Carbon-13 n.m.r. study of 31P?13C spin–spin coupling constants in dichloro- and monochloro-vinyl phosphate derivatives

Carbon-13 chemical shifts and J(PC) coupling constants of 29 vinyl phosphate derivatives are presented. In the series of compounds (R₁O)₂P(O)OC¹(R)?C²X₂ (where 3 in R indicates the first carbon of the R₂ substituent) large differences were found between the ³J(P, O, C-1, C-3) and ³J(P, O, C-1, C-2) coupling constants of the chlorinated (X?CI) and the unsubstituted (X?H) derivatives. A possible explanation of this phenomenon is given on the basis of Jameson's s bond character theory. Strong stereospecificity of ³J(P, O, C-1, C-3) coupling constants was observed in the series of compounds (R₁O)₂ P(O)OC¹(R)?C²HR₃. Coupling constants varied between 3.2–4.9 Hz in the E isomers, while peaks could not be resolved in the Z isomers. The ³J(P, O, C-1, C-2) coupling constants were regularly 20–30% greater in the Z than in the E isomers.     

Spectroscopic studies on N,N-dimethylamides—III Amide-rotational barriers of N,N-dimethylbenzamides and -cinnamamides. A comparison of the NMR intensity ratio- and iterative total line shape method

Amide-rotational barriers of a series of para-substituted N,N-dimethylbenzamides and -cinnamamides have been obtained by both the intensity ratio method, according to Rogers and Woodbrey,² and an iterative total line shape analysis. From a comparison of the results, it is concluded that a discussion of rotational barriers obtained with the intensity ratio method should preferably be based on ΔG values, which are nearly equal for both methods, rather than on ΔG^? at other temperatures.     

13C shieldings of several diacetylenic alcohols

The ¹³C spectra of several conjugated diynols and symmetrical diynediols, represented by the parent compounds 2,4-pentadiyn-1-ol and 2,4-hexadiyne 1,6-diol, respectively, have been determined and individual resonances assigned. The data show close similarities to those observed with related 2-yn-1-ols. Mutual shielding interactions between the conjugated triple bonds result in upfield shifts of both the α- and β-sp-hybridized carbon nuclei by c. 4.5 ppm, relative to the ethynyl prototype. The deshielding γ effect induced by the hydroxyl group at the interior sp-hybridized carbons is discussed.     

The torsional barrier in aromatic carbonyl compounds. V. Carbon-13 DNMR study of stable protonated p-substituted acetophenones

The barrier to internal rotation around the phenyl–carbonyl bond in a series of stable protonated p-substituted acetophenones has been determined by means of low temperature ¹³C NMR and total bandshape analysis resulting in a value of ΔG = 48.1 kJ mol^?1 for the unsubstituted protonated compound. The energy results and the ¹³C chemical shifts are in good agreement with the quasi-planarity of the ground state in p-substituted protonated acetophenones. The barrier heights are discussed in terms of contributions from steric and resonance effects.     

Étude en RMN du 13C de Dérivés de la Pipéridone-4 Stériquement Encombrés et des Oximes Correspondantes

The regiospecific differences in chemical shifts caused by the oximino group, such as Δδ as Δδ parameters, are useful to estimate conformational distortions directly from ¹³C spectra. Particularly, twist-boat conformations resulting from oximation of sterically hindered piperidones are unambiguously detected.     

The relationship between 19F substituent chemical shifts and electron densities: meta- and para-substituted benzoyl fluorides

The ¹⁹F substituent chemical shifts (SCS) of meta- and para-benzoyl fluorides are found to correlate well with substituent parameters using the dual substituent parameter (DSP) equation, indicating that they reflect electronic perturbations induced by the substituent. The direction of the SCS values is such that donating substituents cause upfield shifts whilst acceptors cause downfield shifts. STO-3G calculations indicate that substituents induce only very small changes in π-electron density about the fluorine atom, but that these changes correlate reasonably well with the observed SCS values. For the para series, the slope of the relationship between δq and ¹⁹F SCS is 5000 ppm/electron, indicating the great sensitivity of the flourine atom to small changes in electron density.     

Applications of NMR spectroscopy of chiral association complexes. 6. rotation about the C(sp2)—C(aryl) bond in 2,6-disubstituted pivalophenones

For the investigation of the barrier to rotation about the C(sp²)—C(aryl) bond in non-planar pivalophenones five derivatives were prepared and their ¹H and ¹³C NMR spectra assigned. Methyl and bromine groups in the 3-position have opposite substituent effects on the chemical shifts of the ¹H and ¹³C signals of Me² and Me⁴. The ΔG values were determined from the coalescence temperatures of the signal splittings generated by the addition of optically active shift reagents. The accuracy of this method was estimated by using different signals of 3-bromo-2,4,6-trimethylpivalophenone and by computer simulation of the line shape. A buttressing effect of substituents in the aromatic ring was observed. A change of the twist angle by the substitution of methyl by bromine in the tert-butyl group was suggested in order to explain the changes in ΔG and the chemical shifts.     

Spectroscopic studies on N,N-dimethylamides—V Amide-rotational barriers from iterative total line shape analysis of N,N-dimethylcyclopropanecarboxamides

Amide-rotational barriers in some 2-substituted N,N-dimethylcyclopropanecarboxamides in CDCl₃ as a solvent (0·25 M) were obtained with an iterative total line shape analysis. N,N-dimethylcyclopropanecarboxamide shows a barrier ΔG = 16·72 ± 0·01 kcal/mole. Para-nitro substitution in (trans)-2-phenyl-N,N-dimethylcyclopropanecarboxamide raises the barrier ΔG from 17·08 ± 0·01 to 17·40 ± 0·02 kcal/mole.     

13C-NMR. Spectra of 4-Substituted Quinuclidines. Polar effects,Part V

¹³C-NMR. sepctra of 37 4-substituted quinuclidinium perchlorates, 15 4-substituted quinuclidines and the corresponding 1-methylquinuclidinium iodides have been measured. The chemical shifts δ for all compounds lie in the expected range. No correlation is found between δ and the inductive substituent constant σ of the substituent. Abnormal shift differences between quinuclidines bearing a nucleofugal group and the corresponding protonated or N-methylated quinuclidinum salt are observed for the bridgehead carbon C(4). These differences are ascribed to incipient fragmentation, i.e. C, C-hyperconjugation in the ground state.     

Dynamic NMR study of chair–chair interconversion in N,N -dimethylpiperidinium iodide in aqueous solution using 13C T1p measurements

¹³C T_1ρ values measured for N,N-dimethylpiperidinium iodide yield activation parameters (ΔH^≠ and ΔG values of 43.7 and 41.6 kJ mol^?1) for the chair-chair interconversion in aqueous solution similar to those measured for 1,1-dimethylcyclohexane. The potential of ¹³C T_1ρ values for studying dynamic processes in aqueous solutions is illustrated.     

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.     

Kernresonanzuntersuchungen am 1.10-Phenanthrolin und seinen Komplexverbindungen—IV: 13C-Resonanzen des freien und protonierten 1.10-Phenanthrolins

The ¹³C chemical shifts of free and protonated 1,10-phenanthroline have been measured and interpreted by means of π- and (σ + π)-MO-electron calculations as described in Parts I¹ and III.² For the protonated molecule a high field ¹³C shift of the C-atom adjacent to the N-atom is characteristic; this effect is explained in terms of a decrease of the N–C overlap population. For the mono- and diprotonated 1,10-phenanthroline the best correlation with the experimental shift values was obtained by assuming a small formal charge on the nitrogen atom (Δ = 0·0 to 0·3).     

A tropisomerism: Racemization barrier of 13-substituted oxyprotoberberines

The two possible conformations of 13-R-substituted oxyprotoberberines are enantiomeric. The racemization barriers are determined for three of these compounds (R=OMe, OCOMe, OCOPh) using variable temperature ¹H NMR. It was found that ΔG = 39.0 kJ mol^?1 when R = OMe, ΔG = 63.5 kJ mol^?1 when R = OCOMe and ΔG = 63.7 kJ mol^?1 when R = OCOPh. When R = H, however, the barrier is well below 35 kJ mol^?1, suggesting that one of the factors influencing these racemization barriers (as in 2,2′-bridged biphenyl systems) is steric in origin. Such low barriers to racemization do not allow easy handling of the enantiomers after resolution, but could allow better complexation of the one desired enantiomer to the receptor.     

Nuclear magnetic resonance studies of internal rotation—III Rotational barriers in META- and PARA-substituted N,N-dimethylcinnamamides

The rotational barriers about the C? N bond of eight m- and p-substituted N, N-dimethyl cinnamamides have been determined by the iterative total line shape NMR method. The ΔG values have been correlated with the substituent constants σ, σⁿ and σ⁺. By comparison of the results with literature data, some conclusions about the accuracy of the barrier determination as well as the transmittance of polar effects in conjugated amides have been drawn.     

INDOR study of long range cross-ring couplings in heptahelicene: XX

INDOR experiments on heptahelicene have revealed the existence of positive ⁵J (epi), negative ⁴J (peri) and |⁴J| (peri) cross-ring couplings.     

Application of 13C and 15N spectroscopy to the study of electronic delocalization in N?N bonds: Nitrosamines,hydrazones, triazenes and related protonated species

¹³C and ¹⁵N spectroscopies at natural abundance have been applied to the study of nitrogen lone-pair delocalization in N? N containing compounds: nitrosamines, nitramines, hydrazines, hydrazones and triazenes. Structure-chemical shift correlations have been derived for nitrosamines; the ¹³C upfield effect of a γ substituent has been used for assigning the configuration of both diastereoisomers in N,N-unsymmetrically substituted nitrosamines. Equations have been computed which permit the prediction of the electronic delocalization, expressed in terms of free enthalpy of activation ΔG, as a function of δ¹⁵N and of the length of the N? N bond. ¹⁵N spectroscopy has also been applied to the study of the protonated species of nitrosamines and of acceptor-donor complexes of nitrosamines with Lewis acids. The behaviour of such N? N containing compounds is compared to that of amides.     

13C nuclear magnetic resonance studies on acetophenones: Barriers to internal rotation

The barrier to internal rotation in a series of p-substituted acetophenones has been determined by means of low temperature carbon-13 n.m.r. and total bandshape analysis, resulting in: ΔG = 5·4 ± 0·1 kcal mol^?1 (22·4 ± 0·4 kJ mol^?1) for the unsubstituted acetophenone. The substituent effects on the barrier are found to be the same as for the corresponding benzaldehydes. The barrier height is discussed in terms of contributions from resonance and steric effects.     

Dynamic 13C NMR studies on the internal rotation in tetraarylallyl anions

Proton noise decoupled ¹³C NMR spectra of lithium 1,1,3,3-bis (2,2′-biphenylylene)propenide and lithium 1,1,3,3-tetraphenylpropenide have been studied, and complete assignments have been made based on several methods, e.g. ¹³C? ¹³C coupling constants of selectively labelled compounds and low-temperature splitting of temperature dependent signals. The barriers to rotation about the partial double bonds of the allyl group have been determined by ¹³C DNMR resulting in the thermodynamic parameters of activation ΔG = 57 kj mol^?1, ΔH^≠ = 56.5 kj mol^?1, ΔS^≠ = ?3J mol^?1K^?1 and ΔG = 55.5 kj mol^?1, ΔH^≠ = 63 kj mol^?1, ΔS^≠ = 26 kJ mol^?1K^?1, respectively. The results are discussed, including the hitherto known data about the topomerisation process.     

EPR study of the alimemazine cation radical

The EPR spectrum of alimemazine cation radicals (ALMZ⁺) in a perchlorate single crystal has been studied at 293 K. Since strong exchange interactions between the radicals did not allow the determination of the hyperfine splitting constants, the spectrum of an ALMZ⁺ solution, frozen at 77 K, was also studied. The results, checked by computer simulation, indicate orthorhombic symmetry of the g(2.0075, 2.0059, 2.0023) and hfs (A=3.6G; A=2.0G; A=15.6G) tensors.     

Some thermodynamic parameters for hydroxy amino acids: Bicine and tricine

pK values of N,N-dihydroxyethylglycine (bicine) and N-[tris(hydroxymethyl)methyl]-glycine (tricine) have been determined by the Irving-Rossotti method in an aqueous medium at 25, 30, 35, 40, 45, and 50°C and at different ionic strengths (I = 0.1, 0.5, and 1.0). Plots between pK_a(NH) and 1/T for various ionic strengths have been obtained and the values of slopes have been used to calculate the ΔH, ΔS, and ΔG for the dissociation reactions of bicine and tricine. The ΔH, ΔS, and ΔG values for bicine were found to be 10.6 ± 0.6 kcal mol^?1, ?1.9 ± 1.8 e.u., and 11.1 ± 0.06 kcal mol^?1, respectively, and for tricine 11.2 ± 0.6 kcal mol^?1, 1.6 ± 1.6 e.u., and 10.7 ± 0.06 kcal mol^?1, respectively. The pK_a(NH) values decrease with rise in temperature but the influence of ionic strength is not significant.