Carbon-13 NMR spectra of a series of para-substituted phenyl isothiocyanates. Linear free energy relationships for carbon-13 substituent chemical shifts

Carbon-13 chemical shifts are reported for 16 para-substituted phenyl isothiocyanates measured at 1 and 10 mol % in chloroform-d solution. Data for the ? N?C?S group were not obtained at 1 mol %, but concentration effects for the other resonances were negligible. Hammett, dual substituent parameter (DSP) and DSP-nonlinear resonance (DSP-NLR) analyses were used to evaluate substituent effects on the substituent chemical shifts (SCS) for the ipso-carbon (C-1), C-2, and the ? N?C?S carbon atoms. A good Hammett correlation was observed for C-1 (ν_p⁺ = 8.1 ppm, r = 0.98 at 1 mol %) but was improved for the higher order correlations with the following results, DSP:ρ _I = 5.4, ρ_R° = 22.2, r = 0.998; DSP-NLR: ρ_I = 5.6, ρ_R° = 20.5, ? = ?0.22, r = 0.999. The 10 mol % data were very similar except the value of ? was ?0.26 and confirms the phenyl-bonded ? N?C?S moiety as a mild electron acceptor substituent. Hammett correlations were unsuccessful for the C-2 data, but fairly good results were obtained from the higher order analyses. For the 1 mol % data, DSP: ν_I = 1.6, ν_R° = ?2.0, r = 0.976; DSP-NLR: ν_I = 1.8, ν_R° = ?2.6, ? = 1.1, r = 0.982. Excellent correlations were obtained for the 10 mol % ? N?C?S carbon data. Hammett: ν_p° = 6.2, r = 0.997; DSP: ν_I = 5.9, ν_R° = 7.0, r = 0.997; DSP-NLR: ν_I = 5.8, ν_R° = 7.6, ? = 0.25, r = 0.997. The positive ν values in these three correlations contrast the negative values usually observed for carbonyl and thiocarbonyl carbons, and more closely parallel results previously reported for the β-carbon of styrenes and benzylidene anilines with para-substituents in the aniline ring.     

Carbon-13, nitrogen-15 and oxygen-17 NMR chemical shifts of substituted 1-phenyl-2-pyrrolidinones

The carbon-13 and nitrogen-15 NMR chemical shifts and the direct carbon—proton coupling constants of 1-phenyl-2-pyrrolidinone and its 2′-methyl, 3′-methyl, 4′-methyl, 2′-chloro, 3′-chloro, 4′-chloro, 3′-methoxy, 4′-methoxy and 4′-nitro derivatives were measured in dimethyl sulfoxide. The oxygen-17 NMR chemical shifts of some of the compounds were determined in acetone.The effect of substituents on the chemical shifts of carbonyl carbons correlates well with the Hammett substituent parameters and the nitrogen chemical shifts seem to follow a similar trend. The variation of the oxygen chemical shift due to the substituents is small. The chemical shifts of aromatic carbons can mainly be derived using the substituent parameters of benzene; some deviation probably due to steric effects is observable, however.     

Study of rotation barriers in N,N-dimethylnicotinamide and quaternary salt derivatives by proton NMR. A correlation of rotation barriers of N,N-dimethylamides with carbon-13 chemical shifts

The rotational barrier in the N,N-dimethylamide group is studied by proton NMR for N,N-dimethylnicotinamide and two quaternary salts in aqueous solution. The carbon-13 NMR spectra of a number of aromatic N,N-dimethylamides are discussed. A roughly linear correlation is found between the ΔG^≠ values of the rotation barriers and the chemical shifts of the carbon atom substituted by the amide group.     

Linear free energy relationships for carbon-13 NMR chemical shifts of substituted benzoic acids1

In order to examine ¹³C-SCS of substituted benzoic acids, chemical shifts of the acid form (I) and the dissociated form (II) have been obtained separately. Single substituent parameters, σ⁰ or σ⁺ are correlated with the shifts for the carboxyl (δ_co) or ipso carbons (δ_ipso), respectively. Among the available five equations which are developed for the analysis with dual (or divided) substituent parameters (DSP), the Swain-Lupton equation (eqn 3) and the Taft-Swain-Lupton equation (eqn 4) give much better correlations, not only for δ_co and δ_ipso but also for the results for ring carbons (C(2), C(5), C(6)), except for those attached to or neighboured by substituents. It is concluded that the SCS of aromatic compounds are best analyzed with substituent parameters derived from reactions or equilibria on the basis of linear free energy relationships.     

A Carbon-13 and nitrogen-15 NMR study of some nitrogen heterocycles

A number of phenyl-substituted nitrogen heterocycles and the corresponding 2,6-diethylphenyl derivatives were examined by 13 C nmr in order to verify that the heterocyclic ring system was the same in both series. The heterocyclic ring carbon shifts did not differ substantially between the phenyl and 2,6-diethylphenyl derivatives for all systems except one, where substantial differences were also seen in the ¹⁵N chemical shifts. Acetylated derivatives were prepared to confirm the cyclization mode in this latter case.     

13C and 15N NMR chemical shifts of alkylsubstituted benzonitriles

¹³C NMR data of 14 and ¹⁵N NMR data of four alkylsubstituted benzonitriles are reported and discussed in relation to substituent effects and stereochemistry.     

Carbon-13 nuclear magnetic resonance spectra of isomeric oxaziridines. Effects of the nitrogen lone pair on carbon-13 chemical shifts

The carbon-13 n.m.r. spectra of several oxaziridines were measured. Aliphatic and aromatic ipso carbon atoms trans to the lone pair of nitrogen in oxaziridines were shifted upfield by c. 9 ppm, and 3.4 ppm, respectively, in comparison with isomers of inverted configuration. The results suggest that the nitrogen lone pair is partially responsible for the observed upfield shifts.     

The carbon-13 and nitrogen-15 nuclear magnetic resonance spectra of uroporphyrinogens I and III

Due to their sensitivity to light and air, porphyrinogens are not normally isolated, but are routinely analyzed by oxidation to the corresponding porphyrin. We report herein the ¹³C- and ¹⁵N-NMR spectra of uroporphyrinogens I and III in their “native state”, multiply labelled with ¹³C and ¹⁵N, and at natural abundance (¹³C only).     

Solid-state nitrogen-15 NMR and quantum chemical study of N,N-dimethylaniline derivatives

In this study the components of the nitrogen chemical shift (CS) tensor are examined for a series of para substituted N,N-dimethylaniline derivatives. This is done through measurement of the 15N NMR spectra of powder samples and through quantum chemical calculations on the isolated molecules. Experiments and calculations show that the isotropic CS, delta(iso), decreases with increasing electron donating ability of the para substituent, in agreement with previous solution studies. More importantly, this study shows that this decrease in the isotropic (solution) CS is due to decreasing values of the CS tensor component delta(11) and component delta(33). The component delta(22) is essentially invariant to the electron donating/withdrawing ability of the para substituent. Through Ramsey's theory of nuclear magnetic shielding, it can be seen that the variation in delta(11) and delta(33), and hence delta(iso), is due to changes in the n-pi* and the sigma-pi* energy gaps in N,N-dimethylaniline. This, in turn, is a result of the change in the energy of the pi* molecular orbital with change in the pi-electron donating ability of the para substituent. The effects of nitrogen inversion on the components of the nitrogen CS tensor components are also discussed. This study also shows the feasibility of performing 15N cross-polarization experiments on nonspinning powder samples at natural isotopic abundance.     

Carbon-13 fourier transform NMR studies of organotin compounds : III. Carbon-13 chemical shifts and tin-119—carbon-13 spin—spin coupling constants in acyclic,monocyclic and bicyclic organostannanes

Carbon-13 NMR parameters for 33 organotin compounds with a variety of structurul features were investigated in order to obtain information about the relationship between their structure and ¹³C NMR parameters. It was found that the substitution of a proton by a trialkyltin group generally produces an upfield shift for the directly bonded carbon. The γ-nuclei usually resonate at lower fields except where there is appreciable steric strain while the β-carbons undergo relatively constant shifts of approximately 3.5 to 4.5 ppm to lower fields. The magnitude of direct bond coupling Jz-sfnc;¹J(¹¹⁹Sn¹³C)z-sfnc; is influenced by the hybridization of the tin and the directly attached carbon atoms. In rigid organitins, the vicinal coupling constants show a Karplus type variation. In aliphatic organotins, the values of the vicinal¹¹⁹Sn¹³C coupling indicates a flexible molecular framework with a clear cut preference for certain conformations.     

Carbon-13 chemical shifts and 13C?15N coupling constants of 3,4-dihydroisoquinoline-15N,its 15N-oxide and their conjugate acids

On protonation of 3,4-dihydroisoquinoline-¹⁵N ¹J(¹³C—1, ¹⁵N) is increased by a factor of five and ¹J(¹³C—3, ¹⁵N) changes its sign, while on protonation of 3,4-dihydroisoquinoline-¹⁵N-oxide the coupling constants, including the relatively large ¹J(¹³C—1, ¹⁵N), remain practically constant, although significant alterations of the ¹³C chemical shifts take place.     

NMR study of organosilicon compounds. III—Silicon-29, nitrogen-14, carbon-13 and proton NMR spectra of silylakylamines

²⁹Si, ¹⁴N ¹³C and ¹H NMR data are presented for a series of homologous (methylethoxysilyl)alkylamines of the type (CH₃)_3?n(C₂H₅O)_nSi(CH₂)_mNH₂(n=o to 3; m = 1 to 4). The measured ¹³C and ¹H chemical shifts correlate with the total net charges Q_A on the corressponding atoms, estimated by the Del Re method. ¹⁴N and ²⁹Si chemical shifts which do not show simple linear relationships to the charges are found to correlate with the relative basicities of the compounds. The influence of the remote substituent (? NH₂ and others) on the ₂₉Si chemical shifts is shown to depend on the number and nature of substituents directly on the silicon atom. Argyments for d-orbital participation in the Si? O bounds are given. The chemical shifts of ²⁹Si, ¹⁴N and ₁₃C nuclei are not consistent with the fromation of intramolecular ‘long bonds’ between the solicon and nitrogen atoms in aliphatic silymethylamines.     

13C and 15N NMR spectra of high‐energy polyazidocyanopyridines

¹³C and ¹⁵N NMR spectra of high‐energy 2,4,6‐triazidopyridine‐3,5‐dicarbonitrile, 2,3,5,6‐tetraazidopyridine‐4‐carbonitrile and 3,4,5,6‐tetraazidopyridine‐2‐carbonitrile are reported. The assignment of signals in the spectra was performed on the basis of density functional theory calculations. The molecular geometries were optimized using the M06‐2X functional with the 6‐311+G(d,p) basis set. The magnetic shielding tensors were calculated by the gauge‐independent atomic orbital method with the Tao–Perdew–Staroverov–Scuseria hybrid functional known as TPSSh. In all the calculations, a polarizable continuum model was used to simulate solvent effects. This approach provided accurate predictions of the ¹³C and ¹⁵N chemical shifts for all the three compounds despite complications arising due to non‐coplanar arrangement of the azido groups in the molecules. It was found that the ¹⁵N chemical shifts of the N_α atoms in the azido groups of 2,4,6‐triazidopyridines correlate with the ¹³C chemical shifts of the carbon atoms attached to these azido groups. Copyright © 2016 John Wiley & Sons, Ltd.     

Carbon-13 NMR spectra of all the isomeric methyl hydroxy- and acetoxyoctadecanoates. Determination of chemical shifts by deuterium isotope effects

Carbon-13 NMR spectra of the 17 isomeric methyl hydroxyoctadecanoates and the corresponding acetate derivatives have been measured and chemical shifts assigned to most carbons. Sixteen specifically deuterated hydroxy esters, and their acetates, were employed to make unambiguous assignments from the deuterium isotope effects on the spectra. When substituents are separated from the ends of the chain by 2–3 methylene groups their effects are largely additive. Long range effects of the hydroxyl group were γ, +0.01; δ, ?0.09; ε, ?0.11; ζ, ?0.06; η, ?0.05; and θ, ?0.04 ppm, and of the acetate group were γ, ?0.20; δ, ?0.20; ε, ?0.16; ζ, ?0.11; η, ?0.08 and θ, ?0.07 ppm, showing that they extend across seven methylene groups.     

Carbon-13 NMR studies. 96—carbon-13 spectra of several polyhydroxylated 9,10-dihydrophenanthrene and phenanthrene derivatives

The ¹³C spectra of eleven 9,10-dihydrophenanthrene and twelve phenanthrene derivatives were recorded in order to examine the effects of hydroxy and methoxy substitution on the skeletal carbon shieldings. For each series, the parent compounds were the 2,4-disubstituted derivatives with the others bearing 5-, 6- and/or 7-substituents, including the naturally occurring orchinol and loroglossol. The data should be helpful for examinations of related compounds. Some examples of particularly facile hydrogen-deuterium exchange at aryl positions were encountered. Since these occur under apparently neutral conditions, suitable caution may be required in examinations of related compounds.     

Direct hydrogen-1, carbon-13, and nitrogen-15 NMR study of magnesium(II)-isothiocyanate complexing

A hydrogen-1, carbon-13, and nitrogen-15 NMR study of magnesium(II)-isothiocyanate complexation in aqueous mixtures has been completed. At temperatures low enough to slow proton and ligand exchange, separate¹H,¹³C, and¹⁵N NMR signals are observed for coordinated and bulk water molecules and anions. The¹H NMR spectra reveal signals for the hexahydrate and the mono-through triisothiocyanato complexes, as well as two small signals attributed to [Mg(H₂O)₅(OH)]¹⁺ and [Mg(H₂O)₄(OH)(NCS)]. Accurate hydration numbers were obtained from signal area integrations at each NCS^– concentration. In the¹⁵N NMR spectra, signals also were observed for the mono-through triisothiocyanato complexes, and a small signal believed to be due to [Mg(H₂O)₄(OH)(NCS)]. Coordination number contributions for NCS^– were measured from these spectra and when combined with the hydration numbers they totalled essentially six at each anion concentration. Signals for [Mg(H₂O)₅(NCS)]¹⁺ through [Mg(H₂O)₃(NCS)₃]^1– also were observed in the¹³C NMR spectra and the area evaluations were comparable to the¹⁵N NMR results. An analysis of the magnitude and sign of the coordinated NCS^– chemical shifts identified the nitrogen atom as the anion binding site. All spectra indicated [Mg(H₂O)₅(NCS)]¹⁺ and [Mg(H₂O)₄(NCS)₂] were the dominat isothiocyanato complexes over the entire range of anion concentrations. The inability to detect evidence for complexes higher than the triisothiocyanato reflects the competitive binding ability of water molecules and perhaps the decreased electrostatic interaction between NCS^– and negatively charged higher complexes.