Enols of 2‐nitro‐ and related 2‐substituted malonamides

The structures of 2‐substituted malonamides, YCH(CONR¹R²)CONR³R⁴ (Y = Br, SO₂Me, CONH₂, COMe, and NO₂) were investigated. When Y = Br, R¹R² = R³R⁴ = HEt; Y = SO₂Me, R¹–R⁴ = H and for Y = CONH₂ or CONHPh, R¹–R⁴ = Me, the structure in solution is that of the amide tautomer. X‐ray crystallography shows solid‐state amide structures for Y = SO₂Me or CONH₂, R¹–R⁴ = H. Nitromalonamide displays an enol structure in the solid state with a strong hydrogen bond (O^…O distance = 2.3730 Å at 100 K) and d(OH) ≠ d(O^…H). An apparently symmetric enol was observed in solution, even in appreciable percentages in highly polar solvents such as DMSO‐d₆, but K_enol values decrease on increasing the solvent polarity. The N,N′‐dimethyl derivative is less enolic. Acetylmalonamides display a mixture of enol on the acetyl group and amide in non‐polar solvents, and only the amide in DMSO‐d₆. DFT calculations gave the following order of pK_enol values for Y: H > CONH₂ > COMe ≥ COMe (on acetyl) ≥ MeSO₂ > CN > NO₂ in the gas phase, CHCl₃, and DMSO. The enol on the C?O group is preferred to the aci‐nitro compound, and the N? O? H^…O?C is less favored than the C?O? H^…O?C hydrogen bond. Copyright © 2009 John Wiley & Sons, Ltd.     

Substituent effects on the 13C NMR chemical shifts of the imine carbon in N‐(4‐X–benzylidene)‐4‐(4‐Y–styryl) anilines

Long‐range electronic substituent effects were targeted using the substituent dependence of δ_C(C═N), and specific cross‐interactions were explored extendedly. A wide set of N‐(4‐X–benzylidene)‐4‐(4‐Y–styryl) anilines, p‐X–C₆H₄CH═NC₆H₄CH═CHC₆H₄–p‐Y (X = NMe₂, OMe, Me, H, Cl, F, CN, or NO₂; Y = NMe₂, OMe, Me, H, Cl, or CN) were prepared for this study, and their ¹³C NMR chemical shifts δ_C(C═N) of C═N bonds were measured. The results show that both the inductive and resonance effects of the substituents Y on the δ_C(C═N) of p‐X–C₆H₄CH═NC₆H₄CH═CHC₆H₄–p‐Y are less than those of the substituents Y in p‐X–C₆H₄CH═NC₆H₄–p‐Y. Moreover, the sensitivity of the electronic character of the C═N function to electron donation/electron withdrawal by the substituent X or Y attenuates as the length of the conjugated chain is elongated. It was confirmed that the substituent cross‐interaction is an important factor influencing δ_C(C═N), not only when both X and Y are varied but also when either X or Y is fixed. The long‐range transmission of the specific cross‐interaction effects on δ_C(C═N) decreases with increasing conjugated distance between X and Y. The results of this study suggest that there is a long‐range transmission of the substituent effects in p‐X–C₆H₄CH═NC₆H₄CH═CHC₆H₄–p‐Y. Copyright © 2012 John Wiley & Sons, Ltd.     

Comparison of the 13C (C = N) chemical shifts of substituted N‐(phenyl‐ethylene)‐anilines and substituted N‐(benzylidene)‐anilines

Comparison of ¹³C NMR of C = N bond chemical shifts δ_C(C = N) in substituted N‐(phenyl‐ethylene)‐anilines XArC(Me) = NArY (XPEAYs) with that in substituted N‐(benzylidene)‐anilines XArCH = NArY (XBAYs) was carried out. The δ_C(C = N) of 61 samples of XPEAYs were measured, and the substituent effect on their δ_C(C = N) were investigated. The results show the factors affecting the δ_C(C = N) of XPEAYs are quite different from that of XBAYs. A penta‐parameter correlation equation was obtained for the 61 compounds, which has correlation coefficient 0.9922 and standard error 0.12 ppm. The result indicates that, in XPEAYs, the inductive effects of substituents X and Y are major factors affecting the δ_C(C = N), while the conjugative effect of them have very little effect on the δ_C(C = N) and can be ignored. The substituent‐specific cross‐interaction effects between X and Y and between Me of C = N bond and substituent Y are important factors affecting the δ_C(C = N). Also, the excited‐state substituent parameter of substitute Y has certain contribution to the δ_C(C = N). Copyright © 2015 John Wiley & Sons, Ltd.     

Substituent effect study on δc values of the bridge group carbons in disubstituted cinnamyl aniline series

The ¹³C nuclear magnetic resonance (NMR) chemical shifts δ_c of bridge group carbons (C‐β, C‐α, and C═N) were measured in this work for a wide set of substituted cinnamyl anilines p‐XC₆H₄CH═CHCH═NC₆H₄Y‐p (X = NO₂, Cl, H, Me, MeO, or NMe₂; Y = NO₂, CN, CO₂Et, Cl, F, H, Me, MeO, or NMe₂) and were used to study the substituent effect. In the study on ¹³C NMR chemical shifts of the titled compounds with single substituent changed, for every bridge carbon δ_c, the effect of cinnamyl substituent X is opposite to that of aniline substituent Y. That is, the action of the same substituent on different aromatic rings is different from the ¹³C NMR chemical shifts, and for C‐β, C‐α, and C═N, the choice of correlation equation depends on the ratio ρ_F(Y)/ρ_R(Y). When the ratio ρ_F(Y)/ρ_R(Y) is close to 1, the chemical shifts of bridge carbons can be well correlated with the single‐parameter equation; otherwise, it is better to adopt the dual‐parameter equation for correlation, and the further the values of ρ_F(Y)/ρ_R(Y) stray from 1, the more suitable the corresponding δ_c values are to be correlated with the dual‐parameter equation. In the study on δ_c of model compounds with simultaneous variations of substituents X and Y, for δ_c(C═N), a multi‐parameter correlation equation is obtained, and the substituent cross‐interaction item Δσ² is suitable to scale the interaction between substituents; however, for δ_c(C‐α and C‐β), the substituent cross‐interaction item Δσ² is perhaps too small to be observed. The multi‐parameter correlation equations can be recommended to predict well the corresponding δ_c values of disubstituted cinnamyl anilines. Copyright © 2012 John Wiley & Sons, Ltd.     

The nature of the electronic factor governing diastereofacial selectivity in remotely substituted (X) 2‐adamantyl cations: 5‐X versus 4‐X substitution

A limited series of 4^eq‐substituted (X) 2‐methyleneadamantanes ( 6 , Y?CH₂, X?F, Cl, Br, I, and SnMe₃) has been synthesized and diastereoselectivities for their hydrochlorination (HCl/CH₂Cl₂) have been determined. Diastereoselectivities for the fluorination (DAST/CH₂Cl₂) of secondary alcohol mixtures, obtained from the hydride reduction of the precursor ketones ( 6 ,Y?O) to the alkenes, have also been measured. A comparison of this selectivity data for nucleophilic trapping of 4^eq‐substituted (X) 2‐adamantyl cations ( 4 , R?H and Me) with the corresponding information for 5‐substituted (X) 2‐adamantyl cations ( 1 , R?H and Me) has revealed important distinctions between the two series. In particular, whereas extended hyperconjugative effects appear to be the predominant electronic effect governing facial selectivity in the 5,2‐series, electrostatic influences prevail in the 4,2‐disposition. Copyright © 2007 John Wiley & Sons, Ltd.     

4‐Alkyl‐2,2,6,6‐tetramethyl‐1,4,2,6‐oxaazadisilinanes: synthesis,structure, and conformational analysis

4‐Alkyl‐2,2,6,6‐tetramethyl‐1,4,2,6‐oxaazadisilinanes RN[CH₂Si(Me)₂]₂O [R = Me ( 1 ), i‐Pr ( 2 )] were synthesized by two methods which provided good yields up to 84%. Low temperature NMR study of compounds ( 1 ) and ( 2 ) revealed a frozen ring inversion with the energy barriers of 8.5 and 7.7 kcal/mol at 163 and 143 K, respectively, which is substantially lower than that for their carbon analog, N‐methylmorpholine. DFT calculations performed on the example of molecule ( 1 ) showed that N? Me_ax conformer to exist in the sofa conformation with the coplanar fragment C? Si? O? Si? C, and its N? Me_eq conformer in a flattened chair conformation. Copyright © 2009 John Wiley & Sons, Ltd.     

On structural and spectroscopic differences between quinoline‐2‐carboxamides and their N‐oxides in the solution and solid state

Intramolecular hydrogen bonding in the primarily and secondarily substituted quinoline‐2‐carboxamides and their N‐oxides has been studied in the solution by multinuclear NMR spectroscopy. Hydrogen bonding patterns and supramolecular arrangement in the solid state have been determined by single crystal X‐ray analysis. In quinoline‐2‐carboxamides weak, nonlinear intramolecular N? H…N hydrogen bond is present, but in the solid state the intermolecular hydrogen bonds and packing forces are the factors that decide on the properties of 3D structures. In quinoline‐2‐carboxamide N‐oxides the most important structural features are the intramolecular hydrogen bonds. Details of different weak interactions and resulting 3D arrangement of molecules are discussed. In the solution, two separate ¹H signals are observed for the primary quinoline‐2‐carboxamides in the range from ca. 5.8 to 8.1 ppm. The chemical shifts of the NH group's proton for studied R′‐quinoline‐2‐R‐carboxamides are in the range from 8.1 to 8.4 ppm. For the N‐oxide of 4‐R′‐quinoline‐2‐carboxamides (R′ = H, Me, OPh, Cl and Br), the values of the proton chemical shifts of the NH group in the range from 10.78 to 11.38 ppm (for primary amides) indicating that this group forms hydrogen bonds with the oxygen of the N‐oxide group. This bond is stronger than the N? H…N bond in quinoline‐2‐carboxamides. For the secondary amide N‐oxides, the δ(NH) values are increasing from 11.25 to 11.77 ppm in the sequence of substituents 4‐Br < 4‐Cl < 4‐H < 4‐Me < 4‐OPh. For 4‐substituted compounds these values depend also on the substituent effect. Copyright © 2009 John Wiley & Sons, Ltd.     

Enhancing the efficiency of TiO2–perovskite heterojunction solar cell via evaporating Cs2CO3 on TiO2

In a TiO₂–perovskite heterojunction solar cell (TiO₂–PHSC), besides the perovskite CH₃NH₃PbX₃, TiO₂ as one side of the TiO₂/CH₃NH₃PbX₃ heterojunction also plays an important role in the photovoltaic effect. In order to improve the performance of the TiO₂–PHSC with the structure of glass/FTO/compact TiO₂/mesoporous TiO₂/CH₃NH₃PbI_3–xCl_x /poly‐TPD (poly(N,N ′‐bis(4‐butylphenyl)‐N,N ′‐bis(phenyl)benzidine))/Au, a 2 nanometer thick Cs₂CO₃ layer is thermally evaporated on the mesoporous TiO₂ layer. The short‐circuit current density (J_sc) raises from 17.7 mA cm^–2 to 18.9 mA cm^–2, the open‐circuit voltage (V_oc) from 0.81 V to 0.87 V, and the fill factor (FF) from 55.2% to 67.3%; as a result, the power conservation efficiency (PCE) increases from 8.0% to 11.1% under AM 1.5G solar illumination (100 mW cm^–2). Moreover, in a TiO₂–PHSC free of mesoporous TiO₂, where Cs₂CO₃ is evaporated on the compact TiO₂ layer, the J_sc, V_oc, FF and PCE values increase from 16.0 mA cm^–2, 0.83 V, 50.8% and 6.7% to 17.9 mA cm^–2, 0.90 V, 59.3%, and 9.5%, respectively. The reasons of the PCE increase for either the first kind of TiO₂–PHSC or the mesoporous‐TiO₂‐free TiO₂–PHSC with a nanometer‐thick Cs₂CO₃ layer on mesoporous TiO₂ or compact TiO₂ are discussed. (© 2014 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Hartree–Fock and density functional theory study of remote substituent effects on heterolytic Fe―C bond energies of p‐G‐C6H4CH2Fe(CO)2(η5‐C5H5) and p‐G‐C6H4(H)(CN)CFe(CO)2(η5‐C5H5)

Knowledge of the strength of the metal–ligand bond breaking and formation is fundamental for an understanding of the thermodynamics underlying many important stoichiometric and catalytic organometallic reactions. Quantum chemical calculations at different levels of theory have been used to investigate heterolytic Fe―C bond energies of para‐substituted benzyldicarbonyl(η⁵‐cyclopentadienyl)iron, p‐G‐C₆H₄CH₂Fp [1, G = NO₂, CN, COMe, CO₂Me, CF₃, Br, Cl, F, H, Me, MeO, NMe₂; Fp = (η⁵‐C₅H₅)(CO)₂Fe], and para‐substituted α‐cyanobenzyldicarbonyl(η⁵‐cyclopentadienyl)iron, p‐G‐PANFp [2, PAN = C₆H₄CH(CN)]. The results show that BP86 and TPSSTPSS can provide the best price/performance ratio and more accurate predictions in the study of ΔH_het(Fe―C)'s. The good linear correlations [r = 0.98 (g, 1a), 0.99 (g, 2b)] between the substituent effects of heterolytic Fe―C bond energies [ΔΔH_het(Fe―C)'s] of series 1 and 2 and the differences of acidic dissociation constants (ΔpK_a) of C―H bonds of p‐G‐C₆H₄CH₃ and p‐G‐C₆H₄CH₂CN imply that the governing structural factors for these bond scissions are similar. And the excellent linear correlations [r = ?1.00 (g, 1c), ?0.99 (g, 2d)] between ΔΔH_het(Fe―C)'s and the substituent σ_p^? constants show that these correlations are in accordance with Hammett linear free energy relationships. The polar effects of these substituents and the basis set effects influence the accuracy of ΔH_het(Fe―C)'s. ΔΔH_het(Fe―C)'s(1, 2) follow the Capto‐dative Principle. The detailed knowledge of the factors that determine the Fp―C bond strengths would greatly aid in understanding reactivity patterns in many processes. Copyright © 2011 John Wiley & Sons, Ltd.     

Diastereofacial selectivity in some 4‐substituted (X) 2‐adamantyl derivatives: electronic versus steric effects

π‐Facial selectivity data for the reduction and methylation of some 4^ax‐substituted (X) 2‐adamantanones ( 3 , Y = O) as well as the nucleophilic trapping of secondary and tertiary 4^ax‐substituted (X)‐2‐adamantyl cations ( 4 ; R = H and CH₃, respectively) and the 4‐methylene‐2‐adamantyl radical ( 8 ) are presented. The pronounced anti‐face selectivities observed for ( 3 , Y = O and 4 , R = CH₃) emphasize the importance of the steric factor as expected for systems with a strong steric bias. However, the dominant syn‐face capture of 4 (R = H) was completely unexpected and highlights a subtle interplay between steric and electronic effects. Finally, the very high anti‐face stereoselectivity for the trapping of ( 8 ) with the trimethylstannyl anion (Me₃Sn^?) is rationalized in terms of an electrostatic effect overwhelming the steric factor. Copyright © 2007 John Wiley & Sons, Ltd.     

Theoretical investigations on 4,4′,5,5′‐tetranitro‐2,2′‐1H,1′H‐2,2′‐biimidazole derivatives as potential nitrogen‐rich high energy materials

The ―NH₂, ―NO₂, ―NHNO₂, ―C(NO₂)₃ and ―CF(NO₂)₂ substitution derivatives of 4,4′,5,5′‐tetranitro‐2,2′‐1H,1′H‐2,2′‐biimidazole were studied at B3LYP/aug‐cc‐pVDZ level of density functional theory. The crystal structures were obtained by molecular mechanics (MM) methods. Detonation properties were evaluated using Kamlet–Jacobs equations based on the calculated density and heat of formation. The thermal stability of the title compounds was investigated via the energy gaps (?E_{LUMO ? HOMO}) predicted. Results show that molecules T5 (D = 10.85 km·s^?1, P = 57.94 GPa) and T6 (D = 9.22 km·s^?1, P = 39.21 GPa) with zero or positive oxygen balance are excellent candidates for high energy density oxidizers (HEDOs). All of them appear to be potential explosives compared with the famous ones, octahydro‐1,3,5,7‐tetranitro‐1,3,5,7‐tetraazocane (HMX, D = 8.96 km·s^?1, P = 35.96 GPa) and hexanitrohexaazaisowurtzitane (CL‐20, D = 9.38 km·s^?1, P = 42.00 GPa). In addition, bond dissociation energy calculation indicates that T5 and T6 are also the most thermally stable ones among the title compounds. Copyright © 2014 John Wiley & Sons, Ltd.     

A DFT‐GIAO and DFT‐NBO study of polar substituent effects on NMR 17O chemical shifts in some rigid polycyclic alkanes

¹⁷O NMR shieldings of 3‐substituted(X)bicyclo[1.1.1]pentan‐1‐ols ( 1 , Y = OH), 4‐substituted(X)bicyclo[2.2.2]octan‐1‐ols ( 2 , Y = OH), 4‐substituted(X)‐bicyclo[2.2.1]heptan‐1‐ols ( 3 , Y = OH), 4‐substituted(X)‐cuban‐1‐ols ( 4 , Y = OH) and exo‐ and endo‐ 6‐substituted(X)exo‐bicyclo[2.2.1]heptan‐2‐ols ( 5 and 6 , Y = OH, respectively), as well as their conjugate bases ( 1 – 6 , Y = O^?), for a set of substituents (X = H, NO₂, CN, NC, CF₃, COOH, F, Cl, OH, NH₂, CH₃, SiMe₃, Li, O^?, and NH) covering a wide range of electronic substituent effects were calculated using the DFT‐GIAO theoretical model at the B3LYP/6‐311 + G(2d, p) level of theory. By means of natural bond orbital (NBO) analysis various molecular parameters were obtained from the optimized geometries. Linear regression analysis was employed to explore the relationship between the calculated ¹⁷O SCS and polar field and group electronegativity substituent constants (σ_F and σ_χ, respectively) and also the NBO derived molecular parameters (oxygen natural charge, Q_n, occupation numbers of the oxygen lone pairs, n_o, and occupancy of the C? O antibonding orbital, σ*_CO(occup)). In the case of the alcohols ( 1 – 6 , Y = OH) the ¹⁷O SCS appear to be governed predominantly by the σ_χ effect of the substituent. Furthermore, the key determining NBO parameters appear to be n_o and σ*_CO(occup). Unlike the alcohols, the calculated ¹⁷O SCS of the conjugate bases ( 1 – 6 , Y = O^?), except for system 1 , do not respond systematically to the electronic effects of the substituents. An analysis of the SCS of 1 (Y = O^?) raises a significant conundrum with respect to their origin. Copyright © 2010 John Wiley & Sons, Ltd.     

Solution conformational preferences of glutaric, 3‐hydroxyglutaric, 3‐methylglutaric acid,and their mono‐ and dianions

Conformational preferences of glutaric, 3‐hydroxyglutaric and 3‐methylglutaric acid, and their mono‐ and dianions have been investigated with the aid of NMR spectroscopy. In contrast to succinic acid, glutaric acid displays essentially statistical conformational equilibria in polar and non‐polar solutions of high and low hydrogen‐bonding ability with no clear evidence for intramolecular hydrogen‐bonding interactions. The acid ionization constant ratios, K ₁/K₂, in D₂O and DMSO of glutaric, 3‐hydroxyglutaric, and 3‐methylglutaric acids also indicate that intramolecular interactions are much less important than, or indeed insignificant, for shorter‐chain acids. FTIR studies on 3‐methylglutaric acid indicate some preference for either association with solvent or dimerization, depending on the solvent, rather than intramolecular hydrogen bonding. Copyright © 2008 John Wiley & Sons, Ltd.     

Higher reactivity of 3‐pyridinium boronic acid compared with 3‐pyridinium boronate ion toward 4‐isopropyltropolone in acidic aqueous solution: fundamental reaction analyses for an effective organoboron‐based chemosensor

The pK_as of 3‐pyridylboronic acid and its derivatives were determined spectrophotometrically. Most of them had two pK_as assignable to the boron center and pyridine moiety. The pK_a assignment performed by ¹¹B nuclear magnetic resonance spectroscopy revealed that both boron centers in 3‐pyridylboronic acid [3‐PyB(OH)₂] and the N‐methylated derivative [3‐(N‐Me)Py⁺B(OH)₂] have strong acidities (pK_a = 4.4 for both). It was found that introduction of a substituent to pyridine‐C atom in 3‐pyridylboronic acid drastically increased the acidity of the pyridinium moiety, but decreased the acidity of the boron center, whereas the introduction to pyridine‐N atom had no influence on the acidity of the boron center. Kinetic studies on the complexation reactions of 3‐pyridinium boronic acid [3‐HPy⁺B(OH)₂] with 4‐isopropyltropolone (Hipt) carried out in strongly acidic aqueous solution indicated that the positive charge on the boronic acid influenced little on its reactivity; 3‐HPy⁺B(OH)₂ reacts with Hipt and protonated H₂ipt⁺, and its reactivity was in line with those of a series of boronic acids. Kinetics in weakly acidic aqueous solution revealed that 3‐HPy⁺B(OH)₂ reacts with Hipt faster than its conjugate boronate [3‐HPy⁺B(OH)₃^–], which is consistent with our recent results. The reactivity of 3‐(N‐Me)Py⁺B(OH)₂ towards Hipt was also examined kinetically; the reactivities of 3‐(N‐Me)Py⁺B(OH)₂ and 3‐(N‐Me)Py⁺B(OH)₃^– are almost the same as those of their original 3‐HPy⁺B(OH)₂ and 3‐HPy⁺B(OH)₃^–, respectively. Copyright © 2012 John Wiley & Sons, Ltd.     

Anion effect on the conformational equilibrium of sulfamide and its N,N′‐diindolyl derivative: Insights on anion transportation

The relatively high acidity of the sulfamide hydrogens suggests a potential for the development of sulfamide derivatives as novel anion receptors. The interactions of sulfamide with F^?, Cl^?, CH₃COO^?, and H₂PO₄^? anions were spectroscopically (¹H and ¹⁹F NMR) and theoretically (density functional theory) analyzed, and the complexation through hydrogen bonds was confirmed by changes in the NMR signals and theoretical calculations. The replacement of 2 sulfamide hydrogens with indolyl groups yields the N,N′‐diindolylsulfamide ( DIS , N‐1H‐indol‐4‐yl‐N′‐1H‐indol‐7‐ylsulfuric diamide), whose bond rotations allow the interaction of 4 H(N) atoms with anions. The conformational preferences of DIS change upon the presence of anions, but they are practically insensitive to the anion type. According to the quantum theory of atoms in molecules, natural bond orbital analysis, and NMR chemical shifts, as well as to a thermodynamic cycle, the complex with fluoride is the most stable, followed by the oxoanion‐derived models.     

Substituent effect investigation of 3‐(2, 4‐dichlorophenyl)‐1‐(4′‐X‐phenyl)‐2‐propen‐1‐one. Part 1. Correlation analysis of 13C NMR chemical shifts

A series of substituted chlorinated chalcones namely, 3‐(2,4‐dichlorophenyl)‐1‐(4′‐X‐phenyl)‐2‐propen‐1‐one, have been synthesized, X being H, NH₂, OMe, Me, F, Cl, CO₂Et, CN, and NO₂. Dual substituent parameter (DSP) models of ¹³C NMR chemical shift (CS) have revealed that π‐polarization concept could be utilized to explain the reverse field effect at CO, the enhanced substituent field effect at CO, C‐2, and C‐5, and the decreased sensitivity of substituent field effect at C‐6. Chlorine atoms dipole direction at the benzylidene ring either enhances or reduces substituent effect depending on how they couple with the substituent dipole at the probe site. The correlation of ¹³C NMR CS of C‐2, C‐5, and C‐6 with σ and σ indicates that chlorine atoms in the benzylidine ring deplete the ring from charges. Both MSP of Hammett and DSP of Taft ¹³C NMR CS models give similar trends of substituent effects at C‐2, C‐5, and C‐6. However, the former fail to give a significant correlation for CO and C‐6 ¹³C NMR CS. MSP of σ_q and DSP of Taft and Reynolds models significantly correlated ¹³C NMR CS of C_β. MSP of σ_q fails to correlate C‐1′ ¹³C NMR CS. Investigation of ¹³C NMR CS of non‐chlorinated chalcones series: 3‐phenyl‐1‐(4′‐X‐phenyl)‐2‐propen‐1‐one has revealed similar trends of substituent effects as in the chlorinated chalcones series for C‐1′, CO, C_α, and C_β. In contrast, the substituent effect of the non‐chlorinated chalcone series at C‐2, C‐5, and C‐6 did not correlate with any substituent constant. Copyright © 2010 John Wiley & Sons, Ltd.     

Density functional theory study of substituent effects on gas‐phase heterolytic Fe–O and Fe–S bond energies of m‐G‐C6H4OFe(CO)2(η5‐C5H5) and m‐G‐C6H4SFe(CO)2(η5‐C5H5)

The knowledge of accurate bond strengths is a fundamental basis for a proper analysis of chemical reaction mechanisms. Quantum chemical calculations at different levels of theory have been used to investigate heterolytic Fe–O and Fe–S bond energies of (meta‐substituted phenoxy)dicarbonyl(η⁵‐cyclopentadienyl) iron [m‐G‐C₆H₄OFp ( 1 )] and (meta‐substituted benzenethiolato)dicarbonyl(η⁵‐cyclopentadienyl) iron [m‐G‐C₆H₄SFp ( 2 )] complexes. In this study, Fp is (η⁵‐C₅H₅)Fe(CO)₂, and G is NO₂, CN, COMe, CO₂Me, CF₃, Br, Cl, F, H, Me, MeO, and NMe₂. The results show that Tao–Perdew–Staroverov–Scuseria and Becke's power‐series ansatz from 1997 with dispersion corrections functionals can provide the best price/performance ratio and accurate predictions of ΔH_het(Fe–O)'s and ΔH_het(Fe–S)'s. The excellent linear free energy relations [r = 1.00 (g, 1e), 1.00 (g, 2b)] among the ΔΔH_het (Fe–O)'s and δΔG⁰ of O?H bonds of m‐G‐C₆H₄OH or ΔΔH_het(Fe–S)'s and ΔpK_a's of S?H bonds of m‐G‐C₆H₄SH imply that the governing structural factors for these bond scissions are similar. And, the linear correlations [r = ?0.97 (g, 1 g), ?0.97 (g, 2 h)] among the ΔΔH_het (Fe–O)'s or ΔΔH_het(Fe–S)'s and the substituent σ_m constants show that these correlations are in accordance with Hammett linear free energy relationships. The inductive effects of these substituents and the basis set effects influence the accuracy of ΔH_het(Fe–O)'s or ΔH_het(Fe–S)'s. The ΔΔH_het(Fe–O)'s(g) (1) and ΔΔH_het(Fe–S)'s(g)(2) follow the capto‐dative Principle. The substituent effects on the Fe–O bonds are much stronger than those on the less polar Fe–S bonds. Insight from this work may help the design of more effective catalytic processes. Copyright © 2016 John Wiley & Sons, Ltd.     

Selectivity of nitro versus fluoro substitution in arenes in their reactions with charged O‐ and S‐nucleophiles

The relative mobility of nitro and fluoro substituents in 1,3‐dinitro‐ and 1‐fluoro‐3‐nitrobenzenes, 3,5‐dinitro‐ and 3‐fluoro‐5‐nitrobenzotrifluorides under the action of the nucleophiles (2ArYH)·K₂CO₃ and ArY^?K⁺ in solution and the nucleophiles ArYH·K₂CO₃ (Y = O, S) under heterogeneous conditions was studied by a competitive method in DMF at 40–140 °C. The unique dependences of ΔΔH^≠ on ΔΔS^≠ and ΔΔH^≠ on ΔΔG^≠ were determined for all the substrates and nucleophiles. The dependence of the mechanistic pathway on the nucleophile is discussed. Two results are relevant to the reactions studied: (i) substituent effects in the nucleophiles (2ArYH)·K₂CO₃ and ArYH·K₂CO₃ on the activation entropies suggest that the entropy favours the displacement of nitro group; (ii) the negative signs of ΔΔH^≠ and ΔΔS^≠ for the reactions of the nucleophiles ArY^?K⁺ indicate that the enthalpy determines the displacement of nitro group. It is concluded that the selectivity of the reactions with aryloxide and arylthioxide ions cannot be explained by the hard–soft acid–base principle only. Copyright © 2007 John Wiley & Sons, Ltd.     

D2O‐catalyzed proton transfer selectivity of 4 different NH protons in the same molecule

When N‐benzyl‐N′‐methylacetamidinium hydrochloride (pKa=11.8) is dissolved in D₂O/DCl(1 M), an equilibrium of 2 54:46 stereoisomers in an ~2:1 =(R)N^δ+H(D) D/H ratio is formed. Therefore, 2 R =N‐benzyl (E and Z) and 2 R =N‐methyl (E and Z) groups attached to the corresponding H(D) (Z and E) for a total of 8 ¹H‐NMR signals are observed. Consequently, their rates of H and D transfer to D₂O can be measured by means of the ¹H‐NMR broadness (line shape) of the =(R )N^δ+H doublets and =(R )N^δ+D broad singlets. Acidity selectivity is observed for both processes. In fact, the relative proton and deuterium transfer rates follow the acidity order: =(PhCH₂)N^δ+‐H(E) > =(PhCH₂)N^δ+‐H(Z) > =(Me)N^δ+‐H(E) > =(Me)N^δ+‐H(Z). Proton transfer rates are in the range of 8 to 0.5 s^‐1 with α = .92. This tendency is independently supported by the observed experimental chemical shift deuterium isotopic perturbation. The rate‐limiting step for proton exchange is the breaking of the hydrogen bond due to the fast amidine reprotonation (~10¹¹ s). =(R)N^δ+D/=(R)N^δ+H equilibration is reached at ~80 s, and it can be measured by the relative =(R) N^δ+H versus =(R) N^δ+D signal integrations. The equilibrium of the 4 =(R)N^δ+H(D) centers is shifted toward deuterium, but they are further shifted in the more basic centers. Equilibrium is completely shifted toward D in the 4 centers when OD^? contributes with the exchange process at pD > 3.     

Modelling the specific pathway of CH4 and CO2 formation using carbon isotope fractionation: an example for a boreal mesotrophic fen

We described mathematically the process of peat methanization in a boreal mesotrophic fen. Gaseous and dissolved CH₄ and CO₂ as well as their δ¹³C signatures were considered in the dynamic equations for incubation bottles. In accordance with the model, acetate, H₂, and CO₂ were produced during cellulose hydrolysis and acidogenesis. ¹³C/¹²C in CO₂ was a key variable reflecting dynamic changes in the rates of cellulose hydrolysis and acidogenesis, acetoclastic and hydrogenotrophic methanogenesis. As CO₂ is the substrate in hydrogenotrophic methanogenesis, δ¹³C–CO₂ increased from the start till the dissolved hydrogen concentration became very low. Thereafter, the rate of acetoclastic methanogenesis with the significant current acetate concentration dominated over the rate of hydrogenotrophic methanogenesis leading to the decreasing δ¹³C–CO₂ and the increasing δ¹³C–CH₄. The model was validated by describing the system’s dynamics under strong and weak inhibition of acetoclastic and hydrogenotrophic methanogenesis by methyl fluoride, respectively. During peat methanization at the lowered temperature of 10?°C, the processes of hydrogenotrophic methanogenesis and homoacetogenesis competing for H₂ may occur. However, based on dynamics of the carbon isotope signatures, especially on dynamics of δ¹³C–CO₂, the model showed no significant contribution of homoacetogens in peat methanization.