Kinetics of hydrolysis and self condensation reactions of silanes by NMR spectroscopy

The hydrolysis of four alkoxy-silane coupling agents, 3-methacryloxypropyl trimethoxy silane (MPMS), 3-mercaptopropyl trimethoxy silane (MRPMS), octyl triethoxy silane (OES) and 3-aminopropyl triethoxy silane (APES) was carried out in an ethanol/water 80/20 (w/w) solution under acidic, alkaline and neutral conditions and followed by ¹H, ¹³C and ²⁹Si NMR spectroscopy. It was found that the kinetic rate of the hydrolysis of the silanes under neutral conditions was very low, except for APES, which displayed the fastest reaction speed. The addition of TEA catalyzed both silane hydrolysis and self condensation reactions. Acidic conditions enhanced the hydrolysis and the ensuing silanol entities were quite stable. In fact, these conditions slowed down the rate of the self condensation reactions, as deduced from in situ ¹H and ¹³C NMR. Thanks to in situ ²⁹Si NMR spectroscopy, the nature of the intermediary species versus reaction time was established.     

Investigation of the degradation mechanism of cross-linked polyethyleneimine by NMR spectroscopy

The degradation pathway of a cross-linked polyethyleneimine (c-PEI) suitable as a gene carrier was studied at different pH values by real-time ¹H NMR and diffusion-weighted ¹H NMR spectroscopy. The c-PEI was synthesized by cross-linking PEI segments with 2,4-pentanediol diacrylate (PDDA). The experimental results show that under basic, neutral and acidic conditions, the degradation of c-PEI occurs through hydrolysis of the ester moieties of the PDDA linkers. The degradation half-life of the polymer is 22, 48 and more than 720 h at pH 10.2, 7.4 and 4.6, respectively, showing that the degradation of c-PEI is highly pH sensitive. By using a modified version of diffusion-weighted ¹H NMR experiment, the variation of the apparent molecular weight of c-PEI in the degradation process was monitored. Furthermore, a Monte Carlo simulation was applied to simulate the relation between the average molecular weight and the number of chain ends during the degradation process of a model system of c-PEI. By comparing the NMR results with those obtained from simulation, the mechanism of degradation under various pH conditions is discussed. The present work demonstrates that the combination of real-time ¹H NMR, diffusion-weighted ¹H NMR spectroscopy and Monte Carlo simulation is a useful strategy for characterizing the degradation process of degradable polymers.     

Remarkable thermodynamic stability toward hydrolysis of tripodal titanium alkoxides

The monomeric titanium(IV) hydroxide complex, LTi(OH)(LH(3)= tris(2-hydroxy-3,5-di-tert-butylbenzyl)amine), which is sterically inhibited from condensation to a mu-oxo dimer, cannot be prepared by hydrolysis of the alkoxide, with K(eq)= 0.012 for hydrolysis of the titanium methoxide in THF.     

Polyoxomolybdate promoted hydrolysis of a DNA-model phosphoester studied by NMR and EXAFS spectroscopy

Hydrolysis of (p-nitrophenyl)phosphate (NPP), a commonly used phosphatase model substrate, was examined in molybdate solutions by means of (1)H, (31)P, and (95)Mo NMR spectroscopy and Mo K-edge Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy. At 50 °C and pD 5.1 the cleavage of the phosphoester bond in NPP proceeds with a rate constant of 2.73 × 10(-5) s(-1) representing an acceleration of nearly 3 orders of magnitude as compared to the hydrolysis measured in the absence of molybdate. The pD dependence of k(obs) exhibits a bell-shaped profile, with the fastest cleavage observed in solutions where [Mo(7)O(24)](6-) is the major species in solution. Mixing of NPP and [Mo(7)O(24)](6-) resulted in formation of these two intermediate complexes that were detected by (31)P NMR spectroscopy. Complex A was characterized by a (31)P NMR resonance at -4.27 ppm and complex B was characterized by a (31)P NMR resonance at -7.42 ppm. On the basis of the previous results from diffusion ordered NMR spectroscopy, performed with the hydrolytically inactive substrate phenylphosphonate (PhP), the structure of these two complexes was deduced to be (NPP)(2)Mo(5)O(21)(4-) (complex A) and (NPP)(2)Mo(12)O(36)(H(2)O)(6)(4-) (complex B). The pH studies point out that both complexes are hydrolytically active and lead to the hydrolysis of phosphoester bond in NPP. The NMR spectra did not show evidence of any paramagnetic species, excluding the possibility of Mo(VI) reduction to Mo(V), and indicating that the cleavage of the phosphomonoester bond is purely hydrolytic. The Mo K-edge XANES region also did not show any sign of Mo(VI) to Mo(V) reduction during the hydrolytic reaction. (95)Mo NMR and Mo K-edge EXAFS spectra measured during different stages of the hydrolytic reaction showed a gradual disappearance of [Mo(7)O(24)](6-) during the hydrolytic reaction and appearance of [P(2)Mo(5)O(23)](6-), which was the final complex observed at the end of hydrolytic reaction.     

Investigation of particle isolation in Li-ion battery electrodes using Li NMR spectroscopy

⁷Li MAS NMR spectroscopy was used to study the failure mechanisms of LiNi_0.8Co_0.15Al_0.05O₂ electrodes in Li-ion cells. Three sets of electrodes with different degrees of power fade (0%, 9% and 23%) were studied. The three electrodes were charged to various states of charge (0%, 40%, 60%, 80% and 100%) in pouch cells which were subsequently disassembled for NMR analysis. The lithium NMR shifts of the positive electrodes in the different states of charge were investigated. The results indicate that NMR spectroscopy can be used to probe particle isolation in these electrodes. Particle isolation is responsible for the capacity and power fades since some of the active material particles are disconnected from the matrix. This study also clearly showed the loss of electrochemically active lithium as the power fade increased.     

Investigation of the microstructure of metallocene-catalyzed norbornene–ethylene copolymers using NMR spectroscopy

Norbornene–ethylene copolymers were prepared using the metallocene catalyst ethylene bis (indenyl) zirconium dichloride with MAO, and their microstructure was characterized with ¹H-NMR and ¹³C-NMR methods. From a Cosy ¹H-NMR spectrum it was found that all norbornene units are enchained in the exo-configuration. The sequence distribution of norbornene units was investigated using ¹³C-¹H correlations, hmqc for one-bond correlations, and hmbc for two- or three-bond correlations. It was shown that norbornene diads were formed at a high norbornene content (45 mol %). When further increasing the norbornene incorporation (66 mol %) a number of new signals were obtained. A Cosy ¹H-NMR spectrum revealed a new crosspeak which, according to the corresponding ¹³C-NMR shifts (hmqc), correlated well with a terminal unit of a trimer of norbornene. This means that at very high norbornene contents, norbornene triads can be formed. Because the formation of isotactic norbornene triads is very difficult to understand from a sterical point of view, an epimerization process causing stereoirregularities in the norbornene triad is proposed. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 1633–1638, 1998     

Investigation of phenyl derivatives of group IV elements using carbon-13 NMR spectroscopy

The tetraphenyl–metal, hexaphenyl–dimetal, and triphenylchloro–metal derivatives of the Group IV elements were examined using ¹³C NMR methods. Chemical shifts and metal–carbon coupling constants were obtained for all compounds studied.     

Investigation of substituted 1,3-oxazolidines using 1H and 13C NMR spectroscopy

The characteristics of the ¹H and ¹³C NMR spectra of 1,3-oxazolidines with substituents in the 2-, 3-, and 5-positions have been studied. The relation of the spectral characteristics to the structure and configuration of the compounds has been examined, and information on the transmission of the substituent effect through the atoms of nitrogen and oxygen has been obtained.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 9, pp. 1255–1260, September, 1986.     

Investigation of the stereochemistry of N-substituted 2,5-dimethyl-4-piperidinones by NMR spectroscopy

The conformations of the cis and trans isomers of N-substituted 2,5-dimethyl-4-piperidinones were studied by means of the¹H and¹³C NMR parameters. It was established that in the case of bulky and electron-acceptor substituents attached to the N atom the cis isomers are virtually completely represented by the chair (2a,5e) conformation, while the trans isomers are characterized by the chair (2e,5e) twistboat (2a,5e) chair (2a,5a) conformational equilibrium. It is demonstrated that 1-tert-butyltrans-2,5-dimethyl-4-piperidinone hydrochloride has the twist (2a,5e) conformation.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 8, pp. 1760–1768, August, 1990.     

Investigation of the reactions of pyrocatechol derivatives with AlBr3 by NMR spectroscopy

Alkylpyrocatechols are dealkylated by AlBr₃ in CH₂Br₂. The formation of stable carbenium ions (tert-butyl, methylcyclopentyl, and l-adamantyl) was detected by¹H NMR spectroscopy. Pyrogallol and 2,3-dihydroxynaphthalene are coordinated with AlBr₃ in the tautomeric keto form, and 2-mercapto-4-methylphenol forms a complex in the hydroxy form. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1158–1160, June, 1997.     

Investigation of isomeric forms of 2-aminobenzothiazoles by 13C NMR spectroscopy

A method is presented for distinguishing the amino and imino isomers in a series of benzothiazoles by ¹³ C NMR. It is shown that this method is suitable for determination of the position of the side chain.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 9, pp. 1265–1267, September, 1991.     

Investigation of the association and flexibility of cationic lipidic peptide dendrons by NMR spectroscopy

The cationic peptide dendrons synthesized and studied are lower generation polylysine-based partial dendrimers with or without lipid chains in the core. The dendrons with lipidic chains can be utilized as protein and liposomal mimics because of their unique structural properties. The full assignments of three different dendrons (L)7(NH2)8, (C14)1(L)7(NH2)8 and (C14)3(L)7(NH2)8 were obtained in D2O and H2O/D2O using a 500 MHz NMR spectrometer. The hydrophobic lipidic core of branched polylysine dendrons was found to induce aggregation upon increasing concentration. Because non-lipidic dendrons do not self-assemble, the behaviour and internal structural features of two different dendrons with one and three C14 hydrocarbon chains were explored. The critical association concentration clearly depends on the number of core hydrophobic residues and the association starts at 0.025 mM for (C14)1(L)7(NH2)8 and 0.05 mM for (C14)3(L(7(NH2)8. Chemical shift analysis also revealed that the hydrophobic chains of the dendrons associate in the core, whereas the polar head groups (NH2) are mainly located at the surfaces of the aggregates. The T1 relaxation time measurements showed that the mobility of the hydrocarbon chain is greater with the monomeric form of dendron (C14)1(L)7(NH2)8) than that of monomer (C14)3(L)7(NH2)8. The inter-chain hydrophobic interactions restrict the flexibility of the dendron with three hydrocarbon chains. As expected, the flexibility of the monomeric form is higher than that of the aggregated state for both of the dendrons.     

Investigation of dipolar-mediated water-protein interactions in microcrystalline Crh by solid-state NMR spectroscopy

Water-protein interactions play a major role in protein folding, structure, and function, and solid-state NMR has recently been shown to be a powerful tool for the site-resolved observation of these interactions in solid proteins. In this article we report investigations on possible water-protein dipolar transfer mechanisms in the microcrystalline deuterated protein Crh by a set of solid-state NMR techniques. Double-quantum (DQ) filtered and edited heteronuclear correlation experiments are used to follow direct dipolar water-protein magnetization transfers. Experimental data reveal no evidence for "solid-like" water molecules, indicating that residence times of solvent molecules are shorter than required for DQ creation, typically a few hundred microseconds. An alternative magnetization pathway, intermolecular cross-relaxation via heteronuclear nuclear Overhauser effects (NOEs), is probed by saturation transfer experiments. The significant additional enhancements observed when irradiating at the water frequency can possibly be attributed to direct heteronuclear water-protein NOEs; however, a contribution from relayed magnetization transfer via chemical exchange or proton-proton dipolar mechanisms cannot be excluded.     

Unraveling complex small-molecule binding mechanisms by using simple NMR spectroscopy

Heteronuclear NMR spectroscopy provides a unique way to obtain site-specific information about protein-ligand interactions. Usually, such studies rely on the availability of isotopically labeled proteins, thereby allowing both editing of the spectra and ligand signals to be filtered out. Herein, we report that the use of the methyl SOFAST correlation experiment enables the determination of site-specific equilibrium binding constants by using unlabeled proteins. By using the binding of L- and D-tryptophan to serum albumin as a test case, we determined very accurate dissociation constants for both the high- and low-affinity sites present at the protein surface. The values of site-specific dissociation constants were closer to those obtained by isothermal titration calorimetry than those obtained from ligand-observed methods, such as saturation transfer difference. The possibility of measuring ligand binding to serum albumin at physiological concentrations with unlabeled proteins may open up new perspectives in the field of drug discovery.