Stabilization of the cyclopropenum and cyclopropenone ring systems by ferrocene : A 13C nuclear magnetic resonance study

The ¹³C NMR spectra of diferocenylcyclopropenone (1) and the triferrocenylcyclopropenium (2) and ferrocenyldiphenylcyclopropenium (3) cations in non-acidic media are analyzed with special reference to the information they afford concerning charge distribution in these species. A delocalization of the positive charge into the remote unsubstituted five-membered rings of 2 and 3 is indicated. The role of the metal as a conduit for this effect is suggested.     

Probing interactions by means of pulsed field gradient nuclear magnetic resonance spectroscopy

Molecular self-diffusion coefficients (D) of species in solution are related to size and shape and can be used for studying association phenomena. Pulsed field gradient nuclear magnetic resonance (PFG-NMR) spectroscopy has been revealed to be a powerful analytical tool for D measurement in different research fields. The present work briefly illustrates the use of PFG-NMR for assessing the existence of interactions in very different chemical systems: organic and organometallic compounds, colloidal materials and biological aggregates. The application of PFG-NMR is remarkable for understanding the role of anions in homogenous transition metal catalysis and for assessing the aggregation behaviour of biopolymers in material science.     

A study of the structure of chemical compounds by nuclear magnetic resonance spectroscopy

The PMR spectra of the condensation products of N-arylsulfonylmonoimino sulfur dioxide with the dienes were studied. It was established that the reaction goes according to the condensation pattern of the Diels-Alder reaction, involving the nitrogen sulfur double bond with the formation of 1-oxo-2-arylsulfonyl-3, 6-dihydro-1, 2-thiazines.     

A first principles theory of nuclear magnetic resonance J-coupling in solid-state systems

A method to calculate NMR J-coupling constants from first principles in extended systems is presented. It is based on density functional theory and is formulated within a planewave-pseudopotential framework. The all-electron properties are recovered using the projector augmented wave approach. The method is validated by comparison with existing quantum chemical calculations of solution-state systems and with experimental data. The approach has also been applied to the silicophosphate, Si(5)O(PO(4))(6), giving (31)P-(29)Si-couplings which are in excellent agreement with experiment.     

The study of domain structure in polyurethanes by nuclear magnetic resonance

Polyurethanes are block copolymers composed of hard and soft segments which often undergo microphase separation with the resultant formation of hard and soft domains. The nuclear-magnetic-free induction decay of such systems is shown to consist of a fast Gaussian component corresponding to glassy domains and a slow exponential component corresponding to rubbery domains. Thus, by being sensitive to the microscopic mobility of a material, nuclear magnetic resonance (NMR) has the potential to determine when a polyurethane has separated into domains and the relative amounts of material in each domain. After annealing the free-induction decay of a linear polyurethane showed a distribution of relaxation times indicating that mixing of the domains had taken place and a continuum of compositions exist in the material. The free-induction decay of a cross-linked polyurethane was unaffected by annealing, confirming the results of an earlier study.     

A nuclear magnetic resonance study of hindered rotation in 8-phenylpurines

In the PMR spectrum of 8-phenylpurines, the multiplet of the o-protons appears downfield of the multiplet, characteristic for m,p-protons. The separation of the centres of these two signals (Δ-value) diminishes with increasing steric interference between the phenyl ring and substituents in the imidazole moiety. The contribution of the purine ring current to the chemical shifts of the aromatic protons was calculated according to the theory of Johnson and Bovey, and the torsion angles θ between the phenyl ring and the plane of the purine system were derived. For 8-phenylpurines with an NH-group in the imidazole ring, θ is 10–15°; for compounds with an N-methyl group in this ring, θ ≈ 35–45°; in 3,9-dimethyl derivatives, Δ becomes zero, while θ is about 50°.     

A study of molecular motion in a liquid-crystalline copolyester by broad-line nuclear magnetic resonance

Broad-line nuclear magnetic resonance (NMR) measurements have been carried out for the proton resonance in an oriented liquid-crystalline copolyster prepared from 4-hydroxybenzoic acid (HBA) and 2-hydroxy-6-naphthoic acid (HNA). The Second moments were determined as a function of the angle γ between the specimen orientation direction and the static magnetic field. The low-temperature NMR results are consistent with a rigid structure of random copolymer chains arranged on a hexagonal lattice. As the temperature is raised the NMR signal changes and can be modeled very satisfactorily by considering that there is free rotation of the benzene ring residues about the 1-4 substitution direction. At more elevated temperatures the observed NMR spectra are consistent with rotation of both HBA and HNA groups.     

Quantitative determination of aluminium in tea by means of aluminium-27 nuclear magnetic resonance spectroscopy

A method for determining the Al levels in tea leaf digests and tea infusions by means of 27Al nuclear magnetic resonance (NMR) spectroscopy is described. Quantitative results down to 2.0 micrograms cm-3 of Al in 0.5-cm-3 acidified samples are readily obtainable in standard 5-mm NMR tubes. An acceptable quantitative determination is achieved by using an internal Al standard in a 1-mm capillary in conjunction with europium nitrate as a shift reagent.     

A study of association by hydrogen bonding in the system phenol-CCl4 by the nuclear magnetic relaxation method

The system phenol-CCl₄ was studied by measuring proton and deuteron magnetic relaxation rates and self-diffusion coefficients at 25°C. From these data intermolecular relaxation rates have been calculated. By means of an association parameter A, association of phenol molecules with respect to various parts of the molecule has been established. Closest distances of approach between protons and configurations of maximum occurrence probabilities are reported.     

Electrical conductivity of lithium chloride solutions in the system isopropyl alcohol-water

Electrical conductivity of lithium chloride solutions in the system isopropyl alcohol-water in the temperature range 290–323 K was studied. The dependences of the specific and molar electrical conductivity of the solutions on the concentration of water and lithium chloride were determined.     

Substituent effects in heterocyclic systems by carbon-13 nuclear magnetic resonance. Isoxazoles

The carbon-13 nmr spectra of a series of 3-aryl-5-phenylisoxazoles (I) and 3-phenyl-5-arylisoxazoles (II) have been recorded and the signals assigned. Carbon-13 data for series I show little effect of substituent on the chemical shift of the isoxazole ring carbons. However, a plot of the carbon-13 chemical shift of carbon-5 in the isoxazole system I versus the chemical shift of carbon-3 in the 3-(4′-aryl)-1-phenylpropenones gives a straight line (r = .989) with a slope of 0.35. In series II, the chemical shifts of both carbon-4 and -5 are relatively sensitive to substituent effects. Fair correlations between Hammett sigma values and the chemical shifts of these two carbons are found; dual substituent parameter treatment improves the correlations. The results obtained from correlations with carbon-4 in series II are similar to those obtained from β-carbons of a number of styrene systems. The data show that carbon-4 in series II is approximately 20% less sensitive to substituent effects than the previously reported data for carbon-3 of 2-arylfurans. Transmission of substituent effects in the isoxazole system compare well with those of the benzothiazole system.     

A proton and carbon-13 nuclear magnetic resonance study of neopentylmercury compounds

Substituent effects on ¹⁹⁹Hg¹H and ¹⁹⁹Hg¹³C spinspin coupling constants have been studied for neopentylmercury derivatives, (CH₃)₃CCH₂HgR(or X), where R is covalently bonded Me, Et, t-Bu, neopentyl, and vinyl, and X is easily ionizable CN, Br, Cl, OCOCH₃, and ONO₂. Linear relationships exist between the methylene J(¹³CH) and ²J(HgH), ⁴J(HgC) and ²J(HgC) and ³J(HgC); but deviations from linearity occur for the chloride, bromide, acetate, and nitrate in the relationships between ²J(HgH) and ⁴J(HgH), ²J(HGH) and ²J(HGC). These deviations are discussed in terms of hyperconjugative p_πd_π bonding between the methylene CH bonds and mercury.     

A carbon-13 nuclear magnetic resonance study of compounds substituted by a perfluoroalkyl chain

The ¹³C NMR spectra of alkanes, alkanones and cyclohexanones substituted by perfluoroalkyl groups, R_F, have been studied. The influence of the perfluoroalkyl group on the chemical shifts of other carbons of the molecules is the same regardless of the R_F chain length.     

Electroconductivity of potassium iodide solutions in the system ethyl alcohol-water

The electrical conduction of potassium iodide solutions in the ethanol-water system in the temperature range 292–323 K was studied using the conductometric method. Specific and molar electrical conductions of the solutions depending on water and potassium iodide contents were determined.     

Characterizing nitrilimines with nuclear magnetic resonance spectroscopy. A theoretical study

The 13C chemical shifts in selected nitrilimines, nitriles, acetylenes, allenes, and singlet carbenes have been calculated using density-functional theory [PBE0/6-311++G(2df,pd)] and the gauge including atomic orbital (GIAO) method. The effects of substitution on the 13C chemical shifts in nitrilimines, R1-CNN-R2, have been examined. The carbon nucleus is generally found to be deshielded by substituents in the order CH3 < NH2 < OH < F. Comparison with nitriles, acetylenes, and allenes shows that this effect is related to the presence of the cumulated functionality, C=N=N. Terminal N-substitution is found to have a larger effect than C-substitution due to a large increase in chemical shielding anisotropy. The electronic structure of nitrilimines has recently been shown to possess a carbene component whose resonance contribution varies widely with substitution, and, as previously reported, insight into the electronic structure can be gained by an analysis of the shielding tensor, especially for carbenes. Accordingly, the components of the diagonalized 13C shielding tensor for nitrilimines and stable singlet carbenes have been examined. This analysis suggests that diaminonitrilimine, H2N-CNN-NH2, may be a stable carbene, and, to the best of our knowledge, it would be the first acyclic, unsaturated stable carbene ever reported. Finally, a detailed analysis of the 13C chemical shifts shows that an increase in the dipolar character of nitrilimines induces a shielding at the carbon nucleus, while an increase in allenic or carbenic character tends to cause a deshielding.     

A nuclear magnetic resonance study of molecular anisotropy in strained networks and network blends

The splitting of the nuclear magnetic resonance line, seen in strained networks, is shown to originate from the anisotropic screening of the excluded volume interaction by density fluctuations. In a blend concentration fluctuations also contribute to the screening and are greatly enhanced if the phase separation point is approached. The effect on the line shape has been calculated and is shown to be sensitive to the thermodynamics of the phase separation.     

A dynamic nuclear magnetic resonance study of nitrogen inversion in N,N'-dimethylimidazolidine

The 251 MHz ¹H and the natural-abundance 63.1 MHz ¹³C NMR spectra of N,N'-dimethylimidazolidine have been measured from ?50 to ?170°C. Below about ?140°C. nitrogen inversion in the compound becomes slow on the NMR time scale and both the ¹H and the ¹³C spectra indicate that it exists in solution as a mixture of cis and trans conformations having nearly the same energies. The free-energy barrier (ΔG^≠) for nitrogen inversion in N,N'-dimethylimidazolidine is 6.4 kcal mol^?1, a value which is 1.5 kcal mol^?1 lower than that for N-methylpyrrolidine.