The chemistry of indole

The possibility of an electrophilic substitution reaction in the lilolidene series is shown. It is established that, when 2, 3-dimethyl-1, 7-trimethyleneindole (9, 10-dimethyl-9-lilolidene) is nitrated, the main product is the 5-nitro derivative with some of the 6-nitro isomer. The structure of the compounds obtained is established by the combined use of UV and NMR spectra. The conditions for the dehydrogenation of lilodines to lilolidenes are found. It is shown that the indoline system is dehydrogenated more readily than the tetrahydroquinoline system. The nitro compounds obtained are reduced to the corresponding amines by hydrazine hydrate in the presence of Raney nickel.     

Theoretical study on metal NMR chemical shifts: Germanium compounds

Germanium chemical shifts were studied theoretically by the ab initio molecular orbital method. The compounds studied were GeMe_4?xCl_x and GeMe_4?xH_x(x = 0–4). The calculated values of the germanium chemical shifts agreed well with the available experimental values. The germanium chemical shift is due to the p-electron mechanism that reflects the ligand electronic effect on the p-p* excitation term in the second-order paramagnetic term. For GeMe_4?xH_x, the chemical shift is almost linear to the number of the ligand, x. On the other hand, a U-shaped dependence is predicted for the chemical shifts of the GeMe_4?xCl_x series and is shown to be caused by the strong and nonadditive electron-withdrawing ability of the Cl ligand. The diamagnetic contribution is relatively small for the chemical shift and is determined solely by a structural factor. © 1994 John Wiley & Sons, Inc.     

Natural abundance 17O NMR spectroscopy of heterocyclic N-oxides and Di N-oxides. Structural effects

The ¹⁷O chemical shift data for a series of azine N-oxides, diazine N-oxides and di-N-oxides at natural abundance are reported. Isomeric methyl substituted quinoline N-oxides exhibited chemical shifts which are interpreted in terms of electronic and compressional effects. The ¹⁷O chemical shift for 8-methylquinoline N-oxide (370 ppm) is deshielded by 25 ppm more than predicted, based upon electronic considerations. The ¹⁷O chemical shift for the N-oxide of 8-hydroxyquinoline (289 ppm) is substantially shielded as a result of intramolecular hydrogen bonding. The relative ¹⁷O chemical shifts for diazine N-oxides of pyrazine, pyridazine and pyrimidine follow predictions based on back donation considerations. Because of solubility limitations, spectra of only two N,N′-dioxides were obtained. The chemical shift of benzopyrazine di N-oxide in acetonitrile was shielded by 18 ppm compared to that of its mono N-oxide.     

Saturated amine oxides: part 10. hydroacridines: part 32. linear 17O/13C and 13C/13C chemical shift correlations between saturated azaheterocyclic N‐methylamine N‐oxides,and the methiodides of their parent amines

Two kinds of good linear correlations were found between the chemical shifts of saturated six‐membered azaheterocyclic N‐methylamine N‐oxides and the chemical shifts of the methiodides of their parent amines. One of the correlations occurs between the ¹⁷O chemical shift of the N⁺―O^– oxygen in the N‐oxides and the ¹³C chemical shift of the N⁺―CH₃ methyl group analogously situated in the appropriate methiodide (r = 0.9778). This correlation enables unambiguous configuration assignment of the N⁺―O^– bond, even if the experimentally observed ¹⁷O chemical shift of only one N‐epimer is available, provided the ¹³C chemical shifts of both N⁺―CH₃ groups in the methiodide are known and assigned; furthermore, it can be used also for the estimation of ¹⁷O chemical shifts of the N⁺―O^– oxygens in N‐epimeric pairs of N‐oxides, for which observed ¹⁷O data hardly become available. The second correlation is observed between the ¹³C chemical shift of the N⁺―CH₃ methyl group in the N‐oxides and the ¹³C chemical shift of the N⁺―CH₃ methyl group analogously situated in the appropriate methiodide (r = 0.9785). It can be used for safe configuration assignment of the N⁺―CH₃ group and, indirectly, also of the N⁺―O^– bond in an amine N‐oxide, even if no ¹⁷O NMR data, and the ¹³C chemical shift of only one N‐epimer is available. Copyright © 2015 John Wiley & Sons, Ltd.     

Determination of the ionic radii by means of the Kohn–Sham potential: Identification of the chemical potential

Under the Kohn–Sham theory, we examine solutions for the equations δT_S/δρ(r) = 0 and δT_S/δρ(r) = ν_KS(r) that link the chemical potential of the electronic system with the effective Kohn–Sham potential through μ = ν_KS(r) + δT_S/δρ. For single ions, we identify the chemical potential with the eigenvalue of the frontier orbital when the atom is in the limit of full ionization. For the case of cations, the chemical potential is found above ?(I + A)/2 and has the property of grouping ions with the same chemical characteristics. For the anion instead, the chemical potential is fixed at the ionization energy. By solving the above equations numerically, two radial points called r_? and r₊ are obtained and compared with the Shannon–Prewitt ionic radius. Moreover, we found for the halide series, that r_? is numerically equivalent to rm, the radii where the electrostatic potential has its minimum, but shows different behavior upon charge variation. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006     

Theoretical Study of 13C Chemical Shifts Structures of Some ortho-Substituted 3-Anilino-2-nitrobenzo[b]thiophenes and 2-Anilino-3-nitrobenzo[b]thiophenes and Comparison with Experimental

¹³C NMR chemical shifts have been calculated for structures of some substituted 3‐anilino‐2‐nitrobenzo‐[b]thiophenes ( 2 o) and 2‐anilino‐3‐nitrobenzo[b]thiophenes ( 3 o) derivatives containing OH, NH₂, OMe, Me, Et, H, F, Cl and Br. The molecular structures were fully optimized using B3LYP/6‐31G(d,p). The calculation of the ¹³C shielding tensors employed the GAUSSIAN 03 implementation of the gauge‐including atomic orbital (GIAO) and continuous set of gauge transformations (CSGT) by using 6‐311++G(d,p) basis set at density functional levels of theories (DFT). The isotropic and the anisotropy parameters of chemical shielding for all compounds are calculated. The predicted ¹³C chemical shifts are derived from equation δ=δ₀+δ where δ is the chemical shift, δ is the absolute shielding, and δ₀ is the absolute shielding of the standard TMS. Excellent linear relationships have been observed between experimental and calculated ¹³C NMR chemical shifts for all derivatives     

Carbon-13 NMR and conformational analysis of meso- and dl-α,α′-disubstituted succinic acids

meso-and dl-Diastereomers of a number of α,α′-disubstituted succinic acids have been shown to give different ¹³C NMR chemical shifts. The results can be satisfactorily explained on the basis of their conformational analyses. A discussion of the observed chemical shifts is presented, and the preferred conformation for each of several compounds is predicted on the basis of these chemical shifts.     

Assessment of the extended Koopmans' theorem for the chemical reactivity: Accurate computations of chemical potentials,chemical hardnesses,and electrophilicity indices

The extended Koopmans' theorem (EKT) provides a straightforward way to compute ionization potentials and electron affinities from any level of theory. Although it is widely applied to ionization potentials, the EKT approach has not been applied to evaluation of the chemical reactivity. We present the first benchmarking study to investigate the performance of the EKT methods for predictions of chemical potentials (μ) (hence electronegativities), chemical hardnesses (η), and electrophilicity indices (ω). We assess the performance of the EKT approaches for post‐Hartree–Fock methods, such as Møller–Plesset perturbation theory, the coupled‐electron pair theory, and their orbital‐optimized counterparts for the evaluation of the chemical reactivity. Especially, results of the orbital‐optimized coupled‐electron pair theory method (with the aug‐cc‐pVQZ basis set) for predictions of the chemical reactivity are very promising; the corresponding mean absolute errors are 0.16, 0.28, and 0.09 eV for μ, η, and ω, respectively. © 2015 Wiley Periodicals, Inc.     

15N chemical shifts as a conformational probe in enaminones A variable temperature study at natural isotope abundance

The high sensitivity of ¹⁵N shielding to the displacement of the lone pair electrons makes it a useful conformational probe for remote parts of a conjugated molecule. Thus, the chemical shifts are observed for different rotamers of enaminones in the slow exchange limit. The interpretation of the ¹⁵N chemical shifts in terms of the non-planarity of the E, s-E rotamers is in accord with ¹³C chemical shifts and ¹J(CH) coupling constants.     

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.     

Selenate—An internal chemical shift standard for aqueous 77Se NMR spectroscopy

The ⁷⁷Se NMR spectra of selenate were studied under various circumstances, such as concentration, pH, temperature, ionic strength, and D₂O:H₂O ratio, in order to examine its potential as a water-soluble internal chemical shift standard. The performance of selenate as a chemical shift reference and that of other attempted ones from the literature (dimethyl selenide, tetramethylsilane/TMS, and 3-(trimethylsilyl)propane-1-sulfonate/DSS) was also explored. The uncertainty in the resulting chemical shift relative to the effective spectral width is comparable to that of DSS. Compared to the currently prevalent water-soluble external chemical shift reference, selenic acid solution, the properties of internal selenate are much more favorable in terms of ease of use. We have also demonstrated that selenate can be used in reducing media, which is inevitable for the analysis of selenol compounds. Thus, it can be stated that sodium selenate is a robust internal chemical shift reference in aqueous media for ⁷⁷Se NMR measurements; the chemical shift of this reference in a solution containing 5 V/V% D₂O at 25°C and 0.15 mol·dm⁻³ ionic strength is 1048.65 ppm relative to 60 V/V% dimethyl selenide in CDCl₃ and 1046.40 ppm relative to the ¹H signal of 0.03 V/V% TMS in CDCl₃. In summary, a water-soluble, selenium-containing internal chemical shift reference compound was introduced for ⁷⁷Se NMR measurements for the first time in the literature, and with the aforementioned results all previous ⁷⁷Se measurements can be converted to a unified scale defined by the International Union of Pure and Applied Chemistry.     

Normal (decoupled) representation of electronegativity equalization equations in a molecule

The decoupled (normal) representation of the electronegativity (chemical potential) equalization equations, in which the hardness tensor {η_ij}={?μ_i/?N_j} becomes diagonal, is examined in the atoms-in-a-molecule (AIM ) approximation; μ_i=?E/?N_i is the chemical potential of the i-th AIM , N_i is its electron population, and E is the system energy. All relevant chemical potential, hardness, softness, and Fukui function quantities corresponding to the normal electron redistribution channels, Q_y, are discussed and expressed in terms of the canonical AIM parameters. The normal chemical potentials, μ _γ=?E/?Q_γ, provide a natural classification of the normal modes into three groups: (a) acceptor normal modes (μ _a < 0, positive mode Fukui function, hardness, and softness), (b) donor normal modes (μ _d > 0, negative mode Fukui function, hardness, and softness), and (c) polarization normal modes (μ _p=0, zero mode Fukui function, hardness, and softness). The implications of the normal mode analysis for the theory of chemical reactivity are briefly summarized.     

The 1H and 13C chemical shifts for some tetrakis(para-substituted phenyl)ethylenes

The proton and carbon chemical shifts for a series of tetrakis(p-substituted phenyl)ethylenes are described. Assignments followed routine substituent chemical shift trends. Both proton and carbon chemical shifts ortho to the varying substituent follow the empirical parameter, Q. The ethylene carbon chemical shifts are proportional to those at the position para to the varying substituent.     

Geometrical isomerism and 13C spectra of the β-substituted Enones R1?C(1)O?C(2)HC(3)H?R2

The ¹³C chemical shifts of the unsaturated carbons were measured in 31 cis and trans pairs of β-substituted enones R₁? C(1)O? C(2)H?C(3)H? R₂. In these polarized ethylenes the chemical shifts of the olefinic carbons are simply related by the equation δ_c=δ_t+A. The steric and electronic effects introduced by the R₁ and R₂ substituents influence the chemical shifts of C-2 and C-3 in both isomers. It is shown that the sign and magnitude of the intercept A mainly reflect the π-charge electronic density changes which arise in the cis isomer and are transmitted via the π-framework. The effect of the steric interaction on the chemical shift of C-3 in the cis isomers is postulated to be related to the symmetry of the substituents. Therefore, the differential shielding of C-3 is indicative of the conformational structure of the cis molecule.     

15N n.m.r.: Substituent effects on nitrogen chemical shifts in anilinium ions

¹⁵N chemical shifts were measured in a series of anilinium fluorosulfonate salts and compared with chemical shift data from a comparable series of ¹⁵N-enriched aniline derivatives. A smaller overall range of nitrogen chemical shifts was observed for the protonated aniline series compared with that for the unprotonated anilines and is attributed to the elimination of nitrogen lone pair delocalization in the former series. Further-more, it was found that the range of nitrogen chemical shifts in the protonated anilines is determined primarily by substituent electronic effects from the ortho ring position with almost negligible contributions from the para position.     

NMR-Untersuchungen über inter- und intramolekulare Wechselwirkungen in substituierten Trifluormethylphenylisocyanaten,Fluorphenylisocyanaten, in Fluoranilin und Fluornitrobenzol

The temperature and solvent concentration (carbon tetrachloride) dependence of the chemical shifts of the fluorine and proton signals of substituted trifluoromethylphenylisocyanate, fluorophenylisocyanate, fluoroaniline and fluoronitrobenzene are investigated. The temperature effect on the chemical shifts of the fluorine signals of the ortho-substituted trifluoromethylphenylisocyanate originates from the intramolecular interaction of the trifluoromethyl groups and the ortho-substituents, resulting in an increase of the chemical shifts with rising temperature. The temperature dependence of the electron distribution of meta- and para-substituted trifluoromethylphenylisocyanates is mainly attributed to intermolecular interactions, causing a decrease of the chemical shifts of fluorine signals with increasing temperature. The protons of the investigated compounds, being localized on the ‘molecular surface’, show a temperature dependence of the chemical shifts due to the van der Waals effect. Increasing the amount of the solvent (carbon tetrachloride) leads to a weakening of the anisotropy effect and causes an increase of the chemical shifts of the fluorine and proton signals of all investigated compounds.     

13C NMR study of ortho-, meta- and para- substituted phenyldiphenylamines: Substituent effect correlations

The ¹³C chemical shifts of 11 substituted triphenylamines have been determined and the assignment of these resonances made using intensities, ¹H and ¹⁹F couplings and predictions from bond additivity relationships. ¹³C chemical shifts at carbons bearing the substituent and at carbons ortho to the substituent correlated reasonably well with the Q parameter. A multiple regression analysis of chemical shifts with the field and resonance parameters of Swain and Lupton and the Q parameter produced significantly better correlations than those obtained when Q was omitted for these positions. ¹³C chemical shift correlations for carbons meta and para to the substituent were not significantly better than when Q was omitted. Significant correlations were obtained between field and resonance parameters and ¹³C chemical shifts of C-o and C-p, and C-i, C-o, C-m and C-p of the non-substituent bearing phenyl rings in ortho- and para-substituted phenyldiphenylamines, respectively.     

Quantum chemical calculation of nickel and copper atomic valencies in crystalline oxides

A new quantum chemical definition is proposed of the full atomic valence, V_A, taking into account both the covalent and ionic parts of the chemical bonds formed by atoms in molecules and crystals. The full atomic valencies, covalencies, and charges on atoms are calculated for nickel–oxygen crystalline compounds in the CNDO band theory approximation. A comparison of the chemical bonding in nickel and copper crystalline oxides is given. © 1994 John Wiley & Sons, Inc.     

Carbon-13 NMR study of substituted benzyl N,N-dimethylcarbamates

The ¹³C NMR chemical shifts of m- and p-substituted benzyl N,N-dimethylcarbamates were measured in CDCl₃. The meta and para ¹³C substituent chemical shifts were analysed by means of dual substituent parameter (DSP) equations. Good correlations were obtained, especially for the para-carbon substituent chemical shifts. The computed transmission coefficients, ρ_I and ρ_R, are consistent with the general features of the fitting parameters. It has been shown that no significant electron demand is imposed by the ? CH₂OCON(CH₃)₂ substituent.     

Electronic properties of oxides: Chemical and theoretical approaches

An original analysis of the electronic and chemical properties of oxides is proposed based on the electronegativity χ and the chemical hardness η. This model which has been applied to various oxide based metals, degenerate semiconductors and optical properties of transition metal oxides allows explaining their electronic behaviors: Strong electronegativity and weak chemical hardness characterize oxides of transition elements with high oxidation state. Strong electronegativity and strong chemical hardness feature insulators with a large optical gap. Weak electronegativity and moderate chemical hardness describe alkali and alkaline earth oxides and weak electronegativity and strong chemical hardness are for ionic oxides with a relatively large optical gap. For a few illustrative case studies, ab intio electronic band structure calculations within the density functional theory framework are used.