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.     

An 1H-NMR. Spectroscopic Study of Alloxazines and Isoalloxazines

¹H-NMR.-spectra of a series of alloxazines and isoalloxazines and certain cationic derivatives were investigated (Tab. 2 and 5). Unequivocal assignment of all resonance signals was achieved in some compounds by selective deuteriation also by double resonance technique. The coupling constants were verified by computer simulation. Considerable enhancement of the signals due to H-C(9) and H-C(6) is found on decoupling of H₃C-C(7), H₃C-C(8) and H₃C-N(10), resp. These results are compared with those obtained with FAD. The methyl resonance signal of the H₃C-C(7) compounds give doublets due to coupling with H-C(6). The difference in chemical shifts observed upon successive formal introduction of methyl groups into the benzene nucleus of (iso)alloxazines indicates that the molecule becomes less planar thereby. The pyrimidine ring of (iso)alloxazines does not contribute to the ring current except by indirect effects via the carbonyl groups. The experimental data are compared with published MO calculations and discussed.     

The solvent effect on polar compounds containing acidic hydrogens

The effect of aromatic and non-aromatic solvents on the proton chemical shifts of 23 polar compounds has been determined. The protons which are activated by electron-withdrawing groups show large highfield shifts in benzene (relative to the isotropic solvents). Based on evidence provided by infrared data, the highfield shifts of the acidic protons in benzene solution are interpreted in terms of a model involving C? Hπ hydrogen bonding. This model successfully interprets the data reported previously for steroidal ketones. The same model can be extended to benzene solutions of other polar compounds containing strongly electron-deficient sites to which alkyl groups are attached. It is observed that the use of CCl₄ as a reference solvent in studies of benzene induced shifts may have greater significance, since these two solvents have similar dielectric constants.     

Carbon-13 NMR studies on some 5-substituted quinoxalines

Eleven 5-substituted quinoxalines (NO₂, NH₂, COOH, OCH₃, CH₃, OH, F, Cl, Br, I, CN, the latter five not reported previously) have been synthesized by standard methods. Their ¹³C NMR spectra have been measured in DMSO-d₆ and assigned on the basis of substituent parameters, by line widths and by intensities. The chemical shifts compare favorably with those calculated using benzene substituent parameters, and are very close to those of corresponding carbons in 1-substituted phenazines. The correlation with the chemical shifts of the corresponding positions in 1-substituted naphthalenes is also close except for those of carbons 4a and 8a in the quinoxalines which, due to their proximity to nitrogen, are downfield (in some cases 12 ppm) of the signals of the corresponding carbons in naphthalene. 5-Fluoroquinoxaline was also measured in CDCl₃, CD₃COCD₃, CD₃CN, CD₃OD, C₆D₆ and CD₃COOD. In all solvents an abnormally low ²J(CF) (～ 12 Hz) was found for C-4a and no C? F spin-spin splitting could be detected for the three-bond coupling of C-8a. Similar abnormalities were found in 2-fluoroaniline and 2-fluoroacetanilide. There are linear relationships between the Q parameter of the substituent and the chemical shift of carbons 4a, 5 and 6. A linear relationship also exists between the chemical shift of C-8 (‘para’ position) and the Hammett σ_p parameter of the substituent.     

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).     

Model studies of trialkyltin–protein interactions: 13C NMR analysis of solution equilibria of the complex between trimethyltin and methyl N-benzoyl-l-leucyl-l-histidinate

¹³C NMR spectra for the 1:1 complex between methyl N-benzoyl-l-leucyl-l-histidinate and the trimethyltin moiety in d-chloroform (CDC1₃), d₄-methanol (CD₃OD) dimethyl sulphoxide (DMSO) and d₆-DMSO/H₂O solvents are reported, and contrasted with those for the free ligand. The spectra are interpreted in terms of a variety of solution equilibria illustrating the nature of the interaction between the trimethyltin species and primarily the imidazole ring of the histidine residue. Evidence for the preferential stability of pentacoordinate solution structures about tin is presented.     

Solvent effects on the proton magnetic resonance spectra of thio- and dithio-oxamides

The chemical shifts of the N-methyl signals for a number of thio-oxamide and oxamide derivatives are unambiguously determined in the solvents chloroform, carbon tetrachloride, benzene, chlorobenzene, o-dichlorobenzene and nitrobenzene. The ASIS effect is discussed. The temperature dependence of the absolute shift and of the relative shift differences of both methyl groups of the N,N-dimethyl derivatives are studied.     

Polymerization of aromatic nuclei. VIII. Molecular weight control in benzene polymerization

The molecular weight of p-polyphenyl prepared from benzene—aluminum chloride—cupric chloride, was affected by solvent, concentration, and temperature. Relative molecular weights were measured by polymer solubility in chloroform, and positions of the infrared para band and ultraviolet reflectance λ_max. The order of effectiveness of the solvents in reducing molecular weight was: o-C₆H₄Cl₂ > 1,2,4-C₆H₃Cl₃ > SnCl₄ ～ CS₂ > [C₆H₆]. Degradative oxidation revealed that o-dichlorobenzene solvent was incorporated as an endgroup to only a minor extent. In general, the molecular weight of p-polyphenyl decreased with increasing temperature and with decreasing concentration. The theoretical aspects are treated.     

Benzene dilution shifts and steric interactions in N,N-dialkylamides

The NMR spectra of ten disubstituted amides have been recorded at 0°C in carbon tetrachloride and in benzene solutions. The benzene dilution shifts (ASIS) and proton spin decoupling were used to make the chemical shift assignments. A time-averaged solvent cluster model for the association between the amide and benzene is consistent with the observed ASIS values. The assignments for the N-methine and N-methyl resonance peaks in RCON[CH(CH₃)₂]₂, where R is methyl, ethyl or propyl are inverted from the assignments for N,N-diisopropylformamide (R ? H).     

The Effect of Electron-Donating and Electron-Withdrawing Substituents on 1H-and 13C-NMR chemical shifts of novel 7′-aryl-substituted 7′-apo-β-carotenes

The synthesis of 7′-aryl-7′-apo-β-carotenes, where aryl (Ar) is Ph, 4-NO₂C₆H₄, 4-MeOC₆H₄, 4-(MeO₂C)C₆H₄, C₆F₅, and 2,4,6-Me₃C₆H₂, is described. NMR Chemical shifts of all H- and C-atoms are presented, together with specific examples of the spectra. In contrast to ¹H chemical shifts which, except for H? C(8′) and H? C(7′), did not differ greatly from those of β,β-carotene, considerable variations in ¹³C chemical shifts were observed. Signals of the C(α) atoms of the polyene chain [C(β)? C(α)] +_n Ar were shielded, those of the C(β) atoms were deshielded, with some exceptions when n = 1; the effects decreased with increasing n.     

Study of the intrinsic and conformational solvent effects on the chemical shift of the aldehydic proton in furfural and thiophenealdehyde

The chemical shifts of the aldehydic proton in furfural, thiophenealdehyde and benzaldehyde have been measured in fourteen solvents. The correlation of the chemical shifts of thiophenealdehyde and benzaldehyde is excellent (r = 0·996) while it is lower for furfural–benzaldehyde (r = 0·956). The long range coupling constant J_α5 of furiurai has been measured in twelve solvents and the rotameric mole fractions determined. The chemical shifts of individual rotamers are calculated and shown to correlate with benzaldehyde, (r = 0·992; 0·993). Only one rotamer is predominant for thiophenaldehyde in all solvents. The intrinsic solvent effects of the three aldehydes are similar.     

The use of 13C satellites in proton spectra for a study of the rα structure and of the internal rotation of p-chlorotoluene partially oriented in nematic phases

The proton magnetic resonance spectra with natural abundance ¹³C satellites of p-chlorotoluene in isotropic as well as in oriented solvents are analyzed. The indirect C? H and H? H couplings are determined from the isotropic sample and one bond ¹³C isotope effects on the proton chemical shifts are measured. The direct couplings derived from the oriented samples are corrected for harmonic vibration and used to determine the molecular r_α structure with a 6-fold hindering potential for the methl group. The resulting C? H and C? C bond lengths have errors which are smaller than 0.002Å. They deviate from the unsubstituted benzene molecule by between ?1.1 and ?0.5% for the C? H and between +0.3 and +0.5% for the C? C distances. It is not possible to obtain conclusive information about the hindering potential.     

Effects of fused benzene rings on tautomerizations and inversions of benzo, azabenzo, and oxabenzocycloheptatrienes at theoretical levels

Biologically important bicyclic species, including 6H-, 6H-6-aza-, and 6-oxabenzocycloheptatrienes (in which the benzene moiety is fused meta with respect to the tetrahedral constituents: –CH₂–, –NH–, and –O–, respectively), show strong shifts of tautomerizations in favor of the corresponding tricyclic benzonorcaradienes (with ΔH values of −11.49, −14.55, and −19.20 kcal mol⁻¹, respectively), at B3LYP/6-311++G**//B3LYP/6-31G*, and MP2/6-311++G**//MP2/6-31G* levels, and at 298 K. In contrast, such shifts are strongly disfavored by the isomeric bicyclic species in which the benzene moieties are fused ortho or para with respect to –CH₂–, –NH–, and –O–, respectively. Hence for species with ortho benzene rings including 5H-, 5H-5-aza- and 5-oxabenzocycloheptatrienes, tautomerization ΔH values are 30.76, 31.89, and 25.27 kcal mol⁻¹, respectively, while for species with para fused benzene moieties including 7H-, 7H-7-aza-, and 7-oxabenzocycloheptatrienes, tautomerization ΔH values are 24.12, 26.00, and 19.55 kcal mol⁻¹, respectively. NICS calculations are successfully used to rationalize these results. The calculated energy barriers for inversion of the seven-membered rings of bicyclic species predict a dynamic nature for all the structures except for the virtually planar 6H-6-aza- and 6-oxabenzocycloheptatrienes. Finally, our theoretical data are compared to the experimental results where available. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Computational investigation of solvent effect on the structure,spectroscopic properties (13C, 1H NMR and IR,UV), NLO properties and HOMO–LUMO analysis of Ru(NHC)2Cl2(=CH-p-C6H5) complex

This paper presents, a theoretical study of the structural, ¹³C and ¹H NMR chemical shifts, electronic transitions, vibrational analysis, and first hyperpolarizability for Ru(NHC)₂Cl₂(=CH-p-C₆H₅) complex in gas phase and different solvents. The solvent effect on structural parameters, frontier orbital energies, Ru=C_carbene and C_carbene-H stretching frequencies, and chemical shifts of C_carbene, C_NHC and H_carbene of complex was explored based on Polarizable Continuum Model (PCM). The wavenumbers of υ(Ru=C_carbene) and υ(C_carbene-H) of complex in different solvents were correlated with the Kirkwood–Bauer–Magat equation (KBM). As well as, the polarizability and the first order hyperpolarizability values of the investigated compound were computed in various solvents.     

Effet de Solvant en RMN du Carbone-13. III—Cas d'un Soluté Polaire: l'Acétone dans Divers Solvants et en Phase Gazeuse

The chemical shifts of acetone carbons are measured in the gas phase and in nineteen solvents, thus allowing the separation of the screening constant terms arising from the different kinds of solute–solvent interactions. It is shown that for methyl carbons the van der Waals term σ_w, interpreted using Rummens's theory, is more important than the specific solute–solvent interaction term σ_H. In contrast, for the carbonyl carbon, the term σ_H, which is eight times greater than for the methyl carbons, dominates when dipole–dipole interactions and hydrogen bonding occur. No evidence for an electric field term proportional to the Onsager reaction field is found. But when there are dipole–dipole interactions, σ_H is proportional to the electric field of the dipole of a solvent molecule interacting with the dipole of a solute molecule, the two dipoles being antiparallel. The variation of σ_H with the acetone concentration in a non-associating solvent is interpreted as a consequence of the displacement of the acetone in self-association equilibrium, leading to the determination of the equilibrium constant.     

Studies on tacticity of polyacrylonitrile. II. High-resolution nuclear magnetic resonance spectra of 2,4-dicyanopentanes

The NMR spectra of 2,4-DCNP were measured in CCl₄, NaCNS–D₂O, DMSO-d₆, and other solutions. The spectra of the meso form show no significant change with the solvent, but the racemic form shows two kinds of spectra, one of which is observed in a solvent for PAN and the other in a nonsolvent. In the solution, the meso 2,4-DCNP is considered to have two equivalent conformations, TG and G'T, which are the mirror images with each other. The racemic 2,4-DCNP, however, might have predominantly either the TT or GG conformation in CCl₄, pyridine, and benzene, while it has the two conformers with almost equal probability in NaCNS–D₂O and DMSO-d₆. The results obtained from the calculation assuming appropriate constants are in fairly good agreements with the observed spectra of the 2,4-DCNP isomers. The values of chemical shifts and coupling constants used in the calculation correspond to those of PAN which were obtained previously from the analysis of the NMR spectra.     

Solid‐state CP/MAS 13C NMR studies on conformational polymorphism in sertraline hydrochloride,an antidepressant drug

The C(2) isotropic chemical shift values in solid‐state CP/MAS ¹³C NMR spectra of conformational polymorphs Form I (δ 28.5) and III (δ 22.9) of (1S,4S)‐sertraline HCl ( 1 ) were correlated with a γ‐gauche effect resulting from the respective 162.6° antiperiplanar and 68.8° (+)‐synclinal C(2)? C(1)? N? CH₃ torsion angles as measured by X‐ray crystallography. The similarity of the solution‐state C(2) chemical shifts in CD₂Cl₂ (δ 22.8) and DMSO‐d₆ (δ 23.4) with that for Form III (and other polymorphs having C(2)? C(1)? N? CH₃ (+)‐synclinal angles) strongly suggests that a conformational bias about the C(1)? N bond exists for 1 in both solvents. This conclusion is supported by density functional theory B3LYP/6‐31G(d)‐calculated relative energies of C(1)? N rotameric models: (kcal) 0.00 [73.8 °C(2)? C(1)? N? CH₃ torsion angle], 0.88 (168.7°), and 2.40 (?63.4°). A Boltzmann distribution of these conformations at 25 °C is estimated to be respectively (%) 80.3, 18.3, and 1.4. Copyright © 2002 John Wiley & Sons, Ltd.     

The effects of σC—Se–π hyperconjugation in 1‐methoxy‐ and 1‐nitro‐4‐[(phenylselanyl)methyl]benzene

The structures of 1‐methoxy‐4‐[(phenylselanyl)methyl]benzene, C₁₄H₁₄OSe, (1), and 1‐nitro‐4‐[(phenylselanyl)methyl]benzene, C₁₃H₁₁NO₂Se, (2), were determined at 130 K. The two structures, which differ in that (1) contains an electron‐rich aromatic ring and (2) contains an electron‐deficient aromatic ring, both adopt conformations which allow for σ_C—Se–π hyperconjugation. However, although there are significant differences in the ⁷⁷Se chemical shifts for these two compounds, they do not display significantly different H₂C—Se or H₂C—C_ar bond lengths, suggesting that the effects of σ_C—Se–π hyperconjugation in (1) and (2) are not strong enough to be manifested in measurable differences in the structural parameters.     

High-resolution 13C NMR study of free and metal-complexed ionophores in the solid state: Conformation and dynamics of the macrocyclic ring and the effect of intermolecular short contact of methyl groups on spin–lattice relaxation times and displacements of chemical shifts

High-resolution ¹³C NMR spectra of 15 samples of uncomplexed and metal-complexed tetranactin and nonactin were recorded in the solid state, revealing characteristic displacements of peaks due to complex formation and the effect of crystalline packing on the ¹³C chemical shifts and spin–lattice relaxation times of the methyl groups. The C-1 ¹³C chemical shifts of uncomplexed and complexed tetranaction and nonactin are well related to the variation of nearby torsion angles characteristic of the macrocyclic conformation, as determined by x-ray diffraction. The existence of short intermolecular contact of methyl groups (<3.8 Å) at the surface of the molecules results in either prolonged ¹³C spin–lattice relaxation times in the laboratory frame (T₁^C) or substantial upfield displacement of peaks (up to 6 ppm). In addition, significantly reduced T₁^C values in uncomplexed nonactin (one order of magnitude smaller than those of other compounds) was ascribed to the presence of a puckering motion of the tetrahydrofuran ring and fluctuation of the macrocyclic ring in the solid state (with a time scale of 10⁻⁸ _s). Finally, how the conformations of these compounds in the solid are retained in chloroform solution was examined in view of the differences in the ¹³C chemical shifts between the solid and solution.     

Hydrogen migrations in mass spectrometry. IV—formation of [C6H7]+ in the chemical ionization mass spectra of alkylbenzenes

Using specific deuterium labelling the mechanisms of the olefin elimination reactions leading to formation of [C₆H₇]+ in the H₂ and CH₄ chemical ionizatin mass spectra of ethylbenzene and n-propylbenzene (and to [C₂H₅C₆H₆]+ in the CH₄ chemical ionization mass spectra) have been investigated. The results show that the reaction does not occur by specific migration of H from the β position of the alkyl group to the benzene ring. For ethylbenzene 23–29% of the migrating H originates from the α-position, while for n-propylbenzene H migration from all propyl positions is observed in the approximate ratio, position 1:position 2:position 3=0.30:0.22:0.48. It is proposed that the results can be explained on the basis of competing H migration from each alkyl position involving cyclic transition states of different ring sizes, rather than by H randomization within the alkyl chain.