Constants of keto-enol equilibrium of acetoacetyl- and 1,1′-bis(acetoacetyl)ferrocene in aprotic solvents at 20°

The keto-enol equilibrium of 2 × 10⁻³ M solutions of (acetoacetyl)ferrocene and 1,1′-bis(acetoacetyl)ferrocene was studied by ¹H NMR spectroscopy in a series of aprotic solvents such as DMSO-d ₆, (CD₃)₂CO, CDCl₃, CD₂Cl₂, CCl₄, and C₆D₆ at a temperature of 20°C. It was established that the calculated enolization constants increase with a decrease in the polarity of solvent molecules. The results complement and combine known empirical rules about the effect of the medium on keto-enol equilibria.     

Studies on the tacticity of polyacrylonitrile. III. NMR spectra of stereoisomers of 2,4,6-tricyanoheptane as model compounds of polyacrylonitrile

The three stereoisomers of 2,4,6-tricyanoheptane were separated and their NMR spectra were studied as the three-unit model compounds of polyacrylonitrile. The chemical shifts and coupling constants obtained from the NMR spectra were compared to those of the two-units model compounds. The geminal protons for the isotactic methylene are not equivalent, but the difference of the chemical shifts is smaller than that in the meso two-unit model. The racemic methylene signals appear at higher field than the meso methylene in the heterotactic three-unit model as in the case of the racemic and the meso two-unit models. The signals of the methinic protons in triads appear in the order: isotactic CH, heterotactic CH, and syndiotactic CH from the high field. From the observed values of the vicinal coupling constants, the chain conformations of the model compounds are also discussed.     

1H, 13C, 15N NMR and IR Spectroscopic Studies of a Rh(II) Complex of Thiourea

A rhodium(II) complex of thiourea (Tu), Rh₂(OAc)₄Tu₂, has been prepared from Rh₂(OAc)₄(H₂O)₂ and characterized. A shift of the ν(N?H) vibration towards higher frequencies in the IR spectrum is consistent with sulfur coordination to rhodium(II). ¹³C NMR spectra recorded in DMSO-d ₆ reveal that thiourea is replaced by DMSO-d ₆ solvent, followed by replacement of acetate ions by free thiourea. ¹⁵N NMR indicates some nitrogen involvement in coordination to form an S?N chelate.     

1H NMR study of cyclo-L-Thr-L-Thr and cyclo-L-Thr-L-His conformations

Cyclodipeptides containing L -Thr and L -His residues have been studied by ¹H NMR in D₂O and DMSO-d₆. In the neutral form in D₂O as in DMSO-d₆, the folded form of the L -His residue is not unique. The diketopiperazine ring seems to be not strictly planar.     

A novel use of cobalt(III) meso-tetraphenylporphyrin as a chiral shift reagent

The NMR spectra of the diastereomeric complexes formed by the coordination of nitrogenous enantiomeric bases with cobalt(III) meso-tetraphenylporphyrin (CoTPP) allow the immediate differentiation of these complexes. The spectra are interpreted on the basis of symmetry (RR and SS) and pseudo-chirality (RS and SR) considerations. The effects are observed in both the proton and carbon spectra and the complexes are stable in both CDCl₃ and DMSO-d₄ solution. This technique, in principle, allows the ready determination of the optical purity of multifunctional ligands.     

The NMR spectra of the porphyrins: 23—Metalloporphyrins as diamagnetic shift reagents,chemical applications

Some applications illustrating the use of zinc (II) meso-tetraphenylporphyrin (ZnTPP) as a diamagnetic shift reagent are described. The complexation shifts (ΔP) in a series of substituted pyridines are shown to be determined by both the basicity of the coordinating nitrogen atom and steric effects of ortho-substituents. The selectivity of ZnTPP (N»O) in the simplification of the spectra of multifunctional ligands is demonstrated. Addition of the reagent neither affects the rotamer populations of the Inderal side-chain nor produces any distortion of the 2-benzylmorpholine ring. In addition, the substrate-reagent solution may be shaken with D₂O to identify exchangeable protons. The application of ZnTPP in structural problems involving substituted imidazo[2,1-b]thiazoles and 2-benzyl-1,3-dioxopyrrolo[3,4-c]pyridines allows unambiguous identification of the possible structural isomers. To overcome the solvent limitations of ZnTPP, the more soluble Co(III)TPPBr can be used successfully as a shift reagent in DMSO-d₆, CD₃OD and acetone-d₆ solutions.     

Carbon-13 nuclear magnetic resonance studies of some 9-alkoxy- and 9-alkylthio-acridines

Carbon-13 NMR parameters were determined for some 9-alkoxy- and 9-alkylthio-acridines as solutes in DMSO-d₆ or D₂O.     

Carbon-13 NMR chemical shifts in some substituted 1,2,4-triazol-3-ones

Carbon-13 NMR data for 15 substituted 1,2,4-triazol-3-ones are presented and discussed with regard to enolization in the neutral molecules. The coupling constant and chemical shift data show that the proton at N-2 is not exchanging rapidly in the DMSO-d₆ solvent. Using D₂O? OD^? as a solvent, it is found that the C-3 and C-5 resonances are shifted downfield by nearly the same amount, suggesting that the proton at N-4 is being removed. Enolization in the neutral molecules does not occur to any significant extent.     

Vicinal interproton couplings through two heteroatoms. II—Coupling 3J(H?N?O?H) occurring in m-Nitrophenylhydroxylamines

The proton NMR spectra of certain arylhydroxylamines in DMSO-d₆ show a vicinal coupling through nitrogen and oxygen. The assignment of OH and NH signals was made using a ¹⁵N labelled derivative.     

The stereoregularity of polyacrylonitrile and its dependence on polymerization temperature

The stereoregularity of polyacrylonitrile was studied by NMR spectroscopy. The methylene protons in isotactic configuration are equivalent in dimethylformamide-d₇ solution and are nonequivalent in dimethylsulfoxide-d₆ solution. In the case of the latter solution the difference in chemical shift between the isotactic methylene protons is 6.6 cps. The stereoregularity of polyacrylonitrile-α-d instead of polyacrylonitrile was determined on the dimethylformamide-d₇ solution. In the radical polymerization all the polyacrylonitrile-α-d's that polymerized at temperatures between 80 and ?78°C. have a configuration consisting of about 50% of isotactic diads and, accordingly, the stereoregularity of polyacrylonitrile does not depend upon the polymerization temperature. Analysis of the NMR spectra of isotactic polyacrylonitrile prepared from acrylonitrile–urea canal complex was also carried out. The NMR spectra of meso- and dl-2,4-dicyanopentanes, dimer models of isotactic and syndiotactic polyacrylonitriles, respectively, were analyzed by a computer program proposed by Bothner-By.     

Conformational analysis. Part 20—conformational analysis of 4-deoxy-4-fluoro-D-glucose and 6-deoxy-6-fluoro-D-galactose in solution

The ¹H and ¹⁹F NMR spectra of the α- and β-pyranose anomers of 4-deoxy-4-fluoro-D -glucose (4FG) and 6-deoxy-6-fluoro-D -galactose (6FGA) in methanol-d₄, DMSO-d₆, acetone-d₆ and D₂O solution are reported. Computer analysis of the ABMX spectra of the CH CH₂F fragments gives accurate vicinal HH and HF coupling constants. An iterative computational analysis of the observed vicinal couplings in this fragment for 6FG, 6FGA and other molecules allows the determination of both the individual rotamer couplings and the rotamer populations. Consideration of the derived rotamer couplings strongly suggests that the correct assignment for the prochiral C-6 methylene protons in 6FG is that with the 6S proton having the larger coupling to H-5. This is the reverse of the assignment of these protons in D -glucose. In contrast, the assignment of these protons in 6FGA follows that given previously for D -galactose. The relative energies for the conformations about the C-5 C-6 bond for 4FG, 6FG and 6FGA are given from the derived rotamer populations. For 6FGA the rotamer in which the fluorine is antiperiplanar to C-4 is particularly favoured. For 4FG the rotamer with OH anti-periplanar to the ring O is highly unfavoured, but the other two rotamers are of almost equal energy. Consideration of the effect of replacing hydroxyl by fluorine in these molecules indicates that any hydrogen bonding involving the C-4 or C-6 hydroxyls plays little part in determining the conformer energies of glucose or galactose in polar solutions.     

High-resolution NMR spectra of the polysulfones of six monolefins

100-MHz NMR spectra are reported for solutions of the 1:1 copolymers of sulfur dioxide with hex-1-ene, cis- and trans-but-2-enes, cyclohexene, cyclopentene, and norbornene, including the deuterated polymers made from hex-1-ene-2-d₁, cis- and trans-but-2-ene-2,3-d₂, and cyclohexene-3,3,6,6-d₄. The resolution of the spectrum of poly-(hex-1-ene sulfone) was very poor in CCl₄ as solvent but good in polar solvents. The main-chain CH₂ protons are nonequivalent, and their chemical shifts show sensitivity to dyad structure; the α-CH₂ protons are also non-equivalent. The spectra of most of the other polymers are discussed in terms of possible preferential modes of addition as well as tacticity effects.     

Synthesis of 2-(1-Alkoxyvinyl)oxazolidines by Condensation of 2-Alkoxypropenals with 2-Aminoalkanols and Ring-Chain Tautomerism of the Products

Reactions of 2-alkoxypropenals with 2-aminoalkanols afforded tautomeric mixtures of previously unknown 2-(1-alkoxyvinyl)oxazolidines and imino alcohols. The condensation takes 2 h at room temperature (89-100%) or 1-5 min under microwave irradiation. The tautomeric equilibrium shifts toward the open-chain structure with increase in the solvent polarity (CDCl₃, CD₂OD, DMSO-d ₆, D₂O) and temperature. The presence of substituents in the oxazolidine ring raises the stability of the cyclic tautomer.     

Intermolecular interactions of 1-vinyl-1,2,4-triazole in various solvents

The solvent effect on intermolecular interactions of 1-vinyl-1,2,4-triazole was estimated by ¹H NMR spectroscopy. It was shown that 1-vinyl-1,2,4-triazole molecules form homoassociates (molecular stacks) which are broken on dilution with CCl₄. In polar solvents (dimethylacetamide-d ₆, D₂O), monomer-solvent heteroassociates prevail.     

Photo-oxidation of the 2,4-dichloropentanes and PVC

The photo-oxidation of PVC and its model compounds, the dl- and meso-2,4-dichloropentanes, were studied by high-resolution NMR spectroscopy. The structure, ? CHCl? CH₂? CO? CH₂? was found to be present in PVC after photo-oxidation. Two peaks, which corresponded to the CHCl and CH₂ groups next to the carbonyl group, were observed in carbon disulfide–acetone-d₅ mixture, at 6.15 and 7.25τ, respectively. From the NMR spectra, the ratio of the CH₂ to CHCl protons in PVC and the model compounds were found to increase linearly with the time of photo-oxidation after induction periods. The initial rates of the model compounds were comparable with that of PVC; i.e., 1.6 × 10^?2 hr^?1, at 30°C. Since similar gaseous products were also detected by mass spectrometry in the initial stage, their photo-oxidation probably assumed the same rate-determining step. According to the results that the dl-2,4-dichloropentane was photo-oxidized 1.5 times faster than the meso-compound, it would be likely that the syndiotactic sequences in PVC might be more easily photo-oxidized than the isotactic sequences.     

Three nuclei n.m.r. spectroscopy of dimethoate compounds. A large solvent effect on the 31P?S?C?1H vicinal coupling

Proton, phosphorus and carbon magnetic resonance spectra of dimethoate, dimethoxon, des-N-methyldimethoate, ω-hydroxydimethoate, trimethyldithiophosphate and O,O-dimethyldithiophosphate in different solvents have been measured. Most of the n.m.r. parameters were characteristic of the structural environment of the corresponding nucleus and solvent-independent. However, in the amide structures dimethoate, dimethoxon, des-N-methyldimethoate and ω-hydroxydimethoate the ³¹P? S? C? ¹H vicinal coupling showed an unusually large solvent effect of a 2.3 to 6.5 Hz decrease when CDCI₃ was replaced by acetone-d₆ or DMSO-d₆.     

Synthesis of BINOL-containing oxacalix[4]arenes

Unprecedented chiral oxacalixarenes were obtained from the reaction of (±)-1,1′-binaphthyl-2,2′-diol (BINOL) with 1,5-difluoro-2,4-dinitrobenzene. Structural elucidation of C_2h-symmetric meso (R,S)-oxacalix[4]arene and D₂-symmetric racemic (as well as enantiopure) (R,R)- and (S,S)-oxacalix[4]arene was carried out by means of 1D and 2D NMR spectroscopy, ESI-MS, X-ray diffractometry, and circular dichroism spectroscopy.     

Halide ion effect on the 1H NMR chemical shifts of the residual protons in commonly employed deuterated solvents with tetra-n-butylammonium chloride – Part 2

Many ¹H NMR spectroscopic studies involving supramolecular binding of tetra-n-butylammonium halides (TBAX) with a variety of molecular receptors have been reported to date. Previously we demonstrated that the reference residual proton signal of the deuterochloroform solvent itself in TBAX solutions shifted downfield in a linear TBAX concentration-dependent relationship. We now report that a similar downfield chemical shift behaviour of the residual protons of other commonly employed deuterated solvents with TBACl can be seen for dichloromethane-d₂ and acetonitrile-d₃, but in acetone-d₆, methanol-d₄ and DMSO-d₆, upfield shifts are observed. A hypothesis based on Density Functional Theory (DFT) modelling is presented to account for this behaviour.     

Novel determination of the total phenolic content in crude plant extracts by the use of H NMR of the –OH spectral region

A novel method for the determination of the total phenolic content using ¹H NMR spectroscopy in the –OH spectral region is presented. The use of DMSO-d₆, which is an aprotic and strongly hydrogen bonding solvent, allows the “appearance” of the relative sharp resonances of phenolic hydroxyl protons in the region of 8–14 ppm. The determination of the total phenolic –OH content requires three steps: (i) a 1D ¹H NMR spectrum is obtained in DMSO-d₆; (ii) a subsequent 1D ¹H NMR spectrum is recorded with irradiation of the residual water signal which results in the elimination or reduction of the phenolic –OH groups, due to proton exchange; and (iii) 1D ¹H NMR spectra are recorded with the addition of a progressively increased amount of salt, NaHCO₃, which results in extensive linebroadening of the COOH resonances thus allowing the discrimination of the phenolic from the carboxylic acid signals. Integration, with respect to the internal standard TSP-d₄, of the signal resonances between 14 and 8 ppm in spectrum (i) which are either eliminated or reduced in intensity in steps (ii) and (iii) allows the quantitation of the total phenolic content. The method was applied to model compounds, a mixture of them and several extracts of natural products. The results of the proposed ¹H NMR method were compared to the Folin-Ciocalteu (FC) reagent method. Additionally, since ¹H NMR refers to the total phenolic hydroxyl protons, a reaction factor, A_e, is proposed that corresponds to the hydroxyl reactivity. The ¹H NMR method is rapid and accurate bearing the inherent advantages of the NMR spectroscopy and can be applied directly in complex extracts. Furthermore, it can be applied in a wide range of matrixes from crude plant extracts and food products to biological samples.     

The proton magnetic resonance spectra of d-aldosterone and its acetylated derivatives

PMR spectra of d-aldosterone, its acetylated derivatives and related compounds were studied using 60 and 100 MHz spectrometers at various temperatures, in CDCI₃, DMSO-d₆, CD₃OD and D₂O. The aldehyde form ( 1 ) was not found. The solutions of d-aldosterone and aldosterone-21 acetate contain a mixture of the cyclic forms with one (11–18) hemi-ketal bridge ( 2 ) and two (11–18, 18–20) hemi-ketal bridges ( 3 ). Preliminary results concerning modifications of the relative concentration of 2 and 3 obtained by varying solvents and temperature are given. Aldosterone-diacetate exists in only one form, most likely form 2 . There is restricted rotation of the group at C-21 in d-aldosterone and in form 3 of aldosterone 21-acetate. One molecule of water is probably bound to d-aldosterone.