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. studies on intramolecular hydrogen bonding in anilides of 6-methylpicolinic acid N-oxide

The ¹H n.m.r. spectra of eleven anilides of 6-methyl-picolinic acid N-oxide in chloroform were obtained. The influence of temperature, concentration and substituents on the chemical shifts of the N? H protons was investigated. The structure of the anilides is discussed.     

VT 1H NMR investigations of resonance-assisted intramolecular hydrogen bonding in 4-(dimethylamino)-2'-hydroxychalcone

[reaction: see text] Resonance-assisted intramolecular hydrogen bonding in both polar aprotic and nonpolar solutions of 4-(dimethylamino)-2'-hydroxychalcone (DMAHC) has been investigated by variable-temperature proton NMR spectroscopy. In both nonpolar and polar solvents, the signal for the phenolic hydrogen moves downfield as the temperature is lowered. In each solvent system studied, a linear relationship between chemical shift and temperature was observed.     

Anion binding versus intramolecular hydrogen bonding in neutral macrocyclic amides

Although amide groups are important hydrogen-bond donors in natural and synthetic anion receptors, studies on structure-affinity relationships of amide-based macrocyclic receptors are still very limited. Therefore, we synthesized a series of macrocyclic tetraamides 5-8 derived from 1,3-benzenedicarboxylic (isophthalic) acid and aliphatic alpha,omega-diamines of different lengths. (1)H NMR titrations in DMSO solution show that the anion affinity of these receptors decreases with increasing size of the macrocycle irrespective of the anion, and this suggests a minor role of geometric complementarity. Comparison with their previously studied pyridine congeners reveals that the isophthalic acid based macrocycles are less potent, in contrast to what was found for simple model diamides. Combined theoretical and experimental structural studies were carried out to determine the reasons behind this behaviour. The results show that the unexpectedly low anion binding ability of the isophthalic acid-based receptors is due to the self-complementary nature of the isophthalic bis-amide fragments: when two such moieties are present within a sufficiently flexible macrocycle, they adopt syn-anti conformations and bind each other by two strong intramolecular hydrogen bonds that close the macrocyclic cavity. Nevertheless, anion binding is able to break these hydrogen bonds and switch a macrocycle into a convergent all-syn conformation. Despite the ill-preorganized conformation, 20-membered receptor 6 is better than either its open-chain analogue (macrocyclic effect) and/or its isomer having differently placed carbonyl groups. The crystal structures of four anion complexes of the macrocyclic receptors are reported. X-ray studies and solution NMR data confirmed the inclusive nature of the complexes and pointed to strong involvement of aromatic CH hydrogen atoms in anion binding.     

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.     

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.     

NMR and FTIR studies of coordinate-bonding and intramolecular and intermolecular hydrogen bonding in zinc(II)(3,5-diisopropylsalicylate)2

Carboxylate and salicylic OH coordinate bonding as well as intramolecular and intermolecular hydrogen bonding of bis-3,5-diisopropylsalicylatozinc(II), [Zn^II(3,5-DIPS)₂], with Lewis bases were studied to determine mechanisms accounting for antioxidant reactivity of Zn^II(3,5-DIPS)₂. Apparent thermodynamic parameters: K _eq, ΔS ⁰, ΔH ⁰, and ΔG ⁰ were determined for these equilibria with bonding of two molecules of dimethyl sulfoxide-d₆ (DMSO) or ethyl acetate-d₈ (EA) to the Zn^II using NMR and FTIR. We conclude that addition of two equivalents of DMSO or EA to non-polar solutions of Zn^II(3,5-DIPS)₂ results in bonding of DMSO or EA to Zn^II via sulfoxide or ester carbonyl oxygen atoms with ternary complex formation, leading to weakening of carboxylate and salicylic OH coordinate bonding to Zn^II and strengthening intramolecular hydrogen bonding between protons of salicylic OH groups and carboxylate oxygens. Subsequent addition of two or three additional equivalents of DMSO or EA leads to intermolecular hydrogen bonding between protons of salicylic OH groups.     

A 1H NMR study of hydrogen bonding and complexation processes of trimethylacetic acid in acetone

The carboxyl proton chemical shifts of trimethylacetic acid, which was dissolved in acetone and in binary mixtures of acetone and cyclohexane, have been measured as a function of concentration at temperatures of 0, +15, +30, +45 and +60 °C. The results obtained for the system studied, using the equilibrium constant method for interpretation of experimental data, indicate that the molecular association through hydrogen bonds occurs as a monomer-cyclic dimer-1:1 complex equilibrium process in the low concentration range (up to 0·08 mol fraction). The equilibrium constants of complexation and dimerization, the chemical shifts of the acid in the complex, and the thermodynamic parameters of complexation and dimerization have been calculated.     

1H and 13C NMR studies on N-(cyclohex-2-en-1-ylidene)amines

¹H and ¹³C NMR spectroscopy allow the identification of diastereomeric Z- and E-N-(cyclohex-2-en-l-ylidene)amines and the assignment of their configuration.     

Investigations of intramolecular hydrogen bonding in three types of Schiff bases by 2H and 3H NMR isotope effects

Hydrogen bonding within the structures of three Schiff bases (1-3), obtained by condensation of 4-methoxy-, 5-methoxy- and 4,6-dimethoxysalicylaldehyde with methylamine, was investigated by measuring deuterium and tritium NMR isotope effects. The primary deuterium and tritium isotope effects (delta(XH)-delta(XD/T)) and secondary one-bond nitrogen deuterium effect appear to be very useful parameters for defining the character of intramolecular hydrogen bonds. The tritium isotope effects were also determined for nitrogen-hydrogen one-bond coupling constants for both 4-methoxy and 4,6-dimethoxy derivatives. These parameters are seen to be highly sensitive to hydrogen bond characteristics and can be used to distinguish localized and tautomeric hydrogen bonds.     

An NMR, IR and theoretical investigation of (1)H chemical shifts and hydrogen bonding in phenols

The change in (1)H NMR chemical shifts upon hydrogen bonding was investigated using both experimental and theoretical methods. The (1)H NMR spectra of a number of phenols were recorded in CDCl(3) and DMSO solvents. For phenol, 2- and 4-cyanophenol and 2-nitrophenol the OH chemical shifts were measured as a function of concentration in CDCl(3). The plots were all linear with concentration, the gradients varying from 0.940 (phenol) to 7.85 (4-cyanophenol) ppm/M because of competing inter- and intramolecular hydrogen bonding. Ab initio calculations of a model acetone/phenol system showed that the OH shielding was linear with the H...O=C distance (R) for R < 2.1 A with a shielding coefficient of - 7.8 ppm/A and proportional to cos(2)phi where phi is the H...O=C--C dihedral angle. Other geometrical parameters had little effect. It was also found that the nuclear shielding profile is unrelated to the hydrogen bonding energy profile. The dependence of the OH chemical shift on the pi density on the oxygen atom was determined as ca 40 ppm/pi electron. This factor is similar to that for NH but four times the value for sp(2) hybridized carbon atoms. The introduction of these effects into the CHARGE programme allowed the calculation of the (1)H chemical shifts of the compounds studied. The CHARGE calculations were compared with those from the ACD database and from GIAO calculations. The CHARGE calculations were more accurate than other calculations both when all the shifts were considered and also when the OH shifts were excluded. The calculations from the ACD and GIAO approaches were reasonable when the OH shifts were excluded but not as good when all the shifts were considered. The poor treatment of the OH shifts in the GIAO calculations is very likely due to the lack of explicit solvent effects in these calculations.     

Synthesis of dinitrosyl iron complexes (DNICs) with intramolecular hydrogen bonding

HSCH₂CONHCH₃ and HSCH₂CON(CH₃)₂ containing a peptide bond are prepared for the synthesis of DNICs with/without intra-molecular hydrogen bonding, respectively. The IR ν(NO) bands of [(NO)₂Fe(SCH₂CONHCH₃)₂]⁻ (2) appears at 1751, 1700 cm⁻¹. In complex 2, the presence of intramolecular [NH?S] hydrogen bonding was verified by the observation of IR spectroscopy with N−H stretching frequency 3334 cm⁻¹ (CDCl₃) and subsequently confirmed by single-crystal X-ray diffraction showing N−S distance of 2.94 Å. Complex 2 displays the rhombic EPR spectrum with g₁ = 2.039, g₂ = 2.031 and g₃ = 2.013 at in frozen H₂O. Complexes 2 and 3 rapidly release NO when exposed to light. The time needed for photolysis reactions of 2 is two times faster than that of 3 in less polar solvent. Representative time courses for the photolability of 2 and 3 in THF display the NO-off ability: 2 > 3.     

Vibrational and 1H NMR spectra of N-(2-pyridyl)-thioformamide and N-(2-pyridyl)thioacetamide

The Raman and infrared spectra of N-(2-pyridyl)thioformamide and N-(2-pyridyl)-thioacetamide have been measured. The assignment of the bands is aided by the complete normal coordinate treatment for all the vibrations of N-(2-pyridyl)thioformamide and its N-deuterated molecule using a Urey—Bradley force function for the in-plane vibrations and a valence force function for the out of plane vibrations. Variable temperature ¹H NMR study of the two pyridylthionamides has also been performed. It is inferred that while N-(2-pyridyl)thioformamide favours a cis —CSNH— group, the other compound favours a trans —CSNH— grouping at ambient temperature.     

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.     

Matrix-isolation FT-IR and DFT theoretical studies of the intramolecular hydrogen bonding in Mannich bases

FT-IR Ar-matrix isolated spectra were studied for dichloro- (Cl₂-MB) and tetrachloroderivatives (Cl₄-MB) of the ortho Mannich base. The spectra were analyzed based on the DFT calculated frequencies and intensities and compared with those recorded in CCl₄ solution in the region of the ν(OH) and ν(OD) vibrations. The matrix-isolated spectra are characterized by narrower ν(OH) and ν(OD) bands with much better resolved fine structure than in solution. The fine structure originates from the anharmonic coupling with the low frequency modes as well as from Fermi resonance. The ν(OD) band shapes can be reproduced exclusively by assuming the Fermi resonance with overtones and summation of the frequencies of modes into which the bridge atoms are involved. The frequency isotopic ratio (ISR) is for both compounds 1.33 while the half-width ratios are equal to 1.82 and 1.94, for Cl₂-MB and Cl₄-MB, respectively.     

Multinuclear NMR (1H,13C,15N,17O) studies of intramolecular interactions in 1-vinylpyridones and quinolones

¹H and¹³C NMR was used to estimate the dihedral angles between the planes of the vinyl group and the hetero ring for 1-vinylpyridones and quinolones. The¹H,¹³C,¹⁵N, and¹⁷O NMR data showed the existence of a specific intramolecular weak hydrogen bond between the -H atom of the vinyl group and the O atom in 1-vinyl-2-pyridones and 2-vinyl-1-isoquinolone.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 7, pp. 1539–1547, July, 1990.     

Hydrogen bonding between phenols and fatty acid esters: (1)H NMR study and ab initio calculations

[structure: see text] (1)H NMR measurements and ab initio calculations were used to study the interactions between hindered/nonhindered phenols and carboxylic acid esters. The dihedral angle (phi) between the OH group and a plane of the aromatic ring is close to 0 degrees in the hydrogen-bonded nonhindered phenols, whereas for 2,6-di-tert-butyl-4-methylphenol the OH group is completely twisted out of the aromatic plane (phi approximately 90 degrees ).