The effect of solvents on the 13C NMR chemical shifts of the carbonyl carbon and the rotational barriers of N,N-dimethylbenzamide

The ¹³C solvent induced chemical shifts (SICS) of the carbonyl carbon and the thermodynamic barriers to rotation about the C? N bond of N,N-dimethylbenzamide are linearly related to the solvent parameter, E_T(30). A multi-parametric solvent parameter approach indicates that the SICS are influenced equally by polar effects and hydrogen-bond donor effects. Rotational barriers for N,N-dimethylbenzamide may, in principle, be determined by measurement of the ¹³C chemical shift of the carbonyl carbon in a particular solvent.     

15N-, 1H-, and 13C-NMR chemical shifts and electronic properties of aromatic diamines and dianhydrides

We measured the ¹⁵N-, ¹H-, and ¹³C-NMR chemical shifts for a series of aromatic diamines and aromatic tetracarboxylic dianhydrides dissolved in DMSO-d₆, and discuss the relationships between these chemical shifts and the rate constants of acylation (k) as well as such electronic-property-related parameters such as ionization potential (IP), electronic affinity (EA), and the energy ε of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). The ¹⁵N chemical shifts of the amino group of diamines (δ_N) depend monotonically on the logarithm of k (log k) and on IP. We inferred the reactivities of diamines whose acylation rates have not been measured from their δ_N, and we propose an arrangement of diamines in the order of their reactivity. The ¹H chemical shift of amino hydrogens (δ_H) and the ¹³C chemical shift of carbons bonded to nitrogen (δ_C) are roughly proportional to δ_N, but these shifts are not as closely correlated with log k and IP. Although the ¹³C chemical shifts of the carbonyl carbon of dianhydrides (δ_C,) varies much less than the δ_C and δ_N of diamines, δ_C, can be an index of acylation reactivity for dianhydrides because it is closely correlated with ε_LUMO. These facts indicate that the chemical shifts of diamines and dianhydrides are displaced according to their electron-donor and electron-acceptor properties, and that these chemical shifts can be used as indices of the electronic properties of monomers. Changes in reactivity caused by the introduction of trifluoromethyl groups into diamines and dianhydrides are inferred from the displacements of δ_N and δ_C © 1992 John Wiley & Sons, Inc.     

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.     

13C NMR spectroscopic studies of the behaviors of carbonyl compounds in various solutions

¹³C NMR spectroscopic studies were performed for carbonyl compounds having a hydroxyl group, a carboalkoxy group, an acetoxy group, or a carboxyl group in various solvents with different polarities for observation of their behaviors of ¹³C NMR chemical shifts of carbonyl carbons in solutions. It was found that the chemical shifts of the carbonyl carbons in ¹³C NMR have good correlation with the empirical parameter for solvent polarities, E_T^N, depending on the structures. Inter- or intramolecular hydrogen bonding and dipolar-dipolar interactions appear to play a key role in this observation.     

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.     

Determination of aromaticity indices of thiophene and furan by nuclear magnetic resonance spectroscopic analysis of their anilides

A series of m‐ and p‐substituted anilides of benzoic acid, 2‐thienoic acid, and 2‐furoic acid were prepared and their ¹H and ¹³C nmr spectroscopic characteristics were examined. In general, good correlations were observed between the chemical shifts of proton and carbon signals of the acyl aromatic rings and the Hammett σ. Plots of the chemical shift values of the carbonyl carbons of the benzanilides against those of the 2‐thienamides and 2‐furamides gave an excellent correlation and the values of the slopes are 0.79 and 0.52, respectively, in dimethyl sulfoxide‐d₆. The slopes could be considered as a set of aromaticity index.     

Unusual deuterium isotope effects in 13C NMR spectra of trans-stilbene

A detailed analysis of the ¹³C NMR spectra of trans-stilbene and ten deuteriated trans-stilbenes has been undertaken. Some unusual deuterium isotope effects on carbon–hydrogen spin–spin coupling constants could not be explained by the ordinary primary and secondary isotope effects. The positive and negative changes of ⁿJ(CH) were interpreted in terms of a steric effect, the vibrational influence of the C? D bond and the para-effect induced by deuterium. In this respect, deuterium behaves as a real substituent with electronic properties different from those of hydrogen. The deuterium isotope effects on ¹³C NMR chemical shifts and carbon–deuterium coupling constants have also been determined.     

Einflüsse von substituenten in 6-stellung auf die 13C-chemischen verschiebungen der kohlenstoffatome des purins

The ¹³C chemical shifts of purines substituted in the 6 position are reported. Signals are assigned on the basis of general chemical shift rules and by proton “off-resonance” decoupling. Substituent effects (Z_6i) of the substituent X in the 6 position of purine on the ¹³C chemical shifts of purine ring carbon atoms are determined. A linear correlation exists between the substituent effects of X on C-6 (Z₆₆) and Pauling's electronegativity values E_x of the substituent X.     

Substituent and solvent effects on tautomeric equilibria of barbituric acid derivatives and isoterically related compounds

N-Mono and N,N-dialkyl/diarylbarbituric acids exist in solution as a single tautomer. The ¹³C nmr spectroscopy shows that they are present in the triketo form in a number of polar and non-polar solvents. 2-Thiobarbituric acid derivatives, however, show extensive tautomerization. Their ¹³C chemical shift assignments were achieved by utilizing models 11a , 11c , 12b and 12d and from which relative tautomer distribution ratios were determined. These ratios were correlated with the dielectric constant of the various solvents (?). Thio-barbituric acids also formed adducts with solvents having carbonyl groups, characteristic observed only with barbiturates possessing the thione or thiophenolic group. 6-Amino and 6-methyluracils and thiouracils exist in DMSO solution as stable “ene” forms as do orotic acid, 24 , and its thio analogue 25. Compound 25 undergoes disproportionantion and tautomerization when heated or on prolonged standing in solution. Literature contradictions regarding the structure of “4,6-dihydroxypyrimidine,” 26 , were resolved and its tautomers in solution correctly assigned by ¹³C nmr. Anions of barbiturics and related systems exist in one of the two possible types A and B, depending on whether ring nitrogens are substituted (type A), or not (type B). Rapid H/D exchange at C₅ was evident from C-deuterium coupling. The redistribution of charge through C₄(C₆) carbonyl groups shown by ¹³C shifts of carbonyl carbon atoms of up to 10 ppm as compared to the CO carbons of the neutral species was evident.     

13C-NMR.-Spektren N-acylierter Indoline. Einfluss der Orientierung der Carbonylgruppe auf die chemische Verschiebung

¹³C-NMR. spectra of N-acylated indolines. Effect of the orientation of the carbonyl group on the chemical shift Large downfield shifts are observed in the ¹³C-NMR. spectra of indolines and related compounds induced by electric field effects of carbonyl and oxime groups.     

13C and 1H chemical shifts in 2-benzylidene[3]ferrocenophane-1,3-diones. Elucidation of the substituent dependence of carbonyl chemical shifts in different carbonyl derivatives

The ¹³C and ¹H chemical shifts of the ferrocene moiety, as well as the carbonyl carbons and styrene moiety, of substituted 2-benzylidene[3]ferrocenophane-1,3-diones have been assigned. Correlations of ¹³C substituent chemical shifts of both carbonyl carbons with the Hammett constants have been found, and the effect of the transmission of substituent effects on these chemical shifts through the styrene moiety is discussed. An explanation is given for the different sensitivities of the carbonyl carbon chemical shifts to the electronic effect of substituents in mono- and dicarbonyl derivatives.     

13 C NMR spectroscopy of substituted xanthones—II: 13C NMR spectral study of polyhydroxy xanthones

The ¹³C NMR chemical shifts of eleven hydroxy-, two hydroxymethoxy xanthones, and xanthone-C-glucoside, mangiferin, are presented and analyzed. Hydroxy substituent effects depending on substituent position as well as on shielded ring carbon position have been evaluated. Hydroxy substituent increments for xanthones are proposed. Effects of hydroxylation on carbonyl carbon shift and the methylation of hydroxy group and the corresponding shift increments which are of diagnostic value have been observed and discussed.     

1H and 13C NMR study of 8-hydroxyquinoline and some of its 5-substituted analogues

¹H and ¹³C NMR spectra of 8-hydroxyquinoline (oxine) and its 5-Me, 5-F, 5-Cl, 5-Br and 5-NO₂ derivatives have been studied in DMSO-d₆ solution. The ¹H and ¹³C chemical shifts and proton–proton, proton–fluorine, carbon–proton and carbon–fluorine coupling constants have been determined. The ¹H and ¹³C chemical shifts have been correlated with the charge densities on the hydrogen and carbon atoms calculated by the CNDO/2 method. The correlation of the ¹H and ¹³C chemical shifts with the total charge densities on the carbon atoms is approximately linear (r_H² = 0.85, r_C² = 0.84). The proton in peri position to the nitro group in 5-NO₂-oxine is an exception.     

Substituent chemical shifts of the organotin moiety in compounds of the type RSnMenCl3 − n (n = 0 to 3; R = n-alkyl)

The ¹³C and ¹¹⁹Sn NMR spectra of 33 organotin compounds of the type RSnMe_nCl_{3 ? n} and related types are discussed. The substituent effects of the groups SnMe₃, SnMe₂Cl, SnMeCl₂ and SnCl₃ (and of some related groups) on the carbon chemical shifts in the alkyl group R have been determined; the SnMe₃ group causes a small upfield shift of the carbon attached to it, while the other groups cause downfield shifts. The shifts show a monotonic change on replacing methyl groups in Me₃Sn by chlorine atoms. The effects on carbons further removed from the tin atom are discussed. Variation in R causes little change in ⁿJ(Sn? C) or δ(¹¹⁹Sn).     

Substituent effects on 13C and 1H chemical shifts in 3-benzylidene-2,4-pentanediones

Substituent effects on the ¹³C and ¹H chemical shifts have been studied for derivatives of 3-benzylidene-2, 4-pentanedione. A significant correlation has been found between chemical shifts of the Z carbonyl group (C-2) and Hammett constants, while no correlation has been found for the E carbonyl group (C-4). Attempts have been made to determine the structural factors which influence these effects. The conformation of 3-benzylidene-2, 4-pentanediones has been determined by ¹³C and ¹H NMR spectroscopy.     

13C NMR investigation of tautomeric and acid-base equilibria of pyrazolone T and three analogs

Pyrazolone T and three derivatives have been characterized by ¹³C and, in part, ¹⁵N nmr at several pH values. The ¹³C chemical shifts have been assigned at, or near, the equivalence points and pK_a values of these four compounds. Closely situatued quaternary carbon signals were assigned by means of a heteronuclear chemical shift correlation (FLOCK) experiment which is sensitive to, and was optimized for, 3-bond C-H couplings. The ¹³C chemical shift data indicate the existence of both tautomeric and acid-base equilibria and demonstrate that the four congeners exist in surprisingly different forms at certain common pH values.     

Cholesterol and phospholipids in membranes: The hydrogen bonding problem

Cholesterol and phospholipids are major components of biological membranes. The role of cholesterol in membranes is not metabolic and is known to be a regulator of membrane fluidity which in turn regulates various biological phenomena. We have studied the nature of cholesterol and phospholipid interaction in artificial membranes using ¹³C NMR spectroscopy. This involved preparation of phospholipids specifically labeled with ¹³C in the ester carbonyl group. Though the chemical shift data did not provide very useful information the T₁ and T₂ measurements indicated that previously proposed H bonding between the ester carbonyl group and hydroxyl group of cholesterol seems unprobable.     

Radical cyclocopolymerization of divinyl ether and maleic anhydride. A 13C-NMR study of the polymer structure

The structure of the 1:2 copolymer of divinyl ether and maleic anhydride was investigated by ¹³C-NMR spectroscopy. The polymer contains the bicyclic unit composed of one molecule of each monomer and the maleic anhydride unit. The carbon chemical shift for these units was calculated on the basis of the chemical shift of many model compounds. The major peaks of the cyclopolymer prepared in chloroform were consistent with the presence of the symmetrical bicyclic unit with cis junction and the trans monocyclic anhydride unit. The carbonyl carbon spectrum for the copolymer obtained in a mixed solvent of acetone and CS₂ suggested the predominant formation of the unsymmetrical bicyclic unit. The polymerization process was discussed on the basis of these results.     

13C n.m.r. investigation on the first and second nitrogen protonation in the diazanaphthalenes

The ¹³C chemical shifts of the diazanaphthalenes have been recorded as a function of the pH value, providing classical titration curves. From these curves the pK₁ and pK₂ values have been determined taking into account the activity coefficients. The changes in ¹³C chemical shift under the influence of nitrogen protonation (Δδ) can be described by two protonation parameter sets.     

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.