Intramolecular Hydrogen Bonds of the CO…︁H?O Type as Studied by 17O-NMR

The ¹⁷O-NMR spectra of 1,4-naphthoquinone and 5-hydroxy-1,4-naphthoquinone (juglone) have been recorded in CDCl₃ solution at 40°. In juglone the ¹⁷O resonance of the carbonyl peri to the OH group was displaced by 70 ppm to low frequency relative to the resonance in the para-position. It is shown that this chemical shift arises mainly from intramolecular H-bonding, the substituent and steric effects being one order of magnitude smaller. Large carbonyl ¹⁷O chemical shifts between ?34 and ?100 ppm were also observed in a series of aromatic aldehydes and ketones where intramolecular H-bonds of the C?O…?H? O type are formed. The H-bond-induced carbonyl ¹⁷O chemical shifts were linearly correlated with both the ¹⁷O and ¹H chemical shifts of the OH groups. They represent a most sensitive measure of the strength of intramolecular H-bonds. The ¹⁷O resonances of the OH groups were directed to high frequency on H-bonding. Analysis of the ¹⁷O chemical shifts in 2,2′-dihydroxy-benzophenone showed clearly that the two OH groups build H-bonds simultaneously to the single carbonyl group. The ¹⁷O linewidths decreased strongly on H-bonding; the linewidth of the H-bonded carbonyl O-atom in juglone, for example, was reduced by 25% with respect to that of the free carbonyl O-atom. The carbonyl O-atom quadrupole coupling constants in juglone, evaluated from the combined use of ¹³C and ¹⁷O relaxation times, were 9.5 and 11.0 MHz, respectively. No correlation was observed between the H-bond-induced ¹⁷O chemical shifts and the variations in ¹⁷O quadrupole coupling constants.     

17O NMR studies on (E)-3-arylidenechromanone and -flavanone derivatives

¹⁷O NMR data for some 3-arylidenechromanones and -flavanones are discussed in terms of mesomeric and steric substituent interactions. The transmission of long-range substituent effects was studied. ¹⁷O NMR chemical shifts were correlated with Hammett σ_p⁺ values for 4′-substituted derivatives. Copyright © 2001 John Wiley & Sons, Ltd.     

Ab initio MO calculations and 17O NMR at natural abundance of para-substituted acetophenones

¹⁷O NMR chemical shifts and calculated (ab initio MO theory) electron densities are reported for a series of para-substituted acetophenones, X? C₄H₆? COCH₃, where X = NH₂, OCH₃, F, Cl, CH₃, H, COCH₃, CN, NO₂. The ¹⁷O shifts are very sensitive to the para substituent and cover a range of some 51 ppm. Donors induce upfield shifts and acceptors downfield shifts. The substituent chemical shifts (SCS) correlate precisely with σ_I and σ_R⁺ using the Dual Substituent Parameter (DSP) method. The derived transmission coefficients ρ_I and ρ_R indicate that polar and resonance mechanisms contribute approximately equally to the observed substituent effects. The shifts also correlate well with calculated π-electron densities (slope = 1500 ppm per electron) confirming their electronic origin. λ values are also reported, and the role of the average excitation energy, ΔE, in determining ¹⁷O SCS values is discussed. It is concluded that variations in ΔE are minor and that the local Δ-electron density is the dominant feature controlling ¹⁷O SCS values.     

13C NMR STUDY OF SUBSTITUENT EFFECTS IN 1,2,4-OXADIAZOLE AND 1,2,4-THIADIAZOLE DERIVATIVES

¹³C chemical shifts of C═N, C═O and C═S carbons of 3,4-disubstituted-1,2,4-oxadiazole-5-ones(thiones) and 3,4-disubstituted-1,2,4-thiadiazole-5-ones have been determined in CDCl₃ solution. Exceptionally good Hammett correlations of ¹³C NMR chemical shifts of these carbons with σ were obtained. The negative ρ values observed (inverse substituent effects) indicate π-polarization of the C═N, C═O and C═S bonds. As expected, the long distance C═O and C═S ¹³C chemical shifts were found less susceptible to substituent-induced electronic changes.     

Oxygen-17 NMR spectroscopy: σI dependence for the substituent chemical shifts of 3-substituted pyridine 1-oxides

¹⁷O NMR spectra were measured at natural abundance in DMSO for a series of 3-substituted pyridine 1-oxides and their substituent chemical shifts were found to be correlated with σ_I substituent constants and not with ΔpK_a values.     

Saturated amine oxides: part 10. hydroacridines: part 32. linear 17O/13C and 13C/13C chemical shift correlations between saturated azaheterocyclic N‐methylamine N‐oxides,and the methiodides of their parent amines

Two kinds of good linear correlations were found between the chemical shifts of saturated six‐membered azaheterocyclic N‐methylamine N‐oxides and the chemical shifts of the methiodides of their parent amines. One of the correlations occurs between the ¹⁷O chemical shift of the N⁺―O^– oxygen in the N‐oxides and the ¹³C chemical shift of the N⁺―CH₃ methyl group analogously situated in the appropriate methiodide (r = 0.9778). This correlation enables unambiguous configuration assignment of the N⁺―O^– bond, even if the experimentally observed ¹⁷O chemical shift of only one N‐epimer is available, provided the ¹³C chemical shifts of both N⁺―CH₃ groups in the methiodide are known and assigned; furthermore, it can be used also for the estimation of ¹⁷O chemical shifts of the N⁺―O^– oxygens in N‐epimeric pairs of N‐oxides, for which observed ¹⁷O data hardly become available. The second correlation is observed between the ¹³C chemical shift of the N⁺―CH₃ methyl group in the N‐oxides and the ¹³C chemical shift of the N⁺―CH₃ methyl group analogously situated in the appropriate methiodide (r = 0.9785). It can be used for safe configuration assignment of the N⁺―CH₃ group and, indirectly, also of the N⁺―O^– bond in an amine N‐oxide, even if no ¹⁷O NMR data, and the ¹³C chemical shift of only one N‐epimer is available. Copyright © 2015 John Wiley & Sons, Ltd.     

17O NMR investigation of p,π-interactions in α,β-unsaturated and aromatic ethers

The ¹⁷O chemical shifts have been measured for 51 α,β-unsaturated and aromatic ethers. A good linear relationship is found between the ¹⁷O chemical shifts in a series of dialkyl and the corresponding alkyl vinyl ethers. Hence, the extent of p,π-interaction, between the oxygen atom and the vinyl group in the latter series does not, apparently, depend upon branching at the α-carbon atom in the alkyl moiety of these ethers. The PhOBu^t ether, however, as compared to the other alkyl phenyl ethers, shows significantly weakened p,π-interaction, which is apparently related to the steric hindrance of this interaction. The effects of two unsaturated groups upon the ¹⁷O chemical shifts in the corresponding ethers are non-additive. This is undoubtedly a result of ‘rivalry’ between these groups for conjugation with the lone electron pairs on the ethereal oxygen. The ¹⁷O chemical shift ranges of substituted methyl and vinyl phenyl ethers are nearly equal (≈30 ppm). An analysis of the ¹⁷O shielding for cyclopropyl ethers shows no observable p,σ-conjugation in these compounds. Excellent correlation (r>0.99) between the values of ¹⁷O chemical shifts and the calculated (MO LCAO SCF, CNDO/2) π-electron charges on the corresponding oxygen atoms look promising for experimental estimations of π-electron densities on the ethereal oxygen.     

Chemical shift correlations in the 13C, 17O and 31P NMR spectra of some Mo(CO)4((PPh2O)2Y(R)R′) (Y(R) = P(O), Si(Me); R′ = alkyl,haloalkyl, aryl) and [Mo(CO)4(PPh2O)2]2Si complexes

The syntheses and ¹³C, ¹⁷O, ²⁹Si and ³¹P NMR spectra of a series of Mo(CO)₄((PPh₂O)₂Y(R)R′) (Y(R) = P(O), Si(Me); R′=alkyl, haloalkyl, aryl) and [Mo(CO)₄(PPh₂O)₂]₂Si complexes are given. The chemical shift ranges of the cis and trans carbonyl ¹³C and ¹⁷O, phenyl C(1) ¹³C and ³¹P resonances are relatively large and, with the exception of the cis carbonyl ¹⁷O chemical shifts, the correlations between the chemical shifts of the various resonances are excellent. These correlations are consistent with the model of metal carbonyl ¹³C and ¹⁷O chemical shifts proposed by Bodner and Todd. In addition they allow the model to be extended to include the diphenylphosphinite ³¹P chemical shifts in these complexes. The excellent correlations may be due to the presence of the chelate ring which limits the rotation around the molybdenum-phosphorus bond and to the fact that all three groups directly bonded to the phosphorus remains constant.     

17O, 13C and 1H NMR and IR spectral study of crowded ketones: possible intramolecular C—H···O interactions

Unusual behaviour was observed in the study of the ¹⁷O, ¹³C and ¹H NMR and IR spectra of crowded (1‐adamantyl)alkyl ketones. As the size of the alkyl substituent is increased, abnormal upfield chemical shifts in the ¹³C NMR and downfield shifts in the ¹⁷O NMR of the carbonyl group, as well as downfield shifts in the ¹H NMR of the adamantyl γ'‐protons, are found. In the IR spectrum, the ν_C?O stretching frequencies of the ketones with bulky substituents show considerable red shifts. Correlation of the NMR shifts with the number of γ‐carbon atoms of the alkyl substituents and comparison with the IR results indicated that there is an intramolecular through‐space CH···O interaction in crowded ketones. This was supported by the results of ab initio calculations. Copyright © 2002 John Wiley & Sons, Ltd.     

17O NMR spectroscopy of nucleoside derivatives; bonding characteristics of pyrimidine carbonyls

Various uridine derivatives have been selectively enriched (≈ 50%) at the 04 and 02 oxygens, and the ¹⁷O chemical shifts and linewidths measured. The chemical shifts, which are primarily dependent on π-bond-order and hydrogen bonding, show effects that are selective for the 02 and 04 oxygens. In particular, significantly more H-bonding to D₂O and higher π bond order are found for the 04 oxygen.     

NMR Spectra of Cyclic Nitrones. 7. The Influence of Substituents and a Hydrogen Bond on <Superscript>14</Superscript>N and <Superscript>17</Superscript>O Chemical Shifts in Derivatives of 3-Imidoazoline 3-Oxide

Derivatives of 3-imidazoline 3-oxide have been studied by ¹⁴N and ¹⁷O NMR methods. Regularities of the influence of substituents and of a hydrogen bond on chemical shifts have been made apparent. The range of changes of the chemical shifts of the nitrogen and oxygen nuclei of the nitrone group has been determined. Both in the ¹⁷O and in the ¹⁴N NMR spectra the signals of the amino derivatives are the highest field signals for the nitrone group, and the lowest field signals are the signals of the cyano derivatives in the series of derivatives investigated. Depending on the substituent (from amino to cyano group) the ¹⁷O chemical shifts varied over a range ∼155 ppm, but the interval of change of the ¹⁴N chemical shifts for the same substituents was ∼110 ppm. __________ Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 9, pp. 1336–1341. September, 2005.     

17O NMR study of isomeric 2-R-2-oxo-1,3,2-dioxaphosphorinanes

Oxygen-17 NMR spectra were obtained from the four pairs of isomeric 2-R-2-oxo-1,3,2-dioxaphosphorinanes, where R=OMe (2), NMe₂ (3), H (4) or Me (5). The isomerism has been previously shown to be configurational at phosphorus, with one isomer of each pair having an equatorial phosphoryl oxygen (a isomer), and the other an axial orientation for phosphoryl oxygen (b isomer). Only data for the phosphoryl oxygen are reported. Substitution of OMe or NMe₂ for H or Me produced upfield shifts of 27.9-41.8 ppm. In all cases, the chemical shifts of the a isomers were upfield of the b analogs, with differences of 7.9, 18.0, 20.3 and 8.6 ppm for 2—5, respectively. The absolute values of ¹J(³¹P¹⁷O) were 5–9 Hz larger for the a isomers.     

1H NMR spectra. Part 29§: proton chemical shifts and couplings in esters—the conformational analysis of methyl γ‐butyrolactones

The ¹H NMR spectra of 35 cyclic and acyclic esters are analysed to give the ¹H chemical shifts and couplings. The substituent chemical shifts of the ester group were analysed using three‐bond (γ) effects for near protons and the electric field, magnetic anisotropy and steric effect of the ester group for more distant protons. The electric field is calculated from the partial atomic charges on the O?C = O atoms, and the asymmetric magnetic anisotropy of the carbonyl group acts at the midpoint of the C = O bond. The values of the anisotropies Δχ_parl and Δχ_perp were for the aliphatic esters 10.35 and ?18.84 and for the conjugated esters 7.33 and ?15.75 (×10^?6 ų/molecule). The oxygen steric coefficients found were 104.4 (aliphatic C = O), 45.5 (aromatic C = O) and 16.0 (C–O) (×10^?6 Å⁶/molecule). After parameterisation, the overall RMS error for the data set of 280 entries was 0.079 ppm. The strongly coupled ¹H NMR spectra of the 2‐methyl, 3‐methyl and 4‐methyl γ‐butyrolactones were analysed and the methyl conformational equilibrium obtained from the observed couplings. The observed versus calculated density functional theory (DFT) ΔG(ax‐eq) was 1.0 (1.01), 0.34 (0.54) and 0.65 (0.71) kcal/mol res. The shielding effect of a methyl cis to a proton in the five‐membered lactone rings is ?0.40 ±0.05 ppm and deshielding trans effect 0.12 ±0.05 ppm, which is common to both five and six membered rings. The cis/trans isomerism in the vinyl esters methyl acrylate, crotonate and methacrylate and methyl furoate was examined using the ¹H chemical shifts. The calculated shifts of both the cis and trans isomers were in good agreement with the observed shifts. Copyright © 2012 John Wiley & Sons, Ltd.     

Oxygen-17 and Carbon-13 Nuclear Magnetic Resonance. Chemical shifts of unsaturated carbonyl compounds and acyl derivatives

The ¹⁷O and ¹³C chemical shifts (δ) of 14 α,β-unsaturated aldehydes and ketones and 33 acyl derivatives RXC = O (X = Cl, OH, OMe, OEt, NH₂ and R = H or alkyl) have been measured. In the unsaturated carbonyl series, a correlation exists between δ ¹⁷O and the π electron density at the β-carbon atom. From this correlation, an δ ¹⁷O of 530 ppm was extrapolated for the loss of one electron at the oxygen atom. In the acyl series, the δr ¹⁷O were also sensitive to changes in the polarity of the carbon-oxygen bond. A partial correlation between ¹⁷O-NMR. chemical shifts and the nuclear quadrupole coupling constants exists for aldehydes, ketones, esters and amides but not for acyl chlorides.     

Ion pairing in H<Subscript>2</Subscript>O and D<Subscript>2</Subscript>O solutions of lead nitrate,as determined with <Superscript>207</Superscript>Pb NMR spectroscopy

The temperature and concentration dependences of ²⁰⁷Pb NMR chemical shifts of Pb(NO₃)₂ in D₂O are reported. The results are analyzed in terms of exchange between a solvated lead ion and the Pb(NO₃)⁺ contact-ion pair. Predictions of the chemical shift difference between the aquated ion and contact-ion pair are carried out for the gas-phase entities and for the solvated species with a DFT calculation. Previously reported data on ²⁰⁷Pb NMR chemical shifts of Pb(NO₃)₂ in H₂O are reevaluated. From the analysis, the enthalpy of dissociation of the contact-ion pair is found to be–42.3±1.0 kJ/mol.     

17O-enriched α-azohydroperoxides: 17O NMR spectroscopy, 17O-labeling reagents

¹⁷O-enriched α-azohydroperoxides, prepared by autoxidation, are efficient ¹⁷O-labeling reagents; ¹⁷O NMR (CD₃CN) of 2 showed broad signals at δ 254 and 204 PPM; the solvent dependence of the ¹⁷O chemical shifts and the kinetics of ionic oxidations are interrelated.     

A 17O NMR study. On the correlation of ionization potentials and 17O chemical shifts in 4-substituted pyridine-N-oxides and 4-substituted N-(benzylidene)phenylamine-N-oxides

The approximate linear correlation between first ionization potentials and ¹⁷O chemical shifts for 4-substituted pyridine-N-oxides and 4-substituted N-(benzylidene)phenylamine-N-oxides is presented. The obtained results are used to estimate the ¹⁷O chemical shift in 4-N,N-dimethyl pyridine-N-oxide. The obtained results are discussed in relation to similar correlations in substituted benzenes and pyridines. The linear correlation between oxygen atomic net charge and ¹⁷O chemical shift for 4-substituted pyridine-N-oxides and 4-substituted N-(benzylidene)phenylamine-N-oxides is also presented.     

Struktur-Reaktivitätsuntersuchungen mit heterosubstituierten Nitrilen—VII: 13C-, 14N- und 19F-NMR-Spektroskopische Untersuchungen an Verbindungen vom Typ Ar?X?CN

The chemical shifts of aromatic nitriles of the general structure para-Y? C₆H₄? X? CN with X = O, S, Se and N(CH₃) have been investigated by the ¹³C NMR technique. For cyanates (X = O) the ¹⁴N shifts and for Y = F the ¹⁹F shifts were likewise measured. The chemical shifts and the corresponding ¹³C shift increments Δ_n have been found to correlate with the appropriate substituent constants σ_R⁰, σ_p⁰ and σ_I, as well as with the π-electron densities calculated in the PPP approximation.     

All‐atom Molecular Dynamic Simulations Combined with the Chemical Shifts Study on the Weak Interactions of Ethanol‐water System

All‐atom molecular dynamics (MD) simulation combined with chemical shifts was performed to investigate the interactions over the entire concentration range of the ethanol (EtOH)‐water system. The results of the simulation were adopted to explain the NMR experiments by hydrogen bonding analysis. The strong hydrogen bonds and weak C–H···O contacts coexist in the mixtures through the analysis of the radial distribution functions. And the liquid structures in the whole concentration of EtOH‐water mixtures can be classified into three regions by the statistic analysis of the hydrogen‐bonding network in the MD simulations. Moreover, the chemical shifts of the hydrogen atom are in agreement with the statistical results of the average number hydrogen bonds in the MD simulations. Interestingly, the excess relative extent of η_rel^E calculated by the MD simulations and chemical shifts in the EtOH aqueous solutions shows the largest deviation at x_EtOH≈0.18. The excess properties present good agreement with the excess enthalpy in the concentration dependence.     

Über die pH-Abhängigkeit der 31P- und 31C-NMR-Spektren von Cyclohexan-, Cyclohexen- und Benzenphosphonsäuren

About the pH-Dependence of ³¹P and ¹³C NMR Spectra of Cyclohexane-, Cyclohexene-, and Benzenephosphonic Acids ³¹P and ¹³C chemical shifts as well as P? C coupling constants of phosphonic acids are affected by the pH of the investigated solutions. This dependence was measured for aqueous solutions of cyclohexane-, cyclohex-1-ene-, cyclohex-2-ene-, cyclohex-3-ene-, and benzenephosphonic acid. It is shown that the ³¹P chemical shift and various ¹³C chemical shifts are well suited for the determination of pK_a values.