NMR spectra of pyrimidines effect of substituents on the chemical shift of the para protons in 2- and 5-substituted pyrimidines

The chemical shifts of the protons of the pyrimidine ring in the 2 position for a series of 5-substituted pyrimidines and in the 5 position for a series of 2-substituted pyrimidines in solutions in dimethyl sulfoxide were determined. The correlation equations that link the relative chemical shifts with the F and R substituent constants were calculated. The correlation equations were analyzed by comparison with the corresponding correlation equation for a series of monosubstituted benzenes. The reasons for the change in the conductivity of the electronic effects of the substituents via inductive and conjugation mechanisms in the pyrimidine ring as compared with the conductivity in the benzene ring are discussed.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 4, pp. 526–529, April, 1978.     

B环对位取代异黄酮化合物的核磁共振研究

对14个合成的B环对位取代异黄酮化合物核磁共振氢谱进行了研究.利用超导核磁共振归属了B环无取代异黄酮质子的化学位移,根据取代基化学位移的变化影响规律考察了取代基对分子的影响方式.研究结果表明,2'(6'),3'(5')位质子共振迁移分别与取代基参数σ_p和S_o线性相关,说明4'位取代基主要通过电子效应影响其间位质子,其磁各向异性仅影响邻位质子,该取代基对A环质子影响不大,而对C环尤其是对2-H影响较明显.     

NMR spectra of pyrimidines. Effect of substituents on the chemical shift of the protons of the amino group of p-substituted 2- and 5-aminopyridines and anilines

The chemical shifts of the protons of the amino groups of p-substituted anilines and 2- and 5-aminopyrimidines in dimethyl sulfoxide were measured, and their dependence on the inductive and conjugative constants of the substituents was studied. On the basis of an analysis of the correlation equations it was concluded that there is a rather close similarity in the pyrimidine and benzene rings with respect to their conductivity of the inductive effect of substituents in the para position. A difference in the conductivity of the electronic effects of substituents over the system of bonds of the pyrimidine ring as a function of the relative orientation of the substituents and the reaction center is demonstrated.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 12, pp. 1683–1686, December, 1979.Original article submitted April 26, 1979.     

Carbon-13 NMR investigation of 2,5-diaryl-1,4-dithiins

The ¹³C NMR spectra of eight 2,5-diaryl-1,4-dithiins were recorded and signals were assigned. A linear correlation was observed between the electronegativity of the substituent groups on C-10,10′ and the chemical shifts of C-10,10′ after applying corrections for the magnetic anisotropic effect of the substituents. A Hammett correlation was found between the ¹³C chemical shifts of C-3,6 and C-7,7′ and the σ_p⁺ parameter associated with the substituents on C-10,10′. Extended electronic interaction between the π system of the aryl group and the π system of the dithiin ring was suggested by the observance of an alternating behavior in the magnitude of the substituent effects on the ¹³C shifts of C-2,5 and C-3,6. An alternating effect was also noted in the magnitude of the long-range ¹³C? F coupling constants for these same carbon signals in 2,5-(10,10′-difluoro)diphenyl-1,4-dithiin.     

Transmission of substituent effects in 1,2,3-selenadiazole derivatives—I

Chemical shifts are reported for the heterocyclic ring hydrogen of a series of para-substituted 4-aryl-1,2,3-selenadiazoles and substituent effects are evaluated statistically by means of the Swain-Lupton multiple correlation analysis. A satisfactory understanding of the various substituent effects is obtained by this approach. Excellent correlations were obtained for a number of varying aliphatic solvents, no serious hindrances to correlation being encountered. An approach was also made towards an understanding of aliphatic- and aromatic-induced solvent shifts, as far as these are modified by substituents.     

Proton magnetic resonance spectra of thenyl derivatives—II. Chloromethylthiophenes and some dithienylmethanes

PMR spectra of thirty-eight chloromethylthiophene and seven dithienylmethane derivatives were observed at 60 or 40 MHz. The chemical shifts of methylene protons were 4·63 to 5·25 ppm for monosubstituted 2-chloromethylthiophenes and 4·37 to 4·56 ppm for monosubstituted 3-chloromethylthiophenes, respectively, with reference to TMS. Those for 2,2′ -dithienylmethanes, which have one substituent in each ring, were 4·12 to 4·34 ppm. These shifs are useful for determination of the positions of the methylene groups in the related compounds. The long-range coupling constants observed for methylene proton signals are also useful for the determination of the positions of substituents.     

Correlation of substituted aromatic β‐diketones' characteristic protons chemical shifts with Hammett substituent constants

Influence of dibenzoylmethane's substituents in meta and para positions on chemical shift values of tautomers' characteristic protons was investigated in four solvents with ¹H NMR spectroscopy: acetone‐d₆, benzene‐d₆, CDCl₃ and deuterated dimethyl sulfoxide (DMSO‐d₆). It was proved that the influence of substituents on chemical shifts strongly depends on the kind of the solvent; the greatest changes were observed in benzene‐d₆ and the smallest in CDCl₃. In acetone‐d₆ and DMSO‐d₆, the influence of substituents on chemical shifts is similar and the most regular. It allowed a fair correlation of chemical shifts of para‐substituted dibenzoylmethane derivatives' characteristic protons with Hammett substituent constants in these solvents. In CDCl₃, characteristic protons' chemical shifts were near ¹H NMR spectroscopy measurement error limits, and, therefore, correlation with Hammett substituent constants in this solvent was unsatisfactory. In benzene, although the changes of chemical shifts are the most evident, the changes are also the most irregular, and, therefore, correlation in this solvent failed completely. Results of meta‐substituted derivatives were much more irregular, and their correlation with Hammett substituent constants was poor in all investigated solvents. Copyright © 2013 John Wiley & Sons, Ltd.     

The NMR spectra of porphyrins—19: 13C and proton NMR spectra of metal-free porphyrins with the Type-IX substituent orientation,and of their zinc(II) complexes

The ¹³C and proton NMR spectra of six porphyrins bearing the substituent orientation characteristic of the natural “Type-IX” arrangement are reported and assigned. Significant concentration effects in the spectra of the free base porphyrins, together with the broadening of the C_α (and occasionally C_β) carbon resonances due to NH tautomerism caused a significant loss of data in these spectra. However, the spectra of the corresponding zinc(II) porphyrins (with addition of excess pyrrolidine) show that both these extraneous effects are completely removed to give well-resolved spectra with accurately reproducible chemical shifts. These spectra are assigned and an analysis of the chemical shifts allows the deduction of substituent chemical shift (SCS) parameters for the peripheral substituents at the beta and meso carbons. There is no global effect of these beta substituents, the beta carbon SCS being confined to the immediate pyrrole ring, and the meso carbon SCS to the two adjacent pyrrole rings. The SCS parameters are analyzed and it is shown how they can be used to predict the peripheral and meso carbon chemical shifts of any porphyrin bearing the substituents discussed.     

Correlation of ring nitrogen substituents with carbon-13 nuclear magnetic resonance data in azoloazines

Carbon–13 nuclear magnetic resonance data have been acquired on 22 azoloazines. Chemical shifts have been correlated by a step–wise linear multiple regression with nitrogen substituents in both the 5- and 6-membered rings using pyrrolo[1,2-α]pyridine as the reference for chemical shift correlation. The data demonstrate that a highly correlated set of chemical shift parameters exist. Nitrogen substitution in the five–membered ring produces larger cross–ring effects than are oberved in the five–membered ring when substitution occurs in the six–membered ring. Within the six–membered ring a constant para- substituent parameter is noted. The meta- and para- parameters are more complex and fall into two groups for each parameter. Within the five–membered ring, a highly regular chemical shift pattern is observed which reflects an attenuated perturbation from nitrogen substitution in the six–membered ring.     

Carbon-13 NMR spectra of DDT,its analogues and related compounds

Carbon-13 NMR spectra of mono- and disubstituted aromatic compounds including DDT, its analogues, homologues, derivatives and certain model compounds have been studied. The Savitsky scheme of carbon chemical shifts in disubstituted benzenes is applicable to these compounds. The data obtained show that in mono- and disubstituted aromatic compounds containing two different substituents in the α- and β-positions of the side chain, the substituted ring carbon atom shifts follow the additivity rule and can be calculated from substituent increments. Mutual effects of substituents in the ring and in the side chains are analysed. The chlorine atoms in α-position to the phenyl ring give rise to an additive α-effect of about 25 ppm, as in perchloroalkanes. The influence of a β-chlorine atom in the side chain on the substituted carbon atom in the ring is, however, only 3 ppm as against the usual value of about 10 ppm for the β-effect in alkyl chains. Moreover, the first β-chlorine substituent has no noticeable influence on the substituted ring carbon chemical shift: the effect of 3 ppm is transferred to the para-carbon atom almost without attenuation. The ring substituted carbon atom signal shifts caused by the γ-effect of chlorine in the side chain are similar to those observed in aliphatic chains. The ortho-chlorine substituents shift the side chain α-carbon atom signal by 3.6-5.2 ppm to high field compared to para-chlorophenyl compounds. This is similar to the chlorine γ-effect in aliphatic chains.     

NMR study of the spatial structure of synthetic pyrethroids

The spatial isomers of the new synthetic analogs of ethyl permithrinic ether and permethrin were investigated by NMR (¹H, ¹³C, DEPT (distortionless enhancement by polarization transfer), COSY (correlation spectroscopy), CHCORR (heteronuclear (C, H) shift correlation spectroscopy), ROESY (rotating-frame Overhauser effect spectroscopy)). Several tendencies were revealed in the ¹H and ¹³C chemical shifts of the α atoms of the substituents in the 2nd and 3rd positions of the cyclopropane ring. For substituents cis-orientated relative to the ester group, the spectra show a paramagnetic shift of the ¹H signals and the diamagnetic shift of the ¹³C signals relative to the trans-orientated substituents. The ¹H and ¹³C chemical shifts of the α atoms of the substituents in the 2nd and 3rd positions of the cyclopropane ring permit an unambiguous determination of the stereochemistry of ethyl permethrinic ether and permethrin analogs.     

Substituent influences on the stability of the ring and chain tautomers in 1,3-O,N-heterocyclic systems: characterization by 13C NMR chemical shifts, PM3 charge densities, and isodesmic reactions

Substituent effects on the stabilities of the ring and chain forms in a tautomeric equilibrium of five series of 2-phenyloxazolidines or -perhydro-1,3-oxazines possessing nine different substitutions at the phenyl moiety have been studied with the aid of 13C NMR spectroscopy and PM3 charge density and energy calculations. Reaction energies of the isodesmic reactions, obtained from the calculated energies of formation, show that electron-donating substituents stabilize both the chain and ring tautomers but the effect is stronger on the stability of the chain form than on that of the ring form. The 13C chemical shift changes induced by the phenyl substituents (SCS) were analyzed by several different single and dual substituent parameter approaches. The best correlations were obtained by equation SCS = rhoFsigmaF + rhoRsigmaR. In all cases the rhoF values and in most cases also the rhoR values were negative at both the C=N and C-2 carbons, indicating a reverse behavior of the electron density. This concept could be verified by the charge density calculations. The 13C chemical shifts of the C=N and C-2 carbons show a normal dependence on the charge density (q(tot)), but the charge density shows a reverse dependence on substitution. Correlation analysis of the 13C chemical shifts, solvent effect (CDCl3 vs DMSO-d6) on the NMR behavior as well as the effect of substituents on the electron densities and on the stabilities of the ring and chain tautomers show that the substituent dependence of the relative stability of the ring and chain tautomers in equilibrium is governed by several different electronic effects. At least intramolecular hydrogen bonding between the imine nitrogen and the hydroxyl group as well as polarization of the C=N bond seem to contribute in the chain form. Stereoelectronic and electrostatic effects are possible to explain the increase in stability of the ring form by electron-donating substituents.     

Substituent cross-interaction effects on the electronic character of the C=N bridging group in substituted benzylidene anilines--models for molecular cores of mesogenic compounds. A 13C NMR study and comparison with theoretical results

13C NMR chemical shifts delta(C)(C=N) were measured in CDCl3 for a wide set of mesogenic molecule model compounds, viz. the substituted benzylidene anilines p-X-C6H4CH=NC6H4-p-Y (X = NO2, CN, CF3, F, Cl, H, Me, MeO, or NMe2; Y = NO2, CN, F, Cl, H, Me, MeO, or NMe2). The substituent dependence of delta(C)(C=N) was used as a tool to study electronic substituent effects on the azomethine unit. The benzylidene substituents X have a reverse effect on delta(C)(C=N): electron-withdrawing substituents cause shielding, while electron-donating ones behave oppositely, the inductive effects clearly predominating over the resonance effects. In contrast, the aniline substituents Y exert normal effects: electron-withdrawing substituents cause deshielding, while electron-donating ones cause shielding of the C=N carbon, the strengths of the inductive and resonance effects being closely similar. Additionally, the presence of a specific cross-interaction between X and Y could be verified. The electronic effects of the neighboring aromatic ring substituents systematically modify the sensitivity of the C=N group to the electronic effects of the benzylidene or aniline ring substituents. Electron-withdrawing substituents on the aniline ring decrease the sensitivity of delta(C)(C=N) to the substitution on the benzylidine ring, while electron-donating substituents have the opposite effect. In contrast, electron-withdrawing substituents on the benzylidene ring increase the sensitivity of delta(C)(C=N) to the substituent on the aniline ring, while electron-donating substituents act in the opposite way. These results can be rationalized in terms of the substituent-sensitive balance of the electron delocalization (mesomeric effects). The present NMR characteristics are discussed as regards the computational literature data. Valuable information has been obtained on the effects of the substituents on the molecular core of the mesogenic model compounds.     

NMR studies of substituted pyridines

Proton magnetic resonance chemical shifts and coupling constants for a series of 2-substituted-5-nitropyridines and 2-substituted-3-nitropyridines, both in the free base and protonated forms, are reported. The substituents were chloro, bromo, iodo, and hydroxyl. The effect of the substituents on the chemical shifts are substantial but not unusual. The coupling constants are less sensitive to the nature of the substituent and are perturbed only slightly when the ring nitrogen is protonated. This latter observation is consistent with the results of LCAO-MO calculations.     

13C NMR spectra of 9-methylcarbazoles and electron conductivity of the carbazole ring

The effect of substituents in the ring of 9-methylcarbazoles on the ¹³C NMR chemical shifts was determined. Correlation relationships between the inductive and resonance constants of the substituents and the chemical shifts were found. The transmission properties of the carbazole ring with respect to the electronic effects of substituents in the 3 position were evaluated on the basis of the results obtained. Nonadditivity of the effects of the substituents on the NMR chemical shifts within the limits of one phenyl ring of carbazole relative to monosubstituted benzenes was observed.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 2, pp. 204–209, February, 1984.     

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.     

Propagation of polar substituent effects in 1-(substituted phenyl)-6,7-dimethoxy-3,4-dihydro- and -1,2,3,4-tetrahydroisoquinolines as explained by resonance polarization concept

[structures: see text] Propagation of inductive and resonance effects of phenyl substituents within 1-(substituted phenyl)-6,7-dimethoxy-3,4-dihydro- and -1,2,3,4-tetrahydroisoquinolines were studied with the aid of 13C and 15N NMR chemical shifts and ab initio calculations. The substituent-induced changes in the chemical shift (SCS) were correlated with a dual substituent parameter equation. The contributions of conjugative (rhoR) and nonconjugative effects (rhoF) were analyzed, and mapping of the substituent-induced changes is given over the entire isoquinoline moiety for both series. The experimental results can be rationalized with the aid of the resonance polarization concept. This means the consideration of the substituent-sensitive balance of different resonance structures, i.e., electron delocalization, and the effect of the aromatic ring substituents on their relative contributions. With tetrahydroisoquinolines, the delocalization of the nitrogen lone pair (stereoelectronic effect) particularly contributes. Correlation analysis of the Mulliken atomic charges for the dihydroisoquinoline derivatives was also performed. The results support the concept of the substituent-sensitive polarization of the isoquinoline moiety even if the polarization pattern achieved via the NMR approach is not quite the same as that predicted by the computational charges. Previously the concepts of localized pi-polarization and extended polarization have been used to explain polar substituent effects within aromatic side-chain derivatives. We consider that the resonance polarization model effectively contributes to the understanding of the polar substituent effects.     

Carbon-13 nuclear magnetic resonance spectra of some methyl and phenyl substituted 2H-azirines

Carbon-13 n.m.r. spectra have been obtained for some methyl and phenyl substituted 2H-azirines. The higher field resonance of C-2 than that of the corresponding aziridine carbon is interpreted in terms of ring strain. Substituent effects on the chemical shifts of the azirine ring carbons are discussed. A set of additivity parameters for the methyl and phenyl groups are obtained which can be used for the calculation of the chemical shifts of the azirine ring carbons. The substituent effect of an azirine ring on the chemical shift of benzene is also discussed in comparison with those of some other substituents. A high degree of s character (48.5%) in the exocyclic orbital of C-3 is indicated by a large J(¹³C-3,H) value (242.5 Hz).