Use of silver trifluoroacetate together with lanthanide shift reagents for simplification of 1H NMR spectra of aromatic hydrocarbons

The equimolar mixtures of typical lanthanide shift reagents such as Eu(fod)₃, Pr(fod)₃ or Yb(fod)₃ with silver trifluoroacetate, previously used to induce paramagnetic shifts in the ¹H NMR spectra of alkenes, have been successfully applied to simple aromatic hydrocarbons such as benzene, toluene, ethylbenzene and xylenes. In benzene and p-xylene the signals of all the aromatic protons are shifted identically. In other substituted benzenes the magnitude of the induced shift depends on the distance between the proton and the substituents. In addition, the different behaviour of the signals of the methyl groups in meta-and para-xylene on the addition of the complex shift reagent allows the quantitative analysis of the two xylenes in their mixtures.     

The 1H and 13C nmr spectra of the “onium” salts of dithiacyclophanes

The ¹H and ¹³C nmr spectra of three cyclic sulphonium salts ol dithiacyclophanes have been recorded and compared with those of the open-chain analogue. The ¹H spectra indicate that methylation of the dithiacyclophanes gives a mixture of eonformational isorners with slightly different chemical shifts for the benzylic protons. The ¹³C spectra indicate that the smaller rings adopt an unsymmetrical conformation with respect to the aromatic ring but that in the larger ring studied, the aromatic carbon atoms are magnetically equivalent. The relevance of these results to the reactivity of the aromatic ring is discussed.     

Shift reagent 1H NMR study of methoxycoumarins

The complete assignment of the proton chemical shifts of coumarin ( 1 ), all monomethoxy derivatives and the six possible dimethoxyl substituted compounds at the aromatic ring was achieved at 60 MHz in a quantitative study utilizing Pr(fod)₃ as the shift reagent. It was found that in addition to the complexation at the lactone carbonyl a second interaction of the lanthanide shift reagent occurs when two methoxyl groups are found in an ortho distribution. The results are discussed in comparison to data published for flavones in the presence of Pr(fod)₃ shift reagent.     

Microwave spectroscopy and nuclear magnetic resonance study of the molecular geometry of 2-fluorophenylisocyanate

Analysis of the microwave spectra of the vibrational ground state of 2-fluorophenyl-isocyanate in the gaseous state shows the existence of a single planar cis conformer. Rotational spectra of the excited torsional states out of the plane and spectra of in-plane vibrations have also been studied. MINDO III type theoretical calculations corroborate the experimental results. Similarly, the configuration of 2-fluorophenylisocyanate in the liquid phase has been studied by NMR using the chemical shift reagent, Eu(fod)₃. Induced paramagnetic shifts have been measured on the ¹³C spectra, for different quantities of the reagent. The results clearly show a higher paramagnetic shift for the carbon bonded to the fluorine atom than for the other nuclei of the aromatic cycle (with the exception of the quaternary carbon), indicating the probable presence of the cis isomer.     

NMR studies of polymer solutions. II. Polyethylene

The intramolecular structure of polyethylene in solution was studied by a high-resolution nuclear magnetic resonance technique. Highly purified n-alkanes (99.5%) from C₅H₁₂ to C₃₆H₇₄ were used as its oligomers. The NMR spectra of the polyethylenes (oligomers) are very sensitive to the solvents used. The internal methylene protons of all polyethylenes of various chain length resonance at an identical frequency in carbon tetrachloride. A sharp transition in the NMR spectrum of polyethylene in α-chloronaphthalene at 35°C. was observed at n-C₁₇H₃₆, above which there exist two distinguishable NMR peaks for internal methylene protons, and below which (fewer carbons) only a single peak was seen. The NMR spectra of the internal methylene protons of the polyethylenes (oligomers) taken in benzene are very similar to those taken in pyridine. They are not as easily resolved as those NMR spectra taken in α-chloronaphthalene solutions. The effect of the size of the aromatic solvent molecule on the NMR spectra of the internal methylene protons of the polyethylenes (oligomers) in solutions was demonstrated by using aromatic solvents of various sizes, such as chlorobenzene, α-chloronaphthalene, and 9-chloronathracene. The results indicate that the formation of polymeric structure of the internal methylene groups in the polyethylene chain is very sensitive to the size of the solvent used. The interaction of the solvent with the methylene groups of the polyethylenes varies as a function of chain length; it is stronger for those low member n-alkanes and decreases gradually to an asymptotic value.     

A novel method for correlation of substituent constants in some aromatic esters using the shift reagent Eu(fod)3

The lanthanide induced chemical shifts of the ester methyl group were studied in eighteen m-, and p-substituted aromatic methyl esters with varying amounts of added shift reagent, Eu(fod)₃. Each time the lanthanide induced shift (LIS) was plotted against the molar ratio of shift reagent (SR)/substrate and a straight line was obtained upto one mole ratio of added shift reagent. The slopes of these plots correlated well with the Hammett's σ constants and yield a value of ?4.95 for the reaction constant ?. The method is therefore one of the simplest ways of correlating the substituent constants in aromatic esters.     

An investigation of the utility of using n.m.r. lanthanide shift reagents for 13C assignments in selected aromatic ethers and ketones. A detailed description of the lanthanide induced shift ratio method for lanthanide shift reagent–substrate geometry analysis

The ¹³C and ¹H spectra of a series of aromatic ketones, ethers and keto-ethers were studied using the lanthanide shift reagent (LSR) Yb(fod)₃ and the data analyzed using the lanthanide induced shift (LIS) ratio method. Studies were carried out on phenalenone, 6-methoxyphenalenone, 1- and 2-methoxynaphthalene, 1-indanone, 5- and 6-methoxy-1-indanone and flavone and the data obtained subjected to LSR-substrate geometry analysis as a means of making ¹³C assignments. The utility and limitations of the LSR method for making ¹³C assignments is discussed and the LIS ratio method for LSR-substrate geometry analysis is described in detail.     

Proton magnetic resonance studies of actinomycin-related peptides containing N-methyl groups

The PMR spectra of several cyclic and acyclic dipeptide derivatives representing portions of the actinomycin structure have been studied. In the spectra of the acyclic compounds temperature-dependent duplicity resulted from the equilibration of conformers possessing cis and trans peptide bonds. The lanthanide shift reagent Eu(FOD)₃ was utilized to distinguish N-methyl groups in the two conformations and observations were made on the steric dependence of the observed shifts. The origin of the wide variation in chemical shift of N-methylvalyl α-protons is discussed in relation to the conformation of actinomycin.     

Chemical shift reagents in the study of polycyclic alcohols IX—1H NMR spectra of myrtenol and some other primary alcohols

Structure proof of (—)-myrtenol, 3-cyclopentenyl-1-methanol and 5-norbornene-2-endo-methanol has been obtained from their proton magnetic resonance spectra in carbon tetrachloride containing different added amounts of tris(dipivaloylmethanato)europium. For each alcohol, a 1:1 complex structure with Eu(dpm)₃ could be computed, in which the calculated pseudocontact shift effects on all skeleton protons of the ring system were consistent with the observed shift effect values. A considerable contact contribution of opposite sign to that of the pseudocontact part of the effect could be estimated for the methylene protons of the CH₂OH group.     

Investigation of a resorcinol–formaldehyde resin by 13C-NMR spectroscopy and intrinsic viscosity measurement

Various molecular weight fractions and oligomers isolated from a novolac type resorcinol–formaldehyde (RF) wood adhesive resin were studied by ¹³C-NMR, gel permeation chromatography (GPC), and intrinsic viscosity measurements. The ¹³C-NMR results indicate that methylene groups occur mostly between C₄–C′₄ and C₂–C′₄ carbons and in minor amounts between C₂–C′₂ carbons of resorcinol rings. By suppressing the nuclear Overhauser effect (nOe) and using a long delay time for ¹³C-NMR measurements, reasonable methylene/aromatic carbon ratios were obtained from the aromatic C₁ carbon patterns that have resulted from different numbers of methylene groups bonded on the aromatic ring. This analysis results indicate that the RF resin has an appreciable amount of branch structures and compare favorably with the gel permeation chromatography and intrinsic viscosity measurement results. © 1993 John Wiley & Sons, Inc.     

The stereochemistry of 4,8-dimethyl-6-phenyl-5,6,7,8-tetrahydro-4H-1,3,6-dioxazocin-2-ones using NMR

Two isomeric 4,8-dimethyl-6-phenyl-5,6,7,8-tetrahydro-4H-1,3,6-dioxazocin-2-ones were prepared, and their pmr and cmr spectra were examined. From the tris(dipivaloylmethanato) europium (Eu(dpm)₃) shift reagent and the dihedral angle, the title compounds are found to exist in the eq-ax and eq-eq methyl groups with boat-chair form.     

Magnetic resonance study of diastereomeric interactions of nitroaromatic sulfxides with alcohols in solution

The synthesis of optically active (1R,2R)- and (1S,2R)-2-deuterio-2,3-dihydro-2-methyl-6-nitrobenzothiophene-1-oxide (3 and 4) is described. Several racemic aryl methyl carbinols in the presence of 3 or 4 give different chemical shifts for enantiomeric CH, CH₃ and OH protons. These chemical shift differences are larger than those observed in analogous sulfoxide–alcohol pairs without nitro groups. Quinine and cinchonidine cause observable non-equivalence of methylene and aromatic protons in racemic 4. The magnitude of the diastereomeric shift difference is sensitive to temperature, solvent, concentration, nature of the π system and steric effects.     

Synthesis and equilibrium studies of new dihydrothiophene sulfoxides

Oxidation of the Knoevenagel condensation products 2 of 3-oxo- tetrahydrothiophene (1) and active methylene compounds of the type Z-CH₂-COOH with an H₂O₂/V₂O₅/t-BuOH reagent furnishes the corresponding sulfoxides 3 in excellent yields. The isomer distributions of the sulfides 2 and the sulfoxides 3 reflect peculiarities of hydrothiophenes as compared to similar open-chain systems and establish that the stabilizing effect of the sulfoxide group on β, γ-double bonds is greater than the corresponding effect of so strongly stabilizing groups as CN and COOR.     

Deuterated Epoxides and Their Polymers. I. Synthesis of cis- and trans-Propylene Oxide-1-d and Their NMR Spectra

Abstract

cis- and trans-Propylene oxide-1-d were synthesized from the corresponding propenyl halides via propenyllithium and propylene bromohydrin. Their NMR spectra (about 3% in benzene) were analyzed in terms of ABX₃ spin system and the assignment was carried out. The chemical shift of the methylene proton in cis position to the methyl group was found to be Δ = 1.86 ppm and that of the methylene proton in trans position Δ = 2.18 ppm (downfield from HMDS). The H-D couplings were observed in the methine-methylene part spectra decoupled from methyl protons. The observed J_HD's agree with the theoretical relation: J_HH = 6.5 14 X J_HD.     

Novel polycyclic heterocycles. XII. Reactions of 1,2-dihydro-11-(trifluoromethyl)-3H-7H-quino[8, 1-cd] [1,5]-benzoxazepin-3-one with aromatic aldehydes

The Claisen-Schmidt condensation between 1,2-dihydro-11-(trifluoromethyl)-3H,7H-quino-[8,1 -cd][1,5]benzoxazepin-3-one, 1 , and aromatic aldehydes has been investigated. The acid catalyzed reactions yielded the trans-2-benzylidene derivatives, 4 ; the structures and configurations of the group of compounds represented by 4 have been confirmed by pmr in conjunction with the Eu(fod)₃ shift reagent. In contrast, catalysis with sodium hydroxide gave the isomeric 2-benzyl-endocyclic α,β-unsaturated ketones, 3. Finally, the 4 could be isomerized to the corresponding 3 by means of sodium hydroxide. The ir, uv, and pmr spectra of these compounds are discussed.     

Glycoluril‐Derived Molecular Clips are Potent and Selective Receptors for Cationic Dyes in Water

Molecular clip 1 remains monomeric in water and engages in host–guest recognition processes with suitable guests. We report the K_a values for 32 1? guest complexes measured by ¹H NMR, UV/Vis, and fluorescence titrations. The cavity of 1 is shaped by aromatic surfaces of negative electrostatic potential and therefore displays high affinity and selectivity for planar and cationic aromatic guests that distinguishes it from CB[n] receptors that prefer aliphatic over aromatic guests. Electrostatic effects play a dominant role in the recognition process whereby ion–dipole interactions may occur between ammonium ions and the C=O groups of 1 , between the SO₃^? groups of 1 and pendant cationic groups on the guest, and within the cavity of 1 by cation–π interactions. Host 1 displays a high affinity toward dicationic guests with large planar aromatic surfaces (e.g. naphthalene diimide NDI+ and perylene diimide PDI+) and cationic dyes derived from acridine (e.g. methylene blue and azure A). The critical importance of cation–π interactions was ascertained by a comparison of analogous neutral and cationic guests (e.g. methylene violet vs. methylene blue; quinoline vs. N‐methylquinolinium; acridine vs. N‐methylacridinium; neutral red vs. neutral red H⁺) the affinities of which differ by up to 380‐fold. We demonstrate that the high affinity of 1 toward methylene blue (K_a=3.92×10⁷ m ^?1; K_d=25 nm ) allows for the selective sequestration and destaining of U87 cells stained with methylene blue.     

Propellanes—XXXIII : Conformations of various heterocyclic propellanes

Eu(DPM)₃ has been used as a shift reagent in studying the NMR spectra of various propellanes. The PDIGM program has been used to calculate the preferred conformations of these substrates.     

Discrimination by NMR spectroscopy of isomeric ar,ar′-disubstituted [2.2] paracyclophanes carrying identical substituents

 Four diastereomers are formed when precursors of the type 2-R-1,4-(CH₂X)₂C₆H₃ are coupled in the synthesis of symmetrical [2.2]paracyclophanes with one substituent R per aromatic ring. The stereochemistry of the diastereomers is derived from the symmetry of their ¹H NMR spectra and from NOE difference and 2D C,H-COSY and -COLOC spectra for the case of R=CO₂CH₃. This substituent causes only small ¹H chemical shift effects in the distant aromatic ring; so the results of chemical shift increment calculations should be regarded with caution. For the present compounds, the use of ¹³C shift increments is safer, but the disadvantage of increment calculations is the need for the mono-substituted derivative and the analysis of its spectra. Received: 24 May 1996/Accepted: 1 July 1996     

1H NMR Study on Solubilization of Aromatic Solutes in Gemini Cationic Surfactant Micelles

The ¹HNMR spectra of gemini cationic surfactant 1,3‐Propane diaminium N, N‐didodecyl‐2‐hydrxyl‐N,N,N′N′‐tetramethyl‐dichloride (ClC₁₂H₂₅‐(CH₃)₂N‐CH₂CH(OH) CH₂N(CH₃)₂ C₁₂H₂₅ Cl), abbreviated to GC₁₂NCl solutions were determined in the presence of toluene, phenyl ethane, benzyl alcohol, or 2‐phenylethanol. Based on observations, probable solubilization sites of the substrate molecule within the micelles are discussed. The examined results indicate that the polarity of aromatic solutes affects their solubilization site in the micelle solution. The methylene in long chain of GC₁₂NCl shows a single signal in the presence of toluene and phenyl ethane, however, when benzyl alcohol or 2‐phenylethanol is in existence, the peak of ethylenes in long chain Gemini surfactant splits into an upfield signal and a downfield signal in the solubilization system with enhancement of molar ratio of aromatic solutes to the sufactant. The addition of benzyl alcohol and 2‐phenylethanol causes a downfield shift of the methyl signal, the other signal were up field shifted by the addition of the phenyl derivatives. From the dependence of chemical shifts of GC₁₂NCl on solubilizate concentration, it is ascertained that at low solubilizate contents the aromatic solutes are solubilized by adsorption at the micelle‐water interface. With further addition of aromatic solutes, the solubilization site of the solubilizate varied gradually from the mantle of the surfactant micelle to its inner core.     

Carbon-13 nuclear magnetic resonance studies of leonurine hydrochloride and its analogues

The ¹³C NMR spectra of leonurine hydrochloride and thirteen of its analogues in DMSO-d₆ have been analyzed. Changes in the aromatic substituents have no significant effect on the chemical shifts of the side chain methylene carbons indicating that they do not influence the conformation of the latter. Observed deviations from additivity of substituent effects for the methylene carbon chemical shifts suggest that the methylene side chains of these compounds may be more tightly coiled than are the corresponding n-alkanes. In representative cases no change in conformation is evident in 50% aqueous DMSO-d₆ solutions, indicating that similar considerations may apply in aqueous media.