Carbon-13 nuclear magnetic resonance spectroscopy: VIII—aliphatic hydrocarbon derivatives

Carbon-13 NMR chemical shifts of several series of aliphatic hydrocarbon derivatives–-substituted methanes, ethanes, isopropanes, n-propanes and n-butanes–-were found to have a linear relationship with σ-electron densities (Q^σ) calculated by the method of σ-included ω-HMO. A plot of the ¹³C NMR chemical shift of a given carbon in a substituted propane versus that of the corresponding carbon in a substituted butane showed a good linearity with a slope of unity. The values of the ¹³C chemical shifts of the n-butyl derivatives converged rapidly to a constant value as the distance from the substituent increased. Accordingly, the value for the δ-carbon was found to be constant regardless of the substituent. These results show that the ¹³C NMR chemical shifts of aliphatic hydrocarbon derivatives are mainly dependent on inductive effects. The convergence shown by the experimental results is supported by the calculated results of the Q^σ values of the n-butyl derivatives.     

Inductive substituent constants σji from olefinic geminal 1h, 1h nmr coupling constants

Theoretical and experimental arguments are presented supporting the postulate that olefinic NMR coupling constants ²J_HH are insensitive to β-substituent influences on the olefinic π orbital. A new set of substituent constants, σ^J_I, is proposed to measure directly the inductive substituent effect transmitted by σ-bonds. The previously available range of inductive substituent constants can be appreciably extended in this way. Comparisons of σ^J_I with other observables and parameters for selected substituents are made as a test of consistency.     

A unified correlation of substituent effects with carbon,proton and fluorine chemical shifts in aromatic and olefinic systems

At the present time no completely satisfactory quantum mechanical calculations exist for carbon, proton or fluorine chemical shifts in various substituted aromatic or olefinic systems. However, the chemical shifts in such systems have been shown to be well correlated by a linear multiple regression analysis with the Swain and Lupton field and resonance para meters ? and ?, and the semiempirical parameter Q. The utility of Q in testing substituent stereochemistry has been exemplified previously. Here the applications of the complete regression analysis to a wide variety of different systems for the three nuclei are given. The correlation is also shown to apply to selenium in substituted selenophenes. The ¹³C chemical shifts for a series of ortho substituted toluenes are presented and comparisons made with other ortho disubstituted benzenes.     

Reactivities of Dialkyl Dithiol Mesaconates in Radical Copolymerization with Styrene

Nine novel types of dialkyldithiol mesaconates (DRTM, M₁) were synthesized and copolymerized with styrene (Ma) in tetrahydro-furan at 60 °C in order to clarify the polymerization behavior of DRTM and the substituent effects on the copolymerization. From the results obtained, the monomer reactivity ratio r₁, r₂ and Q₁, e₁ values were determined. It was found that the relative reactivities l/r₂ of DRTM toward an attack by polystyryl radical were correlated only by the polar substituent constant σ? of the alkyl group in DRTM, but not by the steric substituent constant E, in Taft's equation: log (l/r₂) = σ?σ? + ΔE_s. It was also observed that the Q₁ and e₁ values for DRTM were correlated by Taft's σ? constant. The number-average molecular weights of the DRTM-ST copolymer were found to be between 5.0 × 10³ and 1.2 × 10⁴.     

The Reactivities of α -Substituted Acrylonitriles and Acrylic Esters in Radical Copolymerizations

Abstract

In order to clarify the effect of the substituents in a-substituted acrylonitriles and acrylic esters on their relative reactivities toward a polystyryl radical, the radical copolymerizations of diethyl methylenemalonate, ethyl a-chloroacrylate, ethyl a-bromoacrylate, a-chloroacrylonitrile, methyl a-methoxyacrylate, and a-methoxyacrylonitrile with styrene (M₂) were investigated at 60°C. From the copolymerization parameters obtained in this study and those reported in the literature, it was confirmed that the a substituents additively contributed to the values of log Q and e of the monomers. Hence, the reactivities of a-substituted acrylonitriles and acrylic esters relative to unsubstituted acrylonitrile and methyl acrylate toward polystyryl radical, respectively, were expressed by the following equation: log (rel. react.) = δlog Q_X + 0.83 σp where δ log Q_x and σp are the resonance and polar substituent constants of α substituents, respectively. The values of δ log Q_x were determined for CH₃0, CH₃, C₆H₅, Cl, Br, OCOCH₃, COOCH₃, and CN substituents, and these values were closely related to the other resonance substituent constants such as ER and R of corresponding substituents, except for OCH₃ substituent. This relationship could also be applied to monosubstituted ethylenic monomers by using the same parameters. However, the reactivities of a-substituted styrenes deviated from this relationship because of the low ceiling temperatures of these monomers.     

In-Situ Electronegativity and the Bridging of Chemical Bonding Concepts

One challenge in chemistry is the plethora of often disparate models for rationalizing the electronic structure of molecules. Chemical concepts abound, but their connections are often frail. This work describes a quantum-mechanical framework that enables a combination of ideas from three approaches common for the analysis of chemical bonds: energy decomposition analysis (EDA), quantum chemical topology, and molecular orbital (MO) theory. The glue to our theory is the electron energy density, interpretable as one part electrons and one part electronegativity. We present a three-dimensional analysis of the electron energy density and use it to redefine what constitutes an atom in a molecule. Definitions of atomic partial charge and electronegativity follow in a way that connects these concepts to the total energy of a molecule. The formation of polar bonds is predicted to cause inversion of electronegativity, and a new perspective of bonding in diborane and guanine−cytosine base-pairing is presented. The electronegativity of atoms inside molecules is shown to be predictive of pK_a.     

Change of the favored routes of EI MS fragmentation when proceeding from N1, N1‐dimethyl‐N2‐arylformamidines to 1,1,3,3‐tetraalkyl‐2‐arylguanidines: substituent effects

Although series of N¹, N¹‐dimethyl‐N²‐arylformamidines and of 1,1,3,3‐tetraalkyl‐2‐arylguanidines are structurally analogous and similar electron‐ionization mass spectral fragmentation may be expected, they display important differences in the favored routes of fragmentation and consequently in substituent effects on ion abundances. In the case of formamidines, the cyclization‐elimination process (initiated by nucleophilic attack of the N‐amino atom on the 2‐position of the phenyl ring) and formation of the cyclic benzimidazolium [M‐H]⁺ ions dominates, whereas the loss of the NR₂ group is more favored for guanidines. In order to gain information on the most probable structures of the principal fragments, quantum‐chemical calculations were performed on a selected set. A good linear relation between log{I[M‐H]⁺I [M]^+?} and σ_R⁺ constants of substituent at para position in the phenyl ring occurs solely for formamidines (r = 0.989). In the case of guanidines, this relation is not significant (r = 0.659). A good linear relation is found between log{I[M‐NMe₂]⁺/I [M]^+?} and σ_p⁺ constants (r = 0.993). Copyright © 2010 John Wiley & Sons, Ltd.     

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.     

Correlation of the isosteric heat of adsorption of organic molecules over zeolites with equalized electronegativity and chemical hardness

Considering the direct correlation between charge transfer and heat of adsorption, we have equated the isosteric heat of adsorption (Q _st ) with Nalewajski’s charge transfer equation involving equalized electronegativities and chemical hardness given in the literature. The equation is then tested and compared with the experimental heat of adsorption values of organic molecules over zeolites given in the literature with the average percentage deviation of 15·9. Other similar types of equations of charge transfer affinity are also tested. Various semi-empirical equations based on Barrer’s approach of the determination ofQ _st and neural network method have been proposed, tested and compared for the first time     

Dynamic stereochemistry of imines and derivatives: 15—Carbon-13 NMR studies of aryl-substituted imines and oxaziridines

¹³C chemical shifts for several series of cis- and trans-N-alkylimines and oxaziridines bearing para-substituted C-phenyl rings are reported and correlated with dual substituent parameters. The ¹³C?N and oxaziridine ring carbon shifts correlate primarily with the inductive/field parameters, σ₁, whereas both resonance and inductive terms generally contribute about equally to the long-range substituent effects on alkyl side-chain chemical shifts. Correlations on diastereoisomeric imines show that the transmission of substituent effects can be significantly affected by the E–Z configuration. Aromatic carbon chemical shifts in imines are discussed in relation to the E–Z configuration and the conformation around the aryl—imino bond.     

Substituent effects on the charge density in the formyl group

SCF wavefunctions with similar extended Gaussian bases for the series HCOX with X = H, OH (syn and anti), NH₂, CN, and F yield closely similar charge deformation density maps in the formyl region of all six molecules. The differences, measured by moments of partitioned atomic deformation densities, correlate almost linearly with the Hammett substituent parameters σ_I and σ_R of the several substituents X. However, systematic deviations, especially in the carbon fragment, suggest the need for modified values of the inductive parameters σ_I for the correlation of these molecular charge densities.     

Correlation of para substituent carbon-13 chemical shifts in aromatic compounds using polar (inductive) and resonance constants which are identical for aliphatic and aromatic compounds

A correlation of para substituted ¹³C chemical shifts in aromatic compounds with substituent polar (inductive) and resonance constants σ* and σ^r of aliphatic compounds has been studied. It has been shown that the precision of the correlations obtained corresponds to that of the Swain-Lupton and Taft two-parameter equations, but the correlation equation used in this work seems to permit a more exact separation of the substituent effects of aromatic compounds into inductive and resonance contributions. Thus, σ* and σ^r substituent constants are universal parameters which can be used in a correlation analysis of the properties of both aliphatic and aromatic compounds.     

The effect of the structure of substituted phenylphosphonic difluorides on their 19F and 31P NMR spectral parameters

The ¹⁹F and ³¹P NMR spectral parameters of a series of meta and para substituted phenylphosphonic difluorides were found to be linearly related to the substituent parameters: the phosphorus chemical shifts correlated with Hammett's σ constant, the fluorine chemical shifts with Taft's σ_R parameter and the phosphorus–fluorine coupling constants with σ⁺.     

Surface energies of chain molecules from homogeneous nucleation

A model for the interpretation of homogeneous nucleation data for chain molecules is presented. The two surface energies σ_s and σ_e are related to interchain and intrachain bonding. Surface energies calculated from experimental data on n-alkanes from octane to dotriacontane and polyethylene agree with estimated values. The results are discussed in relation to surface energies measured from spherulite growth rates in polymers but these values are not known with sufficient reliability to provide a good basis for comparison.     

A detailed experimental and computational study of monocarbohydrazones

The substituent and solvent effect on intramolecular charge transfer (ICT) of twelve monocarbohydrazones (mCHs) were studied using experimental and theoretical methodology. The effects of specific and non-specific solvent–solute interactions on the UV–Vis absorption maxima shifts were evaluated using linear free energy relationships (LFERs) principles, i.e. using the Kamlet-Taft and Catalán models. Linear free energy relationships in the form of single substituent parameter equation (SSP) was used to analyze substituent effect on UV–Vis, NMR and pK_a change. According to crystallographic data and quantum chemical calculations, the trans (E) form was found to be more stable. A photochromism of compounds with 2-hydroxyphenyl and 2-pyridylimino groups substituted at imine carbon atom results in E/Z isomerization due to creation of intermolecular hydrogen bond in E and Z form, respectively. Multiple stage mass spectrometry (MS-MSⁿ) analysis was applied to define main fragmentation pathways. Furthermore, the experimental findings were interpreted with the aid of ab initio MP2/6–311 G(d,p) and time-dependent density functional (TD-DFT) methods. TD-DFT calculations were performed to quantify the efficiency of intramolecular charge transfer (ICT) with the aid of the charge-transfer distance (D_CT) and the amount of transferred charge (Q_CT) calculation. It was found that both substituents and solvents influence electron density shift i.e. extent of conjugation, and affect ICT character in the course of excitation.     

Substituent effect on the IR and PMR spectra of diphenylamines

PMR chemical shifts of various m- and p-substituted diphenylamines were correlated with σ-constants. However, the NH stretching frequencies of these amines were not linear with the σ- constants, and the irregularity in the ν_NHvs σ plot was interpreted by taking into accounts the following factors: the changes in hybridization of the imino N atoms, the normal electronic effect of the substituent and the steric effect on the planarity of the amine molecules. Results on the CN stretching absorptions were also presented.     

Etudes d'Hétérocycles Pentagonaux Polyhétéroatomiques par RMN du 13C. Effets de Substituant,de la Benzocondensation et Etude de la Tautomèrie Prototropique en Série Benzothiazolique

The ¹³C NMR spectra of 43 benzothiazoles have been recorded in DMSO-d₆. All carbon atoms have been attributed in an unambigous way owing to substituent effects in position 4, 5, 6 or 7. We discuss variations of chemical shifts as a function of the nature of the substituent in position 2 (equation of type: Δδ = aF+bR+cQ+d′), annelation in the benzoheterocyclic series, and prototropic tautomerism in the benzothiazolic series (in the case of the substituent in the 2-position being an OH, SH or NHR group).     

1H NMR studies of solvent and substituent effects on strong intramolecular hydrogen bonds

Chemical shifts of H-bonded protons in tetrabutylammonium hydrogen maleate and 14-substituted picolinic acid N-oxides have been measured in a number of dry solvents, of different activity, in order to distinguish between symmetrical single minimum and asymmetrical hydrogen bonds. In tetrabutylammonium hydrogen maleate the resonance was observed at 20.70 ppm and its was independent of the nature of the solvent used. The chemical shift value of picolinic acid N-oxide varies with the solvent. These observations suggest that the hydrogen bond is symmetrical in tetrabutylammonium hydrogen maleate but that it is asymmetrical in picolinic acid N-oxide. The chemical shifts of substituted picolinic acid N-oxides were correlated with σ_p, σ_m and Δ_pK_a. The substituent and solvent effects are compared and the position of the intramolecular H-bonded protons in picolinic acid N-oxides are estimated and discussed.     

n-π-Interaction in Enamines and Enaminoketones. A 15N-NMR. Study

¹⁵N-Chemical shifts of 32 enamines, 11 enaminoketones and 28 closely related amines have been determined with the isotope in natural abundance. In order to eliminate substituent effects, differential chemical shifts Δδ(N) are defined as δ_N(amine)-δ_N(enamine). This parameter is shown to correlate well with the free enthalpy of activation ΔG# for restricted rotation about the N? C(α) bond in enamines with extended conjugation. Δδ(N) values of substituted anilinostyrenes correlate also with ¹³C-chemical shifts of the β-carbon in the enamine system and with Hammett σ-constants of the aniline substituents. The experimental results suggest that differential ¹⁵N shifts are a useful probe to study n, π-interaction in enamines.     

Looking at orbitals in the laboratory: The experimental investigation of molecular wavefunctions and binding energies by electron momentum spectroscopy

The experimental technique of electron momentum spectroscopy (EMS ) (i.e., binary (e, 2e) spectroscopy) is discussed together with typical examples of its applications over the past decade in the area of experimental quantum chemistry. Results interpreted within the framework of the plane wave impulse and the target Hartree—Fock approximations provide direct measurements of, spherically averaged, orbital electron momentum distributions. Results for a variety of atoms and small molecules are compared with calculations using a range of Fourier transformed SCF position space wavefunctions of varying sophistication. Measured momentum distributions (MD ) provide a “direct” view of orbitals. In addition to offering a sensitive experimental diagnostic for semiempirical molecular wavefunctions, the MD's provide a chemically significant, additional experimental constraint to the usual variational optimization of wavefunctions. The measured MD's clearly reflect well known characteristics of various chemical and physical properties. It appears that EMS and momentum space chemistry offer the promise of supplementary perspectives and new vistas in quantum chemistry, as suggested by Coulson more than 40 years ago. Binding energy spectra in the inner valence region reveal, in many cases, a major breakdown of the simple MO model for ionization in accord with the predictions of many-body calculations. Results are considered for atomic targets, including H and the noble gases. The measured momentum distribution for H₂ is also compared with results from Compton scattering. Results for H₂ and H are combined to provide a direct experimental assessment of the bond density in H₂, which is compared with calculations. The behavior of the outer valence MD ''s for small row two and row three hydride molecules such as H₂O and H₂S, NH₃, HF, and HCl are consistent with well known differences in chemical and physical behavior such as ligand-donor activity and hydrogen bonding. MD measurements for the outermost valence orbitals of HF, H₂O and NH₃ show significant differences from those calculated using even very high-quality wavefunctions. Measurements of MD's for outer σ_g orbitals of small polyatomic molecules such as CO₂, COS, CS₂, and CF₄ show clear evidence of mixed s and p character. It is apparent that EMS is a sensitive probe of details of electronic structure and electron motion in atoms and molecules.