13C-Nmr Spectroscopy of β-Nitrostyrenes. II. Mono-, Bi- and Tri-Methoxy Phenyl-Substitutions and Long Distance Electronic Effects

By means of ¹³C-NMR spectroscopy and AM1 molecular orbital calculations of mono-, bi- and tri-methoxy-β-nitrostyrenes at the meta and para positions, we have characterized a long distance electronic charge transfer pattern on the ethylenic bridge (CH=CH) and on the aromatic ring (Ph) carbon centers, determined by the electron-donor nature of the methoxy-substituent groups.

After a complete spectral assignment of the ¹³C-NMR signals, we have found a functional dependence of the chemical shifts on the C₁ and C_β centers respect to the C₄ and C₃ methoxy subtitution sites on the aromatic ring, while in the same molecular series C_alfa-chemical shifts are practically constants. on the other hand, the ¹³C-NMR chemical shifts of the C₃ and C₄ centers plus the analysis of the AM1 electronic charge density have permitted us determine the long distance charge transfer effect induced by the C₄ methoxy substitutions as well as the attenuation of this effect due to the C₃ methoxy substitutions.     

Bridge Effect in Charge Transfer Absorption Bands. Para-substituted Benzylideneacetones

Abstract

The electronic absorption charge transfer bands in a series of para - substituted benzalketones are analyzed in order to stablish the role of the electron-donor substkuent as well as the electronic properties of the molecular structure of the π-conduction channel.

Absorption bands assignment of the π-π^? electronic transitions in the near ultraviolet spectral region is carry out from an experimental and theoretical point of view. The photo-induced charge transfer spectral bands in these aromatic compounds follow the same spectral pattern than the para-substituted benzaldehydes and acetophenones and the electronic transition takes place in the π,π^?(¹L_a) excited state. However, our semiempirical M.O. calculations show that this charge transfer process involve the electron-acceptor carbonyl group and the olefinic bond bridge as a second electron-acceptor group.     

A bonding evolution theory study on the catalytic Noyori hydrogenation reaction

ABSTRACT

The electronic rearrangements involved in Noyori hydrogenation reactions with double bonds (ethene and formaldehyde) are analysed using the bonding evolution theory. The study and analysis of the changes on the electron localisation function topology along a given reaction path reveals fluxes of electron density, allowing to unambiguously identify the main chemical events happening along the chemical reactions. This analysis shows that the first hydrogen transfer (with hydride character) occurs before the transition state (TS), while the second hydrogen transfer (with proton character) takes places after having reached the TS. The lower energy barrier found for formaldehyde over ethene is explained by two reasons. First, the hydride transfer is favoured for the C?=?O bond over C?=?C due to the electrophilic character of the carbon atom. Second, a negatively charged CH₃–X (X?=?CH₂, O) hidden intermediate is formed in the proximities of the TS region. The oxygen atom is able to stabilise this negatively charged species more effectively than the CH₂ group due to its higher electronegativity and the presence of V(O) lone pairs. The obtained analysis explains and rationalises catalyst chemoselectivity (C?=?O vs. C?=?C). Finally, a curly arrow representation diagram accounting for the electronic rearrangements is proposed on the basis of BET results.     

Can tetra‐tert‐butylethylene be formed by ion–ion recombination or ion–molecule reaction of two di‐tert‐butylcarbene units?—a DFT study

The reaction channels of di‐tert‐butylcarbene ( 2 ), its radical anion, ( 3 ) and its radical cation ( 4 ) were investigated theoretically by using DFT/B3LYP with 6‐31+G(d) basis set and 6‐311+G(2d,p) for single point energy calculations. Conversion of the neutral carbene 2 to the charged species 3 and 4 results in significant geometric changes. In cation 4 two different types of C? (CH₃)₃ bonds are observed: one elongated sigma bond called “axial” with 1.61 Å and two normal sigma bonds with a bond length of 1.55 Å. Species 2 and 4 have an electron deficient carbon center; therefore, migration of CH₃ and H is observed from adjacent tert‐butyl groups with low activation energies in the range of 6–9 kcal/mol like similar Wagner–Meerwein rearrangements in the neopentyl‐cation system. Neutral carbene 2 shows C? H insertion to give a cyclopropane derivative with an activation energy of 6.1 kcal/mol in agreement with former calculations. Contrary to species 2 and 4 , the radical anion 3 has an electron rich carbon center which results in much higher calculated activation energies of 26.3 and 42.1 kcal/mol for H and CH₃ migrations, respectively. NBO charge distribution indicates that the hydrogen migrates as a proton. The central issue of this work is the question: how can tetra‐tert‐butylethylene ( 1 ) be prepared from reaction of either species 2 , 3 , or 4 as precursors? The ion–ion reaction between 3 and 4 to give alkene 1 with a calculated reaction enthalpy of 203.5 kcal/mol is extremely exothermic. This high energy decomposes alkene 1 after its formation into two molecules of carbene 2 spontaneously. Ion–molecule reaction of radical anion 3 with the neutral carbene 2 is a much better choice: via a proper oriented charge–transfer complex the radical anion of tetra‐tert‐butylethylene (11) is formed. The electron affinity of 1 was calculated to be negligible. Copyright © 2010 John Wiley & Sons, Ltd.     

Relationship between substituent effect and aromaticity – Part III: naphthalene as a transmitting moiety for substituent effect

Molecular geometry of 10 isomeric nitronaphtholate ions (excluding peri‐ and ortho‐type substituted systems), 1‐ and 2‐naphtholate ions, 1‐ and 2‐nitronaphthalene, meta‐ and para‐nitrophenolate, phenolate, and nitrobenzene were optimized at B3LYP/6‐311G** level of approximation. Substituent effect stabilization energy (SESE), geometry‐based aromaticity index HOMA, magnetism‐based indices NICS, NICS(1), NICS(1)_zz, and parameters characterizing Bond Critical Points (BCP) (ρ, ?²ρ, ellipticity, ion/cov) of the Bader AIM theory were used to characterize transmitting properties for substituent effect through the naphthalene moiety. It results from our study that the studied systems could be clearly divided into two groups, (i) a para‐type group, where the intramolecular charge transfer between the π‐electron donating and π‐electron accepting substituents can be described by canonical forms with charge separation (as in the case of para‐nitrophenolate) and (ii) a meta‐type group, where this transfer requires using canonical forms with double charge separation (as in the case of meta‐nitrophenolate). Copyright © 2007 John Wiley & Sons, Ltd.     

Quantum chemical studies on a series of transition metal carbon dioxide complexes: Metal–carbon bonding and electronic structures

The bonding features and electronic structures of a series of transition metal carbon dioxide complexes have been studied by density functional theory (DFT) calculations combined with natural bond orbital (NBO) analysis and energy-decomposition analysis (EDA). NBO analysis shows that the interaction between the metal center and the carbon atom of the carbon dioxide ligand (M–C) is stronger than the other interaction between the metal center and the carbon dioxide ligand. Natural hybrid orbital (NHO) analysis gives the detailed bonding features of the M–C bond for each complex. The NBO charge distribution on the carbon dioxide unit in all studied complexes is negative, which indicates charge transfer from the metal center to the carbon dioxide ligand for all studied complexes. The hyperconjugation effect of the metal center and the two C–O bonds of the carbon dioxide ligand has been estimated using the NBO second-order perturbation stabilization energy. It was found that the NBO second-order stabilization energy of C–O?→?nM* is sensitive to the coordinated sphere and the metal center. Frontier molecular orbital (FMO) analysis shows that complexes 1 and 4 may be good nucleophilic reagents for activation of the carbon dioxide molecule. However, the EDAs show that the M–CO₂ bond interaction energy of complex 4 is about two times as large as that of complex 1. The high M–CO₂ bond interaction energy of complex 4 may limit its practical application.     

Substituent effect on the electrochemical behaviour of some ortho-substituted (aryl)(2-nitrobenzo[ b ]thiophen-3-yl)amines. A combined experimental and computational study

The reduction potential values of the title compounds (1Ar,o) have been evaluated by cyclic voltammetry at a platinum electrode in the presence of 0.1?M N(Et)₄BF₄ in DMSO. Compounds 1Ar,o give reversible reduction peaks. The ortho-substituent affects their values basically by electronic effects (good LFER have been observed). The peculiar behaviour of (2′-hydroxyphenyl)(2-nitrobenzo[b]thiophen-3-yl)amine 1Ar,o(i) (showing two quasi reversible systems of peaks) has been explained on the basis of the special effect of the 2′-hydroxy substituent, which is able to assist a proton transfer through a network of hydrogen bonds involving the amine nitrogen atom (proton shuttle). This behaviour is unexpected in anhydrous DMSO and has similarities with the reaction features observed in water. The above hypothesis has been confirmed by quantum-mechanical DFT calculations carried out on 1Ar,o(i). In this theoretical investigation all the possible species, that can form as a consequence of two mono-electron reduction processes and the relevant various chemical transformations involving the intermediate species, have been carefully investigated.     

Interaction of <Superscript>3</Superscript>He<Superscript>2+</Superscript> and Ar<Superscript>6+</Superscript> ions with acetylene,ethylene, and ethane molecules

Time-of-flight mass spectroscopy methods are employed for studying processes occurring during capture of electrons by ³He²⁺ and Ar⁶⁺ multiply charged ions with energy 6z keV (z is the ion charge) from C₂H _n molecules (n = 2, 4, 6) with different multiplicities of C-C bonds. Fragmentation schemes of the molecular ions formed in such processes are established from analysis of correlations of recording times for all fragment ions. The absolute values of the cross sections of capture of an electron and capture with ionization are measured, as well as the cross sections of formation of fragment ions in these processes. The absolute values of total capture cross sections for several electrons are determined.     

Photoluminescence of a-Si:H films grown by plasma-enhanced chemical vapor deposition and doped with erbium from the metallorganic compound Er(HFA)3·DME

Using standard low-temperature (<300 °C) plasma-enhanced chemical vapor deposition (PE CVD) technology, films of a Si(Er): H were obtained that emitted light in the neighborhood of 1.54 μm at room temperature. The Er source was the specially synthesized fluorine-containing metallorganic complex Er(HFA)₃·DME where HFA=CF₃C(O)CHC(O)CF₃ and DME=CH₃OCH₂CH₂OCH₃, which possesses a low transition temperature to the gas phase (of order 100 °C) at working pressures (0.1–0.5 Torr) for the PE CVD method. Distinctive features of the photoluminescence spectrum of a-Si(Er):H were investigated in the range 0.5–1.7 μm for T=77 and 300 K. The presence of photoconductivity in the synthesized films is evidence of their satisfactory electronic quality. Fiz. Tverd. Tela (St. Petersburg) 40, 1433–1436 (August 1998)     

Charge transfer between alkali cluster ions and atoms in the 1 to 10 keV collisional energy range

The cross-sections for collisional charge transfer between singly charged free clusters M _n ⁺ (M = Li, Na; n=1...50) and atomic targets A (cesium, potassium) have been measured as a function of collisional relative velocity in laboratory energy range 1–10 keV. For each cluster size, the experimental values of the charge transfer cross-section are fitted with an universal parametric curve with two independent parameters and v_m, the maximum cross-section and the corresponding velocity. For small size clusters (), the characteristic parameters show strong variations with the number of atoms in the cluster. Abrupt dips observed for n=10 and n=22 are attributed to electronic properties. Charge transfer patterns observed for various collisional systems present similarities, which appear more sensitive to cluster quantum size effects than to collision energy defects. In their whole, the and v_m parameters show differences in both their size evolution and their absolute values discussed in term of projectile and target electronic structures. Received 13 April 2000 and Received in final form 29 June 2000     

BF自由基X1∑+和a3∏态光谱常数和分子常数研究

采用内收缩多参考组态相互作用方法和相关一致基aug-cc-pV6Z, 对BF自由基X¹∑⁺和a³∏ 态的势能曲线进行了研究. 计算是在0.095---1.33 nm的核间距内进行的. 为获得更准确的结果, 计算中还考虑了Davidson修正、相对论修正及核价相关修正对势能曲线的影响. 相对论修正采用的方法是二阶DouglasKroll哈密顿近似, 修正计算是在cc-pV5Z基组水平上进行的. 核价相关修正使用的是cc-pCV5Z基组. 利用得到的势能曲线, 拟合出了各种修正下BF自由基X¹∑⁺和a³∏ 态的光谱常数D_e, R_e, ω_e, ω_ex_e, ω_ey_e, B_e和α_e、并与实验结果进行了比较. 结果表明: 考虑Davidson修正、相对论修正和核价相关修正后得到的光谱常数最接近实验结果. 利用修正后的势能曲线, 通过求解径向振转Schrödinger方程, 找到了转动量子数J = 0时这两个电子态的全部振动态, 并计算了每一电子态前20个振动态的振动能级、惯性转动常数和离心畸变常数, 其值与已有的实验结果较为一致. 本文得到的光谱常数和分子常数达到了很高的精度, 能为进一步的光谱实验提供可靠的参考.     

13C-NMR Spectroscopy of para-Substituted Benzylideneacetones.: I. Long Distance Electronic Effects

para-benzylideneacetones present a characteristic long distance charge transfer pattern, where the olefinic bridge (CH=CH) and the aromatic ring (Ph) carbon centers are perturbed according to the nature of the para-substituent groups.

By means of ¹³C-NMR spectroscopy and AMl molecular orbital calculations we have found that in this molecular series the chemical shifts (Δ) and the charge densities (qAMI) corresponding to the C₃, C₁ and C_β centers follow a functional dependence of the type: Δ = a qAMl + Δ°, while C₂, C_α and C_CO are practically constants.

On the other hand, after a complete spectral assignment of the ¹³C-NMR signals, an analysis of the electron-donor substituent effect at the para-position of the aromatic carbonyl compounds on the C₄ center, has permitted us to find a good correlation between the C₄ spectral shift and the electronegativity of this vicinal center.     

Theoretical study on the sequential hydroxylation of C82 fullerene based on Fukui function

In the present work semi-empirical PM3 method and ab initio density-functional theory calculations were performed in carbon systems. The condensed Fukui function was calculated and HOMO–LUMO were visualised in order to study the sequence of hydroxylation of two isomers of C₈₂ fullerene for the low coverage regime, with the formula C₈₂(OH) _x where x?=?0???12. It was found that there was a formation of dangling bonds on structures with an odd number of hydroxyl groups on the fullerene surface, which suggests an enhanced reactivity of these molecules. Nevertheless, the coverings with an even number of groups tend to the reconstruction of π bonds, obtaining less reactive molecular structures. With the adsorption of the first group, a narrow HOMO–LUMO gap (1.28?eV) is observed in comparison with the C₈₂(OH)₂ system (1.70?eV), as is found in similar systems, such as C₆₀ fullerenol [E.E. Fileti et al., Nanotechnology 19, 365703 (2008); J.G. Rodríguez-Zavala and R.A. Guirado-López, Phys. Rev. B 69, 075411 (2004)]. Through an analysis of the electronic structure to these coverings, a splitting of electronic energy levels in the structure with one hydroxyl group is observed, which could be one of the factors that causes the narrowing of the energy gap in this structure. On the other hand, with a coverage of 12 hydroxyl groups, the formation of an amphiphilic molecule, where the location of groups in one side of the C₈₂ surface provides an hydrophilic character, is observed, while the uncovered part has an hydrophobic character. This could be important in the formation of Langmuir monolayers. Finally, it is shown that the precise distribution of the OH groups on the fullerene surface plays a crucial role in the electronic structure of the polyhydroxylated fullerenes.     

The Schiff Base Probe With J-aggregation Induced Emission for Selective Detection of Cu2+

Here, three Schiff bases 3a-c, differing by the substitutions (–H, –Cl, and –N(CH₃)₂) on the phenyl ring, have been designed and synthesized via the reaction of ortho-aminophenol with benzaldehyde, 2,4-dichlorobenzaldehyde and para-dimethylamine benzaldehyde in 1:1 molar ratio with favourable yields of 89–92%, respectively. Their structural characterizations were studied by FT-IR, NMR, MALDI-MS and elemental analysis. The fluorescence behaviours of compounds 3a and 3b exhibited a severe aggregation caused quenching (ACQ) effect in EtOH/water system. On the contrary, compound 3c had an obvious J-aggregation induced emission (AIE) feature in EtOH/water mixture (v/v?=?1:1), and exhibited excellent sensitivity and anti-interference towards Cu²⁺ with the limit of detection (LOD) of 1.35?×?10^–8 M. Job’s plot analysis and MS spectroscopic study revealed the 2:1 complexation of probe 3c and Cu²⁺. In addition, probe 3c was successfully applied to the determination of Cu²⁺ in real aqueous samples.

     

Bichromophoric behavior of nitrophenyl‐triazene anions: a resonance Raman spectroscopy investigation

Highly delocalized molecular frameworks with intense charge transfer transitions, known as push‐pull systems, are of central interest in many areas of chemistry, as is the case of nitrophenyl‐triazene derivatives. The 1,3‐bis(2‐nitrophenyl)triazene and 1,3‐bis(4‐nitrophenyl)triazene were investigated by electronic (UV‐Vis) and resonance Raman (RR) spectroscopies. The bichromophoric behavior of 1,3‐bis(4‐nitrophenyl)triazene anion opens the possibility of tuning with visible radiation, two distinct electronic states. The RR profiles of nitrophenyl‐triazene derivatives clearly show that the first allowed electronic state can be assigned to a charge transfer from the ring π system to the NO₂ moiety (ca 520 nm), while the second, as a charge transfer from N₃⁻ to the aromatic ring (ca 390 nm). In the para‐substituted derivative, a more efficient electron transfer and a greater energy separation between the two excited states are observed. Copyright © 2008 John Wiley & Sons, Ltd.     

Effects of neutral and charged substituents on the infrared carbonyl stretching frequencies in phenyl and alkyl benzoates in DMSO

The carbonyl infrared stretching frequencies for 57 meta‐, para‐ and ortho‐substituted phenyl benzoates, C₆H₅CO₂C₆H₄‐X and alkylbenzoates, C₆H₅CO₂R, containing besides neutral substituents the charged substituents in phenoxy and alkoxy part in dimethyl sulfoxide (DMSO) have been recorded. The carbonyl stretching frequencies, ν_CO, for meta‐ and para‐substituted phenyl esters of benzoic acids in the case of neutral substituents were found to correlate well with the substituent constants, σ°. The ν_CO values for ortho derivatives correlated with the inductive substituent constants, σ_I, only. The values of constants for charged substituents, σ°_±, calculated on the basis of the ν_CO and the ¹³C NMR chemical shifts, δ_CO, in DMSO agree well with the σ°_± values for the corresponding ion pairs reported by Hoefnagel and Wepster and those determined from the log k values of the alkaline hydrolysis in 4.4 M NaCl solution at 50 °C. Thus, the values of substituent constants for ion pairs of charged substituents estimated on the basis of aqueous data could be successfully used in non‐aqueous solution (DMSO) simultaneously with neutral substituents in case the charged substituents were not completely ionized and are in ion pair form. Copyright © 2016 John Wiley & Sons, Ltd.     

Charge inversion,condensation and decondensation of DNA and polystyrene sulfonate by polyethylenimine

We study the formation and structure of stable electrostatic complexes between polyanions (DNA and poly(styrene-sulfonate)) and linear polyethylenimine. The charge ratio x of the mixture is tuned by varying the concentration of the polycation at constant concentration of polyanion. In agreement with recent theories, dynamic light scattering and electrophoretic mobility measurements show two distinct regimes of weak and strong complexation. At low polycation concentration, negatively charged small complexes involving a few polyanion chains are observed first. By further increasing x , these small complexes condense at a precise charge ratio x _c < 1 to form large anionic aggregates. The inversion of the charge of the condensed complexes coincides with the maximum of complexation and precedes the dissolution of the aggregates which occurs at a well-defined decondensation threshold x _d > 1. Above x _d , positively charged complexes containing again a few overcharged polyanion chains are observed. The macroscopic phase diagram is qualitatively well corroborated by AFM observation of the complexes. The influence of entropic effects is probed by varying parameters like concentration, polycation molecular weight and ionic strength. Structure of stable negatively charged complexes is investigated at higher concentration using Small Angle Neutron Scattering. In the condensed regime, we observe large soluble bundles with sharp interfaces where the local structure of the polyanions is preserved.     

The pKa theoretical estimation of C―H,N―H,O―H and S―H acids in dimethylsulfoxide solution

A scheme for the pK_a estimation of organic acids in dimethylsulfoxide (DMSO) solution based on quantum chemical calculations is proposed. The procedure of pK_a calculation requires several steps. The first is the calculation of the gas phase acidity of the compound. The G3MP2B3, G4MP2 as well as CBS‐QB3 composite methods made it possible to estimate values of gas phase acidities of an extensive set of structures with a high confidence level (standard deviations equal to 1.15, 1.13 and 1.29 kcal mol^?1, respectively; the test set included 91 compounds). The second step is the computation of the solvation correction with the integral equation formalism version of polarizable continuum model (IEF‐PCM)–B3LYP/6‐311+G(d,p) approximation. Within the bounds of our approach, the medium properties were covered only by the PCM model, i.e. the proposed procedure neglects specific interactions between DMSO and the solute. It was determined that the approach to pK_a estimation mentioned above is the most balanced in terms of accuracy, resource intensity and computation time cost. In the third step, the error of the pK_a calculation was decreased by correlation allowances. Correlation allowances were determined for each acid class (62 С―Н, 55 N―Н, 24 O―Н and 5 S―Н acids) in the range of 50 units in terms of logarithmic scale using the test set including 146 compounds. Seven O―H acids showing the ability to form cyclic dimers were separated into a discrete group. The proposed methodology was applied to the estimation of pK_a for trans‐ and cis‐dimethyl‐4,5‐dihydro‐3H‐pyrazol‐3,5‐dicarboxylates as well as for 5‐fluorouracil subject to competitive dissociation, the latter by N1―H or N3―H bonds. Copyright © 2014 John Wiley & Sons, Ltd.