Conformational equilibria in isomeric methylcyclohexanols studied by 13C NMR spectroscopy

The conformational equilibrium constants for isomeric methylcyclohexanols (cis- and trans-1,2-, 1,3- and 1,4-methylcyclohexanols) have been determined from peak area measurements in the completely proton decoupled low temperature ¹³C NMR spectra of the individual conformers. The ¹³C chemical shifts are discussed in terms of the additive model.     

Conformations and 13C NMR chemical shifts of some polyamides in the solid state as studied by high-resolution 13C NMR spectroscopy

High-resolution ¹³Carbon nuclear magnetic resonance (NMR) spectra of Nylons 4, 6, and 66 in the solid state were measured over a wide range of temperature. From the results, it was found that resonance lines of crystalline and noncrystalline components were separable and their chemical shifts were determined. The ¹³C chemical shift behavior is closely related to their conformation. The origin of the conformational effects on the chemical shifts is discussed.     

13C NMR spectroscopy of four tertiary methyl norbornenols and norbornanols

Carbon chemical shifts and direct ¹³C? ¹H coupling constants of 2-endo-methyl-5-norbornen-2-exo-ol, 2-exo-methyl-5-norbornen-2-endo-ol, 2-endo-methylnorbornan-2-exo-ol and 2-exo-methylnorbornan-2-endo-ol have been measured from single samples using a dual probe pulse Fourier transform method.     

13C NMR spectroscopy of tautomeric conversions in imidazole compounds

A study has been made of¹³C NMR spectra of several series of imidazoles and their salts in solution and in the solid state. It has been shown that in compounds containing a pyridine ring, prototropic tautomerism on the NMR time scale may be retarded. The possible mechanism of these processes is discussed.V. I. Vernadskii Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, 11795 Moscow. Torino University, Italy. Translated from Izvestiya Akademii Nauk, Seriya Khimicheskaya, No. 2, pp. 299–308, February, 1992.     

Protolytic properties of polyamine wasp toxin analogues studied by 13C NMR spectroscopy

Acid-base properties of the natural polyamine wasp toxin PhTX-433 (1) and seven synthetic analogues [PhTX-343 (2), PhTX-334 (3), PhTX-443 (4), PhTX-434 (5), PhTX-344 (6), PhTX-444 (7), and PhTX-333 (8)], each having four protolytic sites, were characterized by 13C NMR spectroscopy. Nonlinear, multiparameter, simultaneous fit of all chemical shift data obtained from the NMR titration curves yielded macroscopic pKa values as well as intrinsic chemical shift data of all differently protonated macrospecies. Analyses of the chemical shift data demonstrated strong interactions between all four sites and provided information about complex relationships between chemical shift values and protonation state. Deprotonation of fully protonated forms starts at the central amino group of the polyamine moiety, and the extent of this trend depends on the distance to the flanking, protonated amino groups. The pKa1 values of 1-8 are in the range 8.2-9.4. Hence, some of the toxins are incompletely protonated at the pH and ionic strength conditions used for assessment of their interactions with ionotropic glutamate and nicotinic acetylcholine receptors, and the degree of protonation is expected to have pharmacological importance in the ion-channel binding event.     

Polymer electrolyte poly(ethylene oxide)/LiCF3SO3 studied by solid-state 13C NMR spectroscopy and ab initio calculations

Solid-state ¹³C NMR spectra and ab initio calculations of ¹³C NMR chemical shifts show that poly(ethylene oxide) (PEO) carbons in complex with LiCF₃SO₃ (both in crystalline and amorphous phase) are more shielded in comparison with neat PEO, due to the coordination to the Li⁺ cation. The results obtained from ¹³C NMR cross-polarization dynamics are in agreement with the published X-ray crystal structure of the PEO/LiCF₃SO₃ complex. The mobility of PEO in the crystalline complex is lower than in neat crystalline PEO.     

The crystalline structures of ethylene-dimethylaminoethyl methacrylate copolymers as studied by solid-state high-resolution 13C NMR spectroscopy

The crystalline structures of ethylene-dimethylaminoethyl methacrylate (EDAM) copolymers, which were either melt-quenched (mq) or isothermally crystallized (iso), were studied by solid-state high-resolution ¹³C NMR spectroscopy. It revealed that the crystalline structures of EDAM copolymers are greatly dependent on the comonomer content, crystallization condition and the storage time after treatment. The ratio of monoclinic to orthorhombic crystal (M/O) increases with the increase in the dimethylaminoethyl methacrylate content. Higher crystallinity and lower monoclinic content were observed for iso samples compared to the mq ones. The monoclinic crystal was found to melt at lower temperatures compared to the orthorhombic one during the heating process. The degree of crystallinity as well as the contents of monoclinic and orthorhombic crystals and the M/O value are found to increase after storage at room temperature for a month.     

Determination of enantiomeric excess by 13C NMR spectroscopy

Oxazolidine derivatives of β-amino alcohols such as ephedrine have been resolved by ¹³C NMR spectroscopy using Eu(hfc)₃ as a chiral shift reagent. The method is quantitative in the determination of enantiomeric excess, and is advantageous where ¹H NMR is of limited use owing, for example, to significant line broadening.     

Structure of ultrahigh-molecular-weight polyethylene in the solid state as studied by variable-temperature 13C CP/MAS NMR spectroscopy

In order to obtain some insight into the structure of an ultrahigh-molecular-weight polyethylene sample, ¹³C CP/MAS NMR experiments have been carried out at temperatures from 23 to ?108°C. The peak for the crystalline component moves upfield with a decrease in temperature, which is contrary to what was reported previously for a melt-crystallized polyethylene sample. On the basis of x-ray diffraction results and quantum-chemical calculations, it is suggested that the methylene carbons are in the distorted orthorhombic form at low temperatures.     

Electron-donating ability of triarylphosphines and related compounds studied by 31P NMR spectroscopy

The influence of aryl, heterocyclic, amide, alkyl, alkoxyl, thioalkoxyl, and ferrocenyl substituents at the phosphorus atom on its electron-donating ability was studied by the measurement of direct ³¹P—⁷⁷Se spin-spin coupling constants for the corresponding selenides. Series of diphenylorganylphosphines and their selenides were studied.     

Structure of polymer networks in liquid crystals studied by 13C NMR

We report on 13C NMR measurements above and below the clearing temperature of the liquid crystal 4-n-hexyloxyphenyl 4-methoxybenzoate constrained to an oriented, low concentration polymer network. The network is obtained by UV-irradiation of the reactive monomer 1,4-di-[4-(6-acryloyloxyhexyloxy)benzoyloxy]-2-methylbenzene which is in admixture with the liquid crystal. The characterization of orientational order of the mixtures before UV-radiation, and hence before polymerization, reveals the high order of the components at the polymerization temperature. The chosen geometry explains the LC director orientation only by the aligned network. Above the nematic-isotropic transition a strong pretransitional order is detected. Fast molecular translational diffusion averages the order over dimensions smaller than 1 mum. The Landau-de Gennes theory predicts a relation between pretransitional order and the lateral dimension of the LC regions. The experimental data are successfully explained by pore diameters of 35 and 98 nm for concentrations of 20 and 8 mol% of monomer, respectively. The results support the model of nearly cylindrical shaped liquid crystal domains surrounded by thin walls of crosslinked network.     

Characterization of monofluorinated polycyclic aromatic compounds by 1H, 13C and 19F NMR spectroscopy

Monofluorinated polycyclic aromatic hydrocarbons (F-PAHs) have attracted much attention in analytical, environmental, toxicological and mechanistic studies because of their physico-chemical properties, which are closely similar to those of the parent PAHs. Because of this, full NMR characterization has become of interest. Complete 1H, 13C and 19F NMR chemical shifts, and also 1J(H,C), (n)J(C,F), (n)J(H,F) and (n)J(H,H) coupling constants, have been assigned for the F-PAHs 1-fluoronaphthalene, 2-fluorofluorene, 5-fluoroacenaphthylene, 2-fluorophenanthrene, 3-fluorophenanthrene, 3-fluorofluoranthene, 1-fluoropyrene, 1-fluorochrysene, 2-fluorochrysene, 3-fluorochrysene and 9-fluorobenzo[k]fluoranthene. To allow comparison with the corresponding parent PAHs, the 1H and 13C chemical shifts of acenaphthylene, phenanthrene, fluoranthene, pyrene and benzo[k]fluoranthene were determined. Chemical shift increments and the effects on the coupling constants from the fluorine substitution are discussed.     

Differentiation of diastereomeric bornane derivatives by 13C NMR spectroscopy

Three diastereomeric bornane derivatives have been assigned configurational structures by ¹³C NMR measurements in the presence of Yb(dpm)₃.     

Protonation of pyrimidotriazinediones by1H and13C NMR spectroscopy

On the basis of an analysis of the changes in the chemical shifts of the signals in the¹H and¹³C NMR spectra on the pyrimidotriazinedione and trifluoric acid concentrations in CDCl₃ it was established that the protonation of rheumycin and fervenulin takes place at the N(₂) atom, whereas the protonation of isofervenulin takes place competitively at the N(₁), N(₂), and O(₆) atoms. The equilibrium constants of the investigated protonation processes were measured.A study of the chemical peculiarities of the behavior of Ia, b in aqueous acidic media (H₂O-HCl, D₂O-DCl) was described in [6].Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 11, pp. 1532–1538, November, 1988.     

13C NMR spectroscopy of cyclopropane derivatives. 1—monocyclic compounds

The ¹³C NMR data of an appreciable number of cyclopropane derivatives have been collected. Most of the spectra were recorded by ourselvesand some were taken from the Literature. With a view to furthering the useof ¹³C NMR spectroscopy as a diagnostic tool in this field, we havemeasured not only the chemical shifts,δ, but also the coupling constants ¹J(CH). It is shown that both embody valuable structural information.     

NMR study of orgnasilicon compounds : X. The transmission of substituent effects in phenoxysilanes as studied by 29Si and 13C NMR

²⁹Si and ¹³C NMR chemical shifts for a series of meta and para substituted phenoxytrimethylsilanes are given and compared with those in phenyltrimethylsilanes using the formal single and dual substituent parameter analysis of substituent effects.The silicon chemical shift is found to be about twice as sensitive to substituent effects in phenoxytrimethylsilanes as in phenyltrimethylsilanes. The chemical shift sensitivity to substituent effects, ?, is considered to be a product of two factors, ?^el and ?^shield, which describe the sensitivity of the electron density to substituent effects and the sensitivity of the shielding to the electron density, respectively.Using ¹³C chemical shifts and CNDO/2 net atomic charges, it is shown that the substituent effects propagate within XC₆H₄ fragment of phenoxysilanes no better than in phenylsilanes. The ¹³C chemical shifts of the terminal methyl groups are affected by the substituents in the former series of compounds much less than in the latter. An increase in the relative basicity of oxygen is accompanied by an increase in silicon shielding in phenoxytrimethylsilanes.According to CNDO/2 calculations, the substituents cause larger changes in net atomic charges on the silicon atom if it is bonded directly to the benzen ring rather than via the oxygen bridge. In spite of the fact that the possibility of a dative O·→Si interaction, not reflected by the CNDO/2 calculations, cannot be completely excluded, the results that the increased silicon shift sensitivity to substituent effects in phenoxysilanes is due to higher sensitivity of silicon shielding (?^shield) to electron density in these compounds rather than to a bettr transmission of electronic effects (?^el). The existing theory of silicon shielding must be improved or refined if it has to accomodate the increased sensitivity in the phenoxysilanes.