Molecular aggregation in liquid acetic acid: insight from molecular dynamics/quantum mechanics modelling of structural and NMR properties

Quantum mechanics/molecular dynamics approaches have been applied to unveil the anomalous upfield shift of the 1H NMR signal of acetic acid by going from low-concentration solution in cyclohexane to the neat liquid.

Cinnamoylphenethylamine 1H-NMR Chemical Shifts: A Concise Reference for Ubiquitous Compounds

1H-NMR data of 25 cinnamoylphenethylamine derivates were recorded and compared in order to assign signals unequivocally without additional spectroscopic data. The spectra provide a key for the rapid identification of these ubiquitous natural products. The compounds isomerize rapidly in UV light, producing a characteristic upfield shift of the olefinic protons consistent with distorted planarity of the Cis cinnamate, and this requires special attention during preparative work.

Synthesis, spectral characterization and antibacterial studies of palladium(II) complexes of heterocyclic thiones

Reactions of K2[PdCl4] with heterocyclic thiones in molar ratios of 1:2 and 1:4 in water-methanol medium yielded the palladium(II) complexes with the general formula of either [Pd(L)Cl2], [Pd(L)2]Cl2or [Pd(L)4]Cl2where L ═ Imidazolidine-2-thione (Imt), 2-Mercaptopyridine (Mpy), 2-Mercaptopyrimidine (Mpm), 6-Mercaptopurine (6-Mp) and Thionicotinamide (Tna). The complexes were characterized by elemental analysis and spectroscopic (IR,1H and13C NMR) methods. An upfield shift in the >C═S resonance of thiones in13C NMR and downfield shift in N–H or aromatic proton resonances in1H NMR are consistent with the sulfur coordination to palladium(II). The complexes were screened for antibacterial activity, and the results showed that the complexes exhibited moderate activities as compared to that of a standard drug.

Solid State NMR Study of Calcium Fluoroaluminosilicate Glasses

Fluorine containing calcium aluminosilicate glasses are widely used for a number of technological applications including dental cements, mould fluxes in steel making and in a variety of glass-ceramic systems. Despite of their importance these systems remain quite poorly understood with respect to their composition. To address this question a glass composition corresponding to the equimolar binary system anorthite−fluorite (Ca2Al2Si2O8−CaF2) was chosen as a base point for two series of compositions. One of the series is designed on the anorthite stoichiometry and considered as classically charge balanced. Another series starts from the fluorine free composition of the anorthite−lime (Ca2Al2Si2O8−CaO) stoichiometry and, therefore, is characterized by a disrupted network with at least one non-bridging oxygen (NBO) attached to silicon. A multinuclear 19F, 27Al, 29Si solid state NMR study of the glasses was undertaken. It is shown that in both series fluorine is predominantly coordinated by calcium, F−Ca(n), and in addition interacts with aluminium forming Al−F−Ca(n) complexes, where n denotes the number of first neighbouring calcium cations. Small amounts of high coordinated aluminium grows with increasing fluoride content in both glass series. However, the high coordinated aluminium may not be solely due to the formation of the Al−F−Ca(n) complexes. Glasses of the first series displayed systematic upfield shift of 29Si NMR resonance while substituting fluoride for oxide, starting from the fluorine free composition. This upfield shift is interpreted as the lack of cations in the network, due to formation of the F−Ca(n), which drives silicon network to polymerize toward a higher Qn structure. Contrary to the first series, the 29Si NMR resonance remains constant for fluorine containing compositions of the second series but differs downfield from the initial anorthite glass. The latter is explained by the excess of cations in the network due to addition of the fluorite resulting in formation of NBO on the silicon. Binding of fluorine with silicon is considered negligible in these systems. Thus, fluorine and calcium both define the degree of network polymerization and are considered as a cause for the changes in silicon and aluminium networks.

2"-Chartreusin-monoacetate, a New Natural Product with Unusual Anisotropy Effects from the Marine Isolate Streptomyces sp. B5525, and its 4"-Isomer

The marine Streptomycete isolates B5342 and B5525 forms, beside large amounts of the antitumor antibiotic chartreusin (1a), two chartreusin monoacetates as trace components. Whereas the chemical shift of the acetate methyl in 1c is in the normal range, the methyl group of the 2"-acetate residue in 1b and also in the tetra- and pentaacetates shows an extreme upfield shift. The structures of the monoacetates were confirmed by extensive NMR experiments, the anisotropic shift is explained by semi-empirical calculations.

The T, p-Dependence of the Chemical Shift of the Hydroxyl Protons in Deeply Supercooled Methanol and Water

The hydroxyl proton chemical shifts δ (H-O) of supercooled methanol (Tmin= 149 K) and water have been determined (Tmin= 183 K), and the pressure dependence of these shifts was measured up to 200 MPa. In both compounds the downfield shift of δ (H-O) continues down to the lowest temperatures reached. This result disagrees with the two state models for the hydrogen bond formation in both liquids. The isotherms δ(H2O) show for T< 273 K an upfield shift that becomes more pronounced with decreasing temperature. For δ ( H-O-CH3), increasing p causes at all temperatures a downshift

33S NMR spectra of some sulfonated naphthalenes, naphthols, and their anions

The 33S NMR spectra of 11 naphthalenesulfonates, 11 hydroxynaphthalenesulfonates, two dihydroxynaphthalenesulfonates, and their anions have been measured in aqueous solutions. The 33S NMR chemical shifts of these compounds exhibit upfield shifts when related to ammonium sulfate as the standard. Introduction of a further sulfonic group causes a small upfield shift (up to 3 ppm). Introduction of a hydroxyl group causes a downfield shift (up to 5 ppm) unless the ring is further substituted. Formation of the anion from hydroxyl group causes an additional downfield shift (up to 6 ppm). The half-width of 33S NMR signals of sulfonated naphthalenes, naphthols, and their anions vary from 10 to 400 Hz.

15N Nuclear Magnetic-Resonance Spectra of ortho-Substituted Phenylacetanilides: The Origin of γ-syn Effects

The 15N chemical shifts of X α and Y α, i.e., the nuclei through which the side chains, X and Y, respectively, of o-substituted phenylacetanilides (2a) are bonded to the aromatic ring were determined. There was observed a uniform upfield shift (γsyn effect). In order to explore the origin of the γ effect, other o-disubstitutes benzenes (2b-d) were also examined. The values of D, defined as the difference between γ α substituent chemical shifts of the ortho (2) and para (3) series of compounds, and considered as a measure of the γ-syn effect, were correlated with various electronic and steric substituent parameters. Correlations with electronegativity and van der Waals terms were successful. Examination of the correlations indicated that the γ-syn effect is mostly governed by electronegativity. The classic steric compression theory does not seem very convincing as a means of explaining the shielding γ-syn effect observed in the present study.

Determination of the intercarbonyl dihedral angle of 1,2-diketones by 17O nmr

This study describes a method using 17O nmr to determine the intercarbonyl dihedral angle in a homologous series of seven 1,2-diketones. It is shown that the 17O nmr shifts of the 1,2-diketones are linearly dependent on both the electron density at the oxygen atoms and the n→π* excitation energy of the 1,2-diketo chromophore. Reduction of the overlap in the π system of the 1,2-diketo moiety leads to a gradual upfield shift of the 17O nmr signal, due to the increase in both the π-electron density at the oxygen atoms [Formula: see text] and the mean transition energy ΔE.

13C N.M.R. Spectra of ortho-Substituted Phenyl N,N-Dimethyl-Carbamates and N-Methyl Carbamates

We measured 13C nuclear magnetic resonance spectra of O-substituted phenyl N,N- dimethyland N-methyl- carbamates and compared the results with those for the corresponding m- and p-substituted derivatives. The additivity relationship on the basis of the substituent chemical shift for monosubstituted benzenes ( scsphx ) did not hold well because of the ortho effect. However, the scs of C2(ipso), C3(ortho) and C5(para) was correlated excellently with scsphxX . Dual substituent parameter (DSP) analyses of the scs of C5 and the carbonyl carbon in the fixed side chain showed that correlation was very good for C5 but moderate for C=O. These observations suggested that the X substituent lay in almost the same plane as the aromatic ring, whereas the OCONMe2 group may have changed its conformation somewhat. In O-substituted derivatives, the carbon nucleus that is directly attached to the aromatic ring (γ to the oxygen; OCONMe2)had a pronounced upfield shift relative to the corresponding m- and p-substituents by 4-5 ppm . This phenomenon was explained by the γ-effect caused by the oxygen atom.