Invertomers at nitrogen in aziridine carboxylates by mltltinuclear (1H,13C,17O,and15N) NMR study

A confrgurational and conformational study of NH, N-acetyl- and N-sulfonylaziridine carboxylates is performed by¹H ,¹³C,¹⁷O, and¹⁵N NMR spectroscopy. The presence of acetyl and su fonyl groups on the ring nitrogen atom seems to reduce greatly the configurational stability at nitrogen.Published in Khimiya Geterotsiklicheskikh Soedinenii, No. 9, pp. 1226–1234, September, 1995.     

Intramolecular rearrangements of cyclooctadienerhodium(I) fragments supported on a vanadium oxide cluster as evidenced by dynamic17O NMR spectroscopy

Variable-temperature¹⁷O NMR together with⁵¹V and¹⁰³Rh NMR studies on newly prepared vanadium oxide-supported organorhodium(I) fragment(s), [(RhCOD) _n (V₄O₁₂)]^(4–n)– (n = 1, 2; COD = ⁴-1,5-cyclooctadiene) indicate that intramolecular rearrangements of RhCOD fragment(s) on a vanadium oxide surface occur in solution.     

Secondary structure of peptides 16th. Characterization of proteins by means of13C NMR CP/MAS spectroscopy

50.3 or 75.4 MHz¹³C NMR cross-polarization/magic angle spinning spectra of human hair, horse hair, horse hoof, parrot feather, sperm whale myoglobin, and horse heart cytochrome C were measured. The spectra of human hair and horse hair indicate nearly equal mole fractions of-sheets and-helices and a low percentage of amorphous regions, whereas horse hoof contains a higher fraction of amorphous proteins. The parrot feathers contain a small-helix fraction (ca. 10±5 %) in additon to a large-sheet fraction whereas cytochrome C contains 70–90%-helices. The spectrum of myoglobin could not interpreted in terms of defined secondary structures. The usefulness of the¹³C NMR CP/MAS spectroscopy for the characterization of proteins is compared with that of IR spectroscopy and X-ray diffraction.     

A Thermodynamic, 13C NMR,and 17O NMR Study of Isomeric 4,7-Dihydro-1,3-dioxepins and 4,5-Dihydro-1,3-dioxepins

The relative thermodynamic stabilities of 4,7-dihydro-1,3-dioxepin (4,6-dioxacycloheptene, 1a) and 4,5-dihydro-1,3-dioxepin (3,5-dioxacycloheptene, 1b), and of a number of their 2-substituted derivatives, have been determined by base-catalyzed chemical equilibration in DMSO solution. Without exception, the 4,5-dihydro isomer is the dominating species at thermodynamic equilibrium. The relative stability of the b form is promoted by the presence of a single alkyl group on C-2, whereas two alkyl groups on C-2 have an opposite effect. In general, the thermodynamic parameters H _m and _Sm , of isomerization vary unexpectedly with the pattern of substitution at C-2. These trends appear to be derived from significant substituent-induced conformational changes in the b isomer, as suggested by ¹³C and ¹⁷O NMR chemical shift data.     

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.     

Critical comments on accuracy of quantitative determination of natural humic matter by solid state (13)C NMR spectroscopy

Structural information of natural organic matter (NOM) at the molecular level is very essential in understanding their nature and reactivity. Nuclear magnetic resonance (NMR) is an excellent tool for estimating the gross chemical composition of the very complex humic matter (HM). A well-known fact is that the solid state ¹³C NMR spectral analysis is very parameter-sensitive especially in the study of the heterogenous HM (e.g. baseline corrections, different pulse techniques and spinning rates of the rotor vs. different disruptive sidebands in the spectra). This being the case, it has been emphasized the importance of qualitative and quantitative analyses for generating as real spectra as possible by means of different pulse and polarization techniques, sampling spinning rates as well as certain correction factors. In the present study a practical accuracy for quantitative determination of NOM type material by solid state ¹³C NMR spectroscopy was assessed using a known HM sample. Different magnetic-field strengths, sampling spinning rates, single and ramped amplitude cross polarization techniques and TOSS pulse sequence were applied for obtaining a more reliable insight into the disruptive effect of the chemical shift anisotropy (CSA), especially the most disturbing first order spinning side bands (SSB). The results demonstrated that the SSB problem is not so significant as sometimes stated, at least in the context of HM samples and in the light of the overall reproducibility and uncertainty connected with the sample itself.     

New synthesis of mullite. Structural evolution study by 17O, 27Al and 29Si MAS NMR spectroscopy

Mullite has been prepared from a new combination of precursors. An aluminum alkoxide, aluminium isopropoxide, and silicon tetrachloride, are hydrolysed in tetrahydrofuran solution by ¹⁷O enriched water. The resulting powder is chemically homogeneous, crystallizing into mullite at 980°C. The structural evolution has been studied by DTA, TGA, XRD and ¹⁷O, ²⁷Al and ²⁹Si MAS NMR spectroscopy.     

The structure of 1-formyl-3-phenyl-Δ-pyrazoline in the gas phase (DFT calculations), in solution (NMR) and in the solid state (X-ray crystallography)

The molecular structure of 1-formyl-3-phenyl-Δ²-pyrazoline was determined by X-ray crystallography (triclinic, P-1). The geometry thus obtained was compared with that obtained by DFT calculations. The GIAO method was used to calculate absolute shieldings, which agree conveniently with those measured by ¹³C and ¹⁵N NMR. The title compound appears to be an essentially planar molecule.     

17O NMR Investigation of Chemical Homogeneity in Hybrid Systems

¹⁷O NMR experiments using enriched water were performed to followthe hydrolysis-condensation process of dimethyldiethoxysilanetetraethoxysilane and a 1/1 mixture of dimethyldiethoxysilane andtetraethoxysilane (H₂O/OEt = 0.5; pH = 2). The spectrarecorded over several hours time period were simulated to followquantitatively the variations of residual water, hydroxyl groups(Si–¹⁷ H) and oxo bridges (Si–¹⁷ –Si). Presence of a resonance signal due oxo bridges between di- andtetrafunctional Si units clearly demonstrates that co-condensationreactions occur to a large extent between the two alkoxides, and that thesebonds are stable during the aging period.     

13C NMR spectra and the structure of dithizone

The structural characteristics of dithizone in the solid state and in solution were studied by a high-resolution¹³C NMR method. A highly symmetrical structure of dithizone was established in the crystalline state, and it was suggested that it exhibits dual behavior in complexation reactions with metal ions.V. I. Vernadskii Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, 117975 Moscow. Translated from Izvestiya Akademii Nauk, Seriya Khimicheskaya, No. 1, pp. 113–117, January, 1992.     

Structural Assignment of Side Chain Liquid Crystalline Monomer and Polymer by 1-D and 2-D Solution NMR Studies

In the present study, the synthesis and characterization of a methacrylic-based side chain liquid crystalline monomer and its polymer were investigated with the aid of both 1-D and 2-D nuclear magnetic resonance (NMR) techniques. The mesophase characteristics of the monomer and polymer were determined by hot-stage optical polarized microscopy (HOPM). The nematic and smectic phases were observed for the monomer and polymer. Furthermore, ¹³C spin-lattice relaxation time measurements were performed on the above systems in order to monitor molecular dynamics. The present study provides an opportunity to carry out a systematic comparison of the evolution of structural as well as dynamical changes of the monomer and the polymer.     

Molecular weight effect on the complexation of poly(methacrylic acid) and poly(ethylene oxide) as studied by high-resolution solid-state C NMR spectroscopy

Complexes of poly(methacrylic acid) (PMAA) and poly(ethylene oxide) (PEO) with different PEO molecular weight were studied by solid-state high-resolution ¹³C NMR spectroscopy, with the emphasis on the PEO molecular weight effect on inter-polymer interaction, morphology and molecular motion. It is found that the crystalline phase of PEO is completely destroyed in the complex. The results of ¹H transverse relaxation times and ¹³C spin-lattice relaxation times indicate that the chain mobility of both PEO and PMAA are greatly restricted by inter-molecular hydrogen-bonding interactions, especially when the molecular weight of PEO is 1500. The bulk structures of the complexes are found to be closely dependent on the molecular weight of PEO. The fraction of “free” PEO segments without forming hydrogen-bonds with PMAA increases with increasing PEO molecular weight.     

Alkyldiamine bis(selenocyanato)cadmium(II) complexes: Synthesis, Cd, Se, N and C NMR spectroscopy and X-ray structure of a 2D metal-organic framework

Complexes of cadmium(II)-selenocyanate with several alkyldiamine ligands have been synthesized and characterized by IR, ¹¹³Cd, ⁷⁷Se, ¹⁵N and ¹³C NMR spectroscopy. The X-ray structure of the complex [Cd(SeCN)₂-en] reveals two non-equivalent metal ion centers, both with a distorted octahedral geometry. The combined bridging modes of selenocyanate and ethylenediamine with the blocking mode of a chelating ethylenediamine generate a 2D metal-organic framework.     

Electron density distributions in substituted 2,3,4,5-tetraphenyl-1-germacyclopenta-2,4-dienes studied by NMR spectroscopy

The signals in the¹³C NMR spectra of 2,3,4,5-tetraphenyl-1-germacyclopenta-2,4-dienes (R¹=R²=H, Me,cyclo-C₃H₅, SiMe₃, SnMe₃, R¹=Me, R²=H, Cl) were completely assigned using 2D NMR spectroscopy. The pattern of the variation of the chemical shifts in the¹³C NMR spectra indicates that the effects of substituents R¹ and R² on the heterocycle and on the phenyl groups are of inductive rather than mesomeric origin and include the direct through-space polarization of bonds (field effect). Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 1962–1965, November, 1997.     

Correlation between C and O chemical shifts and torsional strain in spiroacetals

The relationship between the ¹³C and ¹⁷O NMR chemical shifts and the dihedral energies (non-bonding interactions) of 1,4-dioxaspiro[4.4]nonane, 1,4-dioxa- and 6,10-dioxaspiro[4.5]decane, 1,4-dioxa- and 6,11-dioxaspiro[4.6]undecane, 1,5-dioxaspiro[5.5]undecane, 1,5-dioxa and 7,12-dioxaspiro[5.6]dodecane and 1,6-dioxaspiro[6.6]tridecane were analyzed. These data showed correlation of the non-bonding interactions with the chemical shift of the spiranic carbon, as well as a linear relationship between ¹³C and ¹⁷O.     

H and C NMR spectroscopy of 9-acridinones

The ¹H and ¹³C NMR spectra of 9-acridinone and its five derivatives dissolved in CDCl₃, CD₃CN and DMSO-d₆ were measured in order to reveal the influence of the constitution of the compounds and features of the solvents on chemical shifts and ¹H-¹H coupling constants. Experimental data were compared with theoretically predicted chemical shifts, on the GIAO/DFT level of theory, for DFT (B3LYP)/6-31G^∗∗ optimized geometries of molecules—also for four other 9-acridinones. This comparison helped to ascribe resonance signals in the spectra to relevant atoms and enabled revelation of relations between chemical shifts and physicochemical features of the compounds. It was found that experimentally or theoretically determined ¹H and ¹³C chemical shifts of selected atoms correlate with theoretically predicted values of dipole moments of the molecules, as well as bond lengths, atomic partial charges and energies of HOMO.     

A multinuclear NMR study in the solid state and in solution of thallium(I) tris-(pyrazol-1-yl)borates (thallium scorpionates)

The ¹H, ¹³C and ¹⁵N NMR spectroscopic properties of six thallium tris-(pyrazol-1-yl)borates, including a tetrakis derivative, were determined. The results in solution were necessary to understand those, more complicated, in the solid state. A collection of ²⁰⁵Tl-¹⁵N and ²⁰⁵Tl-¹³C couplings was measured in the latter state. Among those, a very large coupling constant (between 194 and 282 Hz) has been measured on the carbon at the position 4 of the pyrazole ring in several compounds and particularly for the cyclobutyl derivative [Tl(Tp^Cbu)]. It has been assigned to a direct interaction of the 4-C-H ? Tl type and related to the X-ray structures, when available.     

Search of structure and ligands exchange for palladium(II) complexes with N-allylimidazole; X-ray and solid-state/solution NMR studies

Two coordination compounds of palladium(II) with N-allylimidazole (l) of the general formula [PdL₄]Cl₂ · 3H₂O (1) and trans-[PdL₂Cl₂] (2) have been synthesized. The crystal and molecular structure of complexes 1 and 2 was established by single-crystal X-ray diffraction analysis. The X-ray structural data were supplemented by solid-state ¹³C NMR measurements (CP MAS and PASS 2D). The 1D and 2D NMR studies in solution reveal that complex 1 is unstable at room temperature and undergoes reversible decomposition to 2. The method for how to preserve a complex with four allyl-imidazole ligands in solution is shown.     

Polymorphism of phenylpyruvic acid studied by IR, Raman and solid state C NMR spectroscopy

Polymorphs I and II of phenylpyruvic acid are obtained as mixtures of both crystal forms or relatively pure crystals, from different solvents. Polymorph I is more stable than polymorph II at room temperature. Spectral characteristics of these polymorphs are discussed on the basis of IR, Raman and solid state ¹³C NMR spectra. Also, the assignment of the IR features observed in the 1600–1700 cm⁻¹ region is re-investigated by referring to the spectra of heavy-atom substituted derivatives. It is suggested that the C=O stretching band is split by the crystal field for both polymorphs.     

On the detection of both carbonyl and hydroxyl oxygens in amino acid derivatives: a O NMR reinvestigation

The hypothesis and the conclusions of previous ¹⁷O NMR studies on the detection of both oxygens of the carboxylic group of Boc-[¹⁷O]Tyr(2,6-diClBzl)-OH in DMSO-d₆ solution (Tetrahedron Lett.2000, 41, 8651) are reconsidered. The appearance of two discrete resonances at 340 and 175 ppm of this protected amino acid is not now attributed: (a) to the reduction of the intramolecular conformational exchange rate, due to the effect of intramolecular hydrogen bonding of the hydroxy part of the carboxyl with the carbonyl oxygen of the Boc-group, and (b) to the effect of solvent viscosity, suggested in the mentioned study. The cause of this phenomenon is now attributed to a strong hydrogen bonding of the polar proton acceptor solvent DMSO with the carboxy group, which effectively reduces the proton exchange rate, thus becoming slow on the ¹⁷O NMR time scale.