Two-dimensional NMR studies of cyperene

Combined utilization of ¹H? ¹H homonuclear and ¹H? ¹³C heteronuclear NMR chemical shift correlations, two-dimensional J-resolved ¹H NMR measurements and homonuclear ¹H double resonance experiments allowed specific assignments for both ¹H and ¹³C NMR frequencies to be made for the tricyclic sesquiterpene cyperene. The results also provided information on the conformation of the six-membered ring of this natural product, which was recognized as a distorted chair.     

青蒿素的二维核磁共振研究

报导了采用二维核磁共振技术研究青蒿素的核磁氢谱和碳谱的谱线归属,以今后为^1^3C中间体的生物转化产物提供鉴定的基础.     

Carbon-13 NMR studies on some 5-substituted quinoxalines

Eleven 5-substituted quinoxalines (NO₂, NH₂, COOH, OCH₃, CH₃, OH, F, Cl, Br, I, CN, the latter five not reported previously) have been synthesized by standard methods. Their ¹³C NMR spectra have been measured in DMSO-d₆ and assigned on the basis of substituent parameters, by line widths and by intensities. The chemical shifts compare favorably with those calculated using benzene substituent parameters, and are very close to those of corresponding carbons in 1-substituted phenazines. The correlation with the chemical shifts of the corresponding positions in 1-substituted naphthalenes is also close except for those of carbons 4a and 8a in the quinoxalines which, due to their proximity to nitrogen, are downfield (in some cases 12 ppm) of the signals of the corresponding carbons in naphthalene. 5-Fluoroquinoxaline was also measured in CDCl₃, CD₃COCD₃, CD₃CN, CD₃OD, C₆D₆ and CD₃COOD. In all solvents an abnormally low ²J(CF) (～ 12 Hz) was found for C-4a and no C? F spin-spin splitting could be detected for the three-bond coupling of C-8a. Similar abnormalities were found in 2-fluoroaniline and 2-fluoroacetanilide. There are linear relationships between the Q parameter of the substituent and the chemical shift of carbons 4a, 5 and 6. A linear relationship also exists between the chemical shift of C-8 (‘para’ position) and the Hammett σ_p parameter of the substituent.     

Carbon–13 NMR studies of tautomerism in some 2-substituted imidazoles and benzimidazoles

The ¹³C NMR spectra of several 2-substituted imidazoles and benzimidazoles have been measured. The substituent was CH₃, COOH and CONHR, where R = H, n-Bu, p-tolyl or m-chlorophenyl. Carbons 4 and 5 in the imidazoles and the carbon pairs 8/9, 4/7 and 5/6 become equivalent by proton transfer from N-1 to N-3, possibly through intermolecular association. The rate of this proton exchange increases with concentration and temperature. It decreases with extension of the 2-substituent (rate CH₃?CONH-phenyl > CONH-p-tolyl ? CONH-m-chlorophenyl ? CONH-n-butyl) due to steric hindrance at the site of the (benz)imidazole nitrogen.     

Synthesis, NMR and crystallographic studies of 2-substituted dihydroquinazolinones derived from (S)-phenylethylamine

2,3-Dihydro-3-[(S)-1-phenethyl]quinazolinone and some new 2-substituted derivatives bearing isopropyl, o-nitrophenyl and p-nitrophenyl groups were prepared in 40-90% yield by amidation of isatoic anhydride with (S)-phenylethylamine, followed by condensation with triethyl orthoformate, isopropylaldehyde, o-nitro- and p-nitro-benzaldehyde, respectively. The two 2-subtituted dihydroquinazolinones obtained either by using isopropylaldehyde, o-nitro- or p-nitrobenzaldehyde, were separated and purified before their NMR spectra in CDCl3 solutions were recorded. The detection of the low energy conformation of O=C-N-phenethyl segment in solution allowed the correlation of the NMR data with the configuration of newly stereogenic carbon C-2; thus, one diastereomer was labeled SS while the other was RS. Configurations determined by the NMR method were corroborated by X-ray diffraction analysis. X-ray structures of each diastereomeric series showed characteristic conformational types: a propeller-like for the SS and a hairpin for the RS series. Interatomic distances of the hairpin conformation suggest the existence of intramolecular face-to-face interactions between two aromatic rings.     

Two-dimensional zero-field NMR and NQR

Corrlalations between quadrupolar frequencies in a spin I = 1 system are determined through a two-dimensional version of the zero-field NMR experiment. Results presented on a selectively deuterated polycrystalline solid illustrate this for the quadrupolar lines corresponding to inequivalent deuterium sites.     

14N and15N NMR spectra of 2-substituted 5-nitrofurans

The¹⁴N and¹⁵N NMR spectra of a number of 2-substituted 5-nitrofurans were studied. On the basis of experimental data it was concluded that there is considerable π-electron density on the nitrogen atom of the nitro group. A linear relationship between the chemical shifts in the¹⁴N NMR spectra and the frequencies of the asymmetrical deformation vibrations of the nitro group in the IR spectra was found for the series of investigated 5-nitrofurans. The observed¹⁵N-H spin-spin coupling constants showed that in the 5-nitrofuran molecule transmission of spin information through the ring oxygen atom to the H₂ nucleus is appreciably greater than through the carbon atom to the H₄ nucleus. It was established by measurement of the¹⁵N NMR spectra that the crude adduct formed in the nitration of furfural diacetate with acetyl nitrate is a mixture of trans and cis isomers of 5-nitro-2-acetoxy-2,5-dihydrofurfural diacetate in a ratio of 7∶1. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 6, pp. 741–743, June, 1980.     

Proton-coupled 13C NMR spectra of 2- and 3-substituted pyridines

The proton-coupled ¹³C NMR and PMR spectra of pyridine and the 2- and 3-monosubstituted pyridines NC₅H₄X [where X=CH₃, CN, COCH₃, COOCH₃, N(CH₃)₂, NO₂, OCH₃, Cl, or Br] for one-molar solutions of the compounds in DMSO-D₆, have been analyzed. The signs and values of the ¹³C-¹H HSSCs have been determined. Equations have been obtained connecting the ¹³C-¹H SSCCs in the 2- and 3-substituted pyridines and the monosubstituted benzenes. A satisfactory correlation of ¹JCH with the F and R constants of the substituents has been shown.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 9, pp. 1223–1230, September, 1984.     

13C and 15N NMR spectra of 2,3-substituted 2H-azirines

The ¹³C and ¹⁵N NMR spectra of a series of 2,3-substituted 2H-azirines have been studied. The ¹⁵N chemical shift for the nitrogen in the azirine ring is found at much higher field than in acyclic imines with a considerable electronic effect for the substituents on the double bond. Cooperative steric and electronic effects associated with substituents on the unsaturated carbon atom of the ring were found to influence the shielding of the ¹³C and ¹⁵N nuclei. Reaction constants have been calculated for 2-alkyl(aryl)-3-phenylazirines. It has been shown that the azirine ring has a powerful —I effect (when compared with the phenyl ring) that exceeds the analogous value for the cyano group.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 9, pp. 1181–1183, September, 1986.     

The NMR spectra of the carbanions of xanthene and thioxanthene. Evidence for paratropism

The proton NMR spectra of the carbanions of xanthene [XH]^? and thioxanthene [TxH]^? have been recorded and interpreted. Paratropism in the central rings of [XH]^? and [TxH]^? is inferred from a comparison of the chemical shifts with those of the carbanion of 9,10-dihydroanthracene [AH]^?. The contributions to the chemical shifts arising from n-electron excess charges, local dipoles and magnetic anisotropies are discussed. Numerical values for the various ring currents have been estimated by a least squares analysis of the observed chemical shifts after applying corrections for the excess charge effect. The results point to a strongly increasing paramagnetic ring current in the central ring in the order [AH]^?, [TxH]^?, [XH]^?.     

Two-dimensional NMR characterization of propylene copolymers

Newly developed two-dimensional NMR techniques are used to characterize homopolypropylene and copolymers of propylene with ethylene and butylene. Both homonuclear and heteronuclear shift-correlated experiments have been attempted. The most useful technique for the copolymers is ¹³C-¹H shift correlation, whereby the one-dimensional ¹H-NMR spectra of these copolymers are completely interpreted for the first time. Although the ¹H resonances are heavily overlapped, the use of a computerized “analytical” approach permits pertinent information to be obtained from the one-dimensional ¹H-NMR spectra.     

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.     

2-取代吡啶衍生物的CNDO/2计算和分子光谱研究

本文用CNDO/2方法计算了吡啶及其2-取代衍生物,测了其紫外光谱.通过红外,紫外光谱和量子化学计算的综合分析,确定2-吡啶酮与2-羟基吡啶是互变异构体,并推断2-氨基吡啶的互变异构体存在,从而很好地得到了此系列紫外光谱的线性规律.     

33S NMR spectroscopy 3. Substituent effects on 33S NMR parameters in 2-substituted ethanesulfonates

33S NMR parameters (chemical shifts and linewidths) in 2-substituted sodium ethanesulfonates, XCH2CH2SO3Na (X = H, CH3, OH, SH, NH2, Cl, Br, NH3+) depend upon the electronic properties of substituents. To explain experimental results and obtain additional information on the origin of the observed substituent effect (SE), sulfur isotropic absolute shielding constants have been calculated at DFT level of theory (B3LYP/6-311++G(2d,p)) by gauge-including atomic orbitals (GIAO) method. Data have been interpreted with the aid of natural bond orbital (NBO) method and natural chemical shielding (NCS) analysis. It has been demonstrated that in the class of compounds considered the diamagnetic contribution to sulfur-shielding constant is constant and the observed upfield shift of 33S resonance induced by electron-withdrawing substituents (reverse chemical shift effect) can be related to variations of the paramagnetic contribution. Substituents with different electronic properties cause variations in the polarization of S-C and S-O bonds of the -C-SO3- moiety thus determining changes of the electron density at sulfur nucleus and consequently the expansion or contraction of 3p sulfur orbitals. Also oxygen lone-pairs and sulfur core 2p electrons can play an active role in determining the paramagnetic contribution to sulfur shielding. With regard to linewidth variations, they can be ascribed primarily to changes in the nuclear quadrupole coupling constant values. B3LYP/6-311++G(2d,p) method allows obtaining a good reproducibility of SE on the electric field gradient (EFG) at sulfur, although its values tend to be underestimated significantly. Moreover, 17O shielding constants have been calculated.     

Vibrational spectroscopic studies on some 2-substituted 1,3-dithi a-2-boracyclopentanes

Infrared and Raman spectra are reported for 2-X-1,3-dithia-2-boracyclopentanes, where X = Br, Cl, Ph or NMe₂. In all cases the vibrations of the heterocyclic ring unit can be assigned in terms of C₂ symmetry, corresponding to a “twisted-ring” conformation similar to that found for the related molecule 1,3-dithiolan-2-thione. The internal modes of the B-Ph unit are in agreement with C_2v “local” symmetry, while those for B-NMe₂ suggest a considerably lower symmetry.     

Natural abundance 17O NMR study of 2- and 4-substituted benzoyl chlorides

Natural abundance ¹⁷O NMR chemical shift data for 17 ortho and para benzoyl chlorides recorded in acetonitrile at 75°C are reported. ¹⁷O NMR data for the para substituted benzoyl chlorides are correlated with ¹⁷O NMR data for similarly substituted acetophenones and methyl benzoates. The ¹⁷O NMR signals for ortho isomers are downfield (ca 30 ppm) from their para isomers; the downfield shifts are consistent with torsion angle change. The ¹⁷O NMR data for the para isomers gave good correlations with σ⁺ constants and with dual substituent parameters (DSP).     

2D NMR studies of acrylonitrile-methyl acrylate copolymers

The microstructure of acrylonitrile-methyl acrylate copolymers prepared by the solution polymerization using 2,2′-azobisisobutyronitrile (AIBN) as free radical initiator was investigated by two-dimensional NMR techniques. 2D-heteronuclear single quantum correlation (HSQC) and the total correlation spectroscopy (TOCSY) have been utilized to resolve the complex ¹H NMR spectrum and to establish the compositional and configurational sequences of acrylonitrile-methyl acrylate copolymers. 2D HSQC and TOCSY showed compositional and configurational sensitivity of methine protons of A and M units upto the triad level. Heteronuclear multiple-bond correlation (HMBC) spectroscopy has been used to study carbon (carbonyl/nitrile)-proton coupling. The carbonyl and nitrile carbons showed compositional sensitivity upto the triad level. The values of reactivity ratios were determined by Kelen-Tudos (KT) and non-linear error in variable method (RREVM).     

Two-dimensional Pure Isotropic Proton Solid State NMR

One key bottleneck of solid-state NMR spectroscopy is that ¹H NMR spectra of organic solids are often very broad due to the presence of a strong network of dipolar couplings. We have recently suggested a new approach to tackle this problem. More specifically, we parametrically mapped errors leading to residual dipolar broadening into a second dimension and removed them in a correlation experiment. In this way pure isotropic proton (PIP) spectra were obtained that contain only isotropic shifts and provide the highest ¹H NMR resolution available today in rigid solids. Here, using a deep-learning method, we extend the PIP approach to a second dimension, and for samples of L-tyrosine hydrochloride and ampicillin we obtain high resolution ¹H-¹H double-quantum/single-quantum dipolar correlation and spin-diffusion spectra with significantly higher resolution than the corresponding spectra at 100 kHz MAS, allowing the identification of previously overlapped isotropic correlation peaks.