Complete proton and carbon-13 NMR assignment of complex polycyclic aromatic hydrocarbons

B-B vs. B-H Bond Activation in a (μ-Hydrido)diborane(4) Anion upon Cycloaddition with CO2 , Isocyanates, or Carbodiimides

The intriguing (μ‐hydrido)diboranes(4) with their prominent pristine representative [B₂H₅]⁻ have mainly been studied theoretically. We now describe the behavior of the planarized tetraaryl (μ‐hydrido)diborane(4) anion [1H]⁻ in cycloaddition reactions with the homologous series of heterocumulenes CO₂, iPrNCO, and iPrNCNiPr. We show that a C=O bond of CO₂ selectively activates the B−B bond of [1H]⁻, while the μ‐H ligand is left untouched ([2H]⁻). The carbodiimide iPrNCNiPr, in contrast, neglects the B−B bond and rather adds the B‐bonded H⁻ ion to its central C atom to generate a formamidinate bridge across the B₂ pair ([3]⁻). As a hybrid, the isocyanate iPrNCO combines the reactivity patterns of both its congeners and gives two products: one of them ([4H]⁻) is related to [2H]⁻, the other ([5]⁻) is an analog of [3]⁻. We finally propose a mechanistic scenario that rationalizes the individual reaction outcomes and combines them to a coherent picture of B–B vs. B–H bond activation.

Base Promoted Intumescence of Phenols

The intumescent process of sodium (substituted) phenolates has been studied. The generation of hydrogen radical via a homolytic cleavage of the Ar–H bond and the subsequent hydroarylation of phenolates to cyclohexadienes along with cyclization and elimination reactions of cyclohexadienes are critical steps in the base promoted intumescence of phenols. The substituents show great influence on the intumescence of phenolates. Phenolates substituted with a weak electron donating group enable intumescence while those with an electron withdrawing group or strong electron donating group suppresses intumescence. This distinction can be justified by both electronic and steric effects of substituents on the generation of hydrogen radical and the degree of hydroarylation.

Terminal Diazirines Enable Reverse Polarization Transfer from 15 N2 Singlets

Diazirine moieties are chemically stable and have been incorporated into biomolecules without impediment of biological activity. The ¹⁵ N₂ labeled diazirines are appealing motifs for hyperpolarization supporting relaxation protected states with long-lived lifetimes. The (-CH¹⁵ N₂ ) diazirine groups investigated here are analogues to methyl groups, which provides the opportunity to transfer polarization stored on a relaxation protected (-CH¹⁵ N₂ ) moiety to ¹ H, thus combining the advantages of long lifetimes of ¹⁵ N polarization with superior sensitivity of ¹ H detection. Despite the proximity of ¹ H to ¹⁵ N nuclei in the diazirine moiety, ¹⁵ N T₁ times of up to (4.6±0.4) min and singlet lifetimes T_s of up to (17.5±3.8) min are observed. Furthermore, we found terminal diazirines to support hyperpolarized ¹ H₂ singlet states in CH₂ groups of chiral molecules. The singlet lifetime of ¹ H singlets is up to (9.2±1.8) min, thus exceeding ¹ H T₁ relaxation time (at 8.45 T) by a factor of ≈100.

Analysis of Polycyclic Aromatic Hydrocarbon (PAH) Mixtures Using Diffusion-Ordered NMR Spectroscopy and Adsorption by Powdered Activated Carbon and Biochar

Analysis of polycyclic aromatic hydrocarbons (PAHs) in air and water sources is a key part of environmental chemistry research, since most PAHs are well known to be associated with negative health impacts on humans. This study explores an approach for analyzing PAH mixtures with advanced nuclear magnetic resonance (NMR) spectroscopic techniques including high-resolution one-dimensional (1D) NMR spectroscopy and diffusion-ordered NMR spectroscopy (DOSY NMR). With this method, different kinds of PAHs can be detected and differentiated from a mixture with high resolution. The adsorption process of PAH mixtures by PAC and biochar was studied to understand the mechanism and assess the method.

Transition-Selective Pulses in Zero-Field Nuclear Magnetic Resonance

We use low-amplitude, ultralow frequency pulses to drive nuclear spin transitions in zero and ultralow magnetic fields. In analogy to high-field NMR, a range of sophisticated experiments becomes available as these allow narrow-band excitation. As a first demonstration, pulses with excitation bandwidths 0.5-5 Hz are used for population redistribution, selective excitation, and coherence filtration. These methods are helpful when interpreting zero- and ultralow-field NMR spectra that contain a large number of transitions.

Molecular structure, vibrational spectral assignments (FT-IR and FT-RAMAN), NMR, NBO, HOMO–LUMO and NLO properties of 2-nitroacetophenone based on DFT calculations

The FT-IR and FT-Raman analyses of 2-nitro acetophenone (2NAP) have been carried out by density functional theory (DFT) calculations based on B3LYP level with 6-31G*/6-311[Formula: see text]G** basis set. The gauge-independent atomic orbital (GIAO) method has been used to get 1H NMR and [Formula: see text]C NMR chemical shifts. From DFT calculations, various parameters such as atomic charges, HOMO–LUMO energies and Dipole moment have been obtained. The molecular electronic potential (MEP) has also been derived for 2NAP. In order to find the electronic excitation energies, oscillator strength and nature of the respective excited states, the closed-shell singlet calculation has been utilized. MOLVIB program has been employed to calculate total energy distribution (TED) and normal coordinate analysis. Natural bond orbital (NBO) analysis has also been carried out by DFT calculations with B3LYP/6-311[Formula: see text]G** basis set.

"Perfect echo" INEPT: more efficient heteronuclear polarization transfer by refocusing homonuclear J-coupling interaction

A "perfect echo" based INEPT experiment that demonstrates more efficient heteronuclear polarization transfer over conventional INEPT has been developed. This scheme refocuses the effect of homonuclear (1)H-(1)H J-evolution and simultaneously allows heteronuclear (13)C-(1)H J-evolution to continue during INEPT. This improves one bond heteronuclear polarization transfer efficiency at longer INEPT transfer delays and also enhances the sensitivity of long range INEPT. The refocusing of homonuclear (1)H-(1)H J-coupling could be achieved by doubling the INEPT transfer period leading to a doubling of T2 losses. Therefore, the sensitivity gain is observed when loss of magnetization due to homonuclear (1)H-(1)H J-modulation is more severe than that of T2 decay. However, in general, INEPT transfer period is rather short compared to the longer T2 observed in small molecules. The long range PE-INEPT transfer to carbonyl carbon in beta-butyrolactone, showed much faster build up of C-13 signal than conventional long range INEPT as the long range heteronuclear J-coupling is comparable in magnitude to homonuclear (1)H-(1)H J-coupling in this case. For one bond heteronuclear polarization transfer at shorter delay, PE-INEPT and conventional INEPT displays equal transfer efficiency. Efficient polarization transfer is observed for small molecules dissolved in isotropic as well as weakly aligned media. Further, simulation results obtained using the full propagator and product operator analysis agree well with the experimental observations.

Direct detection of non-proton-bearing 15N nuclei by long-range couplings using polarization transfer

The application of the QDEPT and QHSQC principles to long-range interactions for the direct detection of non-proton-bearing (15)N is proffered as a general solution together with multiselective polarization transfer and triple INEPT for overcoming convoluted responses.

Bioactive Marine Alkaloids

The chapter deals with bioactive marine alkaloids. The chemistry and biological activities of pyridoacridines, pyrroloacridines, indoles, β-carbolines, pyrroles, isoquinolines, and tyrosine derived alkaloids have been discussed and reviewed.

Substituent effects and charge delocalization mode in chrysenium, benzo[c]phenanthrenium, and benzo[g]chrysenium cations: a stable ion and electrophilic substitution study

The first series of persistent carbocations derived from mono- and disubstituted chrysenes Ch (5- methyl- 3, 2-methoxy- 19, 2-methoxy-11-methyl- 20, 2-methoxy-5-methyl- 21, and 9-methyl-4H-cyclopenta[def]chrysene 22), monosubstituted benzo[c]phenanthrenes BcPh (3-methoxy- 23, 3-hydroxy- 24), and monosubstituted benzo[g]chrysenes BgCh (12-methoxy- 25; 12-hydroxy- 26) were generated in FSO3H/SO2ClF or FSO3H-SbF5 (4:1)/SO2ClF and studied by low-temperature NMR at 500 MHz. The methoxy and methyl substituents direct the protonation to their respective ortho positions. Whereas parent Ch 1 is protonated at C-6/C-12, 3 is protonated at C-6 (3aH+) and at C-12 (3bH+) with the latter being the thermodynamic cation. The 2-methoxy-Ch 19 is protonated at C-1 to give two conformationally distinct carboxonium ions (19aH+/19bH+). In the disubstituted Ch derivatives 20 and 21, the 2-methoxy overrides the 5-methyl and the predominant carbocations formed are via attack ortho to methoxy. For the methano derivative 22 (Me at C-9), a 3:1 mixture of 22aH+/22bH+ is formed. For parent BcPh 13, nitration and benzoylation are directed to C-5. With 3-methoxy-BcPh 23, the site of attack moves to C-4, thus producing two conformationally distinct carboxonium ions (23aH+/23bH+), whereas conventional nitration gave a 2:1 mixture of 23aNO2 and 23bNO2. In 3-hydroxy-BcPh 24, the carboxonium ion 24H+ is exclusively formed. For parent BgCh 16, protonation, nitration, and benzoylation are all directed to C-10 (16H+, 16NO2, 16COPh), but presence of OMe or OH substituent at C-12 changes the site of attack to C-11. Charge delocalization mode is probed based on magnitude of delta delta 13Cs and conformational aspects via NOED experiments. Complete NMR data are also reported for several benzoylation/nitration products. Using ab initio/GIAO (and NICS), the NMR chemical shifts (and aromaticity) in model carbocations A-D were evaluated. This work represents the first direct study of the carbocations derived from the methyl-, methoxy-/hydroxy-derivatives of three important classes of bay-region and fjord-region PAHs whose diol-epoxides extensively bind to DNA. It also extends the available data on electrophilic chemistry of BcPh and BgCh.

Bilosespens A and B: two novel cytotoxic sesterpenes from the marine sponge Dysidea cinerea

[formula: see text] Two novel sesterpenes, bilosespens A and B (1 and 2) were isolated from the Red Sea sponge Dysidea cinerea collected in the Dahlak archipelago, Eritrea. The structure of the mixture of the two inseparable compounds was established by spectroscopic analysis, mainly by 1D and 2D NMR measurements. The mixture of bilosespens A and B is cytotoxic to a few human cancer cells.

Conformational analysis of an alpha-galactosyl trisaccharide epitope involved in hyperacute rejection upon xenotransplantation

alpha-Galactosyl epitopes are carbohydrate structures bearing an alpha-Gal-(1-->3)-Gal terminus (alpha-Gal epitopes). The interaction of these epitopes on the surface of animal cells with anti alpha-Gal antibodies in human serum is believed to be the main cause in antibody-mediated hyperacute rejection in xenotransplantation. In this paper, conformational analysis of an N-linked alpha-D-Galp-(1-->3)-beta-D-Galp-(1-->4)-beta-D-Glcp trisaccharide epitope was conducted in terms of each monosaccharide residue conformation, primary hydroxymethyl group configuration, and interglycosidic conformations. Selective 2D J-delta INEPT experiments have been carried out at three different temperatures to evaluate three-bond, long-range 13C-1H coupling constants for the crucial alpha-(1-->3) linkage. The NMR experimental data were complemented by theoretical calculations. The flexibility and dynamics of the trisaccharide have been studied by Metropolis Monte Carlo simulations. Ensemble-averaged three-bond, long-range 13C-1H coupling constants and nuclear Overhauser effects were in good agreement with the experimental data. The alpha-(1-->3) glycosidic linkage has shown a restricted flexibility as indicated by NMR spectroscopy and molecular modeling.

Triterpene saponins from Scabiosa rotata

Four new allose-containing triterpenoid saponosides, scabriosides A, B, C and D were isolated from the roots of Scabiosa rotata. Their structures were established as 3-O-beta-D-xylopyranosyl-28-O-[beta-D-allopyranosyl (1-->6)-beta-D-glucopyranosyl]-pomolic acid, 3-O-[alpha-L-rhamnopyranosyl (1-->2)-beta-D-xylopyranosyl]-28-O-[beta-D-allopyranosyl (1-->6)-beta-D-glucopyranosyl]-pomolic acid, 3-O-[alpha-L-rhamnopyranosyl (1-->2)-alpha-L-arabinopyranosyl]-28-O-[beta-D-allopyranosyl (1-->6)-beta-D-glucopyranosyl]-pomolic acid, and 3-O-[beta-D-glucopyranosyl (1-->3)-alpha-L-rhamnopyranosyl (1-->2)-beta-D-xylopyranosyl]-28-O-[beta-D-allopyranosyl(1-->6) -beta-D-glucopyranosyl]-pomolic acid, respectively, by the help of spectral evidence (IR, 1D- and 2D-NMR, FAB-MS).

Flavonoids and a saponin from Spartium junceum

Two new flavonoids, 5,8-dihydroxy-4'-methoxy-6,7-methylenedioxyisoflavone and carthamidin-7-O-alpha-L-rhamnopyranoside and a new triterpenoid saponin, 3 beta,16 beta,22 beta,24-tetrahydroxy-olean-12- ene-3-beta-yl-]beta-D-glucopyranosyl (1-->2)]-alpha-L-rhamnopyranoside (junceoside), were isolated from Spartium junceum. Their structures were elucidated by means of IR, UV, MS, 1H, 13C NMR, 13C NMR DEPT and 2D NMR experiments. Apigenin-7-O-beta-D-glucopyranoside and genistein-8-C-beta-D-glucopyranoside, never previously found in this plant, were also isolated.