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.     

Carbon-13 nuclear magnetic resonance spectroscopy—VII:para-substituted fluorobenzenes

C-13 and F-19 NMR spectra of seventeen para-substituted fluorobenzenes were measured and the chemical shifts as well as coupling constants with respect to substituents were analysed. The chemical shifts of the fluorine, the C₁ and the C₂ atoms were found to depend on the total electron densities. In the case of the C₃ atom, the chemical shifts seem to depend on π-electron densities rather than the total electron densities. The present calculations also indicate that the chemical shift of the C₄ atom depends mainly on σ-electron densities due to the inductive effects of substituents. The strongest factor influencing the coupling constant, ⁿJ(C? F), is also considered to be the π-electron densities on the carbon atoms. In the case of the direct couplings, ¹J(C? F), the π-bond orders are important.     

1H NMR spectra part 31: 1H chemical shifts of amides in DMSO solvent

The ¹H chemical shifts of 48 amides in DMSO solvent are assigned and presented. The solvent shifts Δδ (DMSO‐CDCl₃) are large (1–2 ppm) for the NH protons but smaller and negative (?0.1 to ?0.2 ppm) for close range protons. A selection of the observed solvent shifts is compared with calculated shifts from the present model and from GIAO calculations. Those for the NH protons agree with both calculations, but other solvent shifts such as Δδ(CHO) are not well reproduced by the GIAO calculations. The ¹H chemical shifts of the amides in DMSO were analysed using a functional approach for near ( ≤ 3 bonds removed) protons and the electric field, magnetic anisotropy and steric effect of the amide group for more distant protons. The chemical shifts of the NH protons of acetanilide and benzamide vary linearly with the π density on the αN and βC atoms, respectively. The C=O anisotropy and steric effect are in general little changed from the values in CDCl₃. The effects of substituents F, Cl, Me on the NH proton shifts are reproduced. The electric field coefficient for the protons in DMSO is 90% of that in CDCl₃. There is no steric effect of the C=O oxygen on the NH proton in an NH…O=C hydrogen bond. The observed deshielding is due to the electric field effect. The calculated chemical shifts agree well with the observed shifts (RMS error of 0.106 ppm for the data set of 257 entries). Copyright © 2014 John Wiley & Sons, Ltd.     

The 1H and 13C NMR chemical shifts of Strychnos alkaloids revisited at the DFT level

The density functional theory calculation of ¹H and ¹³C NMR chemical shifts in a series of ten 10 classically known Strychnos alkaloids with a strychnine skeleton was performed at the PBE0/pcSseg-2//pcseg-2 level. It was found that calculated ¹H and ¹³C NMR chemical shifts provided a markedly good correlation with experiment characterized by a mean absolute error of 0.08 ppm in the range of 7 ppm for protons and 1.67 ppm in the range of 150 ppm for carbons, so that a mean absolute percentage error was as small as ~1% in both cases.     

Synthesis and Reactivity of Difluoroaromatic Compounds Containing Heterocyclic Central Groups

The reaction of trichloroacetaldehyde with fluorobenzene, followed by a series of transformations, gave 4-fluorobenzil and 4,4'-difluorobenzil which were used in the synthesis of new difluoroaromatic compounds with a heterocyclic central group. The ¹H, ¹³C, and ¹⁹F NMR spectra of the newly synthesized difluoroaromatic compounds were studied. The charge densities on the carbon atoms attached to fluorine were calculated in terms of the PM3 and AM1 semiempirical approximations. A correlation was found between the charge on C(F) and the corresponding ¹³C and ¹⁹F chemical shifts. Using this correlation, the reactivity of difluoroaromatic compounds in nucleophilic substitution reactions was estimated.     

Sur le calcul a priori des déplacements chimiques de 1H et de 19F

We propose a new method for calculating ¹H and ¹⁹F chemical shifts in saturated organic compounds. Results show that the shifts are a balance of several contributions and allow an evaluation of these effects: a diamagnetic contribution d(—?) proportional to the net charge ? of the electronic environment of the nucleus considered and paramagnetic contributions p_X of each of substituents, constant attached to the substituent X in any molecule. For both nuclei, d values were deduced from a theoretical calculation of the screen in the free atom. This method, using a restricted number of parameters obtained from experiment, lead to the observed chemical shifts with a mean accuracy of ± 0,32 ppm for ¹H and ± 11 ppm for ¹⁹F. Therefore this method gives a simple quantitative relation between chemical shift and structure from which precise indications about the origin of the contributions from neighbouring electronic surroundings can be obtained.     

Gas-phase 13C chemical shifts in the zero-pressure limit: refinements to the absolute shielding scale for 13C

¹³C chemical shifts have been measured relative to ¹³CO in the zero-pressure limit for over twenty molecules for which theoretical calculations of ¹³C nuclear shielding have recently been reported. Rovibrational averaging effects on the spin-rotation constant in ¹³C¹⁶O have been used to find σ_e(¹³C in ¹³C¹⁶O) = 3.0 ± 1.2 ppm and σ₀(¹³C in ¹³C¹⁶O) = 1.0 ± 1.2 ppm. With the latter, the σ₀ values for the ¹³C nuclei in this work have been determined absolutely and compared with calculated values. Agreement is generally good in most cases except where low-lying n → π transitions contribute significantly to the paramagnetic shielding.     

Acetal formation from formylthiophene-2-carboxylic acids and recalculation of σI and σR substituent constants of the acetal group by 13C NMR chemical shifts

A uv-visible, ¹H and ¹³C nmr study has been carried out to show that 4-, 1, and 5-formylthiophene-2-carboxylic acids, 2, give acetals in methanol. The ms data have confirmed the acetal formation, which has been studied kinetically by the uv technique. The substituent chemical shifts induced on the carboxy carbon atom and on the endocyclic carbon atoms in the acetals formed from 1 and 2 and in the corresponding anions have been used to recalculate σ_I and σ_R values by means of a dual substituent parameter treatment of data.     

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.     

Non-empirical and DFT calculations of <Superscript>19</Superscript>F and <Superscript>13</Superscript>C chemical shifts in the NMR spectra of substituted pentafluorobenzenes

Chemical shifts in ¹⁹F and ¹³C NMR spectra of substituted pentafluorobenzenes are calculated by Hartree-Fock and density functional theory methods. The calculated values are compared with the experimental data known from the literature. It is shown that chemical shifts in non-polar solvents can be predicted sufficiently accurately by the GIAO-DFT(PBE/L22) method. This method is used to predict the ¹⁹F and ¹³C chemical shifts of a heptafluorobenzyl cation in the SbF₅ medium. The best agreement between the calculated and experimental values is achieved when the counterion effect is taken into account.     

13C and 19F NMR substituent chemical shifts (SCS) of some bridgehead-substituted phenyl- and fluorophenylbicyclo [2.2.2] octyltricarbonylchromium(O) derivatives: nature of aryl 19F NMR polar field effects in the Cr(CO)3-complexed benzene ring system

A number of Cr(CO)₃ complexes of bridgehead-substituted phenylbicyclo[2.2.2]octanes and (m- and p-)fluoropheylbicyclo[2.2.2]octanes have been synthesized and their ¹³C and ¹⁹F NMR spectra have been recorded, respectively. The substituent chemical shifts (SCS) of these stereochemically well-defined model systems permit an unambiguous evaluation of polar factors governing ¹³C and ¹⁹F SCS in arene-Cr(CO)₃ complexes. The dual nature of ¹⁹F NMR polar field effects is reaffirmed and the coefficient (A) of the Buckingham equation for linear electric field effects on C(sp²)F bonds in fluoroarene-Cr(CO)₃ complexes has been calculated. A re-examination and re-interpretation of the ¹⁹F chemical shifts of m- and p-substituted fluorophenyltricarbonylchromium derivatives is also reported. New substituent parameters (σ_I and σ^o_R) for C₆H₅ · Cr(CO)₃ as a substituent in the neutral ground state arepresented.     

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.     

Aminoazoles in Heterocycles Synthesis: II. Trifluoromethyl-containing Diketones in the Synthesis of Pyrazolo[1,5-a]pyrimidines

A regioselective synthesis was carried out of 7-trifluoromethylpyrazolo[1,5-a]pyrimidines by reaction of 3(5)aminopyrazoles with 1,3-diketones containing CF₃ group. The characteristic chemical shifts were established for C⁵ and C⁷ atoms of the pyrimidine ring and of substituents thereof in the ¹H, ¹³C, and ¹⁹F NMR spectra of pyrazolo[1,5-a]pyrimidines.     

13C NMR spectra of 1-(N-arylidene)amino-1,2,3-triazoles

The ¹³C nmr spectra of several 1-(N-arylidene)amino-1,2,3-triazoles were determined. The long range influence of substituents on ¹³C resonances of triazole and C = N carbon atoms was examined and a correlation of their chemical shifts with net charge densities (q_{π + σ}) calculated by the CNDO/2 method was made. It was also shown that the dependence of the coupling constants ¹J(CH) for C = N carbon on the substituents is better correlated using electrons in bond values instead of charge densities.     

Application of chemical shifts in 1H and 13C NMR spectra to configuration assignment of Schiff bases in the liquid phase

In ¹H and ¹³C NMR spectra of N-substituted dimethylketimines chemical shifts of protons and carbon atoms of the methyl groups in the cis-position with respect to the unshared electron pair of the nitrogen are larger than those of the CH₃ groups in the trans-position by 0.2–0.4 and 8–11 ppm respectively. This effect is accompanied by the reduction of the corresponding direct spin-spin coupling constant ¹³C-¹³C by 10 Hz. The experimental trends in the variation of the spectral parameters are well reproduced by ab initio quantum-chemical calculations. The discovered stereochemical dependence of the chemical shifts of ¹H and ¹³C may underlie a simple and efficient method of the configuration assignment in various compounds with a C=N bond.     

1H-, 13C- UND 77SE-NMR-STUDIE AN SCHWEFEL- UND SELEN-HALTIGEN CYCLISCHEN 6π-KATIONEN

Abstract

The ¹H, ¹³C and ⁷⁷Se NMR chemical shifts for 1,3-dithiolium, 1,3-thiaselenolium and 1,3-diselenolium tetrafluoroborates which are unsubstituted in the 4 and 5 positions and are unsubstituted or contain ethylseleno, ethylthio or morpholino groups in the 2 positions are reported. The dependence of the chemical shifts on the substituents and ring hetero atoms are discussed and shift increments given. The study includes the ¹³C shift effects for the following series of compounds: (a) thiotropone, ethylthiotropylium cation and tropylium cation; the iso-π-electronic heterocycles (b) thiopyrane-2-thione, 2-ethyl-thiothiopyrylium cation and the thiopyrylium cation, (c) 1,2-dithiole-3-thione, 3-ethylthio-1,2-dithiolium cation and the 1,2-dithiolium cation, and (d) 1,3-dithiole-2-thione, 2-ethylthio-1,3-dithiolium cation and the 1,3-dithiolium cation. Linear correlations between δ (¹H) and δ (¹³C), δ (⁷⁷Se) and δ (¹³C) and δ (¹³C) and δ (¹³C) (i) of neighbouring ring positions and (ii) between ring and substituent atoms proves, that changes in the electron density distribution of the ring systems is the intrinsic reason for the shift effects discussed. In particular the ⁷⁷Se/¹³C shift correlations show, that δ (⁷⁷Se) of double coordinated selenium atoms is determined by the 〈r ^?3〉_4p term in the contribution of paramagnetic screening σ_p(⁷⁷Se).

Es werden die ¹H-,¹³C- und ⁷⁷Se-NMR-chemischen Verschiebungen der in 4/5-Position unsubstituierten 1,3-Dithiolium-, 1,3-Thiaselenolium- und 1,3-Diselenolium-tetrafluoroborate, die in 2-Stellung unsubstituiert sind bzw. den Ethylseleno-, Ethylthio- oder Morpholinrest tragen, mitgeteilt und in Abhängigkeit von den Substituenten und den Ring-Heteroatomen diskutiert.

In die Betrachtung einbezogen sind die ¹³C-Verschiebungseffekte beim Übergang von den Thionen über die SEt-substituierten Kationen zu den Kation-Grundverbindungen in der Reihe Thiotropon/Ethyl-thiotropyliumion/Tropyliumion und der jeweiligen iso-π-elektronischen Schwefelheterocyclen Thiopyran-2-thion/2-Ethylthiothiopyryliumion/Thiopyryliumion, 1,2-Dithiol-3-thion/3-Ethylthio-1,2-dithioliumion/1,2-Dithioliumion und 1,3-Dithiol-2-thion/2-Ethylthio-1,3-dithioliumion/1,3-Dithioliumion.

Aufgefundene lineare Korrelationen zwischen δ (¹H) und δ (¹³C), δ (⁷⁷Se) und δ (¹³C) sowie δ (¹³C) und δ (¹³C) sowohl benachbarter Ringpositionen als auch zwischen Ring- und Substituentenatomen beweisen, daß als Ursache der diskutierten Verschiebungseffekte im Wesentlichen Änderungen der Elektronendichteverteilung der Ringsysteme anzusehen sind. Speziell die ⁷⁷Se/¹³C-Verschiebungskorrelationen zeigen, daß für δ (⁷⁷Se) zweifach koordinierter Selenatome der 〈r ^?3〉_4p-Term im σ_p (⁷⁷Se)-Verschiebungsbeitrag entscheidend ist.     

NMR- und schwingungsSpektroskopische Untersuchungen höherer Indiumtrialkyle

NMR and Vibrational Spectroscopic Investigations on Higher Indium Trialkyls Several isomeric indium tripentyles and trihexyles are synthesized by known methods. The chemical shifts δ of the ¹³C NMR spectra are used together with those of the corresponding alkanes for determining the increments Δδ(¹³C) = δ(InR₃)–δ(RH). By means of these increments and the Grant/Paul-method the chemical shifts δ(¹³C) of any indium trialkyl can be calculated. The vibrational spectra (IR and Raman) of most liquid trialkyls show very obvious rotameric splittings of the InC vibrations between 450–600 cm^?1. Both frequent conformations of the alkyl ligands with three and more C atoms consist of either a βH atom (notations P_H, S_HH, and T_HHH with vInC between 450–500 cm^?1) or a γC atom (P_C, S_CH, T_CHH with vInC between 550–600 cm^?1) in the transposition to indium. The InC stretching modes of all other more rare configurations can be observed between 500 to 550 cm^?1.     

Beziehungen zwischen chemischer Verschiebung (19F und 13C) und Ladungsdichte: III

The diamagnetic and paramagnetic terms of the chemical shift are expressed by semi-empirical CNDO/II charge densities and bond orders. To establish whether a linear functional relationship exists between the calculated terms and the chemical shifts of ¹⁹F and ¹³C atoms a number of relationships in the form of a linear regression have been examined, taking into consideration both s?_dia and s?_dia plus s?_para. It was found that the relationship δ_A = a + bq_A + cQ_AB + dq_A^πQ_AB produces the smallest standard deviation.