The basicity of sulfonamides and carboxamides. Theoretical and experimental analysis and effect of fluorinated substituent

The basicity of a series of sulfonamides and carboxamides with respect to protonation and hydrogen‐bonded complex formation with phenol was investigated by calculations using the Becke three‐parameter hybrid functional combined with Lee–Yang–Parr correlation functional with the 6‐311G** and 6‐311++G** basis sets and by infrared spectroscopy. The effect of fluorinated substituent was studied for the two series. The proton affinity of nitrogen in sulfonamides is higher than oxygen, in contrast to carboxamides, which are protonated at oxygen. The phenyl group in benzenesulfonamide increases the basicity of both heteroatoms, but more strongly of the nitrogen, whereas in benzamide the effect on the two heteroatoms is about the same. The CF₃ group equally decreases the basicity of nitrogen and oxygen atoms in sulfonamides and carboxamides. The second fluorinated substituent decreases the basicity of oxygen in (CF₃CO)₂NH more strongly than of nitrogen. For sulfonamides, the same effect results in the reverse of the center of basicity from nitrogen in (MeSO₂)₂NH to oxygen in (CF₃SO₂)₂NH. All studied carboxamides give H‐complexes via the carbonyl oxygen, whereas for sulfonamides two types of H‐complexes, with the OH···N and OH···O=S, were found theoretically, the latter being more stable. The exception is bisimide (CF₃SO₂)₂NH, for which only the OH···O=S complex is stable. Experimentally, only the oxygen‐bound complexes are observed. Analysis of the natural charges revealed an ‘abnormal’ increase of the electron density on the NH group by electron‐acceptor substituents in CF₃SO₂NHR, which was explained using the natural bond orbital analysis by loosening of the S–N bond because of orbital interactions with the σ*_S?N orbital. Copyright © 2012 John Wiley & Sons, Ltd.     

Sulfonyl nitrenes from different sources: computational study of formation and transformations

The formation of sulfonyl nitrenes RSO₂N (R = Me, p‐Tol, CF₃) from different sources, such as sulfonyl azides RSO₂N₃, N‐halosulfonamides RSO₂NHHal, salts RSO₂NNaCl, N‐hydroxysulfonamides RSO₂NHOH and sulfonylimino‐λ³‐iodanes ArI = NSO₂R or ‐bromanes, by elimination of neutral molecules (N₂, HCl, NaCl, H₂O and ArHal, respectively) was studied theoretically at the density functional theory and second‐order Møller‐Plesset perturbation theory using the 6‐311++G(d,p) and cc‐pVTZ basis sets. The originally formed singlet nitrenes suffer nonradiative intersystem crossing to the triplet state (S₁ ? T₁) or, sometimes, undergo spontaneous barrierless pseudo‐Curtius rearrangement into the corresponding sulfonylamines RN = SO₂. The activation barriers decrease in the order (kcal/mol): RSO₂NHHal ~ RSO₂NHOH (60–70) ≥ RSO₂N(Na)Hal (40–50) > RSO₂N₃ (35) > RSO₂N = IPh (9–13) ≥ RSO₂N = BrC₆H₄CF₃ (4–8). Copyright © 2013 John Wiley & Sons, Ltd.     

Detailed analysis of the charge transfer complex N,N‐dimethylaniline–SO2 by Raman spectroscopy and density functional theory calculations

Although the amine sulfur dioxide chemistry was well characterized in the past both experimentally and theoretically, no systematic Raman spectroscopic study describes the interaction between N,N‐dimethylaniline (DMA) and sulfur dioxide (SO₂). The formation of a deep red oil by the reaction of SO₂ with DMA is an evidence of the charge transfer (CT) nature of the DMA–SO₂ interaction. The DMA–SO₂ normal Raman spectrum shows the appearance of two intense bands at 1110 and 1151 cm⁻¹, which are enhanced when resonance is approached. These bands are assigned to ν_s(SO₂) and ν(ϕ N) vibrational modes, respectively, confirming the interaction between SO₂ and the amine via the nitrogen atom. The dimethyl group steric effect favors the interaction of SO₂ with the ring π electrons, which gives rise to a π–π* low‐energy CT electronic transition, as confirmed by time‐dependent density functional theory (TDDFT) calculations. In addition, the calculated Raman DMA–SO₂ spectrum at the B3LYP/6‐311 + + g(3df,3pd) level shows good agreement with the experimental results (vibrational wavenumbers and relative intensities), allowing a complete assignment of the vibrational modes. A better understanding of the intermolecular interactions in this model system can be extremely useful in designing new materials to absorb, detect, or even quantify SO₂. Copyright © 2009 John Wiley & Sons, Ltd.     

Ab initio SCF CI study of the electronic spectrum of s-tetrazine

Configuration interaction (CI) studies of ground, n→ π* and π→ π* electronically excited states are reported for s-tetrazine. The first n→ π* singlet excited state (¹ B _3u ), which is responsible for the purple-red colour of the molecule, is calculated at 2·80 eV, compared to the experimental transition energy of 2·22–2·70 eV. The singlet-triplet split of the first n→ π* states (¹ B _3u and ³ B _3u states) is calculated to be 0·76 eV.

The interaction of nitrogen lone pair orbitals (n-orbitals) is studied in terms of the ordering of the n π* excited states and found that the SCF orbital ordering is qualitatively in accord with the ordering of the n π* excited states in the CI level.

The first π→ π* excited state (¹ B _2u ) is calculated at 5·99 eV, slightly above the observed range of absorption. Numerous other high-lying singlet states as well as the triplet states have been calculated and they are used to verify several proposals relating to the excited state dynamics in the photo-physical studies of s-tetrazine.     

Counter anion effect on structural,opto-electronic and charge transport properties of fused π-conjugated imidazolium compound

The structure–activity relationship of fused π-conjugated imidazolium cation with three counter anion molecules, BF₄^?, CF₃SO₃^? and (CF₃SO₂)₂N^?, was studied using electronic structure calculations. The structural, opto-electronic and charge transport properties of these complexes were studied. The charge transfer from π-conjugated imidazolium(I) to counter anion was confirmed in all the studied complexes. Interaction energy varies significantly depending on the counter anion and the stability was found higher for I-BF₄ complex than both I–CF₃SO₃ and I–(CF₃SO₂)₂N complexes. The strong (C–H)⁺···F^? hydrogen bond of length 1.95 Å between fused π-conjugated imidazolium and BF^?₄ anion is the driving force for the strongest interaction energy in I–BF₄ complex. The energy decomposition analysis confirms that the interaction between imidazolium and counter anion is mainly driven by electrostatic and orbital interaction. It has been observed that the absorption spectra of the complex are independent of anion nature but the influence of anion character is observed on frontier molecular orbital pattern. The charge transport property of I–BF₄ complex was studied by using tight-binding Hamiltonian approach and found that the hole mobility in I–BF₄ is 1.13 × 10^?4 cm² V^?1 s^?1.     

Chalcogen bonds formed through π-holes: SO3 complexes with nitrogen and phosphorus bases

The possibility of formation of chalcogen bonds through π-holes is examined by means of ab initio calculations at MP2/aug-cc-pVTZ level. We investigate the complexes of SO₃ with a series of electron-donating nitrogen bases (ZN), including NH₃, H₂C═NH, NH₂F, NP, NCH, NCF, NF₃, and N₂, and their phosphorous analogues (ZP). The ZN:SO₃ complexes show shorter chalcogen bond distances than the ZP:SO₃ counterparts, accompanied with the more negative interaction energy in the former than in the latter. To understand the nature of interactions, molecular electrostatic potential analysis is performed. In addition, the quantum theory of atoms in molecules and electron localisation function are employed to analyse the chalcogen bond properties in these complexes.     

A natural bond orbital analysis of aryl‐substituted polyfluorinated carbanions: negative hyperconjugation

Aryl‐substituted polyfluorinated carbanions, ArCHR_f^? where R_f = CF₃ ( 1 ), C₂F₅ ( 2 ), i‐C₃F₇ ( 3 ), and t‐C₄F₉ ( 4 ), were analyzed by means of the natural bond orbital (NBO) theory at the B3LYP/6‐311+G(d,p) computational level. A lone pair NBO at the formal anionic center carbon (Cα) was not found in the Lewis structure. Instead, significant donor/acceptor NBO interactions between π(Cα‐C1) and σ*(Cβ‐F) or σ*(Cβ‐Cγ) were observed for 1 , 2 , 3a (strong electron‐withdrawing substituent, from p‐CF₃ to p‐NO₂), and 4 . Their second‐order donor/acceptor perturbation interaction energy, E(2), values decreased with the increase of the stability of carbanions. A larger E(2) value corresponds to longer Cβ‐F and Cβ‐Cγ bonds and a shorter Cα‐Cβ bond, indicating that the E(2) values can be associated with the negative hyperconjugation of the Cβ‐F and Cβ‐Cγ bonds. In accordance with this, the E(2) values for π(Cα‐C1) → σ*(Cβ‐F) were linearly correlated with the ΔG^o_β‐F values (an empirical measure of β‐fluorine negative hyperconjugation obtained from an increased acidity). In 3b (weak electron‐withdrawing substituents, from H to m‐NO₂) very large E(2) values for LP(Fβ) → π*(Cα‐Cβ) were obtained. This was attributed to the Cβ‐F bond cleavage and the Cα‐Cβ double bond formation in the Lewis structure that is caused by the extremely strong negative hyperconjugation of the Cβ‐F bond.     

The structure and proton affinity of N‐benzyl‐N‐(allenyl)trifluoromethanesulfonamide: FT‐IR,DFT and ab initio study,NBO analysis

The first N‐allenyl derivative of trifluoromethanesulfonamide, N‐benzyl‐N‐(allenyl)trifluoromethanesulfonamide ( 1 ), was studied experimentally by the FT‐IR spectroscopy and theoretically at the DFT and MP2 levels of theory. The intramolecular interaction of the nitrogen atom with the triflyl and the allenyl group was studied in comparison with the analogously substituted vinyl derivatives. Compound 1 in heptane solution at 295–183 K exists as an equilibrium mixture of conformational isomers. Protonation at different basic sites in a series of reference molecules is studied theoretically. The central C² atom of the allenyl group in 1 has the highest proton affinity, which is 16 kcal/mol higher than in the N‐vinyl analogues. The relative ability of the allenyl and vinyl groups to conjugation with an electron‐rich and electron‐deficient nitrogen atom lone electron pair is discussed. From the NBO analysis, the conjugation of the nitrogen lone electron pair with the allenyl group is much stronger than with the vinyl group. Copyright © 2013 John Wiley & Sons, Ltd.     

Solvatochromic behavior of dyes with dimethylamino electron‐donor and nitro electron‐acceptor groups in their molecular structure

Six dyes with N,N‐dimethylaminophenyl and 4‐nitrophenyl or 2,4‐dinitrophenyl groups in their molecular structures were prepared and characterized. These compounds have different conjugated bridges (C?C, C?N, and N?N) connecting the electron‐donor and the electron‐acceptor groups. All compounds are solvatochromic, with reverse solvatochromism occurring. The solvatochromic band observed in each spectrum for the dyes is due to a π ? π* transition, of an intramolecular charge transfer nature, which occurs from the electron‐donor N,N‐dimethylaminophenyl group to the electron‐acceptor group in the molecules, which is reinforced by the structures of the compounds optimized by applying density functional theory, which exhibit high planarity. The reverse solvatochromism was explained considering two resonance structures. The benzenoid form is better stabilized in less polar solvents and characterizes the region displaying positive solvatochromism, while the dipolar form is better stabilized in more polar solvents, in the region of negative solvatochromism. The Catalán multiparametric approach was used to study the contribution of solvent acidity, basicity, dipolarity, and polarizability to the solvatochromism exhibited by the compounds. These compounds are good candidates for the investigation of the polarizability and, to a lesser extent, the dipolarity of the medium, with very little interference from specific interactions of the solvent through hydrogen bonding. Copyright © 2014 John Wiley & Sons, Ltd.     

Acid‐base properties and supramolecular structure of N‐[(hydroxymethyl)triazolyl]triflamides: DFT,ab initio,and FTIR study

For N‐{[2‐(hydroxymethyl)‐2H‐1,2,3‐triazolyl‐4‐yl]methyl}triflamide 1 , N‐{[2‐(hydroxymethyl)‐2H‐1,2,3‐triazolyl‐4‐yl]methyl}‐N‐phenyltriflamide 2 , and N,N‐bis{[2‐(hydroxymethyl)‐2H‐1,2,3‐triazolyl‐4‐yl]methyl}triflamide 3 , the proton affinities of the triazole nitrogen atoms and the hydroxy and sulfonyl oxygen atoms as well as the energies of formation of the conformers with intramolecular H‐bonds and dimers with intermolecular NH?N, OH?N, OH?O═S, and NH?O═S H‐bonds were calculated by density functional theory and second‐order Møller‐Plesset perturbation methods. Quantum Theory of Atoms in Molecules analysis was performed to investigate the nature of H‐bonds. According to Fourier transform infrared spectroscopy, in CH₂Cl₂ solution, the monomeric molecules of 1 to 3 exist in the equilibrium with cyclic dimers having the OH?N hydrogen bonds.     

Thioenols and thioamides substituted by two β‐EWGs. Comparison with analogous amides and enols

Condensation of organic isothiocyanates with active methylene compounds gave nine thioamides RNHCSCHYY′ or their isomeric thioenols RNHC(SH) = CYY′ for substrates in which Y and Y′ are electron‐withdrawing groups (EWG). These included derivatives of Meldrum's acid (MA) which showed 100% thioenol in all solvents. For other compounds the percentages of thioenol in CDCl₃ when R = Ph are 100% when Y = CN and Y′ = CO₂Me or Y′ = CO₂CH₂CCl₃, 6% when Y = Y′ = CO₂CH₂CF₃, and 0% when Y = Y′ = CO₂Me. The chemical shift of SH (highest values 12.0–16.0 ppm) served as a probe for the thioenol structures and also for the extent of hydrogen bonding to the SH group. In contrast to simple ketones and thioketones in which thioenolization is favored over enolization by factors as large as 10⁶, for intramolecular competition K_Thioenol/K_Enol ratios are much lower than for systems not substituted by β‐EWGs. X‐ray crystallography of the 5‐anilido‐MA derivative shows a hydrogen‐bonded thioenol structure. δ(OH), δ(NH), K_Enol, and crystallographic data for analogous thioenol and enol systems are compared. Copyright © 2008 John Wiley & Sons, Ltd.     

Computational design and screening of promising energetic materials: Novel azobis(tetrazoles) with ten catenated nitrogen atoms chain

Density functional theory methods were used to study on 2 N10 compounds, 1,1′‐azobis(tetrazole) and 1,1′‐azobis(5‐methyltetrazole). We systematically investigated 10 novel substituted azobis(tetrazoles) with 10 catenated nitrogen atoms and various energetic groups (–CF₃ 1 , –C(NO₂)₃ 3 , –N₃ 5 , –NH₂ 6 , –NHNH₂ 7 , –NHNO₂ 8 , –NO₂ 9 , –OCH₃ 10 , –OH 11 , –ONO₂ 12 ). The optimized geometry, frontier molecular orbitals, electrostatic potential, Infrared and nuclear magnetic resonance spectrum were calculated for inspecting the molecular structure and stability as well as chemical reactivity. The effects of different substituents on the density, enthalpy of formation, heat of explosion, detonation velocity and pressure, and sensitivity of the azobis(tetrazole) derivatives have been investigated. Compound 9 with nitro was found to have remarkable detonation performances (D = 9.61 km/s, P = 42.14 GPa), which are close to the excellent explosive CL‐20. Results show that compounds 1 , 3 , 4 , 7 , 9 , 11, and 12 have high potential to replace RDX. It is surprising that compounds 1 , 3 , 9, and 12 possess better energetic properties than HMX. These novel substituted azobis(tetrazoles) with unique N10 structure may be promising candidates of HEDMs with outstanding performance and acceptable sensitivities.     

Kinetics and mechanism of the reaction of alkoxymethylidene malonate and malononitrile with hydrazines and anilines

The kinetics and mechanism of the nucleophilic vinylic substitution of dialkyl (alkoxymethylidene)malonates (alkyl: methyl, ethyl) and (ethoxymethylidene)malononitrile with substituted hydrazines and anilines R¹–NH₂ (R¹: (CH₃)₂N, CH₃NH, NH₂, C₆H₅NH, CH₃CONH, 4‐CH₃C₆H₄SO₂NH, 3‐ and 4‐X‐C₆H₄; X: H, 4‐Br, 4‐CH₃, 4‐CH₃O, 3‐Cl) were studied at 25 °C in methanol. It was found that the reactions with all hydrazines (the only exception was the reaction of (ethoxymethylidene)malononitrile with N,N‐dimethylhydrazine) showed overall second‐order kinetics and k_obs were linearly dependent on the hydrazine concentration which is consistent with the rate‐limiting attack of the hydrazine on the double bond of the substrate. Corresponding Brønsted plots are linear (without deviating N‐methyl and N,N‐dimethylhydrazine), and their slopes (β_Nuc) gradually increase from 0.59 to 0.71 which reflects gradually increasing order of the C–N bond formed in the transition state. The deviation of both methylated hydrazines is probably caused by the different site of nucleophilicity/basicity in these compounds (tertiary/secondary vs. primary nitrogen). A somewhat different situation was observed with the anilines (and once with N,N‐dimethylhydrazine) where parabolic dependences of the kinetics gradually changing to linear dependences as the concentration of nucleophile/base increases. The second‐order term in the nucleophile indicates the presence of a steady‐state intermediate ‐ most probably T^±. Brønsted and Hammett plots gave β_Nuc = 1.08 and ρ = ?3.7 which is consistent with a late transition state whose structure resembles T^±. Copyright © 2013 John Wiley & Sons, Ltd.     

Electrochemical properties and radical anions of carbocycle‐fluorinated quinoxalines and their substituted derivatives

Electrochemical reduction (ECR) and oxidation (ECO) of 5,6,7,8‐tetrafluoroquinoxaline ( 1 ) and its derivatives bearing various substituents R (7‐H ( 2 ), 7,8‐H₂ (3 ), 6‐CF₃ ( 4 ), 6‐Cl ( 5 ), 5,7‐Cl₂ ( 6 ), 5‐NH₂ ( 7 ), 6‐OCH₃ ( 8 ), 6,7‐(OCH₃)₂ ( 9 ), 6,7,8‐(OCH₃)₃ ( 10 ), 5,6,7,8‐(OCH₃)₄ ( 11 ), 6‐OCH₃,7‐N(CH₃)₂ ( 12 ), 6‐N(CH₃)₂ ( 13 ), 6,7‐(N(CH₃)₂)₂ ( 14 ), 5,6,7‐(N(CH₃)₂)₃ ( 15 ), and 7,8‐cyclo‐(=CF‐CF = CF‐CF=) ( 16 )) in the carbocycle have been studied by cyclic voltammetry in MeCN. For 1 – 4 and 7 – 15 , the first reduction peaks have been found to be 1‐electron and reversible, thus corresponding to the formation of their radical anions (RAs), which are long lived at 295 K except those of 4 – 6 and 15 , 16 . Irreversible hydrodechlorination has been observed for 5 and 6 at the first step of their ECR confirmed by EPR detection of corresponding RAs of 2 and 5,7‐H₂ derivative of 1 ( 17 ) at the next steps. Electrochemically generated RAs of 1 – 3 , 7 – 14 , and 17 have been characterized in MeCN by EPR spectroscopy together with DFT calculations at the (U)B3LYP/6‐31 + G(d) level of theory using PCM to describe the solvent. A noticeable alternation of spin density on the –NCCN– moiety of quinoxaline has been observed for all RAs possessing R‐substitution asymmetry. The comparative electron‐accepting ability of 1 – 15 has been analyzed in terms of their experimental reduction peak potentials and the (U)B3LYP/6‐31 + G(d)‐calculated gas‐phase first adiabatic electron affinities (EAs). The differences in electron transfer solvation energies for 1 – 15 have been evaluated on the basis of ECR peaks' potentials and calculated gas‐phase EAs. The ECO of 1 – 5 and 7 – 14 has been found to be irreversible.     

Theoretical investigation of the backbone···π and π···π stacking interactions in substituted-benzene||3-methyl-2′-deoxyadenosine: a perspective to the DNA repair

The π-stacking effects of substituted benzenes on the N-glycosidic bond strength of 3-methyl-2'-deoxyadenosine (3-MDA) were studied by quantum mechanical calculations. Although all substituents enhance the stacking interactions, enhancement is higher for the electron-donating (ED) substituents. When the overall binding energy is separated into the π···π (ΔE_π···π) and backbone···π (ΔE_bb···π) contributions, the ED and electron-withdrawing (EW) substituents increase those contributions, respectively. Both the ED and EW substituents decrease the distance between the centres of stacked rings, while the EW ones increase the N-glycosidic bond length. The electron charge density calculated at the C--N bond critical point (ρ_C–N) is in linear correlation with the backbone···π interaction, not with the π···π interaction. This study also shows that the charge transfer from X-Ben to 3-MDA is in linear correlation with the ΔE_π···π and the change in the charge on the sugar ring is in better accordance with the backbone···π interaction. The N7 proton affinity (PA_N7), with a key role in the depurination process, is highly affected by the π···π interactions. Thus, both interactions must be considered because of the balance between the backbone···π and π···π contributions in these biomolecular systems.     

Protonation of captodative trifluoromethylated aminoalkenes and isomerization of their salts. An ab initio study

A study of the regioselectivity of protonation of captodative trifluoromethylated enamines was carried out using MP2/6‐311 + G(d,p) calculations and the natural bond orbital analysis. The central issue of this research concerns the influence of the electron‐withdrawing group, which is not capable of the π,π‐conjugation, on the properties of captodative enamines and their salts. The presence of CF₃ group in such type of enamines levels the energy of their N‐protonated and C‐protonated forms. The transition states were found for both intramolecular and intermolecular processes of the proton transfer. The more possible mechanism of the isomerization of enammonium and iminium cations includes the proton transfer from N‐protonated form to olefinic carbon atom of the starting enamine. The transition state energies, which correspond to intermolecular process, are relatively low (11–13 kcal mol^–1) in contrast to the intramolecular pathway (64–69 kcal mol^–1). Copyright © 2011 John Wiley & Sons, Ltd.     

Vibrational spectra of two BEDT-TTF-based organic conductors: charge order

Infrared and Raman investigations of two phases of bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) based organic conductors with the same CF₃CF₂SO₃⁻ anion: β′-(BEDT-TTF)₂CF₃CF₂SO₃ and δ′-(BEDT-TTF)₂CF₃CF₂SO₃, are shortly reviewed and compared with the most typical infrared properties of the family of (BEDT-TTF)₂RR′SO₃ organic conductors, where R = SF₅, CF₃, and R′ are CH₂, CF₂, CHF, CHFCF₂, and CH₂CF₂. The role of the molecular structur and spatial organization of the counterions is discussed. Presented at 2-nd International Conference on Functional Materials and Devices, ICFMD 2008, June 16–19, 2008, Kuala Lumpur, Malaysia     

FTIR spectra of plasticized high molecular weight PVC–LiCF3SO3 electrolytes

Fourier transform infrared spectroscopy studies have been conducted to investigate the interaction among components in a system of high molecular weight polyvinylchloride (PVC)–lithium trifluoromethanesulfonate (LiCF₃SO₃) incorporated with different type of plasticizers, namely, ethylene carbonate (EC), propylene carbonate (PC), and dibutylphthalate (DBP). Interaction between PVC and LiCF₃SO₃ was confirmed by C–H rocking mode at 1,255 cm⁻¹ for PVC shift to 1,252 cm⁻¹ in PVC–LiCF₃SO₃. The plasticizers’ carbonyl (C=O) oxygen atom which carries lone pair electrons interact with Li⁺ of LiCF₃SO₃ and methine hydrogen of PVC in LiCF₃SO₃–plasticizer system and PVC–plasticizer system, respectively. Changes in peaks assigned to 1,264 cm⁻¹ (ν _as(SO₃)), 1,033 cm⁻¹ (ν _s(SO₃)), 1,181 cm⁻¹ (ν _as(CF₃)), 1,230 cm⁻¹ (ν _s(CF₃)), 765 cm⁻¹ (δ _s(CF₃)), 644 cm⁻¹ (δ _s(SO₃)), 578 cm⁻¹ (δ _as(CF₃)), and 519 cm⁻¹ (δ _as(SO₃)) indicate the occurrence of complexation in the PVC–LiCF₃SO₃ system, LiCF₃SO₃–plasticizer system, and PVC–LiCF₃SO₃–plasticizer system.     

Substituent effects on the conformational stability of allyl fluorides

By the B3P86/6‐311G(3d,2p) method, remote substituent effects on trans‐YCH?CHCH₂F were investigated by examining their conformational stabilities, molecular geometries, and stereoelectronic interactions in this paper. The cis conformer is favored for Y?H, Cl, Me, Vinyl, CF₃, CN, CHO, and NO₂, whereas the gauche is favored for Y?OMe, OH. A correlation of ΔH with the substituent constants σ⁺(Y) shows that the increasing electron‐withdrawing ability of the substituent Y increases the relative stability of the cis conformer. It was found that the substituent effect on the molecule stabilization energies (relative to CH₂?CHCH₂F) is more significant in the gauche conformers than in the cis conformers. In agreement, molecular structures of the gauche conformers were also observed to vary more significantly with the substitution than those of the cis conformers. By the second‐order perturbation energy (E(2)) in NBO analysis, it was found that the total C₂–C₃ vicinal hyperconjugation is determinant in the enthalpy difference and consequently controls the conformational stability. Further analysis shows that the substituent effect on the C₂–C₃ vicinal hyperconjugations is much higher in the gauche conformers than in the cis conformers. The highly sensitive π_C?C→σ*_{C? F} interaction to the substitution in the gauche conformers, is the leading factor in variation of molecular stability and geometry. Copyright © 2010 John Wiley & Sons, Ltd.     

Hydrogen abstraction from H‐donor substrates by the 6‐CF3‐benzotriazol‐N‐oxyl radical (TFNO)

The aminoxyl radical 6‐trifluoromethyl‐benzotriazol‐N‐oxyl (TFNO) has been generated from the parent hydroxylamine 6‐CF₃‐1‐hydroxy‐benzotriazole (TFBT) by one‐electron oxidation with a Ce^IV salt and characterized by spectrophotometry and cyclic voltammetry (CV). Rate constants of H‐abstraction (k_H) by TFNO from a number of H‐donor benzylic substrates have been determined spectrophotometrically in MeCN solution at 25 °C. A radical H‐atom transfer (HAT) route of oxidation is substantiated for TFNO by several pieces of evidence. The kinetic data also testify the relevance of stereoelectronic effects upon the HAT reactivity of TFNO. Copyright © 2010 John Wiley & Sons, Ltd.