental evidence of intermolecular carbon-centered NCIs in solution. The challenge was to find appropriate heterodimers that could sustain room temperature thermal energy and collisions from the solvent molecules. However, after several trial model compounds, the pyridine-N-oxide:dimethyltetracyanocyclopropane (PNO–DMTCCP) complex was found to be a good candidate for the investigation. NBO analyses show that the PNO:DMTCCP complex is stabilized mainly by intermolecular n → π* interaction when a weaker carbon bond gives extra stability to the complex. From the NMR study, it is observed that the NCIs between DMTCCP and PNO are enthalpy driven with an enthalpy change of −28.12 kJ mol⁻¹ and dimerization energy of ∼−38 kJ mol⁻¹ is comparable to the binding energies of a conventional hydrogen-bonded dimer. This study opens up a new strategy to investigate weak intermolecular interactions such as n → π* interaction and carbon bonds in the solution phase.

NMR spectroscopy combined with MD simulation and electronic structure calculations enabled us to quantify the energetics of the intermolecular n → π* interaction and carbon-bonding (n → σ* interaction) in solution.     

An aqueous colloidal silver solution stabilized with carbonate ions

A method has been developed for the preparation of a stable “pure” silver hydrosol containing nanoparticles with an average size of 10 nm and stabilizing carbonate ions. The method consists in the photochemical reduction of silver ions with oxalate ions, which simultaneously generate carbonate ions. Optical spectroscopy, electron microscopy, and dynamic light scattering are used to study the hydrosol     

Acid-base interactions in the methanesulfonic acid-propylene carbonate system

The methanesulfonic acid (MSA)—propylene carbonate (PC) system with component concentrations of 0–100% was studied at 30°C using the Multiple Attenuated Total Reflection (MATR) IR spectroscopy. The formation of a strong 1∶1 molecular complex of MSA with PC was established. In the presence of an excess of the acid, a second MSA molecule adds to this complex to give the molecular complex (2MSA)·PC. When excess propylene carbonate is used, the MSA·PC complex is solvated by a propylene carbonate molecule. No protonation of the base or formation of complexes with a strong symmetrical H bond was observed. Continuous absorption was not detected in IR spectra of the solutions. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 313–318 February, 1999.     

Spontaneous luminescence of hydrogen carbonate solution

The possibility of luminescence of freshly prepared doubly distilled water, settled doubly distilled water, and a bicarbonate solution has been studied. It has been found that the water settled in air can be a source of emission. The contribution of various factors including external background radiation to the luminescence of doubly distilled water has been examined.     

Kinetic model for reaction of aspartic acid with magnesium carbonate in solution

A kinetic model has been developed for describing the non-catalytic liquid-solid reaction between aspartic acid and magnesium carbonate in solution, with parallel dissolution of aspartic acid particles. The effects of size distribution of both the aspartic acid and magnesium carbonate particles have been accounted for by using a population balance model. The parameters of the model have been estimated by fitting them to experimental data taken from literature.

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Spectrophotofluorometric determination of terbium and europium in potassium carbonate solution

The conditions for the spectrophotofluorometric determination of terbium and europium, in solutions of potassium carbonate, have been established. The apparent excitation and fluorescence wavelengths used, respectively, are 245 mmu and 550 mmu for terbium and 400 mmu and 620 mmu for europium. The fluorescence varies linearly with the concentration of terbium and europium in the range 0.3-70 mug, of terbium/ml and 4-800 mug of europium/ml. Large amounts of gadolinium, lutetium and yttrium do not interfere. Cerium(IV) interferes seriously.     

Intermolecular interactions in heteromolecular crystals of tetrazine derivatives with azoles

Heteromolecular crystal structures formed by symmetrically and unsymmetrically 3,6-disubstituted tetrazine derivatives with NH donor azoles are investigated. The main crystal motifs and the intermolecular interactions responsible for their formation are identified.     

Photopolymerization of methyl methacrylate in solution initiated with benzophenone derivatives

Theoretical and Experimental Chemistry -     

Hydrolysis of polyacrylonitrile in aqueous solution of sodium carbonate

The kinetics of homogeneous hydrolysis of polyacrylonitrile in an aqueous solution of sodium carbonate and the chemical structure of the resulting copolymer are studied by FTIR spectroscopy, ¹³C NMR analysis, and titration methods. It is found that hydrolysis in the presence of sodium carbonate does not include the stage of amidine formation and does not result in the complete exhaustion of nitrile groups in a polymer. The designed partial-hydrolysis method permits the use of polyacrylonitrile for the synthesis of the copolymer with a predominant unit alternation; this copolymer is identical in terms of chemical structure to the Rohacell copolymer obtained through block copolymerization of acrylonitrile and methacrylic acid.     

X-ray and solution structures of Cu(II) GHK and Cu(II) DAHK complexes: influence on their redox properties

The Gly‐His‐Lys (GHK) peptide and the Asp‐Ala‐His‐Lys (DAHK) sequences are naturally occurring high‐affinity copper(II) chelators found in the blood plasma and are hence of biological interest. A structural study of the copper complexes of these peptides was conducted in the solid state and in solution by determining their X‐ray structures, and by using a large range of spectroscopies, including EPR and HYSCORE (hyperfine sub‐level correlation), X‐ray absorption and ¹H and ¹³C NMR spectroscopy. The results indicate that the structures of [Cu^II(DAHK)] in the solid state and in solution are similar and confirm the equatorial coordination sphere of NH₂, two amidyl N and one imidazole N. Additionally, a water molecule is bound apically to Cu^II as revealed by the X‐ray structure. As reported previously in the literature, [Cu^II(GHK)], which exhibits a dimeric structure in the solid state, forms a monomeric complex in solution with three nitrogen ligands: NH₂, amidyl and imidazole. The fourth equatorial site is occupied by a labile oxygen atom from a carboxylate ligand in the solid state. We probe that fourth position and study ternary complexes of [Cu^II(GHK)] with glycine or histidine. The Cu^II exchange reaction between different DAHK peptides is very slow, in contrast to [Cu^II(GHK)], in which the fast exchange was attributed to the presence of a [Cu^II(GHK)₂] complex. The redox properties of [Cu^II(GHK)] and [Cu^II(DAHK)] were investigated by cyclic voltammetry and by measuring the ascorbate oxidation in the presence of molecular oxygen. The measurements indicate that both Cu^II complexes are inert under moderate redox potentials. In contrast to [Cu^II(DAHK)], [Cu^II(GHK)] could be reduced to Cu^I around ?0.62 V (versus AgCl/Ag) with subsequent release of the Cu ion. These complete analyses of structure and redox activity of those complexes gave new insights with biological impact and can serve as models for other more complicated Cu^II–peptide interactions.     

Intermolecular interactions of punicin derivatives

The 2- and 4-methyl derivates of punicin [N-(2′,5′-dihydroxyphenyl)-pyridinium chloride] were subjected to Knoevenagel reactions. Starting materials as well as reaction products were examined with respect to homo and hetero-intermolecular interactions. It was found that N-(2′,5′-dihydroxyphenyl)-2-methylpyridinium chloride forms a stable 2:1 complex to hydroquinone. Decomplexation can be accomplished by anion exchange to tetraphenylborate, or by competing complexation with p-benzoquinone. Results of three single crystal X-ray analyses as well as NMR titrations and dilution experiments are presented.     

S---S and S---O interactions in some 4-thiazoline derivatives

Structural data have been obtained by X-ray crystallography for three 4-thiazoline derivatives, 2-benzoylimino-3, 4-diphenyl-4-thiazoline, 3-methyl-4-phenyl-2-thiobenzoylimino-4-thiazoline, and 3,4-diphenyl-4-thiazolin-2-ylidene thioacetophenone. In these compounds the separations between the sulphur atoms of the thiazoline rings and the adjacent oxygen or sulphur atoms of the carbonyl or thiocarbonyl groups approximate to the Huggins “constant energy” distances, suggesting that there is little or no bonding between them.     

Intramolecular charge-transfer interactions in pi-extended tetrathiafulvalene derivatives

New electron-donor (D)-electron-acceptor (A) TTF architectures are presented in which two electron-donating 1,3-dithiole moieties are connected by a pi bridge to the weak electron-accepting quinoxaline moiety (D-pi-A compounds 9a and 9b and also two 1,3-dithiole-2-ylidene moieties are connected by a pi bridge to the electron-accepting thieno[3,4-b]quinoxaline bridge (D-pi-A-pi-D compounds 12a-c). There are through-bond intramolecular charge-transfer (ICT) interactions, predicted in theoretical calculations, and confirmed by UV-vis spectroscopy and cyclic voltammetry measurements. This work constitutes the first use of quinoxalines as electron-accepting moieties in D-pi-A compounds.     

Interaction of three pradimicin derivatives with divalent cations in aqueous solution

This study focused on further analysis of the aggregation behavior of pradimicin derivatives and their interaction with cations in aqueous solution. BMY was compared with two other pradimicin antibiotics (T2 and FB) with the same aglycone moiety but consisting of different substitute groups. The surface tension measurement showed a clear critical micelle concentration at 1-2 mM of the BMY aqueous solution. The role of Zn2+ in replacing the Ca2+ was examined using 1H nuclear magnetic resonance (NMR) method. From changes in the NMR spectrum and precipitability, it was concluded that zinc ion has lower affinity and higher precipitating ability to BMY than the divalent cations of alkaline earth metal. The aggregation behavior of T2 and FB in aqueous solution was also studied using NMR method. The results suggest that the supramolecular behavior of T2 is similar to BMY whether or not Ca2+ ions are present in solution and that there are two binding sites for calcium ions in a T2 molecule. Unlike BMY and T2, the NMR spectrum of FB does not show distinct change upon Ca2+ addition. The interaction of pradimicin antibiotics with divalent metal ions was thought to be related to ionic electronegativity and to the amphoteric property of the antibiotics.     

Sol-gel transition of polyacrylonitrile/propylene carbonate solution

The thermoreversible gelation of polyacrylonitrile (PAN)/propylene carbonate (PC) solution was investigated by calorimetric, thermomechanical, optical, and rheological measurements. Two endothermic signals are observed for PAN/PC gels in DSC thermograms, and the peak temperature of the higher melting endotherm coincides with the gel melting temperature determined from the TMA thermogram. SEM micrographs show that the overall structure is separated into two phases and is composed of sperical domains, which may be formed upon passing through the thermodynamically unstable or metastable state via the liquid-liquid phase separation. The gelation rate increases with the aging temperature. This is also discussed on the basis of the elasticity theory of the fishnet gel. © 1994 John Wiley & Sons, Inc.     

Investigation on lipase-catalyzed solution polymerization of cyclic carbonate

In this study, 26-membered macrocyclic carbonate, cyclobis(decamethylene carbonate) [(DMC)₂] was attempted to undergo ring-opening polymerization by lipase catalysis in toluene. Novozym-435 exhibited even higher catalytic activity towards (DMC)₂ polymerization compared with SnOCt₂ while high molecular weight (M_n) of 5.4 × 10⁴ and yield of 99% was still achieved at ultra-low enzyme/substrate (E/S) weight ratio of 1/200. ¹H NMR spectra demonstrated the existence of terminal hydroxyl group. Solid phase polymerization in the absence of toluene unexpectedly took place at the temperature lower than (DMC)₂’s melting point of 110 °C. Compared with solvent-free case, the addition of toluene solvent resulted in marked increase in reaction rate. As to the polymerization during 48 h with the E/S weight ratio of 1/100, a region existed at around toluene/carbonate (vol/wt, ml/g) ratio of 1∼2 where the polymerizations gave optimal results in terms of both higher molecular weight and monomer conversion. It was found that much higher molecular weight polymers may be obtained by decreasing enzyme concentrations. Plots of ln{[M]₀:[M]_t} versus reaction time were in linear agreement, indicating no chain termination, and monomer consumption follows a first-order rate law. The Novozym-435 catalyzed polymerization of (DMC)₂ in toluene presented pseudo-living characteristic. Compared with 6-membered trimethylene carbonate, much lower reaction activity of large-sized (DMC)₂ is observed, which is opposite to the result concerning the enzymatic polymerization of lactones with different ring-size.