A multinuclear 1H, 13C, and 15N magnetic resonance study of ten 4-nitropyridine N-oxides

The ¹H, ¹³C, and ¹⁵N NMR chemical shifts of ten 4-nitropyridine N-oxide derivatives are assigned. The shielding of the pyridine ring nitrogen is sensitive to ring substitution through inductive effects, steric effects by ortho-substituents, and the possibility for electron delocalisation (resonance energy). In solution, 3-ethylnitrosoamino-4-nitropyridine N-oxide has two tautomers. The proposed reason is the steric crowding between vicinal 4-nitro and 3-ethylnitrosoamino groups, causing a disturbance to amino nitrogen that can delocalize its lone pair to the oxygen atom of the nitroso group.     

Anisochronism of large geminal groups in the 1H and 13C nuclear magnetic resonance spectra

The role of steric factors in magnetic non-equivalence anisochronism of complex geminal groups and of their fragments in compounds of the general formula RCH(Ph)CH(COOR′)₂ has been investigated. CMR anisochronism is more sensitive to conformational and other steric changes than is PMR.     

Structural examination of some polychlorinated 2-phenoxyphenols by 1H and 13C nuclear magnetic resonance

Carbon-13 NMR spectra of some polychlorinated 2-phenoxyphenols have been obtained. The substituent chemical shifts obtained by varying the chlorine substitution pattern of one ring are very similar to those reported for the corresponding diphenyl ethers. Thus, the replacement of a 2-chlorine atom by a hydroxyl group only induces minor shielding changes at the adjacent aryl moiety and the ¹³C chemical shift changes are mainly determined by the preferred conformations governed by the steric demand of the ortho substituents. An ¹H NMR/IR study revealed an equilibrium between intermolecular aggregates and intramolecular OH…π species in the concentration interval 2-0.005 M. Any hydrogen bonding effects on ¹³C NMR shieldings are, therefore, minor compared to shielding variations caused by steric perturbations.     

13Carbon nuclear magnetic resonance of ethylene–propylene–1‐decene terpolymers

Many studies have been reported on the ¹³C NMR characterization of ethylene–α‐olefin copolymers, but only a few have been reported on terpolymers. The incorporation of an α‐olefin into the polyethylene chain changes the structure and, consequently, the properties of the polymer obtained. Looking for new products, we obtained a series of ethylene–propylene–1‐decene terpolymers with the metallocenic system rac‐ethylene bisindenyl zirconium dichloride/methylaluminoxane. We performed a complete ¹³C NMR characterization of these terpolymers qualitatively and quantitatively. Here we present a detailed study of the ¹³C NMR chemical shifts, triad sequence distributions, monomer average sequence lengths, and reactivity ratios for these terpolymers. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 2531–2541, 2003     

13Carbon nuclear magnetic resonance of ethylene–propylene–1‐hexene terpolymers

We report the complete ¹³C NMR characterization of a series of ethylene–propylene–1‐hexene terpolymers obtained with the metallocenic system rac‐ethylene bis‐indenyl zirconium dichloride, with different comonomer ratios. A detailed study of ¹³C NMR chemical shifts, triad sequence distributions, monomer‐average sequence lengths, and reactivity ratios for these terpolymers is presented. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 2474–2482, 2004     

7Li and 13C solid-state NMR spectra of lithium cuprates

7Li and 13C solid-state MAS NMR spectra of three lithium cuprates with known X-ray structures--lithium([12]crown-4)2 dimethyl and diphenyl cuprate (1,2) and lithium(thf)4-[tris(trimethylsilyl) methyl]2 cuprate (3)--have been measured and analysed with respect to the quadrupolar coupling constants of lithium-7, chi(7Li), and the asymmetry parameters of the quadrupolar interactions, eta(7Li), as well as the 6, 7Li and 13C chemical shifts. The chi(7Li) values of 23, 30, and 18 kHz for 1, 2 and 3, respectively, are in line with the high symmetry around the lithium nucleus in the solvent-separated structures and may be used as reference data for this structural motif. Calculations based on charges derived from ab initio 6-31 G* HF computations using the point charge model (PCM) and the program GAMESS support the experimental findings.     

13C, 15N and 29Si nuclear magnetic resonance studies of some aminosilanes and aminodisilanes

¹³C, ¹⁵N (at natural abundance) and ²⁹Si NMR data (chemical shifts and coupling constants) are reported for aminosilanes R₂R′SiNHR¹ (1), bis(silyl)amines Me₂R′SiNHSiMe₃ (2), 1,2-bis(amino)-ethanes (3), bis(amino)silanes RR′Si(NHR¹)₂ (4), 1,2-bis(amino)tetramethyldisilanes (5) and 1,1,2,2-tetrakis(amino)dimethyldisilanes (6). The δ¹⁵N values depend more on the nature of the substituents R¹(H, alkyl, aryl) at the nitrogen atom (in the same way as for other amines) than on different substituents at the silicon atom. A linear correlation between ¹J(²⁹Si¹⁵N) and ¹J(²⁹Si¹³C) is proposed for silanes in which the SiN unit is replaced by the SiCH unit. This correlation comprises all ¹J(²⁹Si¹⁵N) values for aminosilanes R_4-nSi(N)n (n = 1–4) and—most likely—also for aminodisilanes, and it predicts ¹J(²⁹Si¹⁵N)>0 if the corresponding value |¹J(²⁹Si¹³C)|>25 Hz. For the first time a two-bond coupling across Si, ²J(²⁹Si ¹⁵N) = 6.9 Hz, has been observed for 6a. In the case of 6b (R¹ = ^sBu) all resonances for the diastereomers are resolved in the ¹⁵N and ²⁹Si NMR spectra in contrast to the ¹H and ¹³C NMR spectra.     

17O and 13C nuclear magnetic resonance study of polyfluorinated carbonyl compounds

δ¹⁷O- and δ¹³C values are reported for 15 polyfluorinated carbonyl compounds. If the R_f group is separated from the carbonyl group by less than two carbon atoms a marked increase in the nuclear shielding of ¹³C(C=O) and a marked decrease in the nuclear shielding of the ¹⁷O(C=O) nucleus is observed. This is ascribed to the differing effects of the R_f group on the σ and the π system of the carbonyl unit. The effect on the σ-manifold leads to increase in shielding but it may be offset (as in the case of the ¹⁷O nucleus) by destabilization of the π system. UV spectroscopic data for some polyfluorinated carbonyl compounds support these arguments.     

Transmission of the substituent effects in 2-substituted benzimidazoles studied by 1H and 13C nuclear magnetic resonance

The substituent influence on the ¹H and ¹³C NMR chemical shifts in 2-substituted benzimidazoles and their anions and cations has been investigated. The transmission of the electronic effects of substituents from C-2 to C-5 (6) is approximately 20% less effective than that in the opposite direction. The solvent influence on the chemical shifts of protons and transmission effects in the charged forms of 2-substituted benzimidazoles has been studied.     

13C nuclear magnetic resonance studies of cellulose acetate

¹³C-NMR spectra of ring carbons and O-acetyl carbonyl carbons of cellulose acetate (CA) in dimethyl sulfoxide-d₆ were analyzed. The CA samples with the degree of substitution (DS) ranging from 0.84 and 1.91 were prepared by homogeneous acetylation of cellulose with acetic anhydride in a 10% LiCl/dimethyl acetamide solvent. It was found that the use of these low DS samples permitted easier assignments not only of the ring carbon but also of the O-acetyl carbonyl carbon signals. The assignments were confirmed by comparing with the ¹H-NMR spectra of the samples obtained by complete acetylation of the corresponding CA samples with acetyl-d₃ chloride. Two methods for determining the distribution of O-acetyl groups of CA, i.e., the relative DS at the three different types of hydroxyl groups, were developed. One is based on the measurements of the relative intensities of the signals for the ring carbons and the other is based on the measurements of the relative intensities of the signals for the O-acetyl carbonyl carbons.     

13C nuclear magnetic resonance of organophosphorus compounds XI—aminophosphines

Natural abundance ¹³C NMR studies have been carried out on a series of organophosphorus compounds possessing P? N bonds. For the first time a one-bond temperature-dependent ¹³C—³¹P nuclear spin coupling was observed for the P-phenyl carbons in bis(N,N-dibenzylamino)phenylphosphine (0-9 Hz) and bis(N,N-diethylamino)phenylphosphine (0–2 Hz). This temperature-dependent behavior can be rationalized in terms of free rotation about the phenyl phosphorus bond with concomitant hindered rotation about the P? N bonds. A conformational preference for the nitrogen and phosphorus lone pairs to exist in the trans orientation is indicated. In the similarly substituted 5-membered heterocyclic ring compound, 1,3-dimethyl-2-phenyl-1,3-diazaphospholidine, the phenyl one-bond coupling increases to (?) 42.1 Hz and becomes temperature independent. These data suggest that ¹J(PC) is very responsive to electronic effects.     

Lithium-7 nuclear magnetic resonance and calorimetric study of lithium crown complexes in various solvents

Lithium-7 NMR studies have been carried out on lithium ion complexes with crown ethers 12C4, 15C5, and 18C6 in water and in several nonaqueous solvents. In all cases the exchange between the free and complexed lithium ion was fast on the NMR time scale, and a single, population average, resonance was observed. Both 1:1 and 2:1 (sandwich) complexes were observed between lithium ion and 12C4 in nitromethane solution. The stability of the complexes varied very significantly with the solvent. With the exception of pyridine, the stability varies inversely with the Gutmann donor number of the solvent. In general, the stability order of the complexes was found to be 15C5·Li⁺>12C4·Li⁺>18C6·Li⁺. Calorimetric studies on these complexes show that, in most cases, the complexes are both enthalpy and entropy stabilized.     

Infrared and 13C MAS nuclear magnetic resonance spectroscopic study of acetylation of cotton

The acetylation of commercial cotton samples with acetic anhydride without solvents in the presence of about 5% 4-dimethylaminopyridine (DMAP) catalyst was followed using Fourier transform infrared (FTIR) and 13C MAS NMR spectroscopy. This preliminary investigation was conducted in an effort to develop hydrophobic, biodegradable, cellulosic materials for subsequent application in oil spill cleanup. The FTIR results provide clear evidence for successful acetylation though the NMR results indicate that the level of acetylation is low. Nevertheless, the overall results indicate that cotton fibres are potential candidates suitable for further development via acetylation into hydrophobic sorbent materials for subsequent oil spill cleanup application. The results also indicate that de-acetylation, the reverse of the equilibrium acetylation reaction, occurred when the acetylation reaction was prolonged beyond 3 h.     

1H and13C NMR spectroscopy in the study of flavan-3-ols, proanthocyanidins, and their derivatives III.13C nuclear magnetic resonance spectroscopy of flavan-3-ols and proanthocyanidins

This review discusses the possibilities of using carbon-13 nuclear magnetic resonance spectroscopy in the investigation of flavan-3-ols, proanthocyanidins, and their derivatives, and reports differences in the spectral behavior of monomers, oligomers and their derivatives and features of the spectra of high-molecular-mass proanthocyanidins.Translated from Khimiya Prirodnykh Soedinenii, No. 4, pp. 545–568, July–August, 1997.     

1H and 13C nuclear magnetic resonance study of (D3)-trishomocuban-4-ol and (D3)-trishomocubanone

The complete assignments of the ¹H and ¹³C NMR spectra for (D₃)-trishomocuban-4-ol (3) and (D₃)-trishomocubanone (4) are reported. The difference between the ¹H and ¹³C chemical shifts of 3 and 4 and those of the hydrocarbon (D₃)-trishomocubane, and the substituent effect of the hydroxy group, can be adequately explained by the unique stereochemistry of these compounds.     

Solid-state 13C nuclear magnetic resonance characterization of MDI-based polyurethanes

¹³C solid-state nuclear magnetic resonance (NMR) experiments on linear polyurethanes and poly(ether-urethane) block copolymers demonstrate that ¹³C spin-lattice relaxation experiments in the laboratory [T₁(C)] and rotating [T_1p(C)] frames provide the most information about domain morphology in these microphase-separated polymer systems. T₁(H) TCH, and T_1p(H) data are less useful in a 4,4′-methylene bis(p-phenyl isocyanate)-1,4-butanediol (MDI/BD) hard-segment material, the MDI bridging methylene and the MDI urethane carbonyl T₁(C and T_1p(C) times fall in characteristic ranges for crystalline, amorphous, interfacial, and dissolved species. BD methylene carbons have short T_1p(C) for crystalline and long T_1p(C) for amorphous hard-segment aggregates. The distinct T_1p(C) and T₁(C) fractins observed are attributed to the presence of several crystalline polymorphs. Both T₁(C) results and DSC endotherms indicate that the crystalline polymorphs present in the poly(ether-urethane) are less ordered than the types seen in the pure hard-segment material. © 1993 John Wiley & Sons, Inc.     

13C nuclear magnetic resonance data of bile acid derivatives

The 13C-NMR spectra of well over 100 bile acid derivatives have been analyzed and summarized. A diagnostic gamma-oxygen shielding effect has been identified.