1H NMR studies of intramolecular hydrogen bonding in N-(pyridyl)amides of 6-methylpicolinic acid N-oxide

The ¹H NMR spectra of seven N-(pyridyl)amides of 6-methylpicolinic acid N-oxide in chloroform were obtained. The influence on the chemical shifts of the N? H protons of temperature, concentration and the CH₃ substituent in the pyridine ring was studied. The N? H protons were found to be shifted to low fields (～14 ppm) owing to the formation of strong intramolecular hydrogen bonding. The influence of the pyridine ring on the chemical shift of the N? H proton is comparable with the inductive effect of the p-nitrophenyl group. The hindered rotation around the N-pyridyl bond of N-(α-pyridyl)amides of 6-methylpicolinic acid in solution is discussed.     

1H NMR studies on intramolecular hydrogen bonding in perchlorates of N-(pyridyl)amides of 6-methylpicolinic acid N-oxide

The ¹H NMR spectra of perchlorates of N-(pyridyl)amides of 6-methylpicolinic acid N-oxide (PYAP) in CD₃CN at 100 MHz show two proton signals belonging to two distinct intramolecular hydrogen bonds. The position of these signals is independent of concentration and temperature. That of the proton of the N? H ?O bond in PYAP is shifted to still lower field than in N-(pyridyl)amides of 6-methylpicolinic acid N-oxide (PYA) due to the inductive effect of the pyridine cation and the formation of another intramolecular hydrogen N⁺? H ?O bond. The proton of the N⁺? H ?O bond interacts strongly with its environment and is highly sensitive to traces of water. Presumably, water leads to dissociation of the intramolecular bond.     

1H n.m.r. study of the self-association of acetone phenylhydrazone by hydrogen bonding

Proton magnetic resonance dilution studies of acetone phenylhydrazone were carried out in n-heptane, carbon disulphide and hexachlorobutadiene solutions. Equilibrium constants and enthalpy values for the predominant self-associated species are reported. Linear trimeric aggregates predominate in n-heptane solutions. The monomeric units are associated by intermolecular hydrogen bonding of the NH proton to the iminic nitrogen. The enthalpy per hydrogen bond is 10.04 kJ (2.4 kcal). In carbon disulphide and hexachlorobutadiene solutions complexation of acetone phenylhydrazone with the solvent competes with self-association.     

1H n.m.r. spectra of 6-acetoxy-2,4-cyclohexadienones

¹H n.m.r. spectra of 36 derivatives of 6-acetoxy-2,4-cyclohexadienones were analysed. All available evidence indicates that all members of the series have similar conformations which do not depart significantly from planarity. Substituent-induced chemical shifts and interproton coupling constants correlate well with those in the analogously substituted ethylene and benzene derivatives. In particular, it appears that a substituent on the central carbon atom exerts a similar influence on ⁴J(HH) across a ‘W’ path when the three intervening carbon atoms are a part of a benzene ring, an allylic system (i.e. sp³–sp²–sp²), a localized sp²–sp²–sp² system or a saturated (i.e. sp³–sp³–sp³) system.     

1H n.m.r. spectra of N-phenylmaleimide in isotropic and nematic phases

¹H n.m.r. spectra of N-phenylmaleimide have been investigated in isotropic as well as nematic phases; the chemical shifts, the direct dipolar and the indirect spin–spin coupling constants have been determined. The direct dipolar coupling constants are consistent with rapidly interconverting energetically equivalent twisted conformations of C₂ symmetry. Under the assumption that only two such conformers are predominantly present, the angle between the phenyl and the maleimide planes is determined as 52.9±0.9°.     

1H n.m.r. investigation on the site of hydration in the asymmetric diazanaphthalenes

The site of hydration for 1,6- and 1,7-naphthyridine has been determined by specific line broadening in the ¹H n.m.r. spectra on the addition of water. The site of hydration appears to be the same as the site of protonation (β-nitrogen atom). The site of hydration in quinazoline has been shown to be the α-nitrogen atom. This strongly indicates that the site of protonation in this compound will also be at that nitrogen.     

2,3,5,6-Tetramethylmorpholine: II—1H n.m.r. structural studies of the isomers of 2,3,5,6-tetramethylmorpholine

The structures of the six isomers of 2,3,5,6-tetramethyl-morpholine have been determined by means of proton nuclear magnetic resonance studies at 100 MHz. The spectra have been analysed in terms of spin–spin coupling constants and population distributions of the possible conformers at low temperatures. Also included in this paper are results from n.m.r. studies on the six 4-benzyl-2,3,5,6-tetramethylmorpholine derivatives, which give information about the stereochemistry of the methyl groups α to the amine group.     

Carbon-13 n.m.r. studies of methylquinolines and methylisoquinolines

Spectra are reported for all seven methylquinolines, nine dimethylquinolines, six methylisoquinolines and one dimethylisoquinoline. Substituent effects for the α, ortho, meta, para, vinylogous para and peri positions are reported for the monomethylcompounds. Additivity of these substituent effects is demonstrated for the dimethyl compounds which also exhibit a vicinal dimethyl steric effect.     

1H NMR studies of solvent effects on hydrogen bonding in some pyridine trifluoroacetates

The chemical shifts of hydrogen bonded protons in complexes of 11 substituted pyridines with trifluoroacetic acid were examined, in five dry solvents of different activity, with respect to proton transfer and aggregation effects. The results were correlated with ΔpK_a, the Kirkwood function and E _T parameters. The solvent effect on the intersection point obtained from the plot of the chemical shift of the hydrogen bonded protons against ΔpK_a can be used, similar to an isotopic effect, to differentiate strong hydrogen bonds. The aggregation of acid–base complexes can lead to downfield or upfield shifts; the variation of chemical shift with aggregation depends on the position of the proton in the hydrogen bridge.     

Localized orbital studies of hydrogen bonding: III. The intramolecular hydrogen bond in 4-methylimino 2-pentanone

Geometry optimizations using the INDO molecular orbital method are carried out on the three possible isomeric forms of 4-methylimino- 2-pentanone. It is found that the molecule strongly prefers a structure containing a hydrogen-bonded chelate ring. Of the two possible isomers containing an intramolecular hydrogen bond, only the structure with the chelated proton closer to the oxygen corresponds to an energy minimum, although the energy surface connecting these two isomers is very flat. The results are interpreted using experimental and calculated NMR properties and energy localized molecular orbitais.     

1H NMR studies of solvent and substituent effects on strong intramolecular hydrogen bonds

Chemical shifts of H-bonded protons in tetrabutylammonium hydrogen maleate and 14-substituted picolinic acid N-oxides have been measured in a number of dry solvents, of different activity, in order to distinguish between symmetrical single minimum and asymmetrical hydrogen bonds. In tetrabutylammonium hydrogen maleate the resonance was observed at 20.70 ppm and its was independent of the nature of the solvent used. The chemical shift value of picolinic acid N-oxide varies with the solvent. These observations suggest that the hydrogen bond is symmetrical in tetrabutylammonium hydrogen maleate but that it is asymmetrical in picolinic acid N-oxide. The chemical shifts of substituted picolinic acid N-oxides were correlated with σ_p, σ_m and Δ_pK_a. The substituent and solvent effects are compared and the position of the intramolecular H-bonded protons in picolinic acid N-oxides are estimated and discussed.     

Solvent effect on the intramolecular hydrogen bond in 8-quinolinol N-oxide

Solvent effect on intramolecular hydrogen bond in 8-quinolinol N-oxide has been studied by IR, UV,¹H NMR and¹³C NMR spectroscopy, dipole moment measurements and quantum-mechanical calculations. The solute-solvent interactions are of local character and they vary considerably over the range of solvent under study. The results suggest that formation complexes with solvent molecules weaken the intramolecular hydrogen bond in 8-quinolinol N-oxide.     

1H n.m.r. studies of palladium(II) complexes of 2,9-dimethyl-1,10-phenanthroline and 8-hydroxyquinoline

Summary The structures in solution of a series of palladium(II) complexes have been determined by¹H n.m.r. and i.r. spectroscopy. Dicyanobis-(8-hydroxyquinoline)palladium(II) has acis-square-planar configuration, the unidentate 8-hydroxyquinoline molecules bonding to Pd through the nitrogen atoms. Dicyanobis-(2,9-dimethyl-1,10-phenanthroline)-palladium(II) has acis-square planar arrangement about Pd with respect to the nitrogen atoms of the two heterocyclic ligands. The cyanide groups bond to the two apical positions apparently giving rise to a six-coordinate PdlI atom. Dihalo-2,9-dimethyl-1,10-phenanthrolinepalladium(II) (X = Cl, Br, I) exhibits the usualcis-square-planar arrangement of Pd^II, whereas the halobis-(2,9-dimethyl-1,10-phenanthroline) - palladium(II) ion (X = Cl, Br) has a trigonal bipyramidal structure with the halogen atom in the trigonal plane.     

N.m.r. studies in the heterocyclic series: XVI—determination of the structure of 3-azidoindazole by 1H and 13C n.m.r. spectroscopy

The 1-NH structure for 3-azidoindazole has been demonstrated by the observation of ¹H? ¹H and ¹³C? ¹H couplings involving the hydrogen atom attached to nitrogen. A comparison between 3-azidoindazole and indazole shows that both compounds have the same tautomeric structure.     

1H n.m.r. conformational studies of glycyl-L-phenylalanine and L-phenylalanyl-glycine dipeptides and their complexes with palladium (II)

The conformations of Gly-Phe and Phe-Gly dipeptides and those of their complexes with the Pd(II) ion has been established by means of ¹H n.m.r. spectra over various pH ranges. In the most stable rotamer of both dipeptides the bulkiest groups, i.e. carboxylate (or carbonyl) and the aromatic ring, are in trans position to each other. In the 1:1 molar ratio complexes, the dipeptides are the tridentate ligands within the 3–10 pH region. In the predominant conformer the aromatic ring lies over the complex plane, which suggests its interaction with the metal ion. At pH above 13, the carboxyl group becomes non-coordinated. The dipeptides are then bidentate and the most stable rotamers are the same as those found for the metal-free ligands, i.e. the carboxyl or carbonyl group is in the trans position to the aromatic ring. The glycine chelate ring in the Pd(II) Gly-Phe complex is planar up to pH 10 and its distortion from planarity follows when the ligand becomes bidentate.     

Spectroscopic and computational studies of the intramolecular hydrogen bonding of 2-indanol

2-Indanol in its most stable form is stabilized by internal hydrogen bonding, which exists between the hydroxyl hydrogen atom and the pi-cloud of the benzene ring. A comprehensive ab initio calculation using the MP2/cc-pVTZ level of theory showed that 2-indanol can exist in four possible conformations, which can interchange through the ring-puckering vibration and the internal rotation of the OH group on the five-membered ring. A potential energy surface in terms of these two vibrational coordinates was calculated. Density functional theory calculations were used to predict the vibrational frequencies and to help in normal mode assignments. Fluorescence excitation spectra of 2-indanol confirm the presence of the four conformers in the electronic ground and excited states. The spectral intensities indicate that, at 90 degrees C, 82% of the molecules exist in its most stable form with the intramolecular hydrogen bonding. The other isomers are present at approximately 11, 5, and 3%. The MP2/6-311++G(d,p) calculation predicts a distribution of 70, 13, 9, and 8% at 90 degrees C, the experimental sample temperature.     

Carbon-13 n.m.r. studies of substituted bisalkylamino-s-triazines in acidic media

The carbon-13 n.m.r. spectra of six substituted bisalkylamino-s-triazines are reported. The site of protonation of 2-methoxy-s-triazines is shown to be at N-1 or N-3. The effect of trifluoroacetic acid on 2-chloro-s-triazines is also discussed.